Effect of Overload on SCC Growth in Stainless Steels in High Temperature Water
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カテゴリ: 第6回
He XUE, Qunjia PENG and Tetsuo SHOJIFracture and Reliability Research Institute, Tohoku University, Sendai 980-8579, Japan,Tel: +81-22-795-7520, Fax: +81-22-795-7543
1. Introduction
Stress corrosion cracking (SCC) is a failure mechanism that is caused by environment, susceptible material and tensile stress near the crack tip area. Since various stainless steel or nickel based alloy components are subjected to SCC behaviors in light water reactor (LWR), many efforts have been done to understand the underlying mechanism of SCC of core nuclear power engineering materials [1]. Among all mechanism proposed, the film slip-dissolution/oxidation model is widely accepted as a reasonable description of SCC of austenitic alloys in high temperature oxygenated aqueous systems [2]. This mechanism attributes the crack growth to a result of the oxidation at the crack tip that occurred periodically following the rupture of the oxide by crack tip strain. Efforts have been done to utilize the model to quantify the SCC growth rate [3].The effect of overload on fatigue crack growth has been investigated that indicated the overload could delay retardation of the crack propagation and reduce the crack growth rate [4]. This draws the interests of researchers in the SCC field that whether the overload could also affectCorresponding author: He XUE, Fracture and Reliability Research Institute, Tohoku University, 6-6-01, Aoba-ku, Sendai 980-8579, Tel. +81-22-795-7520 e-mail: xue_he@hotmail.comthe SCC growth rate of structural materials in high temperature water. By incorporating the film slipdissolution/oxidation model that describes the SCC growth mechanism, the elastic-plastic finite element method (EPFEM) and the former SCC experimental data, the effect of the overload on SCC growth rate of stainless steel in high temperature water is discussed in this paper2. Calculation Model1000SUS316L in 288 A°C(0.=535 MPa)True stress o (MPa)Loanding curveUnloading curve00.0050.0150.020.01True strain Iμ (%) Fig.1 Material mechanical property duringloading and unloading courseOne inch compact tension (1T-CT) specimen with a constant load Kj is usually used in SCC experiments in high temperature water. Therefore, to investigate the effect of the overload on SCC growth rate, a simulated SCC numerical test with 1T-CT specimen is performed in this30study. The numerical analysis process satisfies American Society for Testing and Materials (ASTM) A‰ 813 standards [5].The material mechanical property of a 20% cold worked SUS316L stainless steel at high temperature (2889) is adopted in this simulation calculation. The mechanical relation of the material during loading and unloading processes is depicted by Fig.1, where the loading mechanical relation is obtained by a tensile strength test and the unloading mechanical relation is simply represented by a linear elastic relation.1000Crack advanced length Aa (um)Loading stageCrack propagating stage0.TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT0510153540455020 25 30 Calculation stepa) Crack advanced lengthA? (MPa.m““Norminal K, without overload- Norminal K with 10% overload -- Norminal K with 20% overload -- Norminal Kwith 40% overload8Norminal K without overload Norminal K with 10% overload Norminal K, with 20% overload - Norminal K with 40% overload88 Stress intensity factor K, (MPa.m““)82510203040501900/02/1825+TomorsomCalculated stepb) Applied loadFig.2 Parameters change in calculation processThe plastic strain ahead of the crack tip and the SCC growth rate are investigated with the nominal load Kj of 30 MPa.m/2 in this study. The 10%, 20% and 40% overload are separately applied on the specimen during the crack being stationary, which is known as the loading stage.And then the crack propagates 20 um in each step, and the final length is 1.0 mm, which takes 50 steps in this period. This course is known as crack advanced stage. As will be shown in the following, the total crack increment of 1 mm is generally larger than the overload affected zone that ensures the disappearance of the overload effect at the crack tip at the end of the simulation. The crack increment and applied Ky vs. calculation steps during the simulated numerical process are shown in Fig. 2 (a) and Fig. (b). As can be seen in Fig.2 (b), the applied Ky is slightly increased by the crack propagation, which simulates a process of crack propagation under constant load.Fig.3 Finite element model of specimen (plane strain)A commercial FEM code ABAQUS is used in this simulated analysis [6]. A half of the specimen is calculated based on the symmetry condition. Mesh of specimen is shown in Fig.3. 