WHEEL SEPARATION

By: G. Wayne Maltry, AMT, BSAE, MSME, P.E.

Occasionally, an analysis is dismissed on the assumption that it is too academic, insinuating that it would not be useful in a “real world” situation. In forensic science, that assumption may not hold true, especially when several so-called academic observations point to the same conclusion.

A left-rear wheel separated from a bucket truck while being driven on an interstate highway. The driver lost control and rolled the truck as a result (truck wreck). A passenger in the bucket truck was injured in the incident. It was asserted that a truck repair company who last worked on the truck was responsible because they had not properly retightened (under torqued) the wheel nuts after servicing the brakes. Although this assertion seemed reasonable on the surface, there were several inconsistent issues that were immediately evident.

Inspection of the bucket truck revealed that it did, in fact, exhibit indications of having been rolled (Figures 1 and 2). In addition, all of the wheel studs on the incident wheel hub had snapped off. (Figure 3). The fractured wheel stud surfaces,

although weathered, still exhibited a flat granular appearance and included circumferential ratchet marks consistent with a reversed bending, fatigue fracture mode. (Figure 4). It was known, however, that fractured wheel studs are not typical to left side wheel separations. Left side wheel separations usually occur after the wheel nuts spin-off and right side wheel separations tend to occur after the wheel studs break off. 1

1 Bailey and Bertoch, "Mechanisms of Wheel Separations," SAE, 2009.

In simplistic terms, the left side loosening effect is attributed to the counterclockwise direction of wheel rotation which, coincidentally, is the same direction threaded fasteners are turned to loosen them. Fractured wheel studs on the left rear wheel, therefore, was not consistent with the condition of under-torqued wheel nuts.

A second observation while inspecting the bucket truck was excessive oxidation of the rear wheel hubs. While most often a result of general surface corrosion due to long-term exposure to ambient moisture, excessive oxidation to steel components can also result from excessive heating. This excessive oxidation condition, at first made off-hand, later became more significant when a paint mark was identified consistent with the spacing between the parking brake pads (Figure 5).

This paint mark width indicated that the bucket truck’s wheel hubs, which at one time had been painted, had also been stopped with overheated parking brake pads, an indication of parking brake misuse. This observation prompted removal of the wheel hub for a first-hand inspection of the parking brake pads. It was determined that the parking brake pads were worn smooth with less than 4 mm of the original 12 mm pad thickness remaining (Figures 6 and 7). This worn parking brake pad condition was inconsistent with having had new brake pads applied to the bucket truck and also inconsistent with typical use of parking brakes wherein the pads are applied against a stationary wheel hub in order to keep the bucket truck still. The wheel hub and brakes, therefore, were removed from the bucket truck and secured for laboratory examination.

It was reported that the bucket truck had been driven for nearly a month, from SC, where the brake work was performed, to TX, where it remained onsite for several weeks, and then had covered most of the ground on its way to NC before the truck wreck took place. The distance traveled since the repair was determined to be 2,971 miles. In addition, 225/70R19.5 size tires had been in service. These tires have an overall diameter, minus 5% load deflection, of 30.3 inches, or 2.5 feet, which translates to 7.9 feet in circumference. In other words, for each revolution of the tire, and load cycle on the wheel stud, the bucket truck would travel 7.9 feet.

These figures (2,971 miles and 7.9 feet per wheel revolution) correspond to nearly two million load cycles on the wheel studs. Had the wheel studs been left loose when the brakes were serviced, one bending load (flex) would have resulted on each wheel stud every time the wheel rotated. Repeated flexing results in metal fatigue, and just like when you bend a paper clip back and forth until it breaks, flexing metal wheel studs will also break, or fracture. But the fracture will only take place if the flexing exceeds certain load and cycle limits.

The cycle limit is 10 6 , or one million, rotations. In other words, if the load is such that the wheel stud sustains one million rotations, then the wheel stud is not likely to fatigue and fracture if additional wheel rotations are experienced. It bears repeating that the wheel studs in this incident fractured at two million load cycles.

