Experimental Determination of Crack Growth Rate during Thermal Cycling on NiTi Shape Memory Alloys

There is an increasing demand to compact and lightweight actuators in aerospace and automotive industries. The solid state actuators are promising candidates for replacing some existing actuators or making it possible to put actuators where it was impossible or impractical with current commercially available methods. Shape memory alloys (SMAs) have the highest work output for a specific volume among all active materials. They can create high forces and large strokes. In addition they can be used as multifunctional components thanks to their high strength. Which is beneficial because that would reduce complexity, weight and volume.

It is important to characterize the proposed materials and designs against fatigue and fracture. The proposed actuators will operate by cycling temperature, thus to get representative fatigue life or crack growth behavior thermal cycling must be considered. In this work binary Ni₅₀Ti₅₀ has been tested for thermal actuation crack growth. Analogues to Paris-Erdogan law for mechanical cycling dependence of applied stress intensity factor or force on crack growth rates are investigated. This type of analysis may make it possible to incorporate damage tolerant design against fatigue for SMA actuators. Preliminary results suggest that crack growth under thermal cycling is significantly faster than that of with mechanical cycling. In addition to crack growth rates, strain field are determined with digital image correlation (DIC) and finite element analysis (FEA) has been conducted to better understand characteristics of crack growth.