The anisotropic nature of composite materials, specifically fiber reinforced plastics (FRPs), constitutes them a material category with adaptable mechanical properties, appropriate for the application they are being designed for. The stacking sequence choice of FRP laminates allows for the optimization of their strength, stiffness, and weight to the desired design requirements. The anisotropic nature of composites is also responsible for the different failure modes that they experience, which are based on the accumulation of damage, rather than crack initiation and propagation as the majority of homogeneous isotropic materials. This chapter discusses the background theory for determining the stress distribution in a laminated FRP, the possible failure modes occurring in composites, the failure criteria predicting the onset of failure, as well as cumulative damage models predicting the fatigue life of laminates.
Part of the book: Failure Analysis and Prevention
The strength of Fiber Reinforced Plastic laminated structures is strongly dependent on the stacking sequence of the laminate, and consequently the fiber orientations of the individual laminae (also referred to as layers or plies). Classical Lamination Theory (CLT) is a theoretical tool providing the strain and stress distribution in a laminate based on its stacking sequence and material properties. On the other hand, first ply, and consequent ply failure can be approximated with interactive failure criteria, such as the Tsai-Hill and Tsai-Wu. Technological advances often require material alternatives to metallic structures, and FRPs constitute optimum solutions to such selections. However, these structures are no longer just plain laminates with unidirectional fibers in their laminae, they include geometric discontinuities allowing ease of assembly. Such discontinuities become stress concentration regions, which require extra attention upon design against failure. This chapter discusses the extent to which the traditional analysis of FRP failure, using CLT and interactive failure criteria is adequate in structures with discontinuities, and suggests extra analysis steps to be considered when designing against failure in the area of the discontinuity.
Part of the book: Engineering Failure Analysis