Relation between interlaminar damage extension and fibre orientation with acoustic signal: quasi-static and cyclic loading

Abstract

The employment of composite laminates in aerospace and aeronautic industries is justified by its properties of high specific strength, high stiffness and low density (compared to metals). However, due to the anisotropic nature of composites, this material presents low resistance to out-of-plane stresses, which make them vulnerable to delamination. Delamination is one of the most critical damage mode of composites, hampering their employment for some applications.Considering aircraft structures, the design philosophy called "no growth" is widely used in the design of composite laminate structures, in which no delamination would be developed in the structure. This design philosophy leads to structures with bigger dimensions and, consequently, heavier structures, which is incoherent to the aim of the composite's employment.In the last ten years, a philosophy design that allow the development of damage in the structure until a critical size have been used. This approach improves the life cycle of structural components and enables the application of higher loads since the crack behaviour during its propagation is well understood.With the purpose of applying the damage tolerant philosophy, a great knowledge about damage evolution is needed to enable a reliable prediction of material fatigue life time. Crack growth models based on physical concepts are suitable to this design approach. However, models based on the physics of delamination are still limited in the literature. Therefore, the aim of the present work is to explain physically the mechanisms developed during crack propagation, through mechanical tests (quasi-static and fatigue) in composite laminate materials under mode I loading.To this aim, the damage mechanisms developed during induced delamination growth will be evaluated by scanning electron microscopy, optical microscopy and acoustic emission. These features will be correlated to variation of fiber orientation at the interface of delamination growth. (AU)