The Influence of Energy absorbing Electro-rheological Fluid on Fibre-reinforced Composites

U. Igwe*, E. SioresInstitute of Materials Research & Innovation, University of Bolton, UK

Abstract

Impact tests have been carried out on a set of electro-rheological (ER) fluid-filled fibre-reinforced

(FR) composite structures at low velocity. The study focussed on verifying the flow behaviour of

the fluid under electric field and its effect on FR composites under impact loading. The impact

test was carried out with a drop weight impact tester, Dynatup 9250HV for a height of 2.5m.

Electric field strengths were supplied by a voltage source via two copper electrodes and varied

between 0KV/mm and 5KV/mm for the fluid’s responses. With the fluid characterised as

extremely viscous liquid, the rheological data obtained were explained relative to structure. From

the results, the load bearing capability of the fluid and its effect on the failure mechanism of the

composites revealed considerable inherent stiffness as energy absorbing medium, with respect to

on- and off-states. Hence, the average failure performance of the composite hybrid was found to

be localised deformation with uniform through-thickness damage at increased field strength.

Keywords: Hybrid; Smart material; Impact behaviour; Mechanical testing

1. Introduction

ER fluids are materials which possess flow properties that can reversibly be changed in an

electrical field to various magnitudes. As density-matched suspensions, they contain conductive

or polarizable micron-sized solids or particles in dielectric fluids, which afford them the

characteristics that are efficient for different smart applications; exhibiting fascinating rheological

behaviors, often modelled as Bingham solids [1, 2]. Hence, in the absence of an electric field the

fluid is weak and unable to support load, but under an electric field it changes from liquid to

‘solid’ state with reasonable strength and response time in milliseconds [3, 4].

To the advent of this, structures made up of ER fluid have been prepared, studied and their

properties reported to give rise to a new set of materials with the advantage of ease in change of

properties. The change offers the possibility of controlled (through the adjustment of rheological

properties by an electric field) damping and stiffness mostly unavailable in many engineering

materials. As a result, the engineering applications of ER fluids include actuators [5, 6], vibration

absorbers [7, 8], clutches and accurate abrasive polishing [9, 10], US army combat suite [11],

1*Corresponding: E-Mail: a90thplcs@yahoo.com; Tel: +44(0)7949128152; Fax: +44(0)1204414355

new type of portable and controllable active knee rehabilitation orthotic device[12], tactile

displays [13], hydraulic valves [14], brakes [15], shock absorbers [16], mobile device

applications [17], and energy-transducing devices as they all employ the yield stresses of the

fluids under electric field.

Furthermore, the performances of composites made up of ER fluids have been reported [16, 18].

This suggests that they could serve as support members while acting as dampers or offer discrete

vibrational control without supporting spring, to closely simplify any design complexity and

system flexibility. These characteristics, therefore, make them efficient for different composite

material applications with embedded smart capabilities. Therefore, through proper practical

selection of control strategies the best response of dynamic systems can be achieved based on the

reversibility of the fluid. For instance, taking advantage of the continuous phase system of ER

fluid suspension composites can be evaluated with respect to uneven distribution of energy to

wide areas.

Supportively, the unequal energy distribution is as a result of discontinuous surfaces (inherent

properties or introduced during mechanical or chemical treatment of fabrics), weakness of fibres,

weave pattern, crossover points, non-orthogonal response of fibres, poor damping factor of

constituting materials, fibre slippage, crimp frequency, poor fibre gripping and bonding, surface

finish, inter-laminar mismatch and many more means composites fail with different damage

mechanisms or are susceptible to failure. For this reason, this work is focussed on evaluating a

mode of improving the impact performances of composites. To supply required and regulated

electric field strength, a voltage supply unit was used with two single-faced copper electrodes for

the sandwiched fluid. Thus, by a simple approach the dynamic behaviour of ER fluid-filled

composite structures subjected to low velocity impact is presented based on material selection,

fabrication and architecture to realize the objectives of this study.

