Thesis (Ph.D.)--Australian National University, 2001.

No description available.

Thermoplastic composite filament winding is an on-line consolidation process, where the composite experiences a complex temperature history and undergoes a number of temperature history affected microstructural changes that influence the structure's subsequent properties. These changes include melting, crystallization, void formation, degradation and consolidation. In the present study, models of the thermoplastic filament winding process were developed to identify and understand the relationships between process variables and the structure quality. These include models that describe the heat transfer, consolidation and crystallization processes that occur during fabrication of a filament wound composites structure. A comprehensive thermal model of the thermoplastic filament winding process was developed to calculate the temperature profiles in the composite substrate and the towpreg temperature before entering the nippoint. A two-dimensional finite element heat transfer analysis for the composite-mandrel assembly was formulated in the polar coordinate system, which facilitates the description of the geometry and the boundary conditions. A four-node 'sector element' was used to describe the domain of interest. Sector elements were selected to give a better representation of the curved boundary shape which should improve accuracy with fewer elements compared to a finite element solution in the Cartesian-coordinate system. Hence the computational cost will be reduced. The second thermal analysis was a two-dimensional, Cartesian coordinate, finite element model of the towpreg as it enters the nippoint. The results show that the calculated temperature distribution in the composite substrate compared well with temperature data measured during winding and consolidation. The analysis also agrees with the experimental observation that the melt region is formed on the surface of the incoming towpreg in the nippoint and not on the substrate. Incorporated with the heat transfer analysis were the consolidation and crystallization models. These models were used to calculate the degree of interply bonding and the crystallinity achieved during composite manufacture. Bonding and crystallinity developments during the winding process were investigated using the model. It is concluded that lower winding speed, higher hot-air heater nozzle temperature, and higher substrate preheating temperature yield higher nippoint temperature, better consolidation and a higher degree of crystallization. Complete consolidation and higher matrix crystallization will result in higher interlaminar strength of the wound composite structure. / Master of Science

No description available.

No description available.

Thermoplastic composites using natural fibres are studied intensively and widely used in applications including automotive, packaging, consumer goods and construction. Good balance of mechanical properties, processability and low cost are great advantages of these materials on top of the environmental benefits. Recently, there have been various efforts to amplify the positive effects on the environment by replacing the conventional polymers by bio-derived renewable polymers in the composites. Recent studies conducted from our research group showed competitiveness of plant fibre-thermoplastic composites. Implementing the promising results and experience, a new composite design using renewable polyethylene as the matrix material was studied. This polyethylene is a renewable thermoplastic that was derived from sugar cane ethanol. The objectives of this study were to employ renewable high density polyethylene (HDPE) into composites using wheat straw and flax fibre to extend the range of properties of the HDPE while keeping the amount of renewable content to nearly 100%. The chemical resistance of these materials has not been reported before and it was investigated here by measuring and comparing the properties before and after accelerated chemical ageing. Both wheat straw and flax fibre had two different grades in size. Each of them was compounded with HDPE and additives (antioxidant and coupling agent) in a co-rotating twin screw extruder. The concentrations of fibres were varied from 0 to 30 wt-%. Then, injection molded samples were prepared for measurement of properties: tensile, flexural, impact tests. The effects of reinforcing fibre size were studied first. Both length and aspect ratio were considered. For both types of fibre composites, a general trend was observed. There was no clear evidence of improvements in flexural (strength and modulus) and tensile (strength, percentage elongation at break) properties with respect to the change in fibre size. However, impact (IZOD impact strength, Gardner impact failure energy) properties showed some improvements. This result was due to no substantial difference in size and aspect ratios in post-processed fibres that were actually residing in the matrix. There were remarkable improvements in flexural strength and modulus when the fibre content increased. However, minor decreases in tensile properties were observed. Furthermore, the impact properties were very sensitive to the concentration of fibres. As the fibre concentration went up, there were significant decreases in both IZOD impact strength and Gardner impact failure energy. Chemical resistance of these composites was studied by exposing them in six different chemical solutions (hydrochloric acid, acetic acid, sodium hydroxide, ethyl alcohol, industrial detergent, water) for up to thirty days. The increase in weight and leaching behaviour was observed. As the fibre content increased within the composites, the weight gain was more rapid during chemical ageing. Because there were more fibres exposed on the surface after chemical ageing, it is likely that they contributed to the higher flux of liquids (used for chemical ageing) inside the sample. Among the physical properties, tensile properties were most susceptible to the chemical ageing. One possible reason could be due to the exposed surface area to volume ratio, which was the highest in tensile bars and therefore faster mass transfer taking place into the matrix per volume. Finally, morphological study using scanned electron spectroscopy (SEM) revealed the damage on the surface when exposed to the chemicals. The fibres on the surface had been leached out in the sodium hydroxide solution leaving empty spaces. The fractured surface was also monitored via SEM. Though there was not enough evidence of strong interfacial interactions between the fibre and the polymer, good dispersions were observed.

