Evidence of second order stresses of plastic origin in Zircaloy-4 sheets

Complementary methods have been used to analyse residual stresses in Zircaloy-4 (zy4) thin sheet heat treated and which had undergone uniaxial plastic deformation: Electronic Speckle Pattern Interferometry (ESPI) and X-Rays Diffraction (XRD). ESPI is a non-contact optical technique that enables measurements of full-field small displacements. Measurements obtained from the surface progressively removed by chemical etching allow analysis of the macroscopic residual stress distribution. By this method, distortions, observed in classical macroscopic methods and due to mechanical removing, are avoided. ESPI results show small macroscopic residual stress values (lower than 10 MPa). This is consistent with the method used to deform sheets. Residual stress analysis can also be performed by XRD. Macroscopic, mesoscopic and microscopic strains are obtained from peak position (I-st and IInd order stress) and line breadth (IInd and IIInd order stress) measurements. In this work, measurements have been performed with K alpha copper radiation in rolling and transverse directions. XRD analysis in transverse direction shows compression stress levels which are significantly different for (20.5) and (10.6) planes respectively. Both results have shown that first order stresses are negligible and that the stresses calculated from XRD peak positions are IInd order ones. The obtained stress level are too strong to be IInd order stress of thermal origin. So the measured strains were probably generated by an anisotropic plastic deformation of crystallites.