Extension of Slitting Method for Simultaneous Measurement of Residual Normal and Shear Stresses in Polymer Composites

The thermal residual stresses generated during curing process of laminated polymer composites can negatively affect their mechanical strength and performance. Premature failure, delamination, out-of-plane deformation and matrix cracking are negative effects of residual stresses. In order to predict the residual stresses effects on the performance of composite components, it is important to develop techniques to measure those stresses. The slitting method is one of the most conventional methods of residual stress measurement. In this method, a slit is created incrementally through the thickness of the stressed part and the released strains in each incremental depth are then measured by a strain gauge. A major assumption of
this method is that the measured strains during the slitting experiment are only caused by the residual stress component perpendicular to the slit face. However, slitting process will also release two shear stress components in the slit plane, which may influence the values of the measured strains. This paper investigates the effect of the released in-plane and out-of-plane residual shear stresses in the slitting process on the measured strains for a carbon/epoxy laminate as well as a steel specimen. The results
of the finite element simulations show that both components of the residual shear stress may significantly affect the measured strains during the slitting experiment. For simultaneous determination of residual normal and shear stresses, a new method using two strain gauges attached in both sides of the slit is presented. Finally, the validity of the simulation results was demonstrated by carrying out the slitting experiments on two carbon/epoxy composite specimens.