This observation indicated an event took place between the time that the brakes were serviced and the rollover incident. No maintenance related to wheel removal was reported subsequent to the brake service work. It was, therefore, theorized that parking brake misuse culminated in the wheel separation incident.

To either prove or refute this parking brake misuse theory, a couple of questions were asked:

1. How much heat would it take to cause the wheel nuts to loosen?

2. Is there any evidence that the wheel hub experienced this level of heating?

Question 1 was answered through additional measurements and calculations. It was known that wheel nuts maintain secure attachment of the wheel assembly to the wheel hub by providing clamping force. In other words, wheel nuts sufficiently tight on wheel studs clamp the wheel to the wheel hub. This clamping force is provided by tightening the wheel nuts just enough to stretch the wheel studs but not so much as to induce permanent deformation. This level of tightening is referred to as preload. Because metal studs become longer when they are heated, the preload on a wheel stud can be reduced if the wheel studs become overheated after they have been tightened.

The wheel studs were removed from the hub by cutting the wheel hub flange, thereby saving the fracture surface from mechanical damage (Figure 8). Once removed, the wheel studs were determined to be 10.9 Class (alloy steel quenched and tempered), size M14 x 1.5 pitch, flat head, ribbed-necked studs. The fractured wheel stud’s original thread length of 1.85 inches was determined by researching and procurement of the required replacement part from an authorized parts supplier (Figure 9).

Using the published values of major diameter, pitch diameter, thread pitch, tensile stress area, stud length and modulus of elasticity for this wheel stud material, a 140 ft. lb. applied torque force and stretch of 20,039 lbs. and 0.0066 inches, respectively, were calculated. The calculations, therefore, revealed that the wheel stud strain (stretch) would be just short of 0.007 inch when the specified torque of 140 ft. lbs. (preload) is applied to the wheel nuts.

That stretch value applied to the thermal expansion equation for steel corresponds to a temperature of just 616°F for the 20,039 lb. of wheel stud preload to be completely diminished (no-preload temperature). So if the wheel hub becomes heat soaked such that the wheel studs reach a temperature of 616 °F, the wheel studs lose all preload and, as a result, begin to flex and can become unthreaded from the wheel studs while the bucket truck was being driven. However, the bending only takes place while the wheel studs are heat soaked. Once temperatures return to ambient, bending ceases and eventual fracture is not likely to take place.

To answer the question whether there was any evidence that the wheel studs reached this temperature, a metallurgical analysis of the incident wheel stud material was performed. The metallurgical analysis included the following activities:

1. Clean and photograph wheel stud fracture surfaces

2. Section fractured and exemplar wheel studs

3. Hardness test fractured and exemplar wheel studs

4. Chemically test fractured and exemplar wheel stud materials

5. Metallurgically mount fractured and exemplar wheel stud materials

6. Use optical and metallographic microscopy to examine fractured and exemplar wheel stud materials

The wheel stud fracture surfaces, once a majority of surface oxidation had been removed, exhibited a clearer view of its circumferential ratchet marks and, in addition, provided a view of residual beach marks (Figure 10).

Once again, these features were consistent with a reversed bending fatigue fracture mode. Hardness Testing is used to determine whether the structure, and therefore the strength of the material has changed. A lower hardness corresponds to a weakened and more ductile, or softer, material. The hardness of the incident wheel studs were, therefore, compared to that of the exemplar wheel stud material. Hardness for fastener material is measured by Rockwell C-scale units. Two incident studs, Nos. 2 and 6, and the one exemplar stud were tested in both macrohardness, with a 1 KG load, and microhardness, with a 500 gram OD to core load.

