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Effect of rest time after application of single overload cycle on fatigue life
662
Fatigue Abstracts
Fatigue properties of high strength steels containing retained austenite Yokoi, T., Kawasaki, K., Takahashi, M., Koyama, K. and Mizui, M. JSAE Review (1996) 17 (2), 210-212
Fatigue life distribution and its simulation in spheroidal graphite cast irons Tokaji, K. and Ogawa, T. Mater. Sci. Res. Int. (Mar. 1996) 2 (1), 39-45
In order to apply high strength steels to automotive parts, the effects of strengthening mechanisms on the fatigue properties were investigated. The following results were obtained: (1) the fatigue limit and fatigue strength at 500000 cycles of TRIP steel are superior to those of DP steel and Nb containing HSLA steel. (2) TRIP steel has higher resistance to cyclic softening than DP steel and Nb containing HSLA steel. (3) These results indicate that the compressive residual stress caused by transformation from retained austenite to martensite under the cyclic load can retard the propagation of the microcracks. Graphs, 4 ref.
Statistical fatigue tests have been carried out using smooth specimens of a spheroidal graphite cast iron (SGI) with ferritic microstructure and the fatigue life distributions were examined. It was found that fatigue cracks were initiated from casting defects at the early stage of fatigue life and thus fatigue life could be regarded as crack growth life. The scatter in fatigue lives was primarily attributed to the scatter in casting defect size. The fatigue life prediction was performed using experimentally determined crack growth characteristics. The obtained results agreed well with the experimental data. From a practical viewpoint, similar prediction was conducted using the growth characteristics for large cracks (CT specimen). When the crack opening stress was assumed to be zero, the prediction showed longer fatigue lives than the experimental data at high stress level, and a good agreement at low stress level. Furthermore, a Monte Carlo simulation was performed using measured distribution function of casting defect size, and similar results were obtained. Additional fatigue tests were conducted on both pearlitic and bull's eye materials prepared with heat treatment of different SGI from ferritic SGI. It was found that the fatigue strength of both materials was almost the same and also similar to that of ferritic material. The distribution of casting defect size in pearlitic material agreed closely with that in ferritic material. This indicated that if the distribution of the sizes of casting defects from which crack was initiated were the same in different SGI's, then the fatigue strength would be independent of microstructure. Graphs, photomicrographs, 14 ref.
Rolling contact fatigue of three pearlitic rail steels Beynon, ,LH., Garnham, ZE. and Sawley, K.,L Wear (1996) 192 (1-2), 94-111 The effects of varying contact pressure and creepage on the early stages of rolling contact fatigue in three pearlitic rail steels have been investigated in the laboratory under water lubricated conditions. In the test machine used, a non-contacting eddy current probe detected very small fatigue cracks and was used to stop tests before spalling could destroy the fatigue initiation sites in order that the metallography of crack initiation could be undertaken. Crack morphologies similar to those found in wheel-rail contact have been observed. Except at the highest creepages and contact stress used, there was little evidence that manganese sulphide inclusions contributed to crack initiation. In contrast, hard oxide inclusions did contribute to initiation. The head-hardened grade eutectoid steel had the best resistance to rolling contact fatigue. The naturally hard eutectoid steel was the next best, while the lowest strength steel gave the worst performance. Crack networks were formed in the specimens tests at nominal contact stresses of 1200 MPa and 1500 MPa. Isolated cracks were produced in the specimens tested at 1800 MPa. Graphs, photomicrographs, 16 ref.
Analysis of the fatigue failure of tee pieces forming part of a header in a descaling system in a hot strip rolling mill Watson, J.F., Rance, J.M. and Anderson, H.J. Int. J. Pressure Vessels Piping (1996) 68 (1), 12t-126 Several fatigue failures occurred in tee pieces forming part of a header which supplies high pressure water for descaling steel slabs. The failures were in the form of through cracks which were initiated at the inside of the tee radius. Analysis of the stresses in the tee piece indicated a significant local stress intensification at the tee radius. This stress intensification is not taken into account in the standard design codes, however, the analysis indicates that it should be a main parameter in designing tee pieces for pressurized applications. The reason for the failures was shown to be due to the cyclic nature of the loading resulting in a fatigue failure. A failure model has been established which interfaces fatigue crack initiation with crack propagation. The model gave a predicted life which agreed well with the actual life of the tee pieces. Graphs, 3 ref.
