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Improvement in resistance to bending fatigue: prestress shot blasting of case-hardened steel gearing
E s t i m a t i o n o f f a t i g u e s t r e n g t h a n d life. Proskovec, J., Kepka, M. and
Rehor, P. Skoda Rev. 1990 (2), 2 - 5
The fatigue analysis of public transportation vehicles by the SKODA works is described. Testing of both models and prototypes is used to determine the dynamic response of the whole system, end testing of critical components gives the limits of the various critical components. Typical tables of data, loading cycles and damage calculations are presented for a rear axle suspension part. Simulations for various types of road surface are developed. Based on the results, several modifications were developed for the system. Graphs. I m p r o v e m e n t in resistance t o b e n d i n g fatigue: prestress s h o t blasting OT case-hardened steel gearing. Couratin, D. and Guimier, A. Galvano-Organo-Traitements de Surface M a y 1990 606, 4 3 9 - 4 4 4 (in French ) For many years prestress shot blasting has been used to improve the fatigue limits in steel (e.g. 16CD4). it was used originally for springs and the improvements suggested the process might be adapted for pinions and gears, where a high surface residual stress could improve their lives. Peening tests were made using both round and irregularly shaped steel and irregularly shaped corundum at temperatures between ambient end 140 °C. It was found that the corundum was most effective, irragularfy shaped steel was better than round, the angle of blasting was very important especially at the root of the teeth, and temperatures up to 80 °C do not have any effect on residual stresses. The major problem with corundum was that it increased surface roughness. Graphs. 2 refs. The interaction o f creep and f a t i g u e in l e s d - t i n solders. (Dissertation).
Hendrix, B,C. Diss. Abstr. Int. Sept. 1990 51, (3), 96 pp
The creep and fatigue damage mechanisms interact in Pb-Sn solder alloys thatare used in the surface mounting of electronic components. This study concentrates on identifying and characterizing the damage mechanisms that impact on the design of accelerated tests and on the application of these tests to life prediction. Cyclic creep tests were performed on bulk cast solder of 63Sn-37Pb, the eutectic composition, and 81Pb-19Sn. In the eutactic solder, mean stress was varied from 1-21 Mpa, stress range was varied from 5-28 MPe, temperature was varied from 0-42 °C, and hold time on load was varied from 0.3-300 s and compared to static tests. Tests with unbalanced hold times, that is with the off-loed hold time or the on-load hold time removed, were also performed. Strain-controlled fatigue tests were performed from 2000-20000 I~ with hold times from 0.3-300 s. The 81-19 alloy and joints of the eutectic alloy were tested at hold times from 0.3-300 s and compared to static tests. It is found that damage is stored as a function of time on load and stress, that is cyclic creep damage. Because creep is the dominant damage mechanism, the severity of cyclic creep tests should not be ranked by an effective stress defined as the average of the stress raised to the nth power, where n is the stress sensitivity exponent of the creep equation. At shorter hold times in both alloys and eutectic joints, a significant part of the strain is recoverable, time dependent, and non-damaging, that is anelestic, By this anelastic strain storage mechanism, life, measured either by cycles to failure or by time on load, is increased by at least a factor of five. The unbalanced hold time test results show definitively that the anelaatic mechanism is responsible for the increase in life rather than some other effect of cycling the solder. In the strein-controiled tests, the anelestic effect is exhibited by a decrease in stress relaxation rate with decreased hold time. The implications of these findings on accelerated testing and life prediction schemes are discussed. The amount of crsep-fatigue damage that the solder has experienced has also been measured by the examination of grain boundary surfaces for void nucleation and growth. The volume of grain boundary voids is found to be proportional to the number of cycles of damage that have been stored. Porosity and crack initiation during l o w cycle fatigue. Gerard, D.A. and
Koss, D.A. Mater. ScL Eng. A Oct. 1990 A129, (1), 7 7 - 8 5
The influence of porosity on crack initiation during low-cycle fatigue has been examined by both experimental observations and theoretical modelling. Experimental data based on powder-procassed Ti indicate a porosity-induced enhancement of crack initiation which contributes to significant reductions in the low-cycle fatigue life. All the levels of porosity examined, which range from 0.4-6 vol.%, cause an order-of-magnitude or greater decrease in the number of cycles to initiate a 15 l~m long crack. Assuming that porosity located at a surface can be modelled as a through-thickness hole under plane stress conditions, adapting a Coffin-Manson law as a failure criterion and applying cumulative damage theory, a theoretical analysis has been developed for predicting the number of cycles for microcrack initiation in the presence of porosity. The predictions, which rely on pore shapes and the low-cycle fatigue response of the full density matrix, accurately predict the number of cycles necessary to initiate a 15 p,m crack adjacent to both isolated and interconnected pores during low-cycle fatigue. Graphs, photomicogrephs. 43 refs. Fatigue b e h a v i o u r o f C M S X 2 superalloy (001) single crystals at h i g h t e m p e r a t u r e . I. L o w cycle f a t i g u e o f notched specimens. Defresne, A.
