Harpur Hill, Buxton Derbyshire, SK17 9JN T: +44 (0)1298 218000 F: +44 (0)1298 218590 W: www.hsl.gov.uk

Dry Contaminants Scoping Study

HSL/2006/23

Project Leader: Dr. Marianne Loo-Morrey Phd., Msc., BSc.

Author(s): Dr. Marianne Loo-Morrey Phd., Msc., BSc.

Science Group: Human Factors

© Crown copyright (2006)

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ACKNOWLEDGEMENTS The work reported here was carried out at the request of Mr. Stephen Taylor, Construction Division Technology Unit in collaboration with the Tribology Research Lab at the University of Sheffield.

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CONTENTS

1 INTRODUCTION ...................................................................................................1

2 EXPERIMENTAL ..................................................................................................2 2.1 Low-speed friction rig ...........................................................................................2 2.2 Pendulum friction rig.............................................................................................3 2.3 Application of Contaminants.................................................................................3 2.4 Test Specimens & Contaminants...........................................................................4

3 KEY FINDINGS......................................................................................................6 3.1 Dry Contaminant Lubrication Mechanisms ..........................................................6 3.2 Effect of Surface Roughness .................................................................................7 3.3 Reproducibility of Data and Transport of Contamination.....................................9 3.4 Particle cohesiveness and its importance ............................................................10 3.5 Slip Mechanism Map...........................................................................................13

4 CONCLUSIONS....................................................................................................15

5 FUTURE WORK...................................................................................................16

6 REFERENCES ......................................................................................................17

7 APPENDICES........................................................................................................18 Appendix 1 – University of Sheffiled Report “ The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination.........................................................................18

8 GLOSSARY ...........................................................................................................70

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EXECUTIVE SUMMARY Pedestrian slipping in the presence of a fluid contaminant can be understood in terms of squeeze film theory. The understanding of this theory provides a rigorous scientific underpinning as to why the pendulum test is suitable for assessing the slip resistance of wet floors, and a means by which HSE/HSL can give informed advice to duty holders on flooring requirements based on the viscosity of likely contaminants.

There is currently no such fundamental understanding of the mechanisms involved in pedestrian slipping in the presence of dry contaminants.

Objectives

The aims of the scoping study were:

• To investigate the effects of particulate contaminants on coefficient of friction. • To generate simple mechanisms to describe the observed behaviour. • To generate an idea of the quantities of contaminant required to increased slip potential. • To provide a framework for interpreting slip resistance data. • To highlight potential routes for further research.

Main Findings

Adhesive friction is significantly affected by the presence of solid contaminants, but hysteretic friction is not.

Smooth floor (Rz of the order of a few m) are highly susceptible to the effects of dry contaminants. Even small quantities of dry contaminant can significantly reduce available friction and result in a high slip risk.

Three distinct dry contaminant mechanisms have been identified with the contact during slipping: Sliding: This mechanism occurs when the limiting shear stress at the particle-

shoe interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer. The particles than all slide with the shoe.

Shearing: This mechanism occurs when the layer of particles itself shears.

Tumbling/Rolling: This mechanism occurs for larger single particles layers. The particles tend to roll or tumble through the contact.

Results indicate that the sliding and rolling mechanisms on smooth floors pose the most significant risk to pedestrians.

Dry contaminants with a particle diameter of approximately 50 m or less (e.g. Flour,Talc, Coco, and Baking Powder) have higher levels of cohesion and therefore are likely to clump and promote the sliding mechanism in the contact.

Pendulum tests showed that rougher surfaces break down the sliding particle layer and promote shearing of the particle layer.Floor surface micro-roughness (Rz) may be a means of controlling the slipping hazard posed by dry contaminants.

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Recommendations

The current scoping has highlighted a number of potential avenues for future research including:

• To identify the physical characteristics of flooring and footwear which promote good hysteretic friction to better inform advice to duty holders regarding controlling the hazards posed by dry contaminants

• To enlarge the database of laboratory data on slip resistance in particle contaminated conditions using the pendulum and sled tester.

• To further understand the mechanisms by which solid particles cause reduction in friction between the shoe and the floor.

• To reconsider the current suggested site investigation test procedure in cases where there is dry particulate contamination to ensure it is the best approach to take.

• To define simplified criteria and standards for advice on flooring and footwear requirements to deal with dry contaminants

• Examine the possibility of adapting current testing methods for assessing the anti-slip performance of footwear for use with dry contamination

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1 INTRODUCTION

The work reported here was carried out at the request of Mr. Stephen Taylor, Construction Division Technology Unit in collaboration with the Tribology Research Lab at the University of Sheffield.

Mr. Taylor received an unsolicited project proposal for the development of a slip resistance test method for use in dry contaminated conditions. The author was requested to review the proposal and give an opinion as to it’s suitability for further consideration by HSE. While there were numerous technical and financial reasons for not pursuing the project proposal as received by Mr. Taylor, it did highlight the limited work conducted into dry contaminants by HSL / HSE in the past.

Previous dry contaminant research [Lemon et al.] has been largely empirically based, and focused on establishing the suitability of the pendulum test method for use in dry contaminated situations. This work clearly demonstrated that the pendulum test could distinguish between clean dry floors and floors with dry contaminants. The pendulum test data indicated that the presence of dry contaminants could reduce the level of available fiction to the point where pedestrian slipping incidents could reasonably be expected to occur.

Consideration of the research proposal received by Mr. Taylor highlighted a significant knowledge gap in HSE’s understanding of the pedestrian slipping problem. Pedestrian slipping in the presence of a fluid contaminant can be understood in terms of squeeze film theory [Richardson and Griffiths, Lemon and Griffiths]. The understanding of this theory provides a rigorous scientific underpinning as to why the pendulum test is suitable for assessing the slip resistance of wet floors, and a means by which HSE/HSL can give informed advice to duty holders on flooring requirements based on the viscosity of likely contaminants.

There is currently no such fundamental understanding of the mechanisms involved in pedestrian slipping in the presence of dry contaminants.

It was therefore decided to conduct a small scoping study to determine the feasibility of developing a model for pedestrian slipping in the presence of dry contaminants. The aims of the scoping study were:

• To investigate the effects of particulate contaminants on coefficient of friction. • To generate simple mechanisms to describe the observed behaviour. • To generate an idea of the quantities of contaminant required to increased slip potential. • To provide a framework for interpreting slip resistance data. • To highlight potential routes for further research.

The development of a model for dry contaminant slipping would require a thorough understanding of contact mechanics and an understanding of how to model solid debris within a contact. It was therefore decided to conduct this work in conjunction with the Tribology Research Lab at the University of Sheffield as they have an established track record in researching contact mechanics and modelling contaminants within a contact. The aim of this report is to summarise the scoping study, which is reported in full in the University of Sheffield report, which is reproduced in Appendix 1.

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2 EXPERIMENTAL

Dry contaminant slip resistance tests were conducted using two different test methods, the pendulum test and the University of Sheffield low speed friction rig.

2.1 LOW-SPEED FRICTION RIG

Four-S samples were cut from standard pendulum slider samples (see Figure 2.6; 76 x 6.35 x 25.4mm) with the dimensions of 25 x 25mm (±1mm) and mounted on 1 inch diameter steel buttons using double sided tape (proved in preliminary tests to generate sufficient shearing resistance). The floor tile samples were cut to dimensions of 50 x 150mm and also fixed with double-sided tape after cleaning with acetone.

Figure 2.1: Low speed friction rig apparatus photo

The required load was positioned on the pivoting arm of the LSFR (see Figures 2.1 and 2.2) and Four-S ‘button’ was placed in its housing and aligned 10mm from the edge of the counter-face. Zeroing of the force transducer was performed and test initiated with a sample capture rate of 20Hz. Three runs were performed for each configuration of the apparatus.

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380

45

LOADSLIDER

SPECIMEN

COUNTERBALANCE FORCE

TRANSDUCER

BI-AXIALPIVOT

(a) Lateral view

(b) Plan view

FRICTIONALLOAD

MEASUREDFORCE

Figure 2.2: Sliding mechanisms observed on LSFR and pendulum tests

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2.2 PENDULUM FRICTION RIG

Preparation of the pendulum apparatus, shown in Figure 2.3, was conducted in accordance with the United Kingdom Slip Resistance Group (UKSRG) Guidelines [UKSRG], which are typically used by HSL for forensic site investigations.

Particles were applied by two methods, one equivalent to that used for the LSFR and the other using a paintbrush or finger to apply a very fine layer to the slider only. Two series of eight tests were performed for each configuration. The first set involved re-application of contaminant after every stroke whilst in the second set contaminant was applied prior to the first stroke only.

2.3 APPLICATION OF CONTAMINANTS Contaminants were applied to the floor tile and forced into depths of 500, 1000, 1500 and 2000µm by the use of specially made ‘combs’, see Figure 2.5. The resulting particle tracks had a width of 40mm allowing a 30% overlap each side of the slider. Problems were encountered with the smaller particles caused by high cohesiveness with sweeping occurring at all depths, preventing the generation of even layers. Subsequently a degree of compaction was required though the pressure applied was always less than that applied by the pendulum or low speed friction rig (LSFR).

High speed camera

Pendulum tester

Glass counter-face and powder

Slider

Figure 2.4: High speed video apparatus for Pendulum Tester

PENDULUM ARM

INITIAL POSITION

FOUR-S RUBBERSLIDER

SRV SCA LE SRV INDICATOR

126mm (±1mm) CONTACT STROKE

FLOOR SPECIMEN

Lateral view

PENDULUM ARM

INITIAL POSITION

FOUR-S RUBBERSLIDER

SRV SCA LE SRV INDICATOR

126mm (±1mm) CONTACT STROKE

FLOOR SPECIMEN

Lateral view

Figure 2.3: Stanley Pendulum Tester schematic

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Figure 2.5: ‘Combs’ used for generating specific particle thickness’

2.4 TEST SPECIMENS & CONTAMINANTS In both test configurations standard Four-S rubber specimens have been used (IRHD hardness 94° – 98°), as shown in Figure 2.6.

Figure 2.6: LSFR Four_S samples

Preparation of the Four-S sample followed a similar protocol to that adopted for pendulum tester where by the contact surface was initially conditioned with P400 abrasive paper followed by a series of unidirectional runs along wet 3µm pink lapping paper. It is suggested that this produces a similar surface to that seen in ‘bedded-in’ shoes. Table 2.1 shows the test contaminant particles used. These were chosen to be typical of particles found in factory situations and have been associated with slip risks. The particles sizes were measured using a laser scattering analyser. The full report in Appendix 1 gives photographs of the particles and the particle size distributions.

