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Finite-element study of residual stresses in a plate T-joint fatigue specimen
Subsurface c l a c k i n i t i a t i o n in high-cycle f a t i g u e o f T i - - g A I - 4 V s l a y s a t
cryogenic temperatures. Umezawa, 0., Nagai, K. and Ishikawa, K. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 2 4 - 9 3 1 (in Japanese) In Ti alloys subsurface fatigue crack initiation apparently occurs without the existence of any defects such as inclusions or pores. Subsurface crack initiation and subcrecks for "ri-6AI-4V alloys at cryogenic temperatures, and discussed the growth mechanism of microcreck. As the maximum stress was reduced the morphology of the subsurface crack initiation sites was changed from consisting of one facet to more than one facet. The facet was identified as a cracked (=-phase by comparing the chemical composition and morphology. Suberecks were also produced in the (z-phsee. The subsurface crack initiation sites and the suberacks were not perpendicular to the applied stress. Hence it is concluded that a microcreck initiates in an a grain, grows into neighbouring j~-pletes and ¢¢-greins, and finally forms an initiation site for the main fatigue crack. The lower the maximum stress, the greater the size of the subsurface crack initiation sites. The size of the projection of the subsurface crack initiation site on the main crack propagation plane was adopted as a shape parameter of the three-dimensional cracks. Using this parameter, the dependence of the initiation site size on the maximum stress range can be accounted for by assuming that the microcreck growth is controlled by a threshold condition. Photomicrographs, graphs. 16 refs.
S a t u r a t i o n o f d a m a g e in f i ~ t i n g f a t i g u e o f h i g h - s t r e n g t h steels in seawater. Nakazawa, K., Sumite, M. and Maruyama, N. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 1 7 - 9 2 3 (in Japanese) The saturation behaviours of damage in fretting fatigue at frequencies of 1 and 20 Hz in seawater under freely corroding conditions were studied using highstrength steels having tensile strengths of 490, 690 and 880 MPa. Fretting damage saturated beyond a certain number of fretting cycles, and this number varied drastically with the steel and the frequency. The saturation behaviours of damage could be divided into two groups, that is the smallest numbers of fretting cycles to cause the saturation were less than 0.1% of the fretting fatigue life (group 1) and more than 40-60% (group 2). They had a strong relationship to crack initiation and growth behaviours. In group 1, cracks propagated normal to the alternating stress axis from the early stage of crack propagation. However, in group 2 the cracks propagated at angles less than 90° to the alternating stress axis, but changed direction to become normal as they propagated beyond a certain length. This relationship could be explained from the mechanism that the decrease in the fretting fatigue lives of group 1 resulted from the decrease in crack initiation life caused by the acceleration of corrosion pit formation, and that of group 2 from crack initiation and growth caused by fretting. Graphs, photomicrographs. 13 refs.
Rnlta-element
s t u d y o f residual stresses in a plate T-joint f a t i g u e
r~oc.mee. Mok, D.H.B. and Pick, R.J. Inst. Mech. Eng. C, J. Mech. Eng. Sci. 1990 204, (C2), 1 2 7 - 1 3 4 Using the finite-element code Abequs to make numerical predictions, this paper studies the crack shape development and thickness effects on a series of welded plate T-joint fatigue specimens. It was found that redistribution of the residual stress only occurs in a small region near the weld toe, the extent depending greatly on the magnitude of the applied load. Graphs. 18 refs.
Environmentally i n d u c e d cracking. Scott, P.M.
