Letter to the Editor
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Letter to the Editor Michael B. Connelly
Published online: 20 June 2007 ASM International 2007
Dear Editor, In reviewing the August 2006 edition of Journal of Failure Analysis and Prevention, I became excited reading the article on ‘‘Metallurgical Failure Analysis of Fasteners in an Impeller Assembly.’’ Since I am employed by a fastener distributor and a NIST accredited fastener test laboratory, I always look forward to articles that help me improve my knowledge base. I applaud the author’s efforts in trying to establish a root cause of the failure when several pieces of critical information were missing. I would like to add some observations that may help the author zero in on a root cause. Figure 4 is a micrograph of a proprietary thread form and is designed as an all metal reusable locking thread form. It is usually applied to internally threaded fasteners. The angled ramp area adjacent to the fracture is designed to embed itself into the tip of the mating fastener. This design changes the loading dynamics by loading the crest of each mating thread instead of the usual load distribution encountered in fasteners. It is a very successful design that has seen service in the space shuttle, orthopedics implants, exhaust manifolds, etc. The pitting that the author encounters in the Type 1 fasteners is most likely in the spots where the mating thread engaged the ramp angle. It does not surprise me that each thread shows cracks, especially if the bolt was overloaded. If the load distribution is roughly equal on all of the threads and the screws were overloaded I would expect to see cracks at the root of each thread.
Figure 5 reveals that the Type 2 bolts appear to have their threads rolled and not cut. The clean looking thread profile and the uniform thread radius indicate rolled threads, as does the tensile strength. Does the author know if strain hardened wire was used? An etched micrograph would clear up the confusion. In Fig. 5, if you count the threads from the left, the fourth thread (highest crack) shows signs of failing by a shearing action. The fifth thread also shows the same deformation. You will see that type of deformation in a threaded cross section that has been tensile tested—especially if you don’t have enough thread engagement. Bolts are not supposed to fail in the head—they fail in the first thread engaged in the internal threads. I did notice in the micrographs that the fasteners may be fully threaded to the head. If that is true, are the head failures listed in the article true head failures? Did they
M. B. Connelly (&) Casey Products, 11230 Katherine’s Crossing, Woodridge, IL 60517, USA e-mails: [email protected]; [email protected]
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encounter any lubrication on the fasteners, or were they plated? We keep running into this problem from time to time. The torque coefficient changes after the bolt is lubricated, and even with a calibrated torque wrench the assembler winds up over loading the bolt. If the manu-
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