Ian J. Davies

Fabrication and Properties of Recycled Cellulose Fibre-Reinforced Epoxy Composites

Epoxy matrix composites reinforced with recycled cellulose fibre (RCF) were fabricated and characterized with respect to their flexural and impact properties. Reinforcement of the epoxy by RCF resulted in a significant increase in the strain at failure, fracture toughness and impact toughness but only a moderate increase in flexural strength and flexural modulus. The effect of accelerated exposure to seawater on the flexural and impact properties was also investigated. The salient toughening mechanisms and crack-tip failure processes were identified and discussed in light of observed microstructures, in particular the orientation of RCF sheets to the applied load.

Microstructural investigation of low-density carbon-carbon composites

The microstructure of low-density (0.13–0.64 Mg m−3) carbon-carbon composites was investigated using optical microscopy, scanning electron microscopy and image analysis. All samples initially contained varying proportions of rayon precursor carbon fibres, recycled fibrous material and phenolic resin precursor matrix, and were manufactured utilizing a vacuum moulding technique. Some of the composites were densified using the chemical vapour deposition (CVD) of pyrolytic carbon. All the composites were shown to have a two-dimensional planar random microstructure, with a distinct layering effect being seen on the microscopic (and sometimes macroscopic) level. The degree of layering in the composites was quantified utilizing image analysis and was found to be most pronounced in samples containing no recycled material, and least pronounced in samples containing all of its fibrous constituent as recycled material. The composites were found to be very porous, the pores consisting of mainly interconnecting open pores (typically 65–85% of the sample volume). In non-CVD samples the fibrous material was held together by thin (<5 μm) discrete “matrix bonds”, with a few large (typically 100 μm×200 μm×800μm) fibre bundles also existing within the structure. In the CVD-processed material the deposit coat on the fibres was of even thickness throughout the composite and joined together fibrous material that was not previously in contact.

Flexural Properties of E Glass and TR50S Carbon Fiber Reinforced Epoxy Hybrid Composites

A study on the flexural properties of E glass and TR50S carbon fiber reinforced hybrid composites is presented in this paper. Specimens were made by the hand lay-up process in an intra-ply configuration with varying degrees of glass fibers added to the surface of a carbon laminate. These specimens were then tested in the three-point bend configuration in accordance with ASTM D790-07 at three span-to-depth ratios: 16, 32, and 64. The failure modes were examined under an optical microscope. The flexural behavior was also simulated by finite element analysis, and the flexural modulus, flexural strength, and strain to failure were calculated. It is shown that although span-to-depth ratio shows an influence on the stress-strain relationship, it has no effect on the failure mode. The majority of specimens failed by either in-plane or out-of-plane local buckling followed by kinking and splitting at the compressive GFRP side and matrix cracking combined with fiber breakage at the CFRP tensile face. It is shown that positive hybrid effects exist for the flexural strengths of most of the hybrid configurations. The hybrid effect is noted to be more obvious when the hybrid ratio is small, which may be attributed to the relative position of the GFRP layer(s) with respect to the neutral plane. In contrast to this, flexural modulus seems to obey the rule of mixtures equation.

Mechanical properties in compression of CVI-densified porous carbon/carbon composite

Compressive properties of a porous 2-D planar-random carbon/carbon (C/C) composite preform densified by chemical vapour infiltration (CVI) of pyrolytic carbon were determined and compared to those obtained previously for similar C/C composite densified using a non-CVI technique. Stress/strain curves and failure modes of the CVI composite were similar to those seen in the non-CVI C/C composite. For specimens of equivalent density, the CVI technique resulted in improved in-plane properties, but inferior out-of-plane properties, when compared to the non-CVI composite. Compressive property anisotropy ratios for the CVI composite were slightly reduced compared to the uninfiltrated preform state whilst comparison between compressive and (previously determined) flexural properties showed general agreement. A microstructural model of the composite developed by the authors was used to explain the results.

Flexural properties of a hybrid polymer matrix composite containing carbon and silicon carbide fibres

The flexural properties of hybrid unidirectional fibre reinforced polymer (FRP) composites containing a mixture of carbon (C) and silicon carbide (SiC) fibres were evaluated at span-to-depth (S/d) ratios of 16, 32, and 64. The flexural strength generally increased with increasing S/d ratio with a maximum value of 2316 MPa being achieved for the specimen with nominally equal volume fractions of C and SiC fibre. However, even replacing 12.5 vol% of the C fibres by SiC fibres increased the flexural strength by 22%. The mechanical property most strongly influenced by the incorporation of SiC fibres was the work of fracture with a maximum value of 206.5 kJ m-2 (compared to 78.8 kJ m-2 for the specimen containing only C fibres). First estimate values for the SiC fibre compressive strength, elastic modulus, and strain to failure were 3.46 GPa, 157 GPa, and 0.018, respectively.

