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Effect of stress ratio on effective stress range ratio and crack growth in 6061-T6 aluminium alloy
often present in these alloy types. Calabrese and Laird have shown, in the case of AI 4Cu with coarse 0', that cell formation depends on interparticle spacing Furthermore. some authors have shown that the to and from movement of dislocations could enhance the formation of dislocation loops. On Cu. Feltner has demonstrated that the flip flop movement of prismatic loops could accommodate both cyclic stress and strain An attempt is made to apply, on 2024 alloy, the approach developed by Feltner on cyclic behaviour The cyclic behaviour of AI alloy 2024 results from dislocation loop multiplication provided by cross slip mechanisms. Both cyclic strain and stress fairly agree with a process based on the interaction of mobile dislocations with primsatic loops 7 refs
Modelling delay and thickness effects in fatigue. Clayton, J.Q. Eng. Fract. Mech. 1 9 8 9 32, ( 2 ) , 2 8 9 - 3 0 8 A simple Dugdale theory is used to investigate the effect of material (eg 2024-T3) thickness on the retardation due to a fatfgue overload cycle. In the first part, crack growth through a standard Dugdale plastic zone is analysed, and a crack closure model derived from the analysis is tested for predictive accuracy against published experimental data. The merits and limitatons of the model in practical crack growth prediction are discussed In the second part, the overload problem is re-analysed, taking a more detailed account of the stresses likely to be present in the plastic zones surrounding a fatigue crack tip. Rather than dealing specifically with plane stress, as implied by the basic Dugdale concept, this approach considers a modified stress state in which the conventional tensile yield stress, (~v, is replaced by a flow stress, o o, which is dependent on the level of threedimensional constraint developed in a particular material thickness, Relationships for the crack-opening loads as a function of overload magnitude, average level of constraint developed for a given material thickness, and position of the crack tip within the constrained plastic zone are presented. Predictions of crack growth made by this simple model are shown to be in good agreement with experimental data. particularly at high overload ratios and in material of various thicknesses, where other overload models such as the Willenborg model appear deficient 48 refs.
A cycle-combining method for creep fatigue analysis. Debaene, J.P. and Permezel, F~ Int. J. Pressure Vessels Piping 1989 36, ( 1 ) , 7 5 - 9 0 During the operational life of an industrial installation, the operating conditions are modified, voluntarily or involuntarily, as a result of normal, upset or emergency conditions. The order in which these various events occur in time influences the cumulative creep fatigue damage and is not known in advance. For this reason, the codes require the succession of events which will lead to the maximum creep fatigue damage to be taken into account. The purpose is to present such a method. According to the data given by the process and the thermomechanical analysis, it is considered, as a starting point, to build the complete load histogram of the component to be analysed. The problem to solve is how to take out the oberetional cycles which will lead to the maximum total creep fatigue damage, A computer program which evaluates all the possible operational cycles is developed. Taking into account the relaxation of stresses theoretically, the equations lead to the optimum distribution of hold time, both between the same operational cycle and between different operational cycles. Then, practical implementation of these equations and a complete practical example with stainless steel are given 2 refs.
