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Trailer Wheel Stud Failure

J.E.I. Metallurgical, Inc. was requested to assist in the evaluation of metallurgical evidence resulting from a fatal trailer wheel stud accident. During the accident, the wheel and tire on the subject left rear axle wheel and tire disengaged from the twin axle, 16 foot trailer. The rogue, unguided, detached trailer wheel and tire traveled north in the grassy median of an interstate highway. Two workers, who were installing median cables between the north and southbound lanes, were struck and fatally injured by the subject detached wheel.

Summary

Based on the investigation that was conducted, several conclusions and opinions could be stated. These conclusions and opinions were stated and believed to be true to a reasonable degree of engineering probability. The opinions were:

  • The wheel lugs on the left rear axle of the trailer involved in this matter failed and allowed the left rear wheel to detach from the vehicle.
  • As stated in sworn testimony the vehicle was traveling at a speed of approximately 78 miles per hour, above the posted speed limit of 70 miles per hour.
  • At the accident scene investigation by highway patrol, the troopers revealed that the trailer tire air pressure varied from a low of 38 psi to 56 psi. Thus, the tire pressure in the subject trailer tires was 14 to 42% below the recommended/required 65 psi tire pressure.
  • Low tire pressure and exceeding the legal speed limit undoubtedly contributed to the subject left rear trailer wheel departing the subject trailer at the time and location of the accident.
  • The subject trailer was observed by others using the same northbound highway to be swaying from lane line to lane line.
  • The wheel exited the subject trailer, fatally injuring both workers.
  • All of the left rear lug bolts failed as a result of metal fatigue.
  • All of the lug bolt holes in the subject left rear wheel exhibited extraordinary amounts of wheel lug hole oblation and wallowing out.
  • The right side rear wheel lugs, three of which were found hand loose by the investigating State Trooper, exhibited absolutely no evidence of fatigue crack initiation or fatigue cracking in the lug bolt threads, although they had experienced the same or almost the same alternating stress environment during the trip, preceding the accident.
  • The tightness or torque of the subject left rear axle wheel lugs had been neglected and had been loose for days and possibly weeks prior to the accident. No evidence was presented that the driver or work crew actually checked the lug bolts for tightness and/or proper torque during this entire trip.
  • The lack of lug bolt tightness would have been obvious had the driver and/or any member of the work crew checked the wheel lugs for tightness and proper torque at any of the fuel or food stops. This is especially important since the vehicle occupants were technically trained and had a torque wrench available in the towed trailer..
  • The work crew testified and stated to highway patrol troopers that they had conducted a walk around inspection and had “kicked” the tires prior to leaving just 24 minutes prior to the accident.
  • Had the work crew actually conducted a walk around inspection and kicked the tires, it would seem that these highly trained mechanics would have detected the missing/fractured lug studs and lug nuts and would have visually observed the wallowed out wheel lug holes.
  • The height of the wheel standoff bosses on the right rear, right front and left front wheels were all insufficient to prevent wheel contact and rubbing with the wheel hub lug bolt reinforcement bosses.
  • The subject wheel did not exhibit a wheel standoff boss and therefore, the trailer wheels were mismatched. It is possible that the subject rim actually was intentionally cast without the wheel lug hole standoff boss and therefore, the wheel rims were mismatched.
  • It is also possible that the lug holes in the left rear wheel were not machined for tapered lug nuts. Thus, the use of improperly tapered lug nuts would have only exasperated an already bad situation, thereby accelerating the wheel lug hole wallowing.
  • The trailer wheel mismatch, and continued interference between the inner wheel surface and the wheel hub bolt reinforcement probably caused a continual problem of loose lug nuts and precipitated re-tightening by whoever would have serviced the trailer, i.e., shop or outside service organization. The use of an air impact wrench probably induced over-tightening and/or over-torquing and attendant high tensile stress in the failed lug studs.

Investigation

A perspective view of the left front axle trailer wheel and the subject left rear axle hub assembly are shown in Photograph A.

Perspective view of the left fron axle trailer and the subject left rear axle hub assembly
Photograph A Left front axle trailer wheel and the subject left rear axle hub.

The subject detached wheel and tire were also produced for examination.

Wheel Lug Hole Evaluation

Close-up view of the outer wheel surface with wallowed out wheel lug holes, clockwise from the 12 o’clock position, is shown in Photograph B.

Overall view of deformed wheel lug
Photograph B Deformed wheel lug at about the 12 o'clock location.

The results shown in Photograph B dramatically illustrate the extreme wear and wallowing out that occurred around each lug hole.

