The critical element model: A modeling philosophy

Abstract The advent of composite primary structures has presented the technical community with several challenges. Not the least of these is the need to answer the question, “How long will this composite component last?”. One of the most basic issues associated with this question is the matter of how to describe (and predict) behavior that is influenced and controlled by complex combinations and interactions of the multiple damage modes know to occur in composite materials. The present paper suggests a mechanistic approach to this problem which is generally known as the “critical element model” (CEM). The CEM is not so much a single model as a modeling philosophy which proposes that the damage process that ultimately leads to initiation of the fracture event can be modeled by using a micro-mechanical analysis to determine local stress redistribution caused by the failure of “subcritical elements”, and phenomenological (constitutive) information to characterize the condition of the “critical elements” which control fracture. This approach has a variety of advantages, and provides a framework for the incorporation of improved descriptions and analysis of micro-events as they become available. At the same time, it is presently operable at the elementary engineering level in convenient coded form and has been used to predict the behavior of composite coupons subjected to a variety of tensile and compressive load cycles including block loading. Some 270 tests have been compared with the predictions with encouraging results. A summary of these activities and the general philosophy will be presented in the paper.