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Fatigue properties of AlSiCuNiMg casting alloys at elevated temperatures
Fatigue p r o p e r t i e s o f A i - S i - C u - N I - M g casting alloys st elevated t e m p e r a t u r e s . Hasegawa, N., Miyabe, T. and Yamada, T. J. Japan Inst. Light Met. M a y 1991 41, (5), 3 1 1 - 3 1 7 (in Japanese) Rotating-banding fatigue tests of modified AI-Si-Cu-Ni-Mg eutectic casting alloys (ACaA) and hypereutectic casting alloys (AC9A and AC9B) were carried out in a temperature range between room temperature and 400 °C. The fatigue strength of Na-modified ACBA is superior to that of phosphorus-modified AC8A. The fatigue strength of AC9B with a lower content of Si is greater than that of AC9A. The decrease in fatigue strength results from a coarse primary St. It is concluded that the fatigue properties of AI-Si-Cu-Ni-Mg casting alloys are influenced by the primary Si grains. Graphs, photomicrographs. 4 refs. E l e v a t e d - t e m p e r a t u r e f a t i g u e crack propagation in h o t w o r k i n g t o o l steels. Tang, N.-Y. and Plumtree, A. Fatigue Fract. Eng. Mater. Struct. 1991 14, (9), 9 3 1 - 9 3 8 Cyclic tests have been carried out on a hot working tool steel (H13) over a range of temperatures using different waveforms. At a temperature of 125 °C a slow-fast wave resulted in a lower fatigue crack propagation behaviour in which the slow-fast wave was more damaging than a balanced waveform at the same frequency. Dynamic strain ageing is attributed to the reversed crack propagation effect at 125°C. Graphs, photomicrographs. 17 refs.
Fracture mechanics analysis o f f a t i g u e resistance o f s p o t - w e l d e d coachFa/~tt/gljoints. Wang, P.-C. and Ewing, K.W. ue Fract. Eng. Mater. Struct. 1991 14, (9), 9 1 5 - 9 3 0 Resistance spot welding is the most widely used joining method in automobile manufacture. The number, location and quality of welds are some of the factors that influence the performance of welded subassemblies and body panel structures. Therefore, design optimization requires a knowledge of not only the sheet metal behaviour, but also the behaviour of welded material under service Ioadings. A linear elestic fracture mechanics approach was employed to estimate the fatigue lives of spot welds subjected to tearing loads in a coach-peel specimen. Using a finite-element method (FEM), the initial J-integral values for five coach-peel joints, each with different geometries, were calculated. Fatigue tests conducted on the same weld geometries provided the life data. The experimental data were used to derive a relationship between the initial elastic J-integral values (8J¢) and the fatigue life. It was found that the total fatigue life (Nf) of a weld at one applied stress range is related to its range of J-integral values such that a 8Jc against Nf log-log plot gives a straight-line relationship. This relationship can be used to evaluate the effects of geometrical variables on the fatigue life of coach-peel joints. The results show that, within the dimension range studied, the effects of geometrical variables on the fatigue resistance can be ranked in the following, decreasing, order: weld eccentricity, sheet thickness and weld nugget diameter. Three sheet materials were used: a galvanized 1006 AKDQ steel (1.27 mm thickness), a galvanized, high-strength low-alloy (HSLA) steel (1.4mm thickness), and a bare, HSLA steel (1.52mm thickness). The HSLA steel is SAE-960X, an AI-killed, microalloyed steel exhibiting a fine-grain, polygonal-ferritic microstructure. 25 refs. Interaction o f overloads on fatigue crack growth for 40CrNi steel. Yang,
S. and Li, H. Fatigue Fract. Eng. Mater. Struct, 1991 14, (9), 9 0 7 - 9 1 3
This work was designed to improve the general understanding of loading sequence effects on fatigue crack growth and lead to the development of improved methods for predicting crack propagation behaviour. Two loading histories were selected: a baseline amplitude with periodic overloads or underloads; and several overloads without interactive effects. The specimen used was of a wedge opening loading type, and the material was a low-alloy high-strength steel, ie 40CrNi. It was found that the linear summation of damage (LSD) assumption could be applied in predicting fatigue crack growth rate (FCGR) under periodic overloads or underloads within the scetterband of the constant-amplitude data obtained using an alternating current potential drop technique for measuring the crack lengths for multiple specimens with several load amplitudes. A discrepancy existed between the FCGR predicted from LSD and the one actually measured during several hundreds of loading cycles immediately following every non-interactive overload of the latter loading history, although the overload ratio was the same as that of the periodic overloading. The causes of this phenomenon are discussed. Graphs. 11 refs.