Effects of treatments on mechanical properties of titanium alloy ca=tings. Eylon, D., Frees, F.H. and Barice, W.J. SAMPEQ. Jan. 1989 20, ( 2 ) , 4 2 - 4 5 Refinement of the alpha phase is one of the most effective methods for improving tensile and fatigue properties of alpha + beta Ti alloys, However. microstructure refinement in Ti alloy net-shabe products, like castings, requires different approaches than those used for wrought material, since work is not applied during net-shape processing, Refinement and modification of the alpha structure in Ti-6AI-4V castings by eight different methods is discussed and fatigue life and tensile properties are presented and correlated to the modified structures, 21 refs
Localization of plastic strain and fatigue behaviour of AI-Li-AIIoys. Velten, B., Vasudevan, A.K. and Hornbogen, E. Z. Metallkd. Jan. 1 9 8 9 80, ( 1 ) , 2 1 - 2 5 The influence of shearable coherent particles on fatigue behaviour of two different AI-Li alloys is investigated, The localization of plastic strain, caused by the shearing of those particles, favours an early crack initiation, as was also found in Ni superalloys or stainless steels. Crack growth, however, is decelerate d by crack branching and reversible slip. The degree of strain localization can be influenced by the addition of further elements eg Cu, which form semi-coherent precipitates. In the range of low and intermediate stress amplitudes the alloy exhibiting an intensely localized strain shows the highest fatigue life 17 refs
A criterion for contact fatigue of ion-nitrided gear. Wu, Y.S., Ma, B.T., Hello, J.W. and Lye, B.Z. Wear Jan. 1989 135, (1), 13-21 ion nitriding with a hardened layer of 0.¢-Q5 mm thickness can be employed for heavy duty gear (eg 35CrMo) with contact stresses as high as 950 MPa. Endurance lives may reach an order of 10', For the criterion of contact fatigue strength for case hardened gear, it is suggested that the ratio of the shear fatigue strength to the loading shear stress parallel to the surface be used, The thin ion nitriding layer is not very brittle under static loading and can last 105 cycles of pure roiling under 4000 MPa contacting stress, This layer is not as brittle as expected and can carry an appreciable contact load 11 refs
Evaluation of leek flow rate end jet impingement related to leek before break. Yano, T., Mamushima, E. end Okamoto, A. Nucl. Eng, Des. Jan. (11), 1 9 8 9 111, ( 1 ) , 1 9 7 - 2 0 6 Leak flow rate and jet impingement load are experimentally evaluated using very narrow and short artificial slits which simulate a through-wall fatigue crack. The following items are clarified in the experiments under BWR fluid conditions; the leak flow rate from the rectangular slit is experimentally clarifi~l and it agrees well with the analytical result when the estimated thermodynamic non-equilibrium parameter N in the present experiment is introduced; the orifice effects are experimentally evaluated concerning the through-wall fatigue crack, in which the discharge coefficients Co between 0,75-0.25 are obtained as a function of the exit to the stagnation area ratio AcAo; when very narrow and short slits are used in the jet impingement experiments, the maximum pressure on the target
286
is higher than that of a large diameter pipe and it increases with the decrease of the flow path length to hydraulic diameter ratio LID of the slit because of the thermodynamic nonequilibrium effect Examples of STS42 steel are given 12 refs
Surface small crack growth behaviour in low-cycle fatigue at elevated temperature and application limit of macroscopic crack growth l a w . Okazaki, M., Yada, T. and Endoh, T Nuc/. Eng. Des. Jan. (11) 1989 111, (1), 1 2 3 - 1 3 4 Surface small crack growth behaviour of type 304 stainless steel in low cycle fatigue under fast fast and slow-fast cycling was investigated at a temperature of 873 K. using smooth specimens with various grain sizes It was shown that. once the small cracks had grown up to a few grains size, they predominantly propagated with strain cycling. while most of small cracks stopped propagating when they grew up to one grain size. It was also Shown that the small crack growth rate significantly slowed down where the crack rength was integral multiple of the grain size. The above behaviour resulted from the grain boundaries temporarily impeding the small crack growth. The crack length below which the grain boundaries affected the small crack growth rate was also given as function of the relative length to the grain size Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fas~fast cycling, the small crack growth rate was about ten times as targe as the macroscopic crack growth rate, where its length was comparable to the grain size Based on the results thus obtained. the application limit of macroscopic crack growth law to the surface small crack growth was discussed The macroscopic crack growth law was n'ot applicable to the small crack growth, until the crack length was about ten times average grain size in fas~fast and slow fast cycles 22 refs
Realistic cycle fatigue strength of spherical graphite cast iron (sphsroguss) under the influence of corrosion. Fischer, E., Mahnig, I~ and Wafter, H.P. Konstr. Giessen 1988 13, (2), 1 6 - 2 4 (in German) Specimens of heet-treatable steel Ck45V (049 carbon. 0 34Si, 0.73Mn, 0.26Cu) and spherical graphite cast iron GGG-40 (3.63C, 2.42SL 0.48Mn, 0.037 Mg) were fatigue tested in air and in NaCI solution under both uni-directional and completely reversed bending Some tests were performed under constant stress cycling, others with a pre-selected loading program The effect of the corrosive environment under constant cycle loading was large for the steel and almost as large for the cast iron. Use of realistic cycles shifts the fatigue limit to a factor of 100 higher number of cycles; the difference between the two materials is almost completely eliminated. Under realistic cycling, corrosion fatigue strength of GGG40 at 2 x 10~ cycles is plus/minus 265 N/mm = as compared with plus/minus 88 N/mm 2 under constant cycles. Shot biasing has a beneficial effect on corrosion fatigue under both constant and realistic cycling. 13 refs.
