Theoretical and experimental investigation of strain gradients for transversal shear stress determination in GRP sandwich panels

Abstract

This report describes a theoretical and experimental investigation for transversal shear stress determination in the core of a sandwich panel. The outlined method is based on strain gradient measurements on the surface of the faces. The method is applicable both to isotropic and anisotropic sandwich plates. Experiments have been carried out on a fully clamped, rectangular plate with a concentrated load in centre. Strain gauges have been cemented at areas where one expects the highest shear stresses to occur, ie two points located near the clamping. Two tests have been carried out, one with load 5000 N and one with load 10000 N. Gradients have been measured in both x-, y- and 45 degree direction, but it is shown that it is only necessary to measure the steepest gradient to obtain acceptable values. This, together with exploitation of symmetry, simplifies the instrumentation of the plate. Measured strains and calculated shear stresses are compared to corresponding values from a finite element model of the same plate problem. The values match quite well, but it is difficult to measure the steepest gradients. The analytical stress values are in general somewhat higher than the simulated values, but this can be caused by weaknesses in the finite element model. For future measurements optical Bragg gratings will be used and not strain gauges as in this report. This as an attempt to measure the steepest gradients more accurate. Optical Bragg gratings are also the foundation of the existing CHESS project at FFI