In order to investigate the interior-induced fatigue crack propagation behavior of spring steel (SUP7), fatigue tests in axial loading were performed for 4 kinds of specimen with different hardness (tempered at 450 ℃ or 386 ℃) or residual stress (finished by grinding, electrochemical polishing or shot peening). Harder specimen has indicated longer fatigue life resulting from its mechanical properties of matrix. Ground specimen and shot-peened specimen also provided longer fatigue life than electrochemical-polished specimen, and the fatigue life of shot-peened specimen was almost same as one of ground specimen. After fatigue tests, fracture surfaces were observed using a scanning electron microscope (SEM). In some cases, non-metallic inclusion was not observed at interior fracture origin, but granular microstructure was observed. Profile analysis and crack-path analysis with FRASTA (Fracture surface topography analysis) method were performed to investigate the fatigue fracture mechanism induced by the granular microstructure. As results of these analyses, it was clarified that a facet of the granular microstructure was inclined from 33 to 42 degrees to the fracture surface. It was another finding that small countless cracks emanated discretely inside the granular microstructure during fatigue process.
The fractography is one of the useful technique for finding the cause of breakage in the engineering par ts. However, it is not easy to find an appropriate point on the fracture sur face, and also the distinction of fracture surface morphology, such as dimple, river-pattern, striation and so on, needs highly skilled technique. To assist unskilled engineers, there are many studies for quantitative analysis of fracture surface morphology by means of Fractal dimension measurement. It is ver y interesting that fracture surface possesses fractal structure, but it is still difficult to distinguish the fracture surface morphology by using fractal dimensions measurement. On the other hand, Recurrence plots introduced by Eckmann et al. provide a way to visualize the periodic nature of events such as fracture surface s characteristic feature. Therefore, by applying the image recognition technique to Recurrence plots obtained from fracture surface, it is expected to see the fractographical feature extraction for distinguishing fracture surface morphology. In this study, in order to recognize the Intergranular fracture on fatigue-fractured spring specimen in a salt water environment, the image recognition technique is applied to Recurrence plots obtained from fracture surface. Since we obtained appropriate judgements for intergranular fracture, it is found that Recurrence plots and image recognition technique could be useful for a computer aided fractographical new technique.
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