INFLUENCE OF VISCOPLASTICITY ON THE RESIDUAL-STRESS AND STRENGTH OF ATITANIUM MATRIX COMPOSITE AFTER THERMOMECHANICAL FATIGUE

Modifications to the Bodner-Partom (B-P) constitutive equations provid e improved flexibility in fitting a larger strain-rate range than prev iously available. Using the modified B-P equations, the importance of good matrix characterization becomes apparent for a SCS-6/Timetal(R)21 S composites under thermomechanical and isothermal fatigue, as well as sustained loading. An interesting observation is that the residual st ress after thermomechanical fatigue (TIMF) can be significantly differ ent than that found in the as-received composite. This phenomenon is c ause by prestraining the fiber at high loads and temperatures during T MF. This phenomenon also provides an excellent opportunity to compare the predictive capabilities between the older and newer B-P models. Wh en the maximum and minimum strains during TMF are obtained with the tw o models and compared to experimental measurements, the difference bet ween the two models is minimal. However, inspection of the shifts in r esidual stress from the as-received condition shows significant differ ences between the older and newer B-P formulation. Lastly, pre-straine d fibers caused by thermomechanical loading can cause a loss of room t emperature tensile strength. To show this a rudimentary fiber failure model is defined. This model is used to show that the loading type str ongly influences residual strength. The loss of residual strength is s ignificantly more sensitive to loading type, rather than the viscoplas tic characterization. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.