A quantitative analysis of tensile fracture surface was performed on JIS FCD400 and FCD500 spheroidal graphite cast irons. The unnotched and circumferentially notched specimens with different notch acuity were employed to obtain a wide range of fracture strain. All specimens were loaded up to break under tensile loading at room temperature.
The fracture strain decreased markedly with increasing stress concentration factor for both FCD400 and FCD500 cast irons. The relative amount of dimple area on the fracture surface was measured using a scanning electron microscope associated with fracture strain. Fractographic examination revealed the fracture mechanism of FCD400 with ferritic matrix to be significantly different from that of FCD500 with 42% pearlitic matrix, depending upon matrix structure. For FCD400, dimple fracture was predominant for a series of tensile specimens. Whereas, cleavage fracture predominates for FCD500, and the area fraction covered by dimples was about 30% for the unnotched specimen.
On the other hand, the asperity on the fracture surface reduced significantly as the fracture strain decreased. Then, the roughness of the fracture surface, expressed in terms of the maximum height on the fracture surface, was examined with a laser microscope associated with fracture strain. As a result, the maximum height on the fracture surface reduced with decreasing fracture strain for both FCD400 and FCD500 cast irons, irrespective of matrix structure. Hence, the roughness of the fracture surface is expected to provide a more positive and quantitative information regarding the fracture behavior of ductile cast irons.
