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A miniature 4-kN servohydraulic three-point bend load frame coupled to a scanning electron microscope (SEM) was developed to allow direct observation of fatigue and fracture processes in ceramic materials at magnifications up to X20 000. Two series of fatigue crack growth experiments were conducted on Norton/TRW NT-154 silicon nitride, one using the in-situ three-point bend system and the other using compact tension specimens in a conventional test system. The objectives of the work were to ascertain whether crack growth under cyclic loading is a manifestation of a load-level dependent mechanism or a true cyclic effect, and to identify mechanisms of fatigue crack propagation at a microstructural level. Tests were conducted at room temperature and load ratios of 0.1 to 0.4, both in air and vacuum. Results of both series showed a marked load ratio effect and a distinct cyclic loading effect. Crack propagation was highly discontinuous, occurring on individual cycles at a rate approaching that for fracture and arresting between these growth increments for hundreds or thousands of loading cycles. Between growth increments there were no detectable changes at the crack tip; however, crack wake features such as bridges and interlocking grains decayed and lost their ability to transfer load.
ceramics, silicon nitride, fatigue (materials), scanning electron microscopy, crack propagation, residual stress, advanced materials
Senior mechanical engineer, Sarcos Research Corporation, Salt Lake City, UT
Professor of Mechanical Engineering, Quality and Integrity Design Engineering Center, The University of Utah, Salt Lake City, UT