Constr. Build. Mater. J u n e 1990 4, (2), 9 8 - 1 0 6
A review of current research topics in the UK on environmentally induced or assisted cracking is given with particular emphasis on how these research interests are related to current individual applications. Although the review cannot be completely comprehensive, some particularly topical items are described in greater detail including corrosion fatigue of offshore structural materials, pipeline cracking, highstrength fasteners end deserator cracking. Several key generic issues ere highlighted such as the relevance of short-term test methods to long-term service experience and the increasing reliance on fracture mechanics analyses with their strengths and weaknesses for facilitating run/repair decisions. 25 refs. Fatigue s t r e n g t h characteristics o f h i g h - s t r a n g t h steel. Asami, K. and
Emura, H. JSME Int. J. J u l y 1990 I 33, (3), 3 6 7 - 3 7 4 In order to clarify the fatigue characteristics of low-alloy steels tempered at low temperature (150-200 °C) Or carburized in generated gas, rotating-bending fatigue tests were conducted in laboratory air and dry air. The influence of the moisture in air was only observed for the cerburized steel. The low-temperature-tempered steel revealed two knees in the S-N diagram, and no fatigue limit was found up to 10s cycles (9 months). The cerburized steel revealed three knees and the fatigue limit was found to be over 108 cycles. When the fatigue test on carburized steel was carried out in dry air or the surface structure anomalies were removed, the fatigue strengths were improved, but neither knees nor fatigue limits were found up to 10s cycles. The reasons for this were discovered from the results of fractogrephy and the equations proposed by Murekami et al for the prediction of the fatigue limit of steel with microdefects. Graphs, photomicrographs. 6 refs. Prediction o f p r o p a g a t i o n and n o n - p r o p a g a t i o n o f s h o r t f a t i g u e cracks at notches u n d e r m e a n s U m . Akiniwa, Y., Tanaka, K. and Taniguchi,
N. JSME Int. J. J u l y 1990 I 33, (3), 2 8 8 - 2 9 6 A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, Kopth, and the non-propagating crack length, Cnp, was approximated by Kopth = K;pth[(Cnp-Cr)/(C2-C1)]1/2(CI=~Cnp~C2)and Kopth = K;pth (C2'~Cnp), where K~oth Wee the value for long cracks, C2 was a linear function of K;pth, and C1 was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the non-propagating crack length. The fatigue limit for crack initiation of a stage-II crack
Int J Fatigue May 1991
wee almost constant and equal to the value predicted for the case of R = -1. Graphs. 16 refs. resistance in e i u m i n i u m alloys. Jono, M. and Sugete, A. J. Jpn. Inst. Light Met. J u l y 1990 40, (7), 5 4 3 - 5 5 3 (in Japanese) Fatigue crack g r o w t h
During metal fatigue, crack initiation and propagation are very different. When designing high-rigidity machinery components, damage tolerance is to be considered. Aluminium alloy fatigue crack growth beheviour based on the theory of fracture mechanics is reviewed. Data and theory from the literature are provided and discussed. Fatigue failure of AI alloys is divided to two stages: microcreck and macrocreck propagation until fracture. A mathematical analysis of the stress concentration in crack tips is given. The crack propagation beheviours of ingot metallurgy alloys and powder metallurgy alloys are studied, followed by an emphasis on the alloys AI-Si and AI-Li. 7075 and ZK141 alloy fatigue crack propagation under dynamic loading is also discussed. Graphs, photomicrographs. 32 refs. T h e r m a l f a t i g u e u n d e r m u l t i a x i a i stresses. Feng, J., Bian, M. and Dang, Z. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 5 2 5 - 5 3 4 Thermal fatigue under multiaxiel stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with the axial mechanical fatigue loading. The axial forces on the specimens (e.g., 3Cr2W8V) were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elastoplastic finite-elemeot method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of the cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another. Graphs, photomicrographs. 6 refs.
Fatigue microcracks in t y p e 304 stainless steel at elevated t e m p e r a t u r e .
Suh, C.M., Lee, J.J. and Kang, Y.G. Fatigue Frect. Eng. Mater. Struct. M a y 1990 13, (5), 4 8 7 - 4 9 6 Microcrecking of type 304 stainless steel at 538°C has been studied, in particular, the initiation, growth and coalescence of fatigue microcrecks on smooth specimens via surface replicas and photomicrographs. Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, was obtained. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue life assessment and the evaluation of damage in structures at elevated temperatures, Graphs. 22 refs.
