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Microstructural and mechanical factors affecting creep-fatigue properties of heat resisting cast steels
since they prevented PPB failure. Furthermore, the usage of fine powders is preferable because it can reduce the size of pores and inclusions. Photomicrographs, graphs. 11 refs.
Microstructural and mechanical factors affecting creep-fstigue propertie,= of heat resisting cost steels. Yokomaku, T. and Saori, M. J. Soc. Mater. Sci., Japan Feb. 1990 3g, (437), 181-187 (in Japanese) Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP and their modified alloys containing a small amount of Zr, Nb and/or Ti. The failure mechanism was established based on microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life are discussed, by considering the crack propagation law. The main results obtained are as follows: (1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked. (2) Cracks propagate along the intensive sub-boundaries, on which coarse M23Cs carbides precipitate during the LCF test. (3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub*boundaries and then result in the dispersed precipitation of coarse M23C6carbides. Accordingly, the addition of these elements improves the LCF and FCP properties. (4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities, which are obtainable based on J-integral analysis of a cracked body. Graphs, photomicrographs. 6 refs.
Acceleration of fatigue crack growth for HTa0 and SUS304 steels in 3% NaCI aqueous solution under very-low-frequency cyclic loading. Shimodaira, M., Matsuoka, S., Masuda, H. andNishijima, S. J. Soc. Mater. Sci., Japan Feb. 1990 38, (437), 1 6 2 - 1 6 8 (in Japanese) Fatigue crack growth properties under freely corroding and cathodic protection in 3% NaCI aqueous solution were studied under very-low-frequency cycling for HTe0 high tensile and SUS304 stainless steels. The crack growth rate in 3% NaCI solution was accelerated as compared with that in air. The acceleration was dependent on material, environment, ~K level and frequency, and the following three types of acceleration were observed. ( 1) The CF-type acceleration was inherent to corrosion fatigue in free corrosion and the rate was accelerated about twice at frequencies less than 0.3 Hz independently of material and ~K level. (2) The HE(IG) type was related to the intergranular cracking due to hydrogen embrittlement and appeared in the range of &K = 4-20 MPa m 1/2 and at frequencies 10-3-102 Hz only for HT60 steel. The maximum acceleration rate became about five times at &K = 10 MPa m 1!2 and f = 0.3 Hz. (3) The HE(TG) type was related to the transgranular and brittle fracture due to H embrittlement and the rate became constant at frequencies less than 0.03 Hz. This type was also dependent on material, environment and ~K level. The maximum acceleration rate was nearly 30 and ten times for HT80 and SUS304 steels, respectively. Graphs, photomicrographs. 9 refs.
Investigations on small crack growth and life property in fatigue of a circumterentiaUy-notched component of pure copper subjected to combined Ioedings. Hoshide, T., Yokota, K. and Inoue, T. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 144-149 (in Japanese) Fatigue tests were conducted using solid cylindrical specimens of pure Cu with circumferential blunt notches subjected to combined axial-torsional Ioadings. The behaviour of crack growth was observed by a plastic replication technique, and the property of fatigue life was investigated. The cracking at notch root was identified as an intergranular type. The dominant morphology of crack growth was found to be the coalescence of distributed small cracks, while some difference was observed depending on the stress multiaxiality. The fatigue life was correlated with the equivalent stress of the Von Mises type and the maximum shear stress. The fatigue life for the same value of each stress parameter became longer with increasing shear component in the stress state at the notch root. An analytical procedure for the crack growth at the notch root was established by using a model based on the competition between the coalescence growth and the propagation of a dominant crack. Characteristics of intergranular cracking at the notch root and its dependence on the stress state were simulated very well by the present analytical model. Fatigue lives in several test conditions were statistically estimated by a simulation of the Monte Carlo type. The prediction with the simulated scatter-bands almost coincided with the experimental results. Graphs. 12 refs.
were as follows: (1) The acceleration behaviour of the crack propagation rate was observed just after changing the stress level from low to high, and the crack propagation rate became two times that under the constant loading. This acceleration behaviour continued for 500-1000 cycles of high-stress loading. (2) The retardation behaviour of the crack propagation rate was observed just after changing the stress level to tow from high. The number of cycles of low-stress loading needed for the crack propagation rate to return to that of constant loading was approximately 10000-70000. (3) It was difficult to elucidate the acceleration and retardation behaviour only by the change of crack opening stress level. (4) It is necessary to consider the acceleration effect of the crack propagation rate for the estimation of fatigue life under service loading conditions when a higher stress is intermittently loaded. Graphs. 14 refs.
