  We present a computational framework to explore the effect of microstructure
and constituent properties upon the fracture toughness of fibre-reinforced
polymer composites. To capture microscopic matrix cracking and fibre-matrix
debonding, the framework couples the phase field fracture method and a cohesive
zone model in the context of the finite element method. Virtual single-notched
three point bending tests are conducted. The actual microstructure of the
composite is simulated by an embedded cell in the fracture process zone, while
the remaining area is homogenised to be an anisotropic elastic solid. A
detailed comparison of the predicted results with experimental observations
reveals that it is possible to accurately capture the crack path, interface
debonding and load versus displacement response. The sensitivity of the crack
growth resistance curve (R-curve) to the matrix fracture toughness and the
fibre-matrix interface properties is determined. The influence of porosity upon
the R-curve of fibre-reinforced composites is also explored, revealing a
stabler response with increasing void volume fraction. These results shed light
into microscopic fracture mechanisms and set the basis for efficient design of
high fracture toughness composites.
