A study on the mechanical properties and performance of selected thermoplastic and thermosetting composites for engineering applications


Student thesis: Doctoral Thesis

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  • Kai Kin YEUNG


Awarding Institution
Award date15 Jul 2011


Fibre reinforced composites, a combination of reinforcing fibre and matrix, offer many advantages over traditional materials and therefore found wide application in the aerospace and construction industry. Among the advantages that traditional thermosetting composites offer, the most citied are weight saving, high modulus, high strength-to weight ratio, corrosion and fatigue resistance. Recently, the development of high performance thermoplastics composites has been accountable for increasing their demand. Whereas the developed thermosetting composites have created environmental concerns because of hazardous fumes generated during fabrication of composites and, most importantly, they are non-recyclable. Lack of previous experience on the durability of thermoplastic composites and the demand for more understanding of their properties in the planning and design stage is increasing. Such advances have created in inescapable need for establishing an understanding and comparison of the mechanical behaviour of thermoplastic and thermosetting composites. The main aim of this research is to investigate and explore the potential advantages and feasibility of thermoplastic composites in typical applications that are traditionally deploying thermosetting composites. In addition, the study also widens the scope of designing composites in a simple and direct way by industry persons, without the need for complex mathematical analysis. This study also covers issues of environmental aging, durability and cost analysis for thermoplastic and thermosetting composites that can serve as a ready reference for the industry. The objectives of this research are: 1) to develop a class of thermoplastic-based Boron, Kelvar-49 and Carbon reinforced fibre composite laminates; 2) to study the mechanical behaviour of the developed composites and explore the application of theoretical models for predicting the mechanical behaviour of these composites, and 3) to study the value, benefit and potential application of thermoplastic composites for the industry. The selection of fibres for this study was based on an initial study using some simple theoretical models for predicting the mechanical properties of a large number of fibres with combination of several thermoplastic and thermosetting matrixes. The fibre and matrix combinations are chosen such that low, medium and high strength composites are developed for further study. The three types of fibres selected (Boron, Kevlar-49 and Carbon) are combined with three types of thermoplastics (Styrene Acrylonitrile (SAN), Acrylonitrile Butadiene Styrene (ABS) and Polyethylene (PE)) and two types of thermosetting matrices (Polyimide (PI) and Low Modulus Polyester). Firstly, the prediction of mechanical behaviour of composites with these chosen combinations of constituents has been made. Theoretical prediction of mechanical properties of thermoplastic and thermosetting composites from their constituent's properties has been made by using the following micromechanical models: (1) Rule of Mixtures (ROM), Halpin-Tsai, Eshelby's Inclusion model, Modified Rule of Mixtures (Modified ROM) and Self-Consistent model for uniaxial longitudinal tensile strength; (2) Rosen - extensional mode, Rosen - shear mode and Xu-Reifsnider Model for longitudinal compressive strength and; (3) Inverse Rule of Mixture (IROM) and Modified Inverse Rule of Mixtures (Modified IROM) for transverse strength. All the tested specimens were prepared in sheet form by modified pultrusion process which is being widely used in the manufacture of thermosetting composites. The fibre volume fraction was maintained at 61% which is commonly used for most structural composites. The experimental work to evaluate the mechanical properties of the developed composites included uniaxial tension test following British Standards BS 2782 Part 3 Method 321:1994, compression test based on BS 2782 Part 3 Method 345A:1993 standard, and three-point bending test following BS 2782 Part 3 Method 335A:1993 standard. The consolidated results indicate that all experimentally obtained strength values are the highest for Boron composites when compared with Carbon and Kevlar-49 thermoplastic and thermosetting composites. The tensile strengths are nearly the same for all composites prepared with the same fibre but with different matrix materials, indicating that they are fibre dominated. Their respective compressive strengths are lower than tensile strengths as they appear to be matrix dependent. The flexural strengths are between the tensile and compressive values, whereas the flexural modulus values are the lowest among the three types of loading, except Boron-Polyimide composite having its flexural strength higher than tensile strength. In assessing the standing of theoretical models which predict the mechanical properties of Boron, Carbon and Kevlar-49 thermoplastic and thermosetting composites in terms of their constituents, it is often difficult to discriminate between the validity of assumptions made in the models and constructional imperfections within the specimens. Results indicate the theoretically estimated tensile strengths using ROM model are dramatically higher by an order of magnitude than the experimentally obtained tensile strengths. The tensile strengths are controlled and dominated by fibre alone and the matrix contribution to their strengths is negligible. From the compression tests, it is found that micro-buckling approach has made little success in predicting the compressive strength by Rosen's model whereas Xu-Reifsnider model has shown good agreement with the experimental values which are only about 10% lower than theoretically obtained compressive strengths. The matrix properties seem to be the major contributors of the prepared composites. It is found that the flexural strength values obtained from the three-point flexural tests are about 14-17% lower than the corresponding theoretical values using IROM models. These properties have more complex dependency with respect to the properties of participating fibre, matrix and manufacturing. Systematic and numerical costing analysis of respective thermoplastic and thermosetting composites has been made for managing their application. The total product cost is established based on factors such as material, labour, overhead, selling expenses, general administrative expenses, taxes as well as net profit. The results indicate that the production cost is highest for Boron based thermoplastic composites among all the selected composites whereas thermosetting composites with Carbon are the lowest. From the comparison of cost per unit strength analysis, it is found that Boron-Polyethylene, Carbon-Polyimide and Boron-SAN gave the best results in tensile properties, flexural properties and compressive properties, respectively. The outcomes can assist the fabricator to choose suitable composites in order to suit their specific loading requirement during design stage. In conclusion, the micromechanical models can hardly be used for exact practical analysis of composite materials and structures because even the most vigorous models cannot describe adequately the real composites evolved during manufacturing. Many features such as voids, microcracks, and randomly damaged and misaligned fibres cannot be formally reflected in mathematical models. In such circumstances, micromechanical models may be used for initial qualitative analysis, providing an understanding of how the constituent parameters affect the mechanical properties followed by experimental determination of these properties. Unintentional variations in fibre alignments provide unforeseeable complications and variations to the experimental results and the problems are enhanced in fabric reinforced blocks. Even with these reservations and complications, the results obtained in the present investigation fall into a clearly identified pattern and the results obtained are readily applicable in the composites industry.

    Research areas

  • Thermoplastic composites, Mechanical properties, Thermosetting composites