Fatigue crack propagation in pseudoelastic TiNi smart microcomponents

Abstract

Mechanisms of fatigue crack propagation in pseudoelastic TiNi thin strips are similar in air, physiological 0.9 NaCl, and aerated Hank's solution, when tested at 40 degreesC and a loading frequency of 15 Hz. The threshold values DeltaK(th), the Paris-Erdogan exponents, and the maximum applied DeltaK at instability prior to failure were found to be similar for all three environments. In a good correlation with finite element analysis, B19'martensite forms at the crack tip during cycling with applied external loads significantly lower than the critical stress required for stress-induced phase transformation. Formation of martensite may be eliminated due to work hardening at the crack tip, which contrasts with former assumptions. Cracks typically propagate by bridging the crack tip with microvoids formed in front of the advancing crack at locations with maximum stress intensity. TiNi thin strips exhibited a lower fatigue crack growth resistance when compared to data previously published for bulky TiNi.