In-Plane Shear Characterization of Sandwich Laminates Using a Picture-Frame Test Configuration

Abstract

An investigation was conducted to characterize the accuracy of “picture-frame†test configurations for measuring the in-plane shear strength of sandwich constructions as may be limited by local instability of the face sheets including wrinkling and dimpling. Two configurations were studied: a conventional configuration featuring corner pins which extend between the two sides of the specimen and fixture, and a modified configuration in which the corner pivot axes coincide with the corners of the square gage area bounded by edge doublers. Shear strains were predicted using finite element analysis and measured experimentally with strain gages and digital image correlation. The conventional configuration produced shear strain at the specimen center approximately 15% below the nominal value predicted for uniform shear stress, and the strain was significantly non-uniform over the gage area. The modified configuration provided a much more uniform shear strain distribution, at a strain level corresponding closely to the nominal value. Ultimate strength measurements are provided for four sandwich designs featuring both plain foam core and web reinforced core, tested in both configurations. Practical details of the hardware implementation for the modified configuration are presented.