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The influence of prestrain and ageing on near-threshold fatigue crack propagation in as-rolled and heat-treated dual-phase steels
Root failure resulting from toe treatment of non-loed-carrying cruciform
fillet w e l d e d joints. Ohta, S., Asai, K. and Ohya, S. Quart. J. Japan Welding Soc. M a y 1991 9, (2), 8 0 - 8 6 (in Japanese)
The smoothing of the fillet toe results in an improvement in the fatigue strength, but at the same time it seems to develop a liability to root failure occurrence. The purpose of this study is to investigate the relation between root failure and stress concentration in a toe-smoothed non-load-carrying-type cruciform fillet welded joint end the effects of lack of joint penetration on the fatigue strength of the fillet joint. Lack of joint penetration in the welds was deliberately introduced by a special method of putting s half-cut core wire on the weld joint. After welding, toes were TIG dressed. In fatigue teats the maximum stress of the fatigue cycle was fixed to the yield stress of the material (589 MPa). The minimum stress was variable. The root failure seems to be apt to oCcur if the elastic-stress concentration factor at the TIG dressed toe is approximately 1.6 in the present tests. The difference in heights of lack of joint penetration, 0.92 and i.64 ram, shows no significant effect on fatigue strength. The root failure occurs in general at a lower stress range than the stress range where a toe failure follows a root crack. The most important condition for preventing a root failure is to allow no lack of joint penetration (rib side), that is a slit perpendicular to the loading direction. Graphs. 13 refs.
b e h a v i o u r o f a l u m i n i u m alloy components or weldments. Bornpard, S., Bramat, M. and Uhry, A. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 4 1 - 4 7 (in French) Fatigue
This paper reviews the statistical investigations on the fatigue behaviour of AI alloy components or weldments: a file including 1057 recordings characterizing 123 test series performed on butt, fillet and cruciform joints was used; the materials considered were AI-Mg , AI-Si-Mg and AI-Zn alloys in the thickness range of 8-15 ram. The problems raised by the fatigue of both full-scale components and notched or unnotched specimens are outlined (the influence of surface condition, defects and stress levels). It was shown that for a number of cycles N ~>105 the fatigue behaviour is not affected by the type of alloy, whereas it is moderately by the stress ratio Omin/Omaxand significantly by the joint geometry, the welding process and the residual-stress level. Graphs. 14 refs.
shock t r e a t m e n t o f h i g h - s t r e n g t h steels. Castellucci, P. Troufiakov, V.I., Mikheev, P.P. Stetnikov, E. Ch. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 3 1 - 3 7 (in French)
Ultrasonic
The effect of a treatment on the fatigue behaviour of welded T-joints was investigated. The applied technique was ultrasonic prestress shock treatment; tests were performed on 10 mm thick E-692-type steel in the quenched and tempered delivery state; weld beads were deposited by GMA welding. The shock treatment procedure is outlined and the equipment briefly described. The residual-stress field resulting from the treatment is determined. The comparison of fatigue test results on joints both in the as-welded condition and after ultrasonic shock treatment was satisfactory; ultrasonic prestress shock treatment is a simple and low-cost method as compared with other treatments. Graphs, photomicrographs. 14 refs.
Fatigue s t r e n g t h o f p a r t i a l - p e n e t r a t i o n w e l d m e n t s . Janosch, J.J. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 1 7 - 3 0 (in French) The effect of partial penetration on the fatigue strength of partial-penetration butt welds in T-joints under tension and bend loading was investigated. Conditions for the slow propagation of microcracks originating at the weld toe were determined accurately. A mathematical model was developed for the effect of partial root penetration on the fatigue behaviour in terms of the bead geometry and loading conditions. Graphs, photomicrographs. 5 refs.