11961 8-node biquadrate plane strain quadrilateral elements are adopted in this model. The mesh in the vicinity of the growing crack tip is observably refined in order to investigate the detailed plastic strain and plastic strain rate in front of the crack tip. X-axis is the opposite direction of the crack growth and Y-axis is the normal direction of the crack growth in the coordinate system. The course of steadily growing crack is simulated by the node release method in ABAQUS. The stationary crack and steadily growing crack are calculated under the elastic-plastic loading and linear-elastic unloading condition, respectively.3. Results and Discussions3.1 Plastic strain and plastic strain rate in front of the crack tipThe normal plastic strain in front of the stationary and growing crack tip is shown in Fig. 4. The plastic-310.0060.0060.0050.005-Without overload With 10% overload With 20% overload - With 40% overloadWithout overload - With 10% overloadWith 20% overload - With 40% overload0.004)0.002Normal plastic strain &Normal plastic strain &0.001 -MUTTWHITE00.000+0.20.20.80.4 0.60.8 Distance in front of crack tip (mm)0.40.6 Distance in front of crack tip (mm)a) Aa=0 umb) Aa=100 um0.0060.0060.0050.005Without overload With 10% overloadWith 20% overload - With 40% overloadTIT----Without overload - With 10% overloadWith 20% overload ...With 40% overload0.00410Normal plastic strain ENormal plastic strain e0.001 -00.80.4 0.60.8 Distance in front of crack tip (mm)0.6 Distance in front of crack tip (mm)c) Aa=200 umd) Aa=400 um Fig.4 Normal plastic strain ahead of stationary and growing crack tipstrains close to the stationary crack tip are increased after the overload is applied, Fig.4 (a). Once the crack starts to propagate, however, the overload decreases the plastic strain at the vicinity of the growing crack tip, Fig. 4(b) and Fig. (c). Further, Fig. 4(a) through Fig. 4 (d) also show that the effect of overload on the plastic strain ahead of the growing crack tip is gradually weaken by the crack propagation until the effect disappears. In addition, the overload affected zone is larger at higher overload level. At 40% overload, the affected zone at the crack tip is about 0.4 mm.The normal plastic strain at a characteristic point in front of the growing crack tip is shown in Fig. 5, where the distance from the characteristic point to the growing crack tip, ro adopts 10 um in this study. The effect of the overload on the plastic strain in front of the growing cracktip could be clearly distinguished in Fig.5. Therefore, the 10 um could be a reasonable distance from the growing crack tip for investigating the plastic strains in the growing crack tip under the mechanical consideration. Because the applied load Ky increase with the crack propagation, the normal plastic strain at a characteristic point in front of the growing crack tip is slowly increased during the crack propagation.The effect of the overload on the normal plastic strain rate at the characteristic point in front of the growing crack tip could also be clearly distinguished, Fig.6. Therefore, the 10um could also be a reasonable distance from the growing crack tip for investigating the plastic strain rate in the growing crack tip under the mechanical consideration320.010 -0.009Normal plastic strain &Without overload With 10% overload With 20% overload With 40% overload0.0040.0030.002+TTTTTTTTTTTTTTTTT20*260870Crack advanced length Aa (mm)Fig.5 Normal plastic strain at characteristic pointahead of growing crack tip (ro=10um)0.355Normal plastic strain rate de /da (1/mm)+ Without overload- With 10% overload- With 20% overload -- -- With 40% overload0.00 I2040.6modoCrack advanced distance Aa (mm)Fig.6 Normal plastic strain at characteristic pointahead of growing crack tip (ro=10um) 3.2 Estimation of SCC growth rate of stainless steel in high temperature waterThe crack tip strain Iμct is the main mechanical affecting SCC growth rate in the film slip-dissolution /oxidation model. Because the cracks tip strain Ect is difficulty obtained directly, it is proposed to replace ect by the plastic strain &p at a characteristic distance ro in front of the crack tip in FRI model [2]. The estimation formulation of the SCC growth rate by incorporating the FRI model and EPFEM is as following [3].