Lug Fracture Evaluation

All six wheel lugs on the subject trailer failed by metal fatigue. An overall view of the subject rear axle wheel hub is shown in Photograph C. The failed wheel stud locations discussed below will be referred to the clock positions in this picture.

Overall view of subject rear axle wheel hub.
Photograph C Rear axle wheel hub.

The failed lug, illustrates very classic fatigue fracture characteristics. However, some interesting anomalies are also present. The bright shiny circular area, denoted with white arrows in Photograph D, is believed to be an earlier rim or halo of fatigue fracture at the root of the wheel lug threads. This halo rim contained ratchet marks, fatigue striations, and beach marks also referred to as clam shell marks.

Close-up view of failed lug.
Photograph D Failed lug located at the 6 o'clock location.

This circumferential zone of fatigue initiated the lug separation and contains many areas of intermittent crack propagation. An enlargement at higher magnification shows numerous regions where the wheel lug fatigue crack propagated, then arrested creating a beach mark, then propagated again, arrested and then again propagated. Further magnification is shown in Photograph E. A very fine pattern of progressive crack extension is denoted with yellow brackets in Photograph E.

Fine pattern of progressive crack extension
Photograph E Fine pattern of progressive crack extension is denoted with yellow.

A careful examination shows numerous start/stop beach marks. The various start/stop areas are denoted with arrows in Photograph F.

Various start/stop beach marks
Photograph F Various start/stop beach marks are noted with white arrows.

These start and stop beach marks are a fingerprint of changes in the loading imposed on the subject failing wheel lugs, for example, traveling at 75 mph, pulling off to refuel the towing vehicle, pulling back onto the highway, etc. In between each of these beach marks are numerous fatigue striations resulting from crack propagation during similar individual stress cycles, such as wheel rotations. The mottled gray area in Photograph D does not represent final overload or final separation of the wheel lug but represents areas where the fatigue crack is enlarging in amounts dependent on the stresses being imposed and the direction in which that stress is imposed during driving stopping re-starting of the towing vehicle. The ONLY way to see these individual fatigue striations could be correlated with the probable stress history of the vehicle would be to conduct an investigation of the lug fractures in the scanning electron microscope. Another feature of this particular fractured wheel lug is the halo or rim fracture around the lug perimeter. The halo rim is shown in Photograph G. This halo rim has unusual features not characteristic of wheel lug failures.

Distinct beach marks
Photograph G Distinct beach marks are denoted with arrows.

In Photograph G at least five start stop beach marks are noted within the halo with arrows. The possible source and timing of the halo rim, along with evidence described below, can easily be explained. It is clearly evident that the halo rim is of a relatively constant thickness throughout the fracture circumference of this failed lug. It is also unarguable that the halo rim originates at the thread root. The existence of the halo rim is believed to be the result of tensile stress, possibly introduced at the thread root by over torquing of the subject lug or set of lugs. The lack of significant oxidation argues for a recent formation of the halo rim. None of the truck’s work crew mentioned nor acknowledged any torquing of the subject lug nuts during the most recent work trip involving the subject trailer. Since few work orders or outside vendor receipts for maintenance work on the subject trailer were received, the torquing and re-torquing could have been accomplished at the trailer maintenance facility. The fact that five distinct beach marks are identified in the halo rim suggests that there were at least five distinct stress events that resulted in the cessation and restarting of the stress responsible for the halo fracture pattern formation. Between these stop start events, the fracture mode is most likely metal fatigue. The reason necessitating to repeatedly re-torque, assuming the need to re-torque was the result of loose lug nuts, the subject lug nuts, is clearly from conditions on the subject wheel, the wheels from the other axles, and all of the axle hubs. Fatigue at the lug fracture origin indicates that the pre-load torque had been compromised. The beach marks in the halo rim have been the result of re-torquing and or re-initiation of a cyclic stress.

Wheel Lug Reinforcement Boss Wear

The wheel lug reinforcement bosses, which reinforce and distribute stress transmitted from the wheel lugs, contain incriminating evidence. The incriminating evidence was clearly and easily observed in HP scene photographs. One of the first highway patrol photographs of the left rear hub assembly is shown in Photograph H.

Overall view of left rear hub assembly
Photograph H Left rear hub assembly. White arrows mark and denote abrasion.

Areas of fresh wear on each of the left rear hub assembly wheel lug reinforcement bosses are denoted with arrows in Photograph H. When the subject trailer was first viewed, those same areas were highlighted by subsequent rusting. An overall view of the subject, left rear axle wheel assembly is shown in Photograph I.