Fatigue crack growth behaviour in welded joints of high-strength steel at l o w t e m p e r a t u r e . Kitsunai, Y. and Yoshihisa, E. JSME Int. J. J u l y 1991 Ser. I, 34, (3), 3 3 9 - 3 4 6 Fatigue crack growth rates in the base metal and welds of welded joints of HT80 steel were evaluated at temperatures ranging from room temperature down to 93 K. For the welds the crack growth rates between room temperature and 173 K were dominated by residual stresses rather than temperatures. As temperatures decreased below 123 K, the crack growth rates of the base metal and welds increased markedly because of cyclic cleavage during striation growth. The crack growth rates of the welds were correlated with an effective stress intensity factor range, AKeffR, which was estimated by superposition of the respective stress intensity factors for the residual-stress fields and for the applied sresses when the crack growth was dominated by striation formation. Photomicrographs, graphs. 21 refs.
Cyclic fatigue-crack propagation along c e r a m i c / m e t a l interfaces. Cannon, R.M,, Dalgleish, B.J., Dauskardt, R.H., Oh, T.S., and Ritchie, R.O. Acta Metell. Mater. Sept. 1991 39, (9),2145-2156 The integrity of ceramic/metal joints is investigated under mechanically applied cyclic stresses using double-cantilever-beam, and compect-tension, sandwich test specimens. Specifically, fatigue-crack propogstion rates for interfacial cracks are characterized over a range of velocities from 10-9 to 10-4 m s -1 for glass/Cu and alumina/AI-alloy interfaces tested in moist air. Compared with the corresponding (stress-corrosion) results under sustained loading, it is found that true interracial cracks in glass-Cu joints are significantly accelerated under cyclic loads. In addition, the crack extension force (G) thresholds for interfacial crack growth under cyclic loads are approximately 45% lower than under sustained loads and are typically over six times lower than the interface toughness (Gc). Conversely, for the alumina/AI-alloy system (AI-4Mg) fracture never occurs in the interface region; under monotonic loading, cracking progresses near the interface in the ceramic layer, whereas under cyclic loading, failure may occur either in the ceramic or in the metal. Based on a comparison with fatigoa-crack growth data in bulk alumina and bulk AI alloys, it is found that the near-interfacial crack-growth rates in the metal are much lower than those of the bulk ceramic and show a far higher dependence on the range of G than their behaviour in the bulk metal. Graphs, photomicrographs. 30 refs.
Int J Fatigue May 1992
Mechanical properties Initial crack size in f a t i g u e life e s t i m a t i o n of f i l l e t - w e l d e d j o i n t s
Sakano, M. and Mikami, /. Technol. Rep. Kansai Univ. Mar. 1991 33, 1 8 7 - 1 9 4 In order to define the initial crack size in fatigue life estimations of fillet-welded joints, long life fatigue tests of specimens with a longitudinal attachment were carried out under constant-amplitude loading and computer-simulated highway variable loading, and the initiation and early-stage propagation behaviour of fatigue cracks at the fillet weld toe were investigated. Facet-like fracture surfaces were observed where fatigue cracks were initiated and their depth was found to be independent of the stress range. Fatigue crack propagation life analyses applying the fracture mechanics concept were performed on the assumption that the initial crack is such a fracture surface. Good agreement was obtained between the estimated fatigue life and the experimental one. 9 refs.
The influence of prestTain and ageing on near-threshold fatigue crack propagation in as-rolled and heat-treated dual-phase steels. Zheng, Y., Wang, Z. and Ai, S. SteelRes. M a y 1991 62, (5), 2 2 3 - 2 2 7 The influence of prestrain and ageing on near-threshold fatigue-crack propagation (FCP) in as-rolled and heat-treated dual-phase steels (DPS) was investigated over a wide range of 10-1° - 10-7 mm/cycle in laboratory air under a load ratio of R = 0. It was found that the value of the fatigue crack propagation threshold increases with increasing grain size and decreasing yield stress, and that a combination of 10% prestraining with 175 °C/30min ageing showed almost no effect on the threshold level of as-rolled dual-phase steel, but decreased that of heat-treated dualphase steels by more than 37%. This different type of behaviour was suggested to result from the remarkable differences in grain size and volume fraction of martensite (Vrn) in these two kinds of dual-phase steels. An estimation of the plastic-zone size of the fatigue crack tip supported this suggestion. Graphs, photomicrographs. 18 refs.
