An Approach for MMC Component Design

Abstract

Titanium based metal matrix composites (MMCs) with continuous fibers first gained prominence as enabling structural materials for the National Aerospace Plane (NASP). Some of the peculiar deformation and damage characteristics of MMCs were identified, explained, and modeled as part of the NASP program activities in the early 1990s. Much was discovered and learned regarding the behavior of MMCs under a variety of thermal and mechanical load combinations. Analytical and numerical models of deformation and damage were developed and validated. Significant progress was achieved in proper viscoplastic characterization of the matrix materials and micromechanics based deformation and damage modeling of MMCs. These research activities, which actually outlasted the NASP program, resulted in major advances in understanding, characterization, and modeling of MMCs. However, virtually all-modeling activity remained focused on unidirectional loading of MMCs. To some extent, virtually all aircraft propulsion system and structural components are subjected to multiaxial stresses. Therefore, while the understanding and the modeling techniques developed under the NASP program represented a major step forward, additional steps were needed to reach a level where models could be used in the design and life prediction of actual components. Overlapping NASP related activities, efforts were underway by the major jet engine manufacturers under the Integrated High Performance Turbine Engine Technology (IHPTET) program to design and test propulsion system components involving the use of MMCs. These activities were mainly focused on fabricating and testing metallic rotors with unidirectional MMC inserts. The inserts were toroids of various cross-sections, with fibers along the circumferential direction. Initial sub-component and component level testing of rings and rotors indicated that the existing design and stress analysis methods were inadequate in predicting the deformation and damage behavior of MMCs under biaxial (hoop and radial) stresses. Instigated by IHPTET needs, a project was initiated with the following objectives: (a) develop a theory for predicting MMC damage and deformation response under multiaxial stress states caused by general time dependent thermomechanical loading, (b) validate the theory by comparing predictions with laboratory test measurements, and (c) incorporate the theory in a stress analysis procedure that can be used by design engineers. The project was focused on unidirectional titanium based matrix composites with silicon carbide fibers. The paper will consist of a summary of progress that has been achieved in the above project. Specifically, outline of a theory for predicting deformation and damage of MMCs under multiaxial stress states will be presented. The theory includes consideration of dominant damage mechanisms associated with titanium matrix composites, processing induced residual stresses, and time, temperature, and strain rate dependent inelastic deformation. This will be followed by a description of how the theory is implemented in a nonlinear finite element analysis procedure. Finally, examples of comparison between theoretical predictions and experimental measurements will be presented.