X-Ray Stress Measurement in Graphite/Epoxy Composites

Small amounts (< 2 mg/cm2) of Al and Nb filler powders were incorporated between the first and second plies of 6-ply unidirectional graphite/epoxy laminates. Cu Ka1 X-rays were diffracted from specific crystallographic planes in these fillers: 333 + 511 for Al and 411 + 330 for Nb, giving peaks in the back reflection region. The peak positions shifted linearly with stress applied to the laminates in the fiber direction and had stress sensitivities of 8.52 and 3.92 x lo-4 deg 26/MPa for Al and Nb respectively. Elastic strains in the filler particles measured by X-rays were found to be proportional to the corresponding composite strains measured by strain gages, in agreement with the model of H. T. Hahn. Residual strains and stresses in filler particles were also measured.