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Contaminant Ref.

No. Mean particle size (µm)

[Approximate] Uni-axial particle

compressive strain (at 125.6 kPa)

Talcum Powder 1 10 0.4 Cocoa Powder 2 15 0.2

Flour 3 25 0.25 Baking Powder 4 30 0.15

Bicarbonate of Soda 5 50 0.04 Table Salt 6 500 0.01

Ballotini (glass spheres) 7 500 0.01 Sand 8 1000 0.01

Table 2.1: Particle contaminant data

Floor materials were selected to achieve a range of surface roughness (shown in Table 2.2). In these tests, the roughness parameter Rz (mean peak to valley height) has been used. This is the parameter that HSL/HSE routinely records during site investigations to determine the roughness of a flooring material. The full report in Appendix 1 shows some photographs of the tile specimens used.

Tile Ref. Code Mean Rz (µm) Perlina polished marble PED/05/156 0.51

Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32

Riven slate PED/05/159 34.18 Table 2.2: Tile counter-face data

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3 KEY FINDINGS

3.1 DRY CONTAMINANT LUBRICATION MECHANISMS Detailed observations of the LSFR and pendulum tests have identified three mechanisms for dry contaminant behaviour during a slip 1) sliding, 2) shearing and 3) tumbling and rolling which are illustrated in Figure 3.1.

Sliding: This mechanism occurs when the shear stress at the particle-shoe

interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer.

Shearing: This mechanism occurs when the shear stress at the particle-flooring interface is greater than the shear stress required to shear the particles in the layer, but less than the shear stress at the particle-shoe interface.

Slider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

SliderPowder

Slider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

SliderPowder

Slider

Floor counter-face

Shear planeSlider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

Slider

Floor counter-face

SliderPowder

Figure 3.1: Sliding mechanisms observed on LSFR and pendulum tests, where τps is the

shear stress at the particle-shoe interface, τpf is the shear stress at the particle-floor interface and τcrit is the shear stress required to shear the particles within the layer

SLIDING τps > τpf < τcrit : Entire powder layer has velocity of slider with extremely small surface layer of fine particles.

SHEARING τpf > τcrit < τps : Shear failure of particle layer occurs resulting in contact thinning and material seen in wake of slider.

ROLLING / TUMBLING For larger single particle layers, with counter face roughnesses to allow rolling at shear stresses < τpf, particles will roll and tumble rather than slide

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Tumbling/Rolling: This mechanism occurs for larger single particles layers, where the floor surface roughness is small compared to the particle diameter, such that rolling occurs at shear stresses less than the shear stress at the particle floor interface.

Slip resistance tests indicated that of the three dry contaminant mechanisms identified, sliding was generally associated with the greatest reduction in slip resistance, followed by rolling/tumbling and finally shearing which resulted in the smallest reduction in available friction.

These results imply that when attempting to control the slip risk posed by dry contaminants measures should be taken to select flooring and footwear, which will promote a shearing type mechanism for the particles in the shoe/floor contact.

3.2 EFFECT OF SURFACE ROUGHNESS

3.2.1 Components of Friction The friction in sliding contacts involving an elastomer (any rubber like material) is composed of two parts; adhesive friction (also known as adhesion, see glossary) and hysteretic friction (also known as hysteresis, see glossary) [Moore]. The total sliding friction is given by the sum of these two components:

HystereticAdhesiont fff += (1) Adhesive Friction When an elastomer, such as the pendulum slider or a shoe sole, comes into contact with a counter-face (a flooring material), temporary adhesive bonds are formed between them at the molecular level. When subjected to sufficient shear stress, the temporary bonds will stretch, break and reform in a cyclic slip-stick fashion. Adhesive friction is a measure of how difficult it is to break the temporary bonds between the elastomer and the counter-face. The more difficult it is to break the bonds the higher the level of adhesive friction. The magnitude of the adhesive force depends on the area of the elastomer in contact with the counter-face, which is directly related to the surface roughness of the counter-face, see Figure 3.2.

Figure 3.2: Schematic of effect of roughness on contact area

From Figure 3.2 it can be seen that increasing the surface roughness of the counter face reduces the area of contact between the elastomer and the counter-face. The highest levels of adhesive friction can therefore be reasonably be expected for the smoothest surfaces.

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Hysteretic Friction

The hysteresis component of friction is a measure of the way energy is absorbed internally by the elastomer. It arises mainly from the way in which the elastomer deforms around the asperities (peaks in the surface, see Appendix 1) on the counter-face surface. As the surface roughness of the counter face increases, the size of the asperities on the surface increases and the deformation of a elastomer in contact with it is increased. It would therefore be reasonable to expect the level hysteretic friction to be higher for rougher surfaces.

If an ideal symmetrical asperity is considered, see Figure 3.3a, the pressure profile developed will be symmetrical in the static case. However, if there is relative motion between the elastomer and asperity, as during the pendulum swing or when a pedestrian walks, the viscoelastic nature of the elastomer causes a non-symmetric pressure profile biased towards the direction of movement to be developed, see Figure 3.3b.

The asymmetric nature of the pressure profile generated when there is relative motion between the elastomer and the counter-face results in a net retardation force, which is responsible for the hysteretic component of friction.

The total friction measured in a sliding contact between an elastomer and a counter-face is combination of these two components, adhesive friction and hysteretic friction.

3.2.2 Pendulum Results

The Rz surface roughness values for the tiles used in this project are given in Table 3.1.

Tile Ref. code Mean Rz (µm) Perlina polished marble PED/05/156 0.51

Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32

Riven slate PED/05/159 34.18 Table 3.1: Tile counter-face data

AsperityElastomerElastomer

Symmetric pressure profile(no relative motion)

Retarding bias (relativemotion)

a) Static case b) Relative motion between elastomer and counter-face.

Figure 3.3: Schematic of hysteretic deformation of an elastomer around and asperity

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Pendulum data for the tiles in the clean and dry condition and when contaminated with baking powder are shown in Figure 3.4.

Figure 3.4 Graph comparing PTV and coefficient of friction pendulum data for the flooring samples in the clean dry condition and in the contaminated condition. Rz surface roughness of the flooring samples increases from left to right.

In the clean dry condition the highest pendulum values and therefore the highest levels of total friction were achieved for the smoothest tiles. As the surface roughness increases the PTV value decreases, as might be reasonably expected from the reduced effect of adhesive friction. When the dry contaminant was introduced (baking powder) the PTV values measured were lower for all the floors studied. However the reduction in PTV observed decreased as the surface roughness of the flooring increased.

This finding suggests that the presence of a dry contaminant significantly reduces the effect of adhesive friction, which is the dominant friction component for smooth floor, but that the hysteretic friction, which is the dominant friction mechanism for rougher floors is relatively unaffected by the presence of a dry contaminant.

The pendulum test results suggest that surface roughness may have a significant role to play in controlling the slip risk posed by dry contaminants.

3.3 REPRODUCIBILITY OF DATA AND TRANSPORT OF CONTAMINATION

During tests with both the pendulum test and the LSFR apparatus dry contaminant particles tended to adhere to the slider surface. The adhesive nature of the sliders tends to cause particles to adhere to it. Even if the Four-S specimen was well cleaned (with a dry cloth), the PTV was lower than the water-cleaned sample by up to 10 - 20 PTV, particularly for the finer, more cohesive powders.

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The action of the pendulum tends to sweep contaminant particles away from the floor test area. Repeated test swings without the application of fresh contaminant generated similar results to those in which the contaminant was reapplied between each swing of the pendulum. Tests on the Perlina tile showed that on average the PTV for the different contaminants increased by approximately 3 points over a series for eight test swings of the pendulum without the application of fresh contaminant. Such smooth floors result in particle sliding for almost all cases (outlined in detail in the full report Appendix 1) with very small volumes of contaminant required to significantly reduce the friction measured by the pendulum (up to approximately 70 points). The pendulum results indicate that only a very small amount of dry contaminant particles are needed to appreciably reduce friction. The practical consequences of these results are that reductions in friction will be sustained for a number of steps on a smooth floor. This is analogous to the effects of liquids where a wetted slider will exhibit low friction coefficients on a smooth dry floor for a number of swings, but to a lesser degree.

This observation has serious implications for real workplace situations. It suggests that there is a high probability of pedestrians transporting dry contaminants such as dust over considerable distances into a building on their footwear. It again highlights the importance of entrance matting systems as a first line of defence for preventing the ingress of contamination in to building and further supports HSE/HSL’s position that the minute levels of contamination carried on a pedestrian’s footwear can be sufficient for certain types of flooring to pose a high potential for slip.

3.4 PARTICLE COHESIVENESS AND ITS IMPORTANCE

It was observed in both test methods that smaller particles form a compacted layer and a clean, ‘swept’ region behind the slider. Particles ahead of the slider do not get entrained and are instead forced along or displaced around the edges of the slider. This sliding mechanism causes low friction and warrants further investigation. For this condition to be observed, the following criteria must be satisfied:

rppf ττ < and critpf ττ < Where τpf is the shear stress at the particle-floor interface τrp is the shear stress at the rubber- particle interface τcrit is the required shear stress to shear the particles within the layer.

Thus the shear occurs at the particle floor interface, i.e. the dry contaminant particles and the pedestrian’s shoe stick together and slide.

All particles below approximately 50 m diameter, tended to clump together. This can be attributed to cohesive forces. Cohesiveness is a measure of the mechanical shear strength of a powder. A coulomb powder will generally follow a stress-strain curve as shown in Figure 3.5a, [Orband and Geldart]. When the shear strength of the particles is reached, failure occurs causing relative particle movement. There is a critical particle diameter below which, the shear strength rapidly increases (represented in Figure 3.5b). This critical particle diameter was found to be between 50 and 60µm.

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ε

σs

D

σs

Increasingconsolidation

pressure

Critical diameter Figure 3.5: (a) Generic stress-strain curve and (b) variation of critical stress with particle

diameter for powders Industrial experience suggests that particles below around 55µm cause increased handling problems as a result of this effect. With many food and industrial contaminants falling within this size range (e.g. Cocoa ~ 15µm, Baking Powder ~ 30 µm and Talc ~ 10 µm) the effects of cohesiveness are significant. In real workplace situations, the depth of the contaminant layer is likely to be small. The weight of the particles is therefore almost negligible and all layers of the powder can be thought of as uncompressed. In this situation the effects of cohesiveness generate macroscopic particulate structures that support the weight of the higher particle layers. The effect of these structures is to reduce the bulk density of the particles and decrease the macroscopic powder stiffness. Volumetric strain under compression can be a method for comparing the cohesiveness of particles and predicting their effect on CoF. Uni-axial compression was used to compare the particles and was performed using a syringe of known volume. Subjected to a pressure approximately equivalent to that used in the pendulum and sliding rig, the particles behaved as shown in Figure 3.6. It can be seen that a marked increase in the compressive strain occurs as the particle diameter decreases. The transition point occurs within the shaded band between approximately 45 and 60µm, which corresponds well the results obtained by Orband and Geldart [Orband and Geldart].

