MICROCOMPRESSION HIGH CYCLE FATIGUE TESTS UP TO 10 MILLION CYCLES

摘要

Nanomechanical tests are moving beyond hardness and modulus to encompass host of different mechanical properties like strain rate sensitivity, stress relaxation, creep, and fracture toughness by taking advantage of focused ion beam milled geometries. Adding high cycle fatigue to this list will be useful to extend the gamut of properties studied at the microanoscale. However, this presents inherent challenges like low oscillation frequencies, long duration of tests and large thermal drift when attempted with standard indenters. This presentation will report, for the first time, the development of micropillar compression-compression high cycle fatigue tests going up to 10 million cycles. This has been made possible by the development of a novel piezo-based nanoindentation technique that allows accessing extremely high strain rates (>10~4 s~(-1)) and high oscillation frequencies (up to 10 kHz). The associated instrumentation and technique development, design of the fatigue tests at the micron scale, data analysis methodology, experimental protocol and challenges will be discussed. Validation data on single crystal silicon, a reference material, will be presented to demonstrate the reliability of the designed high cycle fatigue tests. Finally, case studies of compression-compression high cycle micropillar fatigue on nanostructured materials will be presented and their results will be discussed in light of existing literature data, particularly the operative deformation mechanism(s). The convolution of time dependent plasticity in such tests will also be addressed. It is hoped that this study will pave way for routine high cycle fatigue tests of metals at the micron scale and provide clues for designing a similar indentation fatigue test.