Mittuniversitetet

This paper examines the compressive failure mechanism in edge-to-edge loaded corrugated sandwich panels. The formation of face wrinkles is specifically considered. A detailed finite element model of face sheets and web core of a sandwich panel was developed to provide insight on the failure mechanism. A gradient enhanced continuum damage theory was implemented to capture length effects caused by the material microstructure including formation of damage in the face sheets and core. Distributions of strains in the face sheets determined from finite element analysis (FEA) are compared to experimentally measured strains. The predicted location and orientation of the face wrinkle, as indicated by high values of the second principal strain, agrees well with experimental observations.Load vs. out-of-plane deflection curves obtained from FEA with the gradient enhanced damage material model are compared to those obtained from a linear-elastic material model and experimentally determined curves. The gradient enhanced solution gives qualitatively better agreement with experimental results, although the magnitudes of strains are less than those determined experimentally.

Detailed structural nonlinear finite element modeling of a sandwich panel with corrugated core is performed in this study. A simply supported panel is loaded in uniaxial compression well into the regimes of global panel buckling and local face sheet buckling. The highly nonlinear load versus in-plane and out-of-plane displacement responses obtained from finite element analysis agree reasonably well with experimental results, but the model slightly overpredicts the maximum load. The difference between experiments and predictions is attributed to damage of the corrugated paper web introduced during manufacture of the core and corrugated board. Computations of the buckling also results in a slight thickness reduction of the panel for a large range of face and web thicknesses identify lower thickness limits when the web loses its ability to contribute to the compressive strength of the panel. The highly nonlinear response associated with local and global buckling also results in thickness reduction of the panel.

This study examines thickness changes in web-core sandwich panels under edgewise compressive loading. Both undamaged and damaged panels were examined. Three-dimensional full-field digital image correlation systems were used to determine deflections on both sides of loaded panels. The change in thickness at any given point in the panel was obtained as the difference between the two displacement fields. It was observed that the thickness was reduced in the post-buckling regime. Damage introduced into the corrugated core by lateral compression proved to significantly reduce the load-carrying capability panels and elevate the thickness reduction of the panels.

The microstructural effect on the mechanical behavior of idealized two-dimensional random fiber networks subjected to in-plane compression is studied. A finite element model utilizing nonlinear beam elements assuming a linearly elastic material is developed. On a macroscopic level, random fiber networks often display an asymmetric material behavior when loaded in tension and compression. In mechanical models, this nonlinearity is traditionally described using continuum elastic-inelastic and/or damage models even though using a continuum approach risks overlooking microstructural effects. It is found that even though a linear elastic material model is used for the individual fibers, the network gives a nonlinear response in compression. The nonlinearity is found to be caused by buckling of individual fibers. This reversible nonlinear mechanism is limited in tensile loading and hence offers an alternative explanation to the global asymmetry of random fiber networks.