Estimation of service loading from the width and height of fatigue striations o f 2017-T4 aluminium alloy. Murakami, Y., Shiraishi, N. and Furukawa, K. Fatigue Fract. Eng. Mater. Struct. 1991 14, (9), 8 9 7 - 9 0 6 A case-study method is proposed to determine the service loading from fracture surface striations. Although it is well known that striation spacing relates to the crack growth rate, it is considered impossible to determine maximum and minimum loads under service loading conditions, and which define the stress ratio R, from an analysis of the striation spacing. A new method of determining the maximum and minimum loads is presented, which is based on the experimental fact that the relative height of the striations H to the striation spacings s is strongly influenced by the stress ratio. The results of fatigue tests on the 2017-T4 AI alloy showed a definite correlation between these parameters. The difficulty of measuring striation height was overcome by taking the measurements in a scanning electron microscope after sectioning the specimens at a large inclined angle. Striations under a stress ratio of R = - 1 are compressed and flat, and the ratio of H/s is small. With increasing R, the value of H/s becomes large. Therefore, a standard laboratory test that determines the relationship between s and AK, and also between H/s and R, enables one to estimate the service Ioadings from fatigue fracture surfaces. Graphs, photomicrographs. 17 refs. T i m e - d e p e n d e n t f a t i g u e failure: t h e c r e e p - f a t i g u e interaction. Ellyin, F.
and Asada, Y. Int. J. Fatigue Mar. 1991 13, (2), 1 5 7 - 1 6 4
Most metals show some form of time-dependent bahaviour at elevated temperatures, which results in a reduced fatigue life. Based on the governing failure mechanisms of the rate-independent and rate-dependent parts of the phenomenon, a fatigue failure criterion is proposed. The number of cycles to failure is related to a rate-
Int J Fatigue May 1992
independent plastic-strain energy, and a rate-dependent plastic-strain energy. The material constants, four in all, can be determined from a few simple tests. The predicted results are compared with the experimental data of various symmetric and asymmetric wave forms, and the agreement is found to be fairly good. The material mentioned was 2.25Cr-1Mo steel. Graphs. 30 refs.
Frectographic observations and predictions on fatigue crack growth in an s i u m i n i u m alloy u n d e r m i n i T W l S T f l i g h t - s i m u l a t i o n loading. Seigl, J., Schijve, J. and Padmadinata, U.H. Int. J, Fatigue Mar. 1991 13, (2), 139-147 Sheet specimens from the 2024-T3 AI alloy were tested under two miniTWlST toad histories and a special load sequence, which combines the most severe flights from miniTWlST and batches of constant-amplitude cycles. Fractographic observations were made to determine the fatigue crack growth increments in the most severe flights. The bands caused by the severe flights could be distinguished on the fracture surface and an accurate reconstitution of the crack growth curve could be made. The crack increments occurring in the most severe flights were measured and compared with the values predicted by the modified CORPUS model. Graphs, photomicrographs. 16 refs.
