MONOTONIC AND THERMOMECHANICAL TESTING OF P/M NiTi

Abstract
P/M NiTi for the test program was manufactured by the Special Metals powder metallurgy process and converted to wire by hot swaging and wire drawing. The mechanical test program was undertaken in order to investigate the performance of NiTi both for scientific reasons and for better understanding of material performance as it specifically pertains to shape memory effect (SME) devices. Two types of mechanical tests, monotonic deformation/recovery and thermomechanical cyclic deformation, were used to characterize performance. As anticipated, monotonic mechanical properties were equivalent to cast/wrought properties. Individual NiTi wire specimens were deformed in an incremental series up to 30 percent total engineering strain. These prestrained specimens followed three experimental thermal cycles which monitored deflection, load, and energy during the shape recovery-heating cycle. The aforementioned three techniques were used to provide a more complete understanding of the shape recovery response. Each measurement technique demonstrated a maximum in the recovery resulting from an 8 to 12 percent total input strain. Monotonic results indicate that the maximum recovery strain is a measure of SME capacity to provide a single output of deflection (or force), which are potentially useful data for SME devices performing monotonically. Thermomechanical testing was conducted using a servo-hydraulic test machine in conjunction with a controlled temperature environmental chamber in an attempt to simulate a cyclic SME device. After deformation to a predetermined strain, stroke position was held constant as the specimen was thermally cycled above A_f and the resulting load was monitored (strain control). This test mode resulted in permanent damage and loss of SME with each thermomechanical cycle, manifested as a decrease in load and available work delivered by the specimen. In load control, the specimen was deformed to a predetermined strain, the load reduced, and maintained at approximately zero during the thermal cycle above A_f. This load control technique proved to be much less damaging to the specimen. Load control testing was clearly the superior technique for evaluating performance of cyclic SME devices.