Modeling the statistical lifetime of glass fiber/polymer matrix compositesin tension

In this paper we present a viewpoint on modeling the lifetime of glass fibe r/polymer matrix composite structures loaded primarily in tension along the fiber axis. In many applications such components may sustain, over many ye ars and in deleterious environments, stress levels that are a significant f raction of their ultimate tensile strength. Thus the failure phenomenon of concern is creep rupture. Ideally, a comprehensive model should incorporate such features as environmentally driven, statistical degradation mechanism s in the glass fiber (such as stress corrosion cracking), creep and microcr acking of the polymer matrix, slip at the fiber/matrix interface near fiber breaks, local residual stresses from processing, including their complex m icromechanical interactions. Such a model should yield overall distribution s for lifetime in terms of the overall applied stress field; the overall vo lume of material, and boundary effects. Parameters of the model should refl ect subtle scaling relationships among microstructural variables (e.g., fib er packing geometry), parameters of the statistics of fiber strength and de gradation, matrix and interface creep exponents, rate factors in the stress -corrosion chemistry, and applied stress level. Particular attention must b e paid to the character of the extreme lower tails of the strength and life time distributions since these are crucial in establishing load levels that result in the extremely high reliability levels important in life-safety a pplications. For example, the model should be able to predict the steady lo ad level in a composite specimen with an effective loaded volume that yield s a given lifetime (e.g., 25 years) at an extremely low probability of fail ure (e.g., 10(-6)). This essentially rules out mean field approaches so pre valent in the mechanics and physics community. A model of this sort would a lso be useful in the development of strategies for effective accelerated te sting and data interpretation using special time-temperature scalings and m aster curves. Lastly, the model would have value in guiding strategies for quality control, materials processing, and component architecture during ma nufacture. Of course, such a comprehensive model is well beyond the present stale of the art. Nevertheless, a surprising amount of progress has been m ade in developing the necessary conceptual and computational framework incl uding the micromechanics, chemistry and physics of the fundamental failure mechanisms. In this paper we will review some of the relevant literature an d suggest directions that should be fruitful in yielding useful models. (C) 1999 Elsevier Science Ltd. All rights reserved.