Short-fatigue-crack growth in a nickel-based superalloy at room and elevated t e m p e r a t u r e s . Healy, J.C. Grabowski, L. and Beevers, C.J. Int. J. Fatigue Mar. 1991 13, (2), 133-138 An optical system, developed to monitor short-crack growth at elevated temperature, was used to study the fatigue behaviour of Waspaloy. Tests conducted at 19 and 500 °C revealed that the dominant mechanism of crack formation was slip band cracking. Crack formation was also associated with coarse carbide particles within the matrix. The dominant failure mechanism at 19 and 500 °C was one of mixed mode-I and -II fracture, with the mode-II shear displacements giving rise to nonclosure induced by surface roughness. Oxide- and plasticity-induced non-closure processes made only a minor contribution to the overall growth process. Shortfatigue-crack growth measured at R = 0.1 was faster at 500 °C than at 19 °C at an equivalent value of ~,K1. This was attributed to a change in slip character from highly planar to one involving increasing amounts of cross-slip at 19 and 500°C, respectively. Graphs, photomicrogaphs. 18 refs. T e m p e r a t u r e and humidity effects on f a t i g u e life d i s t r i b u t i o n of carbon
steel. Sakai, T., Rarnulu, M. and Suzuki, M. Int. J. Fatigue Mar. 1991 13 (2), 117-125 The effects of temperature and humidity in the atmosphere on the fatigue life distribution of carbon steel leg, $35C) for machine structural use were examined by performing statistical fatigue tests in the rotating-bending mode. The temperature was a predominant factor in determining the fatigue life distribution even if its variation was slight, as expected in a normal indoors atmosphere, whereas the humidity had no significant effect on the fatigue life distribution. The temperature dependence of the fatigue life distribution was quantitatively determined by means of a Weibull distribution modified by a saturated probability. Based on this experimental evidence, an analytical procedure was developed to evaluate the fatigue life distribution in the actual atmospheres encountered. Graphs, 33 refs. Fatigue crack initiation and stage-I p r o p a g a t i o n in polycrystalline materials. I. M i c r o m e s h a n i s m s . Provan, J.W. and Zhai, Z.H. . Int. J. Fatigue Mar. 1991 13, (2), 9 9 - 1 0 9 The microscopic mechanisms involved in fatigue crack initiation and stage-I fatigue crack propagation in polycrystalline materials are investigated, and models of the observations are proposed. A qualitative description of changes in crack profile in the transition region between the initiation and propagation is also proposed. The results of an experimental study that involved a combination of replication techniques and post-mortem fractographic surface examination by scanning electron microscopy carried out on smooth cylindrical specimens of polycrystslline OFHC Cu at room temperature provide support for the models proposed. Photomicrographs. 17 refs. Fatigue crack initiation and stage-I propa~lation materials. II. M o d e l l i n g Provan, J,W. and Zhal, Z,H. Int. J. Fatigue Mar. 1991 13, (2), 110-116
in polycrystslline
An analytic model of stage-I fatigue crack initiation and the subsequent stage-II crack propagation based on a plastic-strain intensity factor, AKp, is proposed. In this model, the stage-I crack initiation process is simplified as a straight line on a log-log plot of da/dN against AKp. An inflection point that divides the crack growth into the initiation and propagation categories is introduced as the plastic crack propagation threshold, AKpth, whose value is determined by [(AJ)E]~h2, where AJ is the range of the J-integral and E is the Young's modulus. Following this, the integration of the crack growth rate as a function of the plastic-strain intensity factor yields a fatigue crack initiation life estimate whose error is less than 5%. For comparison, the experimental data presented are also discussed on the basis of the parameters AK and AKeff that have been proposed by other researchers. Graphs, photomicrographs. 18 refs.
