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Fatigue life enhancement of specimens with stress concentrators using a thermo-mechanical technique
intensity factor. The fatigue threshold in synthetic sea water was about one-half of that in air. The fatigue crack propagation rates under random loading could be well estimated by the results obtained from constant amplitude tests assuming a linear cumulative damage law. Graphs. 12 refs.
Fatigue Me enhencement of specfmonr with stress concentrators e thermo-mechunicel technique. Co/e, G.K. and Lam, Y.C. Scri. Metal/. Mater. 26 12 (Dee 1991) 2849-2853
using
There has been a debate as to whether fatigue life in welds is mostly dependent upon crack initiation, or may be legitimately described using fracture mechanics and fatigue crack growth methodologies. For this reason it was decided that, before examining the affect of the thermomechanical treatment on the fatigue life of welds, exploratory tests on specimens containing stress concentrations would be carried out. It was found that, at the 95% confidence level, the thermomechanical treatment led to an imwovement in the fetiaue life of wecimans containina holes. This improvement appeared to be more significant at st;esses approaching ihe endurance limit. Although not conclusive, there was indication that the treatment had no detrimental effect on the fatiaua life of specimens without stress concentrations. Graphs. 8 refs.
A demo e mechanics model of fatigue crack initiation in notched pletes. Chow !L. and Wei Y. The& Appl. fract. bech. 16 2 (Nov 1991) 123-133 A damage model is proposed to characterize fatigue crack initiation in notched plates. This is accomplished bY assuming e damage fracture criterion and effective instantaneous tangent moduli matrix which accounts for damage accumulation. A finite element package is used to perform the numerical analyses for an edgenotched plate specimen made of Al alloy 2024-T3 under cyclic loading of varying constant amplitudes. The estimated fatigue crack initiation life agrees satisfactorily with those determined axp%rimantallY. An important observation from the results of the proposed damage mode is its ability to identify tha phenomenon of atrass redistribution due to material degradation in two-dimensional specimens during fatigue loading. Graphs. 10 refs.
Fatigue crack rowth rate in ferrite-martensite dual-phase steel. Deng, R.Y. and Ye i?J. Theor. A& Frac?. Mech. 16 2 (Nov 1991) 109-122 An empirical study is mada on the fatigue crack growth rate in ferrite-martensite dual-phase IFMDP) steel fag 16 f&RI. Particular attention is given to the effect of ferrite content in the range 24.2-41.5% where good fatigue resistance was found at 33.896. Variations in ferrite content did not effect the crack growth rate da/dN when plotted against the effective stress intensihl factor range A&e which was assumed to follow a linear relation with the crack tip stress inter&v factor range AK. A high Ah corresponds to uniformly distributed small si;e ferrite and martensite. No other appreciable correlation could be related to the microstructure morphology of the FMDP steal. The closure stress intansitY factor &I, however. is affected by the ferrite content with &I/K,,,~~ reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite, dividing tha fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in AKand the latter would increase with AKdepending ratio. The same data when correlated with the strain energy on the R = u,i./o,.,: density factor range AS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter (I involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that tha daldN data for different R would collapse on to a sing!+ curve with a narrow scaner band when plotted against CLAS. Graphs, photomicrographs. 17 refs.
A review of fatigue creek growth anelyses. Liu, H. W Theor. Appl. fract. Mech. 16 2 (Nov 1991) 91-108 Stress intensihl factor range AK has been shown to correlate well with fatigue crack growth rate daidN. A number of fatigue crack growth theories have been developed for such correlations. Often, conjectural theories of fatigue crack growth are constructed from experimental data. On the other hand, fatigue crack growth theories can also be derived rigorously with deductive logic. Four such deductive theories are reviewed: (1) that for the growth of a smell crack in a very wide homogeneous plate; (2) the theory of similitude for the correlation of daldN with AK; (3) a thew of crack growth in homogeneous materials in small-scale yielding; and 14) the unzipping theory of fatigue crack growth. These four theories are synthesized into a logic framework useful for fatigue crack growth analysis. The deductive theories and the conjectural theories complement each other in the advancas of the understanding of fatigue crack growth behaviour and their applications to defining the complex &ues oj the grov.?h of small cracks and crack growth in composites are illustrated. The materials examined were Cu. 7075 and 2024 Al. and Fe-2.4Si. Graphs, photomicrographs. 44 refs.
