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Department of Mechanical Engineering University of Canterbury Telephone: +64-3-366 7001 Private Bag 4800 Facsimile: +64-3-364 2078 Christchurch Website: www.mech.canterbury.ac.nz _______________________________________________________________________________

Alex Hannon 28 May 2014 PO Box 123456 CHRISTCHURCH ___________________________________________________________________________________

Tow Ball Failure Summary Dear Mr. Hannon, I have examined your tow ball and found that it has failed due to reverse bending fatigue. Stress and fatigue analysis proved the design to be safe and within regulations and there is no obvious flaws in the microstructure. It is therefore inconclusive as to the particular cause for fatigue initiation. Mixed mode ductile and brittle failure was observed to occur in the final fracture region. The metal appears to be AISI 4340 alloy steel, which contains a microstructure of proeutectoid ferrite and lamellar pearlite. There is indication of inclusions in the microstructure, however the composition and nature of these particles are unknown. Further investigation into the history of the tow ball is required before further investigation into this failure can be completed. Angus Malcolm

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Table of Contents

1.0 Background.......................................................................................................................................................... 3

2.0 Procedure ............................................................................................................................................................ 4

2.1 Initial Sample Preparation............................................................................................................................... 4

2.2 Visual Examination .......................................................................................................................................... 5

2.3 Metallography ................................................................................................................................................. 5

2.4 Microhardness Testing .................................................................................................................................... 5

2.5 Fractography ................................................................................................................................................... 6

2.6 Energy Dispersive Spectography (EDS) ........................................................................................................... 6

3.0 Results ................................................................................................................................................................. 6

3.1 Visual Examination .......................................................................................................................................... 6

3.2 Metallography ................................................................................................................................................. 8

3.3 Fractography ................................................................................................................................................. 10

3.4 Energy Dispersive Spectography ................................................................................................................... 13

3.5 Microhardness Testing .................................................................................................................................. 13

4.0 Stress and Fatigue Analysis……………………………………………………………………………………………………………………………15

5.0 Discussion .......................................................................................................................................................... 20

Conclusions ............................................................................................................................................................. 23

References ............................................................................................................................................................... 24

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Background

A tow ball that had fractured at the neck was obtained. Unfortunately, the source of the tow ball was

unable to be reached for comment on its mechanical history.

The tow ball has a 50mm radius, is rated to withstand 3.5 tonnes, and was manufactured by Lumen, an

Australian automotive company, which is stamped on the tow ball. By Australian and New Zealand law,

this is the largest mass allowed to be towed by a ball tow bar. The failed component, is pictured in figure

1, and the expected loading conditions in figure 2.

When a vehicle towing a trailer accelerates or brakes, Newton’s  second  law  states  that  it must also provide

a force to accelerate/brake the trailer (and contents), equal to the rate of change of momentum of the

trailer and its contents. The force is directly proportional to the mass of the trailer (and contents), as well

as the magnitude of acceleration or deceleration. This force is transmitted directly through the tow ball.

The force distribution direction across the tow ball is opposite for acceleration and braking cases as the

force provided is in opposite directions. This leads to a cyclic loading case with regions undergoing cycles

Figure 1. Tow ball in fractured state Figure 2. Expected loading conditions

Fbraking Facceleration

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of tension and compression. This force is also dependant on the angle between the tow ball and trailer.

The angle of contact between the tow ball and the trailer is dependent on the type of trailer as well as the

height that the tow ball is relative to the ground. Other forces include the lateral forces that are

transmitted through the tow ball during cornering as well as the smaller amplitude but higher frequency

cyclic loading due to inconsistencies in the road, rolling resistance of the trailers tires, friction in the

trailer’s  wheel  bearings,  and  aerodynamic  drag  forces.    

2.0 Procedure

2.1 Initial Sample Preparation

Two Samples were prepared for scanning electron microscopy (SEM) and metallography. The tow ball

was measured and photographed before cutting, to aid in stress analysis, visual examination, and to

preserve an image of the tow bar in its original post fracture state. Vernier callipers were used to measure

the sample’s dimensions to a high precision, and photographs were taken of the sample.

An abrasive saw was then used to section a small slice containing the fracture surface so that it could fit

and mount within the scanning electron microscope. Another smaller section was removed from

remaining material for metallography. Figure 3 shows the approximate locations of the cuts.

