The exponential increase in the use of composites for structural applications in the past decades comes with a constant need to increase their performance for them to remain an attractive substitute to conventional metals. In particular, despite their lightweight and high specific stiffness and specific strength, composites generally suffer from a lack of damage tolerance. Their brittle nature restrains the use of composites in industry, or at least leads to over-conservative designs.

This work aimed at designing and modelling novel discontinuous microstructures, inspired by damage tolerant naturally-occurring discontinuous composites. This was achieved by considering discontinuous hybrid and discontinuous hierarchical microstructures.

In a first part, a virtual testing framework was developed to perform virtual testings of non-hybrid and hybrid aligned-fibre discontinuous composites. The virtual framework, made of analytical models combining statistics and micromechanics, captured and explained different effects such as hybrid effects, size effects, and the effect of variability. The models were used to identify microstructures which maximise the strength, failure strain and stiffness of hybrid discontinuous composites.

In a second part, hierarchical discontinuous carbon-fibre reinforced polymers were designed, modelled numerically and tested experimentally. They exhibited a stable failure mechanism and dissipated energy stably, before failure, through diffuse damage (unlike most conventional composites). This study also showed that non-self-similar microstructures could achieve better damage tolerance and provide a clearer warning before failure than self-similar microstructures.

This work shows the potential for different types of discontinuous composites to overcome the inherent brittleness of conventional composites. The models developed throughout this work can be used to support material design, perform parametric studies and identify optimal designs.