Overall view of subject left rear axle wheel assembly. Arrows denote rusted/abraded tips of each reinforcement boss.
Photograph I Rear axle wheel assembly. Arrows denote rusted abraded tips.

The abraded areas in Photograph I are also denoted with white arrows. Additional confirmation of this boss abrasion problem was apparent as the trailer wheels were sequentially removed from each wheel hub. The left front trailer wheel was removed from the axle wheel hub assembly.

Inside Wheel Surface Wear

The inside surface of the wheel rims from the left front trailer axle and the right rear trailer axle is shown in Photograph J.

Close-up view of abraded/rusted areas where contact was made between the inside surface of the wheel rim and the tips of each wheel lug boss.
Photograph J Abraded rusted areas where contact was made between the inside surface of the wheel rim and the tips of each wheel lug boss.

Circular contact/abrasion rust marks, just outside the wheel lug holes are denoted with arrows in Photograph J.

A close-up view of the inside surface of one of the left front wheel lug holes is shown in Photograph K. Lipping or lug hole edge deformation is noted with white arrows. A lug hole standoff boss, cast into the wheel rim, is noted with black arrows. The circular contact/abrasion rusting mark is noted with red arrows.

Close-up view of lug hole
Photograph K Lug hole (exhibiting lipped hole edge).

Two features are noteworthy; the first feature is the lipping or edge deformation around each lug hole. The lipping is denoted in Photograph K with white arrows. This lipping edge of the lug hole abrasion is the beginning stage of wallowing out. The second feature is the presence of a cast standoff boss around each wheel lug hole. The apparent purpose of the standoff boss around each lug hole is to distance or push away the wheel rim sufficiently far from the wheel hub lug reinforcement boss to eliminate any contact between the two surfaces. The wheel lug hole standoff bosses are denoted with black arrows in Photograph K. The areas of abrasive interference between the inside surface of the trailer wheel and the surface of the wheel lug assembly, because the boss height evidently is insufficient, are each denoted with red arrows in Photograph K. Residual lines of rusty water probably road splash, or some other undetermined liquid, attest to the extended contact between the wheel lug retainer boss and the inside surface of the left front trailer wheel and the right rear trailer wheel. The subject failed wheel presents two anomalies. The first anomaly noted is the fact that the regions of abrasion on the inside wheel surface are larger in circumferential length and width than those exhibited by the left front axle wheel and the right rear axle wheel. The second anomaly is the total lack of any standoff boss to push off or distance the surface of the wheel hub lug boss is clearly evident. With less or zero standoff, more contact with the subject wheel hub, and thus a larger area of abrasion, would occur.

Other Failed Subject Lugs

Another failed lug from the subject left rear wheel assembly is shown in Photograph L.

Overall view of failed wheel lug
Photograph L Overall view of failed wheel lug from 1:30 o'clock position.

The gray area on this particular lug fracture surface visually appears to be a fast fracture final overload zone, created when that particular wheel lug separated. This fracture surface presents the more typical appearance of the final fracture zone than was evident in the mottled gray areas shown in Photograph E. If the gray area is indeed a final fast fracture zone, then the gray area should display a dimple rupture pattern in the SEM at a magnification of 500x to 2000x, confirming the final, fast fracture, failure mode. Again numerous beach marks, start stop indicators, are present across this wheel lug fracture surface indicating that this lug failure did not occur in the 18 miles prior to the accident scene. Typically striations, from individual crack extensions in wheel lug failures, often require magnifications of 40,000x to 50,000x to resolve. The lug fracture in Photograph M presents many of the same features, for example., partial halo rim, ratchet marks, early very slow crack growth, common crack front formation and a rather large gray area which probably is the final/fast fracture zone.

Overall view of the failed wheel lug
Photograph M Failed wheel lug from the 10:30 o'clock position.

Two of the failed lugs on the subject accident wheel hub assembly fractured deep in their respective lug holes. It was visually evident that both of these lugs failed in or at the lug splines as the spline form is clearly visible. Exemplar wheel lugs, removed from the right rear wheel assembly, are shown in Photograph N. The arrows in Photograph N denote the spline area where the deep failure in the subject two lugs occurred.