Creep-fatigue life prediction method using Diercks equetion for Cr-Mo steel. Sonoya, K. Nonaka, 1. and Kitagawa, M. Tersu-to-Hagane IJ. Iron Steel Inst. Jpnl 76 5 (May 1990) 775-782 A method to predict the creep-fatigue life properties of Cr-MO steels is proposed on the basis of the Diercks equation which correlates the creep-fatigue lives of SUS 304 sseels under various temperatures, strain ranges, strain rates and hold times. The accuracy of the proposed method we8 compared with that of the existing methods. The following results were obtained. (1) Fatigue strength and creep rupture strength of Cr-MO steel are different from those of SUS 304 steel. Therefore in order-to epply the Diarcks equation to creep-fatigue prediction for Cr-MO steel, the difference of fatigue strength was found to be corrected by the fatigue life ratio of both steels and the difference of creep rupture strength was found to be corrected by the equivalent temperature corresponding to equal strength of both steels. (2) Creep-fatigue life can be predicted bY the modified Diercks equation within a factor of two which is nearly as precise as the accuracy of the strain range partitioning method. Required test and analysis procedure of this method are not so complicated as the strain range partitioning method. Graphs.
4.20
Hardening mechenisms in e dynamic strain e ing alloy, Hestelloy X, during isothermal and thermcmechenicel cycle 1 defermetion. Mmer, R. V. and Caste//i, M.G. Metal/. Trans. A 23A 2 (Feb 1992) 551-561 Isothermal cyclic deformation tests were conducted on Hastelloy X with a total strain range of + 0.3% at several temperatures and strain rates. Cyclic hardening exhibited a broad peak between appox 200 and 700 “C, with a maximum near 500 “c of approx 80% increase in stress amplitude, Aol2, at failure. The present work examines the mechanisms contributing to this marked cyclic hardening. Cr& precipitation on dislocations contributed to hardening, but only with sufficient time at > approx 500 “C. The substantial hardening rate at lower temperatures or shorter times was attributed to solute drag. The coniribution of solute drag was evidenced in tests at both 400 and 600 “C bv a continuallv decreasina strain rate sensitivitv of the Aui2. Solute drag alone produced vef~ ‘considerable cyclic hardening. ihe increase in Au/2 after 1000 cycles at 427 “C was 75% of the maximum observed at higher temperatures where carbides did precipitate. Additionally, thermomechanical tests ware conducted between various temperature limits, but with the same * 0.3% mechanical strain range. Hardening was bounded by isothermal behaviour at the temperature limits of the thermomechanical cycles, except for tests between 400 and 600 “C which exhibited extreme hardening. However, microstructural examination did not suggest a cause. Specimens subjected to thermomachanical cycles appeared similar to those isothermally cycled at the maximum temperature of the thermomechanical cycle, including those from 400-600°C tests. Graphs, photomicrographs, diffraction patterns. 25 refs.
Inclusion size effect on the fetiv creek pro~feM;U~rrrT;~ F,tture mecherua of a supera lay. Denda, Metal/. Trans. A. 23A 2 (Feb
1992)
519-526
Low-cycle fatigue life of Ni-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependences between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grainad IN718, which is a representative turbine disc materiel for jet engines. There is a faceted-striated crack transition on thefraaure wrfaces.This faceted-striated transition also appears in the daldN vs. crack length curves. It is observed that the faceted crack propagation time can be 250% of total lifetime in the low-cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given. Graphs, photomicrographs. 12 refs.
High tempereture low cycle feti ue pwrtles of e thii-ssctfon 9 wt.% Cr-1 wt.% MO ferritii steel forg Bng. Choudhary, 6.K. Sankara Rao, K.B. and Mannan, S.L. Mater. Sci. Eng. A. A148 2 (Dee 1991) 267-278 Total-axial-strain-controlled fatigue tests have been conducted in air to ascettain the influence of temperature (723, 773, and 793 K) on the lowcycle fatigue behevfour of thick-section 000 mm) SCr-1Mo tube plate forging (where the Cr end MO contents are in approximate wt.%l. The alloy was tested in a simulated postAd heat treatment condition. A symmetrical triangular waveform end a constant strain rate of 1 x 10-s s-’ were employed for all the tests performed over strain amplitudes in the ranae from -t 0.25 to 1.00%. The crack initiation and Drorwation modes were studied. &formation and damage mechanisms which influe& ihe stress response and endurance have been identified. A reduction in fatigue life was observed at all the strain amplitudes with increasing temperature. The temperature effect on life was more pronounced at low strain amplitudes. The reduction in fatigue life at elevated temperatures was attributed to the combined effects of increased inelastic strain and fatigue-oxidation interactions. Thick-section forged SCr-1Mo steel exhibited inferior fatigue resistance compared with the hot-rolled material either in normalized-plus-tempered or in simulated thick-section heat treatment condiiions. The poor fatigue resistance of tube plate forging was ascribed to its coarse grain size. The cyclic stress response of tube plate forging varied as a complex function of temperature and Strain amplitude. The cyclic~stress-strain behaviour could be described bv a wwer law relationshio end strain amplitude. The cvclic stress-strain behaviour c&b be described by a p&er law relatidnship at 773 bnd 793 K. At 723 K, the alloy exhibited a two-slope cyclic stress-strain curve. The crack initiation and propagation modes remained transgranular at all the condiiions investigated. Graphs, photomicrographs. 50 refs.
