  This paper investigates the effect of moisture content upon the degradation
behaviour of composite materials. A coupled phase field framework considering
moisture diffusion, hygroscopic expansion, and fracture behaviour is developed.
This multi-physics framework is used to explore the damage evolution of
composite materials, spanning the micro-, meso- and macro-scales. The
micro-scale unit-cell model shows how the mismatch between the hygroscopic
expansion of fibre and matrix leads to interface debonding. From the meso-scale
ply-level model, we learn that the distribution of fibres has a minor influence
on the material properties, while increasing moisture content facilitates
interface debonding. The macro-scale laminate-level model shows that moisture
induces a higher degree of damage on the longitudinal ply relative to the
transverse ply. This work opens a new avenue to understand and predict
environmentally-assisted degradation in composite materials.
