Maximising the hybrid effect in unidirectional hybrid composites

• A longitudinal strength model for unidirectional hybrid composites incorporating realistic fibre packings was developed.
• The apparent failure strain increase of the carbon fibres, the so-called hybrid effect, was investigated.
• Larger strength scatter of the low elongation fibres leads to strong increases in the hybrid effect.
• The hybrid effect initially increases with increasing failure strain of the high elongation fibre, but then levels off.
• A stiffer higher elongation fibre can slightly increase the hybrid effect.

The failure strain of fibre-reinforced composites can be increased by fibre hybridisation. A recently developed model for unidirectional composites was extended to hybrid composites to analyse this synergetic effect, called the hybrid effect. The model predicts individual fibre breaks and interactions among clusters of fibre breaks. Three key parameters were studied to understand how they can maximise the hybrid effect, namely low elongation fibre strength scatter and hybridisation fibre stiffness and failure strain. Larger strength scatter of the low elongation fibres leads to larger hybrid effects, as the scatter spreads out the cluster development over a larger strain interval. The failure strain ratio of the two fibre types should be above 2 for the properties used here, but a higher ratio did not yield any additional benefits. Increasing the stiffness of the hybridisation fibre reduces the stress concentrations on the low elongation fibre and may also enlarge the hybrid effect. These conclusions provide guidelines for designing optimal hybrid composites.

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