Fatigue of Composites

Composite materials are inhomogeneous and anisotropic by nature, in contrast to the assumptions which are normally made for metallic components. Any FE or fatigue analysis made for composite material components must take this into account.

Inhomogeneous and Anisotropic Materials

Properties of a composite may in general be different for every element in the model and, for layered composite shell elements, also vary through the thickness of each element. In addition, properties including static and fatigue strength are strongly dependent on the direction of loading relative to the fibre orientations. For a given composite system (combination of matrix and reinforcing fibres), the properties of the composite are principally influenced by the fibre orientations, which in turn are determined by the manufacturing process. Although general composite fatigue theories focuse on injection-moulded short fibre reinforced thermoplastics, the nature of the FE and fatigue analysis methods is such that they could also be applied to laminates where a simple ply-by-ply approach is considered adequate.

Short Fibre Composite

To assess fatigue of short fibre composites we use a stress-life approach for the analysis of anisotropic materials such as glass fibre filled thermoplastics. The stress tensor for each layer and section integration point throughout the shell thickness is read from FE results. The material orientation tensor describing the “fibre share” and direction at each calculation point is provided by mapping a manufacturing simulation to the Finite Element model. This orientation tensor can be read from the FE results file or supplied from an ASCII file.

The short fibre composite analysis requires standard materials data of two or more S-N curves for differing fibre orientations. We use this data to calculate an appropriate S-N curve for each calculation point and orientation.

Composite Analysis

In a composites fatigue analysis we evaluate the strength of a structure against industry standard composite failure criteria. Rather than limiting this evaluation to a small number of load cases or steps, stresses can be assessed using the chosen failure criteria throughout realistic duty cycles (quasi-static or dynamic). This allows critical locations, load combinations and associated design reserve factors to be readily identified. In addition, selected location loading paths may be visually compared with the material failure envelope.

The following methods can be used individually or combined to give the most conservative result:

  • Maximum stress
  • Maximum strain
  • Norris
  • Hoffman
  • Tsai-Hill
  • Tsai-Wu
  • Franklin-Marin
  • Hashin
  • Hashin-Rotem
  • Hashin-Sun
  • Modified NU
  • Norris-McKinnon
  • Christensen
  • User-defined custom methods via Python