2. Experimental techniques

2.1 Materials and laminate preparation

In this, pre-impregnated 4-harness satin woven Kevlar®49 fabric and 2×2-twill woven standard

modulus Carbon fabric in E722 epoxy matrix system, UD Dyneema®SK75 fabric, and ER fluid

(37.5% by weight of 45μm Lithium salt from resorcinol/formalin and 39.5% of Chloro-fluoro

polymer particles in 23% silicone-based oil) were used. The fabric materials were supplied by

Amber Composites® UK and DSM® Netherlands, while the fluid by Smarttec® UK. The fabrics

were hand-laid, inter-laminarly stacked in 0°/90° sequence and cured according to the

2

manufacturer’s specifications to obtain the respective laminates. The numbers of plies were

chosen as a test range and each laminate was formed at 120°C under a three-stage-pressure slow

press process of 0.5MPa for 5min, 1MPa for 10min, and finally 1.5MPa for 10min. To fabricate

the recessed laminate for sandwiching the fluid, two individual parts were manufactured: first, a

recessed Kevlar-Dyneema laminate of 2.5mm depth and 110mm diameter and then a second

laminate of Carbon to serve as the rear sheet. The recess was made by pressing thick high density

polyethylene (HDPE) sheets to shape, removed and trimmed to shape.

Accordingly, 50ml of the fluid and two 0.16mm thick copper electrodes (both covering the fluid)

of approximate diameter with the recess were sandwiched, sealed with a cold curing contact

epoxy and pressed at 0.5MPa for 24hrs at room temperature. During the fabrication, direct

contact of the electrodes with the Carbon laminate was avoided, because of suspected electrical

conductivity that can induce short circuit; the insulated face of an electrode was glued to the

laminate, while the conductive face was exposed to the fluid. By the approach, three control

specimens (recessed without the fluid) and fifteen ERF-filled composite structures of 8.2mm

nominal thickness each were fabricated for this study. The summary of the given structure is

detailed in Table 1.

Table 1: Summary of fabricated laminates

Composite Constituting materials

Ply thickness (mm)

Number of plies

Nominal thickness of the

fluid-filled laminate (mm)

Fluid-filled fiber-

reinforced structure

Kevlar 49 0.30 98.2

Dyneema SK75

0.24 9

ERF 50ml covering a recess of 2.5mm depth×110mm

diameter

No Ply: only fluid medium

Carbon (SM) 0.65 6

2.2 Material characterisation

2.2.1 ER fluid properties

3

An electro-rheometer with a central Couette-type cup and bob was used to characterize the fluid.

The Couette was made up of two concentric cylinders (outer-cup and inner-bob) and 1.5mm

annular gap. The rheometer consisted of a temperature control mechanism and voltage power

source, which can provide over 5KV. For characterisation, the ER fluid was placed in the gap,

while the walls of the cylinders were induced by the required voltage (and hence electric field) for

an initial 3min (to obtain relatively consistent field effect) before the application of shear. On the

equipment, a custom suite of programs developed in-house fitted the rheological data to Bingham

plastic model and provided a print-out of the parameters derived with 95% confidence limits.

All measurements, yield stresses and the current densities were made at a bath temperature of

30±2.5C and shear rate range of 10-1 to 103s-1. The yield stress data recorded were derived from a

shear stress-shear rate relationship, while the current density was recorded as the electric current

per unit cross-sectional area, measured in the direction perpendicular to the direction of the fluid

flow. The current density is a vector quantity used in predicting the power consumption of the

fluid. On the other hand, the yield stress was characterized by two limits: static, at which flow

was initiated in the fluid; and dynamic (of interest, recorded and referred to in this study), below

which it was insufficient to cause the fluid to flow. Following the above described procedure, the

fluid responses shown in Table 2 for Figures 1, 2 and 3 were obtained.

Table 2: ERF property evaluation

Electric field

(KV/mm)

Log10 of Electric field

(KV/mm)

Dynamic yield stress

(KPa)

Log10 of Dynamic

yield stress (KPa)

Current density(10-2 ΜA/ mm2)

0.00 - 0.00 - 0.000.20 -0.70 0.05 -1.30 0.050.40 -0.40 0.13 -0.89 0.110.60 -0.22 0.27 -0.57 0.210.80 -0.10 0.60 -0.22 0.301.00 0.00 0.89 -0.05 0.421.20 0.08 1.30 0.11 0.651.40 0.15 1.51 0.18 1.202.00 0.30 1.66 0.22 4.152.60 0.42 1.75 0.24 8.203.00 0.48 2.27 0.36 11.603.50 0.54 3.29 0.52 16.104.00 0.60 3.92 0.60 21.804.50 0.65 4.53 0.66 27.525.00 0.70 4.82 0.68 34.80