Evaluation of actual fatigue stress by using striation spacing and hardness, Ishida, S., Tokumasu, M,, Nakasa, K. and Ishihara, H. J. Soc. Mater. ScL, Japan Jan, 1990 39, (436), 6 3 - 6 9 (in Japanese) The relation between the range of stress intensity factor &K and striation spacing S was obtained by using CT specimens of various kinds of structural steels with different hardnessss, and the actual fatigue stress of square or round bars under repeated three-point bending loads, which was assumed to constitute an actual machine part, was evaluated from the Vickers hardness HV and the striation spacing measured in the bars. The material constants, Cs and ms, obtained from the relation between S (mm) and &K (MPa~/m) for the CT specimens, S = Cs(&K)ms, were able to be approximately expressed as Cs = 1.68 × 10-1° (HV) 18s and ms - 19.1(HV) o,53respectively. The actual fatigue stress of the square bar under three-point bending agreed relatively well with the stress calculated from the striation spacing measured on the bar and the values of Cs and ms, which were obtained from the above equations by substituting the hardness. For the round bar, however, the calculated stress was larger or smaller than the actual fatigue stress when the crack length was short or long, respectively. Photomicrographs, graphs. 12 refs.
Development of two-step varying load rotating-bending fatigue testing machine and some t e s t results. Hoshi, S., Nakamura, H. and Nagai, M. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 5 8 - 6 2 (in Japanese) In order to obtain fatigue strength data in the long life range over 108 stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for a uniform rotating-bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with a short interval of the higher stress level of approximately 100 cycles. It was confirmed, through experiments done by using JIS $45C round-bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cumulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, that is fatigue lives became considerably longer than estimated values or vice versa, especially when the number of under-stressing cycles was sufficiently large. To reveal such an inconsistent effect of under-stressing, and further, to clarify the fatigue behaviours in the long life range up to 2 × 109 stress cycles, the authors intend to conduct a series of experiments systematically. 18 refs.
Considerations of grain orientation and work hardening on shortfatigue-crack modelling, de los Rios, E.R. and Navarro, A. Philos. Mag. A Mar. 1990 61, (3), 4 3 5 - 4 4 9 Methods that consider the effect of grain orientation and work hardening on shortfatigue*crack growth are examined. Fatigue cracks are assumed to form in the most favourably oriented grains on the surface of the specimen and propagate through less well oriented grains. A method of deriving the sequence of orientation factors as the crack front traverses an increasing number of grains is presented. Grain boundaries act as barriers to plastic flow in the zone ahead of the crack tip. This has the effect of hindering dislocation movement in the plastic zone, which can be expressed in terms of an increase in the frictional stress. A method of calculating this work-hardening rate is proposed. The condition for the transition from short to long fatigue cracks is established. 18 refs.
Fatigue life prediction method for gas turbine rotor disc alloy FV535. Horikawa, T,, Okada, T. and Tsunenarl, T. J. Soc. Mater. ScL, Japan Jan. 1990 39, (436), 8 2 - 8 8 (in Japanese)
Cyclic stress-strain response and low-cycle fatigue life in metallic materiels. Hatanaka, K. JSME Int. J. I Jan. 1990 33, (1), 1 3 - 2 5
A rotor disc of jet engine and/or gas turbine is subjected to intermittent stress waves, that is the repetition of centrifugal stress at the start and stop of the machine with superimposed vibrating stress. The serrated part and bolt hole of the rotor disc are important parts needed for fatigue strength evaluation. A number of studies have been conducted on the fatigue strength under the intermittent stress wave and the fatigue strength estimation of notched specimens. The fatigue strength evaluation method under the intermittent stress proposed by the authors is being used for the fatigue life prediction of the piston crown of a diesel engine. However, the adaptability of the method to the rotor disc has not been verified yet, Verification of the adaptability of the method for a rotor disc made of martensite stainless steel FV535 was therefore studied. The following conclusions were obtained from this experimental study. ( 1) The fatigue crack initiation life of a notched specimen under constant stress amplitude was well predicted by Koe's or Neuber's method. (2) The fatigue life prediction method proposed predicts the fatigue life of a smooth specimen well in the region of stress level higher than the fatigue limit obtained by using the interaction coefficient C of 0.3. However, in the region of stress near the fatigue limit, the method predicted a very conservative life and the coefficient C was greater than 5. Graphs. 12 refs.