Effect of rest time after application of single overload cycle on fatigue life Kumar, R., Kumar, A. and Singh, K. Eng. Fract. Mech. (1996) 54 (1), 147-153 Experiments on IS-1020 steel for single tensile overload applied at 12 mm crack length with and without rest time were carried out. The observed retardation effects are presented here and suggested that overload retardation is primarily due to residual compressive stress generated in the crack tip region. On the basis of these experimental results some power laws are developed to predict the decreased number of delay cycles (No) with rest time. The life of the specimen decreased by increasing the rest time after overload. As the magnitude of overload increased the decrease in life is larger for same rest time as compared to lesser overload. Graphs, 11 ref.
Theoretical modelling of fatigue threshold for aluminium alloys Li, X.-D. and Edwards, L. Eng. Fract. Mech. (May 1996) 54 (1), 35-48 The fatigue limit of aluminium alloys is logically modelled as edge dislocations against grain boundaries. A fictitious shear crack is envisioned to represent the fatigue limit condition. By using continuous configuration in the dislocation pile-up, the crack tip sliding displacement can be determined for the shear crack and the fatigue limit is attained when the shear crack is just onset to grow across the grain boundary. The fatigue crack growth threshold AK,, is simulated to be composed of two components: the microstructural threshold A Kth~ and the physical threshold AK~h. Crack tip blunting is considered, regardless of closure mechanisms, to be a fundamental physical phenomenon associated with the fatigue crack growth threshold. The AK,h condition is modelled as no microcrack initiation occurs at the blunted crack front. As a result, a micromechanical model is developed that predicts the AK,h at different load ratios without invoking crack closure. A novel feature of the model is that the fatigue crack growth threshold can be predicted in terms of mechanical and cyclic fatigue properties. The model also shows a strong dependence of the fatigue crack growth threshold of AI alloys upon local microstructure. Finally, consistent predictions of AK~h are demonstrated for AI alloys. Graphs, 66 ref.
The significance of crack tip deformation for short and long fatigue cracks Hammouda, M.M.I., El-Sehily, B.M. and de los Rios, E.R. Fatigue and Fract. Eng. Mater. Struct. (1996) 19 (4), 475-484 The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic twodimensional elastic-plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode 1 cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN-9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates. 21 ref.
Evaluation of Kn using a copper foil when assessing frictional forces on crack surfaces Egami, N. and Kitaoka, S. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (4), 4 5 1 4 5 9 The relation is examined between the strain sensitivity and the grain size of a grown copper foil gauge which has a preferred orientation to slip. We adopted this gauge to evaluate the model II stress intensity factor KH of a through-thickness crack in a plate subjected to cyclic torsion. The K , values obtained are lower than the calculated values because of the frictional force on the crack surfaces. By using the experimentally determined K , value for torsion and the calculated K~ values for plane bending the threshold condition for crack propagation under the combination of bending and torsion is predicted on the basis of the strain energy density criterion. Graphs, photomicrographs, 7 ref.
Variable amplitude loading of small fatigue cracks in 6261-T6 aluminium alloy James, M,N. and de los Rios, E.R. Fatigue Fraet. Eng. Mater. Struct. (1996) 19 (4), 413-426 Constant and variable amplitude (VA) loading fatigue studies were carried out on a 6261 aluminium alloy using cylindrical plain hour-glass specimens. Crack growth was monitored via surface replication using cellulose acetate. Crack growth results at constant amplitude loading show the typical intermittent high and low periods of growth rate associated with crack microstructure interactions. Acceleration in growth rate during an overload block depends on crack length and stress amplitude ratio. It appears to pass through a maximum at a crack length corresponding to the first microstructural barrier. Microstructural-based modelling is therefore required for small fatigue cracks, rather than solely closure-based modelling. The Navarro--de los Rios model of short fatigue crack growth appears able to provide good indications of crack growth rates under VA block loading, and gives reasonable life predictions. For short cracks (surface length < 80 ~m) and a small overload ratio (6.7%), crack growth may show severe retardation during the overload block. This is ascribed to crack tip blunting being more important than the increase in stresses when closure is low. It appears from a Miner's rule type exercise, that VA block loading has its major effect on growth at a surface crack length of 20/.~m. This means that the crack initiation period cannot be ignored in life prediction models for small fatigue cracks. Graphs, photomicrographs, 17 ref.