-1where Ka is the crack tip oxidation rate constant, which is crack tip is increased and the plastic strain and plastic-1determined by the electric-chemical environmental and material in the crack tip area, dE?p/da is the variation of tensile plastic strain with crack growth at a characteristic distance ro in front of the crack tip, m is the exponent of the current decay curve in the crack tip.SCC growth rate of a 20% cold worked 316L stainless steel in high temperature water at 288A°C is investigated in this study. Because the main focus of this research is the effect of the overload on SCC growth rate, the crack tipoxidation rate constant K, is simply appointed as5.5x10-?, and the exponent of the current decay curve m is appointed as 0.5 based on the experimental data adopted in the similar material and similar experiment conditions [7].4.0x10')3.5x10'0x10EAC growth rate da/dt (mm/s)Without overload With 10% overloadWith 20% overload ---- With 40% overload5.0x1000.20.4 0.60.8 Crack advanced length sa (mm)Fig. 7 Effect of overload on EAC crack growth rateThe calculated SCC growth rate of SUS316L stainless steel in high temperature water at 288A° using Eq. (1) is shown in Fig. 7. As can be seen, the overload affected zone is about 400 um in front of the growing crack tip, and the SCC growth rate in the overload affected zone is observably reduced by the overload. The degree of the decrease in SCC growth rate is increased as the overload level increases.4. Conclusions1. The overload will availably affect the plastic strain and the plastic strain rate at both the stationary and growing crack tips. Further, the plastic strain close to the stationary strain rate at the vicinity of the growing crack tip will be33decreased after the overload is applied. 2. SCC growth rate of 20% cold worked 316L stainless steel in high temperature water at 288A°C is decreased in the overload affected zone ahead of the growing crack tip. Therefore, a reasonable overload could availably reduce the SCC growth rate during a certain in-service period.AcknowledgementThis work was performed in the Tohoku-Hokkaido Cluster as a part of National Research Project on Aging Management and Maintenance of Nuclear Power Plant in FY2008 sponsored by Nuclear and Industrial Safety Agency (NISA).References[1]O.K. Chopra, H.M. Chung, and etc., a??Current research on environmentally assisted cracking in light water reactor environmentsa??, Nuclear Engineering and Design, Vol.194, 1999, pp.205-223. Q.J. Peng, J. Kwon, and T. Shoji, a??Development of a fundamental crack tip strain rate equation and its application to quantitative prediction of stress corrosion cracking of stainless steels in high[2][2][3]temperature oxygenated watera??, Journal of Nuclear Materials, Vol.324, 2004, pp.52-61. H. Xue, Y. Sato and T. Shoji, a??Quantitative estimation of the growth of environmentally assisted cracks at flaws in light water reactor componentsa??, Transactions of the ASME- Journal of Pressure Vessel and Technology, Vol.131, 2009, pp.61-70 C. Bichler and R. Pippan, a??Effect of single overloads in ductile metals: A reconsiderationa??, Engineering Fracture Mechanics, Vol.74, 2007, pp.1344a?“1359 ASTM Standard E399-90, Standard test method for plane strain fracture toughness of metallic materials, Annual book of ASTM Standards, Vol. 03.01, ASTM International, 2002 ABAQUS/Standard User's Manual. Version 6.5, Pawtucket, RI, Hibbitt, Karlsson & Sorensen, Inc. 2004 P.L. Andresen, M.M. Morra, and W.R. Catlin, ““Effects of yield strength, corrosion potential, composition and stress intensity factor in SCC of stainless steelsa??, Corrosion 2004, NACE, Paper 04678, 2004.34“ “Effect of Overload on SCC Growth in Stainless Steels in High Temperature Water“ “He XUE, Qunjia PENG, Tetsuo SHOJI