ADVANCED MATERIALS & PROCESSES | JANUARY 2026 26 writing and readers are encouraged to continue their study of these terms and their usage. BASIC FAILURE MECHANISM DEFINITIONS There are several terms related to failure mechanisms that are frequently referred to in a failure investigation. Corrosion. Corrosion is the degradation of a component or system due to environmental effects. Erosion/Wear. Erosion and wear failures of a component are caused by degradation of the surface from inter- action of particles, other components, or a medium. Fatigue. Failure of a component due to cyclic loading that results in crack formation that progresses through the component until final rupture and fracture. Overload. Excessive load/stress applied to a component beyond the component’s strength or design limits. BASIC FAILURE MODE DEFINITIONS When discussing failure modes, familiarization with its related terminology is helpful. Fatigue Fracture. Fatigue fracture is the result of cyclic loading and unloading of a material or component resulting in the initiation and propagation of a fracture through the material or component. The loading causing a fatigue failure initiation and propagation need not be greater than the strength of the material. A fatigue fracture is generally characterized by a relative smooth fracture surface extending from the point(s) of origin to some percentage of the entire cross- section of the component or material then exhibiting a different failure mode for the final rupture. The percentage of fatigue fracture to the area of final rupture is a general indication of loading on the component. There are many subgroups of fatigue failures as determined by type of loading and crack initiation. Some examples of these subcategories include low stress high cycle fatigue, high stress low cycle fatigue, multiple origin fatigue, singular origin fatigue, bending fatigue single direction, bending fatigue dual direction, rotating corrosion, thermal fatigue, erosion failure, yielding failure, compressive failure, and creep failure. As with failure mechanisms, failure modes can have multiple subcategories. While continuing this discussion of failure mechanisms and failure modes it becomes apparent that three of the four failure mechanisms listed are also identified as failure modes. Those three being fatigue, corrosion, and erosion/wear. This can also be a source of confusion when attempting to under- stand and describe a failure event. Even though the terms erosion, wear, and corrosion are primarily nouns, they also can be used to describe the “how” (mechanism) as well as the “what” (mode) of a failure event. The distinction between the two failure phases of fatigue has been previously defined. Futher, the distinction between the two failure phases of corrosion and erosion/ wear failures is illustrated through the usage of the terms as follows. To Identify a Failure Mechanism. 1)The corrosion of the component was due to the caustic operating environment; 2) erosion on the identification of the pipe caused the system failure; and 3) the bearings in the unit failed due to abrasive wear. To Identify a Failure Mode. 1) The component was found to have failed due to corrosion; 2) the pump impeller failed due to erosion of the vanes; and 3) the excessive wear on the shaft was caused by poor lubrication. It is vitally important that the distinction between a failure mechanism and failure mode is completely understood. Only by having a clear understanding of the two phases of a failure can the results of the investigation be properly reported. Relating a failure mode to a failure mechanism is the very essence of the failure investigation and the objective for determining the source (TRC) of the failure. The following brief descriptions of each of the failure mechanisms and modes are presented to provide a cursory understanding of each. A complete discussion of failure mechanisms and modes is beyond the scope of this FATIGUE FRACTURE Visual Examination: The secondary sun gear, shown here, failed due to tooth bending fatigue that initiated at the root of the gear teeth. Note also the high level of wear and polish on the non-fractured teeth indicating high operating loads. This fractograph shows a fatigue fracture in a gear. Courtesy of Auburn Gear LLC Metallurgical Lab Report MLR 7282.
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