Quasi-steady-state creep crack growth in a 3.5NiCrMoV steel
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I.
INTRODUCTION
IN the creep crack growth analysis of creep ductile materials, the crack growth rates (az ) are characterized using the macroscopic load parameters, such as the C* integrals, C(t), and Ct parameters.[1,2,3] The C* integral and C(t) are crack tip field parameters valid for the steady-state and transient (including the steady-state) creep conditions, respectively, while the Ct parameter extended the stress-power release rate definition of C* into the small-scale creep and transient creep regions. These parameters are usually obtained by continuously measuring crack length (a) and load pin displacement (D) under the constant load but seldomly z under constant displacement rate (D) conditions. The constant load test is most favored due to obvious reasons. However, load parameters are continuously changing during the constant load tests, and true steady-state crack growths are never reached even for a material undergoing the steady-state creep. Therefore, questions remain whether the results of the constant load test, particularly az and its correlation with load parameters, are accurate and can be taken as the steady-state results. In the present analysis, quasi-steady-state crack growth rates are measured by conducting creep crack growth tests under constant Ct conditions. The Ct parameter is chosen because it is easily measurable and approximately represents the crack tip fields even for a material undergoing elastic/primary and secondary creep deformation. Then, the results are compared with those of the constant load and z constant displacement-rate (D ) tests. Good agreements with the results of the constant Ct test would attest to the validity of constant load test. II.
EXPERIMENTAL PROCEDURE
A low-carbon steel with the composition shown in Table I was chosen due to its well-defined secondary creep be-
S.H. RYU, Graduate Student, and JIN YU, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 201 Chongryang, Seoul, 130-650 Korea. S.H. HONG, Research Scientist, is with the Material and Damage Analysis Team, Power Engineering Research Institute, Korea Power Engineering, Co., Inc., 360-9 Mabuk-Ri, Kusong-Myon, YounginGun, Kyunggi-Do, 449-910, Korea. Manuscript submitted November 22, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
havior.[4] The heat treatment includes austenitization of one hour at 1373 K followed by oil quenching and subsequent tempering of five hours at 963 K before air cooling. The microstructure after the heat treatment was tempered martensite with the hardness of Rc 25 and grain size of ASTM No. 4–5. Standard smooth bar creep specimens were tested under constant load at 823 K, and typical creep curves of the material are presented in Figure 1. The primary creep curve of the material was assumed to follow the power-law strain-hardening model:[5]
εz 5 B1 ε 2p s m(l1p)
[1]
where B1, m, and p are material parameters. From the creep curve of the specimens tested under 175 and 155 MPa, these para
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