Crack composite

Zhang, J., J. Fan and C. Soutis. 1994. Strain energy release rate associated with local delamination in cracked composite laminates . Composites, 25(9) 851-862. 

Bya] Metallography, SEM, EPMA, XRD, diflfiuctometry, impression tests using Palmqvist cracks Composition structure fracture toughness microhardness... 

A comprehensive approach to model the stress transfer requires simultaneous treatment of all the above-mentioned effects elastic shear transfer, frictional slip, debonding, and normal stresses and strains. Unfortunately, such a unified approach is complex. Therefore, in this chapter, each of these effects will first be discussed separately, based on models developed for fibres of a simple shape, usually straight fibres with a circular cross section. The stress transfer in uncracked and in cracked composites will also be dealt with separately. 

Kelly and Zweben [20] indicated that although Eq. 3.20 may predict a compressive normal stress due to the Poisson effect, this may not occur in some special cases, in which fibres are being pulled out of a matrix (i.e. a pull-out test, or fibres bridging across a crack in the composite). In these instances, the tensile strain in the fibre as it enters the matrix is high, while that of the matrix is low. At the cracked composite surface (or at the matrix surface in the pull-out test) the matrix is practically stress-free. Therefore, in these regions the normal stress across the... 

The analytical treatment of this problem is different for uncracked and cracked composites. The first case is applicable to the contribution of the fibre to the modulus of elasticity the second is more relevant to strength. 

Once first cracking has taken place in the brittle matrix, the fibres serve to inhibit unstable crack propagation. At this stage, the cracking patterns are complex, with discontinuous micro-cracks present ahead of the principal crack. This can be deduced from the various analytical models [28-30] and has also been observed microscopically (Figure 4.15) [31]. Thus, in the cracked composite, it is difficult to define the true crack tip. The simplistic definition of a traction-free crack (as assumed in LEFM) is not applicable to FRC. Stress is transferred across the crack by a variety of mechanisms, as can be seen from the idealization of a crack proposed by Wecharatana and Shah [32], in Figure 4.16. Three distinct zones can be identified ... 

Figure 4.40 Strain distribution in the matrix and fibres in multiple cracked composite with 2x crack spacing during loading (a) and after unloading (b) (after Keer [73]).

---