Influence of cyclic to mean load ratio on creep/fatigue crack growth
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I.
INTRODUCTION
MOSTengineering components which operate at elevated temperatures are subjected to nonsteady loading during service. Depending upon the material of manufacture and operating conditions creep, fatigue, and environmental processes may contribute to failure. The dominating mode of failure in a particular circumstance will depend upon such factors as material composition, heat treatment, cyclic to mean load ratio, frequency, temperature, and operating environment. This paper is concerned with the behavior under superimposed cyclic and steady loading of components which may contain cracks. Attention will be focused on the socalled intermediate (stage II) region of cracking. Near threshold behavior will not be examined. Materials of interest to the gas turbine, chemical, and electrical power generation industries will be considered. Most emphasis will be given to examining the influence of cyclic to mean load ratio on crack propagation to augment previous studies j-j2 of the effects of frequency of loading on crack growth rate. Although environmental effects can exist when load is cycled at room temperature, 13 typically, in the absence of environmental complications, crack growth is cycle dependent and governed by fatigue processes. The mode of fracture is normally transgranular and crack growth/cycle (da/dN), where a is crack length and N the number of cycles, can be described in terms of stress intensity factor range AK by the Paris Law~4 da/dN c~ AK"
[1]
In this equation m is a material dependent constant which usually has a value between 2 and 4 with the proportionality factor being sensitive to the minimum to maximum load ratio R. 15' 16.17 • An increase in R usually produces an increase in da/dN. V. DIMOPULOS, Research Student, K. M. NIKBIN, Research Assistant, and G. A. WEBSTER, Reader, are with the Department of Mechanical Engineering, Imperial College, London, SW7 2BX, United Kingdom. This paper is based on a presentation made in the symposium "Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. METALLURGICALTRANSACTIONS A
When load is cycled at elevated temperatures time dependent crack growth is more likely to be observed. The time dependent cracking can be governed by environmental or creep processes. Generally, when time dependent mechanisms control, fractures are intergranular and when cycle dependent fatigue processes dominate they are transgranular. As at room temperature, when fatigue mechanisms control, da/dN can usually be described in terms of the Paris Law but when creep mechanisms dominate it is more appropriate to use nonlinear fracture mechanics concepts and express cracking rate a as a function of the creep fracture mechanics parameter C* as c~ C .6
[2]
where ~b is a number that has been observed experimentally to be slightly less than unity. 18.19.20The proportionality factor is determined chiefly by the material creep ductility and the constrain
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