Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Crack growth rates have been measured in single edge-notched specimens of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaaiaaikdadaWcbaWcbaGaaGymaaqaaiaaisdaaaaaaa!3E80!\[2\tfrac{1}{4}\]Cr 1 Mo steel loaded in tension at 565 dgC. It is shown that the creep crack growth rate in tests of up to about 2,000 hours duration can be described in terms of the sharp crack stress intensity factor. A correlation is obtained over five orders of magnitude in crack growth rate and the relationship of these results to conventional creep tests is discussed. For the particular test conditions the time required to initiate a crack from a notch is not significant compared with the total times for specimens to fail. Under some conditions high rates of stable crack growth rate can be obtained. The work has obvious potential in the assessment of the integrity of components which contain defects and operate under creep conditions. A possible application of the data to the analysis of a particular thermal fatigue situation is suggested.

Crack propagation under creep conditions in a quenched % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaaiaaikdadaWcbaWcbaGaaGymaaqaaiaaisdaaaaaaa!3E80!\[2\tfrac{1}{4}\]chromium 1 molybdenum steel

Subcritical crack growth behavior in Alloy 718 was studied under creep conditions at 538, 649, and 760°C (1000, 1200, 1400°F) and crack growth rates were correlated using both linear and nonlinear elastic fracture mechanics. The results show that for a given stress intensity value crack growth rate increases significantly with increase in temperature from 538 to 649°C but either decreases or increases slightly with further increase in temperature to 760°C. On the basis of these results it is concluded that creep crack growth results from a balance of two competing processes, diffusion of point defects which contributes to crack growth, and creep deformation process that causes retardation of crack growth and even its arrest. Significance of these concepts in relation to enhancing the resistance of a given material to creep crack growth is discussed in detail.

Creep crack growth in alloy 718

The application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered. From tensile creep rupture tests on single edge notched and notched centre hole specimens of solution treated A.I.S.I. type 316 stainless steel, it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rates. These observations have been explained by considering the creep relaxation that takes place at the notch root, smoothing out the local stresses and thus making the LEFM stress distribution inapplicable. The resulting stress distribution supports the observation that the net section stress is a successful criterion on which to predict creep rupture in stainless steel.

The validity of various fracture mechanics methods at creep temperatures

A computer simulation of the time dependent development of the plastic zone ahead of a crack loaded in uniform tension was performed. The material was assumed to deform according to a creep law relating the local strain rate to the local stress. The plastic zone was modelled by an array of edge dislocations coplanar with the crack. For a given time the stress was found to be uniform in a region ahead of the crack. This region increased and the local stress decreased with increasing time. The distribution of dislocations in the zone at a given time was found to be almost the same as that given by the Bilby, Cottrell and Swinden model (1963) if the friction stress in that model was replaced by an apparent friction stress equal to the uniform stress ahead of the crack. This apparent friction stress is dependent on both the applied stress and time. Assuming a critical crack opening displacement (COD) or a critical value of theJ integral,J
c, to be the criteria for the onset of the creep crack growth the initiation time can be calculated using the results of this study. A good agreement between the theory and experiment is obtained for two different CrMoV steels. This comparison with experiments suggests that the COD is an appropriate crack growth initiation parameter for both ductile and brittle materials whilstJ
cdoes not seem to be applicable in creep fracture.

A theory of the initiation of creep crack growth

IN recent years there has been a growing awareness that the techniques of linear elastic fracture mechanics provide a powerful method for quantifying the behaviour of engineering components containing crack-like defects. In the fast fracture of high strength, low toughness materials1 and in fatigue crack growth2, it is now established that data from laboratory tests can be confidently applied to predict the integrity of a structure or crack extension in service using sharp crack stress intensity factors. More recently, a study of fatigue crack initiation from sharp notches in mild steel3 has shown that the number of cycles to initiate a crack can also be calculated using stress intensity factors. In components which operate at elevated temperatures there is a need to develop methods which describe the growth of crack-like defects in creed conditions. This is particularly relevant to welded joints where defects can be evident after stress relief and assessments are required of fitness for use.

A. T. Price

Papers

Crack growth under creep conditions.