Multiple-cracking phenomenon of the galvannealed coating layer on steels under thermal and tensile stresses
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I. INTRODUCTION
THE hot-dipped galvannealed (GA) steels, consisting of an Fe-Zn intermetallic coating layer and substrate steel, are widely used as architectural and car-body materials due to their high corrosion resistance and weldability.[1,2] The intermetallic coating layer is, however, brittle, and, therefore, it exhibits multiple cracking under thermally induced residual stress and externally applied stress.[3,4,5] The multiple-cracking phenomenon has been observed and discussed for low-failure-strain coating layers on the highfailure-strain substrates,[6–19] for low-failure-strain layers in layered materials,[19,20] and for low-failure-strain fibers embedded in high-failure-strain matrix composites.[21–26] To account for this phenomenon, the following models are mainly used. One is the strength model, in which the cracking of the lowfailure-strain layer (fiber) takes place when the exerted tensile stress exceeds the strength.[7,8,16–18,23–25] Another is the energy model, in which the cracking takes place when the energyrelease rate exceeds the critical value.[9–14,19,29] The energy model is based on linear-elastic fracture mechanics and can account for the cracking behavior of the coating layer on the elastic substrate properly. The present samples are, however, pulled up to 20 to 30 pct, and the substrate becomes fully plastic at such high strains. The large-scale yielding at the crack tip and full plastic deformation at high strains of the present substrate steels make the situation complex. S. OCHIAI, Professor, and H. OKUDA, Associate Professor, International Innovation Center, and S. IWAMOTO, T. TOMIDA, and T. NAKAMURA, Graduate Students, M. TANAKA, Associate Researcher, and M. HOJO, Researcher, Graduate School of Engineering, are with Kyoto University, Kyoto 606-8501, Japan. Contact e-mail: [email protected] Manuscript submitted June 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
On the other hand, the strength model is applicable for both the elastic and plastic states of the substrate, but it is not rigid in comparison with the energy one. It presumes that only the tensile stress in the tensile direction causes cracking and that other stresses are low enough, while the stress state in the coating layer is not uniaxial. Thus, when the stresses other than the tensile one play a dominant role in the cracking behavior, the application of this model is inadequate. However, when the tensile stress dominantly controls the behavior, it gives a good approximation, as has been demonstrated for the analysis of the multiple cracking of fibers in composites.[17–23] This model actually contributes to the description of the relation between the crack spacing, fiber strength, and interfacial strength in fiber composites. In addition, it is emphasized that Leterrier et al.,[15] Andersons et al.,[16] and Yanaka et al.[17] have demonstrated that the multiple-cracking behavior of the brittle SiO2 film on ductile polyethylene terephthalate is described well with this model up to the large applied strains (se
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