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Application of the overstress concept to inelastic behaviour and evaluation of creep-fatigue damage for modified 9Cr-1Mo steel
Analysis of fatigue crack growth behaviour at sea and its comparison with laboratory test results. Iwadate, T. and Abe, T. J. Soc. Mater. Sci., Japan Aug 1990 39, 1 1 3 9 - 1 1 4 4 (in Japanese) In order to study the life prediction method of structural material for use in a sea environment, e field testing facility to measure the fatigue crack growth rate, caused by sea waves, of the materials was developed. The tests of fatigue crack growth rate measurements using 3.5% Ni-Cr-Mo-V structural steels were performed in cathodic protection and free corrosion environments. The field test data were analysed using the rainflow method and equivalent load (Poq) equation. When the yield strength and the stress ratio increase the fatigue crack growth rate increases. The fatigue crack growth rate of field test results was a little different from indoor test results obtained using ASTM sea water. This difference may be caused by the change of stress ratio and frequency conditions. The crack growth rate of laboratory tests using field test conditions coincided well with the field test results, in observations of the fracture surface of the specimens, intergranular fractures containing fatigue cracks were observed. The higher yield strength material showed a larger number of intergranular fractures. This study demonstrates that the fatigue crack growth behaviour in a real environment at sea can be obtained using laboratory test data. Graphs. photomicrographs. 4 refs.
Influenced prior loading on the creep-fatigue damage accumulation of heat-rasictant steels. Kloos, K.H., Granacher, J. and Scholz, A. Materialwissen. Werkstofftech. Dec. 1990 21, (12), 4 7 2 - 4 8 0 (in German) On two heat-resistant powder plant steels the influence of prior strain cycling on the creep-rupture behaviour and the influence of prior creep loading on the strain cycling behaviour are investigated. These influences concern the number of cycles to failure and the rupture time, being the reference values of the generalized damage accumulation rule, and they are used for a creep-fatigue analysis of the results of long-term service-type strain cycling tests. Graphs. photomicrographs. 11 refs.
Effect of crack initiation mode on low-cycle fatigue life of type 304 stainless steel with surface roughness. Lee, J.M. and Nam, S.W. Mater. Lett. Dec. 1990 10, (6), 2 2 3 - 2 3 0 To investigate the effect of surface roughness and grain size on the fatigue life, low-cycle fatigue (LCF) tests were carried out on type 304 stainless-steel specimens with three different modes of surface roughness and two different grain sizes (50 and 500 p,m) at the test temperatures of 300 and 873 K. The fatigue cracks for an electropolished specimen are initiated at grain boundaries regardless of grain size and testing temperature, but the fatigue crack initiation site for a specimen with a rough surface depends on the grain size, the surface roughness and temperature. The surface roughness effect is decreased with increasing grain size and temperature because of the change in the crack initiation mode. Photomicrographs. 18 refs. A thermal-transient test of an FBR piping-bellows model. Saito,
Umeda, H., Kanazawa, S., Watashi, K. and Imazu, A. Int. J. Pressure Vess. Piping 1990 44, (1), 1 1 7 - 1 3 5
T.,
The results are described of a thermal-transient test of the piping-bellows models consisting of an internally pressurized type of bellows and an externally pressurized type of bellows. The piping bellows were subjected to cyclic cold and hot thermal transients by Na in a temperature range from 250-600 °C. Heat transfer and thermal elastic stress analyses were carried out for evaluating the creep-fatigue strength. The method of evaluating creep-fatigue damage in the Design Guide of Japan was demonstrated to be applicable to ensuring a high degree of integrity of the specific structure of the bellows type of expansion joints. The bellows material was 304 stainless steel. Graphs, photomicrographs. 5 refs.