Fibre and interfacial properties measured in situ for a 3D woven SiC/SiC-based composite with glass sealant

Abstract In situ fibre strength Weibull parameters and fibre pullout length distributions were investigated for a 3D woven SiC/SiC-based composite with glass sealant oxidation protection system after tensile testing at 1000°C and 1200°C in air. Results were similar to those previously observed for unsealed specimens tested in vacuum (1200–1380°C), but significantly different from the case of unsealed specimens tested in air (1100–1200°C). Overall, it was concluded that the glass sealant provided excellent oxidation protection for the composite at the test conditions employed, i.e. short-term exposure up to 1200°C in air.

Fibre strength parameters measured in situ for ceramic-matrix composites tested at elevated temperature in vacuum and in air

Abstract In situ fibre fracture characteristics have been investigated for Si–Ti–C–O fibres after tensile testing up to 1380°C in vacuum and in air. Specimens tested in air at 1100 and 1200°C generally had flat fracture surfaces with less than 20% of fibres exhibiting fracture mirrors: this is attributed to oxygen ingress into the fibre bundles. Fibre strength characteristics normalised to a 10−3 m gauge length indicated that fibres tested in air at elevated temperature have significantly lower strengths and average Weibull parameter, m, compared to the room-temperature, 1200 and 1300°C/vacuum cases, and this is attributed to oxygen damage of the fibre together with oxidation of the fibre/matrix interface. The fibre/matrix interface shear strength, τ, was low for the room-temperature specimens and increased slightly with temperature when tested in vacuum, possibly as a result of a change in the thermal mismatch between fibres and matrix. Values of τ for specimens tested at 1100 and 1200°C in air were an order of magnitude greater than those for room-temperature specimens, indicating a significant degree of oxidation damage at the fibre/matrix interface to have occurred.

Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre

Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length ∼0.5 mm) and 0–10 mol% of Al4C3as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al4C3to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al4C3addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al4C3addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al4C3: 5 mol%) was 3.2–3.4 MPa · m1/2and approximately 1.5 times higher than that (2.39 MPa · m1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al4C3and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO2layer at and near exposed surfaces.

Flexural Failure of Unidirectional Hybrid Fibre-Reinforced Polymer (FRP) Composites Containing Different Grades of Glass Fibre

The flexural behaviour of 6-ply unidirectional hybrid fibre-reinforced polymer (FRP) matrix composites containing a mixture of E-glass and S2-glass fibres was investigated. A high performance epoxy system comprising of Kinetix® R240 epoxy resin (combined with Kinetix® H160 epoxy hardener) was utilised for the composite matrix. Flexural testing was conducted in accordance with Procedure A of the ASTM D790-03 test standard on a universal testing machine equipped with a three-point bend test rig. In addition to varying the stacking configurations of the composite prepregs, the influence of span-to-depth ratio on the flexural properties and failure mechanisms was also studied. The failure mechanisms of the resulting fractured specimens were characterised using optical microscopy and compared with those noted by the authors in previous work.

Tensile creep behavior of 3-D woven Si-Ti-C-O fiber/SiC-based matrix composite with glass sealant

The present work investigates the tensile creep behavior (deformation and rupture) at 1100–1300°C in air of a 3-D woven Si-Ti-C-O (Tyranno™) fiber/SiC-based matrix composite with and without glass sealant. The composite contained Si-Ti-C-O fibers with an additional surface modification in order to improve interface properties. Although a significant decrease in tensile strength was observed in the unsealed composite beyond 1000°C in air (and attributed to oxidation of the fiber/matrix interface), the composite with glass sealant possessed excellent mechanical properties for short-term (<1 hr.) exposure in air. In this study, tensile creep testing was conducted at 1100–1300°C in air and the effect of glass sealant on medium- and long-term strength was investigated. In addition, chemical stability of the glass sealant was evaluated by X-ray diffraction analysis (XRD) and energy dispersive X-ray spectrometer (EDS). The creep rupture behavior of the composite with glass sealant under long-term exposure is suggested to depend on several factors including decomposition, evaporation, and crystallization of the glass sealant material, in addition to the applied stress.