Evaluation of thermomechanical damage in silicon carbide/titanium composites

Composite specimens of Ti-15V-3Al-3Cr-3Sn matrix reinforced with continuous SCS-6 silicon carbide fibers were tested under a variety of thermal and mechanical loadings. A combined experimental/finite element approach was used to estimate the effective in situ modulus of the matrix material and to evaluate changes in modulus due to the applied loads. Several fiber orientations were tested. The results indicate that the effect of thermal loads on composite stiffness varies with fiber orientation. Applications of this method to test specimens damaged by uniaxial tension, thermal cycling, and isothermal fatigue loadings are used to illustrate that, by monitoring overall structural behavior, changes in stiffness caused by thermomechanjcal loading can be detected. HE mechanical behavior of composite materials depends on the properties of the fiber and matrix constituents. Because of inhomogeneiti es due to the addition of the fibers and due to thermal loadings that occur during the fabrication process, the in situ matrix properties of the SiC/Ti-15-3 com- posite can vary significantly from those of the monolithic ma- trix.1 Matrix stiffness properties, as well as the integrity of the fiber/matrix bond, will also change as the composite material deforms under applied mechanical and thermal loads. In this paper an indirect measurement of the effective in situ matrix modulus is obtained using the results of experimental tests together with finite element analysis of the composite test specimens. The term "effective matrix modulus" is used to account for changes in the measured stiffness of the compos- ite. These changes could be due to a variety of damage modes. In this article, however, all damage will be incorporated into and accounted for by changes in the matrix modulus. The resonant frequencies of the individual specimens are measured by vibration testing. Based on the measured frequencies, the effective modulus of the test specimens is determined by finite element analysis. This indirect measurement technique is ap- plied to each individual test specimen in its original, as-fabri- cated condition, as well as after a heat treatment, and thermal or mechanical lo^ds are applied. The results therefore show how the effective matrix modulus changes with the different applied loads.