The cyclic stress-strain response and the test method-dependent characteristics of metallic materials were discussed from the viewpoint of such metallurgical factors as the motion of screw dislocations, the formation of strain-induced martensite and dynamic strain aging. Furthermore, their low-cycle fatigue life was examined in terms of the total strain and the plastic strain, and the existence of the unified total strain-based life curve common to the various steel materials was suggested. The plastic strain-based low-cycle fatigue life was successfully estimated through surface crack growth analysis, which was made using the elastic-plastic fracture mechanics; the inflection appearing in the log &~p-log Nf relationship of the high-strength-lowductility materials as well as the Coffin-Manson law embodied in the ductile materials was satisfactorily described through the crack growth analysis. Finally, it was shown that the equivalent stress and plastic strains generated at the notch root were the predominant factors in governing the crack initiation life of the grooved cylinder components. 73 refs.
Acceleration and retardation of crack propagation in high-strength steel under variable loading. Takashina, N. and Misawa, H. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 7 0 - 7 5 (in Japanese) To clarify the interaction effect of stress changing on the crack propagation behaviour, fatigue tests in which the stress sequence was high to low and low to high were carried out on the high-strength steel JIS SNCM439. The results obtained
92
Room temperature fatigue behaviour of a normalized steel SAE 4140 in torsion. Klumpp, S.,Eifler, D. and Macherauch, E. Mater. Wiss. Werkst. Tech. M a y 1990 21, (5), 2 0 3 - 2 1 3 (in German) Cyclic deformation behsviour of a normalized steel SAE4140 in shear-straincontrolled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed onto an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear-strain*controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains. Graphs. 12 rsfs.
Int J Fatigue January 1991
Microstructural and mechanical factors affecting creep-fstigue propertie,= of heat resisting cost steels. Yokomaku, T. and Saori, M. J. Soc. Mater. Sci., Japan Feb. 1990 3g, (437), 181-187 (in Japanese) Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP and their modified alloys containing a small amount of Zr, Nb and/or Ti. The failure mechanism was established based on microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life are discussed, by considering the crack propagation law. The main results obtained are as follows: (1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked. (2) Cracks propagate along the intensive sub-boundaries, on which coarse M23Cs carbides precipitate during the LCF test. (3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub*boundaries and then result in the dispersed precipitation of coarse M23C6carbides. Accordingly, the addition of these elements improves the LCF and FCP properties. (4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities, which are obtainable based on J-integral analysis of a cracked body. Graphs, photomicrographs. 6 refs.
Acceleration of fatigue crack growth for HTa0 and SUS304 steels in 3% NaCI aqueous solution under very-low-frequency cyclic loading. Shimodaira, M., Matsuoka, S., Masuda, H. andNishijima, S. J. Soc. Mater. Sci., Japan Feb. 1990 38, (437), 1 6 2 - 1 6 8 (in Japanese) Fatigue crack growth properties under freely corroding and cathodic protection in 3% NaCI aqueous solution were studied under very-low-frequency cycling for HTe0 high tensile and SUS304 stainless steels. The crack growth rate in 3% NaCI solution was accelerated as compared with that in air. The acceleration was dependent on material, environment, ~K level and frequency, and the following three types of acceleration were observed. ( 1) The CF-type acceleration was inherent to corrosion fatigue in free corrosion and the rate was accelerated about twice at frequencies less than 0.3 Hz independently of material and ~K level. (2) The HE(IG) type was related to the intergranular cracking due to hydrogen embrittlement and appeared in the range of &K = 4-20 MPa m 1/2 and at frequencies 10-3-102 Hz only for HT60 steel. The maximum acceleration rate became about five times at &K = 10 MPa m 1!2 and f = 0.3 Hz. (3) The HE(TG) type was related to the transgranular and brittle fracture due to H embrittlement and the rate became constant at frequencies less than 0.03 Hz. This type was also dependent on material, environment and ~K level. The maximum acceleration rate was nearly 30 and ten times for HT80 and SUS304 steels, respectively. Graphs, photomicrographs. 9 refs.