In situ s h o r t f a t i g u e crack characterization o f a nickel-based superalloy at a m b i e n t and elevated t e m p e r a t u r e s . Stephens, R.R. Diss. Abstr. Int. Aug. 1991 52, (2), 325 pp Fatigue experiments were performed using an Ni-based superalloy at various temperatures in a load frame attached to a scanning electron microscope. An elevated temperature stage was designed, constructed, and coupled to the apparatus, allowing temperatures greater than 700 °C. Experiments on Wespeloy at 25, 500, and 700°C showed similar crack nucleation characteristics. The cracks had a tendency to nucleate along preferentially oriented slip bands. The slip band cracks showed a tendency to nucleate within large grains at 25 and 700 °C, and mixed grain sizes at 500 °C. This was attributed to the slip character experienced at each temperature. The dominant mechanism of fatigue crack growth for the short cracks at 25 and 5O0°C was one of mixed modes I and II by slip band cracking. At 7go °C, crack growth proceeded by a stage-II cracking process showing very little signs of crystallographic growth. Discontinuous crack growth rates at 25 and 500°C were attributed to microstructural barriers such as grain boundaries, twin boundaries,
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Fracture mechanics analysis o f f a t i g u e resistance o f s p o t - w e l d e d coachFa/~tt/gljoints. Wang, P.-C. and Ewing, K.W. ue Fract. Eng. Mater. Struct. 1991 14, (9), 9 1 5 - 9 3 0 Resistance spot welding is the most widely used joining method in automobile manufacture. The number, location and quality of welds are some of the factors that influence the performance of welded subassemblies and body panel structures. Therefore, design optimization requires a knowledge of not only the sheet metal behaviour, but also the behaviour of welded material under service Ioadings. A linear elestic fracture mechanics approach was employed to estimate the fatigue lives of spot welds subjected to tearing loads in a coach-peel specimen. Using a finite-element method (FEM), the initial J-integral values for five coach-peel joints, each with different geometries, were calculated. Fatigue tests conducted on the same weld geometries provided the life data. The experimental data were used to derive a relationship between the initial elastic J-integral values (8J¢) and the fatigue life. It was found that the total fatigue life (Nf) of a weld at one applied stress range is related to its range of J-integral values such that a 8Jc against Nf log-log plot gives a straight-line relationship. This relationship can be used to evaluate the effects of geometrical variables on the fatigue life of coach-peel joints. The results show that, within the dimension range studied, the effects of geometrical variables on the fatigue resistance can be ranked in the following, decreasing, order: weld eccentricity, sheet thickness and weld nugget diameter. Three sheet materials were used: a galvanized 1006 AKDQ steel (1.27 mm thickness), a galvanized, high-strength low-alloy (HSLA) steel (1.4mm thickness), and a bare, HSLA steel (1.52mm thickness). The HSLA steel is SAE-960X, an AI-killed, microalloyed steel exhibiting a fine-grain, polygonal-ferritic microstructure. 25 refs. Interaction o f overloads on fatigue crack growth for 40CrNi steel. Yang,
S. and Li, H. Fatigue Fract. Eng. Mater. Struct, 1991 14, (9), 9 0 7 - 9 1 3
This work was designed to improve the general understanding of loading sequence effects on fatigue crack growth and lead to the development of improved methods for predicting crack propagation behaviour. Two loading histories were selected: a baseline amplitude with periodic overloads or underloads; and several overloads without interactive effects. The specimen used was of a wedge opening loading type, and the material was a low-alloy high-strength steel, ie 40CrNi. It was found that the linear summation of damage (LSD) assumption could be applied in predicting fatigue crack growth rate (FCGR) under periodic overloads or underloads within the scetterband of the constant-amplitude data obtained using an alternating current potential drop technique for measuring the crack lengths for multiple specimens with several load amplitudes. A discrepancy existed between the FCGR predicted from LSD and the one actually measured during several hundreds of loading cycles immediately following every non-interactive overload of the latter loading history, although the overload ratio was the same as that of the periodic overloading. The causes of this phenomenon are discussed. Graphs. 11 refs.