L i f e t i m e p r e d i c t i o n o n C r - M o - V and 316L steels u n d e r t h e r m a l and mechanical cycling. Degallaix, G., Korn, C. and Pluvinage, G. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 7 3 - 4 8 5 The thermal-mechanical fatigue resistance of a Cr-Mo-V steel and a 316L steel under combined complex mechanical strain and temperature cycling has been evaluated. The applicability to thermal-mechanical lifetime predictions of several isothermal fatigue damage concepts is investigated. From a designer viewpoint, some predictions are quite acceptable, in particular, with the Degellaix thermal activation model, for which a recent development is reported. Graphs. 27 refs. M e r k o v a p p r o x i m a t i o n t o f a t i g u e crack size distribution. Solomos, G.P.
and Lucia, A.C. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 457-471 The distribution of the crack size during the fatigue crack propagation phenomenon is investigated. A Markovian modelling of the crack is introduced through the fatigue crack growth law, and the associated Fokker-Planck equation is written while special care is devoted to specifying its boundary conditions. This equation is solved by the method of separation of variables and the required distribution function is obtained in the form of a convergent infinite series, An illustrative example, demonstrating the applicability of the approach, shows satisfactory agreement between the theoretical results and the experimental data. The test material was 7475-T7351 AI. Graphs. 25 refs. Fractographic analysis o f crack g r o w t h and shear lip d e v e l o p m e n t under s i m p l e v a r i a b l e - a m p l i t u d e loading. Ling, M. R. and Schijve, J. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 4 3 - 4 5 6 Fatigue crack growth experiments were carried out on 2024-T3 sheet specimens, loaded under simple variable-amplitude load sequences, including blocks of overload cycles or underload cycles and blocks of small-amplitude cycles. The purpose was to study interaction effects by analysing striation measurements and shear lip developments. The striations clearly demonstrate delayed retardation. Shear lip measurements indicate significantly slower crack growth rates if the percentage shear lip increases from zero (no shear lips) to 100% (fully in the shear mode). The significance of incompatible shear lips during variable-amplitude loading is discussed. The test results are evaluated with relation to interaction mechanisms and the interpretation given to them by cycle-by-cycle prediction models. Graphs. 19 refs. Crack g r o w t h b e h a v i o u r a n d failure m i c r o m e c h a n i s m s in three h e a t resistant materials s t elevated t e m p e r a t u r e . Hour, K.Y. and Stubbins, J.F. Acts Metell. Mater. Aug. 1990 38, (8), 1463-1474 The crack growth behaviour for three high-temperature materials, alloy 80OH, type310 stainless steel (SS) and Hastelloy X, have been investigated under creep, fatigue and hold time test conditions at 650 +C. All alloys show a predominantly transgrenular crack advance mode when cycled at frequencies greater than 0.5 Hz. At lower frequencies, the failure in 310 SS transitions takes place to a predominantly
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cryogenic temperatures. Umezawa, 0., Nagai, K. and Ishikawa, K. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 2 4 - 9 3 1 (in Japanese) In Ti alloys subsurface fatigue crack initiation apparently occurs without the existence of any defects such as inclusions or pores. Subsurface crack initiation and subcrecks for "ri-6AI-4V alloys at cryogenic temperatures, and discussed the growth mechanism of microcreck. As the maximum stress was reduced the morphology of the subsurface crack initiation sites was changed from consisting of one facet to more than one facet. The facet was identified as a cracked (=-phase by comparing the chemical composition and morphology. Suberecks were also produced in the (z-phsee. The subsurface crack initiation sites and the suberacks were not perpendicular to the applied stress. Hence it is concluded that a microcreck initiates in an a grain, grows into neighbouring j~-pletes and ¢¢-greins, and finally forms an initiation site for the main fatigue crack. The lower the maximum stress, the greater the size of the subsurface crack initiation sites. The size of the projection of the subsurface crack initiation site on the main crack propagation plane was adopted as a shape parameter of the three-dimensional cracks. Using this parameter, the dependence of the initiation site size on the maximum stress range can be accounted for by assuming that the microcreck growth is controlled by a threshold condition. Photomicrographs, graphs. 16 refs.
S a t u r a t i o n o f d a m a g e in f i ~ t i n g f a t i g u e o f h i g h - s t r e n g t h steels in seawater. Nakazawa, K., Sumite, M. and Maruyama, N. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 1 7 - 9 2 3 (in Japanese) The saturation behaviours of damage in fretting fatigue at frequencies of 1 and 20 Hz in seawater under freely corroding conditions were studied using highstrength steels having tensile strengths of 490, 690 and 880 MPa. Fretting damage saturated beyond a certain number of fretting cycles, and this number varied drastically with the steel and the frequency. The saturation behaviours of damage could be divided into two groups, that is the smallest numbers of fretting cycles to cause the saturation were less than 0.1% of the fretting fatigue life (group 1) and more than 40-60% (group 2). They had a strong relationship to crack initiation and growth behaviours. In group 1, cracks propagated normal to the alternating stress axis from the early stage of crack propagation. However, in group 2 the cracks propagated at angles less than 90° to the alternating stress axis, but changed direction to become normal as they propagated beyond a certain length. This relationship could be explained from the mechanism that the decrease in the fretting fatigue lives of group 1 resulted from the decrease in crack initiation life caused by the acceleration of corrosion pit formation, and that of group 2 from crack initiation and growth caused by fretting. Graphs, photomicrographs. 13 refs.
