Numerical and experimental studies of multi-ply woven carbon fibre prepreg forming process

Woven carbon fibre prepreg is being increasingly used in high-performance aerospace and automotive applications, primarily because of its superior mechanical properties and formability. A wide range of forming simulation options are available for predicting material deformation during the prepreg forming process, particularly change in fibre orientation. Development of a robust validated simulation model requires comprehensive material characterisation and reliable experimental validation techniques. This paper presents experimental and numerical methods for studying the fibre orientation in multi-ply woven carbon fibre prepreg forming process, using a double-dome geometry. The numerical study is performed using the commercial forming simulation software PAM-FORM and the material input data are generated from a comprehensive experimental material characterisation. Two experimental validation methods are adopted for fibre shear angle measurement: an optical method for measuring only the surface plies, and a novel CT scan method for measuring both the surface plies and the internal plies. The simulation results are compared against the experimental results in terms of fibre shear angle and the formation of wrinkles to assess the validity of the model

The compounding of short fibre reinforced thermoplastic composites

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.It is generally accepted that the mechanical properties of short fibre reinforced thermoplastics do not correspond with the high mechanical properties of fibres used to reinforce them. A study is made into the methods of compounding reinforcing fibres into thermoplastics to produce short fibre reinforced thermoplastics of enhanced properties. The initial method chosen for investigation is the twin screw extrusion compounding process. Variables such as fibre feeding arrangement and extrusion screw design are found to be factors influencing the properties of carbon and glass reinforced nylon 6,6. Use is made of computer programs to predict properties, assess compound quality and estimate fibre-matrix bond strength. Investigations indicate that the presence of reinforcing fibres with enhanced lengths does not result in the predicted property increases. The reasons for this shortfall are believed to lie in unfavourable fibre orientation in injection mouldings and the reduced strain to break of these materials. Short Kevlar reinforced thermoplastics are compounded and their mechanical properties assessed. The reasons for the poor mechanical properties for these materials are identified as a poor bond strength between fibre and matrix, the formation of points of weakness within the fibres by the compounding and moulding processes and the coiled arrangement of fibres present in injection mouldings. A method suitable for the routine assessment of fibre-matrix bond strength is used to examine combinations of fibre and thermoplastic matrix. A comparison is made of the values derived from this method with values calculated from stress-strain curves of injection mouldings. This allows an understanding of the nature of the fibre-matrix bond yielded by compounding and injection moulding steps. A description is given of a novel method designed to overcome the limitations of conventional compounding routes to produce long fibre reinforced injection moulding feedstock. Further work is necessary before this method is a feasible production technique

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of LID HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the LID HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value (c) 2009 Elsevier Ltd. All rights reserve

Exploiting the optical reflectance behaviour of carbon fibre composites for low-cost inspection and orientations analysis

This work focusses on understanding and exploiting the optical reflectance behaviour of carbon fibre reinforced polymer composites for the non-destructive determination of fibre orientation and surface defects. By taking a series of images under different lighting conditions, fibre reflections have been isolated and MATLAB image analysis has been used to calculate the fibre orientations across the surface of the composite. An average peak method has been developed and validated against manual orientation measurements for two different applications: a machined scarf repair surface and a discontinuous fibre composite. The same experimental method and analysis code showed good results for both cases without modification, when at least 16 images with different directional lighting conditions are used. Hence, this low-cost approach can be applied to a diverse range of quality assurance and research applications where fibre orientations or other surface details need to be characterised. © The Author(s) 2020

Edge Trimming of CFRP- Surface Roughness Measurement and Prediction

Use of carbon fibre composites has been increasing in the aerospace industry. However, there is still a need for finishing operations by conventional machining in the manufacturing of composite parts. Composites have a very different machinability to metals and can suffer from a number of surface defects during machining. The fibres are also highly abrasive and can cause rapid tool wear which in turn leads to increased likelihood of machining defects. This project has focussed on the machined surface quality developed during machining using new surface inspection techniques and additional surface roughness parameters. It is important to be able to accurately measure the surface roughness in order to ensure the integrity of in service components and quantify surface damage from machining. The aim of this project is to develop new numerical modelling techniques for the edge trimming of carbon fibre reinforced plastic (CFRP), and develop methods for the prediction of surface roughness. Different experimental techniques have been used to analyse post-machining damage, including scanning electron microscopy (SEM), computed tomography scanning (CT) and a focus variation system for measuring surface roughness. CFRP specimens have been edge trimmed using a poly crystalline diamond (PCD) cutting tool, and compared for different machining parameters, tool wear and material fibre orientations. Cutting forces were recorded and the surface quality was inspected using the optical focus variation method. Regression models from experimental data have been combined with finite element (FE) models to create a surface roughness prediction tool which includes the effects of tool wear. Areal surface roughness Sa measurements were taken using the optical system and the advantages of the system have been compared with conventional stylus roughness measurement methods. Experimental data was used to validate 3D and 2D FE milling models using MSC Marc. New FE models were developed using adaptive re-meshing, and user subroutine to control the cutting tool movement and simulation idle time. Progressive levels of tool wear have been implemented in the 2D model by using cutting edge radius measurements from experiment. FE and experimental results show that tool wear and material fibre orientation have a significant effect on the cutting forces and surface roughness. Regression models showed that the surface roughness was most affected by tool wear, feed rate and cutting speed. A reasonable comparison has been found between FE and experiment and the FE models were capable of predicting the effects of tool wear due to cutting edge rounding. 3D models were found to better predict thrust forces than 2D FE model. The optical system was found to be useful technique for measuring surface roughness of machined fibrous composite surfaces and is more reliable than conventional roughness measurements. New strategies for roughness measurement have been recommended

Interfacial properties of fibre reinforced thermo-plastics

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3D FEA modelling of laminated composites in bending and their failure mechanisms

keywords: 3D keywords: 3D keywords: 3D keywords: 3D keywords: 3DAbstract This paper developed three-dimensional (3D) Finite Element Analysis (FEA) to investigate the effect of fibre lay-up on the initiation of failure of laminated composites in bending. Tsai-Hill failure criterion was applied to identify the critical areas of failure in composite laminates. In accordance with the 3D FEA, unidirectional ([0]16), cross-ply ([0/90]4s) and angle-ply ([±45]4s) laminates made up of pre-preg Carbon Fibre Reinforced Plastics (CFRP) composites were manufactured and tested under three-point bending. The basic principles of Classical Laminate Theory (CLT) were extended to three-dimension, and the analytical solution was critically compared with the FEA results. The 3D FEA results revealed significant transverse normal stresses in the cross-ply laminate and in-plane shear stress in the angle-ply laminate near free edge regions which are overlooked by conventional laminate model. The microscopic images showed that these free edge effects were the main reason for stiffness reduction observed in the bending tests. The study illustrated the significant effects of fibre lay-up on the flexural failure mechanisms in composite laminates which lead to some suggestions to improve the design of composite laminates