Figure 3. Location of cuts for analysis

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2.2 Visual Examination

Low magnification photographs of the fracture surface, as well as observation by eye, served to emphasize

features such as flaws, possible initiation sites, corrosion, final failure regions and indications of fatigue in

the sample. This gave initial clues into reasons for failure of the component, and indicated features that

were to be observed in greater detail with microscopy.

2.3 Metallography

The metallography sample indicated in figure 3 was set into a mould using a mounting press. Green

Bakelite Powder, a general purpose moulding compound was used as the mould material. After the mould

was formed, the top surface was ground with SiC grit paper in a sequence of; 180, 240, 320, 400 and 600

grits. The surface was then polished in a sequence of 9µm, 3µm and 1µm diamond suspensions. A final

polish was performed with a 0.06µm colloidal silica suspension. To conclude, the mould was etched with

a 4% nital solution compromised of 96% ethanol and 4% nitric acid, for 1 minute, to reveal the sample’s  

microstructure for observation under an optical microscope.

The microstructure of the metallographic sample was photographed using the Olympus BH2-RLA-2 optical

microscope and camera at magnifications between 50x and 1000x. The microstructure was photographed

at several different locations in the sample to ensure consistency. These photographs were then

compared to figures in the ASM metals handbook to determine the phase of the specimen.

2.4 Microhardness Testing

Microhardness testing was then performed on the metallography sample with a Leco M400 H1

microhardness tester, set at a 1.0 kg load. There was no case hardened layer on the specimen, so a series

of hardness tests were performed at random locations throughout the sample. The average hardness was

then able to be converted into an approximate tensile strength using correlation tables.

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2.5 Fractography

The SEM sample, indicated in figure 3, was cleaned in a solution of Decon 90 overnight, which is an

ultrasonic active surface cleaning agent. This was done to ensure no dirt or corrosion remained on the

sample to ensure clarity of the fracture surface in the scanning electron microscope.

Following this treatment, the sample was examined in a JEOL JSM 6100 Scanning Electron Microscope,

with the assistance of a skilled operator. Several images at different magnifications were taken at failure

sites, regions containing suspected fatigue striations, regions of suspected fatigue initiation, and other

regions of interest.

2.6 Energy Dispersive Spectography (EDS)

The Scanning Electron Microscope is equipped with an EDS unit which is capable of determining the

approximate composition of materials to within 1.0% - 2.0% for elements with atomic numbers greater

than 10. This became useful for determining the alloy constituents added into the tow ball, however its

inability to accurately predict carbon content (atomic number 6) limited its potential in determining the

exact composition and steel grade of the tow ball.

3.0 Results

3.1 Visual Examination

Figure 1 shows the location of the failure with respect to the entire structure of the tow ball. The fracture

is seen to occur at the narrower end of the fillet joining the tow ball neck and flange. Failure at this

location is plausible, as this is the location at which the largest bending stresses are likely to occur in the

tow ball.

The red arrows on figure 4 point to ratchet marks on the sample. The cause of ratchet marks is usually

due to multiple fatigue cracks originating on different planes, which eventually combine on a single plane.

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This plane is indicated by the faint line pointed to by

the blue arrows. These ratchet marks are seen to occur

on opposite sides of the outer diameter. This is

coincident with the expected loading in the tow ball

and suggestive of fatigue initiating on each end of the

tow ball. These apparent fatigue cracks appear to

meet near the upper end of the fracture surface,

suggesting that the proposed fatigue cracks did not

propagate the same distance. At this location, a ridge

feature travels across the sample, indicated by the

green arrows. This feature is the region of final failure

of the tow ball.

A final observation from visual examination regards the surface texture of the tow ball. Little effort has

been made in smoothing the surface of the tow ball, as grooves can be seen and felt on the surface which

are the result of a turning process used in its manufacture.

Figure 4. Fracture surface with indications of ratchet marks, combination of crack planes and final failure

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3.2 Metallography

Figures 5 and 6 display images of the microstructure at magnifications of 100x and 500x. These images

reveal the microstructure to contain pro-eutectoid ferrite and fine lamellar pearlite. The pearlite is

characterised by the distinct bands seen at higher magnifications, and the ferrite is characterised by the

white regions. Comparison of the microstructure to microstructures in the ASM metals handbook, Volume

9, suggests that this steel has a low carbon content, and is comparable to microstructure images seen of

1045, 4130 and 4140 steel, which all have carbon contents less than 0.5%. Also distinct in these images

is small black spots throughout the microstructure. It is possible that these spots are oxide or Silicon

inclusions which may affect the material properties.