The effect o f hardness on t h e f r e t t i n g f a t i g u e o f alloy steels. Gao, H., Gu, H. and Zhou, H. Fatigue Fract. Eng, Mater. Struct. J u l y 1991 14, (7), 7 8 9 - 7 9 6 The fretting fatigue behaviour of several alloy steels (eg, 2CR13, 35CrMo and 60Si2Mn) is reported, Fretting fatigue experiments were conducted on flat fretting junctions in axial tension at a stress ratio of 0.1. In all cases, the same materials were fretted against each other. The fretting fatigue strength at a slip amplitude of 45 p,m is rather insensitive to the hardness of the materials. The fretting fatigue strength at a slip amplitude of 10 p~m increases with increase in hardness. As the slip amplitude increases, the fretting fatigue life of 35CrMo steel decreases, the depth of wear scars increases and the wear damage becomes more severe. The reason for the similarity of fretting fatigue to the fatigue of notched specimens is that the effect of wear scars is similar to that of notches. Graphs, photomicrographs. 12 refs. Oxidation-assistud f a t i g u e crack growth behaviour in alloy 718. I. Q u a n t i t a t i v e m o d e l l i n g . Ghonem, H., and Zheng, D. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 4 9 - 7 6 0 A new model to predict the high-temperature, intsrgranular crack growth rate behaviour in alloy 718 at 650 °C is presented. The mode~ is based on the concept that the crack tip oxidation-fatigue damage is a non-linear process governed by the chemical-mechanical interaction along the grain boundary fracture path at the crack tip. The concept of the two*stage oxidation mechanism was used here. This mechanism depends on the rate of formation of the chromia layer in relation to the build-up of other oxide types at the crack tip. The saturation of the Cr203 build-up signifies the occurrence of the oxide pessivation effect. The determination of the amount of Cr203 depends both on the amount of oxygen diffused along the affected grain boundary and Cr transported via a mobile dislocation network. Here, both the grain boundary and effective dislocation pipe are treated as short-circuit diffusion paths along which the diffusion process can be described using Whipple's solution. The model yields sufficient information to correlate the amount of Cr203 with the reduction in the grain boundary ductility within the affected oxide zone. The grain boundary ductility is balanced by the effective strain at the crack tip resulting from the external loading. This balance defines the fracture criterion of the model and permits the calculation of the crack advance/cycle, which is the ultimate goal of this work. 38 refs. Oxidation-assisted f a t i g u e crack growth behaviour in alloy 718. II. Applications. Zheng, D. and Ghonem, H. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 6 1 - 7 6 8 In previous work, a quantitative model based on a two-stags oxidation mechanism was developed to describe the oxidation-assisted crack growth behaviour in alloy 718. This model is used to predict the crack growth rate in this alloy at 650 °C for two different loading conditions: one is a continuous cycle with a hold time duration of 300 s imposed at minimum load, the other is a continuous cycle without a hold time duration. The results obtained from applying the model of these loading profiles were then compared with those obtained experimentally. Good agreement was observed between the two sets of data. Details of the model calculations are discussed, and suggestions to extend the model capabilities further are made. Graphs. 10 refs.