Creep-fatigue crack growth behaviour in 1Cr-IMo-0.25V steel. II. Crack growth behaviour and models Adefris, N., Saxena, A. and McDowell, D.L. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (4), 4 0 1 4 1 1 Creep-fatigue crack growth (CFCG) behaviour of an ex-service I C r - I M o 0.25V steel was investigated for hold times of 100 s, 15 rain and 8 h for a trapezoidal waveform at a temperature of 53g°c. The correlation of the crack
Fatigue Abstracts
Fatigue properties of high strength steels containing retained austenite Yokoi, T., Kawasaki, K., Takahashi, M., Koyama, K. and Mizui, M. JSAE Review (1996) 17 (2), 210-212
Fatigue life distribution and its simulation in spheroidal graphite cast irons Tokaji, K. and Ogawa, T. Mater. Sci. Res. Int. (Mar. 1996) 2 (1), 39-45
In order to apply high strength steels to automotive parts, the effects of strengthening mechanisms on the fatigue properties were investigated. The following results were obtained: (1) the fatigue limit and fatigue strength at 500000 cycles of TRIP steel are superior to those of DP steel and Nb containing HSLA steel. (2) TRIP steel has higher resistance to cyclic softening than DP steel and Nb containing HSLA steel. (3) These results indicate that the compressive residual stress caused by transformation from retained austenite to martensite under the cyclic load can retard the propagation of the microcracks. Graphs, 4 ref.
Statistical fatigue tests have been carried out using smooth specimens of a spheroidal graphite cast iron (SGI) with ferritic microstructure and the fatigue life distributions were examined. It was found that fatigue cracks were initiated from casting defects at the early stage of fatigue life and thus fatigue life could be regarded as crack growth life. The scatter in fatigue lives was primarily attributed to the scatter in casting defect size. The fatigue life prediction was performed using experimentally determined crack growth characteristics. The obtained results agreed well with the experimental data. From a practical viewpoint, similar prediction was conducted using the growth characteristics for large cracks (CT specimen). When the crack opening stress was assumed to be zero, the prediction showed longer fatigue lives than the experimental data at high stress level, and a good agreement at low stress level. Furthermore, a Monte Carlo simulation was performed using measured distribution function of casting defect size, and similar results were obtained. Additional fatigue tests were conducted on both pearlitic and bull's eye materials prepared with heat treatment of different SGI from ferritic SGI. It was found that the fatigue strength of both materials was almost the same and also similar to that of ferritic material. The distribution of casting defect size in pearlitic material agreed closely with that in ferritic material. This indicated that if the distribution of the sizes of casting defects from which crack was initiated were the same in different SGI's, then the fatigue strength would be independent of microstructure. Graphs, photomicrographs, 14 ref.
Rolling contact fatigue of three pearlitic rail steels Beynon, ,LH., Garnham, ZE. and Sawley, K.,L Wear (1996) 192 (1-2), 94-111 The effects of varying contact pressure and creepage on the early stages of rolling contact fatigue in three pearlitic rail steels have been investigated in the laboratory under water lubricated conditions. In the test machine used, a non-contacting eddy current probe detected very small fatigue cracks and was used to stop tests before spalling could destroy the fatigue initiation sites in order that the metallography of crack initiation could be undertaken. Crack morphologies similar to those found in wheel-rail contact have been observed. Except at the highest creepages and contact stress used, there was little evidence that manganese sulphide inclusions contributed to crack initiation. In contrast, hard oxide inclusions did contribute to initiation. The head-hardened grade eutectoid steel had the best resistance to rolling contact fatigue. The naturally hard eutectoid steel was the next best, while the lowest strength steel gave the worst performance. Crack networks were formed in the specimens tests at nominal contact stresses of 1200 MPa and 1500 MPa. Isolated cracks were produced in the specimens tested at 1800 MPa. Graphs, photomicrographs, 16 ref.
Analysis of the fatigue failure of tee pieces forming part of a header in a descaling system in a hot strip rolling mill Watson, J.F., Rance, J.M. and Anderson, H.J. Int. J. Pressure Vessels Piping (1996) 68 (1), 12t-126 Several fatigue failures occurred in tee pieces forming part of a header which supplies high pressure water for descaling steel slabs. The failures were in the form of through cracks which were initiated at the inside of the tee radius. Analysis of the stresses in the tee piece indicated a significant local stress intensification at the tee radius. This stress intensification is not taken into account in the standard design codes, however, the analysis indicates that it should be a main parameter in designing tee pieces for pressurized applications. The reason for the failures was shown to be due to the cyclic nature of the loading resulting in a fatigue failure. A failure model has been established which interfaces fatigue crack initiation with crack propagation. The model gave a predicted life which agreed well with the actual life of the tee pieces. Graphs, 3 ref.