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Figure 3.6 Graph of powder compressive strain against particle diameter. This finding indicates that the powders of small particle size will agglomerate. This will promote the sliding mechanism, leading to a situation of high slip potential.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 100 200 300 400 500 600 700 800 900 1000

Particle diameter

Com

pres

sive

str

ain

Sand

BallotiniBicarbonate of Soda

Baking Powder

Flour

Cocoa

Talcum Powder

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3.5 SLIP MECHANISM MAP

A simple map showing the regions in which the different mechanisms occur is shown in Figure 3.7. This is based purely on particle diameter and surface roughness.

The map shows the following features:

(i) Sliding Occurs on Low Roughness Floors. It has been observed that sliding occurs when the flooring has a low roughness (in the order of a few microns) or when d/Rz is sufficiently large so as to make the flooring roughness negligible. From current studies, a potential transition occurs (between sliding and shearing) at a floor Rz surface roughness of approximately 15 – 20 µm for the tested contaminants. Floors with a roughness of less than this value do not allow sufficient mechanical interlocking between the asperities and contaminant to exceed τcrit and sliding occur, potentially causing a low CoF. Essentially a rough floor cuts through the layer of dry contaminant particles and stops it sliding as one block with the shoe surface.

(ii) The Cohesive Boundary. Particles tend to stick together when their size is below

approximately 55 m. This is approximately the size of bicarbonate of soda particles (and has been termed the ‘cohesive boundary’). Current observations suggest that the floor surface roughness for which the sliding mechanism occurs, decreases with particle size until the cohesive boundary is reached. This is likely to be due to the reduction of the cohesive forces, tending to prevent the agglomeration of particles. The critical level of surface roughness required to prevent the sliding mechanism increases as particle size increases.

(iii) Particle Layer Shearing Takes Place On Rough Floors. Inter-particle shearing

occurs when the surface roughness is large but particle diameter relatively small. Asperities on the floor surface prevent the particle layer sliding in a bulk form. As shearing takes place, the layer between slider (or a pedestrian’s shoe) and floor reduces in thickness as particles are lost. In the case of smaller cohesive particles, the uncompressed

Figure 3.7: Map of particle size, d and tile floor roughness,

Rz, showing regimes of slip mechanisms.

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material ahead of the contact is not entrained meaning that removal and thinning of material in the contact occurs. Increasing the particle diameter such that cohesive effects are negligible causes a reduction in the separation of shoe and floor often followed by particle tumbling and rolling.

(iv) Particle Rolling/Tumbling Occurs for the Larger Particles. Particle tumbling and

rolling occurs when there is sufficient traction to preferentially allow the particles to tumble and roll rather than slide against the floor. This occurs for particles of greater diameter than the cohesive boundary (~ 50 microns). Shape factor (see full report Appendix 1) greatly influences the mechanism, with particles of increased roundness rolling at lower levels of floor roughness.

(v) The Effect of Particle Shape. The map shows the variation of dry contaminant

mechanism with shape factor (see full report Appendix 1). A straight vertical shift of the boundaries has been predicted both below and above the cohesive boundary. This is shown as the red dotted lines on Figure 3.7. Further work is required as it is likely that shape factor has less of an effect below the cohesive boundary with inter-particle bonds being comparable to forces generated by mechanical interlocking.

(vi) Depth of the Particle Layer. The current map does not incorporate any information

about the depth of the particle layer. In the case of small cohesive powders, a range of particle depths will end in the same sliding mechanism. However significant depths may result in a transient progression from shearing to sliding (through depth loss from the shearing process). Shearing to rolling/tumbling will also occur for particles with a tendency to roll. This is discussed in detail in the University of Sheffield Report in Appendix 1 for ballotini particles. Further work is required to incorporate a quantitative depth / surface density function.

Region ‘A’ represents a high-risk situation where adhesive friction dominates. Very little contaminant is required to cause a significant reduction in coefficient of friction. Contamination can originate from areas external to the considered floor and be transported considerable distances by sticking to the sole for pedestrian’s footwear. Additionally, the small size and low volume of contaminants required to pose a slipping hazards on smooth floors, means sighting by pedestrians is unlikely. In a similar manner to liquid contaminants, polished floors in areas such as entrance halls present significant risks without sufficient measures to remove contamination from the shoes and surface in general. In addition, the sliding mechanism means powder is only required at the impact point with no relative motion between slider and powder layer. The rolling or tumbling and (to less of an extent) shearing mechanisms however, require a constant input of contaminant to maintain the effective lubrication and are therefore more dependant on a larger area of contamination.

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4 CONCLUSIONS

Adhesive friction is significantly affected by the presence of solid contaminants, but hysteretic friction is not. Smooth floors ( Rz of the order of a few m) are therefore more susceptible to contamination (up to a 70 PTV point reduction). Even very small quantities of dry contaminant particles can reduce the shoe-floor adhesion and cause a high slip risk for pedestrians. Small particles are able to adhere to the soles of shoes allowing transference of the contamination over appreciable distances and continued reduced friction coefficient for many steps. Three significant slipping mechanisms were observed within the contact; sliding, shearing and rolling/tumbling. Sliding and rolling on smooth floors pose the most significant risk. Sliding: This mechanism occurs when the shear stress at the particle-shoe

interface is greater than the particle-floor interface, but less than the shear stress required to shear the particles within the layer. Sliding contacts require contaminant only at the impact site with no further contaminant required to feed the contact.

Shearing: This mechanism occurs when the shear stress at the particle-flooring interface is greater than the shear stress required to shear the particles in the layer, but less than the shear stress at the particle-shoe interface. Leading the particle layer itself to shear.

Tumbling/Rolling: This mechanism occurs for larger single particles layers. Tumbling rolling contacts require a constant supply of contaminant throughout the slip (though only at a depth of a single particle).

Results indicate that the sliding and rolling mechanisms on smooth floors pose the most significant risk to pedestrians. Dry contaminants with a particle diameter of approximately 50

m or less have higher levels of cohesion and therefore are likely to promote the sliding mechanism in the contact.

Pendulum test results indicate that floor surface micro-roughness (Rz) may be a means of controlling the slipping hazard posed by dry contaminants.

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5 FUTURE WORK

The current scoping has highlighted a number of potential avenues for future research including:

• To identify the physical characteristics of flooring and footwear which promote good hysteretic friction to better inform advice to duty holders regarding controlling the hazards posed by dry contaminants

• To enlarge the database of laboratory data on slip resistance in particle contaminated conditions using the pendulum test.

• To further understand the mechanisms by which solid particles cause reduction in friction between the shoe and the floor.

• To reconsider the current suggested site investigation test procedure in cases where there is dry particulate contamination to ensure it is the best approach to take.

• To define simplified criteria and standards for advice on flooring and footwear requirements to deal with dry contaminants

• Examine the possibility of adapting current testing methods for assessing the anti-slip performance of footwear for use with dry contamination

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6 REFERENCES

P. Lemon and S. Griffiths, “Further Application of Squeeze Film Theory to Pedestrian Slipping.”, HSL report, IR/L/PE/97/9, 1997.

P. W Lemon, S. C Thorpe, S. L Jefferies, C. Sexton and M. Hawkins, “Pedestrian Slipping: Dry Contaminants.”, HSL Report, PE/01/15, Health & Safety Laboratory, 2001

D. F. Moore, ‘The Friction and Lubrication of Elastomers’ Pergamon Press, Oxford, Chapter 2, 1972.

J. L. R. Orband and D. Geldart ‘Direct measurement of powder cohesion using a torsional device’, Powder Technology 92, 25b – 33, (1997).

M. T. Richardson and R. S. Griffiths, “The Application of Squeeze Film Theory to Pedestrian Slipping Research.” HSL report, IR/L/PE/96/4, 1996.

UKSRG, United Kingdom Slip Resistance Group, “The Measurement of Floor Slip Resistance - Guidelines Recommended by the UK Slip Resistance Group”, Issue 3, October 2005.

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7 APPENDICES

APPENDIX 1 – UNIVERSITY OF SHEFFILED REPORT “ THE MECHANISMS OF PEDESTRIAN SLIPS CAUSED BY SOLID PARTICLE CONTAMINATION

The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination.

R. S. Mills R. S. Dwyer-Joyce

The Tribology Research Lab at the University of Sheffield The University of Sheffield

Department of Mechanical Engineering

Mappin Street

Sheffield S1 3DJ

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i Summary

An investigation has been undertaken to consider the effects of solid contaminant particles on

slipping risks to pedestrians in the workplace. Experiments were carried out using a sled-type

friction tester and a pendulum tester. In both cases emphasis was placed on observations of

the mechanisms of particle behaviour and slip. Results show that adhesive friction is

significantly affected whilst hysteretic friction is not. Smooth floors (in the order of a few

microns) can therefore be highly susceptible with a reduction in Pendulum Test Value (PTV)

of up to 70 points. Small particles can adhere to the soles of shoes allowing transference of

contaminant between different places and reducing the friction experienced by a pedestrian

for many steps. Very small amounts of particles were shown to lead to large reductions in

PTV.

Three slipping mechanisms were shown to occur; sliding, shearing and rolling/tumbling.

Sliding and rolling on smooth floors pose the most significant risk (medium to high on the HSL

slip risk classification scale). A simplified map of the regimes (of particle size and roughness)

where these mechanisms occur has been created.

The layer of particles required to cause increased risk depends on the mechanism. Sliding

requires particles at the impact point only, whilst rolling/tumbling requires a layer of fresh

particles for the duration of the slide. Further research into the relationship of different

roughness parameters and the use of a customised rig for the pendulum tester are suggested

to progress the current findings.