and Remy, L. Mater. Sci. Eng. A Oct. 1990 A129, (1), 4 5 - 5 3 The low-cycle fatigue beheviour of shallow notch specimens of (001) superalloy single crystals was studied • 650 °C. Three notch orientations were used for CMSX 2 alloy and only one for MAR-M200 alloy. Crack initiation and growth were monitored using potential drop measurements and a plastic replication technique. A very large scatter in fatigue life was observed and no clear notch orientation dependence of fatigue life could be discerned. Cracks initiate at casting micropores in CMSX 2 and carbides in MAR-M200. These results are discussed using fractographic and metallographic observations. Emphasis is placed on microcreck growth and on the size and location of the initiating defect, Graphs, photomicrographs. 18 refs. Fatigue b e h a v i o u r o f C M S X 2 superalloy (001) single crystals at high t e m p e r a t u r e . IL Fatigue crack g r o w t h . Defresne, A. and Rainy, L. Mater. Sci. Eng. A Oct. 1990 A129, (1), 5 5 - 6 4 The fatigue crack growth behaviour of two-dimensional cracks was studied in (001) single crystals of CMSX 2 Ni-besed superalloy at 650 °C. An influence of the
286
crystallographic orientation of the crack growth direction on the crack growth kinetics was evident for short cracks tested in air. This influence was mainly attributed to differences in crack closure. Comparison between short and long cracks indicated that no short-crack effect exists in these single crystals. Tests in vacuum gave significant differences in crack growth rate and in fatigue fracture mode, especially at low rates. These results are discussed and uesd to rationalize previous observations. Graphs, photomicrographs. 30 refs.
The nucleation and g r o w t h o f s h o r t f a t i g u e cracks b o t h at plain surfaces and notches. Guiu, F. and Stevens, R.N. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 6 2 6 - 6 3 6 The problem of the nucleation and growth of short fatigue cracks is addressed from an energetic point of view. It is explained that vanishingly small cracks can only nucleate and grow at the expense of the release of some locally stored energy during the fatigue deformation. This is necessary because an external loading system alone cannot provide a positive driving force for the growth of a crack whose length is below a critical value. The concept of the local driving force is used to explain the nucleation and growth of short fatigue cracks both at plain surfaces and at notches. With this approach a meaningful definition can be given of a 'short fatigue crack' and a sound physical interpretation of the Kitagawa-Takaheshi plots is provided. The conditions for the existence of non-progagating cracks are clearly established and the relationship between the stress concentration factor at the root of the notches and the fatigue limit is explained. A physical framework is set up for the sound understanding and treatment of short fatigue cracks and the microstructural parameters that control their growth. Tests were conducted on Mo and Cu crystals. Graphs. 30 refs.
