Z-PINNING TECHNIQUES AND MODELING IN COMPOSITE LAMINATES AND X-COR SANDWICH STRUCTURES

Abstract

Z-pinning is a technique where composite laminates are typically reinforced with metal or composite fiber pins inserted vertically into the laminate. The strength and fracture toughness of a Z-pinned laminate is directly dependent on the mechanical interlocking of the pin and the laminate. In the present work, novel approaches to the current Z-pinning technology are investigated to increase mechanical interlocking of the pins. Towards this end, we study pin insertion at an angle to the vertical unlike the traditional vertical pin insertion. In addition, a novel variety of pin, namely the threaded pin, is studied as a candidate for reinforcement. Using threaded pins for reinforcement increases mechanical interlocking between the pin and the laminate as well as the epoxy-pin contact area thus delaying delamination. When smooth metal pins are used for reinforcement, anchoring their ends on to the surface of the laminate before curing delays delamination through pin gripping.. Experiments performed show increase in pullout strengths and fracture toughness when angled, threaded or anchored pins were used. This research also looks at developing a computational-analytical model to represent the behavior of Z-pin reinforced X-Cor composite sandwich panels under out-of-plane compression and shear loading. Parameters important in representing the behavior of the individual components of the sandwich are identified. The softening of Z-pins under compression from geometric and material imperfections, densification of the foam and pin-facesheet interface strengths are incorporated into the model. For validation, the values of the parameters are obtained from experiments performed at UMD, and then for comparison, they are used to estimate the stiffness and strength of the specimens with experimentally obtained results reported in an open literature. Good correlation using these parameters across different specimens has implications on development of a predictive methodology for the behavior of Z-pin reinforced sandwich materials under compression and shear. Cohesive Zone Modeling (CZM) is a numerical technique used to model composite delamination in conjunction with FE models, A material damping based approach is proposed to encounter the typical convergence issues faced by CZM and is implemented on FE models to analyze composite delamination and Z-pin pullout.