Rnlta-element
s t u d y o f residual stresses in a plate T-joint f a t i g u e
r~oc.mee. Mok, D.H.B. and Pick, R.J. Inst. Mech. Eng. C, J. Mech. Eng. Sci. 1990 204, (C2), 1 2 7 - 1 3 4 Using the finite-element code Abequs to make numerical predictions, this paper studies the crack shape development and thickness effects on a series of welded plate T-joint fatigue specimens. It was found that redistribution of the residual stress only occurs in a small region near the weld toe, the extent depending greatly on the magnitude of the applied load. Graphs. 18 refs.
Environmentally i n d u c e d cracking. Scott, P.M.
Constr. Build. Mater. J u n e 1990 4, (2), 9 8 - 1 0 6
A review of current research topics in the UK on environmentally induced or assisted cracking is given with particular emphasis on how these research interests are related to current individual applications. Although the review cannot be completely comprehensive, some particularly topical items are described in greater detail including corrosion fatigue of offshore structural materials, pipeline cracking, highstrength fasteners end deserator cracking. Several key generic issues ere highlighted such as the relevance of short-term test methods to long-term service experience and the increasing reliance on fracture mechanics analyses with their strengths and weaknesses for facilitating run/repair decisions. 25 refs. Fatigue s t r e n g t h characteristics o f h i g h - s t r a n g t h steel. Asami, K. and
Emura, H. JSME Int. J. J u l y 1990 I 33, (3), 3 6 7 - 3 7 4 In order to clarify the fatigue characteristics of low-alloy steels tempered at low temperature (150-200 °C) Or carburized in generated gas, rotating-bending fatigue tests were conducted in laboratory air and dry air. The influence of the moisture in air was only observed for the cerburized steel. The low-temperature-tempered steel revealed two knees in the S-N diagram, and no fatigue limit was found up to 10s cycles (9 months). The cerburized steel revealed three knees and the fatigue limit was found to be over 108 cycles. When the fatigue test on carburized steel was carried out in dry air or the surface structure anomalies were removed, the fatigue strengths were improved, but neither knees nor fatigue limits were found up to 10s cycles. The reasons for this were discovered from the results of fractogrephy and the equations proposed by Murekami et al for the prediction of the fatigue limit of steel with microdefects. Graphs, photomicrographs. 6 refs. Prediction o f p r o p a g a t i o n and n o n - p r o p a g a t i o n o f s h o r t f a t i g u e cracks at notches u n d e r m e a n s U m . Akiniwa, Y., Tanaka, K. and Taniguchi,
N. JSME Int. J. J u l y 1990 I 33, (3), 2 8 8 - 2 9 6 A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, Kopth, and the non-propagating crack length, Cnp, was approximated by Kopth = K;pth[(Cnp-Cr)/(C2-C1)]1/2(CI=~Cnp~C2)and Kopth = K;pth (C2'~Cnp), where K~oth Wee the value for long cracks, C2 was a linear function of K;pth, and C1 was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the non-propagating crack length. The fatigue limit for crack initiation of a stage-II crack
Int J Fatigue May 1991
wee almost constant and equal to the value predicted for the case of R = -1. Graphs. 16 refs. resistance in e i u m i n i u m alloys. Jono, M. and Sugete, A. J. Jpn. Inst. Light Met. J u l y 1990 40, (7), 5 4 3 - 5 5 3 (in Japanese) Fatigue crack g r o w t h
During metal fatigue, crack initiation and propagation are very different. When designing high-rigidity machinery components, damage tolerance is to be considered. Aluminium alloy fatigue crack growth beheviour based on the theory of fracture mechanics is reviewed. Data and theory from the literature are provided and discussed. Fatigue failure of AI alloys is divided to two stages: microcreck and macrocreck propagation until fracture. A mathematical analysis of the stress concentration in crack tips is given. The crack propagation beheviours of ingot metallurgy alloys and powder metallurgy alloys are studied, followed by an emphasis on the alloys AI-Si and AI-Li. 7075 and ZK141 alloy fatigue crack propagation under dynamic loading is also discussed. Graphs, photomicrographs. 32 refs. T h e r m a l f a t i g u e u n d e r m u l t i a x i a i stresses. Feng, J., Bian, M. and Dang, Z. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 5 2 5 - 5 3 4 Thermal fatigue under multiaxiel stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with the axial mechanical fatigue loading. The axial forces on the specimens (e.g., 3Cr2W8V) were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elastoplastic finite-elemeot method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of the cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another. Graphs, photomicrographs. 6 refs.