4

Shear stress-shear rate relationship at 300C for 45µm particles

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 250 500 750 1000

Shear rate (s-1)

Sh

ear

stre

ss (

Pa)

0KV/mm

1.3KV/mm

2.6KV/mm

3.5KV/mm

5.0KV/mm

Figure 1: Shear stress vs. shear rate

Power law expression of yield stress vs electric field at 300C for 45µm particles

y = -0.2075x1.3746

R2 = 0.9596

-1.5

-1

-0.5

0

0.5

1

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Log10(electric field - KV/mm)

Lo

g1

0(y

ield

str

ess

- K

Pa)

Log-log yield stress-electric field Linear (Log-log yield stress-electric field)

Figure 2: Natural log of yield stress vs. electric field expressed in power law

5

R2 = 0.9121

-10

-5

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Electric field(KV/mm)

Cu

rren

t den

sity

x10

-2(µ

A/m

m2 )

Current density-electric field series Linear (Current density-electric field series)

Figure 3: Current density vs. electric field

2.2.2 Fibre-reinforced composite laminate properties

The composite laminates were characterised under quasi-static and dynamic loadings in order to

evaluate and establish the sequence of individual laminate performances. The analyses were

carried out so as to obtain most needed fundamental information before the fabrication and

testing of the fluid-filled laminates. For quasi-static test, 25mm×250mm strips of 150mm gauge

length were cut and trimmed from 300mm×300mm Kevlar®49, Carbon, Dyneema®SK75

composite plates. They were prepared according to ASTM D3039/D3039M, mounted within the

serrated grips of a 250KN Instron® tensile testing machine and pulled at a constant cross head

speed of 1mm/min to failure. All the test results as represented in Figure 4 were taken from the

average of three tests for each specimen.

6

0

100

200

300

400

500

600

0 0.005 0.01 0.015 0.02 0.025 0.03

Strain (dimensionless)

Ten

sile

Str

ess

(MP

a)

Kevlar Carbon Dyneema

Figure 4: Tensile properties of laminate materials

To the dynamic analyses, impact tests were carried out on 130mm diameter circular composite

plates clamped by means of four bolts, 131mm diameter circular specimen holder and supporting

metal ring for stability and accurate location of the specimens. Thus, 100mm diameter of each

composite plate was exposed concentrically to the axis of a 12mm diameter impactor tup with a

hemispherical nose. The tests were carried out according to ASTM D3763 standard with an

Instron® Dynatup 9250HV (designed with impulse control and data acquisition system) impact

tester. Under initial impact energy of 94J, the composite plates were subjected to failure with a

fixed impactor mass of 8Kg dropped from a height of 1.2m for impact velocity of 4.85m/s.

Considering the average thicknesses of the specimens, the given initial impact energy was utilised

to give way into studying their effective damage mitigation schemes and to assist in deciding the

impact energy for subsequent analyses. All the test results (maximum output load, displacement

and energy absorbed) as shown in Figure 5 were taken from the average of three tests for an

identical specimen.

7

0

1

2

3

4

5

0 0.5 1 1.5 2

Deflection (10-3m)

Lo

ad

(K

N)

Kevlar Dyneema Carbon

Figure 5: Dynamic properties of the composite under plate impact

2.3 Impact responses of electric field-induced ER fluid-filled laminates

To facilitate this work, a few assumptions were made: the fluid was considered to be chemically

stable under the circumstances exposed and would not attack engineering materials; particles

were all of the same size; the electrodes were completely and perfectly spanned by straight chains

of particles; the fluid particles and the host oil were assumed to be dielectric, polarisable and

sensitive to electric field; test specimens were fully constrained; constant volume and thickness

were assumed to be unibond of all the layers and matrix as an effect of compaction; and wave

transmission between the impactor and the test specimens were neglected. Moreover, energy

losses in the system were neglected in order to ensure energy balance.