Results from a research program to investigate the long term effects of through-the-thickness fastener clamp-up force (preload) relaxation on the strength of mechanically fastened joints for two graphite/thermoplastic composite materials (Dupont's IM6/KIII and ICI-Fiberite's IM8/APC(HTA)) are summarized and compared with analytical methods. An experimental program was conducted in which 56 mechanically fastened single-shear joints were tested. Phase I static tests established joint bearing strength as a function of clamp-up force for two types of fasteners (protruding head and countersink) with no relaxation of preload. Phase II testing monitored short-term fastener preload relaxation (up to 1 ,000 hours), with special bolt force sensor washers. Inservice parameters included were temperature, in-plane loads, and torque. The jOints were tested to failure at the end of the relaxation time period to determine any subsequent effect on joint strength. Phase I test results indicated that joint bearing strength increased by as much as twenty-eight percent over the clamp-up force range of a Ibs (fingertight) to 3,500 Ibs for both materials. Fastener head type, material, and temperature also affected the resultant bearing strength. For Phase II, fastener clamp-up force at room temperature (78°F) relaxed an average of six percent from the initial value during the short-term test period. The relaxation was projected to be as high as fourteen and sixteen percent at 100,000 hours for HTA and Kill, respectively. The elevated temperature condition (250°F) significantly increased the relaxation rate with the projected 100,000 hour relaxation amount being as high as thirty-seven percent for HTA and sixty percent for Kill. Comparison of the Phase II bearing strengths to the Phase I results indicated that portions of the data correlated well, while others did not. It was concluded that relaxation of the clamp-up force over the short-term time period did not significantly lower the bearing strength of either material, however an extended exposure to 250°F could affect the bearing strength. / Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.

This project is aimed at developing and demonstrating highly conductive, lightweight, and low-cost thermoplastic blends to be used as flow field bipolar plates for polymer electrolyte membrane (PEM) fuel cells. <br><br> The research is focused on designing, prototyping, and testing carbon-filled thermoplastic composites with high electrical conductivity, as well as suitable mechanical and process properties. <br><br> The impact of different types of fillers on the composite blend properties was evaluated, as well as the synergetic effect of mixtures of fill types within a thermoplastic polymer matrix. A number of blends were produced by varying the filler percentages. Composites with loadings up to 65% by weight of graphite, conductive carbon black, and carbon fibers were investigated. Research results show that three-filler composites exhibit better performance than single or two-filler composites. <br><br> Injection and compression molding of the conductive carbon filled polypropylene blend was used to fabricate the bipolar plates. A Thermal Gravimetric Analysis (TGA) was used to determine the actual filler loading of composites. A Scanning Electron Microscope (SEM) technique was use as an effective way to view the microstructure of composite for properties such as edge effects, porosity, and fiber alignment. Density and mechanical properties of conductive thermoplastic composites were also investigated. During this study, it was found that 1:1:1 SG-4012/VCB/CF composites showed better performance than other blends. The highest conductivity, 1900 S/m in in-plane and 156 S/m in through plane conductivity, is obtained with the 65% composite. Mechanical properties such as tensile modulus, tensile strength, flexural modulus and flexural strength for 65% 1:1:1 SG-4012/VCB/CF composite were found to be 584. 3 MPa, 9. 50 MPa, 6. 82 GPa and 47. 7 MPa, respectively, and these mechanical properties were found to meet minimum mechanical property requirements for bipolar plates. The highest density for bipolar plate developed in this project is 1. 33 g/cm³ and is far less than that of graphite bipolar plate. <br><br> A novel technique for metal insert bipolar plate construction was also developed for this project. With a copper sheet insert, the in-plane conductivity of bipolar plate was found to be significantly improved. The performance of composite and copper sheet insert bipolar plates was investigated in a single cell fuel cell. All the composites bipolar plates showed lower performance than the graphite bipolar plate on current-voltage (I-V) polarization curve testing. Although the copper sheet insert bipolar plates were very conductive in in-plane conductivity, there was little improvement in single cell performance compared with the composite bipolar plates. <br><br> This work also investigated the factors affecting bipolar plate resistance measurement, which is important for fuel cell bipolar plate design and material selection. Bipolar plate surface area (S) and surface area over thickness (S/T) ratio was showed to have significant effects on the significance of interfacial contact resistances. At high S/T ratio, the contact resistance was found to be most significant for thermoplastic blends. Other factors such as thickness, material properties, surface geometry and clamping pressure were also found to affect the bipolar plate resistance measurements significantly.

