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

Transient 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, ΔK, during both low-high and high-low block-loading sequences, transients were observed in the subsequent cyclic crack-growth rates, but only under specific conditions. When K min was suddenly varied with K max held constant, no transients were observed. However, when the load changes involved both ΔK and K max, 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 of ∼2 different from the steady-state (baseline) growth rates for load excursions of ∼30–40%, 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.