The impact tests were carried out with a fixed impactor mass of 8Kg attached to a 12mm

diameter hemispherical impactor and dropped from a fixed height of 2.5m (impact velocity of

7.00m/s or initial impact energy of 196J) unto 130mm diameter electric field-induced constrained

specimens. With the tests undertaken at ambient conditions, 100mm diameter of the specimens

was exposed concentrically to the axis of the impactor. The test condition was chosen as a test

8

range considering the limits of the voltage power source and characterised material properties:

electric field strength was varied between 0KV/mm and 5KV/mm so as to verify the actuating

properties of the fluid, while the impact energy was chosen due to its capability to induce failure

and offer grounds for comparisons.

The electric field strengths were introduced through mains voltage source in order to

conveniently manipulate the rheological properties of the fluid under consistent electric field

supply through two copper electrodes connected to electrical leads with in-built crocodile

conducting clips. Before impacting the specimens, the field was introduced for an initial time of

3min (to relatively ensure constant effects on the fluid) and then maintained throughout the

impact exercise. The contact load history was measured by a piezoelectric transducer and then

filtered at a frequency of 68KHz, which enabled the production of reliable curves. Furthermore,

the impact velocity was calculated by the equation below by neglecting friction and heat

generation between the drop mass and the vertical rails:

(1)

where, Vf is the final velocity; g gravity and Δh the change in height.

With the corresponding impact responses presented as the averages of three identical specimens,

the absorbed energies were obtained through the integration of the areas under the load-

displacement curves. Hence, the typical load-displacement curves for the varied test conditions

are shown in Figure 6.

9

0

2

4

6

8

10

12

14

16

18

20

22

0 1 2 3 4 5 6 7 8 9

Deflection(10-3m)

Lo

ad

(KN

)

Control (No fluid) 0KV/mm 1.9KV/mm 3.0KV/mm 3.9KV/mm 5.0KV/mm

Figure 5: Typical load-displacement curves of the ER fluid-filled laminates

3. Results

From observation, the overall ‘complex’ moduli of the composite laminates were substantially

affected by the impact and electric field strength, which resulted in wide range of responses.

From the results, it can clearly be seen that the addition of the fluid considerably influenced the

load, displacement and energy absorbed by the rigid structures relative to increasing field strength

and constant impact energy. Hence, with reference to the control specimen, the load was

enhanced by 14.6%, 32.1%, 47.1%, 47.5%, and 61.3%; displacement by 5.0%, 29.5%, 32.1%,

31.6%, and 31.3%; and the absorbed energy by 29.6%, 40.7%, 85.2%, 100% and 66.7%. From

the data, it can be observed that the load increased with field strength, but displacement and

absorbed energy peaked at 3KV/mm and 3.9KV/mm, respectively. The subsequent decrease in

displacement suggests improved rigidity as the fluid increasingly lost its viscous flexible nature.

In the same way, to the absorbed energy, the decrement relates to the extent of damage(s)

sustained or the amount of energy dissipated, which must have been in the decreasing order.

10

Furthermore, from Figure 5, the decreasing stiffness of the composite specimens is in the order of

1.9KV/mm, 0KV/mm, control, 3.9KV/mm, 5KV/mm, 3KV/mm. Though this observation proves

interesting as the ratio of the initial contact load to displacement, it suggests the possible effects

of inhomogeneity as depicted by the inconsistency of the trend. The trend, on the other hand,

depicts the initial bending resistances of the specimens as energy absorption mechanisms in the

penetration process – i.e. the initial contributory displacements required to attain the maximum

loads and subsequent relaxations with alternating linear and nonlinear elastic behaviours.

From the curves, the discontinuities are resistances presented by opposing materials to the

penetrating impactor; upward trends are inherent enormous contact stiffness exhibited by the

composite hybrid; while the first failures are inter-laminar damages – i.e. the outcomes of load

concentration due to the impacting energy parameter. This, therefore, presents the maximum load

as an important part in the analyses of the behaviours of the specimens, because it is linked with

the crushing of the ‘solid’ fluid and fibre bundles by energy higher than the strength compliance

of the materials. Thus, it is right to state that major damages in the structures were induced

around this point.

The result also shows that as the field strength increased the composites required more impactor

kinetic energy to produce damage, though the moduli of the constituents were insensitivity (could

not easily be detected on the curves) to the impact velocity. This observation also indicates that

considerable amount of energy was required for further penetration beyond the maximum loads;

accordingly, the phenomena of penetration process strongly contribute to the energy absorption

mechanism of test specimens. Hence, the differences in impact responses or failure mechanisms

invariably account for the rheological effects of the fluid structure, which are discussed further

below.

