4

Review of Laminate Strength and Failure Criteria

If the loads applied to a laminate are sufficiently high then the strength of the material is exceeded and the laminate fails. It is, therefore, very important to be able to use the stresses and/or strains calculated in the previous chapter to predict failure. This, however, is complicated by the fact that final failure of a laminate does not always coincide with the onset of damage. Depending on the laminate lay-up and loading, damage may start at a load significantly lower than the load at which final failure occurs. Being able to predict when damage starts and how it evolves requires individual modelling of the matrix and fibres. Usually, damage starts in the form of matrix cracks between fibres in plies transverse to the primary load direction. As the load increases the crack density increases and the cracks may coalesce into delaminations (where plies locally separate from one another) or branch out to adjacent plies [1]. In addition, local stress concentrations may lead to failure of the fibre–matrix interphase. Further increase of the load accumulates this type of damage and causes some fibres to fail until the laminate can no longer sustain the applied load and fails catastrophically. The detailed analysis of damage creation and evolution accounting for the individual constituents of a ply is the subject of micromechanics [2, 3].

In an alternative simplified approach, each ply is modelled as homogeneous, having specific ...

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