Thermomechanical fatigue of shape memory alloys

Abstract

As shape memory alloys (SMAs) gain popularity as high energy density actuators, onecharacteristic that becomes particularly important is the thermomechanical transformationfatigue life, in addition to maximum transformation strain and stability of actuation cycles.In this paper, a novel test frame design and testing protocol are discussed, forinvestigating the thermally activated transformation fatigue characteristics of SMAsunder various applied loads for both complete and partial phase transformation. ANi50Ti40Cu10 (at.%) SMA was chosen for this investigation and the effects of various heat treatments onthe transformation temperatures and the transformation fatigue lives of actuators werestudied. For selected heat treatments, the evolution of recoverable and irrecoverable strainsup to failure under different applied stress levels was studied in detail. The influence ofcomplete and partial transformation on the fatigue life is also presented. The irrecoverablestrain accumulation as a function of the number of cycles to failure for different stress levelsis presented by a relationship similar to the Manson–Coffin law for both partial andcomplete transformations.