Compressive response of notched composite-honeycomb sandwich panels

Abstract

Experimental and numerical work was conducted to understand better the compressive response of notched composite sandwich panels. The quasi-static uniaxial compressive response of notched (circular through hole) E-glass/epoxy- NomexTM sandwich panels were studied experimentally. Two different woven fabric architectures were examined. The key failure mechanism was observed to be linear damage zones (LDZs) emanating from the notch tip (in both materials). LDZ's behaved in a macroscopically similar manner to a bridged crack under tensile loading, and were characterized by semi-stable propagation. Crosssectioning studies revealed the key damage mechanisms operating within the LDZ. Progressive cross-sections indicated that individual fiber microbuckling led to out-of-plane warp tow kinking. The LDZ wake was characterized by kinking in all warp tows and transverse tow splitting. Strain gages were used to measure the in situ damage zone tractions as the LDZ propagated across the width of the specimen; a softening trend was observed. Consistent with observations, a two parameter linear strain softening traction law was used to model the LDZ constitutive behavior. The traction law was treated as a material property. The damage zone modeling (DZM) framework was investigated to determine its validity, specifically its ability to predict three experimentally observed phenomena: the notched strength, local strain distribution, and LDZ growth characteristics. A self-consistent physically-based model should be able to predict all three phenomena. Two models were created in order to interrogate the DZM. The damage growth model was used to determine the ability of the DZM to predict the LDZ growth behavior and notched strength. A finite element model that used discrete nonlinear springs in the wake of the LDZ to model the LDZ as a continuous spring, was implemented to determine if the DZM could predict the local strain distribution. Results showed that the current traction law provided excellent agreement with the phenomenon used to calibrate the traction law, for all specimen sizes. Extension of predictive power to other phenomena resulted in weaker correlations. The modeling framework and methodology established provide a robust tool for investigating the potential of adding physical bases to the DZM.