Investigations on small crack growth and life property in fatigue of a circumterentiaUy-notched component of pure copper subjected to combined Ioedings. Hoshide, T., Yokota, K. and Inoue, T. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 144-149 (in Japanese) Fatigue tests were conducted using solid cylindrical specimens of pure Cu with circumferential blunt notches subjected to combined axial-torsional Ioadings. The behaviour of crack growth was observed by a plastic replication technique, and the property of fatigue life was investigated. The cracking at notch root was identified as an intergranular type. The dominant morphology of crack growth was found to be the coalescence of distributed small cracks, while some difference was observed depending on the stress multiaxiality. The fatigue life was correlated with the equivalent stress of the Von Mises type and the maximum shear stress. The fatigue life for the same value of each stress parameter became longer with increasing shear component in the stress state at the notch root. An analytical procedure for the crack growth at the notch root was established by using a model based on the competition between the coalescence growth and the propagation of a dominant crack. Characteristics of intergranular cracking at the notch root and its dependence on the stress state were simulated very well by the present analytical model. Fatigue lives in several test conditions were statistically estimated by a simulation of the Monte Carlo type. The prediction with the simulated scatter-bands almost coincided with the experimental results. Graphs. 12 refs.
were as follows: (1) The acceleration behaviour of the crack propagation rate was observed just after changing the stress level from low to high, and the crack propagation rate became two times that under the constant loading. This acceleration behaviour continued for 500-1000 cycles of high-stress loading. (2) The retardation behaviour of the crack propagation rate was observed just after changing the stress level to tow from high. The number of cycles of low-stress loading needed for the crack propagation rate to return to that of constant loading was approximately 10000-70000. (3) It was difficult to elucidate the acceleration and retardation behaviour only by the change of crack opening stress level. (4) It is necessary to consider the acceleration effect of the crack propagation rate for the estimation of fatigue life under service loading conditions when a higher stress is intermittently loaded. Graphs. 14 refs.
Evaluation of actual fatigue stress by using striation spacing and hardness, Ishida, S., Tokumasu, M,, Nakasa, K. and Ishihara, H. J. Soc. Mater. ScL, Japan Jan, 1990 39, (436), 6 3 - 6 9 (in Japanese) The relation between the range of stress intensity factor &K and striation spacing S was obtained by using CT specimens of various kinds of structural steels with different hardnessss, and the actual fatigue stress of square or round bars under repeated three-point bending loads, which was assumed to constitute an actual machine part, was evaluated from the Vickers hardness HV and the striation spacing measured in the bars. The material constants, Cs and ms, obtained from the relation between S (mm) and &K (MPa~/m) for the CT specimens, S = Cs(&K)ms, were able to be approximately expressed as Cs = 1.68 × 10-1° (HV) 18s and ms - 19.1(HV) o,53respectively. The actual fatigue stress of the square bar under three-point bending agreed relatively well with the stress calculated from the striation spacing measured on the bar and the values of Cs and ms, which were obtained from the above equations by substituting the hardness. For the round bar, however, the calculated stress was larger or smaller than the actual fatigue stress when the crack length was short or long, respectively. Photomicrographs, graphs. 12 refs.
Development of two-step varying load rotating-bending fatigue testing machine and some t e s t results. Hoshi, S., Nakamura, H. and Nagai, M. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 5 8 - 6 2 (in Japanese) In order to obtain fatigue strength data in the long life range over 108 stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for a uniform rotating-bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with a short interval of the higher stress level of approximately 100 cycles. It was confirmed, through experiments done by using JIS $45C round-bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cumulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, that is fatigue lives became considerably longer than estimated values or vice versa, especially when the number of under-stressing cycles was sufficiently large. To reveal such an inconsistent effect of under-stressing, and further, to clarify the fatigue behaviours in the long life range up to 2 × 109 stress cycles, the authors intend to conduct a series of experiments systematically. 18 refs.
