J. Costa

Analytical and Numerical Investigation of the Length of the Cohesive Zone in Delaminated Composite Materials

An accurate prediction of the length of the cohesive zone ahead of a crack tip is fundamental for the correct simulation of delamination in composite materials under both quasi-static and fatigue loading. To ensure a correct dissipation of energy during delamination propagation, several cohesive finite elements have to span the cohesive zone. The length of the cohesive zone depends on the material properties, the geometry/size of the structure, and on the loading mode. This chapter presents new expressions to estimate the length of the cohesive zone under general mixed-mode loading conditions and for finite-sized geometries. The analytical model is validated by comparing its predictions with numerical results based on cohesive-zone models. The relevance of the proposed analytical solutions to the effective simulation of delamination is demonstrated by simulating delamination growth under mixed-mode loading using meshes with the length of the elements greater than the cohesive zone length.

Two-pheromone Ant Colony Optimization to design dispersed laminates for aeronautical structural applications

The objective of the present study is to find out the effect of using non-conventional fiber orientations (orientations not limited to 0^o, +/-45^o and 90^o) to improve the composite material response. The Ant Colony Algorithm is used to optimize the stacking sequence for biaxial tension and compression loading condition under strength constraints. Moreover, a modified algorithm (two-pheromone algorithm) is used to design a fully dispersed laminate. Results show that dispersed laminates can improve the critical buckling load by up to 8% for the biaxial compression loading case. With respect to the biaxial tensile loading condition, the results show that the matrix cracking failure index can be decreased up to 100% and the fiber tensile failure index can be decreased by 40% using the two pheromone formulation.

Delamination propagation under cyclic loading

Publisher Summary The increased use of advanced composite laminates in primary structures of commercial aircraft requires a thorough understanding of the inelastic response of composites under general loading conditions. One of the most relevant mechanisms that contribute to the loss of stiffness and to the structural collapse of composite structures is delamination. Besides degrading the structural integrity of composites, delamination is also difficult to detect using traditional non-destructive inspection methods. The majority of the analytical and experimental investigations of delamination have been focused on the study of delamination growth under quasi-static loads. However, under cyclic loading, delamination might grow up to a critical size for loads well below the critical load for quasi-static loading conditions. Currently, the design methodologies barely consider the possibility of interlaminar crack growth under fatigue loading, being more oriented to prevent fatigue damage by assuring that a stress or strain threshold for delamination onset is not exceeded. This approach is evolving towards more powerful physically based analyses implemented in advanced computational models. This is a result of the industrial interest to partially replace the experimental tests required to certify new composite components (qualification of materials, design allowables, sub-components and full components) by virtual (numerical) tools able to simulate the inelastic response of the composite materials up to structural failure.

Mode I fatigue behaviour and fracture of adhesively-bonded fibre-reinforced polymer (FRP) composite joints for structural repairs

This chapter discusses the methodologies used to analyse fatigue-induced damage of bonded joints between composite components under mode I cyclic loads. It first reviews the existing types of bonded joints used in manufacturing or repair procedures. Then, the chapter presents some general considerations related to mode I testing of adhesive joints, such as the stick-slip effects or the influence of the failure modes on the results. A description is given of the experimental test used at a coupon level to obtain onset and crack propagation curves. Finally, the chapter includes a concise review of the current numerical techniques used to simulate the behaviour of a bonded joint.