Reliability Analysis of Composite Structures and Materials

Abstract

From the perspective of strength limit states, composite laminate failure may be considered in two major stages, first-ply failure (FPF) and last-ply failure (LPF). The first-ply failure usually corresponds to the commencement of matrix cracking failure; structural ultimate failure or last-ply failure consists of a series of the ply-level component failures, such as matrix cracking, delamination, and fiber breakage, from the first one to the last one. Well-known methods such as the first-order reliability method (FORM), second-order reliability method (SORM), or Monte Carlo simulation can be combined with finite element analysis to compute the component failure probability. The branch-and-bound method can then be employed to search for the significant system failure sequences. When each component failure occurs, the structural stiffness is modified to account for this damage, and the damaged structure is reanalyzed. This proceeds until system failure occurs. Based on the identified significant failure sequences, the system failure probability is determined by means of bounding techniques. In the composite structure, multiple sequences are found to be highly correlated, leading to efficient approximations in the failure probability computation. Section 44.2 to Section 44.4 present these methods and illustrate them with simple examples, with a practical application for a composite aircraft wing presented in Section 44.4.5.The characteristics of fatigue-damage growth in composite materials are different from those of damage growth in homogeneous materials. Continuum damage mechanics concepts have been used to evaluate the degradation of composite materials under cyclic loading. Damage-accumulation models that capture the unique characteristics of composite materials are presented in Section 44.5. The predictions from the models are compared with experimental data.Fatigue leads to delamination in several laminated composite applications, affected by anisotropic material properties of various plies, ply thicknesses and orientations, loads, and boundary conditions. The limit state can be formulated in terms of the strain energy release rate computed based on the virtual crack-closure technique. The critical value of the strain energy release rate, obtained from test data, is seen to be a random function of life of the structure. Once the delamination initiation probability is estimated, the propagation life until system failure is estimated through the exploration of multiple paths through the branch-and-bound enumeration technique. The analysis is then repeated for multiple initiation sites, and the overall probability of failure is computed through the union of the multiple initiation/growth events. These techniques are presented and illustrated in Section 44.6 using a practical applicationfatigue-delamination analysis of a helicopter rotor component.Composite materials are particularly attractive for high-temperature applications, as in engine components. Therefore, creep reliability models for high-temperature comp...