Probabilistic Estimation of the Degradation of Function and Strength of Ti-41.7Ni-8.5Cu (at%) Shape Memory Alloys under Thermo-Mechanical Cyclic Conditions

Abstract

Machines and systems that use shape memory alloys require the optimum design for combining functional properties and structural strength. In the reliability study of functional and structural design, the scatter as well as the behaviors of the functional and mechanical properties of the alloys during practical use should be considered. In this study, considering the application of Ti-41.7 Ni-8.5Cu (at%) to a reciprocating-type heat engine, the increase of the irrecoverable strain with increasing number of thermo-mechanical cycles was clarified experimentally and the irrecoverable strain was used as a parameter for predicting the behaviors of functional and structural properties. Thus, models of the behaviors of irrecoverable strain were made to estimate functional properties such as transformation points and recovery stress as a function of irrecoverable strain. To analyze the scatter of irrecoverable strain, the ratio of experimental data to the value estimated from the model was considered and found to show the Weibull distribution. The probabilistic expression estimated as an inverse function of cumulative probability P was proposed to predict probabilistically the recovery energy and the dispersion energy by using irrecoverable strain sir, as an estimation parameter. The proposed model for the behavior of the functional degradation and fatigue strength properties of Ti-41.7% Ni-8.5% Cu shape memory alloys under thermo-mechanical cyclic conditions can be used for the optimum design that combines the functional properties and structural strength of machines and systems such as a reciprocating-type heat engine that uses shape memory alloys.