Stiffness changes in thermodynamic Nitinol with increasing temperature.

The purpose of this study was to measure and describe the physical properties of a recently-developed thermodynamic Nitinol alloy, type A-138, at intervals of temperature between room and body temperatures. The properties of stiffness, flexure yield strength and permanent deformation were measured, demonstrating that the working range of Nitinol A-138 is directly related to increases in temperature between 75 degrees F and 100 degrees F. As stiffness of the .017'' round A-138 Nitinol wire increases, the yield point decreases. In the loading cycle, yield points dropped from 36 degrees angular deflection at 75 degrees F to 25 degrees at 100 degrees F. In the unloading cycle, the yield point at 75 degrees F was 41 degrees of deflection, with force dropping to zero at 35 degrees; at 100 degrees F, yield point was only 12 degrees and force did not drop to zero until unloaded to 3 degrees angular deflection.