Tensile properties

Hamada and co-workers (1992) [134] investigated woven cloth E- glass/epoxy composite subjected to tensile loading. Gripped areas of the specimens were reinforced with aluminium tabs. They found that the tensile modulus of amino-silane coated glass fibre/epoxy samples was slightly higher than those of acryl-silane coated, but the tensile strength of the former was higher than that of the latter.

In 1999, Deng and colleagues [107] utilised three different cross-sectional geometries, and thus cross-sectional aspect ratios, i.e., circular, ‘peanut’-shaped, and oval, of glass fibre to produce glass fibre-reinforced epoxy composite plates.

Specimens were cut from unidirectional GFRP composite panels containing a particular fibre cross-sectional geometry produced using an autoclave at 0.6 MPa compressive pressure and 120 oC in vacuum atmosphere for 16 hours. To prevent damage in the gripped areas, cross ply CFRP composite tabs were glued onto this area. The fibre volume fraction, Vf, was relatively high and varied between (62.0  1.9)% to (69.8  0.4)%. Tensile testing in both the longitudinal and transverse directions were carried out according to ASTM D3039 [172]. They revealed that longitudinal tensile strength, L-t, and modulus, EL-t were not sensitive to fibre cross-sectional aspect ratio, whereas transverse tensile strength, T-t, and modulus,

ET-t, are sensitive to fibre cross-sectional aspect ratio with the ‘peanut’-shaped fibre cross-section FRP composites exhibiting the lowest T-t and ET-t. The circular fibre cross-section FRP composite specimens subjected to longitudinal tensile loading underwent catastrophic failure due to multiple fibre breakage and transverse matrix cracking, indicating strong fibre-matrix interfacial bonding, immediately after the linear load-displacement plot reached its peak load – the fracture surfaces indicated brittle failure. Such a failure mode has also been reported in other literatures [173, 174] resulting in fibre bundle pull-out [169] because the interfacial bond characteristics control the interlaminar shear and tensile strength [175]. The other two types of fibres experienced progressive fibre breakage and longitudinal matrix cracking producing splitting, as has been previously observed by Hayashi [176], because of poor structural integrity due to a lack of matrix surrounding the fibres induced by the applied compressive pressure during autoclave curing. Such a failure mechanism resulted in ‘broom-like’ failure. Different topographic views were observed on the failure surfaces on the specimens subjected to transverse tensile loading. The circular fibre cross-section FRP specimens exhibited flat fracture surfaces with some matrix stuck on the fibre indicating strong fibre-matrix interfacial bonding producing matrix cracking in the fibre longitudinal direction. Unlike the aforementioned FRP specimens, the other two types of specimens showed matrix channels on the fracture surfaces indicating debonding along the fibre-matrix interface.

Thomason [160] investigated the effect of fibre diameter and fibre aspect ratio on the properties of discontinuous E-glass fibre-reinforced polyamide 6,6

(PA6,6) produced by injection moulding. E-glass fibres of four different diameters, i.e., 10, 11, 14 and 17 m, were chopped into 3.8 mm lengths. Pre- dried PA6,6 pellets were dry-blended with chopped E-glass prior to being fed into an extruder for injection moulding. Thomason observed that increasing the fibre diameter resulted in slight decreases in elastic modulus, strength and strain to failure.

Wonderly et al. [83], in 2005, investigated the mechanical properties of knitted E-glass fabric/vinylester composites. Longitudinal tensile testing was carried out in accordance with ASTM D3039 [172] and open hole longitudinal tensile testing was according to ASTM D5766 standards, while ASTM C297 was employed for transverse tensile tests. All specimens contained end tabs of 45o glass fibre-reinforced vinyl ester composites glued onto their gripped areas, with ‘brooming’ being observed during failure, which represents longitudinal matrix cracking and possibly fibre-matrix debonding, failure similar to those reported by Deng and colleagues [107], with the average failure stress being presented in Table 2.11. They also found that the longitudinal tensile strength of the GFRP specimens, for both the solid and open hole cases, is lower than that of the CFRP specimens, whereas the transverse tensile strength of the GFRP specimens is higher than that of CFRP specimens. This phenomenon may be attributed to the fact that longitudinal properties are fibre-dominated [162] whilst transverse properties are matrix-dominated [177].

Thomason [159] recently reported that the tensile modulus of chopped glass fibre/polyamide 6.6 composite containing between 0-50 wt.% of glass fibre of various diameters, i.e., 10, 14 and 17 m (similar to the case mentioned earlier), is higher than its flexural modulus, with the fabrication and testing procedure being identical to that outlined in reference [160]. In addition to finding that the tensile modulus, which is fibre-dominated [162], is higher than the flexural modulus, Thomason also noted both properties to be more sensitive to fibre content in comparison to fibre length or fibre diameter. In addition, shorter fibre composites lost more of their elastic modulus with a decrease of matrix modulus compared to longer fibre composites. Thomason also observed that, for injection moulded thermoplastic matrix composites, fibre content can be increased by increasing

moulding die diameter while keeping fibre diameter constant, whilst the residual fibre length decreased, which led to a decrease in mechanical properties [170, 178], with increasing fibre content.

The discussion in the previous paragraphs can be summarised as follows. There are some factors that control the tensile properties of GFRP composites, i.e., fibre coating that leads to fibre-matrix interfacial characteristics [134], fibre diameter [159, 160], length and content [159]. Unlike the longitudinal tensile properties of GFRP composites, transverse tensile properties are sensitive to fibre cross-sectional aspect ratio [107]. Deng also pointed out that the longitudinal tensile failure mode is also influenced by fibre cross-sectional aspect ratio and he observed that GFRP specimens underwent catastrophic failure showing fibre breakage and transverse matrix cracking [107], whereas Wonderly [83] observed that longitudinally tensile loaded specimens failed by longitudinal matrix cracking leading to splitting. Inline with transverse tensile properties, the transverse tensile failure mode is also influenced by fibre cross-sectional aspect ratio and GFRP specimens were noted to transversely fail due to matrix failure in the fibre direction [107].