Damage evolution in short fiber reinforced polyamide under fatigue loading: influence of the fiber volume fraction

This paper investigates the influence of the fiber volume fraction on the damage evolution in a short glass fiber reinforced polyamide (PA66) under tension-tension fatigue loading. Uniaxial fatigue tests were carried out on notched specimens characterized by different fiber contents (0%, 15%, 25%, 35%, 50% in weight). The notch was created through an insert within the mold in order to reproduce a typical fiber orientation distribution occurring in the real world components. A damage investigation was performed by using Field Emission Scanning Electron Microscopy (FESEM). Interrupted fatigue tests were carried out to analyze the profile of the crack. Fracture surfaces of specimens failed in the high cycle regime were also examined. The damage analysis is based on the comparison of specific morphological features. The matrix ductility, the degree of fiber-matrix adhesion, fiber failure and fiber pull out were investigated. The results of the damage investigation highlight that the crack profile is strongly influenced by the fiber content. No significant differences in terms of matrix ductility and degree of fiber-matrix adhesion between the reinforced material systems were identified. Instead, it was observed that the length over which the fibers are pulled out decreases with increasing fiber fraction. Finally, a higher percentage amount of fractured fibers was observed with low fiber contents.