Transient fatigue-crack growth behavior following variable-amplitude loading in a monolithic silicon nitride ceramic

摘要

Transient cyclic fatigue-crack growth behavior following variable-amplitude loading sequences has been investigated in a hot-pressed, monolithic Si{sub}3N{sub}4 ceramic (NTK EC-141), for which fracture toughness (resistance-curve) and cyclicfatigue properties are well characterized. Following rapid changes in the applied stress-intensity range, △K, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only underspecific conditions. When K{sub}min was suddenly varied with K{sub}max held constant, no transients were observed. However. when the load changes involved both△K and K{sub}max, subtle transients were detected in the form of retardations followinghigh-low, and accelerations following low-high block-loading sequences. The transient growth rates were typically a factor of ~ 2 different from the steady-state (baseline) growth rates for load excursions of ~ 30-40%. and lasted for several hundredmicrons 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.