3.1 Discussion

In relation to the curves, it can be observed that each initial elastic phase of considerable stiffness

was interrupted by an abrupt drop in contact load. This was followed by a series of failure

phenomena which were not all clearly revealed. However, beyond the maximum load, which

reflects the progression of plug formation and frictional force (as depicted by the tail of the

curves) required in pushing through the specimen, the loads continued to undergo considerable

oscillations with increasing displacement and opposing material resistances due to stored elastic

energies. Thus, the curves are not superimposable, which predicts possible variations in damping

properties.

11

In order to obtain better understanding of the failure responses of the specimens, their physical

characteristics and microscopic cross-sectional views were further examined. From visual

inspections carried out, the characteristic surface failures observed are shown in Figure 6 in the

form of front- and rear-sheets damages from initial penetration and petal rotation, respectively.

(a): Responses of the control specimen

(b): Responses of the specimen with fluid (without E-field)

Figure 6: Surface failures of test specimens

12

Petals

Penetration perimeter with concentric ring

and sheet bending at the edges

(c): Responses of the specimen tested at 3KV/mm

(d): Responses of the specimen tested at 5KV/mm

Figure 6 continued: Surface failures of test specimens

These failures are predominant responses, not an attempt to represent different test status. They

are representatives of local tensile deformations as indicated by bending at the edges, tearing at

the centre of loading and direct impact of the fluid core after penetrating the face sheets. Hence,

the damages appeared not to depend on any additional support from the core to determine the

consequent load resistances or energy absorptions as the specimens experienced consistent face-

sheet penetration. The consistency also suggests that the failure strength of the common face-

sheet was less than the ultimate tensile stress of the impacting surface. Therefore, a common hoop

stress due to the impactor characteristics can be suspected as the resultant failures clearly revealed

penetration/cracking diameters equal to that of the impactor.

Nevertheless, for the fact that the face-sheet of the control specimen failed in a similar fashion

this part of the work remains exciting. It is exciting because in the absence of the fluid a buckling

process was anticipated – as an abrupt failure under large compression, it typically occurs when

the strength at an impact site is lower (due to the unstableness of elasticity) than the maximum

13

strength the structure can resist. On the other hand, the unrelated behaviours shown on the curves

are hypothesized to be the resultant effects of the fluid-structure interactions.

Additionally, a close observation of the rear-sheet failures revealed slight deviations in the form

of different petal rotations. The deviations suggest that the responses depended on the rheological

effect and strength of the impacted fluid core, which can be affirmed by comparing the different

pictures shown above. This, therefore, supports a claim that the face-sheets posed no reasonable

resistances to the contact loads required for the rear-sheet damages. Hence, for the entire failure

mechanisms of the specimens, the responses of the constrained core (due to varied stiffness of the

ER fluid) are suggested to have been characteristic, because in each case they largely determined

the ultimate forces required for perforation – i.e. the ‘rigidity’ of the ‘solid’ fluid controlled the

structural behaviour of the specimens. Accordingly, the improved contact loads at constant

impact energy suggest that when the constrained core was impacted the loading was more

distributed over the rear-sheet and higher forces were required to create diverse failures and

cracking diameters. It is noteworthy that the contribution made by the fluid is viewed from a

perfectly-hardened plastic behaviour, an introduced yield stress for nonlinear elasto-plastic

response.

Furthermore, under the microscope matrix failure, inter-laminar shear failure and debonding were

highlighted as the primary fibres were subjected to local tensile failures through the point of

impact. From the cross-sectional view, two outstanding observations were made. First, seemingly

high shearing responses through the path of the impactor on the control specimens and second, a

progressive failure pattern through the penetration path (especially in the lower halves), including

extended debonding and Mode I crack away from the contact area. The latter failure patterns,

which differed only by the extent, were found in all the fluid-filled specimens at perforation.

This, therefore, means beyond the control specimens a certain value of energy absorption

significantly increased as the failure mechanism departed from shearing processes to tensile

deformations.

