Incorporating Fiber Orientation–Length Dependence for Accurate Elastic Modulus Prediction in Short Fiber Reinforced Polymer Composites

ABSTRACT For short fiber reinforced polymer (SFRP) composites manufactured through extrusion compounding followed by injection molding, fiber length distribution (FLD), and fiber orientation distribution (FOD) serve as pivotal microstructural parameters governing macroscopic mechanical properties, including elastic modulus. Prior predictive models have universally assumed that FOD is statistically independent of FLD, thereby decoupling the two descriptors in mean field or laminate analogy calculations. In this study, x‐ray computed tomography (XCT) of injection molded short glass fiber reinforced polyetherimide (SGF/PEI) composites reveals a clear and measurable dependence between FOD and FLD, indicating that relatively short and long fibers do not share identical orientation distributions. Motivated by this observation, an improved laminate analogy approach (ILAA) was proposed by taking into account the effect of the FOD–FLD dependent relationship. Experimental and finite element simulation results show that ILAA halves the modulus prediction deviations from experimental data compared to the traditional laminate analogy approach. Moreover, a reverse analysis routine enables reconstruction of the intrinsic FOD–FLD relation in seconds from minimal XCT data, offering a rapid design tool. This innovative methodology not only offers a cost effective strategy but also provides a pragmatic approach to more accurately predict the elastic modulus of injection molded SFRP composites with a FOD–FLD dependent relationship.