20

ii Contents The Mechanisms of Pedestrian Slips Caused by Solid Particle Contamination. .................... 19 i Summary .......................................................................................................................... 20 ii Contents ........................................................................................................................... 21 1.0 Nomenclature ............................................................................................................... 23 2.0 Introduction ................................................................................................................... 24

2.1 Aims:...................................................................................................................... 24 3.0 Background................................................................................................................... 25

3.1 Current status of work performed by the HSL....................................................... 25 3.2 Pendulum friction tester......................................................................................... 25 3.3 Low speed friction rig (LSFR): Sled-type tester .................................................... 26 3.4 Forces and velocity in human Gait ........................................................................ 27 3.5 Potential problems with test methods.................................................................... 28

4.0 Experimental technique and apparatus ........................................................................ 30 4.1 Low-speed friction rig ............................................................................................ 30 4.2 Pendulum friction rig.............................................................................................. 32 4.3 Application of Contaminants.................................................................................. 32 4.4 Test Specimens & Contaminants .......................................................................... 33

5.0 Results and Discussion: ............................................................................................... 35 5.1 Particulate lubrication mechanisms in the shoe-floor contact ............................... 35 5.2 Effect of particles on coefficient of friction ............................................................. 40 5.3 Repeat testing (single application of contaminant to slider).................................. 45 5.4 Particle cohesiveness and its importance ............................................................. 46 5.5 Map of Particle Slip Mechanisms .......................................................................... 48 5.6 Effect of applied pressure on traction.................................................................... 51 5.7 Effect of shape factor on traction........................................................................... 52

6.0 Conclusions .................................................................................................................. 54 7.0 Implications of findings .......................................................................................... 55 7.1. Implications for the HSE:........................................................................................... 56

Forensic and site investigations........................................................................................... 56 8.0 Future Work .................................................................................................................. 58 9.0 References.................................................................................................................... 60 10.0 Glossary of terms ...................................................................................................... 61 Appendix 1: Particle photos................................................................................................ 63 Appendix 2: Tile photos ........................................................................................................... 64 Appendix 3............................................................................................................................... 64 Appendix 3............................................................................................................................... 65 Appendix 4............................................................................................................................... 66 Appendix 5: Description of Shape factor (F) ...................................................................... 67

21

Appendix 6: Selected test data:............................................................................................... 68 6.1 LSFR test data....................................................................................................... 68 6.2 Perlina tile test results (Pendulum method)........................................................... 68 6.3 Clean tile tests (Pendulum method) ...................................................................... 69

22

1.0 Nomenclature

Rz

µS

µd

θ

P

R

p

A

E*

δ

D

hx

V

F

τx

Mean peak–valley height roughness (µm)

Static coefficient of friction

Dynamic coefficient of friction

Slider angle of attack (degrees)

Applied normal load (N)

Radius of curvature (m)

Particle perimeter (m)

Particle or contact area (m2)

Modified contact stiffness (Nm-2)

Contact deformation (m)

Particle diameter (m)

Contact separation at position x (m)

Relative contact sliding velocity (ms-1)

Shape factor

Shear stress at interface ‘x’

23

2.0 Introduction

Slips, trips and falls are a significant cause of accidents in the workplace causing as many as

one in three major workplace accidents [1]. Reduction in the coefficient-of-friction (CoF) due

to the presence of a liquid is well understood and models are available (Reynolds equation

[2]) to predict the behaviour of a contact subjected to such contamination. There are also

readily available simple criteria for use by flooring manufacturers, factory operators, and

inspectors for liquid slip resistance.

Solid particulate contaminants pose more of a problem. The particles come in many forms;

they are discrete in nature (they are not a continuum like liquids) and their size can be large

relative to the floor roughness. This leads to many mechanical properties such as stiffness,

fracture toughness and hardness being possible factors determining the CoF when particles

are present. Inter-particle forces which are fundamental for much of the continuum behaviour

of liquids are very much smaller for particles and vary substantially with particle size.

This report is intended to highlight the key parameters involved in slipping on solid particle

contamination, develop simple models for the mechanisms involved and suggest possible

avenues for future research. This study is not exhaustive, it is intended to generate a

framework for interpreting data, accidents, and developing criteria for slip resistance in

particle contamination cases.

2.1 Aims:

• To investigate the effects of particulate contaminants on coefficient of friction.

• To generate simple mechanisms to describe the observed behaviour.

• To generate an idea of the quantities of contaminant required to cause increased slip

potential.

• To provide a framework for interpreting slip resistance data.

• To highlight potential routes for further research.

24

3.0 Background

3.1 Current status of work performed by the HSL

To date, the HSL have only undertaken relatively small projects to investigate the risks

associated with solid particle contaminants [3] and [4] and have concentrated on empirical

data obtained from the Stanley Pendulum Tester and ‘Sled-Type’ testers. The results

illustrated the effects of solid particulate contamination clearly. But the work was not aimed at

mechanistically describing the processes involved. This project has been undertaken to

progress the work and apply simple models to the observed phenomena.

An important finding proposed in [3] was that the roughness of the flooring material would not

play a fundamental role unless it was comparable to the dimensions of the contaminant. The

work of this report suggests that roughness can be important at all scales and controls the

effect of the contaminants on the different friction components (adhesion and hysteretic). It is

acknowledged however that further testing is required to construct a comprehensive

knowledge base.

The current investigation concentrates on friction measurements from high speed impacts

using the Stanley Pendulum Tester and low speed continuous force measurement using a

custom built sled-type rig.

3.2 Pendulum friction tester

The preferred method used by the HSL to determine the CoF in the field is the Stanley

Pendulum Tester [1]. This apparatus consists of a weighted slider fixed to the end of a

pivoting arm. The arm is released from an initially horizontal position and contact with the

flooring material occurs at the bottom of the stroke for a distance of 126 mm. As the slider is

drawn across the flooring material, a portion of the kinetic energy is lost due to the frictional

retarding force, resulting in a lower final arm position. The output scale of ‘Pendulum Test

Value’ (PTV) can be converted to CoF using equation 1 (from BSI 96/104915 [B/208]).

1

31110 −

⎟⎠⎞

⎜⎝⎛ −=

PTVµ (1)

Based on observed PTV results, floor condition is given a grade corresponding to an

expected slipping risk as shown in Table [1].

25

Pendulum PTV Associated ‘slip potential’ 0 - 24 High 25 - 35 Moderate

36+ Low Table [1]: Slip risk classification, based on pendulum test values [1]

For clean dry and wet conditions, the pendulum tester has been shown to be effective for

measuring the friction between the rubber test slider and floor material [1]. However, solid

particle contaminants are more problematic. For all pedestrian-slipping applications, a fluid

can be considered a continuum with the surface roughness being far greater than the size of

the fluid particles. Intermolecular bonding and very short range equalisation (pressure etc.)

means that the properties such as particle stiffness and geometry are far less significant.

Instead the shearing parameters such as viscosity take precedence. Particles on the other

hand, act in a far more discrete manner with stiffness, strength, geometry and size potentially

being significant. This makes the modelling procedure a much more complex task.

3.3 Low speed friction rig (LSFR): Sled-type tester

The low speed friction rig is similar in action to the sled type testers held by the HSL. The

dynamics for such devices are very different from the pendulum tester (the velocity between

slider and tile are around 3 orders of magnitude different). However similar particle movement

mechanisms were observed in both apparatus. Additionally, a transient loading profile can be

obtained which can be used to observe effects such as particle layer thinning.

The basic operation of the device is to apply a constant normal load to the slider specimen.

The tile is then drawn past the slider at constant velocity, generating a retarding force that is

measured by a compression load cell. The transient CoF can then be determined. This could

potentially be of much more use than the pendulum PTV, as it gives information that is

dependent on time and therefore stroke position. Since the critical cause of a slide is likely to

occur near the impact site, the resolved data is useful as it highlights low CoF periods. It

should be mentioned that the future work section outlines a customised rig that could be

incorporated into the pendulum laboratory tests of tile specimens. This would enable

pendulum dynamics (and standard PTV results) to be coupled with transient CoF

measurements that could be resolved in both time and stroke position.

26

3.4 Forces and velocity in human Gait

As the heel of the foot is brought into contact with the ground (heel contact - HC), the typical

impact velocity is 0.2 ms-1 [5] for unrestricted, level walking. If the pedestrian is transporting a

load this value can increase (though Redfern and Rhoades [6] found that for loads of up to

13.5 kg the HC velocity varied between 0.14 and 0.24 ms-1). The stated values are the mean

from a number of subjects but it is important to note that subject build, mobility and age are all

factors that will affect the leg dynamics. The velocity of the pendulum is therefore around one

order of magnitude (ten times) larger than actual heel velocity. The angle of attack of the

slider at contact is 26º which is comparable to the 23.5º [5] seen at HC.

The loading at HC in a human is highly variable and depends upon parameters such as body

mass and gait with some subjects arresting the movement of their foot prior to contact and

others using the ground (Whittle [7]). Redfern et al [5] suggest that for walking on a horizontal

surface, the peak load (expressed as impact force per unit body weight) would be

approximately 10 Nkg-1 (or roughly equal to body weight) occurring at approximately 25% into

the stance phase. However the foot reaches the ‘foot-flat’ (FF) position approximately 15%

into the stance phase at which point the reaction force is around 8.5 NKg-1, the point of slip

initiation is likely to occur before FF whilst the shoe makes a non-zero angle with the floor.

The pressure generated will depend upon the area of shoe in contact with the ground of

which little literature is available (highly shoe and gait dependant). The pendulum generates a

contact pressure of 126 kPa when the chamfered contact edge has a width of 2.33 mm and is

equivalent to the low speed friction rig (LSFR – also used in this work) applying 8 kg to the

rubber specimen. For a human weighing 80 kg, the equivalent area of contact required

immediately prior to FF would be up to 5400 mm2 (approximate are of rear raised portion of

shoe). The larger the angle the foot makes with the floor (i.e. closer to 23º) the smaller the

area of contact, resulting in an increase in contact pressure.

Though the maximum shear force of approximately 1.5 Nkg-1 occurs at a position 19% into

the stance (Redfern et al [5]), the critical position in the stance where the shear to normal load

ratio is largest and contact area smallest is likely to occur within the blue shaded region (A) of

figure [1] (before FF), when the shear stress ranges from 0 to around 1.25 Nkg-1.

27

Figure [1]: Sketch of relative variation of normal and shear reactions over a single step (based

on figure (1) from [3]), HC – Heel Contact, FF – Floor Flat, TO – Toe Off.

Human Pendulum LSFR

Slider Velocity (m/s) 0.2 3.2 0.006

Angle (degrees) 23.5 26 0

Applied pressure (KPa) Variable ≈ 126 126 126

Table [2]: Approximate conditions of the different contacts

3.5 Potential problems with test methods

The Pendulum Test The pendulum test is a convenient and realistic method for determining slip resistance. The

dynamics are closer to the real mechanisms of human slip (i.e. the heel striking the floor).

than conventional sliding friction testers (where one surface slides over another). However,

the impact velocity of the pendulum when it strikes the floor is approximately 3.2ms-1.

It is possible that contact pressures generated in human movement maybe to be larger than

those modelled using the pendulum or low speed friction rig (LSFR) as a result of the small

contact area at and for a short time after impact, meaning that care must be taken when

analysing recorded data.