Fatigue microcracks in t y p e 304 stainless steel at elevated t e m p e r a t u r e .
Suh, C.M., Lee, J.J. and Kang, Y.G. Fatigue Frect. Eng. Mater. Struct. M a y 1990 13, (5), 4 8 7 - 4 9 6 Microcrecking of type 304 stainless steel at 538°C has been studied, in particular, the initiation, growth and coalescence of fatigue microcrecks on smooth specimens via surface replicas and photomicrographs. Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, was obtained. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue life assessment and the evaluation of damage in structures at elevated temperatures, Graphs. 22 refs.
L i f e t i m e p r e d i c t i o n o n C r - M o - V and 316L steels u n d e r t h e r m a l and mechanical cycling. Degallaix, G., Korn, C. and Pluvinage, G. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 7 3 - 4 8 5 The thermal-mechanical fatigue resistance of a Cr-Mo-V steel and a 316L steel under combined complex mechanical strain and temperature cycling has been evaluated. The applicability to thermal-mechanical lifetime predictions of several isothermal fatigue damage concepts is investigated. From a designer viewpoint, some predictions are quite acceptable, in particular, with the Degellaix thermal activation model, for which a recent development is reported. Graphs. 27 refs. M e r k o v a p p r o x i m a t i o n t o f a t i g u e crack size distribution. Solomos, G.P.
and Lucia, A.C. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 457-471 The distribution of the crack size during the fatigue crack propagation phenomenon is investigated. A Markovian modelling of the crack is introduced through the fatigue crack growth law, and the associated Fokker-Planck equation is written while special care is devoted to specifying its boundary conditions. This equation is solved by the method of separation of variables and the required distribution function is obtained in the form of a convergent infinite series, An illustrative example, demonstrating the applicability of the approach, shows satisfactory agreement between the theoretical results and the experimental data. The test material was 7475-T7351 AI. Graphs. 25 refs. Fractographic analysis o f crack g r o w t h and shear lip d e v e l o p m e n t under s i m p l e v a r i a b l e - a m p l i t u d e loading. Ling, M. R. and Schijve, J. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 4 3 - 4 5 6 Fatigue crack growth experiments were carried out on 2024-T3 sheet specimens, loaded under simple variable-amplitude load sequences, including blocks of overload cycles or underload cycles and blocks of small-amplitude cycles. The purpose was to study interaction effects by analysing striation measurements and shear lip developments. The striations clearly demonstrate delayed retardation. Shear lip measurements indicate significantly slower crack growth rates if the percentage shear lip increases from zero (no shear lips) to 100% (fully in the shear mode). The significance of incompatible shear lips during variable-amplitude loading is discussed. The test results are evaluated with relation to interaction mechanisms and the interpretation given to them by cycle-by-cycle prediction models. Graphs. 19 refs. Crack g r o w t h b e h a v i o u r a n d failure m i c r o m e c h a n i s m s in three h e a t resistant materials s t elevated t e m p e r a t u r e . Hour, K.Y. and Stubbins, J.F. Acts Metell. Mater. Aug. 1990 38, (8), 1463-1474 The crack growth behaviour for three high-temperature materials, alloy 80OH, type310 stainless steel (SS) and Hastelloy X, have been investigated under creep, fatigue and hold time test conditions at 650 +C. All alloys show a predominantly transgrenular crack advance mode when cycled at frequencies greater than 0.5 Hz. At lower frequencies, the failure in 310 SS transitions takes place to a predominantly
281