Surface Texture, Fatigue, and the Reduction in Stiffness of Fiber Reinforced Plastics

In this study the influence of machined surface texture on the fatigue strength of a Graphite/Bismaleimide (Gr/Bmi) laminate was examined. Rectangular beam specimens were machined to achieve specific surface roughness $0.2⩽Ra⩽10.0 μm$ and then subjected to fully reversed four-point flexural fatigue. The surface texture resulting from machining was evaluated using contact profilometry and the standard surface roughness parameters were used in estimating the effective stress concentration $K¯t$ and effective fatigue notch $K¯f$ factors for the machined surfaces according to models reported in the open literature. Fully reversed flexural fatigue loading of the laminate was conducted at two separate maximum bend loads corresponding to Tsai-Hill ratios for first ply failure of 0.75 and 0.9. It was found that the reduction in stiffness of the Gr/Bmi laminate at both levels of fatigue loading increased with the magnitude of surface roughness. The apparent fatigue stress concentration factors $Kf$ of the machined surfaces determined from experimental results ranged from 1.0 to 1.2. The effective fatigue notch factors estimated using the Arola-Ramulu model for the machined specimens were within 10% of the corresponding $Kf$ determined from experiments.