The larger impact velocity or preceding air ‘wave’ ahead of the slider may act to scatter the

particles to a greater degree than the heel of a shoe, generating a less accurate

representation of impact conditions. It will be demonstrated in the report this did not prove to

be a major issue.

An inherent limitation of the pendulum is its inability to measure the transient properties of a

slide. All the ‘information’ within the slide is fused into a single PTV measurement. It is not

possible to extract the effects of local phenomena within the slide other than by visualisation.

28

Sled-Type Friction Testers It has been suggested that sled type testers do not accurately mimic the conditions of impact

for a floor that has been contaminated with a liquid. This may lead to results that are not

representative of an actual slipping situation. Though some work has been performed with

respect to particle contaminants [3] further work is required to consider its suitability for such

an application.

The relative velocity between slider and flooring is much lower than that seen in the pendulum

and human gait (the device used in this study had a velocity around 3 orders of magnitude

smaller than that of the pendulum).

The angle generated between floor and slider is 0º which is greatly different to the 23.5º

observed in humans at the point of impact. The effects of a converging contact are therefore

not simulated.

29

4.0 Experimental technique and apparatus

4.1 Low-speed friction rig

Four-S samples were cut from standard pendulum slider samples (76 x 6.35 x 25.4mm) with

the dimensions of 25 x 25mm (±1mm) and mounted on 1 inch diameter steel buttons using

double sided tape (proved in preliminary tests to generate sufficient shearing resistance). The

floor tile samples were cut to dimensions of 50 x 150mm and also fixed with double-sided

tape after cleaning with acetone.

Figure 2: Low speed friction rig apparatus photo

95

380

45

LOADSLIDER

SPECIMEN

COUNTERBALANCE FORCE

TRANSDUCER

BI-AXIALPIVOT

(a) Lateral view

(b) Plan view

FRICTIONALLOAD

MEASUREDFORCE

Figure 3: Low speed friction rig schematic of operation

30

The required load was positioned on the pivoting arm (figure [3]) of the LSFR (figure [2]) and

Four-S ‘button’ was placed in its housing and aligned 10mm from the edge of the counter-

face. Zeroing of the force transducer was performed and test initiated with a sample capture

rate of 20Hz. Three runs were performed for each configuration of the apparatus.

31

4.2 Pendulum friction rig

Preparation of the pendulum apparatus (figure [4]) followed exactly, the standard HSL

procedure [12].

PENDULUM ARM

INITIAL POSITION

FOUR-S RUBBERSLIDER

SRV SCA LE SRV INDICATOR

126mm (±1mm) CONTACT STROKE

FLOOR SPECIMEN

Lateral view

PENDULUM ARM

INITIAL POSITION

FOUR-S RUBBERSLIDER

SRV SCA LE SRV INDICATOR

126mm (±1mm) CONTACT STROKE

FLOOR SPECIMEN

Lateral view

Figure 4: Stanley Pendulum Tester schematic

Particles were applied by two methods, one equivale

other using a paintbrush or finger to apply a very fine la

Two series of eight tests were performed for each c

application of contaminant after every stroke whilst in th

prior to the first stroke only.

4.3 Application of Contaminants

Contaminants were applied to the floor tile and force

2000µm by the use of specially made ‘combs’ (figure

width of 40mm allowing a 30% overlap each side of t

with the smaller particles caused by high cohesiveness

preventing the generation of even layers. Subsequentl

though the pressure applied was always less than that

friction rig (LSFR).

32

High speed camera

Pendulum tester

Glass counter-face and powder

Slider

Figure 5: High speed video apparatus for

Pendulum Tester

nt to that used for the LSFR and the

yer to the slider only.

onfiguration. The first set involved re-

e second set contaminant was applied

d into depths of 500, 1000, 1500 and

[6]). The resulting particle tracks had a

he slider. Problems were encountered

with sweeping occurring at all depths,

y a degree of compaction was required

applied by the pendulum or low speed

Figure [6]: ‘Combs’ used for generating specific particle thickness’

4.4 Test Specimens & Contaminants

In both test configurations standard Four-S rubber specimens have been used (IRHD

hardness 94° – 98°), as shown in Figure [7].

Figure [7]: LSFR Four_S samples

Preparation of the Four-S sample followed a similar protocol to that adopted for pendulum

tester where by the contact surface was initially conditioned with P400 abrasive paper

followed by a series of unidirectional runs along wet 3µm pink lapping paper. It is suggested

that this produces a similar surface to that seen in ‘bedded-in’ shoes.

Table [3] shows the test contaminant particles used. These were chosen to be typical of

particles found in factory situations and have been associated with slip risks. The particles

sizes were measured using a laser scattering analyser. Appendix 1 gives photographs of the

particles.

Contaminant Ref. No.

Mean particle size (µm) [Approximate]

Uni-axial particle compressive strain

(at 125.6 kPa) Talcum Powder 1 10 0.4 Cocoa Powder 2 15 0.2

Flour 3 25 0.25 Baking Powder 4 30 0.15

33

Bicarbonate of Soda 5 50 0.04 Table Salt 6 500 0.01

Ballotini (glass spheres) 7 500 0.01 Sand 8 1000 0.01

Table [3]: Particle contaminant data

Floor materials were selected to achieve a range of surface roughness (shown in Table [4]).

In these tests, the roughness parameter Rz (mean peak to valley height) has been used. This

is the parameter that HSL records in determining the roughness of a flooring material.

Appendix 2 shows some photographs of the tile specimens used.

Tile Ref. code Mean Rz (µm) Perlina polished marble PED/05/156 0.51

Polished Granite PED/05/158 0.85 Limestone PED/05/157 8.69 Olympus PED/05/155 12.32

Riven slate PED/05/159 34.18 Table [4]: Tile counter-face data

34

5.0 Results and Discussion: Results are divided into two parts. The first tests describe experiments performed to visualise

the mechanisms of particle motion in both the pendulum and the sled-type LSFR tester. This

is followed by data obtained using the pendulum tester to investigate the effects of particle

type and floor surface.

5.1 Particulate lubrication mechanisms in the shoe-floor contact

Observations of the LSFR and pendulum tests have highlighted three mechanisms for particle

movement during a slip. These have been termed sliding, shearing and rolling/tumbling. A

schematic for the mechanisms is given in figure [8].

Slider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

SliderPowder

Slider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

SliderPowder

Slider

Floor counter-face

Shear planeSlider

Floor counter-face

Shear plane

Floor counter-face

Slider

Floor counter-face

Slider

Floor counter-face

SliderPowder

SLIDING τps > τpf < τcrit : Entire powder layer hasvelocity of slider with extremely smallsurface layer of fine particles.

SHEARING τpf > τcrit < τps : Shear failure of particlelayer occurs resulting in contact thinningand material seen in wake of slider.

Figure 8: Sliding mechanisms observed on LSFR and pendulum tests

ROLLING / TUMBLING For larger single particle layers, withcounter face roughnesses to allow rollingat shear stresses < τpf, particles will rolland tumble rather than slide

35

Sliding generally results in the lowest friction, then rolling/tumbling, followed by shearing. In

an ideal situation the conditions should be designed to promote shear of the particle layer.

This is achieved by rougher surfaces (compared to the particle size) that cuts through the

layer, or by particles which to not adhere into a clump.

Which of these mechanisms occurs can be deduced by observing the wake and under-side of

the slider at the end of the slip. If no material is present in the wake and a compacted layer is

observed on the slider, the particles must slide against the flooring material and remain as a

bulk layer. A gradually thinning layer suggests shearing is occurring. If the wake contains

particles and the slider has striations there must be relative motion between the particles and

both the slider and the counter-face. This suggests that rolling and tumbling of the particles

must be occurring.

High speed camera photos of pendulum pad Visualisation of particle movement during the pendulum test was obtained using high-speed

video. Images were recorded up through a sheet of toughened float glass as the slider

passed over. Video was recorded at a frame rate of approximately 2500 frames per second

(fps). This enabled the different mechanisms to be viewed directly. Three cases were studied

to demonstrate the mechanisms of sliding, shearing, and rolling/tumbling.

Case 1: Talcum powder

Figure [9] shows the results of talcum powder. The talcum powder layer adheres to the shoe

and slides across the glass.

The path swept by the pad is almost completely devoid of particles as a result of the sliding

layer. This suggests that the volume of material required in maintaining the slip is very small.

The sliding layer of contaminant that slides with the pad can be seen in the magnified view of

figure [9b].

This also means that only particles are needed at the actual point of heel strike for slip to

occur (as they are effectively carried with the shoe). There is no need for continual

replenishment.

36

Figure [9a and b]: High speed images of talcum powder contaminant in pendulum contact

It is interesting to note that immediately after impact; the pad oscillates vertically slightly and

results in the deposition of a small amount of material. This is only seen in the case of the

smallest and most cohesive powders though evidence of this effect (through deposition of pad

material) can be seen on both the P400 abrasive and pink lapping papers.

Case 2: Bicarbonate of Soda

The experiments using bicarbonate of soda showed a transitional situation. Initially shearing

(figure [10a]) occurs immediately after impact. The particles can be seen passing through the

contact and remaining on the glass surface. This occurs whilst the layer of particles is thick.

The layer then shears down to a thin layer. Then sliding takes place (figure [10b]). It is likely

that the initial shearing reduces the depth of the particle layer to a critical height when the

required critical shear force (τCrit) exceeds the critical sliding stress at the particle-floor

37

interface (τpf). Again, in this case, material is only required at the impact as no new material

will be entrained into the contact.

Figure [10a and b]: High speed images of Bicarbonate of soda contaminant in pendulum

contact (a. immediately following impact, b. Close to end of stroke)

Case 3: Ballotini

Ballotini are glass micro-spheres, very close to spherical in shape. In the case of experiments

using these particles (figure [11a and b]) rolling of the particles is observed. The particles

pass through the contact and remain in the swept path. In this case, relative motion of the

particles with the contact results in their ejection at the rear of the pad. This means that a

constant contaminant supply ahead of the pad is required in order to maintain the reduced

CoF.

38

Figure [11a and b]: High speed images of ballotini contaminant in pendulum contact

39

5.2 Effect of particles on coefficient of friction

Particles on a Smooth Tile Pendulum tests have shown that for smoother tiles (in the order of a few microns), a small

volume of contaminant at the heel of the footwear can result in a substantial reduction in

friction. Figure [12] shows the variation of the CoF with conditions for the Perlina

(PED/05/156). All contaminants cause a large drop in frictional resistance with one

constituting a high slip potential, two a moderate slip potential and only one a low slip

potential. This generates similar implications, though less severe, to those seen in fluids,

where a thin film of liquid forms in the converging channel between rubber and counter-face.

