Stress measurement in graphite/epoxy uniaxial composites by X‐rays

Abstract

Abstract X‐rays can be used to measure residual as well as applied stresses in polymers and reinforced polymeric composites by diffracting from filler particles that are embedded before curing. We have investigated various fillers that exhibit suitable stress‐induced shifts of diffraction angle when embedded in uniaxial graphite fiber/epoxy composites. Stresses in the particles determined by X‐rays are proportional to the corresponding composite stresses, in agreement with the model of H. T. Hahn. Results indicate that the stress sensitivity (change in X‐ray strain per MPa applied to the composite) increases in the order W, CdO, Ni, Ag, Nb, Al. The elastic range terminates at a yield point beyond which the stress sensitivity is markedly reduced for metallic fillers, but no such yield point has appeared with CdO particles. Oppositely directed particle strains are seen after unloading from stress levels that have exceeded the yield point. Theoretical predictions for these findings are discussed. Residual stresses in the particles were determined after curing and storage under ambient conditions. In the fiber direction the residual stresses were always found to be positive and larger than the principal stresses orthogonal to these.