The static indentation behavior of composite sandwich panels with thin quasi-isotropic skins

The quasi-static normal indentation of sandwich panels with quasi-isotropic laminated composite skins and honeycomb or foam cores, by spherical indentors, has been investigated using experiments and finite element analysis. The experimental program emphasized the effects of indentor size on the resulting load indentation responses, failure mechanisms in the skin and the core, and the measurement of contact areas between indentor and the target. The sandwich panels were indented up to the initiation of skin fracture and the resulting contact data was used to characterize contact power laws. A non-linear finite element model was developed based on the experimental observations to systematically explore the indentation behavior of diverse sandwich configurations and to investigate the contact pressure distributions. The finite element model was used as an experimental tool in the development of a simple non-dimensional semi-empirical model that was based on the Graeco-Latin-square factorial plan.; The indentation experiments revealed the strong dependence of the indentation response and the failure mechanism on the indentor size and the core type. A characteristic network of contact induced delaminations was observed to form in honeycomb core sandwich panels. A decreasing stiffness type loading response was observed for smaller indentors, while the opposite was observed for larger indentors, the transition indentor size being governed by the relative stiffnesses of the skin and the core. The parameter characterizing the non-linearity in the unloading behavior was observed to be a function of the point from which unloading occurred, contrary to previous observations.; The contact pressure distributions obtained from the finite element analysis indicated the presence of a saddle type distribution at smaller indentations while approaching a uniformly distributed pressure as the indentations increased. The peak contact pressure was observed to translate from the edge of the contact zone towards the center as the indentations increased. An increase in compressive stiffness of the core beyond a certain limit was found to decrease the indentation loads for a given crush stress.; The load-indentation response predictions of the semi-empirical non-dimensional model compared satisfactorily with the finite element and experimental data for similar sandwich configurations. A conceptual guideline for developing semi-empirical models for the contact radius and pressures has also been presented. Additional scope exists for the improvisation of the empirical model for increased accuracy and range of variables in the future.