Application of the overstress concept to inelastic behaviour and evaluation of creep-fatigue damage for modified 9 C r - l M o steel. Taguchi, K., Kanno, E., Ozaki, S. and Uno, T. Int. J. Pressure Vess. Piping 1990 44, (1), 9 9 - 1 1 5 A method was developed for both stimulating the inelastic behaviour and predicting the life of modified 9Cr-lMo steel at elevated temperatures. A unified constitutive equation and a damage-rate equation, based on the overstress concept, have been proposed. These equations are based on the same internal-state variables, which coincide with the back stress and overstress determined experimentally. It is therefore possible, by coupling these equations, to simulate the inelastic behaviour and damage development simultaneously. This method was applied to the results of creep and creep-fatigue tests on modified 9Cr-lMo steel at 550 °C in air. The simulated inelastic behaviour and the predicted lives agreed well with the experimental results. Graphs. 8 refs. Elastic-plastic fatigue-crack growth and tearing instability behaviour under cyclic loads. Mogami, K., Hayashi, T., Ando, K. and Ogura, N. Int. J. Pressure Vess. Piping 1990 44, (1), 8 5 - 9 7 The crack-growth behaviour and tearing instability characteristics under cyclic high stress were studied. The materials used were STS42 carbon steel and A508 class3 low-alloy steel. Tests were performed under both load- and displacement-controlled conditions. For a tearing instability test a high-compliance test apparatus was used. The cyclic J-R curve under lower initial J-levels than Jic showed lower values than a monotonic J-R curve. A wide range of fatigue-crack growth rates (da/dN = (3-1) × 10-5 mm/cycle) can be evaluated by using the equation in which AJ and Jmax are used. In a high-compliance load system, it was shown that the critical J-value at tearing instability under cyclic loading is almost equal to the value of J at an instability value under monotonic loading. Graphs. 10 refs.
Construction o f a f a t i g u e - c r a c k - g r o w t h database for nuclear-component ferritic steels in Japan and its statistical analysis. Kobayashi, H., Nakamura, H., Kasai, K.-I., Saito, M., Funada, T., Shibata, K. and lida, K. Int. J. Pressure Vess. Piping 1990 44, (1), 6 7 - 8 3 A database of the fatigue-crack growth for nuclear-component ferritic steels was constructed. The data collected are for the low-alloy steels (A533B-1, A508-2, A5083) and a carbon steel (STS42), tested in air from room temperature up to 371 °C. The characterization of the fatigue-crack-growth data and the improvement of the conventional design curve were attempted. A proposed design curve is given as follows: da/dN = 3.57 x 10 9 [(1 + 0.30R) (Eo/E)051 - ~K] 302 • (da/dNmm/cycls; AK, MPa ~/m) where da/dN is the fatigue-crack-growth rate, R is the stress ratio, Eo is
Int J Fatigue September 1991
the Young's modulus at room temperature, E is the Young's modulus at the temperature concerned, and ~K is the stress-intensity-factor range. This equation proved more applicable than the one proposed for the revision of section XI of the ASME Boiler and Pressure Vessel Code. Graphs. 6 refs.
Surface c o n d i t i o n effect on the fatigue behaviour of aluminium-lithium alloys. Eftekhari, A. and Talia, J.E. Wichita State University Report No N90-25209/9/XAB 1990 23 pp Production of AI-Li (2090-T3) alloy by powder metallurgy results in a material with a surface that contains micro-sized and macro-sized defects. In addition, scratches could be introduced to this surface during assembly and/or processing with a drastic decrease in fatigue strength. The effects of scratching are so severe that a smooth sample with a 200 ~m scratch will have a fatigue life as a notched sample with K t - 3, a reduction in fatigue life of about three orders of magnitude. To recover this loss, shot peening, polishing, anodizing and some of the combined effects of these techniques have been studied. Depending on the applied stress, the fatigue strength of stratched samples, up to a certain depth, can be partially recovered and some of these treatments have accumulative results on improving the fatigue life. The results are discussed in relation to the potential effects of defects on the nucleation of microcracks and their propagation.
