On the Micromechanism of Fatigue Damage in an Interstitial-Free Steel Sheet
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INTRODUCTION
INTERSTITIAL-FREE (IF) steel sheets are extensively used to meet the severe cold-forming requirements of automotive industries. Detailed reports[1,2] exist that are related to the processing routes and properties, especially formability, etc., on these steel sheets. These sheets are primarily used in automobile structural components, which experience cyclic loading in service; hence, knowledge related to the fatigue properties of these materials is important. The number of research reports on the fatigue behavior of IF steel sheets, however, is limited. Earlier works indicate that efforts are primarily aimed at the estimation of fatigue life,[3] with insignificant emphasis on the understanding of the micromechanism of fatigue damage, in terms of the initiation and growth of fatigue cracks at various microstructural locations. Microstructures play a decisive role in the process of fatigue damage, as the latter is primarily governed by the local stress conditions at and around its various features.[4] Hence, an understanding of the micromechanism of fatigue in an extremely clean ultra-low-carbon ferritic microstructure, in which grain boundaries are depleted of interstitials (such as C, N, SHRABANI MAJUMDAR, Researcher, R&D Division, and D. BHATTACHARJEE, Chief, R&D Division and Scientific Services Division, are with the Tata Steel, Ltd., Jamshedpur, 831 007, India. Contact e-mail: [email protected] K.K. RAY, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, 721302, India. Manuscript submitted August 21, 2007. Article published online April 22, 2008 1676—VOLUME 39A, JULY 2008
etc.), presents an interesting problem from the viewpoint of material science. Studies of this nature can sequentially elucidate the preferred location for the initiation of microcracks, the nature of the growth of these microcracks, and, thus, the weak links in the microstructure that provide the low-energy path in which the cracks prefer to grow. The IF steels are important commercial materials and a systematic study of the micromechanism can provide greater insight into both the alloy design and the consequent engineering applications in steels with a similar microstructure. This report is centered on addressing this issue. The micromechanism of fatigue damage in structural components involves several sequential stages, e.g., (1) the initiation of microcracks, (2) the growth and coalescence of microcracks to form a dominant macrocrack, and (3) the propagation of the macrocrack to cause complete failure. In addition, fatigue damage is commonly associated with the formation of slip bands on the specimen surface, which are known to govern the mechanism of crack initiation and growth.[4] In a flawfree, homogeneous material, a significant fraction of the total lifetime is spent before the first detectable microcrack appears.[5,6] The latter is defined as a discontinuity, all the dimensions of which are small in comparison to the characteristic microstructural
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