Shiqiang Deng

Evaluation of fibre tensile strength and fibre/matrix adhesion using single fibre fragmentation tests

Abstract Single fibre fragmentation tests were conducted to obtain a comprehensive understanding of the fibre fragmentation phenomenon in several carbon fibre/epoxy composite systems. Fibre tensile strength and fibre/matrix interfacial adhesion were evaluated, and some important experimental aspects associated with the evaluation of the test results were addressed. Fibre fragmentation in a matrix is a complex process with many factors interacting with each other, which influence the fibre fragmentation behaviour and, as a result, complicate the evaluation of the fibre/matrix interfacial properties. The failure modes of carbon fibres in epoxy resins were also greatly influenced by the variations of these factors. Without considering the special mechanisms of the fibre fragmentation behaviour for individual fibre/matrix systems, equivocal results may be obtained by simply adopting certain micromechanics model to define interfacial parameters.

Influence of fibre cross-sectional aspect ratio on mechanical properties of glass fibre/epoxy composites. I. Tensile and flexure behaviour

Abstract A comprehensive experimental study was conducted to identify the effects of fibre cross-sectional aspect ratio on tensile and flexural properties and the failure modes of glass-fibre/epoxy composites by using fibres of three different cross-sectional shapes (round, peanut-shaped and oval). It was found that the fibres of peanut and oval cross-sectional shapes tend to align with the long axis of the cross section perpendicular to the direction of the applied pressure or in the plane of a composite laminate. As a result, many fibres overlapped each other, having large contact areas which act as a path for longitudinal crack propagation. For composites with fibres of large cross-sectional aspect ratios, a cumulative damage progression with a high failure strain was observed in tensile and flexure tests in the longitudinal direction. However, the longitudinal tensile modulus and strength were nearly the same for the three composite systems. The transverse tensile strength and strain-to-failure results were similar to those for longitudinal tension, but the transverse tensile modulus was reduced for composites with fibres of large aspect ratios. These results indicate the fibre cross-sectional aspect ratio may not play an overriding role in the structure-property relationship for fibre-reinforced composites. However, the progressive failure process associated with fibres of large aspect ratios may enhance the toughness of composites.

Influence of fibre cross-sectional aspect ratio on mechanical properties of glass-fibre/epoxy composites II. Interlaminar fracture and impact behaviour

A systematic experimental investigation has been conducted on glass-fibre/epoxy composites with fibres of different cross-sectional shapes (round, peanut-shaped and oval) in order to evaluate the influence of the fibre cross-sectional aspect ratio on Mode I and Mode II interlaminar fracture toughness, interlaminar shear strength, and Charpy impact properties. Because of large fibre contact areas induced by fibre overlapping, a low delamination resistance was observed for such composite systems reinforced by glass fibres with large fibre cross-sectional aspect ratios, compared with a composite system reinforced by conventional round fibres. Test results from double-cantilever beam (DCB), end-notched-flexure (ENF) and short-beam-shear (SBS) tests showed the same trend that the resistance to delamination decreased with the increase of fibre cross-sectional aspect ratios. However, the composite system reinforced by glass fibres with the large cross-sectional aspect ratio exhibited better energy-absorbing capacity than the composite system reinforced by conventional round fibres.

Measurement of interfacial shear strength of carbon fibre/epoxy composites using a single fibre pull-out test

Single fiber pull-out tests were conducted for G34-700/ Araldite-F, T700S/Araldite-F and T800H/Araldite-F model composites using an improved experimental set-up to evaluate the interfacial shear strength (IFSS). The modified method made it possible to reduce the formation of resin meniscus and to accurately define the embedded fibre length, and it also provided a practical approach to conveniently estimate the IFSS for comparison of the fibre-matrix adhesion between different composite systems or different fibre surface conditions. A strong interfacial adhesion and unstable debonding process without detectable friction stages were found for all types of model composites with fibres being chemically treated/sized or sizing being removed. The electrochemical oxidation treatment and sizing on the carbon fibres noticeably increase the IFSS of the composites.

