Prediction of creep behavior in short fiber reinforced polymer matrix composites using an elementary volume approach

Creep behavior of short glass fiber reinforced poly(butylene terephthalate) (SFRC PBT) composites was analyzed using plates processed by injection molding and push–pull processing, with fiber contents of 0, 20, and 30 wt%. Tensile test bars were extracted parallelly and perpendicularly to the flow direction to assess short-term mechanical properties, fiber length distribution, and orientation. An elementary volume approach was used to predict the longitudinal and transverse creep compliances, showing that the time dependencies were mainly governed by the PBT matrix. Given the minimal fiber orientation in the thickness direction, a transformation based on RM Jones’ mechanics of composite materials was applied to account for fiber misalignment. This led to the introduction of the unknown shear modulus \(G_{12}\), which was addressed by expressing it in terms of the transverse compliance \(J_{22}\) and shear correction factor. Comparison of predicted and measured creep compliances revealed an underestimation of 15–30% parallelly and 5–15% perpendicularly to the flow direction, attributed to imperfect fiber-matrix adhesion. SEM analysis of fracture surfaces indicated different failure behaviors based on the fiber orientation. This suggests that fiber-matrix adhesion is stress-direction dependent. The time range for accurate prediction of composite creep behavior, governed by matrix creep, is defined by the creep time limit, which decreases exponentially with increasing creep stress.