The delamination resistance of Z-pinned laminates is directly dependent on the strength of the pin–laminate bonding at the interface. In this paper, we investigate novel approaches to the Z-pinning technology in order to increase delamination strength via enhancing mechanical interlocking of the pins. Toward this end, we study the effect of pin insertion at an angle to the vertical in contrast to the conventional vertical pin insertion. Subsequently, a novel variety of pin, namely the threaded pin, is studied as a candidate for reinforcement which increases mechanical interlocking between the pin and the laminate as well as the epoxy-pin contact area, thus delaying delamination. In addition, the effect of anchoring reveals the length of smooth metal pins on to the surface of the laminate before curing on delamination strength is investigated. Experiments performed show increase in tensile pullout strengths when angled, threaded, or anchored pins are used. These experimental results for tensile pullout strengths validate nonlinear finite element models incorporating cohesive zones at the pin–laminate interface. In addition, fracture toughness and delamination resistance under mode-I loading are investigated by performing experiments on double cantilever beam specimens. Results demonstrate the superior delamination resistance properties for angled, threaded, and anchored pin inserts.
This paper attempts 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 in house, 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.
Pin reinforced foam cores with composite facesheets offer a viable means of structural construction owing to their low weight and high stiffness and strength. In this paper, various models are introduced to estimate the stiffness and strengths of such a sandwich structure, called K-Cor™, under out-of-plane compressive loading conditions. ince the reinforcing pins have a high modulus compared to the modulus of the facesheet in the transverse direction, the pins indent into the facesheets affecting the overall compressive stiffness of the sandwich structures. Models incorporating this interaction are constructed and their compressive properties are compared with experimental results for various sandwich specimens. The effect of pin locations and size of the sandwich on its compressive properties is also investigated.
Nomenclature= Cross sectional area of facesheet = Cell spacing = Young's modulus of pin = Core stiffness = Modulus of facesheet = Stiffness of the sandwich structure = Core stiffness of infinite panel = Spring constant of model 2 = Spring constant of model 3 = Core thickness = Facesheet thickness = Thickness of the sandwich structure = Pin spring constant = Spring constant of core = Spring constant of facesheet = Spring constant of the sandwich structure = Interaction spring constant of a spring attached to a single pin = Interaction spring constant of the spring for entire core = Compressive spring constant of a single pin model = Number of active pins in the structures
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