Transient Fatigue-crack Growth Behavior following Variable-amplitude Loading in a Monolithic Silicon Nitride Ceramic
نویسنده
چکیده
ÐTransient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si3N4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclic fatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, DK, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under speci®c conditions. When Kmin was suddenly varied with Kmax held constant, no transients were observed. However, when the load changes involved both DK and Kmax, subtle transients were detected in the form of retardations following high-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of02 dierent from the steady-state (baseline) growth rates for load excursions of 030±40%, and lasted for several hundred microns of crack extension before a steady-state growth rate was achieved. Such behavior is rationalized in terms of grain-bridging mechanisms active in the crack wake. # 1998 Elsevier Science Ltd. All rights reserved KeywordsÐsilicon nitride, fatigue-crack growth, transient crack growth, overloads.
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