THE CONSEQUENCES OF BOLT FAILURES

A bolt is a threaded fastener utilized throughout industry to secure two or more mechanical parts together. There are a wide variety of threaded fasteners available including hex head bolts, carriage bolts, machine screws, studs, tapping screws, socket head bolts and plow bolts. A representative hex head bolt, shown in Figure 1, is characterized by a threaded portion, an unthreaded shank and a hexagonly shaped cap or head. Figure 2 shows the cap stamped with the name of the manufacturer and a symbol indicating it to be a grade 5 bolt with a minimum tensile strength of 120000 psi. Bolts may be subjected to thousands of pounds of tensile force as well as alternating forces at a range of frequencies. When a threaded fastener cannot sustain the expected loading and becomes detached, a significant loss may occur. Although rare, bolt failure has caused wheel detachment on vehicles, structural failure in buildings, and crashes of aircraft. This article presents several examples of bolt failures that have been involved in serious losses. The claims professional is usually involved early in the analysis of the accident and is influential in the early decisions as to the failure analysis of the threaded fastener involved. Through a review of these case studies, the claims professional can gain insight into how to handle future assignments involving bolt induced losses.

FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4

FAILURE AS A RESULT OF AN OVERLOAD

Many accidents can be characterized as an impact with a non-compliant object such as a truck impacting a concrete bridge support. In these cases, bolt failure due to overload can occur. Figure 3 is a view of a bolt that fractured in the threaded area. The 45 degree full-slant fracture surface indicates high tensile loads. The fine, gray appearance of the fracture surface is consistent with a sudden overload failure. In this case, other bolts on the mechanical part had failed, transferring the load to the remaining bolt shown in Figure 3, resulting in an overload. Figure 4 is a view of the fracture surface of a steering gear output shaft of a large truck. The truck was involved in an accident and a question arose as to the role of the steering unit as a possible cause. Microscopic examination of the fracture shows a full-slant fracture surface (about 45 degrees) and evidence of a shear-face tensile fracture, characteristic of an overload. It was concluded that the fracture of the output shaft was most likely a result of the accident and not a cause. Figure 5 is a view of a similar fracture surface at the threaded end of a wheel spindle with it’s characteristic 45 degree fracture surface and fine gray appearance. This is a typical overload during a vehicle rollover accident. Figure 6 shows a typical threaded tie rod end on a vehicle steering system. The severe distortion of the bolt prior to failure suggests that an external force from the impact deformed the tie rod end, causing a failure.

FAILURE FROM LACK OF LOCKING MECHANISM

In order to prevent bolts from loosening over time, various locking mechanisms are employed. They include lock washers, locking nuts, jam nuts, mechanical deformations, wire wrap, cotter pins, metal locks, expansion anchors, helical coils and polymer locking compounds. Machinery that is subject to vibratory environments usually is equipped with some sort of locking mechanism. If the locking mechanism is not applied to the machinery during manufacture, a catastrophic event may result. Figure 7 is a view of a hoist transmission used in a large crane. The bolt shown in Figure 8 was found out of position after the crane transmission “jumped out of gear” dropping a heavy load. In Figure 7, the arrow points to the location of the bolts. Specifications called for a polymer locking compound to be applied to the bolt threads to prevent backing out. No compound was found on the bolt threads or in the threaded hole. Consequently, over a period of time, the bolts loosened, resulting in the loss of control to the shift fork in the transmission.

METAL FATIGUE

Metal fatigue is the phenomenon characterized by progressive crack growth during cyclic loading. A crack is often initiated at a flaw or stress riser (sharp notch) in a part. Cyclic forces such as vibrations or repeated impact cause the crack to increase in size until the part can no longer sustain the load, and a final fracture occurs. Figure 9 is a view of a classical reverse bending fatigue fracture of a bolt. The arrows point to the initiation sites of the fatigue crack. The small lines or striations on the metal surface show the advance of the crack from the exterior to the inside of the bolt. The rutted gray area in the middle of the bolt is the area of final fracture where the bolt cross-section was reduced and the bolt could not carry the load. See Insurance Adjuster Magazine, March 1983, regarding fatigue related failures. Metal fatigue can be a result of a design deficiency as well as improper assembly of the part.

FIGURE 5 FIGURE 6 FIGURE 7 FIGURE 8

FAILURE FROM IMPROPER TORQUE

When threaded fasteners are utilized, the amount of tightening or bolt torque is often important. Motor vehicle wheel studs require torques ranging from about 100 ft-lbs for smaller vehicles to over 400 ft-lbs for large trucks. The appropriate torque is required in order to prevent relative flexing of the two parts being fastened and to assure an acceptable mechanical connection. Bolt failures as a result of improper torque have occurred in automobile applications (See Claims Magazine, December 1995). Figure 10 shows a view of a failed wheel stud compared to a new one. This bolt failed as a result of insufficient torque. Figure 11 shows a part of the stud that was bearing on the wheel rim causing severe wear of the thread, another indicator of insufficient bolt torque.