It is important to note that a degree of variation occurs in the clean stroke. This is likely to

result from minute contamination causing disruption to the adhesive friction component.

Figure [12]: Effect of particles on coefficient of friction, pendulum test data on Perlina tile.

Adhesive and Hysteretic Friction

The friction in sliding contacts involving an elastomer has two parts; adhesion and hysteresis

[9]. The total sliding friction is given by:

HystereticAdhesiont fff += (2)

40

Adhesive bonds on a molecular level occur between the elastomer and counter-face which

when subjected to a sufficient shear stress, stretch, break and reform in a cyclic slip-stick

fashion. The magnitude of the adhesive force depends on the area of the elastomer in contact

with the counter-face, which is directly related to the surface roughness of the counter-face

(figure [13]).

Figure [13]: Schematic of effect of roughness on contact area

Increasing the roughness reduces the area of contact. Therefore high total friction occurs for

the smoothest dry surfaces such as float glass and polished marble. This initially reduces as

roughness increases. However as roughness increases, deformation of the elastomer around

the asperities increases and leads to the hysteresis component of friction increasing (this is

the energy absorbed internally by the elastomer).

If a symmetrical asperity is considered, the pressure profile developed will be symmetrical in

the static case. However if the relative motion between the elastomer and asperity is not zero,

the viscoelastic nature of the elastomer causes a non-symmetric pressure profile biased

towards the direction of movement and generates a net retardation force in addition to any

adhesive retardation, as shown in figure [14].

The EffeTable 1

and con

Tile roug

AsperityElastomerElastomer

Symmetric pressure profile(no relative motion)

Retarding bias (relativemotion)

Figure [14]: Schematic of hysteretic deformation

ct of Rough Tiles shows the tiles used in these tests along with their Rz value. In figure [15] the clean

taminated (with baking powder) PTVs for the different flooring materials are shown.

hness increases from left to right.

41

The highest friction occurs for the clean smooth tiles. As roughness increases this drops. In

all cases the presence of particles causes a reduction in friction. This is because the particles

block the adhesion between shoe and floor. However, this reduction is greatest for the

smooth floor cases. For these tiles the adhesion mechanisms of friction is dominant (and this

has been removed by the particles). For the rougher tiles the hysteretic friction is more

important, this is not affected by the presence of particles.

Figure [15]: Effect of flooring roughness on coefficient of friction

The difference between clean and contaminated friction is shown on the plot. This clearly

decreases with an increase in surface roughness. This may suggest that roughness reduces

the effect a contaminant will have on available friction.

Further testing is required with additional contaminants to generate a comprehensive

understanding though it is likely that all will disrupt the adhesive friction component in a

similar manner.

Tile Roughness Profiles – Roughness on Different Scales Figure [16] shows the measured surface profiles for a sample of each of the floorings. Current

experimental work concentrates only on mean peak to trough height (Rz). However, further

work is required to consider whether parameters such as the local micro roughness and

asperity pitch affect traction.

42

Figure [16]: Surface profiles for selected tiles

A difference in micro-roughness can clearly be seen between the Slate (PED/05/159) and

Olympus (PED/05/155) tiles. It is plausible that the Olympus tile would give a (relatively) lower

friction than the Slate for a given particle size as a result of an increased tendency to slide on

the smoother micro-roughness surface. The micro-rough slate tile would break-up

contaminant layers and cause more shearing.

Particles Adhering to the Shoe Specimen During the tests particles tend to adhere to the shoe surface. The adhesive nature of the

elastomer tends to cause particles to adhere to it. Even if the Four-S specimen was well

cleaned (with a dry cloth), the PTV was lower than the water-cleaned sample by up to 10 - 20

PTV, particularly for the finer, more cohesive powders. It is essential that all specimens were

thoroughly water cleaned between tests.

This phenomenon was also observed with the LSFR where visually clean specimens (cloth

wiped) showed reduced traction after having been in contact with the contaminant (figure

[17]).

43

Figure [17]: Visually clean elastomer specimens tested on steel (having been in contact with

table salt) – water cleaned specimen (highest CoF) in pink

44

5.3 Repeat testing (single application of contaminant to slider)

Repeated sliding without the application of fresh contaminant to the slider generated similar

results to those in which the contaminant was reapplied after each swing of the pendulum

(figure [18]). The action of the pendulum sweeps particles away from the floor. However, this

does not have a big effect on friction. This indicates that only a very small amount of particles

are needed to reduce friction.

The average increase in PTV for the different contaminants was 2.6 for eight passes of the

slider. Such smooth floors result in particle sliding for almost all cases with very small

volumes (in the order of 2 – 3mm3) of contaminant required to significantly reduce the friction

seen by the pendulum (up to approximately 70 points).

Figure [18]: Variation of slip potential for various contaminants against Perlina tile following a

single application of contaminant to elastomer pad.

The practical consequences of these results are that reductions in friction will be sustained for

a number of steps on a smooth floor. This is analogous to the effects of liquids where a

wetted elastomer will exhibit low friction coefficients on a smooth dry floor for a number of

swings.

45

5.4 Particle cohesiveness and its importance

It has been observed in both tests that smaller particles form a compacted layer and a clean,

‘swept’ region behind the pad. Particles ahead of the slider do not get entrained and are

instead forced along or displaced around the edges of the slider. This sliding mechanism

causes a low friction and warrant further investigation. For this condition to be observed, the

following criteria must be satisfied:

rppf ττ < and critpf ττ <

Where τpf is the shear stress at the particle-floor interface

τrp is the shear stress at the rubber- particle interface

τcrit is the required shear stress to shear the particles within the layer.

Thus the shear occurs at the particle floor interface. The particles and the shoe stick together

and slide.

All particles below approximately 50 microns, tended to clump together. This can be attributed

to cohesive forces. Cohesiveness is a measure of the mechanical shear strength of a powder.

A coulomb powder will generally follow a stress-strain curve as shown in figure [19a] [10].

When the shear strength of the particles is reached, failure occurs causing relative particle

movement. There is a critical particle diameter below which the shear strength rapidly

increases (represented in figure [19b]). This critical particle diameter was found to be between

50 and 60µm.

i

ii

iii

ε

σs

D

σs

Increasingconsolidation

pressure

Critical diameter Figure[19]: (a) Generic stress-strain curve and (b) variation of critical stress with particle

diameter for powders

Industrial experience suggests that particles below around 55µm cause increased handling

problems as a result of this effect. With many food and industrial contaminants falling within

this size range (e.g. Cocoa ~ 15µm, Baking Powder ~ 30 µm and Talc ~ 10 µm) the effects of

cohesiveness are significant.

46

In the case of spillages and contaminant situations of potential risk to humans, the depth of

the contaminant layer is small. The weight of the particles is therefore almost negligible and

all layers of the powder can be thought of as uncompressed. In this situation the effects of

cohesiveness generate macroscopic particulate structures that support the weight of the

higher particle layers. The effect of these structures is to reduce the bulk density of the

particles and decrease the macroscopic powder stiffness.

Volumetric strain under compression can be a method for comparing the cohesiveness of

particles and predicting their effect on CoF. Uni-axial compression was used to compare the

particles and was performed using a modified syringe of known volume. Subjected to a

pressure approximately equivalent to that used in the pendulum and sliding rig, the particles

behaved as in figure [20]. It can be seen that a marked increase in the compressive strain

occurs as the particle diameter decreases. The transition point occurs within the shaded band

between approximately 45 and 60µm, this corresponds well the results obtained by [10].

Th

tha

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 100 200 300 400 500 600 700 800 900 1000

Particle diameter

Com

pres

sive

str

ain

Sand

BallotiniBicarbonate of Soda

Baking Powder

Flour

Cocoa

Talcum Powder

Figure 20: Graph of powder compressive strain against particle diameter

e consequences of this finding are that the smaller powders will agglomerate. This means

t sliding will occur, leading to a situation of high slipping potential.

47

5.5 Map of Particle Slip Mechanisms

A simple map showing the regions in which the different mechanisms occur is shown in figure

[21]. This is based purely on particle diameter and surface roughness.

The ma

(i) Sw

s

a

a

th

a

E

b

(ii) Ta

h

ro

dcrit ~ 55µm

Rz crit ~ 15-20 µm

Rolling &Tumbling

Shearing

Sliding

Rz

Surf

ace

roug

hnes

s

Particle diameter

Decreasing FIncreasing F

Figure [21]: Map of particle size, D and tile floor roughness, Rz, showing regimes

of slip mechanisms.

p shows the following features:

liding Occurs on Low Roughness Floors. It has been observed that sliding occurs

hen the flooring has a low roughness (in the order of a few microns) or when d/Rz is

ufficiently large so as to make the flooring roughness negligible. From current studies,

potential transition occurs (between sliding and shearing) at a floor roughness of

pproximately 15 – 20 µm for the tested contaminants. Floors with a roughness of less

an this value do not allow sufficient mechanical interlocking between the asperities

nd contaminant to exceed τcrit and sliding occur, potentially causing a low CoF.

ssentially a rough floor cuts through the layer of particles and stops it sliding as one

lock with the shoe surface.

he Cohesive Boundary. Particles tend to stick together when their size is below

pproximately 55 microns. This is around the size of bicarbonate of soda particles (and

as been termed the ‘cohesive boundary’). Current observations suggest that the floor

ughness for the sliding mechanism to occur, drops with particle size until the

48

cohesive boundary is reached. This is likely to be due to the reduction of the cohesive

forces, tending to prevent the agglomeration of particles. Bigger particles need a larger

roughness to stop them sliding as a result of the d/Rz ratio.

(iii) Particle Layer Shearing Takes Place On Rough Floors. Inter-particle shearing

occurs when the roughness is large but particle diameter relatively small. The

asperities prevent the particle layer sliding in a bulk form. As shearing takes place, the

layer between shoe and floor reduces in thickness as particles are lost. In the case of

smaller cohesive particles, the uncompressed material ahead of the contact is not

entrained meaning that removal and thinning of the contact occurs. Increasing the

particle diameter such that cohesive effects are negligible causes a reduction in the

separation of shoe and floor often followed by particle tumbling and rolling.

(iv) Particle Rolling/Tumbling Occurs for the Larger Particles. Particle tumbling and

rolling occurs when there is sufficient traction to preferentially allow the particles to

tumble and roll rather than slide against the floor. This occurs for particles of greater

diameter than the cohesive boundary (~ 50 microns). Shape factor (see Appendix 5)

greatly influences the mechanism, with particles of increased roundness rolling at lower

floor roughness.

