Carbon fibre failure mechanisms

One common characteristic of all C/SiC composites is their distinct anisotropy in the mechanical as well as thermophysical properties. Considerable lower values of the tensile strength and the strain to failure have to be considered for an appropriate design if the load direction and the fibre alignment are not congment. As the carbon fibres show a different physical behaviour in longitudinal and radial direction, the composite s properties like thermal conductivity and coefficient of thermal expansion differ widely with respect to the in-plane or transverse direction. 

Oya N, Hamada H, Mechanical properties and failure mechanisms of carbon fibre reinforced thermoplastic laminates. Composites Part A-Appl Sci Manuf 28(9-10), 823-832, 1997. 

Cantwell WJ, Zulkifli R, An investigation into mode-II failure mechanisms in carbon fibre reinforced PEEK, J Mater Sci Lett, 16(7), 509-511, 1997. 

Hine PJ, Brew B, Duckett RA, Ward IM, Failure mechanisms in continuous carbon fibre reinforced PEEK composites. Composites Sci Technol, 35, 31 51, 1989. 

Stewart M, Feughelman M, The failure mechanism of carbon fibres, J Mater Sci, 8, 1119-1122,... 

Selzer R, Friedrich K, Mechanical properties and failure behaviour of carbon fibre reinforced polymer composites under the influence of moisture. Composites Part A—Appl Sci Manuf 28(6), 595-604, 1997. 

To estimate the influence of layer thickness on the mechanical data of fibres such as tensile strength a-, Young s modulus E and strain to failure e, carbon fibre bundles of about 10 cm length were coated discontinuously. 

The TEM in its various forms is widely used in materials science, and some of its applications are studies of adhesion. It has helped to elucidate failure and bonding mechanisms in composites (see Fibre matrix adhesion - carbon fibres). An important use is in the examination of the structure of surface oxides. Several techniques have been employed. 

The lap-shear stress distribution, the failure pattern and ultimately the bond strength of FRP joints are also functions of the mechanical properties of the FRP reinforcing fibres. This behavioural dependency is depicted in Fig. 10.3 where lap-shear stress distributions along the bondlength for two identical double-strap CFRP/steel specimens, with different elastic moduli of their reinforcing CF (carbon fibres), are presented. 

Aveston eta/. [50] suggested that this is not necessarily always the case, and that the overall orientation efficiency would also depend on the response of the matrix to the local flexural stresses. If the matrix is sufficiently weak, it will crumble, and the flexural stresses will be effectively relaxed. They thought this to be the case in the carbon fibre reinforced cement tested in their work. Stucke and M umdar [52] applied this mechanism to account for the embrittlement of glass fibre reinforced cement and suggested that the densification of the ageing matrix around the fibres leads to a build-up of flexural stresses in the fibres in the cracked zone, which in turn results in premature failure. In the younger composite, the matrix interface is more porous and weaker, and crumbles before any significant flexural stress can develop in the fibres. 

The measurements of tensile strength Young s modulus E and strain to failure e of coated carbon and glass fibres were perform with monofilaments fixed in a clamping firame of 30 mm inner length. To get average values of the mechanical data at least SO measurements were used. 

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

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