fGriffiths,a J.R. Eclosurefand f
e
c
t
s
o f crack, m e a n stress on t h e t h r e s h o l d stress i n t e n s i t y ctor Tor f a t i g u e o f an a l u m i n i u m casting alloy. Couper, M.J. and
Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 6 1 5 - 6 2 4 Observations have been made of the threshold fatigue properties of an' AI-Mg-Si casting alloy. First, it has been shown that there exists a critical value of the stress ratio, R, below which the threshold stress intensity factor decreases with increasing R but above which it is constant. Second, measurements of the cyclic crack opening displacement have been used to examine crack closure effects and to deduce the effective cyclic stress intensity factor. The two sets of data are compared with existing models for threshold phenomena. Graphs. 29 refs.
A comparison o f nest-threshold f a t i g u e crack propagation in t w o highs t r e n g t h steels. Oni, O. and Bathias, C. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 5 8 5 - 5 9 6 Fatigue crack growth rate data for long and short cracks were obtained from two high-strength steels (e.g. 16NCD13 and 30NCD16) used in the aerospace industry. Differences in the behaviour of the two cracks were noted and explained in terms of the three-dimensional nature of short cracks compared to two-dimensional long cracks, the interaction between cracks, the applicability of linear elastic fracture mechanics (LEFM) and their effect on crack closure. Graphs, photomicrographs. 22 refs.
The use of compliance to investigate f a t i g u e crack retardation by cover plates. Han, M.S., Smith, R.A. and Kim, S.C. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 5 7 9 - 5 8 4 The compliance approach is developed to the problem of load sharing between a cracked plate and a cover plate used to bridge the crack. The material tested was a 0.15% carbon, 0.28% Mn structural steel. The theory is validated by using calculated stress intensity factors for the covered and uncovered cases to reduce experimentally observed growth rates to a common base. Calculations are then made on the effect of cover plate width on fatigue crack retardation to demonstrate the predictive capability of the technique. Graphs. 4 refs. steels. Panzenbock, M., Ebner, R., Locker, U., Aigner, H. and Pohl, H. Fatigue Fract. Eng. Mater. StrucL 1990 13, (6), 5 6 3 - 5 7 8
Fatigue b e h a v i o u r o f austenitic C r - M n - N
Solid solution hardening and strain hardening are the dominating strengthening mechanisms to achieve a high yield stress and ultimate tensile strength in Cr-Mn-N steels for drill collars. The fatigue limit can be improved more effectively by solid solution hardening than by strain hardening, but the attainable hardness owing to soluble elements is restricted by other metallurgical demands. Strain hardening significantly increases the strength and the fatigue limit is improved as well, but to a lesser extent. The reason for this beheviour is the introduction of internal stresses of the I, II and III types by forging the drill collars. This results in macroscopic stresses that vary over the cross section and in stresses on a microscopic scala that lead to an early loss of linearby in the elastic line of the stress-strain curve and to the well known Bauschinger effect. Cyclic softening and hardening is accompanied by a rearrangement of the dislocation structure as revealed by transmission electron microscopy. The results indicate that a muitiaxial and homogeneous cold working to produce a stable dislocation structure and to avoid large directional internal stresses is extremely important to achieve high fatigue strengths. Crack growth and crack closure measurements were performed for the determination of the effective cyclic threshold stress intensity range for the evaluation of the influence of the grain size on the fatigue limit. Graphs, photomicrographs. 18 refs.
