Matrix stress distributions at the crack initiation site in a short fiber reinforced polyamide under fatigue loading

Mechanical components made of Short Fiber Reinforced Plastics (SFRPs) are generally manufactured by injection molding. The fiber orientation distribution depends upon how the melt flow fills the mold cavity. In this work, the analysis of the relationship between the local fiber orientation distribution, the local stress field and the crack profile at crack initiation site is presented. In the mechanical components, geometric discontinuities represent, from a fluid-dynamic point of view, locations where the melt flow changes direction and from a structural point of view, typical areas for crack initiation. In order to provide an example of material microstructure representative of a real component, the fiber orientation distribution around a notch of a specimen was examined. A fatigue test of a notched specimen was carried out up to crack initiation, then a small sample around the notch was extracted and analyzed through the X-ray Computed Tomography (X-ray CT). The real fiber distribution in the material volume was modeled in a Finite Element Method (FEM) code. The analysis gives a valuable insight into the typical micro-stress distributions occurring at the notch tip due to the local fiber orientation.