Effects of Orientation on Strength

When the composite is loaded parallel to the fibres, the fibres carry the majority of the load - if failure of the fibres is responsible for failure of the composite then we know the failure strength of the composite parallel to the fibres, scx; it was calculated in the previous sections.





When the composite fails, the composite will crack perpendicular to the fibre axis and as the angle between the fibre and the applied stress increases, the applied stress must be larger in order that the stress resolved parallel to the fibres reach the failure strength of the composite measured parallel to the fibres.

However, as the angle between the applied stress and the fibre increases, we should also think about the shear stress acting parallel to the fibres and the fibre matrix interface. When the misorientation is 0, the shear stress in the matrix is small because the load being carried by the matrix is small - remember most of the applied force is being carried in the fibres - however, as the fibres are rotated, the shear stress in the matrix becomes significant and the matrix will yield in shear.





Finally, as the angle between the fibre and the applied stress approaches 90°, the stress acting on the fibre and matrix tend to the same value and the composite will fail at the lesser of the fibre failure stress, the matrix failure stress or the fibre matrix interfacial strength.




If we now plot out each of the three equations for the applied stress as a function of misorientation and draw a line which represents the minimum of the three equations - after all failure will occur by which ever of the three mechanisms occurs at the lowest stress - then we can see how the fracture stress varies with orientation.