(v) The Effect of Particle Shape. The map shows the variation of mechanism with shape

factor (section 5.7). A straight vertical shift of the boundaries has been predicted both

below and above the cohesive boundary. This is shown as the red dotted lines on

figure 21. Further work is required as it is likely that shape factor has less of an effect

below the cohesive boundary with inter-particle bonds being comparable to forces

generated by mechanical interlocking.

(vi) Depth of the Particle Layer. The current map does not incorporate any information

about the depth of the particle layer. In the case of small cohesive powders, a range of

particle depths will end in the same sliding mechanism. However significant depths

may result in a transient progression from shearing to sliding (through depth loss from

the shearing process). Shearing to rolling/tumbling will also occur for particles with a

tendency to roll. This can be seen clearly in appendix 3 for ballotini particles. Further

work is required to incorporate a quantitative depth / surface density function.

The red region represents a high-risk situation where adhesive friction dominates. Very little

contaminant is required to cause a significant reduction in traction. Contaminant can originate

from areas external to the considered floor and be transported by sticking to the sole.

Additionally, the small size and low slipping volume means sighting by pedestrians is unlikely.

In a similar manner to liquid contaminants, polished floors in areas such as entrance halls

49

present significant risk without sufficient measures to remove contaminant from the shoes and

surface in general.

In addition, the sliding mechanism means powder is only required at the impact point with no

relative motion between slider and powder layer. The rolling or tumbling and (to less of an

extent) shearing mechanisms however, require a constant input of contaminant to maintain

the effective lubrication and therefore more dependant on a larger area of contamination.

50

5.6 Effect of applied pressure on traction

The pendulum tester is designed such that a pre-defined load is applied to the contact region

for the entire stroke. The applied pressure (neglecting contact area changes due to pad wear)

is therefore constant. This required that the LSFR had to be used for these tests. Appendix 4

gives results for ballotini particles subjected to increasing loads. It can be seen that in all

cases that transition from shearing to rolling occurs as the initial depth of the particles was

greater than their diameter. Following the transition, increasing load only slightly increases the

coefficient of friction. This is likely to be due to the increased depression depth of the particles

within the slider.

Though no data is presented, the effect of increasing load is likely to have the following

effects:

Small particles (below ~ 50 microns):

Increasing contact pressure will increase mechanical interlocking which will generate higher

critical inter-particle shear stresses (τcrit). This means that sliding (as opposed to shearing) will

occur at larger particle depths.

Large particles (above ~ 50 microns):

If particles of high shape factor are considered (i.e. rounder particles) little effect other than

added rolling resistance (due to increased slider deformation) will be observed. However

particles of lower shape factor will tend to want to slide under higher loads, as a result of the

shear requirements to permit their tumbling (τr) being greater than the sliding shear stress

(τpf).

Both of these scenarios (like a decrease in shape factor) will tend to want to push the

transition line in the mechanism map upwards, allowing sliding to be sustained at higher floor

roughness.

51

5.7 Effect of shape factor on traction

Particle shape is an important factor (Appendix 5) in determining how a particle will behave in

a constrained contact. In the case of larger particles, those with circular profiles will have a

tendency to roll while those with large aspect ratios will tumble and slide. As particle size

decreases the shape factor may become important in determining how mechanical

interlocking will affect the internal and interface shear stresses in a layer many particles thick.

Current studies have shown that cohesion is important in particles below the ‘cohesive

boundary’ of approximately 50 microns. At the level of the smallest particles, it is technically

challenging to determine the effect of shape whilst keeping the effective diameter constant.

Analysis of this type may form the focus of future study.

The larger particles have shown shape factor dependency in determining whether particles

will roll, tumble or slide. In the case of the ballotini, the distribution function for the shape

factor is fairly tight about the peak as a result of the manufacturing method. However sand

and realistic contaminants are likely to have much looser distributions. Potentially, if two or

more particle movement mechanisms exist within this distribution then all of these

mechanisms have the potential to occur.

Figure [22]: Effect of particle distribution on preferential mechanism

52

In the case of the larger particles, it has been shown that two possible mechanisms can

occur, namely rolling or tumbling and sliding. Considering these two mechanisms, it is

possible to envisage a number of scenarios. The first, that all particles tumble and a constant

distribution of particles is seen in the wake of the slider. The second is that the particles

tumble and stick depending on the roughness at a particular point on the counter-face

(assuming uniform roughness on the slider). The third is that the particles only slide and in

this case no new particles can become entrained in the contact. For a distribution of shape

factors this means that last possibility may create a ‘wall’ where, after sufficient distance,

sliding becomes the dominant mechanism irrespective of whether there are other particles

favouring an alternative mechanism. Figure [22] outlines this affect though assumes an

unrealistic, instantaneous transition from one mechanism to the other.

Though no quantitative data has been obtained this phenomenon can be seen in the particle

tracks of the LSFR (figure [23]) where a sliding mechanism has given way to rolling in the

ballotini.

Figure [23]: Image of transition from sliding to rolling on LSFR

53

6.0 Conclusions

• Adhesive friction is significantly affected by the presence of solid contaminant, hysteretic

friction is not. Smooth floors (in the order of a few microns) are therefore more susceptible

to contamination (up to a 70 PTV point reduction).

• Even very small quantities of particles can reduce the shoe-floor adhesion and cause a

high slip risk.

• Small particles are able to adhere to the soles of shoes allowing transference of the

contaminant and continued reduced friction coefficient for many steps.

• Three significant slipping mechanisms were observed within the contact; sliding, shearing

and rolling/tumbling. Sliding and rolling on smooth floors pose the most significant risk.

• Sliding only requires contaminant at the impact site with no further contaminant required to

feed contact.

• Rolling/tumbling contacts require a constant supply of contaminant throughout the slip

(though only at a depth of a single particle).

• Load has little effect on larger, rolling particles but has the potential to increase the

roughness at which powders below 50 microns slide and larger particles with smaller

shape factors tumble.

• Larger depths of particles can cause transitions in mechanisms, commonly shearing to

sliding and shearing to rolling/tumbling.

54

7.0 Implications of findings

The work has shown that though solid contaminants can have different mechanisms to

generate a reduction in traction, the effects of the high risk situations occur in a similar

manner to liquid contamination.

Key finding: Particles have a significant effect on adhesive friction but small effect on

hysteretic friction. In cases of low hysteretic friction (i.e. low roughness floors) the traction is

determined by the CoF between the floor and particles.

Two Main High Risk situations:

Case 1: Sliding – High to Medium risk for most particles

Flooring: Smooth (less than approximately 10 microns) and low adhesive properties

e.g. Polished stone, vitreous tiles and polished laminate wood flooring.

Contaminant: Favours sliding and has a low CoF when in contact with flooring. Additional

contaminant transmittance risk or prolonged low traction conditions if particles adhere to sole

of shoes.

Case 2: Rolling / Tumbling – Potentially high risk for particles of large shape factor

Flooring: Risk will reduce as roughness increases

Contaminant: Favours rolling or tumbling (e.g. hard, large). Larger shape factors (i.e.

rounder particles) will provide less rolling resistance and reduce CoF.

The loss of adhesive friction is by far the biggest problem. Since hysteretic friction requires

sufficient floor roughness, it is this parameter that has the strongest control, offering increased

resistance in both cases 1 and 2.

55

7.1. Implications for the HSE:

The work performed will affect the way in which the HSL and HSE conduct investigations

involving solid particulate contaminants. However the education of employers on the matters

of solid contaminants and advice to duty holders regarding flooring and shoe material will also

fall into the responsibility of the HSE. A brief outline of these implications for the HSE is

outlined.

Forensic and site investigations

The following outlines a potential test procedure. Further work needs to be performed to

finalise acceptance/failure criteria.

1. Suspect conditions outlined in cases 1 and 2

2. Follow an investigation procedure similar to the following:

(i) Measure Rz roughness of flooring (in critical orientation)

(ii) Provide a qualitative indication of particle size.

(iii) Rate the visibility of particles (less visible – higher risk)

(iv) Perform Pendulum tests on a clean region with contaminant on slider

only, with repeat swings to consider rate of regain of sufficient friction.

(v) Perform pendulum tests on as-found contaminated areas.

(vi) If any of the tests above generate a high risk condition, remedial action

must be imposed (e.g. rougher flooring, stringent cleaning policies and

transmittance prevention methods).

3. Continue research into effects of specific roughness parameters and constructing

data bank for friction generated by different particle types on different floorings.

Education road-shows and guidance to employers

1. Understanding of why particulate contamination creates risk

2. How to take measures to prevent contamination occurring.

3. What sort of measures should be used to remove contaminant following spillage.

4. What sort of measures should be provided to remove contaminants adhered to soling

(e.g. bristle and sacrificial adhesive mats).

5. Understanding of principles for why rough flooring should be used in areas known to

suffer from contamination (e.g. workshops and kitchens)

56

Currently, some of the concepts obtained from these investigations have been used to make

an educational video (which is being used in road-shows) demonstrating visually the effects of

a small quantity of contaminant on a smooth, hard floor.

Advice to duty holders

Footwear. Hysteretic friction has been shown to be effected less by the presence of solid

particle contamination than adhesive friction. It is therefore suggested that HSL conduct

further investigations and/or develop procedures to test prospective soling materials such that

advice can be given (to footwear manufacturers) regarding the desired characteristics for

appropriate footwear.

Flooring. Further investigation into the effects that different roughness parameters have and

which are most significant for minimizing the effects of contaminants (may be environment

dependent e.g. the flooring required for a kitchen area could be different to that required for a

sand yard). Advice can then be given to those manufacturing and installing floors. All floorings

currently considered have been hard in nature and generally develop low particle-floor

interface shear stresses. Further investigation (by the HSL) into using other types such as

resilient flooring should be carried out.

57

8.0 Future Work

As has been stated, one of the key limitations of the Stanley Pendulum Tester is the inability

to extract localised kinetic information from within the sliding contact. All frictional loads are

fused to generate a single PTV. This cannot show transient variations in normal and shear

loads that could aid the understanding of the slider-particle interactions. Since it is not

possible to visualise these interactions directly for anything other than very smooth surfaces

(a roughness of only a few microns), the only available information is the single PTV and

resulting track.

In this study, a custom built low speed friction rig capable of measuring the shear and normal

forces was used. Though this aids understanding, it possesses very different dynamics to the

pendulum, with velocity alone being almost three orders of magnitude less than at pad

impact.

