Static and cyclic loading, impact, and environmental attack all contribute to the accumulation of damage in composite laminates. The damage can take many forms: delamination and splitting during load cycling, matrix cracking during thermal fatigue, and so on. With this diversity of damage mechanisms, it is no wonder that variability in static strength is significantly enhanced by service in the field. We recognise, therefore, that damage is progressive and is accompanied by a gradual deterioration in strength and stiffness of the laminate. In other words, static strength and life-time are part of the same design phenomenon. One way forward is to identify the broad rules governing fibre composite behaviour. There are two directions: continuum modelling and microscopic modelling. Continuum modelling is useful, but generally demands a formidable experimental programme to determine important design parameters. On a much smaller scale, microscopic modelling provides insight into the damaging mechanisms, but alone is too imprecise to be of much practical use to the design engineer. In parallel, however, they can give guidance towards the development of constitutive laws, the path of model-informed empiricism, which leads to predictive design. In other words, extension of basic damage models of composite failure to generic design features can lead to a formulation of design procedures for composite hardware; this is a powerful route to take.
Surface energies emerging in a microscopic, two-dimensional two-well problem
