Cutting force model for machining of CFRP laminate with diamond abrasive cutter

The article presents a cutting force model for trimming operations of CFRP laminate with diamond abrasive cutters. Those tools are more and more encountered on industrial applications of CFRP trimming, due to their abrasion resistance and their low cost. Contrary to endmills, they consist of a large number of cutting grits, randomly distributed around the tool. To tackle the issue, a continuous model of tool engagement is proposed. Validity of the approach is verified. A mechanical model of cutting forces, adapted to CFRP laminate, is then presented. The evolution of specific cutting coefficient in relation to fibres orientation is investigated through a piecewise constant model. It leads to the proposal of a sine model for the specific cutting coefficients. The simulated forces are in good agreement with the experimental results of cutting tests, carried out in multidirectional CFRP laminate for different fibres orientation and widths of cut. Cutting mechanisms are finally discussed depending on fibres orientation

Advanced Finite Element Strategies for Machining of Long Fibre Reinforced Polymer Composites

This thesis addresses a novel finite element study in machining of long-fibre-reinforced polymers (LFRP). For this sake, the development of sophisticated Fortran VUMAT user-subroutines is performed to insert a new composite damage algorithm in the modelling of composite machining, which accounts for damage propagation and chip fracture. These damage algorithms are based on continuous damage mechanics (CDM) theory linked to fracture computational techniques to simulate damage propagation, while chip fracture is induced using wisely strain-based element deletion criteria. The modelling of two main topics in composite machining have been investigated during this research: composite cutting basics (machining induced damage and chip formation) and tool wear influence in machining forces. The influence of cutting tool morphologies and material in the machining induced damage in composite was investigated using a novel method of inserting the spring-back phenomenon in the numerical analysis. Significant conclusions are extracted from this research. For instance, high relief angles reduce the sub-surface damage, or the tool wear incidence is found not to be critical in the studied range. The following step was the modelling of chip formation mechanisms in composite machining. It was achieved by inserting a strain-based element deletion algorithm in the user-defined finite element (FE) code to allow chip fracture. The numerical assessment of sub-surface damage and chip formation was performed, implementing a strain-based continuum damage mechanics (CDM) approach. The study of five common machining configurations was addressed to model the governing chip fracture mechanism for several fibre orientations. This factor would include substantial improvements in the accuracy of the oncoming works. Finally, a common composite edge trimming operation is successfully modelled to prove the damage algorithm's versatility. Edge trimming has barely been modelled so far because of its complexity and high computational cost required. It was developed an FE model to predict the tool wear influence on the machining forces' increment. Interesting technical applications could be achieved using this FE model. For instance, it could detect the point where the tool should be replaced by just checking the machining forces saving manufacturing time and optimising its use

Anisotropic composite cantilevers with bend-twist coupling in the context of preliminary design and analysis

The development of elastically-tailored self-adaptive structures through the use of anisotropic advanced composite materials continues to attract more interest and new applications. However, the process required to design a structure which exploits the elastic-couplings of an anisotropic material is complex. Considerably more understanding of advanced composite materials is required to successfully deliver a tailored elastic response than is required to design a typical laminate. The research of this thesis has thoroughly assessed the effectiveness of using first-order theories to approximate the deformation response of laminated bend-twist coupled cantilever beams/plates as a method for developing system understanding during preliminary design. The bend-twist coupling response of laminated cantilevered strips was measured to higher levels of accuracy than other examples in the literature with the aid of a 3-D laser scanner. The experimental results were compared with one-dimensional analytical models and finite element numerical simulations (both derived from first-order laminated plate stiffnesses) to assess the accuracy and effectiveness of the first-order approximations for developing an understanding of bend-twist coupled cantilever response. Through this research it was demonstrated that the understanding of a bend-twist coupled cantilever can be significantly enhanced with the use of simple and uncomplicated methods derived from the elementary laminated plate theories. The one-dimensional model was found to provide liberal (overestimated) deflection and twist approximations, the magnitude of which was dependent on planform aspect ratio and material properties. The main contributor to the inaccuracy of the one-dimensional models was attributed to the restraint of rotation at the finite width fixed-end boundary. However, specific insight has arisen from this research which shows that in spite of the systematic overestimation of deflection and twist, a first-order approximation provides considerable insight into the deformation response of bend-twist coupled cantilevers. In addition to deformation approximations, it was also found that there are certain aspects of first-order laminated plate models which are very effective at characterising the bend-twist coupling response of thin cantilevers. The locus of flexural-centres, the bend-twist coupling twist rate and a new parameter, the Cross-Sectional Centre of Twist, were all demonstrated to be accurately approximated by expressions derived from the one-dimensional model