To the control, through the path of the impactor, it was only what appeared to be a neat or clean-

cut penetration (due to a shearing force) to the distal side was observed as shown in Figure 7. As

a result, the damages were more concentrated, confined to the impact region and less severe to

that observed in the fluid-filled specimens. With a localised type of damage, the failure responses

are believed to be the resultant effects of the recess, which reduced any resistances that could be

14

posed by more compact laminate. This also confirms that a good constraint system was used and

the stiff face-sheets were well formulated; arguably, the reason why the specimen did not buckle

under the tensile force of the impactor. Moreover, there were no appearances of delamination

through the central impact region, only loose fibres in the tensile zones.

Figure 7: Cross-sectional failures in the control specimen

In the case of the fluid-filled specimens, the amount of damage caused by the impactor ranged

from clear surfaces in the upper halves to fibre rupture/splitting, delamination, crack formation

and fibre bundle crushing (petal rotation) at the lower halves. For this reason, the interesting part

of the inspected specimens was the lower half due to the resultant differences caused by the

applied tensile force. These facts which are detailed in Figure 8, however, are damages caused by

apparent tensile, compressive and shear stresses; with concentration close to the impact site on

the rear. Nevertheless, the geometry of the penetration diameter was found to be the same with

that of the impactor irrespective of the growth of failures.

15

17mm

Fibre breakage in tensile zone

Impactor penetration

site

(a) Through-thickness failures within the impact site

(b) Extended failures outside the impact site

Figure 8: Graduated and extended through-thickness failures in the fluid-filled specimens

Having examined the two different sets of specimens and their failure variations, it is right to

state that the dynamic responses were the resultant effects of increasing field strength and

16

Electrode

Compressive zone of fibre

breakage/shear failure of the

Carbon laminate

Plug from displaced

primary fibres

15mm

Inter-laminar failure in the Kevlar plies

Fibre-matrix debonding

between carbon and Dyneema

laminates

15mm

consequent variations of the fluid strength; not strain rate dependence in the fibre-reinforced

laminates. In other words, the major deviations observed in this study were as a result of the

electric field effect on the rheological responses of the fluid – i.e. improved properties through

change of states and energy dissipation in the particle chain network. By this, given the increased

field strength, higher loads were required in the fracturing of the laminates because of improved

resistance of the ‘solid’ fluid – i.e. higher strength core in the form of ductility was realised

through varied induced electric field, which introduced the fluid as energy absorbing or load

support member. Hence, the energy absorption mechanism of the structures improved from

localised (shear responses) to globalized failures as in structural integrity protection.

This, therefore, presents the electric field strength (not the constant incident energy utilised,

which could barely counteract the applicability of the field) as an essential parameter in the

impact phenomenon of ER fluid-filled laminates. It also emphasizes on the actuating property of

the fluid through the harvest of electrical energy. As a result, in the case of vibration suppression,

ER fluid can be used together with sensors for ‘sensual-actuation’ as validated by Wei et al. [19].

Furthermore, there are other issues which could play vital roles in the behaviour of the ER fluid:

first, electro-magnetic interference (EMI) or air voltage breakdown - capable of producing sparks

and may affect operating systems; second, fluid stiffness threshold or limit - where brittleness or

shear thinning rather than dilatancy or shear thickening (under very high dipole moment) can

occur. In the future, this is hoped to be verified since it comes in proximity with the findings of

Zhang et al. [20]: configuration of applied electric field perpendicular to the shearing direction of

an ER fluid can lead to shear thinning (at high shear rates) and the loss of ER effect.

3.1.1 ER fluid behaviour under electric field

This study has been necessitated by the fact that results showed that the failure strength of the ER

fluid chiefly contributed in the characteristic behaviours of the fluid-filled composite specimens.

For this reason, in this section, the prediction of the typical behaviour of the fluid under an

electric field is attempted.

ER fluid is a particle suspension in which dipole moments (separation of charges) can be induced

by an electric field applied externally. The inducement can also be described as a process of

permittivity – i.e. the ability of the material to transmit or permit, affect or be affected by an

electric field. This is because ERF particles (sometimes the host oil) are usually dielectric,

electric insulators that can be polarised under an electric field. Therefore, the flow of electric

charges is not as in a conductor, only slight displacement from their average equilibrium positions

17

to opposite directions. Hence, for fluid particles alignment, electrostatic interaction can be

assumed as the dominant interaction if other interactions were to be neglected. In the same way, it

is possible to think that any moments induced were linear – maybe as in electrolyte (ionic

conduction) rather than by a simple dielectrophoresis – exertion of force on a dielectric particle as

a result of non-uniform electric field.