The effects o f microstructure and fracture surface roughness on nesrthreshold fatigue crack propagation characteristics o f a two-phase cast stainless steel. Murakami, R.-/., Kim, Y.H. and Ferguson, W.G. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 4 1 - 7 4 8 The effects of stress ratio, microstructure and fracture surface roughness on the fatigue properties of a two-phase cast (DUPLEX) stainless steel were investigated. This pehaviour was examined by means of the fracture mechanics approach and frsctography. The fatigue crack growth rate decreased with decreasing stress ratio. The stress ratio markedly influenced the fatigue crack growth rate as &K approached the value ~Kth. The roughness of the fracture surface was greater in the as-cast material than in the heat-treated material. Analysis of the crack growth data using
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fillet w e l d e d joints. Ohta, S., Asai, K. and Ohya, S. Quart. J. Japan Welding Soc. M a y 1991 9, (2), 8 0 - 8 6 (in Japanese)
The smoothing of the fillet toe results in an improvement in the fatigue strength, but at the same time it seems to develop a liability to root failure occurrence. The purpose of this study is to investigate the relation between root failure and stress concentration in a toe-smoothed non-load-carrying-type cruciform fillet welded joint end the effects of lack of joint penetration on the fatigue strength of the fillet joint. Lack of joint penetration in the welds was deliberately introduced by a special method of putting s half-cut core wire on the weld joint. After welding, toes were TIG dressed. In fatigue teats the maximum stress of the fatigue cycle was fixed to the yield stress of the material (589 MPa). The minimum stress was variable. The root failure seems to be apt to oCcur if the elastic-stress concentration factor at the TIG dressed toe is approximately 1.6 in the present tests. The difference in heights of lack of joint penetration, 0.92 and i.64 ram, shows no significant effect on fatigue strength. The root failure occurs in general at a lower stress range than the stress range where a toe failure follows a root crack. The most important condition for preventing a root failure is to allow no lack of joint penetration (rib side), that is a slit perpendicular to the loading direction. Graphs. 13 refs.
b e h a v i o u r o f a l u m i n i u m alloy components or weldments. Bornpard, S., Bramat, M. and Uhry, A. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 4 1 - 4 7 (in French) Fatigue
This paper reviews the statistical investigations on the fatigue behaviour of AI alloy components or weldments: a file including 1057 recordings characterizing 123 test series performed on butt, fillet and cruciform joints was used; the materials considered were AI-Mg , AI-Si-Mg and AI-Zn alloys in the thickness range of 8-15 ram. The problems raised by the fatigue of both full-scale components and notched or unnotched specimens are outlined (the influence of surface condition, defects and stress levels). It was shown that for a number of cycles N ~>105 the fatigue behaviour is not affected by the type of alloy, whereas it is moderately by the stress ratio Omin/Omaxand significantly by the joint geometry, the welding process and the residual-stress level. Graphs. 14 refs.
shock t r e a t m e n t o f h i g h - s t r e n g t h steels. Castellucci, P. Troufiakov, V.I., Mikheev, P.P. Stetnikov, E. Ch. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 3 1 - 3 7 (in French)
Ultrasonic
The effect of a treatment on the fatigue behaviour of welded T-joints was investigated. The applied technique was ultrasonic prestress shock treatment; tests were performed on 10 mm thick E-692-type steel in the quenched and tempered delivery state; weld beads were deposited by GMA welding. The shock treatment procedure is outlined and the equipment briefly described. The residual-stress field resulting from the treatment is determined. The comparison of fatigue test results on joints both in the as-welded condition and after ultrasonic shock treatment was satisfactory; ultrasonic prestress shock treatment is a simple and low-cost method as compared with other treatments. Graphs, photomicrographs. 14 refs.
Fatigue s t r e n g t h o f p a r t i a l - p e n e t r a t i o n w e l d m e n t s . Janosch, J.J. Soudage Tech. Connexes M a y - J u n e 1991 45, ( 5 - 6 ) , 1 7 - 3 0 (in French) The effect of partial penetration on the fatigue strength of partial-penetration butt welds in T-joints under tension and bend loading was investigated. Conditions for the slow propagation of microcracks originating at the weld toe were determined accurately. A mathematical model was developed for the effect of partial root penetration on the fatigue behaviour in terms of the bead geometry and loading conditions. Graphs, photomicrographs. 5 refs.
