Investigation on defect distribution and its statistical evaluation for case hardened material states |
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| Affiliation: | 1. State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China;2. University of Chinese Academy of Science, Beijing 100049, China;3. Research Center of Geotechnical and Structural Engineering, Shandong University, Jinan 250100, China;1. Reactor Safety Division, Hall-7, Bhabha Atomic Research Centre, Mumbai 400085, India;2. National Metallurgical Laboratory, Jamshedpur 831 001, India;1. Department of Mechanical Engineering, Pontifical Catholic University of Rio de Janeiro, Rua Marquês de São Vicente 225 – Gávea, Rio de Janeiro, RJ 22451-900, Brazil;2. School of Aerospace Engineering and Applied Mechanics, Tongji University, 1239 Siping Road, Shanghai, PR China;1. Aerospace Division, Defence Science and Technology Group, 506 Lorimer Street, Fishermans Bend, Victoria 3207, Australia;2. Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia |
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| Abstract: | The fracture behaviour of steel components with very hard material states under cyclic loading is mostly defined by non-metallic inclusions present in the material. The inclusion distribution is a result of specific conditions in steel making, casting and rolling processes. Standard methods (e.g. DIN EN 10247:2007) are commonly used for estimation of the distribution of non-metallic inclusions. These methods are based on random metallographic examination of longitudinal microsections and evaluation with standard pictures. For numerical evaluation of the endurance limit of very hard material states the knowledge of defect distribution of the material is necessary. However, the standard methods are not suitable to find the actual inclusion distribution.In a large investigation program on the fatigue strength of very hard material states, thin wall tubes made of a case hardening steel were tested. All fracture surfaces were investigated, size and coordinates of the fracture initiating defects were documented and analysed statistically. The defects appear as inclusion stringers stretched in rolling direction mainly containing Al-oxide. Several distribution functions for statistical analysis were investigated.Furthermore, transverse microsections with a total area of about 2000 mm2 were investigated and the data of the maximum width of inclusion stringers were also analysed statistically. Comparison of the extreme value distribution of inclusions made by metallography and fractography, respectively, show that the metallographic method considerable underestimates the actual size of inclusions. Therefore micrographic methods are not suitable to generate input data for inclusion distribution based on fatigue design. |
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