Probabilistic Estimation of Degradation of Functions and Strength for Ti-41.7Ni-8.5Cu(at%)Shape Memory Alloys under Thermo-Mechanical Cyclic Conditions
Maho HOSOGI Nagatoshi OKABE Toshio SAKUMA and Shuichi Miyazaki
Abstract:Machines and systems applied shape memory alloys require the optimum design fused functions and structure strength. And so the reliability for functional and structural design is required to consider the scatters as well as the degradation behaviors of functional and mechanical properties of the alloys during practical service uses. In this paper, under considering the application of Ti-41.7Ni-8.5Cu(at%) to a reciprocating type heat engine, the increasing behaviors of irrecoverable strain with increasing of the number of thermo-mechanical cycles were clarified experimentally and the irrecoverable strain was used as an estimate parameter for predicting the behaviors of functional and structural strength properties. Thus, the formula-models concerning the behaviors of irrecoverable strain were made so as to estimate the functional properties such as transformation points and recovery stress from an irrecoverable strain. The scatters of irrecoverable strains in their behaviors were analyzed against the ratio of experimental data to the value estimated from model formula and were clarified to show Weibull distribution. The probabilistic estimation expression with inverse function of cumulative probability P was proposed for predicting probabilistically recovery energy and dispersion one by using an irrecoverable strain ƒÃir as an estimation parameter. The proposed model formula for degradation of functions and fatigue failure strength for Ti-41.7%Ni-8.5%Cu shape memory alloys under thermo-mechanical cyclic conditions can be available for the optimum design fused functions and structure strength against machines and systems such as a reciprocating type heat engine applied shape memory alloys. Key Words:irrecoverable strain, formula model, transformation points, reliability designs