Considerations of grain orientation and work hardening on shortfatigue-crack modelling, de los Rios, E.R. and Navarro, A. Philos. Mag. A Mar. 1990 61, (3), 4 3 5 - 4 4 9 Methods that consider the effect of grain orientation and work hardening on shortfatigue*crack growth are examined. Fatigue cracks are assumed to form in the most favourably oriented grains on the surface of the specimen and propagate through less well oriented grains. A method of deriving the sequence of orientation factors as the crack front traverses an increasing number of grains is presented. Grain boundaries act as barriers to plastic flow in the zone ahead of the crack tip. This has the effect of hindering dislocation movement in the plastic zone, which can be expressed in terms of an increase in the frictional stress. A method of calculating this work-hardening rate is proposed. The condition for the transition from short to long fatigue cracks is established. 18 refs.
Fatigue life prediction method for gas turbine rotor disc alloy FV535. Horikawa, T,, Okada, T. and Tsunenarl, T. J. Soc. Mater. ScL, Japan Jan. 1990 39, (436), 8 2 - 8 8 (in Japanese)
Cyclic stress-strain response and low-cycle fatigue life in metallic materiels. Hatanaka, K. JSME Int. J. I Jan. 1990 33, (1), 1 3 - 2 5
A rotor disc of jet engine and/or gas turbine is subjected to intermittent stress waves, that is the repetition of centrifugal stress at the start and stop of the machine with superimposed vibrating stress. The serrated part and bolt hole of the rotor disc are important parts needed for fatigue strength evaluation. A number of studies have been conducted on the fatigue strength under the intermittent stress wave and the fatigue strength estimation of notched specimens. The fatigue strength evaluation method under the intermittent stress proposed by the authors is being used for the fatigue life prediction of the piston crown of a diesel engine. However, the adaptability of the method to the rotor disc has not been verified yet, Verification of the adaptability of the method for a rotor disc made of martensite stainless steel FV535 was therefore studied. The following conclusions were obtained from this experimental study. ( 1) The fatigue crack initiation life of a notched specimen under constant stress amplitude was well predicted by Koe's or Neuber's method. (2) The fatigue life prediction method proposed predicts the fatigue life of a smooth specimen well in the region of stress level higher than the fatigue limit obtained by using the interaction coefficient C of 0.3. However, in the region of stress near the fatigue limit, the method predicted a very conservative life and the coefficient C was greater than 5. Graphs. 12 refs.
The cyclic stress-strain response and the test method-dependent characteristics of metallic materials were discussed from the viewpoint of such metallurgical factors as the motion of screw dislocations, the formation of strain-induced martensite and dynamic strain aging. Furthermore, their low-cycle fatigue life was examined in terms of the total strain and the plastic strain, and the existence of the unified total strain-based life curve common to the various steel materials was suggested. The plastic strain-based low-cycle fatigue life was successfully estimated through surface crack growth analysis, which was made using the elastic-plastic fracture mechanics; the inflection appearing in the log &~p-log Nf relationship of the high-strength-lowductility materials as well as the Coffin-Manson law embodied in the ductile materials was satisfactorily described through the crack growth analysis. Finally, it was shown that the equivalent stress and plastic strains generated at the notch root were the predominant factors in governing the crack initiation life of the grooved cylinder components. 73 refs.
Acceleration and retardation of crack propagation in high-strength steel under variable loading. Takashina, N. and Misawa, H. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 7 0 - 7 5 (in Japanese) To clarify the interaction effect of stress changing on the crack propagation behaviour, fatigue tests in which the stress sequence was high to low and low to high were carried out on the high-strength steel JIS SNCM439. The results obtained
92
Room temperature fatigue behaviour of a normalized steel SAE 4140 in torsion. Klumpp, S.,Eifler, D. and Macherauch, E. Mater. Wiss. Werkst. Tech. M a y 1990 21, (5), 2 0 3 - 2 1 3 (in German) Cyclic deformation behsviour of a normalized steel SAE4140 in shear-straincontrolled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed onto an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear-strain*controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains. Graphs. 12 rsfs.
Int J Fatigue January 1991