Increasing load-bearing capacity of wood-plastic composites by sandwiching natural and glass fabrics:

Aimed to increase the load-bearing capacity of wood-plastic composites (WPC), sisal and glass fiber fabrics were incorporated into wood—high density polyethylene (HDPE) composites by sandwiching them inside the composites during the compression molding of extruded WPC pellets. Experimental results showed that the inclusion of long fibers in less than 20 wt% in the form of fabrics, particularly glass fabrics (13—20 wt%), could effectively enhance the mechanical performance of the wood—HDPE composites with significantly increased strength and modulus as well as the improved creep resistance, compared with their unreinforced counterparts. The reinforced WPC preserved their wood-like appearances, but they have much higher strength and creep resistance. Particularly, the WPC samples reinforced by two layers of glass fabrics could have strengths that were comparable to that of solid wood in the parallel-to-grain direction but much better mechanical performance than that of wood in the perpendicular-to-grain direction. Surface treatments to the fiber fabrics using a silane coupling agent and the incorporation of maleic anhydride-grafted polyethylene into the WPC pellets during the compounding process could improve the interfacial compatibility between the reinforcing fabrics and the HDPE matrix and, thus, resulted in additional increases in the mechanical performance of the composites. On the basis of the experimental results and the simplicity of the processing methods adopted in this study, the approach, which sandwiched natural or inorganic fiber fabrics into WPC, provided a feasible method to effectively improve the load-bearing capability of WPC products, which may allow the reinforced WPC to be potentially used in certain structural applications to replace solid wood.Aimed to increase the load-bearing capacity of wood-plastic composites (WPC), sisal and glass fiber fabrics were incorporated into wood—high density polyethylene (HDPE) composites by sandwiching them inside the composites during the compression molding of extruded WPC pellets. Experimental results showed that the inclusion of long fibers in less than 20 wt% in the form of fabrics, particularly glass fabrics (13—20 wt%), could effectively enhance the mechanical performance of the wood—HDPE composites with significantly increased strength and modulus as well as the improved creep resistance, compared with their unreinforced counterparts. The reinforced WPC preserved their wood-like appearances, but they have much higher strength and creep resistance. Particularly, the WPC samples reinforced by two layers of glass fabrics could have strengths that were comparable to that of solid wood in the parallel-to-grain direction but much better mechanical performance than that of wood in the perpendicular-to-grain dir...

Kenaf–polypropylene composites manufactured from blended fiber mats

Experimental investigations were conducted on kenaf–polypropylene composites manufactured from preformed mats consisting of kenaf and polypropylene fibers in a ratio of 1 : 1 in weight to explore the prospects of annual natural plant fibers as reinforcements for polymer–matrix composites. Surface treatments to the comingled kenaf–polypropylene fiber mats were first carried out using different coupling agents to increase the compatibility of the natural fiber and the polypropylene matrix. Kenaf–polypropylene composite laminates were fabricated from the mats via compression molding. Experimental results indicated that, among the coupling agents chosen, maleated polypropylene was the most effective coupling agent to significantly improve the mechanical performance of the composites. Analyses using Fourier transform infrared spectrometer and scanning electron microscopy revealed that maleated polypropylene was able to promote strong adhesion between the kenaf fibers and the polypropylene matrix at their interfaces. The outcomes of the study indicate that the use of the preformed fiber mats in combination with fiber surface modifications makes it possible to efficiently produce natural fiber reinforced composites with excellent mechanical properties.

Determination of CTEs of cured epoxy and polyester (PET) stitching threads

Coefficients of thermal expansion (CTEs) of constituents in fiber-reinforced composites are among the key thermal parameters that need to be defined for material selection, modeling and structural design. This article presents an experimental approach for determining the CTEs of two constituents in a non-crimp fabrics (NCFs) reinforced composite system by means of dynamic mechanical analysis (DMA), mechanical tests at various temperatures and the principle of composite micromechanics. The constituents include the matrix (a cured epoxy resin) and the polyester (PET) stitching threads that are used to assemble multi-layered reinforcements comprising NCFs. CTEs of the cured epoxy resin were measured by both DMA and thermal mechanical analysis, and CTEs of PET threads were successfully estimated from those of the epoxy resin and a PET/epoxy composite in both transverse and longitudinal directions by means of composite micromechanics. DMA was found, in this study, to be an effective technique that can be used to determine the CTEs of polymers and polymer threads in a wide range of temperatures on the basis of proper calibration.Coefficients of thermal expansion (CTEs) of constituents in fiber-reinforced composites are among the key thermal parameters that need to be defined for material selection, modeling and structural ...