Short-fiber reinforced parts show a distinct anisotropic behavior, caused by the alignment of the fibers during the injection molding process. The injection molding simulation provides the local probability distribution of the fibers (orientation tensor). By applying multi-scale material models it is possible to estimate the local anisotropic stiffness. This leads to considerable more accurate results in a subsequent FEA as with isotropic approaches. This is true even if higher temperatures lead to local plasticity. Tools that enable this integrative simulation approach have been established in the last years. To account for the anisotropic fatigue behavior an interpolation of fatigue strength at a discrete fiber orientation distribution is often used by estimating the anisotropic stiffnesses in direction of the orientation tensor principal directions. This is in most cases not appropriate. In the paper an approach is described that uses a so-called Master SN curve concept which estimates SN curves for varying local fiber orientation distributions. For the application case of high temperatures and local plasticity several enhancements were implemented and tested. The methodology is verified at the example of an oil-filter system under pressure at elevated temperature.

Introduction

Fatigue of metals is studied since the beginning of the 19th century. It started by testing full components like steel chains and later rail axels as these parts had shown failure after the repetition of many load cycles, where each individual load cycle did not damage the part on the macroscopic level. Hence approaches in fatigue for metals typically started from a full macroscopic point of view, the behavior at the microscopic level had not been modelled but taken into account by fitting test data to those microscopic models. For composite structures it is necessary to take the local microscopic structure into account as it defines the basic structural behavior. For injection molded short fiber reinforced plastics the local distribution of the fiber orientation directly influences the (anisotropic) local stiffness of the structure and also defines the damage behavior. Also the global damage behavior of composites is different compared to metals. Whereas metal parts do not show changes in the global stiffness until failure, stiffness losses and global stress redistribution in composite structures can be observed.