Predicting the creep compliances of short fiber reinforced polymer composites from the matrix creep using an elementary volume model

The stress dependent creep of short glass fiber reinforced poly(butylene terephthalate) having fiber contents of 0 wt.%, 20 wt.%, and 30 wt.% is measured parallel and perpendicular to the flow direction. Short‐term mechanical properties, mean fiber length, and fiber orientation are accessed to provide input parameter for the modelling. An elementary volume model is employed to derive model predictions of longitudinal and transverse creep compliances in the main axes system which depend primarily on the matrix creep. As the fiber orientation in the thickness direction is close to zero, a 2D‐transformation could be applied to account for fiber misalignment. This transformation introduces the unknown shear modulus G 12 which, however, could be expressed in terms of transverse compliance and shear correction factor. The predicted creep compliances are underestimated by 15 % to 30 % parallel, and 5 % to 15 % perpendicular to the flow direction. This is good news as scanning electron microscope analyses of fracture surfaces show non‐perfect fiber matrix adhesion and fiber orientation dependent failure behavior what suggests a stress direction dependent fiber‐matrix adhesion. The time ranges for accurate prediction decrease exponentially with increasing creep stress.