Creep behavior of short glass fiber reinforced poly(butylene terephthalate) and its prediction based on the matrix creep using an elementary volume approach

The creep compliances of short glass fiber reinforced poly(butylene terephthalate) composites (SFRC PBT) plates processed by conventional injection molding and push pull processing were determined parallel and perpendicular to the flow direction using a tensile creep stand. The fiber weight contents of the plates were 0, 20 and 30 %. Tensile test bars were taken out of the plates parallel and perpendicular to the flow direction to determine short-term mechanical properties, fiber length distribution and fiber orientation distribution necessary for subsequent modelling. An elementary volume approach was used to calculate the longitudinal and transverse creep compliances in the main axes system showing that their time dependencies are governed by the creep of the PBT matrix only. As the plates hardly exhibit any fiber orientation in the thickness direction, the components of the 2D compliance tensor were transformed according to RM Jones’ Mechanics of composite materials to take into account the fiber misalignment in SFRC. This transformation introduces the unknown shear modulus G12 to the components of the transformed compliances tensor. This problem could be overcome as G12 can be expressed in terms of the transverse compliance J22 and a shear correction factor within the EV approach. Comparing the predicted creep compliances to measured ones resulted in an underestimation of 15 to 30 % parallel and 5 to 15 % perpendicular to the flow direction. This underestimation can be attributed to a large extent to a non-perfect fiber matrix adhesion. Furthermore, SEM pictures of fracture surfaces show different failure behavior parallel to the fiber axis and perpendicular to it. Thus, the fiber matrix adhesion seems to depend on the direction of stress with respect to the fiber axes. The time range of well correspondence of matrix creep and composites creep can be expressed by the creep time limit which decreases exponentially with increasing creep stresses.