Raman Study of Hi‐Nicalon‐Fiber‐Reinforced Celsian Composites: II, Residual Stress in Fibers

Band shifts in Raman spectra were used to assess, on a microscopic scale, the residual strain existing in Hi‐Nicalon silicon carbide fibers reinforcing celsian‐matrix composites. Uncoated as well as p‐BN/SiC‐coated and p‐B(Si)N/SiC‐coated Hi‐Nicalon fibers were used as the reinforcements. We unambiguously conclude that the fibers were in a state of compressive residual stress. Quantitative determination of the residual stress was made possible by taking into account the heating induced by laser probing and by using a reference line, of fixed wavenumber. We found fiber compressive residual stress values between 110 and 960 MPa, depending on the fiber/matrix coating in the composite. A stress relaxation‐like phenomenon was observed at the surface of p‐BN/SiC‐coated Hi‐Nicalon fibers whereas the uncoated or p‐B(Si)N/SiC‐coated Hi‐Nicalon fibers did not show any stress relaxation in the celsian‐matrix composites.