Wave propagation in stressed composites

The performance of high-temperature composites can be significantly affected by the presence of residual stresses. These stresses arise due to the mismatch of thermal expansion coefficients between matrix and fibers during cooling from fabrication temperature to room temperature. This effect is especially pronounced in metal matrix and intermetallic composites. It can lead to plastic deformation, matrix cracking and fiber/matrix interface debonding. Elastic wave propagation in homogeneously stressed media has been frequently addressed in the literature. However, the effect of nonhomogeneous stress distribution has not been investigated. This is especially important since the average residual stresses are zero in equilibrium and thus their distribution is inherently nonhomogeneous. In this paper, the feasibility of using ultrasonic techniques for residual stress assessment in composites is addressed. A theoretical method is presented for determining the velocities of ultrasonic waves propagating through a composite material with a nonhomogeneous distribution of residual stresses. It is based on the generalized self-consistent multiple scattering model. Calculated results for longitudinal and shear ultrasonic wave velocities propagating perpendicular to the fiber direction in SCS-6/Ti composite with and without residual stresses are presented. They show that velocity changes due to the presence of stresses are on the order of 1%.