The effect of cyclic loading on the dislocation structure of fully pearlitic steel. Dollar, M., Bernstein, I.M., Daeubler, M. and Thompson, A.W. Metal/. Trans. A Mar. 1 9 8 9 2 0 A , ( 3 ) , 4 4 7 - 4 5 1 The fatigue behaviour and associated crack initiation of peadite has been correlated with developing dislocation configurations in a cyclically deformed fully pearlitic steel (eg 1080). Under fatigue conditions at low stress amplitudes, dislocations are found to be generated and largely confined to the cementite/ferrite interfaces, most likely due to the development of elastic incompatibility stresses between the cementite and ferrite. This deformation mode encourages fatigue crack initiation parallel to the cementite lamellae. The fatigue limit of pearlite appears not to be influenced by the intedamellar spacing, a result basically different from that found in monotoned formation where yielding and flow are strongly affected by the spacing. The apparent differences are discussed in terms of the different dislocation configurations formed during monotnic and cyclic deformation. 9 refs,
Observations on the faceted fatigue fracture of cast C o - C r - M o alloy used for surgical implants. Zhuang, L,Z and Langer, E.W. Metal/. Trans. A Jan. 1989 20A, (1), 99-103 Fatigue crack propagation behaviour of cast Co-Cr-Mo alloy (Vitallium or HS21) used for surgical implants is studied at room temperature with the stress intensity factor range, ~.K, between 20-50 MPa m 1/2, corresponding to a crack growth rate. da/dN, between 10 -5 10 2 mm/cycle. Faceted fatigue fractures are observed to be the dominant fracture features of the alloy under all tested conditions, The characteristics of the faceted fatigue fractu res are discussed and identified. The development of facets seems to be a consequence of low stacking fault energy of the alloy since this results in a dense structure of stacking fault intersections. Grain size has little effect on the occurrence of faceted fatigue fractures in cast Co-C~Mo alloy. Modifying the composition of the base alloy with Ni additions increases the fracture ductility of the alloy, 26 refs
The near-threshold high R-ratio fatigue crack growth characteristics of SA608 C I III reactor pressure veasel steel. Achilles, R.D. and Bul/och, J.H.
