Stochastic Materials Design of Fiber Reinforced Composite Laminates

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Design-oriented failure criteria of composite materials are applied to construct an evaluation model of probabilistic safety based on the extended structural reliability theory. Typical failure criteria such as maximum stress, maximum strain and quadratic polynomial are compared from the viewpoint of reliability-oriented material design of composite materials. A new design diagram which shows the feasible region on in-plane strain space corresponding to the safety index or failure probability is also proposed. These stochastic failure envelope diagrams which are overlayed onto in-plane strain space enables one to evaluate the stochastic behavior of the composite laminate with any lamination orientation under multi-axial stress or strain condition. Experimental studies such as fractgraphy and acoustic emission are also performed to evaluate the complicated failure mechanism of composite materials. The sequential failure path of each layer of composite laminates is simulated by means of modulus degradation and safety indices under the offaxis load conditions. From the simulation analysis, a typical failure path is evaluated corresponding to an in-plane strain vector. The mechanical behavior, such as decreasing modulus and increasing inplane strain are also simulated in correspondence with each stage of sequential ply failure. It is necessary to select the important failure path whose occurrence probabilities are greater than others, since therc are too many failure paths of multi-layer composite laminates to evaluate. The proposed method of selecting effective failure paths depends upon the heuristic procedure based on one branch and bound rule which enables one to estimate the effective failure path whose occurrence probabilities are greater. An extended study is preformed in order to calculate the failure probabilty of multiple layer laminates resulting from the effective failure path.