The interaction between matrix cracks and interface delamination is a major failure mechanism in composite laminates and has been a subject of active research in recent years. Although coupled failure behaviour of composite laminates between matrix cracks and delamination has been observed in experiments, accurate modelling of this phenomenon for application in composite structures remains challenging. In this work, a three-dimensional separable cohesive element (SCE) is proposed to enable the modelling of interaction between matrix cracking and interfacial delamination in laminated fibre-reinforced composite materials. It is demonstrated that traditional cohesive elements are incapable of modelling the coupled failure mechanisms accurately if partitioning is not allowed. The SCE may be partitioned according to the configuration and geometry of matrix cracks in adjacent plies, thus maintaining appropriate connection between plies. Physically, the original interface is split and new interfaces are formed to bond the homologous cracked solids during fracturing process. The stress concentration induced by matrix cracks and the load transfer from cracked solid elements to interface cohesive element are effectively modelled. The proposed SCE is applied to model progressive failure in composite laminates and the results are found to agree with experiments.

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Tremendous efforts have been put into the study of structural integrity and the understanding of failure mechanisms in composites. Geometric non-linearity, receiving few attention in coupon-level simulations, can play an important role in the design and analysis of larger structures. This paper aims at extending the recently-developed Floating Node Method for damage analysis of laminated composites subjected to large deformations. The kinematics of strong discontinuities including interfacial delamination and matrix cracks are explicitly described in a geometrically nonlinear framework. Interactions between these two kinds of failure patterns are enabled through enriched elements equipped with floating nodes. To verify this proposed method, buckling-induced delamination and low-velocity impact damage are modelled, the results of which show good agreement with results from literature.

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The cohesive element (CE) has been widely used to model delamination in laminated composites. The penalty stiffness, interfacial strength and fracture toughness are three CE parameters that often determine the accuracy of the numerical analysis by finite elements (FEs). The fracture toughness may be experimentally determined through standard fracture tests. The interfacial strength is difficult to obtain experimentally but also plays a crucial role in predictive numerical modelling. The penalty stiffness is a numerical parameter whose range of values may influence computational results. Recently, the CE has been deployed to model the initiation and propagation of delamination in composite structures where no pre-existing cracks or delamination has been assumed. The parameters are often calibrated, but comprehensive studies on the influence of CE parameters and their selection criteria remain limited in the literature. In the present paper, the effects of CE parameters on delamination modelling are systematically examined. The sensitivity of the overall progressive damage process and failure load to the assumed CE strengths and stiffness is studied based on selected examples of fracture and coupon-level tests. With varying CE strengths employed in the FE analyses, two distinct kinds of failure behaviour, namely the strength-sensitive and strength-insensitive delamination processes, have been observed, depending on the geometry and ply layups of composite materials. On the other hand, the penalty stiffness of CE mainly affects the computational efficiency and accuracy of delamination modelling. The failure mechanisms and physics behind are investigated to get better understanding on the numerical effects of CE parameters, and thus provide the selection guidelines in practical analyses.@en

In this paper, the recently-developed floating node method is extended for damage analysis of laminated composites with large deformations. Strong discontinuities including interfacial delamination and matrix cracks are explicitly represented by geometrically nonlinear kinematics. Interactions between these two kinds of failure patterns are enabled through enriched elements equipped with floating nodes. A cohesive zone model is utilized for the damage process zone. A general implicit procedure with user-defined elements is developed for both quasi-static and dynamic analysis. The performance of this formulation is verified with two benchmark simulations, involving buckling-induced delamination and low-velocity impact damage. The results presented show good quantitative and qualitative agreements with results from literature.

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A three-dimensional separable cohesive element (SCE) is proposed to enable the modelling of interaction between matrix cracking and interfacial delamination in laminated fibre-reinforced composite materials. It is demonstrated that traditional cohesive elements are incapable of modelling the coupled failure mechanisms accurately if partitioning is not allowed. The SCE may be partitioned according to the configuration and geometry of matrix cracks in adjacent plies, thus maintaining appropriate connection between plies. Physically, the original interface is split and new interfaces are formed to bond the homologous cracked solids during fracturing process. The stress concentration induced by matrix cracks and the load transfer from cracked solid elements to interface cohesive element are effectively modelled. A comprehensive set of cases of multiple matrix crack configurations from plies of different fiber angles is considered. The proposed SCE is applied to model progressive failure in composite laminates and the results are found to agree with experiments.

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The cohesive element has been widely employed to model delamination and matrix cracking in composite materials. However, an extremely fine mesh along the potential crack path is required to achieve accurate predictions of stresses within the cohesive zone. A sufficient number of cohesive elements must be present within the cohesive zone ahead of the crack tip, resulting in very high computational cost and time for application to practical composite structures. To mitigate this problem, an adaptive floating node method (A-FNM) with potential to reduce model size and computational effort is proposed. An element with adaptive partitioning capabilities is designed such that it can be formulated as a master element, a refined element and a coarsened element, depending on the damage state in the progressive damage process. A relatively coarse overall mesh may be used initially, and by transforming the element configurations adaptively, the local refinement and coarsening schemes are applied in the analysis. The localized stress gradient ahead of the crack front within the refinement zone is captured by the refined elements. The refinement and coarsening operations are performed at the elemental level with fixed nodal connectivity, so that global successive remeshing in adaptive mesh refinement (AMR) techniques is avoided; this is the key difference between AMR and A-FNM. It is demonstrated that, without loss of accuracy, the present method simplifies the modelling procedure and reduces computational cost.

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