Computational Mechanics of fracture on advanced aerospace structures

In this thesis, the computational simulation of cracks in advanced composite structures subjected to biaxial loading is studied. A structural integrity analysis using the eXtended Finite Element Method (XFEM) is considered for simulating the crack behaviour of a chopped fibre-glass-reinforced polyester (CGRP) cruciform specimen subjected to a quasi-static tensile biaxial loading [99]. This is the first time this problem is accomplished for computing the stress intensity factors (SIFs) produced in the biaxially loaded area of the cruciform specimen. SIFs are calculated for infinite plates under biaxial loading as well as for the CGRP cruciform specimens in order to review the possible edge effects. A new ratio relating the side of the central zone of the cruciform and the crack length is proposed. Additionally, the initiation and evolution of a three-dimensional crack are successfully simulated. Specific challenges such as the 3D crack initiation, based on a principal stress criterion, and its front propagation, in perpendicular to the principal stress direction, are conveniently addressed. No initial crack location is pre-defined and an unique crack is developed.
A three-dimensional progressive damage model (PDM) is implemented within a CGRP cruciform structure for modelling its damage under loading [100]. In order to simulate the computational behaviour of the composite, the constitutive model considers an initial elastic behaviour followed by strain-softening. The initiation criterion defined is based on the maximum principal stress of the composite and once this criterion is satisfied, stiffness degradation starts. For the computation of damage, the influence of the fibre and the matrix are taken into account within the damage rule. This is the first time a three-dimensional PDM is implemented into a composite cruciform structure subjected to biaxial loading.
A new approach for dynamic analysis of stationary cracks using XFEM is derived. This approach is capable of addressing dynamic and static fracture mechanics problems. Additionally, by means of this relatively simple approach, it is possible to address correctly the crack pattern of the 10 degrees off-axis laminate manufactured solving the limitation observed with progressive damage modelling. During the whole thesis, the computational outcomes have been validated by means of comparison with theoretical and experimental results.