Buckling Analysis of Delaminated and Stitched Composite Plate System Under Hygrothermal Pressure

In this study, we develop a model for buckling of a partially delaminated composite plate with transverse stitching to resist out of plane deformations. The model applies to carbon fiber/polyimide matrix composites rapidly heated to around 370 °C, where it is known that steam-induced delamination (the popcorn effect) becomes an issue as the pressures generated approach the tensile strength of the matrix. Thus, a key element is the incorporation of this hygrothermal pressure within the formulation. This complex composite structure is modeled as two adhesively connected, specially orthotropic, rectangular plates, and the delaminations with internal vapor pressure are considered as holes in the adhesive layer. The intact regions of the adhesive layer and the stitches are modeled by continuous and discrete linear mechanical springs, respectively. The energy contributions of each component in the system are expressed in terms of out-of-plane displacements. The boundary conditions are that the system is simply supported along all edges so as to permit a Fourier sine series to approximate the transverse displacements. Application of the energy minimization approach gives a system of algebraic equations to determine the unknown weighting coefficients of the functions describing the transverse deflections of each plate layer. Deformed shapes of the system under axial compressive loads are obtained for different hygrothermally induced pressure conditions so as to show that the model works well. Parametric studies on critical buckling loads are performed for a few stitch and delamination configurations. It is found that stitching through delaminated areas can increase critical buckling loads and alter the sequence of corresponding mode shapes.