Int. J. Pressure Vessels Piping 1988 35, ( 5 ) , 363--382 The effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cf III reactor pressure vessel (RPV) steel is described. It is shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth beheviour in an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate'increased by a factor of four in the case of the 3 Hz test, while that for the 0,3 Hz test was increased by a factor of approx 16. This environmental enhancement manifested itself in the form of intergrenular failure. For the 0.3 Hz test the percentage intergranular failure decreased from 18 to < 1% with an increase in AK level, The transition from microstructure-sensitive to microstructure-ineensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size, 22 refs,
Effect of straea ratio on effective e t r e ~ range ratio and crack growth in (1061-T6 e l u m i n i u m a l l o y . Kurnar, R. and Garg, S.B.L Int. J. Pressure Vessels Piping1988 35, (5), 351-361 Crack closure experiments using a COD gauge were performed on 6061-T6 AI-alloy. for various stress ratios, Experimental results show that for a given stress ratio, t7, the crack length increases up to 1 mm after which it is stabilized, On the basis of the experimental results a model for effective stress range ratio U, which was found to be a function of stress ratio R, was developed, This model was fitted to existing constant amplitude crack propagation data for the 6061-T6 AI-alloy. 13 refs,
Int J Fatigue July 1989
Modelling delay and thickness effects in fatigue. Clayton, J.Q. Eng. Fract. Mech. 1 9 8 9 32, ( 2 ) , 2 8 9 - 3 0 8 A simple Dugdale theory is used to investigate the effect of material (eg 2024-T3) thickness on the retardation due to a fatfgue overload cycle. In the first part, crack growth through a standard Dugdale plastic zone is analysed, and a crack closure model derived from the analysis is tested for predictive accuracy against published experimental data. The merits and limitatons of the model in practical crack growth prediction are discussed In the second part, the overload problem is re-analysed, taking a more detailed account of the stresses likely to be present in the plastic zones surrounding a fatigue crack tip. Rather than dealing specifically with plane stress, as implied by the basic Dugdale concept, this approach considers a modified stress state in which the conventional tensile yield stress, (~v, is replaced by a flow stress, o o, which is dependent on the level of threedimensional constraint developed in a particular material thickness, Relationships for the crack-opening loads as a function of overload magnitude, average level of constraint developed for a given material thickness, and position of the crack tip within the constrained plastic zone are presented. Predictions of crack growth made by this simple model are shown to be in good agreement with experimental data. particularly at high overload ratios and in material of various thicknesses, where other overload models such as the Willenborg model appear deficient 48 refs.
A cycle-combining method for creep fatigue analysis. Debaene, J.P. and Permezel, F~ Int. J. Pressure Vessels Piping 1989 36, ( 1 ) , 7 5 - 9 0 During the operational life of an industrial installation, the operating conditions are modified, voluntarily or involuntarily, as a result of normal, upset or emergency conditions. The order in which these various events occur in time influences the cumulative creep fatigue damage and is not known in advance. For this reason, the codes require the succession of events which will lead to the maximum creep fatigue damage to be taken into account. The purpose is to present such a method. According to the data given by the process and the thermomechanical analysis, it is considered, as a starting point, to build the complete load histogram of the component to be analysed. The problem to solve is how to take out the oberetional cycles which will lead to the maximum total creep fatigue damage, A computer program which evaluates all the possible operational cycles is developed. Taking into account the relaxation of stresses theoretically, the equations lead to the optimum distribution of hold time, both between the same operational cycle and between different operational cycles. Then, practical implementation of these equations and a complete practical example with stainless steel are given 2 refs.
Effects of treatments on mechanical properties of titanium alloy ca=tings. Eylon, D., Frees, F.H. and Barice, W.J. SAMPEQ. Jan. 1989 20, ( 2 ) , 4 2 - 4 5 Refinement of the alpha phase is one of the most effective methods for improving tensile and fatigue properties of alpha + beta Ti alloys, However. microstructure refinement in Ti alloy net-shabe products, like castings, requires different approaches than those used for wrought material, since work is not applied during net-shape processing, Refinement and modification of the alpha structure in Ti-6AI-4V castings by eight different methods is discussed and fatigue life and tensile properties are presented and correlated to the modified structures, 21 refs
