Fatigue Prediction of 50% Short Fiber-Reinforced Polyphthalamide Using Equivalent Stress Criteria and Critical Distance Methods

Accurately predicting the fatigue lifetime of short fiber-reinforced polymers (SFRPs) remains a significant challenge in the automotive industry, especially in the presence of sharp geometric discontinuities. This study presents a practical approach for estimating the fatigue life of injection-molded notched SFRP specimens, considering material anisotropy and load ratio dependency. Micro-computed tomography was used to analyze fiber orientation in critical areas for simulating anisotropic behavior. The Point Method, based on the Theory of Critical Distances, was applied to calculate matrix equivalent stresses near the notch root. Both von Mises and Beltrami equivalent stresses were evaluated at the matrix level, highlighting distinct behaviors across specimens and enabling the development of a master SN curve. Additionally, a modified constant life diagram was created to account for load ratio effects, offering a generalized fatigue prediction method. This approach provides a balanced solution between prediction accuracy and computational efficiency.