چكيده به لاتين
One of the most important issues in the design of structures is high strength versus low weight. The sandwich panels can be mentioned as one of these kind of structures. The high mechanical properties of sandwich panels, particularly high flexural stiffness against their low weight cause wide application of these structures in various industries. Therefore understanding of the mechanical behavior, failure mechanisms and ultimate strength of sandwich panels is important to have efficient design. Based on the studies performed by a lots of researchers, the type of loading, constituent materials and geometrical dimensions have the most effects on the strength of these structures. Moreover manufacturing processes and damage caused by unintentional impacts severely decrease ultimate strength of sandwich structures. According to above mentioned points, the main objective of this study is investigating the mechanical behavior, strength and failure mechanisms of composite sandwich beams containing initial defects as core-skin separation and sound specimens under four-point bending load. Experiments were conducted for cross ply and angle ply configurations for intact specimens, while for the specimens with defect only angle ply face sheets were tested. The effects of defect position on the failure mechanisms were inspected by locating the core-skin defect in various positions. Maximum external load and observed failure mechanisms from the experiments were predicted using finite element simulation of constituent material properties. The results of the intact specimens revealed that the layup configuration does not affect maximum failure load, because core indentation is the dominant failure mechanism. Also, layup configuration has less influence on the flexural behavior by decreasing the beam length. By observing the results of the specimens with defects it could be concluded, specimens with defect positioned in compression area of the beam are more critical than those positioned in shear span because of suddenly delamination of core-skin interface. In the numerical simulations failure mechanisms are predicted simultaneously for composite skin using modified Hashin criteria and indentation of foam core by considering crushable foam model. Moreover, extended finite element and cohesive zone model were employed to simulate crack propagation in the foam core and in the interface of core and skin respectively. Nonlinear behavior of the constituent materials were modeled explicitly by defining the slope of stress-strain relation in each increment. Material nonlinearities and modified Hashin criteria for face sheets were conducted via USDFLD subroutine in ABAQUS commercial software. Numerical predictions have good correlation with the experiments and model is capable to predict the load-displacement curve and the failure mechanisms accurately.