Strength & failure

The key reason for evaluating the stresses in a laminate is to assess the conditions under which a laminate will fail. Initial laminate failure for design purposes may be defined as the failure of an individual ply in the laminate under uniform loading conditions. Many failure criteria are available including those based on the maximum stress in a ply and those based on the maximum strain in a ply. These criteria assess whether failure of a lamina in a fibre mode or a matrix mode is expected. The maximum stress criterion typically overestimates the strength of a lamina, because the interaction of stress components is neglected. The maximum strain criterion includes some stress interaction. Quadratic failure criteria such as the Tsai-Hill or Tsai-Wu take into account the interaction between stresses and strain parallel and transverse to the fibre direction. A fracture criterion estimates prosperity of a crack between the plies to grow.

To apply any failure criterion, basic mechanical properties of the unidirectional ply must be assessed:

	Uniaxial tension
	Uniaxial compression
	Transverse tension
	Transverse compression
	In-plane shear

Other forms of failure are possible, however, for the structural component. These include failure mechanisms from out-of-plane loading conditions. Delamination failure is an important mechanism that should be assessed during the design. Further the progressive nature of failure in composites should be assessed, to identify how one failure mode may lead to another leading to final catastrophic failure of the component.