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 0 To understandwhy a part or equipment failed, how it failed must be first diagnosed. Different failure modes such as fatigue, brittle, and corrosion all have very different causes and very different solutions. If an investigation were to guess at the failure mode and guess wrongly, the planned remediation may not address the true cause of failure and additional failures may occur. The only way to diagnose how a part failed is by examining the broken part. The various failure modes form different features which in turn can be helpful for diagnosis. Some damage mechanisms such as complex corrosion cracking require lab-based analysis. Yet for mechanical failures such as ductile, brittle, and fatigue, a lot can be learned just by visual examination of the part. It is possible for reliability workers, mechanical engineers, and inspectors dealing with failures to learn how to diagnose mechanical fractures. To demonstrate the importance of identifying how something failed, consider three failed chain links. All three are from automotive assembly plants’ conveyor chain systems. Their failures represented lost production costs and created safety issues for people assembling the cars. Although chain links are a simple part, they can fail in a variety of manners and for very different reasons. Identifying the failure modes was crucial to understanding why each chain link had failed and to develop effective solutions. With minimal training, the failure modes of all three failures would have been identifiable to engineers and metallurgists had they been properly trained in fracture recognition. Or phrased differently, had someone not studied the links’ fractures, what are the odds that anyone would have guessed how to prevent repeat failures for all three? Remember, if even one guess were made wrong, people might have gotten hurt. CASE STUDY 1 The first chain link was from a line that suffered over a dozen failures in two years. Although each failure had been taken seriously, these historical investigations were conducted without study of the links themselves. Examination of the most recent link’s fracture surface confirmed that the failure mode was fatigue (Fig. 1). With this diagnosis as to how the link failed, the next question is naturally, why did the link fail by fatigue? Fatigue failure only occurs when the repetitive loading exceeds the fatigue limit/durability of the steel. Once the link was shown to be of sound quality, the only way to cause fatigue failure of a perfectly good link was from excessive, repetitive loading. Therefore, chain link failure was from cyclic loading higher than it could sustain. The remainder of the investigation could then be focused on considering the source of elevated loading (excessive line tension, small radius turns, and so on). Also, considering the entire chain had experienced the excessive loading, it was likely that numerous other links had already formed cracks. Therefore, the chain needed to be replaced after the source of elevated loading was removed/reduced. After a dozen failures, had the latest link been simply replaced, the chain STUDYING FRACTURES: RECOGNIZING AND UNDERSTANDING FAILURE MODES Examining a failed part’s fracture surface is a great source of information as to why and how it failed. Shane Turcott* Steel Image, Dundas, Ontario *Member of ASM International CASE STUDIES Fig. 1 — Failed chain link (a) shows signs of fracture (b) due to fatigue. Fatigue fracture features (a) (b)
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