The behaviour o f s h o r t fatigue cracks in a I~-processed t i t a n i u m alloy. Hastings, P.J. Diss. Abstr. int. Nov. 1990 51, (5), 219 pp An investigation has been made into the behaviour of short fatigue cracks in the 6processed Ti alloy, Ti65s. The effect on the material of a number of different variables was studied, particularly microstructure, as well as stress level, specimen design and specimen size. Initiation of cracks was found to occur at similar microstructural features in smooth specimens, regardless of c~-platelet morphology. Cracks initiated from slip bands across small bundles of a-platslets, despite the presence of longer, more intense slip bands across a-colonies; ~-platelet morphology was found to have a significant effect on short-crack propagation rates when the crack length was of the order of, or less than. the s-colony size--the more aligned the ~-platelets, the greater the short-crack propagation rates at equivalent values of AK. Increasing the prior G-grain size (and hence the a-colony size) led to higher short-crack propagation rates at equivalent values of ,~K. Specimen design appeared to have an effect on the short-crack growth rates--a uniaxial tension specimen design gave slightly higher growth rates than a four-point bend specimen design. Variation in specimen size had no effect on short-crack growth rates for the same microstructural condition, but a larger specimen size was found to reduce the scatter in specimen lives observed in smaller specimens. Short-crack growth rates were apparently insensitive to stress level when compared at the same nominal ~K values. Crack shape was found to vary considerably at crack depths less than approximately 0.75 mm. At crack depths greater than this value, cracks took an approximately semicircular form. In keeping with many recent studies of short-crack behaviour short cracks were found to grow faster than long cracks at the same nominal value of AK and at ~K below the long-crack threshold value, ~Kth.
Fatigue crack initiation based on random-irreversible slip and surface roughness in copper single crystals--computer modelling and characterization using scanning tunnelling microscopy. Rosenbloom, S.N. Diss. Abstr. Int. Nov. 1990 51, (5), 266 pp In low-strain fatigue of Cu single crystals, cracks nucleate early in life in persistent slip bands (PSBs), which arise from irreversible slip. Although many attempts have been made to explain crack nucleation, none has been entirely successful. Here, it has been shown that random slip can lead to crack initiation within a number of cycles that is reasonably in agreement with the experimental results. A computer simulation of random slip in PSBs, based on experimentally determined parameters, was developed. This model produced crack initiation within 104 cycles. The results of the fully random model were compared with the results of another computer model designed to simulate completely irreversible random slip. Based on the comparison, the reversibility associated with the fully random model and, presumably, real PSBs, was determined to be very high, 87.5% on average. This evaluation of PSB reversibility is the only known quantitative description based on the macroscopic efficiency with which a PSB develops a crack. Model-generated PSB profiles were characterized by their root mean square (RMS) roughnesses. The curve of RMS roughnesses plotted against number of cycles, from saturation to crack initiation, was determined. The RMS was hypothesized to become saturated after crack nucleation because the developing crack shields the surface of the PSB from further roughening. Scanning tunnelling microscopy (STM) was used to investigate PSB surface topography, as were more conventional techniques such as interferometry and SEM It was possible to use STM by modifying a conventional STM to scan areas as large as 50 by 50 I*m horizontally and 10 i~m vertically. PSBs, cycled to various numbers of cycles prior to crack initiation, were imaged by STM and characterized by their RMS. RMS roughnesses were also calculated for PSBs cycled beyond crack initiation. ARMS against number of cycles plot was constructed that agreed weil with the model-generated plot and the hypothesis of RMS saturation after crack initation in both shape and actual values. It was concluded that surface roughness can be used to predict crack initiation, an idea that has never been exploited.