Figure [24]: Schematic and image of suggested pendulum force measurement device

58

A custom rig for the Stanley Pendulum Tester is suggested to enable measurement of normal

and shear loads directly for the duration of the contact. A schematic of the device is shown in

figures [24a] and [24b]. The basic principle would involve some data-logging software to

record normal and shear loads. A high acquisition rate would be required to obtain sufficient

fidelity in time and therefore it is envisaged that two optical triggers would be used to initiate

and terminate the data capture program. Structurally it is proposed that a simple ‘S’ type

tension/compression load cell be used to measure the compressive normal load. For shear

load measurement a co-axial cell is suggested, with the specimen being supported by thin,

laterally compliant cantilever supports which would enable the transmission of the normal load

but minimise lateral elastic resistance and so allow a better voltage/force ratio.

Though this rig would be laboratory based and require the machining of specific test

specimens, it has the potential to be used for dry, liquid contaminated and particle

contaminated conditions. The ability to analyse the transient impact forces directly, together

with the PTV and pad track data would give a much greater insight into the conditions in the

contact and aid the development of models to describe contact behaviours.

59

9.0 References

[1] ‘The assessment of pedestrian slip risk – The HSE approach’, Health and safety

document, (2004).

[2] Williams, A. J. ‘Engineering Tribology’, Oxford University Press, 232 – 237.

[3] Lemon, P. W., Thorpe, S. C., Jefferies, S. L., Sexton, C. and Hawkins, M.,

‘Pedestrian Slipping: Dry Contaminants’ PE/01/15. Human Factors Group, Health and Safety

Laboratory (2001)

[4] Lemon, P. W., Loo-Morrey, M. and Brown, E., ‘Assesment of Floor Surface

Slipperiness – Woodman Furnature Ltd.’ PS/03/19. Human Factors Group, Health and Safety

Laboratory (2003)

[5] Redfern, M. S., Cham, R., Gielo-Perczak, K., Grönqvist, R., Hirvonen, M.,

Lanshammar, H., Marpet, M. and Pai, C. Y., ‘Biomechanics of slips’ Measuring Slipperiness:

human locomotion and surface factors. Taylor and Francis. 37 – 65.

[6] Redfern, M. S. and Rhoades, T. P., ‘Fall prevention in industry using slip resistance

testing’ in Bhattacharya, A. and McGlothlin J. D. (eds) Occupational Ergonomics, Theory and

Applications (New York / Basle / Hong Kong: Marcel Dekker), 463 – 476, (1996).

[7] Whittle, M. W. ‘Generation and attenuation of transient impulsive forces beneath the

foot: A review’, Gait and Posture 10, 264 – 275, (1999).

[8] BS 7976-2:2002 ‘Pendulum Testers – Part 2: Method of Operation’ British Standards

[9] Moore, D. F., ‘The Friction and Lubrication of Elastomers’ Pergamon Press, Oxford,

Chapter 2, (1972)

[10] Orband, J. L. R. and Geldart, D. ‘Direct measurement of powder cohesion using a

torsional device’, Powder Technology 92, 25b – 33, (1997).

[11] Johnson, K. L., ‘Contact Mechanics’. Cambridge University Press, (2004)

60

10.0 Glossary of terms

Adhesion: Bonding of surfaces in contact by short range molecular

forces (van der Waals) the strength of which depends on the

contact area between shoe and flooring.

Agglomeration: Collecting / massing of particles

Asperity: A smooth surface is never completely flat and if viewed at

sufficient magnification, will show peaks and valleys. These

peaks are called asperities. An analogy can be drawn with a

saw blade in which the tips of the cutting teeth are the

asperities.

Continuum: Material acts as a continuous substance. E.g. a puddle of

water (which is a continuum) is described as a single body of

water because the size of the water molecules is very, very

much smaller than the floor roughness. However a heap of

sand is formed out of many discrete particles where the size

of these particles is comparable to the floor roughness. This

means that each particle must be considered separately and

so the heap cannot be considered a continuum.

Coulomb powder: An ideal powder with a degree of shear resistance. Below the

critical shear strength the particles will move relative to each

other elastically, but by no more than a fraction of the particle

diameter (if the force was removed, they would return to their

starting positions). Above the shear strength, the particles

will move past each other such that if the shear force was

removed, they would be in a different place to their starting

position.

Elastomer: Rubber-like material

Hysteresis: Energy dissipation by the deformation of the shoe material

physically realised as a frictional force.

Uniaxial compression: Compression of a material in a single direction whilst being

constrained in the other two directions

61

Van de Waals forces: A force felt between atoms and molecules at a molecular

level

Visco-elastic: Exhibiting both elastic (spring like) and viscous (think treacle

but very much thicker) properties. This means that if a visco-

elastic material is deformed it takes a little while for it to

regain its original shape. Visualisation of this effect can be

performed with foam, which if impressed by a finger, will take

a few seconds to regain its shape once the finger is

removed.

62

Appendix 1: Particle photos

Baking powder

Bicarbonate of Soda

Cocoa Powder

Flour

Table Salt

600 - 1200 micron sand

Talcum Powder

Ballotini

63

Appendix 2: Tile photos

64

Tiles (in order)

Perlina polished marble (PED/05/156)

Polished granite (PED/05/158)

Limestone (PED/05/157)

Olympus (PED/05/155)

Riven Slate (PED/05/159)

Appendix 3

Experimental runs using 500 micron, glass Ballotini particles (shape factor > 0.95) on steel (Rz ~10 microns) and varying particle depth

0

0.05

0.1

0.15

0.2

0.25

0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0

Dyn

amic

CoF

Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.0 mm

0

0.02

0.04

0.06

0.08

0.1

0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0

Dyn

amic

CoF

Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 0.5 mm

0

0.05

0.1

0.15

0.2

0.25

0 200 400 600 800 1000 1200 1400

Dyn

amic

CoF

Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm

0

0.04

0.08

0.12

0.16

0.2

0 2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0 14 0 0

Dyn

amic

CoF

Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 2.0 mm

.05 Dynamic rolling CoF: 0 65

Appendix 4

0

0.04

0.08

0.12

0.16

0.2

0

Dyn

amic

CoF

0

0.04

0.08

0.12

0.16

0.2

0

Dyn

amic

CoF

0

0.05

0.1

0.15

0.2

0.25

0

Dyn

amic

CoF

0.00

0.04

0.08

0.12

0.16

0.20

0

Dyn

amic

CoF

Experimental runs using 500 micron glass Ballotini particles (shape factor > 0.95) on steel (Rz ~10 microns) and varying normal load

Load: 2 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm

200 400 600 800 1000 1200 1400

Load: 4 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm

2 0 0 4 0 0 6 0 0 8 0 0 10 0 0 12 0 0 14 0 0

Load: 8 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm

200 400 600 800 1000 1200 1400

Load: 20 Kgf, Speed: 2 mm/s, Particle depth: 1.5 mm

200 400 600 800 1000 1200

66

Appendix 5: Description of Shape factor (F)

67

Appendix 6: Selected test data:

6.1 LSFR test data

Powder

talc cocoa flourBaking powder

Bicarb of soda salt sand CLEAN

Tile

characteristic dimension (microns) 10 15 25 30 50 500 900

Perlina 0.51 0.14 0.1 0.14 0.14 0.13 0.1 0.1 1.14 Limestone 8.69 0.1 0.34 0.17 0.26 0.49 0.14 0.18 0.6 Olympus 12.32 0.17 0.21 0.11 0.24 0.32 0.25 0.14 0.62

Slate 34.18 0.42 0.55 0.35 0.71 0.61 0.22 0.43 0.65

6.2 Perlina tile test results (Pendulum method)

Pendulum Test Values for (A) Contaminant application to slider for every swing, (B) single

application of contaminant to slider followed by un-cleaned swings.

SWING Diameter Powder CLEAN 1 2 3 4 5 6 7 8 MEAN

15 Cocoa A 96 30 29 28 28 30 29 29 29 29.0

B 101 29 31 30 30 30 31 31 31

25 Flour A 89 36 36 36 36 36 36 37 37 36.3 B 99 37 38 37 37 36 36 36 36

30 Baking powder

A 100 25 25 25 25 25 25 26 25 25.1

B 98 26 27 29 30 30 29 28 28

50 bicarbonate

of soda A

86 21 22 22 22 22 21.8 B 92 21 22 22 22 23

500 Table salt A 102 36 38 40 34 36 24 46 25 34.9 B 106 25 27 30 31 33 33 33 33

68

6.3 Clean tile tests (Pendulum method)

Tile Rz Direction PTV Mean

Perlina 0.51 a 84 84 84 84 84 83.33 b 84 84 83 84 83 c 81 82 82 83 84

Polished granite 0.85 a 100 98 100 99 98 89.87

b 90 90 91 91 91 c 84 81 79 76 80

Limestone 8.69 a 60 60 60 60 60 60.73 b 62 63 63 64 64 c 59 59 59 59 59

Olympus 12.32 a 60 60 60 60 60 59.93 b 60 60 61 61 61 c 58 59 59 60 60

Riven slate 34.18 a 60 60 60 60 60 60.00 b 72 73 72 72 72 72.20 c 66 66 66 67 67 66.40

69

70

8 GLOSSARY

Adhesion: Bonding of surfaces in contact by short range molecular

forces (van der Waals) the strength of which depends on

the contact area between shoe and flooring.

Agglomeration: Collecting / massing of particles

Asperity: A smooth surface is never completely flat and if viewed at

sufficient magnification, will show peaks and valleys.

These peaks are called asperities. An analogy can be

drawn with a saw blade in which the tips of the cutting

teeth are the asperities.

Continuum: Material acts as a continuous substance. E.g. a puddle of

water (which is a continuum) is described as a single body

of water because the size of the water molecules is very,

very much smaller than the floor roughness. However a

heap of sand is formed out of many discrete particles

where the size of these particles is comparable to the floor

roughness. This means that each particle must be

considered separately and so the heap cannot be

considered a continuum.

Coulomb powder: An ideal powder with a degree of shear resistance. Below

the critical shear strength the particles will move relative

to each other elastically, but by no more than a fraction of

the particle diameter (if the force was removed, they

would return to their starting positions). Above the shear

strength, the particles will move past each other such that

if the shear force was removed, they would be in a

different place to their starting position.

71

Elastomer: Rubber-like material

Hysteresis: Energy dissipation by the deformation of the shoe

material physically realised as a frictional force.

Uniaxial compression: Compression of a material in a single direction whilst

being constrained in the other two directions

Van de Waals forces: A force felt between atoms and molecules at a molecular

level

Visco-elastic: Exhibiting both elastic (spring like) and viscous (think

treacle but very much thicker) properties. This means that

if a visco-elastic material is deformed it takes a little

while for it to regain its original shape. Visualisation of

this effect can be performed with foam, which if

impressed by a finger, will take a few seconds to regain

its shape one the finger is removed.