The c r e e p / f a t i g u e b e h a v i o u r o f t y p e 316 steel u n d e r reverse bending conditions. Wood, S.D. and Banks, P. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 551-561 Reverse bending creep/fatigue tests have been performed on type 316 steel at temperatures of 550 and 600°C for test durations of up to 12000 h. It is shown that the endurances obtained are comparable with those observed under push-pull conditions and that the introduction of a hold period can significantly reduce the endurance. Detailed fractography indicates that creep/fatigue in tension is more damaging than cresp/fatigue in compression. The crack initiation behaviour is shown to be temperature dependent and may result from the combination of tensile and shear stresses, the relative importance of which will depend on the precise
Int J Fatigue May 1991
Rehor, P. Skoda Rev. 1990 (2), 2 - 5
The fatigue analysis of public transportation vehicles by the SKODA works is described. Testing of both models and prototypes is used to determine the dynamic response of the whole system, end testing of critical components gives the limits of the various critical components. Typical tables of data, loading cycles and damage calculations are presented for a rear axle suspension part. Simulations for various types of road surface are developed. Based on the results, several modifications were developed for the system. Graphs. I m p r o v e m e n t in resistance t o b e n d i n g fatigue: prestress s h o t blasting OT case-hardened steel gearing. Couratin, D. and Guimier, A. Galvano-Organo-Traitements de Surface M a y 1990 606, 4 3 9 - 4 4 4 (in French ) For many years prestress shot blasting has been used to improve the fatigue limits in steel (e.g. 16CD4). it was used originally for springs and the improvements suggested the process might be adapted for pinions and gears, where a high surface residual stress could improve their lives. Peening tests were made using both round and irregularly shaped steel and irregularly shaped corundum at temperatures between ambient end 140 °C. It was found that the corundum was most effective, irragularfy shaped steel was better than round, the angle of blasting was very important especially at the root of the teeth, and temperatures up to 80 °C do not have any effect on residual stresses. The major problem with corundum was that it increased surface roughness. Graphs. 2 refs. The interaction o f creep and f a t i g u e in l e s d - t i n solders. (Dissertation).
Hendrix, B,C. Diss. Abstr. Int. Sept. 1990 51, (3), 96 pp
The creep and fatigue damage mechanisms interact in Pb-Sn solder alloys thatare used in the surface mounting of electronic components. This study concentrates on identifying and characterizing the damage mechanisms that impact on the design of accelerated tests and on the application of these tests to life prediction. Cyclic creep tests were performed on bulk cast solder of 63Sn-37Pb, the eutectic composition, and 81Pb-19Sn. In the eutactic solder, mean stress was varied from 1-21 Mpa, stress range was varied from 5-28 MPe, temperature was varied from 0-42 °C, and hold time on load was varied from 0.3-300 s and compared to static tests. Tests with unbalanced hold times, that is with the off-loed hold time or the on-load hold time removed, were also performed. Strain-controlled fatigue tests were performed from 2000-20000 I~ with hold times from 0.3-300 s. The 81-19 alloy and joints of the eutectic alloy were tested at hold times from 0.3-300 s and compared to static tests. It is found that damage is stored as a function of time on load and stress, that is cyclic creep damage. Because creep is the dominant damage mechanism, the severity of cyclic creep tests should not be ranked by an effective stress defined as the average of the stress raised to the nth power, where n is the stress sensitivity exponent of the creep equation. At shorter hold times in both alloys and eutectic joints, a significant part of the strain is recoverable, time dependent, and non-damaging, that is anelestic, By this anelastic strain storage mechanism, life, measured either by cycles to failure or by time on load, is increased by at least a factor of five. The unbalanced hold time test results show definitively that the anelaatic mechanism is responsible for the increase in life rather than some other effect of cycling the solder. In the strein-controiled tests, the anelestic effect is exhibited by a decrease in stress relaxation rate with decreased hold time. The implications of these findings on accelerated testing and life prediction schemes are discussed. The amount of crsep-fatigue damage that the solder has experienced has also been measured by the examination of grain boundary surfaces for void nucleation and growth. The volume of grain boundary voids is found to be proportional to the number of cycles of damage that have been stored. Porosity and crack initiation during l o w cycle fatigue. Gerard, D.A. and
Koss, D.A. Mater. ScL Eng. A Oct. 1990 A129, (1), 7 7 - 8 5
The influence of porosity on crack initiation during low-cycle fatigue has been examined by both experimental observations and theoretical modelling. Experimental data based on powder-procassed Ti indicate a porosity-induced enhancement of crack initiation which contributes to significant reductions in the low-cycle fatigue life. All the levels of porosity examined, which range from 0.4-6 vol.%, cause an order-of-magnitude or greater decrease in the number of cycles to initiate a 15 l~m long crack. Assuming that porosity located at a surface can be modelled as a through-thickness hole under plane stress conditions, adapting a Coffin-Manson law as a failure criterion and applying cumulative damage theory, a theoretical analysis has been developed for predicting the number of cycles for microcrack initiation in the presence of porosity. The predictions, which rely on pore shapes and the low-cycle fatigue response of the full density matrix, accurately predict the number of cycles necessary to initiate a 15 p,m crack adjacent to both isolated and interconnected pores during low-cycle fatigue. Graphs, photomicogrephs. 43 refs. Fatigue b e h a v i o u r o f C M S X 2 superalloy (001) single crystals at h i g h t e m p e r a t u r e . I. L o w cycle f a t i g u e o f notched specimens. Defresne, A.