Effect of rest time after application of single overload cycle on fatigue life Kumar, R., Kumar, A. and Singh, K. Eng. Fract. Mech. (1996) 54 (1), 147-153 Experiments on IS-1020 steel for single tensile overload applied at 12 mm crack length with and without rest time were carried out. The observed retardation effects are presented here and suggested that overload retardation is primarily due to residual compressive stress generated in the crack tip region. On the basis of these experimental results some power laws are developed to predict the decreased number of delay cycles (No) with rest time. The life of the specimen decreased by increasing the rest time after overload. As the magnitude of overload increased the decrease in life is larger for same rest time as compared to lesser overload. Graphs, 11 ref.
Theoretical modelling of fatigue threshold for aluminium alloys Li, X.-D. and Edwards, L. Eng. Fract. Mech. (May 1996) 54 (1), 35-48 The fatigue limit of aluminium alloys is logically modelled as edge dislocations against grain boundaries. A fictitious shear crack is envisioned to represent the fatigue limit condition. By using continuous configuration in the dislocation pile-up, the crack tip sliding displacement can be determined for the shear crack and the fatigue limit is attained when the shear crack is just onset to grow across the grain boundary. The fatigue crack growth threshold AK,, is simulated to be composed of two components: the microstructural threshold A Kth~ and the physical threshold AK~h. Crack tip blunting is considered, regardless of closure mechanisms, to be a fundamental physical phenomenon associated with the fatigue crack growth threshold. The AK,h condition is modelled as no microcrack initiation occurs at the blunted crack front. As a result, a micromechanical model is developed that predicts the AK,h at different load ratios without invoking crack closure. A novel feature of the model is that the fatigue crack growth threshold can be predicted in terms of mechanical and cyclic fatigue properties. The model also shows a strong dependence of the fatigue crack growth threshold of AI alloys upon local microstructure. Finally, consistent predictions of AK~h are demonstrated for AI alloys. Graphs, 66 ref.
The significance of crack tip deformation for short and long fatigue cracks Hammouda, M.M.I., El-Sehily, B.M. and de los Rios, E.R. Fatigue and Fract. Eng. Mater. Struct. (1996) 19 (4), 475-484 The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic twodimensional elastic-plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode 1 cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN-9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates. 21 ref.
Evaluation of Kn using a copper foil when assessing frictional forces on crack surfaces Egami, N. and Kitaoka, S. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (4), 4 5 1 4 5 9 The relation is examined between the strain sensitivity and the grain size of a grown copper foil gauge which has a preferred orientation to slip. We adopted this gauge to evaluate the model II stress intensity factor KH of a through-thickness crack in a plate subjected to cyclic torsion. The K , values obtained are lower than the calculated values because of the frictional force on the crack surfaces. By using the experimentally determined K , value for torsion and the calculated K~ values for plane bending the threshold condition for crack propagation under the combination of bending and torsion is predicted on the basis of the strain energy density criterion. Graphs, photomicrographs, 7 ref.
Variable amplitude loading of small fatigue cracks in 6261-T6 aluminium alloy James, M,N. and de los Rios, E.R. Fatigue Fraet. Eng. Mater. Struct. (1996) 19 (4), 413-426 Constant and variable amplitude (VA) loading fatigue studies were carried out on a 6261 aluminium alloy using cylindrical plain hour-glass specimens. Crack growth was monitored via surface replication using cellulose acetate. Crack growth results at constant amplitude loading show the typical intermittent high and low periods of growth rate associated with crack microstructure interactions. Acceleration in growth rate during an overload block depends on crack length and stress amplitude ratio. It appears to pass through a maximum at a crack length corresponding to the first microstructural barrier. Microstructural-based modelling is therefore required for small fatigue cracks, rather than solely closure-based modelling. The Navarro--de los Rios model of short fatigue crack growth appears able to provide good indications of crack growth rates under VA block loading, and gives reasonable life predictions. For short cracks (surface length < 80 ~m) and a small overload ratio (6.7%), crack growth may show severe retardation during the overload block. This is ascribed to crack tip blunting being more important than the increase in stresses when closure is low. It appears from a Miner's rule type exercise, that VA block loading has its major effect on growth at a surface crack length of 20/.~m. This means that the crack initiation period cannot be ignored in life prediction models for small fatigue cracks. Graphs, photomicrographs, 17 ref.
Creep-fatigue crack growth behaviour in 1Cr-IMo-0.25V steel. II. Crack growth behaviour and models Adefris, N., Saxena, A. and McDowell, D.L. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (4), 4 0 1 4 1 1 Creep-fatigue crack growth (CFCG) behaviour of an ex-service I C r - I M o 0.25V steel was investigated for hold times of 100 s, 15 rain and 8 h for a trapezoidal waveform at a temperature of 53g°c. The correlation of the crack