Residual stress measurements in an SiC continuous fiber reinforced Ti matrix composite

Abstract

During the fabrication of ceramic fiber reinforced metal matrix composites mismatch stresses will be introduced due to differences in thermal expansion coefficients between the matrix and the fibers. Calculations, based on a coaxial cylinder model, [1 and 2] predict that, for a Ti matrix SiC continuous fiber composite, the axial stress in the matrix parallel to the fibers and hoop stress around the fibers are tensile while the radial stress perpendicular to the fiber surface is compressive. Saigal et al. [3] used neutron diffraction to measure residual thermal stresses in an SiC continuous fiber Ti-14Al-21Nb matrix composite. In that paper the two principal matrix stresses in the plane of the cross-section of the fibers are supposed to be approximately equal. X-ray and neutron diffraction measured values for the residual stress parallel to the fibers in the matrix of SiC-Ti alloy matrix composites are reported by James et al. [4]. It was found that for composites with more than 6 plies the axial surface stress measured by X-ray diffraction matches that determined by neutron diffraction and therefore represents the axial stress in the bulk of the matrix. Stresses in the plane of the cross section of the fibers were not measured. The present investigation reports about residual stress measurements on the matrix of a continuous SiC fiber-Ti 1100 matrix composite sheet with a thickness of 1.25 mm, using a high resolution neutron diffraction facility and X-ray diffraction. In this investigation we start from a triaxial stress state in the bulk of the matrix, with three different principal components, as measured with neutron diffraction. The neutron diffraction results are compared with those of the X-ray diffraction measurements with which, because of the low penetration depth of the X-rays, a biaxial stress state at the surface is measured. Also investigated was whether any relation could be observed between the predicted stress distribution around the fibers [1 and 2] and the diffraction line broadening measured with neutron diffraction.