In Situ measurement of strain field during high-temperature low-cycle
fatigue. Olfe, J., Rie, K.-T., Ritter, R. and Wilke, W. Z. Metall. Nov. 1990 81, (11), 7 8 3 - 7 8 9 (in German)
In the low-cycle fatigue range the lifetime is determined mostly by the propagation of cracks. Since the crack growth is a local phenomenon that leads to failure of material (e.g., X20CrMoV12 1) it is necessary to study localized crack tip deformation to get reliable lifetime prediction. In situ measurements of local strain at the crack tip, particularly at high temperature, are not easily made because of inaccessibility. A method is presented that allows one to examine the strain field in the region of the crack tip and to estimate the ductile yield strain at the crack tip during a hightemperature low-cycle fatigue test (in situ). To demonstrate the efficiency and reliability of the method presented, the crack tip behaviour during one cycle is analysed. Further applications are described. Graphs, photomicrographs. 9 refs.
435
Influenced prior loading on the creep-fatigue damage accumulation of heat-rasictant steels. Kloos, K.H., Granacher, J. and Scholz, A. Materialwissen. Werkstofftech. Dec. 1990 21, (12), 4 7 2 - 4 8 0 (in German) On two heat-resistant powder plant steels the influence of prior strain cycling on the creep-rupture behaviour and the influence of prior creep loading on the strain cycling behaviour are investigated. These influences concern the number of cycles to failure and the rupture time, being the reference values of the generalized damage accumulation rule, and they are used for a creep-fatigue analysis of the results of long-term service-type strain cycling tests. Graphs. photomicrographs. 11 refs.
Effect of crack initiation mode on low-cycle fatigue life of type 304 stainless steel with surface roughness. Lee, J.M. and Nam, S.W. Mater. Lett. Dec. 1990 10, (6), 2 2 3 - 2 3 0 To investigate the effect of surface roughness and grain size on the fatigue life, low-cycle fatigue (LCF) tests were carried out on type 304 stainless-steel specimens with three different modes of surface roughness and two different grain sizes (50 and 500 p,m) at the test temperatures of 300 and 873 K. The fatigue cracks for an electropolished specimen are initiated at grain boundaries regardless of grain size and testing temperature, but the fatigue crack initiation site for a specimen with a rough surface depends on the grain size, the surface roughness and temperature. The surface roughness effect is decreased with increasing grain size and temperature because of the change in the crack initiation mode. Photomicrographs. 18 refs. A thermal-transient test of an FBR piping-bellows model. Saito,
Umeda, H., Kanazawa, S., Watashi, K. and Imazu, A. Int. J. Pressure Vess. Piping 1990 44, (1), 1 1 7 - 1 3 5
T.,
The results are described of a thermal-transient test of the piping-bellows models consisting of an internally pressurized type of bellows and an externally pressurized type of bellows. The piping bellows were subjected to cyclic cold and hot thermal transients by Na in a temperature range from 250-600 °C. Heat transfer and thermal elastic stress analyses were carried out for evaluating the creep-fatigue strength. The method of evaluating creep-fatigue damage in the Design Guide of Japan was demonstrated to be applicable to ensuring a high degree of integrity of the specific structure of the bellows type of expansion joints. The bellows material was 304 stainless steel. Graphs, photomicrographs. 5 refs.