and Remy, L. Mater. Sci. Eng. A Oct. 1990 A129, (1), 4 5 - 5 3 The low-cycle fatigue beheviour of shallow notch specimens of (001) superalloy single crystals was studied • 650 °C. Three notch orientations were used for CMSX 2 alloy and only one for MAR-M200 alloy. Crack initiation and growth were monitored using potential drop measurements and a plastic replication technique. A very large scatter in fatigue life was observed and no clear notch orientation dependence of fatigue life could be discerned. Cracks initiate at casting micropores in CMSX 2 and carbides in MAR-M200. These results are discussed using fractographic and metallographic observations. Emphasis is placed on microcreck growth and on the size and location of the initiating defect, Graphs, photomicrographs. 18 refs. Fatigue b e h a v i o u r o f C M S X 2 superalloy (001) single crystals at high t e m p e r a t u r e . IL Fatigue crack g r o w t h . Defresne, A. and Rainy, L. Mater. Sci. Eng. A Oct. 1990 A129, (1), 5 5 - 6 4 The fatigue crack growth behaviour of two-dimensional cracks was studied in (001) single crystals of CMSX 2 Ni-besed superalloy at 650 °C. An influence of the
286
crystallographic orientation of the crack growth direction on the crack growth kinetics was evident for short cracks tested in air. This influence was mainly attributed to differences in crack closure. Comparison between short and long cracks indicated that no short-crack effect exists in these single crystals. Tests in vacuum gave significant differences in crack growth rate and in fatigue fracture mode, especially at low rates. These results are discussed and uesd to rationalize previous observations. Graphs, photomicrographs. 30 refs.
The nucleation and g r o w t h o f s h o r t f a t i g u e cracks b o t h at plain surfaces and notches. Guiu, F. and Stevens, R.N. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 6 2 6 - 6 3 6 The problem of the nucleation and growth of short fatigue cracks is addressed from an energetic point of view. It is explained that vanishingly small cracks can only nucleate and grow at the expense of the release of some locally stored energy during the fatigue deformation. This is necessary because an external loading system alone cannot provide a positive driving force for the growth of a crack whose length is below a critical value. The concept of the local driving force is used to explain the nucleation and growth of short fatigue cracks both at plain surfaces and at notches. With this approach a meaningful definition can be given of a 'short fatigue crack' and a sound physical interpretation of the Kitagawa-Takaheshi plots is provided. The conditions for the existence of non-progagating cracks are clearly established and the relationship between the stress concentration factor at the root of the notches and the fatigue limit is explained. A physical framework is set up for the sound understanding and treatment of short fatigue cracks and the microstructural parameters that control their growth. Tests were conducted on Mo and Cu crystals. Graphs. 30 refs.
fGriffiths,a J.R. Eclosurefand f
e
c
t
s
o f crack, m e a n stress on t h e t h r e s h o l d stress i n t e n s i t y ctor Tor f a t i g u e o f an a l u m i n i u m casting alloy. Couper, M.J. and
Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 6 1 5 - 6 2 4 Observations have been made of the threshold fatigue properties of an' AI-Mg-Si casting alloy. First, it has been shown that there exists a critical value of the stress ratio, R, below which the threshold stress intensity factor decreases with increasing R but above which it is constant. Second, measurements of the cyclic crack opening displacement have been used to examine crack closure effects and to deduce the effective cyclic stress intensity factor. The two sets of data are compared with existing models for threshold phenomena. Graphs. 29 refs.
