Poisson effects 378 fibre stresses

The shear stresses developed parallel to the fibre-matrix interface are of prime importance in control ling the fibre-matrix stress-transfer mechanism, as discussed previously. Yet, one should also consider the effect of strains and stresses that develop normal to the fibre-matrix interface. Such strains and stresses may be the result of the Poisson effect, volume changes, and biaxial ortriaxial loading. They may cause weakening of the interface and premature debonding, and may also induce considerable variations in the resistance to frictional slip, which is sensitive to normal stresses. 

A detailed analysis of the Poisson effect alone was presented by Kelly and Zweben [20], For a simple system the normal stress across the interface can be described by Eq. 3,24, assuming that the entire composite undergoes an axial strain e, and using elastic relations for isotropic fibres and the matrix ... 

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... 

More detailed analysis of push-out data is possible considering the fibre expansion due to the effect of the Poisson ratio, contact length changes during progressive sliding of fibre out of the matrix or even residual stresses and thermal effects. The basic difference between pull-out and push-out techniques is in the influence of the Poisson ratio of the fibre whose influence of the stress distribution around the fibre has opposite signs (Zhu and Bartos 1997). 

---