Application of the overstress concept to inelastic behaviour and evaluation of creep-fatigue damage for modified 9 C r - l M o steel. Taguchi, K., Kanno, E., Ozaki, S. and Uno, T. Int. J. Pressure Vess. Piping 1990 44, (1), 9 9 - 1 1 5 A method was developed for both stimulating the inelastic behaviour and predicting the life of modified 9Cr-lMo steel at elevated temperatures. A unified constitutive equation and a damage-rate equation, based on the overstress concept, have been proposed. These equations are based on the same internal-state variables, which coincide with the back stress and overstress determined experimentally. It is therefore possible, by coupling these equations, to simulate the inelastic behaviour and damage development simultaneously. This method was applied to the results of creep and creep-fatigue tests on modified 9Cr-lMo steel at 550 °C in air. The simulated inelastic behaviour and the predicted lives agreed well with the experimental results. Graphs. 8 refs. Elastic-plastic fatigue-crack growth and tearing instability behaviour under cyclic loads. Mogami, K., Hayashi, T., Ando, K. and Ogura, N. Int. J. Pressure Vess. Piping 1990 44, (1), 8 5 - 9 7 The crack-growth behaviour and tearing instability characteristics under cyclic high stress were studied. The materials used were STS42 carbon steel and A508 class3 low-alloy steel. Tests were performed under both load- and displacement-controlled conditions. For a tearing instability test a high-compliance test apparatus was used. The cyclic J-R curve under lower initial J-levels than Jic showed lower values than a monotonic J-R curve. A wide range of fatigue-crack growth rates (da/dN = (3-1) × 10-5 mm/cycle) can be evaluated by using the equation in which AJ and Jmax are used. In a high-compliance load system, it was shown that the critical J-value at tearing instability under cyclic loading is almost equal to the value of J at an instability value under monotonic loading. Graphs. 10 refs.
Construction o f a f a t i g u e - c r a c k - g r o w t h database for nuclear-component ferritic steels in Japan and its statistical analysis. Kobayashi, H., Nakamura, H., Kasai, K.-I., Saito, M., Funada, T., Shibata, K. and lida, K. Int. J. Pressure Vess. Piping 1990 44, (1), 6 7 - 8 3 A database of the fatigue-crack growth for nuclear-component ferritic steels was constructed. The data collected are for the low-alloy steels (A533B-1, A508-2, A5083) and a carbon steel (STS42), tested in air from room temperature up to 371 °C. The characterization of the fatigue-crack-growth data and the improvement of the conventional design curve were attempted. A proposed design curve is given as follows: da/dN = 3.57 x 10 9 [(1 + 0.30R) (Eo/E)051 - ~K] 302 • (da/dNmm/cycls; AK, MPa ~/m) where da/dN is the fatigue-crack-growth rate, R is the stress ratio, Eo is
Int J Fatigue September 1991
the Young's modulus at room temperature, E is the Young's modulus at the temperature concerned, and ~K is the stress-intensity-factor range. This equation proved more applicable than the one proposed for the revision of section XI of the ASME Boiler and Pressure Vessel Code. Graphs. 6 refs.
Surface c o n d i t i o n effect on the fatigue behaviour of aluminium-lithium alloys. Eftekhari, A. and Talia, J.E. Wichita State University Report No N90-25209/9/XAB 1990 23 pp Production of AI-Li (2090-T3) alloy by powder metallurgy results in a material with a surface that contains micro-sized and macro-sized defects. In addition, scratches could be introduced to this surface during assembly and/or processing with a drastic decrease in fatigue strength. The effects of scratching are so severe that a smooth sample with a 200 ~m scratch will have a fatigue life as a notched sample with K t - 3, a reduction in fatigue life of about three orders of magnitude. To recover this loss, shot peening, polishing, anodizing and some of the combined effects of these techniques have been studied. Depending on the applied stress, the fatigue strength of stratched samples, up to a certain depth, can be partially recovered and some of these treatments have accumulative results on improving the fatigue life. The results are discussed in relation to the potential effects of defects on the nucleation of microcracks and their propagation.
