A finite element simulation for orthogonal cutting of UD-CFRP incorporating a novel fibre - matrix interface model

The rapid increase in industrial utilisation of carbon fibre reinforced plastic (CFRP) composites in recent years has led to growing interest in numerical modelling of material behaviour and defect formation when machining CFRP. The inhomogeneous/anisotropic nature of CFRP however presents considerable challenges in accurately modelling workpiece defects such as debonding between the matrix and fibre phase following cutting operations. Much of the published literature has involved the use of zero thickness cohesive elements to represent the fibre-matrix interface, despite the inability of such elements to model compressive stresses. This paper details a new approach for characterising the interface region in a two-dimensional explicit finite element simulation when orthogonal machining unidirectional (UD) CFRP laminates. A cohesive zone model based on a traction-separation law is applied to small thickness (0.25 µm) interface elements in order to accommodate compressive failure, which is implemented via a bespoke user subroutine. Fibre fracture is based on a maximum principal stress criterion while elastic-plastic behaviour to failure is used to represent matrix damage. The influence of varying fibre orientations (45°, 90°, 135°) on predicted cutting and thrust forces were validated against published experimental data. While the former was generally within 5% of experimental data for workpieces with 90° and 135° fibre directions, predicted thrust forces were typically underestimated by ~30-60%. The corresponding chip formation mechanisms and sub-surface damage due to the different material phases were also investigated. The proposed model was able to predict composite behaviour and defect formation that was comparable to experimental high speed camera images outlined in the literature.