FIGURE 9 FIGURE 10 FIGURE 11 FIGURE 12

FAILURE FROM IMPROPER DESIGN

Figure 12 is a view of the front ski suspension system for a snowmobile. The operator of the snowmobile was badly injured when the sled suddenly veered to one side, throwing him into a tree. As shown in Figure 12, the bolt failed in the threaded section at a shear point in the bracket. It is generally considered poor design to allow significant alternating shear or bending forces in the vicinity of the threaded section of the bolt since the threads form a stress riser and tend to initiate fatigue cracks, as happened in this case. A better design would be to utilize a bolt with a shorter threaded section so that the unthreaded shank material is at the shear area of the bracket. This eliminates the stress riser from the threaded section and increases the effective bolt diameter. Figure 13 is a view of a rock truck that sustained a left front wheel mount failure. Figure 14 shows the initiation of the failure mode where an over-stressed bolt failed, fell out of position, thereby transferring higher loads to the remaining bolts. Eventually the remaining bolts failed, causing detachment of the wheel mount and an accident. Figure 15 shows wear on the bolt threads a result of bolt movement due to insufficient clamping force between flanges.

FIGURE 13 FIGURE 14 FIGURE 15 FIGURE 16

FAILURE FROM IMPROPER MANUFACTURE

Figure 16 is a view of a failed tie rod end bolt, a critical steering system component in an automobile. The vehicle suddenly pulled to the right after traveling over a bump in the road. The fracture surface revealed an area of progressive fracture that had been occurring over time. This was initiated by a heat treating related defect in the outer surface of the tapered shank. The crack grew by metal fatigue and finally failed when traveling over a modest road surface perturbation. Figure 17 is a view of the right rear control arm of a midsize automobile that rolled over while traveling on an interstate highway. The driver suddenly experienced extreme difficulty in steering the vehicle and lost control. In Figure 17, it is apparent that the control arm bolt is out of position and, in fact, fractured near the threaded end. With little evidence of an extreme force application at the right rear suspension, it appeared unusual that a bolt would fracture from an overload in such a manner. The bolt was removed and tested. The exterior surface hardness was found to vary considerably along the bolt length, resulting in a stress discontinuity at the fracture surface. The nonuniformity of hardness occured from improper heat treatment of the bolt during manufacture.

FIGURE 17 FIGURE 18 FIGURE 19 FIGURE 20

CORROSION FAILURE

Corrosion of metals can be disastrous to threaded fasteners (see Claims Magazine, September, 1993). Surface and pitting corrosion attacks threaded fasteners as a result of contact with moisture or other corroding media. Since bolts often carry high loads, stress corrosion cracking (also called environmentally assisted cracking) is another corrosion related failure mode. Corrosion, coupled with forces in a bolt, tends to accelerate cracking. Figure 18 shows a damaged dump trailer after a rollover accident. Figure 19 is a view of a suspension related clamp on the dump trailer. The clamp failed, causing the axle to part from the vehicle and an accident. Views of the fracture surfaces show progressive environmentally assisted cracking as a cause of the bolt failure. Such failures are normally discovered during periodic inspections, a typical maintenance procedure on large vehicles.

FIGURE 21

EVIDENCE HANDLING

As can be seen by the previous examples, the fracture surface plays a significant role in the analysis of threaded fastener failures. Consequently, the appropriate handling of the failed bolt as evidence encompasses the protection and preservation of the fracture surface. A light oil coating on the fracture surface helps reduce corrosion, provided the surface films on the fracture surface are not significant. If surface films must be preserved, the sealing in a dry, air tight container is helpful. Removal of the bolt from the vehicle or piece of machinery requires care. Figure 20 shows the result of improper removal. The person removing this bolt used a hammer and screw driver which damaged the fracture surface. In Figure 21, a torch cut has badly damaged the fracture surface. In some cases, the bolt cannot be dislodged. Then the whole part should be removed and possibly be cut away at a later date.

CLOSING

When threaded fastener failure appears to be a cause of a loss, a few fundamental investigative measures are in order. First, thoroughly photograph the parts involved, preferably in their undisturbed state. Save the mechanical system, ie. the automobile, piece of machinery or device for possible future analysis. If parts must be removed, avoid damage to the fracture surfaces of failed parts. Avoid hammering or torching the parts as depicted in Figures 20 and 21. Save the parts in an environment that intends to inhibit the onset of corrosion and reduces the chances of additional deformation from handling. Obtain as much history as possible on the usage and maintenance of the mechanical system. Finally, place interested parties on notice as to testing and disassembly to avoid the pitfalls of spoliating the evidence (see Claims Magazine, June 1992).

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