Enforsing a system approach to composite failure criteria for reliability analysis

Composite failure criteria have found widespread use in research and industry. In the vast majority of applications the material properties and the stresses, which serve as inputs to the criteria, are defined deterministically. However, when the reliability of composite structures is sought the input to the failure criterion will be random quantities. The reliability is efficiently identified using approximate methods such as First Order Reliability Methods (FORM) [1,2]. FORM involves an iterative optimization procedure to obtain a reliability estimate, which imposes a number of additional challenges with the use of failure criteria, since composite materials are a discontinuous medium, which invoke multiple failure modes. Under deterministic conditions the material properties and the stress vector are constant and will result in a single dominating failure mode. When any of these input parameters are random, multiple failure modes may be identified which will jeopardize the FORM analysis and a system approach should be applied to assure a correct analysis. Although crude Monte Carlo simulation automatically may account for such effects, time constraints limit its useability in problems involving advanced FEM models. When applying more computationally efficient methods based on FORM/SORM it is important to carefully account for the multiple failure modes described by the failure criterion. The present paper discusses how to handle this problem and presents examples where reliability assessment of ultimate failure of fiber-reinforced composites is carried out using three different failure criteria.