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

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

Not availabl

A study on the compressive strength of thick carbon fibre-epoxy laminates

This paper describes an experimental study that examines the effect of specimen size on the axial compressive strength of IM7/8552 carbon fibre/epoxy unidirectional laminates (UD). Laminate gauge length, width and thickness were increased by a scaling factor of 2 and 4 from the baseline specimen size of 10 mm x 10 mm x 2 mm. In all cases, strength decreased as specimen size increased, with a maximum reduction of 45%; no significant changes were observed for the axial modulus. Optical micrographs show that the failure mechanism is fibre microbuckling accompanied by matrix cracking and splitting. The location of failure in most specimens, especially the thicker ones, is where the tabs terminate and the gauge section begins suggesting that the high local stresses developed due to geometric discontinuity contribute to premature failure and hence reduced compressive strength. Two generic quasi-isotropic multi-directional (MD) lay-ups were also tested in compression, one with blocked plies [45n/90n/-45n/0n]s and the other with distributed plies [45/90/-45/0]ns with n=2, 4 and 8. The material used and test fixture was identical to that of the unidirectional specimens with three different gauge sections (30 mm x 30 mm, 60 mm x 60 mm and 120 mm x 120 mm) to establish any size effects. Strength results showed no evidence of a size effect when the specimens are scaled up using distributed plies and compared to the 2 mm thick specimens. All blocked specimens had similar compressive strengths to the sub-laminate ones apart of the 8 mm specimens that showed a 30% reduction due to extensive matrix cracking introduced during the specimen's cutting process. The calculated unidirectional failure stress (of the 0° ply within the multidirectional laminate) of about 1710 MPa is slightly higher than the average measured value of 1570 MPa of the 2 mm thick baseline unidirectional specimen, suggesting that the reduced unidirectional strength observed for the thicker specimens is a testing artefact. It appears that the unidirectional compressive strength in thicker specimens (>2 mm) is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects