Failure Analysis of Bolting/Fastener Issues
Metallurgical Consulting has examined a number of bolting and fastener issues. Proper evaluation of these issues requires an understanding of torque or preload and how it affects service life in addition to metallurgical issues. There is considerable misunderstanding of torque and proper installation among users and applicators of fastening systems. This has resulted in the improper application of fasteners, which is a common problem. Inadequate torque or preload accounts for numerous fatigue failures of fasteners and especially automotive wheel stud failures. Proper preloading of bolts with soft washers or gaskets can produce loss of preload (relaxation) which ultimately causes fastener failure.
Figures 1 and 2 show a highly preloaded stainless steel stud bolt in a brine environment which failed by Stress Corrosion Cracking (SCC). Gasket creep over time reduced the preload or clamp up. This loss of clamp up allowed brine to reach the studs and produce the SCC.
Over torqueing is another cause of failure, however much more rare. One case was examined in which slew ring bolts on an offshore crane were twisting off when tightening with a calibrated system to the specified torque. The torque specified was for dry threads; however Loctite had been applied to the threads prior to torqueing. Calculations showed that lubricity from the Loctite increased bolt stresses to failure levels at the specified torque levels.
Several cases of hydrogen embrittlement have also been evaluated in which bolt heads failed hours to days after installation. In some cases workers have had to wear hard hats to protect against flying bolt heads. Figures 3 and 4 show a large electroplated self-tapping screw. The image shows an SEM intergranular fracture surface characteristic of hydrogen embrittlement. Improperly applied electrolytic zinc plate on the screws likely contributed to the screw failures.
Other similar failures have involved bad heat treatment, excessive hardness, and excessive inclusion amounts in the steel. Design and service conditions frequently play a role in failure of threaded systems.
Figures 5 and 6 show the fatigue failure of an automotive tie rod end; failure in this case was attributed to fatigue which occurred at the first cut thread on the rod. Fatigue occurred in stages suggesting changes in operating conditions throughout the life of the rod.
Images From The Metallurgical Services Provided For This Failure Analysis
(click on thumbnail to enlarge)
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