Fractal and fracture mechanics analyses of fatigue fracture surfaces of metallic materials

In this study fatigue crack propagation tests were performed to obtain fatigue fracture surfaces on compact tension type specimens of A5052 aluminum alloy and S25C carbon steel. The fatigue fracture surfaces were observed using a scanning laser microscope system. Based on the digital data thus obtained, an imaginary fracture surface was reconstructed in a three-dimensional (3D) space using a personal computer. Fractal analysis proposed by Mandelbrot was applied to such 3D surfaces and a hyperbola model was accepted to represent the Richardson effect. Because of the Richardson effect thus analyzed, fractal features were distinguished in the fracture surface irregularity. Results demonstrated that the geometrical irregularity of the surface was well evaluated by combining the fractal dimension and additional indices termed as "indices of fracture surface nature", and that the fractal dimension and the additional indices were associated with the stress intensity factor range of Delta K.