A comparison o f nest-threshold f a t i g u e crack propagation in t w o highs t r e n g t h steels. Oni, O. and Bathias, C. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 5 8 5 - 5 9 6 Fatigue crack growth rate data for long and short cracks were obtained from two high-strength steels (e.g. 16NCD13 and 30NCD16) used in the aerospace industry. Differences in the behaviour of the two cracks were noted and explained in terms of the three-dimensional nature of short cracks compared to two-dimensional long cracks, the interaction between cracks, the applicability of linear elastic fracture mechanics (LEFM) and their effect on crack closure. Graphs, photomicrographs. 22 refs.
The use of compliance to investigate f a t i g u e crack retardation by cover plates. Han, M.S., Smith, R.A. and Kim, S.C. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 5 7 9 - 5 8 4 The compliance approach is developed to the problem of load sharing between a cracked plate and a cover plate used to bridge the crack. The material tested was a 0.15% carbon, 0.28% Mn structural steel. The theory is validated by using calculated stress intensity factors for the covered and uncovered cases to reduce experimentally observed growth rates to a common base. Calculations are then made on the effect of cover plate width on fatigue crack retardation to demonstrate the predictive capability of the technique. Graphs. 4 refs. steels. Panzenbock, M., Ebner, R., Locker, U., Aigner, H. and Pohl, H. Fatigue Fract. Eng. Mater. StrucL 1990 13, (6), 5 6 3 - 5 7 8
Fatigue b e h a v i o u r o f austenitic C r - M n - N
Solid solution hardening and strain hardening are the dominating strengthening mechanisms to achieve a high yield stress and ultimate tensile strength in Cr-Mn-N steels for drill collars. The fatigue limit can be improved more effectively by solid solution hardening than by strain hardening, but the attainable hardness owing to soluble elements is restricted by other metallurgical demands. Strain hardening significantly increases the strength and the fatigue limit is improved as well, but to a lesser extent. The reason for this beheviour is the introduction of internal stresses of the I, II and III types by forging the drill collars. This results in macroscopic stresses that vary over the cross section and in stresses on a microscopic scala that lead to an early loss of linearby in the elastic line of the stress-strain curve and to the well known Bauschinger effect. Cyclic softening and hardening is accompanied by a rearrangement of the dislocation structure as revealed by transmission electron microscopy. The results indicate that a muitiaxial and homogeneous cold working to produce a stable dislocation structure and to avoid large directional internal stresses is extremely important to achieve high fatigue strengths. Crack growth and crack closure measurements were performed for the determination of the effective cyclic threshold stress intensity range for the evaluation of the influence of the grain size on the fatigue limit. Graphs, photomicrographs. 18 refs.
The c r e e p / f a t i g u e b e h a v i o u r o f t y p e 316 steel u n d e r reverse bending conditions. Wood, S.D. and Banks, P. Fatigue Fract. Eng. Mater. Struct. 1990 13, (6), 551-561 Reverse bending creep/fatigue tests have been performed on type 316 steel at temperatures of 550 and 600°C for test durations of up to 12000 h. It is shown that the endurances obtained are comparable with those observed under push-pull conditions and that the introduction of a hold period can significantly reduce the endurance. Detailed fractography indicates that creep/fatigue in tension is more damaging than cresp/fatigue in compression. The crack initiation behaviour is shown to be temperature dependent and may result from the combination of tensile and shear stresses, the relative importance of which will depend on the precise
Int J Fatigue May 1991