Accordingly, through the limited charges generated by the above-mentioned process current (and

hence conduction) is expected to be relatively poor towards the electrodes. The poor conduction

is due to the fact that the supposed charges cross the particles’ interface with respect to the

‘small’ contact area between them, though the effects dielectric properties and electric field

strengths play vital roles. Furthermore, the current (inside, outside and across the particles) can be

assumed to be equal because the charges at the conductive plates are expected to build up to the

point where voltage is just enough to maintain balanced circuit. This suggests that the formation

of particle chains will be relative to the electric field either by dielectrophoretic or electrophoretic

effect for field-particle interaction or particle-particle interaction respectively.

The particle chain formation, on the other hand, can be compared to a series connection because

as long as the dipole regenerates particle chains would be formed proportionally to the field

strength and shape of the electrode. This also suggests that the force of interaction between the

particles, supposedly, the strength (yield stress) of the fluid will remain proportional to the field

strength. However, if the dipole saturates, the yield stress can also be assumed to be proportional

to the field strength though with possible variation in stiffness. Having said all that, it is

noteworthy that the behavioural dependence of the ER fluid on electric field reveals a field-

induced moment, which is popularly referred to as ER effect.

In relation to the yield stress, based on power law, polarization and conduction models have

emerged as the best expressions for explaining most experimental observations of polymer-based

(semiconducting) ER fluids [21]. The former relates the material parameters (such as dielectric

constant) of the fluid to the rheological properties and proposes that the electrostatic force

depends on the dielectric constant mismatch between the particles and the host oil [22]. As a

result, it scales the yield stress to the square of the applied electric field strength,

(2)

where, y and E represent yield stress and electric field, respectively.

However, sometimes the model fails to explain the ER behaviour of some fluid, especially when

the particle and liquid host oil/medium are conductive or when the medium (being dielectric)

18

becomes conductive under very high electric field intensities capable of dissociating it to allow

current flow – i.e. on the basis that the applied electric field is localised on the liquid between the

particles for possible decreased ER behaviour and deviation from electrostatic polarization

model.

Thus, the conductive model comes into play: it does not take into consideration micro-structural

changes as it proposes that the particle interactions depend on the conductivity mismatch of the

fluid constituents. As a result, at high electric field strengths, the yield stress has been reported to

be [23]

(3)

With respect to equations 2 and 3, given Figures 1 and 2, it can be observed that the slope

revealed a critical conduction model: the slope of 1.4 is closer to 1.5 than 2. For this reason, it is

interesting to report that the dynamic yield stresses of the ER fluid studied fitted to a universal

yield stress equation with R-Squared (for the prediction of future results) value of 0.9596.

However, the nonlinear characteristic of the yield stress to electric field suggests that the absolute

viscosity of the fluid was not fully reversible upon any unloading of the applied field – i.e.

hysteresis or phase lag becomes a possibility under assumed direct proportionality

To the current density represented in Figure 3, it is usually expected to be low (with volume that

is proportional to an applied electric field) taking into consideration the dielectric constants of the

fluid constituents. As a result, the following are noteworthy: it is important that the current is able

to determine the size and cost of power supply, because to realise high field strengths high

voltage power sources (which can potentially produce 10mA) are likely to be used; and it is

advisable that ER fluid devices are designed to minimize current, taking into account the fact that

ER fluids do not obey Ohm’s law. This, therefore, suggests that the best representation of current

can either be graphical or by reference to a model, but that can make situations worse because it

will best describe static fluid – i.e. without considering possible drop in current when flow is

involved. By taking all said into account, it looks certain that current density is probably the least

understood property of an ER fluid, though it has proven to be one of the most important

parameters to be analysed when designing ER systems. For instance, it is theoretically difficult to

model: pure polarization mechanism does not predict any current flow and from a chemical point

of view electrode products are hard to find. Hence, the mechanism of current flow is hard to

illustrate.

