Fracture and fatigue behavior of sintered steel at elevated temperatures: Part II. Fatigue crack propagation
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I. INTRODUCTION
PART I of this report discusses the fracture toughness and the fracture mechanism of sintered steel. The analysis in Part I of this article indicates that the stress intensity factor, K, of continuum fracture mechanics can still represent the level of stress at the crack tip of porous sintered steel. Based on the conclusions in Part I, we are more willing to adopt the approach of using the stress intensity factor range, DK, to characterize the fatigue crack propagation (FCP) of porous materials. The major portion of the fatigue life of sintered steels is spent in subcritical crack propagation under cyclic loading. The fatigue crack resistance of sintered steel is generally lower than that of wrought steel, and the crack growth rate is more sensitive to DK.[1,2,3] In general, the rate increases with temperature at a given DK, and the crack growth threshold value, DKth, decreases with increasing temperatures for dense materials.[4,5,6] The FCP at elevated temperatures is a complex problem involving a large number of additional factors such as frequency, hold time, creep, and environment. Moreover, most of the structural materials are metallurgically less stable when they are cyclically deformed at elevated temperatures. Little work, however, has been conducted with the purpose of evaluating fatigue behavior of sintered steels at elevated temperature, even though it is very important in applications because sintered steels are commonly used at elevated temperatures. On the other hand, we note the recent argument on crack closure effects on FCP. For over 20 years, the increase of FCP rate and the decrease of DKth with an increasing R and with the same DK have been attributed to crack closure effects. It is generally accepted that crack closure reduces the applied stress intensity amplitude, DKapp, by a factor, Kcl, related to the stress intensity at which closure occurs, and it is particularly significant at the near-threshold regimes at low load ratios (R , 0.6).[7–11] The magnitude of Kcl is generally measured by observing the change in the compliZHAOHUI SHAN, Research Associate, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332. YANG LENG, Associate Professor, is with the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong. Manuscript submitted June 2, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
ance of load vs displacement curves. The effective stress intensity factor amplitude at the crack tip, DKeff, is given by DKeff 5 Kmax 2 Kcl
[1]
The closure concepts have been used to rationalize the influence of R on DKth by many researchers, and they suggest that the observed effects of R could be explained in terms of the relationship between the experimentally measured DK and DKeff. Thus, the R effects should disappear using da/ dN vs DKeff to describe the crack propagation behavior. More recently, Vasudevan and co-workers strongly object to the idea that crack closure can explain the R
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