Random delamination of graphite/epoxy laminated composites : stress analysis, failure criteria, experimental characterization and stochastic modeling

The delamination failure of laminated composites exhibits two distinct characteristics; (1) the corresponding failure strengths display considerable scatter, and (2) the morphologies of the fracture surfaces and the interlaminar strengths are dependent on the magnitude of the fiber orientation difference (abbreviated as FOD below) angle between the two adjacent plies of the interface. A variational solution technique is developed to determine the three-dimensional edge effect stresses around pin-loaded holes in symmetric laminated composites. The stress state around the holes is divided into two parts, one the in-plane stress state determined using the Classical Laminated Plate Theory (CLPT) and the other the boundary layer stress state arising from the edge effects which only exists in the vicinity of the holes. The equilibrium equations of boundary layer stress state with zeroth-order approximation are derived from a set of power expansions. Analytical solutions for the boundary layer stress state are then obtained by imposing zeroth-order stress functions on the variational principle of complementary energy. The developed variational solution technique is versatile, which can be used to calculate the edge effect stresses at straight free edges, curved free edges and pin-loaded hole edges with or without friction in laminated composites. An experimental investigation is conducted to study the effects of fiber orientations on the interlaminar shear (ILS) strengths of CYTEC ® G40-800/5276-1 graphite/epoxy laminated composites. The ILS strengths at the [straight theta]-interfaces ([straight theta] = 0,̕ 10,̕ 20,̕ 30,̕ 40,̕ 50,̕ 60,̕ 70,̕ 80 ̕and 90)̕, around which the two neighboring plies have a FOD angle [straight theta], are experimentally characterized using the modified double notch shear specimens with an elaborately designed layup. A stochastic simulation model is proposed in a phenomenological sense, in which the probabilistic dependence of ILS strengths on the FOD angles [straight theta] is simulated as a non-Gaussian nonstationary stochastic process within the bounded interval [0, s/2]. Two innovative delamination criteria in terms of quadratic and cubic polynomials in each case, are developed based on the Malmeister strength theory and Hashin assumption. The intent of the developed criteria is to include the effects of [straight theta]-dependent and directionally-dependent interlaminar strengths into the failure evaluation of laminated composites, as demonstrated by the experiments in the present thesis. (Abstract shortened by UMI.