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Having said all that, with respect to impact, two major suggestions are here presented on the basis

of the ‘solid’ fluid responses: first, deformations at increasing load; second, actuation which

depends on loading and ambient conditions imposed on the fluid. To the former, it largely

occurred as a result of chain rearrangement, stretching, breakage (with possible cases of

plasticity) and equilibrium (within elastic limit) between particle reformations or alignments in

the layers of the array. Therefore, as the dynamic yield stress increased (via induced electric field)

the number of chains of particles increased to reflect the proportional chain strength (or particle

concentration) that must be broken or sheared per unit area to introduce flow – i.e. resistant,

strengthened and closely packed fluid structures achieved under increasing electric field strength

were subjected to varied plastic and elastic deformations.

To the latter, actuation relates to change in physical (rheological) properties of the fluid with

respect to the field strength. Hence, as the field intensity increased the stiffness or tenacity of the

fluid also increased. Based on this knowledge, it is reasoned that the time it takes the fluid

structure to respond to an external stimulus differs from the time it takes to assume its natural

state upon its removal, irrespective of its short manoeuvring time – i.e. rheological properties

(e.g. yield stress, viscosity) would be nonlinear (not fully reversible) in response. In the same

way, hysteresis or phase lag would be possible under the punching force of the impactor.

However, this is the case where both ‘dwell time’ and uniform field distribution (though not

always realistic) are considered.

Finally, to the fluid-filled structures developed and tested, it can be reported that dielectrophoretic

effect was the force behind the fluids behaviour under the assumption that it is almost impossible

to realise uniform field in the constrained core. As a result, the strength of the interacting forces

would depend strongly on the fluid’s constituent electrical properties, the shape and size of

particles, and the frequency of the electric field which manipulates the particles with selectivity.

This analysis, arguably, describes the impact behaviour of the constrained ER fluid under varied

electric field strengths.

4. Conclusions

The impact responses of a number of electrorheological fluid (ERF)-filled composite specimens

have been investigated with the fluid characterised as solids under electric field. The rheological

data of the fluid under on- and off-states were collated and have been expounded relative to

structure. Having carried out the work to validate the load bearing capability of the ER fluid as an

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energy absorbing medium, in view of improving the impact resistance of composites, a summary

is presented. Hence, in the range of test conditions, careful observation of the results showed that:

beyond 3.9KV/mm (at 5.0KV/mm) the specimen showed reduced energy absorption. In

suggestion, there could be a limit to the contribution of the viscous fluid with increasing

electric field which might lead to brittleness. However, it is not known if it relates to

shear thinning or thixotropic properties for subsequent ultimate failure of the specimens.

the materials responses (load bearing capability, maximum peak load and failure energy)

were mainly due to the varied stiffness of the fluid and increased field strength, not the

constant impact energy applied. Therefore, with a defined limit as in any other material,

ER fluid is capable of serving as load support member.

for one-shot impact, it is difficult to obtain clear transition of impactor penetration stages.

Hence, the curves were not well superimposed upon each other, even up to the point

where the energy initially supplied to the impactor was completely arrested by the

specimen.

if ER fluid were to be characterised as solids, it suggests that there is hope in the future

for it to be tested mechanically to verify its static and dynamic properties – the

contributions come from the viscous force, with or without electric field.

from the failure assessments, unlike expected complex failure of composite laminates, the

growth of crack (and supposed delamination) was limited, uniform and centralised by the

presence of the fluid. This impressively suggests an effect on the structural integrity of

composite structures through improved impact energy dissipation and modal loss factor.

as an actuator, ER fluid can contribute in the improvement of impact resistance of

composites, especially where structural flexibility is required and in high velocity

applications (which mostly depend on wave propagation) to account for vibration.

it will be difficult to compare this verification with other ER fluids, because of inherent

property variations and fluid compositions, not necessarily due to lack of standard test

conditions.

4.1 Further work

In the future, other relative and valuable properties of the fluid will be examined. This will

involve fluid property improvement and fibre-reinforced alternation, laminae variation and

architectural changes so that high velocity applications will not be undermined.

Acknowledgement

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The authors are grateful to Smarttec® UK for the ER fluid; DSM® Netherlands, and Zoltec®

Europe for the high performance fabrics; and the department of MACE (Manchester University

UK) for the test equipments.

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