Stresses & strains

A stress analysis is required to determine the strength of a laminate for which there is no experimental test data. Simple closed form analysis techniques have been developed for the rapid evaluation of alternate laminates at the preliminary design stage. There are several levels on which the stresses and strains in a structural laminate can be assessed:

• Stresses and strains on a laminate level
• Stresses and strains on a ply/lamina level
• Stresses at the fibre / matrix level

Micromechanics is the study of the relations between the properties of the individual constituents of a composite and the effective properties of the composite. Stresses at the fibre / matrix level are a useful approach to the determination of the strength of a unidirectional composite or ply. This is often used at the preliminary stages of design when experimental data is not available. Laminated plate theory (LPT) is an analysis technique that examines stresses and strains on a ply/lamina level in a given multi-directional laminate. In situations where closed form solutions are not available to model the stresses and strains in composite structures, finite element analysis (FEA) can be used. FEA is usually carried out on a laminate level or on a ply/lamina level.

Important assumptions are made for characterising lamina properties:

• Material homogeneity on a macroscopic scale. The analysis of composite materials can then use effective properties which are based on the average stress and average strain.
• Material orthotropy. This allows lamina to be characterised by four independent elastic constants.
• Material linearity. Some composite material properties are non-linear. However, the stress-strain curves for composite materials are generally assumed to be linear to simplify the analysis.
• Residual stresses. Residual strains are present in the lamina after curing. The corresponding residual stresses are often assumed not to affect the material's stiffness or its ability to strain uniformly.

These assumptions should be kept in mind when selecting and applying failure criteria.