The behaviour o f s h o r t fatigue cracks in a I~-processed t i t a n i u m alloy. Hastings, P.J. Diss. Abstr. int. Nov. 1990 51, (5), 219 pp An investigation has been made into the behaviour of short fatigue cracks in the 6processed Ti alloy, Ti65s. The effect on the material of a number of different variables was studied, particularly microstructure, as well as stress level, specimen design and specimen size. Initiation of cracks was found to occur at similar microstructural features in smooth specimens, regardless of c~-platelet morphology. Cracks initiated from slip bands across small bundles of a-platslets, despite the presence of longer, more intense slip bands across a-colonies; ~-platelet morphology was found to have a significant effect on short-crack propagation rates when the crack length was of the order of, or less than. the s-colony size--the more aligned the ~-platelets, the greater the short-crack propagation rates at equivalent values of AK. Increasing the prior G-grain size (and hence the a-colony size) led to higher short-crack propagation rates at equivalent values of ,~K. Specimen design appeared to have an effect on the short-crack growth rates--a uniaxial tension specimen design gave slightly higher growth rates than a four-point bend specimen design. Variation in specimen size had no effect on short-crack growth rates for the same microstructural condition, but a larger specimen size was found to reduce the scatter in specimen lives observed in smaller specimens. Short-crack growth rates were apparently insensitive to stress level when compared at the same nominal ~K values. Crack shape was found to vary considerably at crack depths less than approximately 0.75 mm. At crack depths greater than this value, cracks took an approximately semicircular form. In keeping with many recent studies of short-crack behaviour short cracks were found to grow faster than long cracks at the same nominal value of AK and at ~K below the long-crack threshold value, ~Kth.
Fatigue crack initiation based on random-irreversible slip and surface roughness in copper single crystals--computer modelling and characterization using scanning tunnelling microscopy. Rosenbloom, S.N. Diss. Abstr. Int. Nov. 1990 51, (5), 266 pp In low-strain fatigue of Cu single crystals, cracks nucleate early in life in persistent slip bands (PSBs), which arise from irreversible slip. Although many attempts have been made to explain crack nucleation, none has been entirely successful. Here, it has been shown that random slip can lead to crack initiation within a number of cycles that is reasonably in agreement with the experimental results. A computer simulation of random slip in PSBs, based on experimentally determined parameters, was developed. This model produced crack initiation within 104 cycles. The results of the fully random model were compared with the results of another computer model designed to simulate completely irreversible random slip. Based on the comparison, the reversibility associated with the fully random model and, presumably, real PSBs, was determined to be very high, 87.5% on average. This evaluation of PSB reversibility is the only known quantitative description based on the macroscopic efficiency with which a PSB develops a crack. Model-generated PSB profiles were characterized by their root mean square (RMS) roughnesses. The curve of RMS roughnesses plotted against number of cycles, from saturation to crack initiation, was determined. The RMS was hypothesized to become saturated after crack nucleation because the developing crack shields the surface of the PSB from further roughening. Scanning tunnelling microscopy (STM) was used to investigate PSB surface topography, as were more conventional techniques such as interferometry and SEM It was possible to use STM by modifying a conventional STM to scan areas as large as 50 by 50 I*m horizontally and 10 i~m vertically. PSBs, cycled to various numbers of cycles prior to crack initiation, were imaged by STM and characterized by their RMS. RMS roughnesses were also calculated for PSBs cycled beyond crack initiation. ARMS against number of cycles plot was constructed that agreed weil with the model-generated plot and the hypothesis of RMS saturation after crack initation in both shape and actual values. It was concluded that surface roughness can be used to predict crack initiation, an idea that has never been exploited.
In Situ measurement of strain field during high-temperature low-cycle
fatigue. Olfe, J., Rie, K.-T., Ritter, R. and Wilke, W. Z. Metall. Nov. 1990 81, (11), 7 8 3 - 7 8 9 (in German)
In the low-cycle fatigue range the lifetime is determined mostly by the propagation of cracks. Since the crack growth is a local phenomenon that leads to failure of material (e.g., X20CrMoV12 1) it is necessary to study localized crack tip deformation to get reliable lifetime prediction. In situ measurements of local strain at the crack tip, particularly at high temperature, are not easily made because of inaccessibility. A method is presented that allows one to examine the strain field in the region of the crack tip and to estimate the ductile yield strain at the crack tip during a hightemperature low-cycle fatigue test (in situ). To demonstrate the efficiency and reliability of the method presented, the crack tip behaviour during one cycle is analysed. Further applications are described. Graphs, photomicrographs. 9 refs.
435