Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes
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HIGH-STRENGTH steels show gigacycle fatigue failure due to fish-eye fractures.[1,2] In general, steels show a surface fracture type of fatigue failure in which fatigue cracks are initiated from the surfaces of specimens. In this case, fatigue limits can be determined at around 107 cycles, and the fatigue limits show a good relationship with the tensile strength and hardness of the steels. On the other hand, high-strength steels, whose tensile strength typically exceeds 1200 MPa,[3,4] show an internally originating type of fatigue failure, i.e., fish-eye fractures. These occur even below the fatigue limit expected from the tensile strength and even at over 107 cycles, so the conventional fatigue limit is eliminated in the case of high-strength steels. The origin of a fish-eye fracture is in most cases an internally located inclusion,[5–11] although the matrix itself can also be the origin in some cases.[12,13] These phenomena are called gigacycle fatigue.[14] The fish-eye fracture properties have a close relationship with the inclusion at the origin. The major factor is the size of the inclusion,[5,6] while some researchers report that the shape[7,8,9] and properties[10,11] of the inclusion also affect the fish-eye fracture properties. Moreover, the presence of an optically dark area (ODA) has recently been pointed out.[15,16] An ODA is a rough Y. FURUYA and H. HIRUKAWA, Senior Researchers, are with the National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan. T. KIMURA, Chief Staff Member, and M. HAYAISHI, Senior Research Engineer, are with Daido Steel Co., Ltd., Aichi 457-8545, Japan. Contact e-mail: [email protected] Manuscript submitted December 7, 2006. Article published online July 13, 2007. 1722—VOLUME 38A, AUGUST 2007
area on the fracture surfaces, formed around the inclusion of the fish-eye fracture origin; the area looks dark under optical microscopy. The nature of ODAs is still unclear, although there are reports that hydrogen trapped at the inclusion generates them. The ODAs are, however, believed to play an important role in the occurrence of the long life fish-eye fracture, because the ODA is typically observed in cases of long life failure at above 107 cycles. More specifically, the size of the area, including the ODA, rather than the inclusion size alone, is likely to limit the fatigue strength.[15] Many researchers have tried to describe the relation between the fish-eye fracture properties and inclusions.[1,7,17] For example, the following equation proposed by Murakami[1] provides a good estimation of the fatigue limit rw¢ (MPa). pffiffiffiffiffiffiffiffiffi r0w ¼ 1:56(HV þ 120)/( area)1=6 ½1 2 where HV (kgf/mm pffiffiffiffiffiffiffiffiffi ) is the Vickers hardness of the matrix and area (lm) is the square root of the projected area of the internal defect. This equation suggests that fatigue strength in the case of a fish-eye fracture depends on the inclusion sizes to the –1/6th power and the effects of the inclusion shape and properties are negligible. However, more experimental results are needed to confirm the validi
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