A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 2 3 1 would have continued to suffer more failures. If the loading on the chain had been reduced yet the chain not replaced, links already cracked would have caused more failures. If the chain had been replaced without reducing the loading, the new chain would have eventually suffered failures, too. CASE STUDY 2 In addition to studying fracture features to identify the failure mode, it is critical to locate and examine where the cracking initiated. The initiation site sometimes provides critical information relevant to how and why a part failed. In the case of this second chain link failure (Fig. 2), the fracture features were also fatigue, indicating the repetitive loading had exceeded the strength of the steel. Yet looking at the initiation region, cracking had started from a forging flaw. This pre-existing, cracklike feature had increased the local stresses, which then caused fatigue cracking. Ultimately, this link failed because of a quality issue with the link from original manufacture. Understanding how the part failed from study of its fracture often includes additional information relevant to why it failed. If the broken link had not been studied, it was unlikely that anyone would have guessed its failure was due to a forging quality issue. CASE STUDY 3 The fracture features of the third chain link were distinctly different because it had failed by a different mechanism. Its fracture features were consistent with brittle failure, later found to be caused by embrittlement (Fig. 3). Due to a quality issue with the steel, it did not have the ability to sustain bending loading without snapping. Although the diagnosis as possible brittle fracture is well within the skill limits that reliability/mechanical engineers should develop, to understand why something had been brittle requires metal testing. Yet upon recognizing the fracture features, at least the investigator would know what steps would be required to ensure that the case was solved. For this link, its embrittlement was determined to be from tempered martensite embrittlement, which is caused by tramp alloying and heat treatment, both associated with part quality. CONCLUSION Components as simple as chain links can fail by numerous failure modes, each caused by different factors. One should never guess why something failed because, if they did, they would sometimes guess wrong. To understand why a part failed, one must first accurately identify how it failed. That can only be done by examining the failed part itself. For anyone involved with investigating or preventing equipment failures, learning to recognize the common mechanical failure modes is extremely helpful to equipment reliability efforts. Note This article is based on Decoding Mechanical Failures: The Definitive Guide to Interpreting Fractures by Shane Turcott, and is available in the ASM bookstore, https://bit.ly/3zRC1Ks. This book introduces fractography and how to decode the fracture features of mechanical failures. It demonstrates how to visually diagnose and interpret ductile, brittle and fatigue failures using numerous examples. It then explains how each diagnosis is used to direct the investigation towards the root cause of failure. There are two additional chapters on advanced fatigue of rotating shafts and static fastener failures. ~AM&P For more information: Shane Turcott, Steel Image, 7 Innovation Dr., Suite 155, Hamilton, ON L7H 7H9, Canada, 905.745.6429, shane@steelimage.ca. Fig. 2 — Looking at the failed chain link (a) revealed that cracking started from a forging flaw (b). Fig. 3 — Failure of the third chain link (a) was consistent with brittle failure (b), later found to be caused by embrittlement. Fatigue fracture features Forging flaw (a) (b) (a) (b)
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