Predicting fetlgue crack growth retee end thresholds tures. Lu, 8. and Zheng, Z. Mater. Sci. Eng. A A148 2 (Dee 1991) 179-188
et low tempere
An anempt was made to offer a new method for predicting fatigue crack growth IFCG) rates and thresholds of metals in low temperature at-rat applications. The experimental results available have indicated’that a decreass in t&nperature can influence the FCG mechanism, the FCG rates, end the fatigue ductilbbrittle transition (FDBT) may occur in some metals. Accordingly, metals can ba divided into two groups, ie alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for tempertires down to 4 K and the crack growth rates in the intermediate region (da/dN = 1O-a-1O-s m cycle -‘) depend mainly upon Young’s modulus. In ths latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature. the FCG rates (especially at higher AK levels) will be enhanced bv the brittle transition of the FCG mechanism and both the strength end the ductil& of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authon and a coworker for FCG rates not only explains the low-temperature FCG behaviour of the metals mentioned above but also predicts the tow-temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict A& at low temperature is tentatively proposed based on the static fracture model. Graphs. 32 refs.
Micromechenism of fatiaue creek arowth in melt-sintemd AI-AIZ03 alloys. Satake, T. and Suzano, M. J. Sot. Mater. Sci. Jpn 46 458 (Nov 1991) 1485-1490 (in Japanese) The melt-sintered AI-Al203 dispersion-strengthened alloy (Al203 dispersed in AlO.l4Fe-0.08Si-O.03Mn) has been fatigued in plane bending at the mscmant frequency
Int J Fatigue November
1992
Fatigue Me enhencement of specfmonr with stress concentrators e thermo-mechunicel technique. Co/e, G.K. and Lam, Y.C. Scri. Metal/. Mater. 26 12 (Dee 1991) 2849-2853
using
There has been a debate as to whether fatigue life in welds is mostly dependent upon crack initiation, or may be legitimately described using fracture mechanics and fatigue crack growth methodologies. For this reason it was decided that, before examining the affect of the thermomechanical treatment on the fatigue life of welds, exploratory tests on specimens containing stress concentrations would be carried out. It was found that, at the 95% confidence level, the thermomechanical treatment led to an imwovement in the fetiaue life of wecimans containina holes. This improvement appeared to be more significant at st;esses approaching ihe endurance limit. Although not conclusive, there was indication that the treatment had no detrimental effect on the fatiaua life of specimens without stress concentrations. Graphs. 8 refs.
A demo e mechanics model of fatigue crack initiation in notched pletes. Chow !L. and Wei Y. The& Appl. fract. bech. 16 2 (Nov 1991) 123-133 A damage model is proposed to characterize fatigue crack initiation in notched plates. This is accomplished bY assuming e damage fracture criterion and effective instantaneous tangent moduli matrix which accounts for damage accumulation. A finite element package is used to perform the numerical analyses for an edgenotched plate specimen made of Al alloy 2024-T3 under cyclic loading of varying constant amplitudes. The estimated fatigue crack initiation life agrees satisfactorily with those determined axp%rimantallY. An important observation from the results of the proposed damage mode is its ability to identify tha phenomenon of atrass redistribution due to material degradation in two-dimensional specimens during fatigue loading. Graphs. 10 refs.