Figure 5. Microstructure at 100x magnification

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Figure 6. Microstructure at 500x magnification

The low carbon content of the sample is verified by figure 7, which displays the Iron-Carbon phase

diagram. For Carbon compositions lower than 0.83%, it can be seen that slow cooling from austenite will

hypo eutectoid ferrite and pearlite, which is consistent with the observed microstructure.

Figure 7. Iron- Carbon Phase Diagram

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3.3 Fractography

Figures 8.1 to 8.6 display areas of interest under the scanning electron microscope including regions

displaying striations, ratchet marks, possible initiation sites and suspected failure regions.

Figure 8.1 displays a ratchet mark observed on the tow ball, which is a likely initiation site of fatigue. The

red arrow points to the initiation site of the crack. Another interesting feature of this image is the visible

surface finish of the component which is observed to be rough, with a circular pattern.

Figures 8.2 & 8.3 display images of general fracture surfaces. Striations can be seen in various locations

on these surface, which establishes beyond doubt that fatigue is the failure mechanism. Figure 8.2 also

displays flat surfaces, which appear to be at higher elevations than the visible fatigued surfaces. This is

suggestive of surface wear in which the flat faces have been compressed against their opposite fracture

surface.

Figure 8.1 Ratchet mark at low magnification

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Figure 8.2. Fracture surface displaying striations and wear

Figure 8.3 General fracture surface displaying striations.

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Figure 8.4 is a high magnification image of the fatigue previously noted. The fatigue striations appear to

be relatively consistent. By extrapolating the counted number of striations within the image scale across

the length of the crack propagation zone, it is estimated that the tow ball experienced about 9x105 cycles

before failure.

Figures 8.5 & 8.6 display failure regions of the fracture surface. It was observed that the component

contained ductile and brittle failure zones. Figure 8.5 displays a typical ductile failure zone, indicated by

microvoid coalescence, whilst figure 8.6 displays a typical brittle failure zone, indicated by a cleavage

surface.

Figure 8.4. Close up image of fatigue

Figure 8.5. Image displaying microvoid coalescence Figure 8.6. Image displaying cleavage

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Figure 9. EDS spectra displaying weight percent composition

3.4 Energy Dispersive Spectography

Energy dispersive spectography allowed for the approximate composition of the material to be

determined (excluding elements with atomic numbers less than 10). Fig 9 displays the spectra obtained

from this analysis, which shows a large presence of Iron, and small amounts of Chromium and Silicon.

3.5 Microhardness Testing

Table 1 displays the Vickers hardness at various locations along the length of the moulded sample and the

correlated tensile strength. The hardness was seen to vary a maximum of 26.5HV, which is reasonably

inconsistent. Using an ATSM conversion chart, the average hardness was converted into an approximate

tensile strength value of 720.6MPa.

. Table 1. Measured hardness and correlated tensile strengths

HV Tensile Strength (MPa) 210.5 676.5 224.7 721.45 236.5 759.5 236.5 759.5 220.6 707.1 221.7 710.9 219.8 709.4

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5.0 Discussion

Fracture Mechanism:

The first indications of fatigue were observed in the visual examination. Striations were not visible by eye,

however the ratchet marks and crack propagation zones provided a good indication that fatigue was a

major factor. It was noted that the fracture surface occurred at the edge of a fillet, a location that was

expected to have a stress concentration. It was also noted that the tow ball had a poor surface finish.

Both of these factors can be major factors in fatigue initiation, hence these were initial theories into why

a fatigue crack initiated primarily. The ratchet marks and crack propagation zones were seen to begin on

opposite ends of the tow ball, before meeting in a final failure region. It was realised that this behaviour

is typical of reverse bending fatigue, a common type of fatigue seen in applications that experience

tension and compression cyclic loading through each element of the fracture. The ridge that indicates the

final failure region is seen to be closer to one side of the tow ball than the other. This is most likely due

to increased loading in one direction in comparison to the other, such as greater forces in accelerating

compared to braking.

