Comparison of probabilistic models for stress rupture failure in continuous unidirectional fiber composite structures

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Stress rupture is an important failure phenomenon in composite overwrapped pressure vessels, which is highly unpredictable other than on a statistical basis. Even then, there are several statistical models, with varying bases in composite micromechanics and molecular failure mechanisms. Choosing among these models is not trivial, even when micromechanical details of the failure process are reasonably well appreciated, and one has available a reasonably large database of strength and lifetime data. As a result, there is little in the way of guidance to choose the most appropriate model. One important issue is that accurate predictions are desired at relatively low service loads compared to the strength, and low probabilities of failure that are far less, e.g., 10−6, than can be directly confirmed using the data itself. In essence, one needs a robust and accurate statistical model free of inconsistencies associated with such low stress levels and probabilities of failure. This paper performs an in-depth comparison of several current models, which have varying physical bases. The models compared differ in the number of parameters to be estimated from data. The results of this study, however, show that over a broad range of parameter values these models give surprisingly similar failure probability predictions. While practitioners may have a preference for one model over another, the basis for such a choice is not easily established, given the fidelity of typical data.