Fatigue of Metals

Cyclic fatigue is the root cause of many catastrophic failures in engineering systems, with notable examples in aircraft, artificial heart valves, prosthetic devices, electronics packages, railways, bridges, offshore platforms, and conventional and nuclear power plants. The weakening of the metallic materials caused by cyclic loading ultimately results in fracture at stresses that are often substantially lower than that necessary to cause fracture under monotonic loading.  Such failures often occur after millions or even billions of cycles, complicating the ability to predict when failure will occur.

Design of safety-critical components for survival beyond a critical number of cycles requires knowledge of the fatigue strength of the material for the required number of cycles. To measure fatigue strength, samples are typically cycled between a minimum and maximum stress to failure in a servohydraulic testing machine at a frequency near 1 Hz. At this frequency, approximately 278h are required to apply a million cycles or 278,000h (about 32 years) for a billion cycles. With the recent development of ultrasonic fatigue testing approaches, fatigue testing can be performed at 20kHz, allowing cycling to a billion cycles in approximately 14h. This accelerated approach to testing, used in the present study, enables rapid fatigue characterization of a broader set of materials at very high cycles and enables more tests of a given material, which is important for capturing the variability in fatigue behavior.