Estimation of service loading from the width and height of fatigue striations o f 2017-T4 aluminium alloy. Murakami, Y., Shiraishi, N. and Furukawa, K. Fatigue Fract. Eng. Mater. Struct. 1991 14, (9), 8 9 7 - 9 0 6 A case-study method is proposed to determine the service loading from fracture surface striations. Although it is well known that striation spacing relates to the crack growth rate, it is considered impossible to determine maximum and minimum loads under service loading conditions, and which define the stress ratio R, from an analysis of the striation spacing. A new method of determining the maximum and minimum loads is presented, which is based on the experimental fact that the relative height of the striations H to the striation spacings s is strongly influenced by the stress ratio. The results of fatigue tests on the 2017-T4 AI alloy showed a definite correlation between these parameters. The difficulty of measuring striation height was overcome by taking the measurements in a scanning electron microscope after sectioning the specimens at a large inclined angle. Striations under a stress ratio of R = - 1 are compressed and flat, and the ratio of H/s is small. With increasing R, the value of H/s becomes large. Therefore, a standard laboratory test that determines the relationship between s and AK, and also between H/s and R, enables one to estimate the service Ioadings from fatigue fracture surfaces. Graphs, photomicrographs. 17 refs. T i m e - d e p e n d e n t f a t i g u e failure: t h e c r e e p - f a t i g u e interaction. Ellyin, F.
and Asada, Y. Int. J. Fatigue Mar. 1991 13, (2), 1 5 7 - 1 6 4
Most metals show some form of time-dependent bahaviour at elevated temperatures, which results in a reduced fatigue life. Based on the governing failure mechanisms of the rate-independent and rate-dependent parts of the phenomenon, a fatigue failure criterion is proposed. The number of cycles to failure is related to a rate-
Int J Fatigue May 1992
independent plastic-strain energy, and a rate-dependent plastic-strain energy. The material constants, four in all, can be determined from a few simple tests. The predicted results are compared with the experimental data of various symmetric and asymmetric wave forms, and the agreement is found to be fairly good. The material mentioned was 2.25Cr-1Mo steel. Graphs. 30 refs.
Frectographic observations and predictions on fatigue crack growth in an s i u m i n i u m alloy u n d e r m i n i T W l S T f l i g h t - s i m u l a t i o n loading. Seigl, J., Schijve, J. and Padmadinata, U.H. Int. J, Fatigue Mar. 1991 13, (2), 139-147 Sheet specimens from the 2024-T3 AI alloy were tested under two miniTWlST toad histories and a special load sequence, which combines the most severe flights from miniTWlST and batches of constant-amplitude cycles. Fractographic observations were made to determine the fatigue crack growth increments in the most severe flights. The bands caused by the severe flights could be distinguished on the fracture surface and an accurate reconstitution of the crack growth curve could be made. The crack increments occurring in the most severe flights were measured and compared with the values predicted by the modified CORPUS model. Graphs, photomicrographs. 16 refs.
