LUMINESCENCE SENSING OF STRESS IN TI AL2O3 FIBER-REINFORCED COMPOSITES/

The high temperature processing of fibrous metal matrix composites has been predicted to result in the development of large residual stresse s because of a difference between the coefficients of thermal expansio n (CTE) of the fibers and matrix. The R1 and R2 optical luminescence l ine shifts of single crystal alpha-Al2O3 fibers embedded in a titanium matrix have been measured using a fiber optic method and related to t he reinforcing fiber's principal stresses. The fiber's axial stress co mponent has been found to be more than twice that predicted by a conce ntric cylinder's (CTE difference) model, whilst the fiber's radial (an d hoop) direction stresses are smaller than expected. We propose that the stress in these composites is affected by two presently unmodeled effects: an elevation of composite's residual stress due to the large CTE difference between the tooling used for processing of the composit e and a relaxation of the transverse stress components by radial matri x cracking. The result has important consequences for the manufacture of metal matrix composites and for efforts to predict their mechanical properties.