Metallography indicated that the microstructure consisted of pro eutectoid ferrite and fine lamellar

pearlite. Comparison with microstructures in Volume 9 of the ASTM metals handbook suggested that the

alloy consisted of about 0.4%Wt Carbon. Small black inclusions were observed in the microstructure,

which were postulated to be oxide or Silicon inclusions. The effect of these particles on the mechanical

properties of the alloy is unknown, however it is possible that they served to decrease the mechanical

properties of the alloy, and decreasing the stress required to initiate fatigue.

Hardness testing revealed the microstructure to be reasonably consistent, with an average Vickers

hardness of 219.8. This correlated to a tensile strength of 709.4MPa.

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Scanning Electron Microscopy proved beyond doubt that reverse bending fatigue was the primary reason

for failure of the tow ball. Striations were seen throughout the fracture surface. These striations were

relatively consistent in spacing, indicating that the tow bar towed the similar mass objects throughout its

service life. It was estimated that the tow ball underwent 9x105 cycles before failure. Scanning electron

microscopy also revealed the final failure mechanism to be mixed mode failure, as cleavage planes as well

as microvoid coalescence were observed.

EDS Spectra revealed the composition of the tow ball to me made largely of iron, with small Chromium

and Silicon constituents. With knowledge of the approximate composition, knowledge of the

microstructure, and knowledge of the tensile strength, the steel alloy was determined to likely be 4340

Steel , which exhibits similar properties and traits to those observed in the tow ball.

Stress Analysis and Fracture Mechanics

Stress analysis was performed to predict the maximum possible loads seen in the towbar provided that

the loads were below the specified maximum load rating of the tow ball. A Solidworks finite element

analysis model was also created to ensure the stress calculations were consistent with another stress

measure. Substituting these maximum possible stresses into fatigue calculations yielded unexpected

results. It was shown that even with these maximum loads (which would not be usual loading), the tow

ball was predicted to have an infinite life, which shows that the tow ball has been designed correctly. This

is a confusing result as it is in direct conflict with the knowledge that the tow ball failed at approximately

106 cycles. A possible reason for this discrepancy is that surface flaws existed at the locations of the

initiation sites, thereby initiating fatigue. However this is a large coincidence that this would occur on

both ends. Another possibility is that the inclusions mentioned previously had a large detrimental effect

on the fatigue properties of the alloy.

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Another likely cause of fatigue initiation was the poor surface finish of the tow ball, which can impose

high local stress concentrations on the component. It is also possible that the towbar has been loaded

beyond its specified duty. A likely scenario is that combinations of these conditions have combined to

create the conditions that have enabled fatigue to initiate and propagate through the tow ball.

Metallurgical Factors

Pearlite can be hard and strong but is not particularly tough. It can also be wear resistant because of a

strong lamellar network of ferrite and cementite. The Stress analysis proved that this material

composition was appropriate for its application, despite the unexplained failure. This alloy is also

acceptable in New Zealand and Australian tow ball standards. The only concern with this alloy is the

unknown inclusions seen in the microstructure. These inclusions could potentially have a large

detrimental effect on the fatigue properties of the alloy, however, as their nature is unknown, this

cannot be stated with confidence. It is equally likely that these particles have a beneficial effect on

these properties.

Prevention and Prediction

As the exact cause of fatigue is unknown, the first measure that needs to be taken to ensure a failure like

this does not happen again is to investigate the history of the tow ball. If this investigation yields that he

tow ball was in fact loaded beyond rating, then the investigation will effectively be over. If this is not the

case, then the material properties and design of the towbar will need to be investigated further to ensure

the design is safe and material is to specifications. A simple solution would be to use a different alloy that

exhibits better fatigue resistance properties to make these tow bars, or increase material in the highly

stressed aspects of the design.

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Conclusions

x The tow ball failed due to reverse bending fatigue.

x The tow ball microstructure consists of pro eutectoid ferrite and fine lamellar pearlite.

x The tow ball alloy is likely to be 4340 steel, and has a tensile strength of 720MPa.

x The stress and fracture mechanics analysis indicates that this component should not have failed.

x Mixed mode failure (ductile and brittle failure) occurred at the final failure region.

x Before a particular reason for this failure can be determined and solution suggested, the

mechanical history of this component needs to be investigated further to ensure the tow bar was

not loaded beyond recommendation.

References

[1] Failure analysis info; http://failure-analysis.info/2010/05/analyzing-material-fatigue/

[2] ASM Handbook, Volume 9

[3] Norman. E Dowling , ‘Mechancial Behaviour of Materials’