Fatigue crack growth behaviour in welded joints of high-strength steel at l o w t e m p e r a t u r e . Kitsunai, Y. and Yoshihisa, E. JSME Int. J. J u l y 1991 Ser. I, 34, (3), 3 3 9 - 3 4 6 Fatigue crack growth rates in the base metal and welds of welded joints of HT80 steel were evaluated at temperatures ranging from room temperature down to 93 K. For the welds the crack growth rates between room temperature and 173 K were dominated by residual stresses rather than temperatures. As temperatures decreased below 123 K, the crack growth rates of the base metal and welds increased markedly because of cyclic cleavage during striation growth. The crack growth rates of the welds were correlated with an effective stress intensity factor range, AKeffR, which was estimated by superposition of the respective stress intensity factors for the residual-stress fields and for the applied sresses when the crack growth was dominated by striation formation. Photomicrographs, graphs. 21 refs.
Cyclic fatigue-crack propagation along c e r a m i c / m e t a l interfaces. Cannon, R.M,, Dalgleish, B.J., Dauskardt, R.H., Oh, T.S., and Ritchie, R.O. Acta Metell. Mater. Sept. 1991 39, (9),2145-2156 The integrity of ceramic/metal joints is investigated under mechanically applied cyclic stresses using double-cantilever-beam, and compect-tension, sandwich test specimens. Specifically, fatigue-crack propogstion rates for interfacial cracks are characterized over a range of velocities from 10-9 to 10-4 m s -1 for glass/Cu and alumina/AI-alloy interfaces tested in moist air. Compared with the corresponding (stress-corrosion) results under sustained loading, it is found that true interracial cracks in glass-Cu joints are significantly accelerated under cyclic loads. In addition, the crack extension force (G) thresholds for interfacial crack growth under cyclic loads are approximately 45% lower than under sustained loads and are typically over six times lower than the interface toughness (Gc). Conversely, for the alumina/AI-alloy system (AI-4Mg) fracture never occurs in the interface region; under monotonic loading, cracking progresses near the interface in the ceramic layer, whereas under cyclic loading, failure may occur either in the ceramic or in the metal. Based on a comparison with fatigoa-crack growth data in bulk alumina and bulk AI alloys, it is found that the near-interfacial crack-growth rates in the metal are much lower than those of the bulk ceramic and show a far higher dependence on the range of G than their behaviour in the bulk metal. Graphs, photomicrographs. 30 refs.
Int J Fatigue May 1992
Mechanical properties Initial crack size in f a t i g u e life e s t i m a t i o n of f i l l e t - w e l d e d j o i n t s
Sakano, M. and Mikami, /. Technol. Rep. Kansai Univ. Mar. 1991 33, 1 8 7 - 1 9 4 In order to define the initial crack size in fatigue life estimations of fillet-welded joints, long life fatigue tests of specimens with a longitudinal attachment were carried out under constant-amplitude loading and computer-simulated highway variable loading, and the initiation and early-stage propagation behaviour of fatigue cracks at the fillet weld toe were investigated. Facet-like fracture surfaces were observed where fatigue cracks were initiated and their depth was found to be independent of the stress range. Fatigue crack propagation life analyses applying the fracture mechanics concept were performed on the assumption that the initial crack is such a fracture surface. Good agreement was obtained between the estimated fatigue life and the experimental one. 9 refs.
The influence of prestTain and ageing on near-threshold fatigue crack propagation in as-rolled and heat-treated dual-phase steels. Zheng, Y., Wang, Z. and Ai, S. SteelRes. M a y 1991 62, (5), 2 2 3 - 2 2 7 The influence of prestrain and ageing on near-threshold fatigue-crack propagation (FCP) in as-rolled and heat-treated dual-phase steels (DPS) was investigated over a wide range of 10-1° - 10-7 mm/cycle in laboratory air under a load ratio of R = 0. It was found that the value of the fatigue crack propagation threshold increases with increasing grain size and decreasing yield stress, and that a combination of 10% prestraining with 175 °C/30min ageing showed almost no effect on the threshold level of as-rolled dual-phase steel, but decreased that of heat-treated dualphase steels by more than 37%. This different type of behaviour was suggested to result from the remarkable differences in grain size and volume fraction of martensite (Vrn) in these two kinds of dual-phase steels. An estimation of the plastic-zone size of the fatigue crack tip supported this suggestion. Graphs, photomicrographs. 18 refs.