An average 5 percent macrohardness decrease, and an average 1.4 percent microhardness decrease for the incident wheel studs was identified. At this point it was argued that although the hardness decreased in both cases, the decrease was within the limits of process variations and not significant enough to indicate excessive heat exposure. To answer this argument, the chemistry and microstructure of the samples must first be examined.

Chemical testing of the wheel stud material revealed and average 0.35 percent carbon content alloy steel with principal alloying elements of Manganese (Mn), Silicon (Si) and Molybdenum (Mo). Certain combinations of alloying elements effect changes in the structure and behavior of the material, especially how it responds to applied loads. The chemical analysis was consistent with ISO898-1 2013 E CL. 10.9 material. This is an international standard that defines the properties of fasteners made of carbon and alloy steel. There were no significant differences between the chemical content of the incident and exemplar wheel stud materials.

The samples were mounted in plastic so they could be polished and examined under a metallographic microscope. This process is necessary to view how the different phases of a material are structured. Like the hardness, the different microstructures result in different mechanical properties wherein strength, wear resistance and endurance properties can vary. The exemplar and incident wheel stud microstructures are shown in Figures 11 and 12.

As expected all the samples exhibited a tempered martensite microstructure. The martensitic structure forms when medium carbon steels are put through a hardening process. Martensite is identified as a matrix of randomly oriented light and dark strips, or laths. The darker laths, which contain a substance called cementite, have higher carbon content than do the lighter laths.

When compared, the incident wheel stud microstructure had a more varied, or mottled appearance, which is typical of the initial stages of a process called spheroidization. Spheroidization takes place when the material is subjected to heat sufficient to start softening the material. An evenly distributed network of particulate spheres could also be identified in the incident samples that were absent in the exemplar samples. These spheres, which are characteristic of carbide precipitants, are accentuated with red circles in Figure 13.

William Smith, in Structure and Properties of Engineering Alloys, 2nd edition, 1993 indicates that cementite in carbon and alloy steels will spheroidize and form carbides at temperatures between 752 to 1,112°F (Smith, Page 60). As shown in the above calculations, this spheroidization temperature is well hot enough to exceed the no-preload temperature of 616°F calculated above.

Moreover, a 0.35% Carbon steel will not experience a significant decrease in hardness unless its initial hardness is in the range of 50-60 HRC. If the 0.35% C steel’s initial hardness is 35 HRC, it will decrease to 32 HRC at 900°F and 28 HRC at 1000°F. This 900°F is well within the range that cementite in the tempered martensite structure will spheroidize (Smith, Page 73).

1. The wheel hub had become heat soaked such that the wheel studs attained a temperature between 752 and 1,112°F.

2. The wheel stud temperature was sufficient to exceed the calculated 616°F temperature at which the left side wheel nut would experience a complete loss of wheel nut preload.

3. The complete loss of preload resulted in wheel nut loosening, a reversed bending fatigue load, and eventual fracture of the left side, rear wheel studs.

4. Given the oxidized condition of the wheel hub, and the brake pad wear that is not typical for parking brakes the wheel hub became heated due to parking brake misuse, wherein the parking brake remained applied while the bucket truck traveled an extended distance.

5. Given the fatigue life calculation, based on the distance traveled and the manner in which the bucket truck was used since brake servicing was completed, the parking brake had been misused well after the brakes were serviced.

Columbia District Office | Columbia, South Carolina

Mr. Maltry is an Aerospace, Mechanical, and Automotive consulting engineer in Columbia, South Carolina.

Mr. Maltry joined EDT in 2007. He offers consulting services in the following areas: fracture and failure analysis, including finite element analysis; aircraft crash and vehicle collision investigation and reconstruction; origin and cause of air and land vehicle, structure and equipment fires; machinery scope of damages; assessment of appliances, machinery, vehicles and equipment (including HVAC); evaluation and analysis of process equipment, plumbing and piping; industrial accident analysis; and lightning damage assessment.

Mr. Maltry is a registered professional engineer in Alabama, Florida, Georgia, Mississippi, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas and Virginia.