Papers by Author: Brian G. Falzon

Abstract: A comprehensive continuum damage mechanics model [1] had been developed to capture the detailed behaviour of a composite structure under a crushing load. This paper explores some of the difficulties encountered in the implementation of this model and their mitigation. The use of reduced integration element and a strain softening model both negatively affect the accuracy and stability of the simulation. Damage localisation effects demanded an accurate measure of characteristic length. A robust algorithm for determining the characteristic length was implemented. Testing showed that this algorithm produced marked improvements over the use of the default characteristic length provided by Abaqus. Zero-energy or hourglass modes, in reduced integration elements, led to reduced resistance to bending. This was compounded by the strain softening model, which led to the formation of elements with little resistance to deformation that could invert if left unchecked. It was shown, through benchmark testing, that by deleting elements with excess distortions and controlling the mesh using inbuilt distortion/hourglass controls, these issues can be alleviated. These techniques contributed significantly to the viability and usability of the damage model.

Abstract: Composite damage modelling with cohesive elements has initially been limited to the analysis of interface damage or delamination. However, their use is also being extended to the analysis of inplane tensile failure arising from matrix or fibre fracture. These interface elements are typically placed at locations where failure is likely to occur, which infers a certain a priori knowledge of the crack propagation path(s). In the case of a crack jump for example, the location of the jump is usually not obvious, and the simulation would require the placement of cohesive elements at all element faces. A better option, presented here, is to determine the potential location of cohesive elements and insert them during the analysis. The aim of this work is to enable the determination of the crack path, as part of the solution process. A subroutine has been developed and implemented in the commercial finite element package ABAQUS/Standard[1] in order to automatically insert cohesive elements within a pristine model, on the basis of the analysis of the current stress field. Results for the prediction of delamination are presented in this paper.

Abstract: Experimental static and fatigue tension-tension tests were carried out on 5HS/RTM6 composite intact coupons and coupons incorporating adhesively-bonded (FM300-2) stepped flush joints. The results show that the adhesive joint, which is widely used in repairs, significantly reduces the static strength as well as the fatigue life of the composite. Both, the static and the fatigue failure of the ‘repaired’ coupons occur at the adhesive joint and involve crack initiation and propagation. The latter is modelled using interface finite elements based on the decohezive zone approach. The material degradation in the interface constitutive law is described by a damage variable, which can evolve due to the applied loads as well as the number of fatigue cycles. The fatigue formulation, based on a published model, is adapted to fit the framework of the pseudotransient formulation that is used as a numerical tool to overcome convergence difficulties. The fatigue model requires three material parameters. Numerical tests show that a single set of these parameters can be used to recover, very accurately, the experimental S-N relationship. Sensitivity studies show that the results are not mesh dependent.