Shear fibre interface

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

Furthermore, the absorption of water by the interphasal polymer can reduce its yield strength below the interfacial bond strength. Thus, the apparent interfacial shear strength will be reduced, and a yield front, rather than a debond, will propagate along the fibre interface modifying the stress transfer micromechanics at a fibre break. A consequence is that the stress concentrations in adj acent fibres to the fibre break will be reduced, and the probability of the formation of a flaw of critical dimensions is also reduced. The number of interacting fibre breaks associated with a flaw of critical dimensions will increase. 

The transverse and shear properties of a unidirectional composite are principally dependent upon the behaviour of the matrix and fibre interface. Consequently they can be orders of magnitude less than their longitudinal counterparts. For GRP the shear modulus and strength are typically 5GPa and 60 MPa and for ultra high modulus carbon fibre composite 4GPa and 50 MPa, respectively. 

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

If the matrix in 3.7 was reinforced with the same volume fraction of glass but in the form of randomly oriented glass fibres rather than continuous filaments, what would be the tensile strength of the composite. The fibres are 15 mm long, have an aspect ratio of 1000 and produce a reinforcement efficiency of 0.25. The fibre strength is 2 GN/m and the shear strength of the interface is 4 MN/m". 

If the fibre-matrbc interface is strong, the pull-out force increases as the loading rate increases or the temperature decreases, because of the increase in shear strength of the matrix. 

A strong fibre-matrbc interface, on the other hand, would set off this interpretation. As a matter of fact water content would reduce the shear strength of the matrix and thus fracture toughness. At any rate, a strong interface can become weak in the presence of water. 

This load is transmitted through the interface from the matrix to the fibre and, as originally described by Kelly and Tyson (2), the fibre breaks into small fragments until a limiting fragment size /c is reached (Figure 3). Knowledge of /c enables one to determine the shear resistance of the interface that is, the capacity of the interface to transfer the stress from the matrix to the fiber. This shear resistance of the interface, x, is therefore a direct measurement of fibre-matrix adhesion. 

Determination of Fibre-Matrix Adhesion. The average shear strength x and maximum shear strength Xmax at the fibre-matrix interface CZ, 22) are given by... 

Utracki and Fisa (1982) and Metzner (1985) review the rheology of (asymmetric) fibre-and flake-filled plastics, noting the importance of the filler-polymer interface, filler-filler interactions, filler concentration and filler-particle properties in determining rheological phenomena such as yield-stress, normal-stress and viscosity profiles (thixotropy and rheo-pexy, dilatancy and shear thinning). 

Assumptions must be made about the matrix and interface behaviour before Eq. (4.27) is integrated. If the matrix remains elastic and the interface does not fail, the shear stress rises to a maximum at the fibre ends, where the tensile strains in the fibre Cf and the matrix differ the most. Flowever, a ductile polypropylene matrix is assumed to yield in shear at a stress Ty = 20 MPa. This will occur near the fibre ends, so the interface shear stress is... 

Figure 4.30 Stress transfer to a single fibre in a thermoset matrix, under tension in the x direction. The graph shows the variation of the fibre tensile stress and the interfacial shear stress, when the interface yields at both ends of the fibre.

Some modes may dominate for example, for large bending strains in a flexible structure, fibre fracture in tension and fibre kinking in compression wdl dominate near both surfaces. Matrix cracks can cause delamination when they reach a ply interface. If the structure is stiff enough to resist with a significant force, then local indentation damage, and shear-driven delamination in the interior, wdl occur. Figure 9.2 shows schematically the different modes of fadure in three zones of a laminate. The peanut shape deformations (3) have this shape because the compression under the impact force suppresses the delaminations. 

The failure strain of a unidirectional fibre composite in the transverse direction is normally low because the matrix resins have relatively low failure strains while the large difference between the moduli of the components magnifies the strain in the matrix under stress. Thus, in an angle ply laminate, the first failure event occurs in the transverse ply or phes. Reloading of the transverse ply via shear stress transfer at the ply interfaces leads to multiple cracking before the fibres reach their failure... 

Three test methods, the fragmentation, pull-out and microbond methods, were used to analyse the micromechanics for carbon fibre/epoxy composites and Raman spectroscopy was used to determine the variation of fibre strain with position along a carbon fibre in a resin. It was demonstrated that the latter technique was capable of revolutionising interpretation of composite micromechanics and the different micromechanical test methods. In particular, it was shown that the technique could be used to distinguish between elastic deformation, interfacial debonding and shear yielding of the matrix at the interface. 19 refs. (FRC 94, Institute of Materials, Newcastle, March 1994)... 

The injection-moulded bar described in Example 6.9 is tested to failure in tension parallel to the axis of the bar. Predict the mean fibre stress when the bar fails, and the tensile strength of the bar. Take the tensile strengths of the carbon fibres and the nylon to be 3200 MPa and 70 MPa, respectively, and assume the shear strength of the carbon fibre-i lon interface to be 32 MPa. 

Figure 19.5 Variation of tensile stress in fibre and shear stress at interface occurring along the fibre length. If the fibre aspect ratio is lower than its critical value, l, the fibres are not loaded to their maximum stress value.

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