Fatigue crack rowth rate in ferrite-martensite dual-phase steel. Deng, R.Y. and Ye i?J. Theor. A& Frac?. Mech. 16 2 (Nov 1991) 109-122 An empirical study is mada on the fatigue crack growth rate in ferrite-martensite dual-phase IFMDP) steel fag 16 f&RI. Particular attention is given to the effect of ferrite content in the range 24.2-41.5% where good fatigue resistance was found at 33.896. Variations in ferrite content did not effect the crack growth rate da/dN when plotted against the effective stress intensihl factor range A&e which was assumed to follow a linear relation with the crack tip stress inter&v factor range AK. A high Ah corresponds to uniformly distributed small si;e ferrite and martensite. No other appreciable correlation could be related to the microstructure morphology of the FMDP steal. The closure stress intansitY factor &I, however. is affected by the ferrite content with &I/K,,,~~ reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite, dividing tha fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in AKand the latter would increase with AKdepending ratio. The same data when correlated with the strain energy on the R = u,i./o,.,: density factor range AS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter (I involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that tha daldN data for different R would collapse on to a sing!+ curve with a narrow scaner band when plotted against CLAS. Graphs, photomicrographs. 17 refs.
A review of fatigue creek growth anelyses. Liu, H. W Theor. Appl. fract. Mech. 16 2 (Nov 1991) 91-108 Stress intensihl factor range AK has been shown to correlate well with fatigue crack growth rate daidN. A number of fatigue crack growth theories have been developed for such correlations. Often, conjectural theories of fatigue crack growth are constructed from experimental data. On the other hand, fatigue crack growth theories can also be derived rigorously with deductive logic. Four such deductive theories are reviewed: (1) that for the growth of a smell crack in a very wide homogeneous plate; (2) the theory of similitude for the correlation of daldN with AK; (3) a thew of crack growth in homogeneous materials in small-scale yielding; and 14) the unzipping theory of fatigue crack growth. These four theories are synthesized into a logic framework useful for fatigue crack growth analysis. The deductive theories and the conjectural theories complement each other in the advancas of the understanding of fatigue crack growth behaviour and their applications to defining the complex &ues oj the grov.?h of small cracks and crack growth in composites are illustrated. The materials examined were Cu. 7075 and 2024 Al. and Fe-2.4Si. Graphs, photomicrographs. 44 refs.
Creep-fatigue life prediction method using Diercks equetion for Cr-Mo steel. Sonoya, K. Nonaka, 1. and Kitagawa, M. Tersu-to-Hagane IJ. Iron Steel Inst. Jpnl 76 5 (May 1990) 775-782 A method to predict the creep-fatigue life properties of Cr-MO steels is proposed on the basis of the Diercks equation which correlates the creep-fatigue lives of SUS 304 sseels under various temperatures, strain ranges, strain rates and hold times. The accuracy of the proposed method we8 compared with that of the existing methods. The following results were obtained. (1) Fatigue strength and creep rupture strength of Cr-MO steel are different from those of SUS 304 steel. Therefore in order-to epply the Diarcks equation to creep-fatigue prediction for Cr-MO steel, the difference of fatigue strength was found to be corrected by the fatigue life ratio of both steels and the difference of creep rupture strength was found to be corrected by the equivalent temperature corresponding to equal strength of both steels. (2) Creep-fatigue life can be predicted bY the modified Diercks equation within a factor of two which is nearly as precise as the accuracy of the strain range partitioning method. Required test and analysis procedure of this method are not so complicated as the strain range partitioning method. Graphs.
4.20
Hardening mechenisms in e dynamic strain e ing alloy, Hestelloy X, during isothermal and thermcmechenicel cycle 1 defermetion. Mmer, R. V. and Caste//i, M.G. Metal/. Trans. A 23A 2 (Feb 1992) 551-561 Isothermal cyclic deformation tests were conducted on Hastelloy X with a total strain range of + 0.3% at several temperatures and strain rates. Cyclic hardening exhibited a broad peak between appox 200 and 700 “C, with a maximum near 500 “c of approx 80% increase in stress amplitude, Aol2, at failure. The present work examines the mechanisms contributing to this marked cyclic hardening. Cr& precipitation on dislocations contributed to hardening, but only with sufficient time at > approx 500 “C. The substantial hardening rate at lower temperatures or shorter times was attributed to solute drag. The coniribution of solute drag was evidenced in tests at both 400 and 600 “C bv a continuallv decreasina strain rate sensitivitv of the Aui2. Solute drag alone produced vef~ ‘considerable cyclic hardening. ihe increase in Au/2 after 1000 cycles at 427 “C was 75% of the maximum observed at higher temperatures where carbides did precipitate. Additionally, thermomechanical tests ware conducted between various temperature limits, but with the same * 0.3% mechanical strain range. Hardening was bounded by isothermal behaviour at the temperature limits of the thermomechanical cycles, except for tests between 400 and 600 “C which exhibited extreme hardening. However, microstructural examination did not suggest a cause. Specimens subjected to thermomachanical cycles appeared similar to those isothermally cycled at the maximum temperature of the thermomechanical cycle, including those from 400-600°C tests. Graphs, photomicrographs, diffraction patterns. 25 refs.
