Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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Materials Characterization Primer FATIGUE TESTING William Mankins Metallurgical Services Inc. Fatigue testing is used to predict the service life of components subjected to cyclic loading as a function of applied stresses and cycles to failure and to determine the fatigue-crack propagation rate of a cyclic-loaded component after a crack or flaw has been identified or produced in testing. It is also used to evaluate actual machine components that undergo cyclic loading service and measure the crack-growth characteristics after initial cracking. Limitations • Creating a fatigue test specimen to duplicate actual practice is a challenge. • Metallurgical variables in fatigue samples make it difficult to duplicate testing results. • Metallurgical processing history (heat chemistries, mill processing, or heat treatments) cause variability in fatigue test results. • Material variations and processing variables make it difficult to achieve statistically significant reproducible fatigue data. Applications include the determination of metallurgical effects (processing, heat treatments, and defects) on the fatigue life of components, determination of fatigue life of components resulting from design errors, determination of the effects of welding/joining processes on the fatigue life of components, and to simulate failure conditions for failure analysis. Samples Form: Solids (metals and alloys, polymers, composites, glasses, and ceramics). Size and shape: Machined test specimens per requirements of standard procedures; actual machine components and large fully operational equipment (e.g., aircraft). Preparation: Specimens can be machined without a defect to determine time/cycles to initiate a crack or cause failure. Specimens can also be precracked with the crack growth time to create a critical or fatal flaw being measured. Estimated Analysis Time • Preparation time for machined samples is estimated to be a few hours (six to eight hours maximum). • Testing time is a function of the applied stress and number of cycles; to determine an endurance limit of 106+ cycles (or orders of magnitude greater) requires an extended period of time. • State of the art electromechanical and servohydraulic fatigue testing systems are capable of high cyclic loading rates and can shorten test times to a few hours. Related techniques Corrosion fatigue and thermal fatigue testing procedures, which are realistic problems encountered in actual applications and exacerbate the problem of obtaining accurate fatigue data. 32 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013

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