Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel
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INTRODUCTION
COMPONENTS operating under creep conditions are often subjected to constrained creep deformation imposed by a sudden change in thickness/cross section, geometry, etc., and also by a steep change in microstructure, having an appreciably different creep deformation resistance as in the fusion welded joints. In a laboratory experiment to study the effect of constraint on creep deformation, notches are usually introduced in creep specimens.[1–6] Different creep rates across the notch root because of continuous changes in crosssectional area impose a constraint on creep deformation to maintain the strain continuity. The degree of constraint imposed on the materials depends on the kind of notch introduced and the profile of the notch root and ductility of the material.[7,8] Both V- and U-notches are incorporated in the cylindrical creep specimen to study the effect of constraint on creep behavior.[8] Circumferential V-notches are found to exhibit a higher degree of constraint than that of circumferential U-notches.[8] For laboratory experiments to study the effect of constraint on creep deformation and fracture, U-notches are SUNIL GOYAL and C.R. DAS, Scientific Officers, K. LAHA, Head, Creep Studies Section, S. PANNEERSELVI, Scientific Assistant, and M.D. MATHEW, Head, are with the Mechanical Metallurgy Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] Manuscript submitted May 2, 2013. Article published online October 12, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
preferred over the V-notches, since the U-notches produce creep cavitation over a relatively larger volume, assisting the investigation.[9] The presence of notches may exhibit notch creep strengthening or weakening depending on the notch shape and acuity (ratio of diameter of minimum cross section to radius of notch root), testing conditions, and the material ductility.[2–4,10] Notch strengthening is expected when the high axial stresses across the notch throat plane redistribute quickly below the applied stress. This kind of behavior is typically observed in ductile materials. However, notch weakening is expected in situations where the very high axial stresses due to the presence of the notch redistribute very slowly and the local accumulated strain exceeds the limit which is required for fracture before attaining the stationary state across the notch throat plane. This behavior is typically observed in brittle materials. The stresses around the notch redistribute during creep deformation and approach a stationary state condition.[8,11] Finite element (FE) analysis coupled with continuum damage mechanics (CDM) has been extensively used by many researchers to understand the stress redistribution and damage accumulation across the notch under creep conditions.[12–15] It was observed that for each notch geometry, there is a skeletal point where the stresses are almost constant irrespective of the value of stress exponent n in Norton’s law relating steady sta
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