Localization of plastic strain and fatigue behaviour of AI-Li-AIIoys. Velten, B., Vasudevan, A.K. and Hornbogen, E. Z. Metallkd. Jan. 1 9 8 9 80, ( 1 ) , 2 1 - 2 5 The influence of shearable coherent particles on fatigue behaviour of two different AI-Li alloys is investigated, The localization of plastic strain, caused by the shearing of those particles, favours an early crack initiation, as was also found in Ni superalloys or stainless steels. Crack growth, however, is decelerate d by crack branching and reversible slip. The degree of strain localization can be influenced by the addition of further elements eg Cu, which form semi-coherent precipitates. In the range of low and intermediate stress amplitudes the alloy exhibiting an intensely localized strain shows the highest fatigue life 17 refs
A criterion for contact fatigue of ion-nitrided gear. Wu, Y.S., Ma, B.T., Hello, J.W. and Lye, B.Z. Wear Jan. 1989 135, (1), 13-21 ion nitriding with a hardened layer of 0.¢-Q5 mm thickness can be employed for heavy duty gear (eg 35CrMo) with contact stresses as high as 950 MPa. Endurance lives may reach an order of 10', For the criterion of contact fatigue strength for case hardened gear, it is suggested that the ratio of the shear fatigue strength to the loading shear stress parallel to the surface be used, The thin ion nitriding layer is not very brittle under static loading and can last 105 cycles of pure roiling under 4000 MPa contacting stress, This layer is not as brittle as expected and can carry an appreciable contact load 11 refs
Evaluation of leek flow rate end jet impingement related to leek before break. Yano, T., Mamushima, E. end Okamoto, A. Nucl. Eng, Des. Jan. (11), 1 9 8 9 111, ( 1 ) , 1 9 7 - 2 0 6 Leak flow rate and jet impingement load are experimentally evaluated using very narrow and short artificial slits which simulate a through-wall fatigue crack. The following items are clarified in the experiments under BWR fluid conditions; the leak flow rate from the rectangular slit is experimentally clarifi~l and it agrees well with the analytical result when the estimated thermodynamic non-equilibrium parameter N in the present experiment is introduced; the orifice effects are experimentally evaluated concerning the through-wall fatigue crack, in which the discharge coefficients Co between 0,75-0.25 are obtained as a function of the exit to the stagnation area ratio AcAo; when very narrow and short slits are used in the jet impingement experiments, the maximum pressure on the target
286
is higher than that of a large diameter pipe and it increases with the decrease of the flow path length to hydraulic diameter ratio LID of the slit because of the thermodynamic nonequilibrium effect Examples of STS42 steel are given 12 refs
Surface small crack growth behaviour in low-cycle fatigue at elevated temperature and application limit of macroscopic crack growth l a w . Okazaki, M., Yada, T. and Endoh, T Nuc/. Eng. Des. Jan. (11) 1989 111, (1), 1 2 3 - 1 3 4 Surface small crack growth behaviour of type 304 stainless steel in low cycle fatigue under fast fast and slow-fast cycling was investigated at a temperature of 873 K. using smooth specimens with various grain sizes It was shown that. once the small cracks had grown up to a few grains size, they predominantly propagated with strain cycling. while most of small cracks stopped propagating when they grew up to one grain size. It was also Shown that the small crack growth rate significantly slowed down where the crack rength was integral multiple of the grain size. The above behaviour resulted from the grain boundaries temporarily impeding the small crack growth. The crack length below which the grain boundaries affected the small crack growth rate was also given as function of the relative length to the grain size Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fas~fast cycling, the small crack growth rate was about ten times as targe as the macroscopic crack growth rate, where its length was comparable to the grain size Based on the results thus obtained. the application limit of macroscopic crack growth law to the surface small crack growth was discussed The macroscopic crack growth law was n'ot applicable to the small crack growth, until the crack length was about ten times average grain size in fas~fast and slow fast cycles 22 refs
Realistic cycle fatigue strength of spherical graphite cast iron (sphsroguss) under the influence of corrosion. Fischer, E., Mahnig, I~ and Wafter, H.P. Konstr. Giessen 1988 13, (2), 1 6 - 2 4 (in German) Specimens of heet-treatable steel Ck45V (049 carbon. 0 34Si, 0.73Mn, 0.26Cu) and spherical graphite cast iron GGG-40 (3.63C, 2.42SL 0.48Mn, 0.037 Mg) were fatigue tested in air and in NaCI solution under both uni-directional and completely reversed bending Some tests were performed under constant stress cycling, others with a pre-selected loading program The effect of the corrosive environment under constant cycle loading was large for the steel and almost as large for the cast iron. Use of realistic cycles shifts the fatigue limit to a factor of 100 higher number of cycles; the difference between the two materials is almost completely eliminated. Under realistic cycling, corrosion fatigue strength of GGG40 at 2 x 10~ cycles is plus/minus 265 N/mm = as compared with plus/minus 88 N/mm 2 under constant cycles. Shot biasing has a beneficial effect on corrosion fatigue under both constant and realistic cycling. 13 refs.