The effect o f hardness on t h e f r e t t i n g f a t i g u e o f alloy steels. Gao, H., Gu, H. and Zhou, H. Fatigue Fract. Eng, Mater. Struct. J u l y 1991 14, (7), 7 8 9 - 7 9 6 The fretting fatigue behaviour of several alloy steels (eg, 2CR13, 35CrMo and 60Si2Mn) is reported, Fretting fatigue experiments were conducted on flat fretting junctions in axial tension at a stress ratio of 0.1. In all cases, the same materials were fretted against each other. The fretting fatigue strength at a slip amplitude of 45 p,m is rather insensitive to the hardness of the materials. The fretting fatigue strength at a slip amplitude of 10 p~m increases with increase in hardness. As the slip amplitude increases, the fretting fatigue life of 35CrMo steel decreases, the depth of wear scars increases and the wear damage becomes more severe. The reason for the similarity of fretting fatigue to the fatigue of notched specimens is that the effect of wear scars is similar to that of notches. Graphs, photomicrographs. 12 refs. Oxidation-assistud f a t i g u e crack growth behaviour in alloy 718. I. Q u a n t i t a t i v e m o d e l l i n g . Ghonem, H., and Zheng, D. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 4 9 - 7 6 0 A new model to predict the high-temperature, intsrgranular crack growth rate behaviour in alloy 718 at 650 °C is presented. The mode~ is based on the concept that the crack tip oxidation-fatigue damage is a non-linear process governed by the chemical-mechanical interaction along the grain boundary fracture path at the crack tip. The concept of the two*stage oxidation mechanism was used here. This mechanism depends on the rate of formation of the chromia layer in relation to the build-up of other oxide types at the crack tip. The saturation of the Cr203 build-up signifies the occurrence of the oxide pessivation effect. The determination of the amount of Cr203 depends both on the amount of oxygen diffused along the affected grain boundary and Cr transported via a mobile dislocation network. Here, both the grain boundary and effective dislocation pipe are treated as short-circuit diffusion paths along which the diffusion process can be described using Whipple's solution. The model yields sufficient information to correlate the amount of Cr203 with the reduction in the grain boundary ductility within the affected oxide zone. The grain boundary ductility is balanced by the effective strain at the crack tip resulting from the external loading. This balance defines the fracture criterion of the model and permits the calculation of the crack advance/cycle, which is the ultimate goal of this work. 38 refs. Oxidation-assisted f a t i g u e crack growth behaviour in alloy 718. II. Applications. Zheng, D. and Ghonem, H. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 6 1 - 7 6 8 In previous work, a quantitative model based on a two-stags oxidation mechanism was developed to describe the oxidation-assisted crack growth behaviour in alloy 718. This model is used to predict the crack growth rate in this alloy at 650 °C for two different loading conditions: one is a continuous cycle with a hold time duration of 300 s imposed at minimum load, the other is a continuous cycle without a hold time duration. The results obtained from applying the model of these loading profiles were then compared with those obtained experimentally. Good agreement was observed between the two sets of data. Details of the model calculations are discussed, and suggestions to extend the model capabilities further are made. Graphs. 10 refs.
The effects o f microstructure and fracture surface roughness on nesrthreshold fatigue crack propagation characteristics o f a two-phase cast stainless steel. Murakami, R.-/., Kim, Y.H. and Ferguson, W.G. Fatigue Fract. Eng. Mater. Struct. J u l y 1991 14, (7), 7 4 1 - 7 4 8 The effects of stress ratio, microstructure and fracture surface roughness on the fatigue properties of a two-phase cast (DUPLEX) stainless steel were investigated. This pehaviour was examined by means of the fracture mechanics approach and frsctography. The fatigue crack growth rate decreased with decreasing stress ratio. The stress ratio markedly influenced the fatigue crack growth rate as &K approached the value ~Kth. The roughness of the fracture surface was greater in the as-cast material than in the heat-treated material. Analysis of the crack growth data using
199