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FORENSIC ENGINEERING TESTING OF A CHAIN THAT FAILED

By Edward S. George, PE [NAFE 621M]

Abstract

An attempt to extricate a tractor stuck in a sandy field using a steel tow chain in combination with a nylon strap was unsuccessful when the steel tow chain failed. The broken chain recoiled back hitting the plaintiff in the face causing extensive injuries. The plaintiff sued the distributor of the chain for damages. The Forensic Engineer assessed the specific allegations that included failure to properly design, manufacture, assemble, test, inspect, label and package subject chain, to ensure that it would not fail in ordinary foreseeable use. The Forensic Engineer reviewed Factors of Safety and performed load tests on both the subject chain, an exemplar chain and an exemplar nylon strap. The Forensic Engineer's investigation resulted in an informed conclusion as to the cause of this failure.

Keywords

Utility chain, webbing sling, dimple rupture, Scanning Electron Microscopy, Factor of Safety (safety factor), Forensic Engineer

Background

The plaintiff in this case was attempting to extricate his 18,000 lb tractor which was stuck in sugar sand. A friend was called upon to use another tractor to pull the tractor that was stuck. They attached a new 3/8ths" diameter Grade 43 steel utility chain, purchased from an industrial equipment dealer to both tractors by wrapping it around the bucket of each tractor. Labels on two sides of the box containing the chain showed that the chain was rated at 5400 lbs. The plaintiff was in the stuck tractor while his friend used the other tractor to pull. After many unsuccessful attempts, his friend applied the brakes of the pulling tractor leaving the chain under tension and got off the tractor to get a drink of water. The plaintiff was moving the bucket of the stuck tractor up and down hoping to move it. The chain broke, recoiled back through the window of the stuck tractor hitting the plaintiff in the face causing extensive injuries. The plaintiff sued the distributer of the chain for damages. The distributer hired a law firm who, in turn, hired the Forensic Engineer, Metallurgical Engineer, to evaluate the chain failed.

Edward S. George, PE, 124 Calle de Leon, St. Augustine, FL 32086

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Testing

The first step by the Forensic Engineer, Metallurgical Engineer, was to purchase an identical chain to use as an exemplar for laboratory testing. The utility chain was rated at 5400 lbs and with a safety factor of three should have been able to withstand over 15,000 lbs. of load without breaking. The exemplar chain was subjected to four pull tests at a certified lab in accordance with ASTM A413/A413M-00 Standard Specification for Carbon Steel Chain. The exemplar chain was pulled to failure every time. It failed at over 17,000 lbs of load in direct tension. It was noted that the fractured ends of the broken link of the subject chain were "offset" while the broken ends of the exemplar chain that was pulled in direct tension were not "offset". Because the subject chain was wrapped around the bucket, the Forensic Engineer performs another pull test on the exemplar chain with several of the links twisted . This time failure occurred at over 13,000 lbs (4,000lbs lower than when pulled in direct tension) and the broken link failed with the two fracture ends offset similar to the subject link. At a later date, the Forensic Engineer performed pull tests on the subject chain in both direct tension and in the twisted configuration with similar results (see Figure #8)

At this point in the investigation, the plaintiff stated that there must have been a defect in the subject chain link because the nylon strap was only rated at 2000 lbs. and was inserted between the tractors and the chain. The strap was supposedly in place as a safety precaution. The thinking was that the strap would fail before the chain.

An exemplar strap was purchased which met the plaintiffs description. The strap was described as a 2" x 6.6' webbing sling Pull tests were performed on the exemplar strap. The strap failed at slightly over 14,000 lbs. of load in direct tension, lower than the utility chain in direct tension (17,000lbs) but slightly higher than the twisted chain (13,000lbs). The subject strap was not recovered, therefore it was not possible to perform pull tests on the subject strap or to observe it's condition i.e. any permanent deformation or the condition of the failed ends. Figures #1 through #7 contain the testing procedures. Figure #8 is a comparison chart for the actual failure loads

The Forensic Engineer performed Scanning Electron Microscopy (SEM) on the pull tested subject chain in the twisted mode, non twisted mode and the actual failed link. SEM allowed the viewer to see the topography of the failure surface at low and high magnifications. This was useful to determine the mode of fracture. All three fracture surfaces appeared to have failed by overload in a ductile manner as evidenced by dimple rupture as seen at 1000X magnification. Figures #9 through #11.

The plaintiff's Forensic Engineer and experts also claimed that there were anomalies on the links of the subject chain in the form of marks or notches which weekend the subject chain. Upon the Forensic Engineer's visual examination of the exemplar chain, notches were noted on all links, in all chains inspected and were concluded to be machine marks produced during the

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manufacturing process. The Forensic Engineer also noted that neither the subject chain nor the exemplar chain failed in the vicinity of these notches.

Conclusion

Based on the Forensic Engineer's education, experience and the results of this investigation, the Forensic Engineer, (Metallurgist) opined there were no metallurgical anomalies in the subject chain which contributed to the failure. Both the exemplar and the subject 3/8" diameter Grade 43 utility chain exceeded the safety specification requirement of 16,200 lbs as prescribed by the applicable ASTM standard during the pull tests. Factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), is a term describing the structural capacity of a system beyond the expected loads or actual loads. Essentially, it is how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.

Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation. The chain packaging was clearly marked 5400 lb limit. Most people are not aware of the safety factor, therefore would not attempt to pull something weighing over 18,000 lbs. with a chain clearly marked with a 5400 lb. limit. Due to the cost of litigation, this case was settled at a minimal figure as a direct result of the Forensic Engineer testing and analysis.

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Figure #1: Machine used in pull tests.

Figure #2: Section of exemplar chain being tested in direct tension.

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Figure #3: Exemplar chain pulled to failure in direct tension.

Figure #4: Broken link of exemplar chain pulled in direct tension.

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Figure #5: Exemplar chain pull tested with twisted link.

Figure #6: Failed link in kinked pull test. Note twisted and offset deformity.

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Figure #7: Failed subject link. Note similar twist and offset deformity as in above photograph.

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Figure #8

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Figure #9: SEM of subject chain pull tested to 13,000lbs, 1000x.

Figure #10: SEM of subject chain pull tested to 17,000lbs, not twisted , 1000x.

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Figure #11: SEM of failed link, subject chain, 1000x.