Inclusion size effect on the fetiv creek pro~feM;U~rrrT;~ F,tture mecherua of a supera lay. Denda, Metal/. Trans. A. 23A 2 (Feb
1992)
519-526
Low-cycle fatigue life of Ni-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependences between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grainad IN718, which is a representative turbine disc materiel for jet engines. There is a faceted-striated crack transition on thefraaure wrfaces.This faceted-striated transition also appears in the daldN vs. crack length curves. It is observed that the faceted crack propagation time can be 250% of total lifetime in the low-cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given. Graphs, photomicrographs. 12 refs.
High tempereture low cycle feti ue pwrtles of e thii-ssctfon 9 wt.% Cr-1 wt.% MO ferritii steel forg Bng. Choudhary, 6.K. Sankara Rao, K.B. and Mannan, S.L. Mater. Sci. Eng. A. A148 2 (Dee 1991) 267-278 Total-axial-strain-controlled fatigue tests have been conducted in air to ascettain the influence of temperature (723, 773, and 793 K) on the lowcycle fatigue behevfour of thick-section 000 mm) SCr-1Mo tube plate forging (where the Cr end MO contents are in approximate wt.%l. The alloy was tested in a simulated postAd heat treatment condition. A symmetrical triangular waveform end a constant strain rate of 1 x 10-s s-’ were employed for all the tests performed over strain amplitudes in the ranae from -t 0.25 to 1.00%. The crack initiation and Drorwation modes were studied. &formation and damage mechanisms which influe& ihe stress response and endurance have been identified. A reduction in fatigue life was observed at all the strain amplitudes with increasing temperature. The temperature effect on life was more pronounced at low strain amplitudes. The reduction in fatigue life at elevated temperatures was attributed to the combined effects of increased inelastic strain and fatigue-oxidation interactions. Thick-section forged SCr-1Mo steel exhibited inferior fatigue resistance compared with the hot-rolled material either in normalized-plus-tempered or in simulated thick-section heat treatment condiiions. The poor fatigue resistance of tube plate forging was ascribed to its coarse grain size. The cyclic stress response of tube plate forging varied as a complex function of temperature and Strain amplitude. The cyclic~stress-strain behaviour could be described bv a wwer law relationshio end strain amplitude. The cvclic stress-strain behaviour c&b be described by a p&er law relatidnship at 773 bnd 793 K. At 723 K, the alloy exhibited a two-slope cyclic stress-strain curve. The crack initiation and propagation modes remained transgranular at all the condiiions investigated. Graphs, photomicrographs. 50 refs.
Predicting fetlgue crack growth retee end thresholds tures. Lu, 8. and Zheng, Z. Mater. Sci. Eng. A A148 2 (Dee 1991) 179-188
et low tempere
An anempt was made to offer a new method for predicting fatigue crack growth IFCG) rates and thresholds of metals in low temperature at-rat applications. The experimental results available have indicated’that a decreass in t&nperature can influence the FCG mechanism, the FCG rates, end the fatigue ductilbbrittle transition (FDBT) may occur in some metals. Accordingly, metals can ba divided into two groups, ie alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for tempertires down to 4 K and the crack growth rates in the intermediate region (da/dN = 1O-a-1O-s m cycle -‘) depend mainly upon Young’s modulus. In ths latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature. the FCG rates (especially at higher AK levels) will be enhanced bv the brittle transition of the FCG mechanism and both the strength end the ductil& of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authon and a coworker for FCG rates not only explains the low-temperature FCG behaviour of the metals mentioned above but also predicts the tow-temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict A& at low temperature is tentatively proposed based on the static fracture model. Graphs. 32 refs.
Micromechenism of fatiaue creek arowth in melt-sintemd AI-AIZ03 alloys. Satake, T. and Suzano, M. J. Sot. Mater. Sci. Jpn 46 458 (Nov 1991) 1485-1490 (in Japanese) The melt-sintered AI-Al203 dispersion-strengthened alloy (Al203 dispersed in AlO.l4Fe-0.08Si-O.03Mn) has been fatigued in plane bending at the mscmant frequency
Int J Fatigue November
1992