E 3 AEROSPACE ENGINEERING RESEARCH SHAPE MEMORY ALLOYS (SMA S ) ¡ E3 Teacher Summer Research Program Aerospace Engineering Texas A & M University By Moses Z. Horton Ali A. Jafry

Introduction to Shape Memory Alloys. Shape Memory Alloys (SMAs) are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. The SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite. In addition, the martensite can be in one of two forms: twinned and detwinned, as shown in Figure. A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs. The key effects of SMAs associated with the phase transformation are pseudoelasticity and shape memory effect.

Schematic of a stress-strain-temperature curve showing the shape memory effect.

Temperature-induced phase transformation of an SMA without mechanical loading.

Thermally-Induced Transformation with Applied Mechanical Load

Temperature-induced phase transformation with applied load.

SMA Stress Strain Temperature Phase Diagram.

Pseudoelasticity The pseudo elastic behavior of SMAs is associated with recovery of the transformation strain upon unloading. The super elastic behavior is observed during loading and unloading above A 0 S and is associated with stress- induced martensite and reversal to austenite upon unloading.

Pseudo elastic stress-strain diagram.

Schematic of a thermo mechanical loading path demonstrating pseudo elastic behavior of SMAs.