Short-fatigue-crack growth in a nickel-based superalloy at room and elevated t e m p e r a t u r e s . Healy, J.C. Grabowski, L. and Beevers, C.J. Int. J. Fatigue Mar. 1991 13, (2), 133-138 An optical system, developed to monitor short-crack growth at elevated temperature, was used to study the fatigue behaviour of Waspaloy. Tests conducted at 19 and 500 °C revealed that the dominant mechanism of crack formation was slip band cracking. Crack formation was also associated with coarse carbide particles within the matrix. The dominant failure mechanism at 19 and 500 °C was one of mixed mode-I and -II fracture, with the mode-II shear displacements giving rise to nonclosure induced by surface roughness. Oxide- and plasticity-induced non-closure processes made only a minor contribution to the overall growth process. Shortfatigue-crack growth measured at R = 0.1 was faster at 500 °C than at 19 °C at an equivalent value of ~,K1. This was attributed to a change in slip character from highly planar to one involving increasing amounts of cross-slip at 19 and 500°C, respectively. Graphs, photomicrogaphs. 18 refs. T e m p e r a t u r e and humidity effects on f a t i g u e life d i s t r i b u t i o n of carbon
steel. Sakai, T., Rarnulu, M. and Suzuki, M. Int. J. Fatigue Mar. 1991 13 (2), 117-125 The effects of temperature and humidity in the atmosphere on the fatigue life distribution of carbon steel leg, $35C) for machine structural use were examined by performing statistical fatigue tests in the rotating-bending mode. The temperature was a predominant factor in determining the fatigue life distribution even if its variation was slight, as expected in a normal indoors atmosphere, whereas the humidity had no significant effect on the fatigue life distribution. The temperature dependence of the fatigue life distribution was quantitatively determined by means of a Weibull distribution modified by a saturated probability. Based on this experimental evidence, an analytical procedure was developed to evaluate the fatigue life distribution in the actual atmospheres encountered. Graphs, 33 refs. Fatigue crack initiation and stage-I p r o p a g a t i o n in polycrystalline materials. I. M i c r o m e s h a n i s m s . Provan, J.W. and Zhai, Z.H. . Int. J. Fatigue Mar. 1991 13, (2), 9 9 - 1 0 9 The microscopic mechanisms involved in fatigue crack initiation and stage-I fatigue crack propagation in polycrystalline materials are investigated, and models of the observations are proposed. A qualitative description of changes in crack profile in the transition region between the initiation and propagation is also proposed. The results of an experimental study that involved a combination of replication techniques and post-mortem fractographic surface examination by scanning electron microscopy carried out on smooth cylindrical specimens of polycrystslline OFHC Cu at room temperature provide support for the models proposed. Photomicrographs. 17 refs. Fatigue crack initiation and stage-I propa~lation materials. II. M o d e l l i n g Provan, J,W. and Zhal, Z,H. Int. J. Fatigue Mar. 1991 13, (2), 110-116
in polycrystslline
An analytic model of stage-I fatigue crack initiation and the subsequent stage-II crack propagation based on a plastic-strain intensity factor, AKp, is proposed. In this model, the stage-I crack initiation process is simplified as a straight line on a log-log plot of da/dN against AKp. An inflection point that divides the crack growth into the initiation and propagation categories is introduced as the plastic crack propagation threshold, AKpth, whose value is determined by [(AJ)E]~h2, where AJ is the range of the J-integral and E is the Young's modulus. Following this, the integration of the crack growth rate as a function of the plastic-strain intensity factor yields a fatigue crack initiation life estimate whose error is less than 5%. For comparison, the experimental data presented are also discussed on the basis of the parameters AK and AKeff that have been proposed by other researchers. Graphs, photomicrographs. 18 refs.
In situ s h o r t f a t i g u e crack characterization o f a nickel-based superalloy at a m b i e n t and elevated t e m p e r a t u r e s . Stephens, R.R. Diss. Abstr. Int. Aug. 1991 52, (2), 325 pp Fatigue experiments were performed using an Ni-based superalloy at various temperatures in a load frame attached to a scanning electron microscope. An elevated temperature stage was designed, constructed, and coupled to the apparatus, allowing temperatures greater than 700 °C. Experiments on Wespeloy at 25, 500, and 700°C showed similar crack nucleation characteristics. The cracks had a tendency to nucleate along preferentially oriented slip bands. The slip band cracks showed a tendency to nucleate within large grains at 25 and 700 °C, and mixed grain sizes at 500 °C. This was attributed to the slip character experienced at each temperature. The dominant mechanism of fatigue crack growth for the short cracks at 25 and 5O0°C was one of mixed modes I and II by slip band cracking. At 7go °C, crack growth proceeded by a stage-II cracking process showing very little signs of crystallographic growth. Discontinuous crack growth rates at 25 and 500°C were attributed to microstructural barriers such as grain boundaries, twin boundaries,
203