The effect of cyclic loading on the dislocation structure of fully pearlitic steel. Dollar, M., Bernstein, I.M., Daeubler, M. and Thompson, A.W. Metal/. Trans. A Mar. 1 9 8 9 2 0 A , ( 3 ) , 4 4 7 - 4 5 1 The fatigue behaviour and associated crack initiation of peadite has been correlated with developing dislocation configurations in a cyclically deformed fully pearlitic steel (eg 1080). Under fatigue conditions at low stress amplitudes, dislocations are found to be generated and largely confined to the cementite/ferrite interfaces, most likely due to the development of elastic incompatibility stresses between the cementite and ferrite. This deformation mode encourages fatigue crack initiation parallel to the cementite lamellae. The fatigue limit of pearlite appears not to be influenced by the intedamellar spacing, a result basically different from that found in monotoned formation where yielding and flow are strongly affected by the spacing. The apparent differences are discussed in terms of the different dislocation configurations formed during monotnic and cyclic deformation. 9 refs,
Observations on the faceted fatigue fracture of cast C o - C r - M o alloy used for surgical implants. Zhuang, L,Z and Langer, E.W. Metal/. Trans. A Jan. 1989 20A, (1), 99-103 Fatigue crack propagation behaviour of cast Co-Cr-Mo alloy (Vitallium or HS21) used for surgical implants is studied at room temperature with the stress intensity factor range, ~.K, between 20-50 MPa m 1/2, corresponding to a crack growth rate. da/dN, between 10 -5 10 2 mm/cycle. Faceted fatigue fractures are observed to be the dominant fracture features of the alloy under all tested conditions, The characteristics of the faceted fatigue fractu res are discussed and identified. The development of facets seems to be a consequence of low stacking fault energy of the alloy since this results in a dense structure of stacking fault intersections. Grain size has little effect on the occurrence of faceted fatigue fractures in cast Co-C~Mo alloy. Modifying the composition of the base alloy with Ni additions increases the fracture ductility of the alloy, 26 refs
The near-threshold high R-ratio fatigue crack growth characteristics of SA608 C I III reactor pressure veasel steel. Achilles, R.D. and Bul/och, J.H.
Int. J. Pressure Vessels Piping 1988 35, ( 5 ) , 363--382 The effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cf III reactor pressure vessel (RPV) steel is described. It is shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth beheviour in an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate'increased by a factor of four in the case of the 3 Hz test, while that for the 0,3 Hz test was increased by a factor of approx 16. This environmental enhancement manifested itself in the form of intergrenular failure. For the 0.3 Hz test the percentage intergranular failure decreased from 18 to < 1% with an increase in AK level, The transition from microstructure-sensitive to microstructure-ineensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size, 22 refs,
Effect of straea ratio on effective e t r e ~ range ratio and crack growth in (1061-T6 e l u m i n i u m a l l o y . Kurnar, R. and Garg, S.B.L Int. J. Pressure Vessels Piping1988 35, (5), 351-361 Crack closure experiments using a COD gauge were performed on 6061-T6 AI-alloy. for various stress ratios, Experimental results show that for a given stress ratio, t7, the crack length increases up to 1 mm after which it is stabilized, On the basis of the experimental results a model for effective stress range ratio U, which was found to be a function of stress ratio R, was developed, This model was fitted to existing constant amplitude crack propagation data for the 6061-T6 AI-alloy. 13 refs,
Int J Fatigue July 1989