This project is aimed at developing and demonstrating highly conductive, lightweight, and low-cost thermoplastic blends to be used as flow field bipolar plates for polymer electrolyte membrane (PEM) fuel cells. <br><br> The research is focused on designing, prototyping, and testing carbon-filled thermoplastic composites with high electrical conductivity, as well as suitable mechanical and process properties. <br><br> The impact of different types of fillers on the composite blend properties was evaluated, as well as the synergetic effect of mixtures of fill types within a thermoplastic polymer matrix. A number of blends were produced by varying the filler percentages. Composites with loadings up to 65% by weight of graphite, conductive carbon black, and carbon fibers were investigated. Research results show that three-filler composites exhibit better performance than single or two-filler composites. <br><br> Injection and compression molding of the conductive carbon filled polypropylene blend was used to fabricate the bipolar plates. A Thermal Gravimetric Analysis (TGA) was used to determine the actual filler loading of composites. A Scanning Electron Microscope (SEM) technique was use as an effective way to view the microstructure of composite for properties such as edge effects, porosity, and fiber alignment. Density and mechanical properties of conductive thermoplastic composites were also investigated. During this study, it was found that 1:1:1 SG-4012/VCB/CF composites showed better performance than other blends. The highest conductivity, 1900 S/m in in-plane and 156 S/m in through plane conductivity, is obtained with the 65% composite. Mechanical properties such as tensile modulus, tensile strength, flexural modulus and flexural strength for 65% 1:1:1 SG-4012/VCB/CF composite were found to be 584. 3 MPa, 9. 50 MPa, 6. 82 GPa and 47. 7 MPa, respectively, and these mechanical properties were found to meet minimum mechanical property requirements for bipolar plates. The highest density for bipolar plate developed in this project is 1. 33 g/cm³ and is far less than that of graphite bipolar plate. <br><br> A novel technique for metal insert bipolar plate construction was also developed for this project. With a copper sheet insert, the in-plane conductivity of bipolar plate was found to be significantly improved. The performance of composite and copper sheet insert bipolar plates was investigated in a single cell fuel cell. All the composites bipolar plates showed lower performance than the graphite bipolar plate on current-voltage (I-V) polarization curve testing. Although the copper sheet insert bipolar plates were very conductive in in-plane conductivity, there was little improvement in single cell performance compared with the composite bipolar plates. <br><br> This work also investigated the factors affecting bipolar plate resistance measurement, which is important for fuel cell bipolar plate design and material selection. Bipolar plate surface area (S) and surface area over thickness (S/T) ratio was showed to have significant effects on the significance of interfacial contact resistances. At high S/T ratio, the contact resistance was found to be most significant for thermoplastic blends. Other factors such as thickness, material properties, surface geometry and clamping pressure were also found to affect the bipolar plate resistance measurements significantly.

Thermoplastic composites using natural fibres are studied intensively and widely used in applications including automotive, packaging, consumer goods and construction. Good balance of mechanical properties, processability and low cost are great advantages of these materials on top of the environmental benefits. Recently, there have been various efforts to amplify the positive effects on the environment by replacing the conventional polymers by bio-derived renewable polymers in the composites. Recent studies conducted from our research group showed competitiveness of plant fibre-thermoplastic composites. Implementing the promising results and experience, a new composite design using renewable polyethylene as the matrix material was studied. This polyethylene is a renewable thermoplastic that was derived from sugar cane ethanol. The objectives of this study were to employ renewable high density polyethylene (HDPE) into composites using wheat straw and flax fibre to extend the range of properties of the HDPE while keeping the amount of renewable content to nearly 100%. The chemical resistance of these materials has not been reported before and it was investigated here by measuring and comparing the properties before and after accelerated chemical ageing. Both wheat straw and flax fibre had two different grades in size. Each of them was compounded with HDPE and additives (antioxidant and coupling agent) in a co-rotating twin screw extruder. The concentrations of fibres were varied from 0 to 30 wt-%. Then, injection molded samples were prepared for measurement of properties: tensile, flexural, impact tests. The effects of reinforcing fibre size were studied first. Both length and aspect ratio were considered. For both types of fibre composites, a general trend was observed. There was no clear evidence of improvements in flexural (strength and modulus) and tensile (strength, percentage elongation at break) properties with respect to the change in fibre size. However, impact (IZOD impact strength, Gardner impact failure energy) properties showed some improvements. This result was due to no substantial difference in size and aspect ratios in post-processed fibres that were actually residing in the matrix. There were remarkable improvements in flexural strength and modulus when the fibre content increased. However, minor decreases in tensile properties were observed. Furthermore, the impact properties were very sensitive to the concentration of fibres. As the fibre concentration went up, there were significant decreases in both IZOD impact strength and Gardner impact failure energy. Chemical resistance of these composites was studied by exposing them in six different chemical solutions (hydrochloric acid, acetic acid, sodium hydroxide, ethyl alcohol, industrial detergent, water) for up to thirty days. The increase in weight and leaching behaviour was observed. As the fibre content increased within the composites, the weight gain was more rapid during chemical ageing. Because there were more fibres exposed on the surface after chemical ageing, it is likely that they contributed to the higher flux of liquids (used for chemical ageing) inside the sample. Among the physical properties, tensile properties were most susceptible to the chemical ageing. One possible reason could be due to the exposed surface area to volume ratio, which was the highest in tensile bars and therefore faster mass transfer taking place into the matrix per volume. Finally, morphological study using scanned electron spectroscopy (SEM) revealed the damage on the surface when exposed to the chemicals. The fibres on the surface had been leached out in the sodium hydroxide solution leaving empty spaces. The fractured surface was also monitored via SEM. Though there was not enough evidence of strong interfacial interactions between the fibre and the polymer, good dispersions were observed.