Dr Lynn Boniface
Academic and research departmentsSchool of Mechanical Engineering Sciences.
The growth of transverse ply cracks in composite laminates has been investigated both theoretically and experimentally. Some of the closed-form strain energy release rate based analyses of this problem in the literature have been compared and extensions to these approaches are presented. These models have been shown to be consistent with an alternative approach based on an approximate expression for the stress intensity factor at the tip of a growing transverse ply crack. An experimental study of transverse ply crack growth has been carried out using a simple model array of transverse ply cracks in a glass/epoxy laminate. By making the transverse ply sufficiently thick, the specimen compliance was found to change measurably as individual cracks grow. Hence, the strain energy release rate could be determined experimentally (via the compliance relationship) and compared with analytical predictions. Agreement was found to be satisfactory.
An experimental and theoretical study of the effects of 90° ply cracking on the thermal expansion coefficients of crossply laminates has been carried out. It has been found experimentally that reductions in the coefficient of thermal expansion of up to 50% are caused by 90° ply cracks induced mechanically, although considerable care is needed in the experimentation. This behavior was modeled using a simple shear-lag analysis, and the resulting analytical expressions are compared with other approaches available in the literature. The growth of matrix cracks in a model GFRP system under severe thermal cycling (77 to 373 K) is investigated. The changes in expansion coefficient are affected by the growth of 0° ply cracks in addition to the 90° ply cracks. The crack growth rate/cyclic strain energy release rate range data are compared with those reported previously for mechanical fatigue cycling of similar material. The two data sets are consistent if plotted in terms of a fracture mechanics parameter which aims to account for the temperature dependence of material properties.
The tensile and compressive mechanical performance of a range of unidirectional and +/-45 degrees/0 degrees laminates, based on solid and hollow glass fibres, has been compared in terms of absolute and specific values (i.e. property normalised by the laminate density). The damage tolerance of both systems during impact loading has also been established. The use of hollow glass fibres lends to reductions in laminate density of up to 23% compared with solid fibre constructions. Although the absolute values of mechanical properties are lower for the hollow fibre materials (with the exception of compressive failure strain), normalised properties compare more favourably, especially in compression. It is confirmed that materials utilising hollow fibres for reinforcement have considerable potential for applications in which weight saving is important, without significant reduction in mechanical properties relative to solid fibre materials.
The growth of individual transverse ply cracks during tension-tension fatigue of (0/90//2)s GFRP and (0/90//3)s CFRP has been observed using transmitted light and penetrant enhanced X-radiography, respectively. The growth rate of cracks in both laminates is independent of crack length but depends on the spacing between the cracks. Very low growth rates occur for small crack spacings or if crack branching occurs. The results are interpreted using a stress intensity factor approach.