Exemplar lugs from right rear axle wheel assembly.
Photograph N Exemplar lugs from right rear axle wheel assembly

It is clear that a bright circular burnish abrasion has occurred OVER and is erasing some of the original lug fracture features.  This circular abrasion is NOT the result of torsional overload. What is visually most apparent is rotational mechanical damage that occurred on the surface fractured lug, AFTER the fracture surface was created.  These features are not fracture features but are probably the result of rotational mechanical damage from the other fractured half of that particular wheel lug. It is also well established that rotational or torsional overload of a splined shaft, when it occurs in the splines, results in a twisting of the splines on both sides of the fracture prior to final separation.  Careful examination of the splines on these two failed lugs reveals NO spline twisting or rotational spline deformation.  Thus, the deep failure of these two wheel lugs in the wheel lug spline was not the result of torsional overload. Possible cracking in the roots (bottom of spline V) of several splines was noted on the spline fractures. It was also noted that the other tips (points) of the splines appeared jammed into the wall of the lug/stud hole. The jammed spline tips are shown in Photograph O.

Close-up view of tips of splines which appear to have jammed or impacted the wall of the wheel lug hole. The jammed/impacted spline tips are denoted with arrows.
Photograph O Tips of splines which have jammed or impacted the the wheel lug hole.

 

Discussion

The limited microscopic examination conducted by the author revealed different interpretations, of the fracture characteristics of the subject failed wheel lug bolts, than previously proposed. Certain facts remain obscure because a full investigation was not permitted. A thorough and complete failure analysis of the subject lug bolt failures was not accomplished. The accident lugs were not even removed from the subject left rear hub assembly for macroscopic visual examination much less the necessary cleaning, microphotography, scanning electron microscopy, metallography, hardness and microhardness testing. The hardness of the subject wheel rim has never been determined.

The author has over 46 years of experience conducting metallurgical failure analysis and has previously conducted numerous investigations into wheel lug stud separations. I have NEVER, in all of that previous experience, observed such a gross amount of wallowing out of lug holes in any vehicular wheel. It is difficult, if not impossible, to believe that the observed amount of lug hole wallowing could occur on wheels supposedly tight and that were checked prior to the subject accident. The right rear wheel, according to an investigating officer, had three lug nuts which could be tightened BY HAND. Yet, a visual examination of the lug holes on that wheel provided a view of lug holes with very little lug hole wallowing. Thus, based on the presumption that the right rear wheel lugs were tight when the vehicle started the day's journey, only three lug nuts loosened to the degree that they presented a hand tightening condition by the investigating officer, but very little lug hole oblation.

The right rear wheel presents the best evidence that the left rear wheel assembly was already loose when the subject trip started. Most of the subject wheel lug connections had to have lost torque before any slippage of the wheel relative to the wheel lugs could occur, i.e., before oblation and then wallowing out could occur. Thus, wallowing out of the subject wheel lug holes was probably the result of days or even weeks of looseness. It is a well-established fact that 50 to 90% of all alternating stress cycles experienced in an alternating stress fatigue situation result in the accumulation of invisible damage in the metal prior to the actual initiation of a fatigue crack. The right rear and left rear axle wheels presumably were subjected to the same stress history during the trip. However, visual and laboratory microscopic examination and metallographic sectioning of the lug bolts from the right rear axle wheel revealed ABSOLUTELY no cracking in any of those right rear wheel studs. The difference in cracking in the wheel studs in the right rear wheel assembly and the subject left rear wheel assembly, presuming both sets of wheel lugs were tight when the trip began, is absolutely astounding.

If one is relying on metallurgical logic, rather than having a complete and full metallurgical failure analysis, then the first place to start is at the last stop made by the work crew. This stop was made approximately 18 miles prior to the subject accident. It is inconceivable that all six wheel lug nuts were still in place on the subject right rear trailer wheel at that time. Thus, if the work crew had indeed done a walk around inspection of the trailer and had kicked the tires, it is incomprehensible that they would not have seen the failed wheel stud lugs, and/or lack of lug nuts and lugs, and the grossly distorted, wallowed out wheel lug holes at that time. The same can be said for each of the stops on the trip. The insufficiency and lack of the interior wheel lug bolt hole standoff bosses appears to have existed, assuming the wheels have never been changed since the trailer was new. This condition, i.e., insufficiency of the wheel hole standoff boss height, especially on the left rear wheel, would have induced abnormal bending stress into the wheel lug joints for the life of this trailer. These stresses probably introduced a continuing problem of lug nut loosening on the subject wheels.

It appears from paint abrasion marks surrounding, especially the left rear wheel lug holes, that an air impact wrench was probably used to torque up the subject lug nuts. Tightening the subject lug nuts with an air impact wrench is an ideal, and most probable, way to introduce an over torque and excessive tensile stress into the subject left rear wheel lugs.

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J.E.I. Metallurgical, Inc.

5514 Harbor Town
Dallas, Texas 75287

Phone: (972) 934-0493
Fax: (469) 737-3938
Email: r.c.jerner@metallurgist.com

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