Interfacial properties of sapphire/epoxy composites: Comparison of fluorescence spectroscopy and fiber push-in techniques

Experiments and analyses leading to the assessment of interfacial sliding friction and residual stresses in sapphire filament reinforced epoxy matrix composite by two different techniques are described in this paper. The first technique was based on measurements of stresses in filaments bridging a steady-state matrix crack using a laser microprobe to stimulate fluorescence from the sapphire filaments. The measured stress variation was consistent with un-bonded, frictionally constrained filaments with a constant friction stress along the sliding zone length. The second technique involved measurements of load–displacement records in filament push-in tests using a mechanical microprobe. The sliding friction stress and the residual stress in the filaments assessed by the two techniques were essentially identical. Analyses of the experimental results with different theoretical models indicated that the assessed values of the sliding friction stress and the residual stresses were not affected significantly by accounting for the mismatch in the Poisson’s ratios of the filaments and the matrix and taking into account the elastic anisotropy of the filaments.