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Local fatigue damage accumulation around notch attending crack initiation
96
Abstracts
torsional moments. General aspects of the proof of fatigue strength and stress analysis are discussed first. Finite element analyses have been done for the whole range of practically important geometries. The results for flush and protruded nozzles are given as stress concentration factors (SCF) representing the maximum yon Mises stress in the intersection region referred to an appropriate nominal stress. Comparisons are made with the curves given in BS 5500 and with analytical results. Graphs, 9 refs.
strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti~tAI-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5AI-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in improvement in fatigue strength in both Ti-6A1-4V and Ti-5A1-2.5Fe. Graphs, photomicrographs, diffraction patterns, 17 refs.
Effects of iron and calcium on impact fatigue characteristics of AC2B-T6 aluminium casting alloys Kobayashi, T., Niinomi, M., Harata, T., Shimomura, Y. and lto, T. J. Jpn. Inst. Light Met. (1995) 45 (2), 88-94 (in Japanese)
High-temperature low-cycle fatigue and lifetime prediction of Ti-24AIl l N b alloy Malakondaiah, G. and Nicholas, T. MetalL Mater. Trans. A (1995) 26A (5), 1113-1121
The effects of iron and calcium additions on impact fatigue characteristics were investigated, comparing with their effects on the usual fatigue characteristics in AC2B-T6 aluminium casting alloys. Impact fatigue strength in the low-cycle life region of a low Fe-content sample is greater than that for high-Fe content. This trend is, however, exactly opposite to the trend in the high-cycle life region. Both impact and usual fatigue strengths, especially in the high-cycle life region, increased by refining and spheroidizing of eutectic silicon particles and refining of AI-Fe-based intermetallic compounds by adding small amount of Ca. They were confirmed by fractographic observation of fatigue cracks around eutectic Si particles and AI-Fe system intermetallic compounds on the fatigue fracture surface. The usual fatigue strength is greater than the impact fatigue strength when fatigue strength is characterized by the cumulative duration time of loading. Graphs, photomicrographs, 11 refs.
The influence of hold time on low-cycle fatigue (LCF) of Ti-24Al-I INb was studied at 650°C. At 0.167 Hz, the alloy exhibits cyclic hardening at all strain levels studied, and obeys the well-known Manson-Coffin behaviour. A 100 s hold at peak tensile or compressive strain at +-0.6% strain has no observable effect on cycles to failure. For hold times at +-0.5% strain, however, the fatigue lives are nearly halved, and specimens show secondary cracking normal to the stress axis. The increase in inelastic strain as a result of hold time appears to be primarily responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with predictions. For the range of test conditions employed, the total and the tensile hysteretic energy per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction. Graphs, photomicrographs, 18 refs.
Rational assessment of flexural fatigue characteristics of ferrocement for reliable design Singh, G. and Guang, J.X. Cem. Concr. Compos. (1995) 17 (1), 47-55
Local fatigue damage accumulation around notch attending crack initiation Iino, Y. Metall. Mater Trans. A (1995) 26A (6), 1419-1430
The shortcoming of a design method based on a stress-life (S-N) plot is pointed out and a new method based on a probability-stress-life (P-S-N) plot is proposed. Rectangular stress distribution is shown to be better for predicting steel stress when designing ferrocement against fatigue by using the P - S - N relationship of wire tested in air. The inclusion of run-outs in regression analysis is found to lead to more reliable P - S - N relationships.
Classification of fatigue crack growth behavior Vasudevan, A.K. and Sadananda, K. Metall. Mater. Trans. A (1995) 26A (5), 1221-1234 A self-consistent theory has been developed to account for the variation in fatigue crack growth rates with load ratio, R, without reference to crack closure concepts. The theory states that: (a) for an unambiguous description of cyclic damage, two loading parameters are required; (b) consequently, there are two thresholds corresponding to each parameter that must be satisfied for a crack to grow; (c) these two thresholds are intrinsic and are independent of specimen geometry; (d) a fundamental threshold curve can be developed that is independent of test methods defining these two thresholds from the asymptotic values; and (e) the two thresholds vary with the degree of slip planarity, microstructure, and environment. Based on these new concepts, we have classified the entire fatigue crack growth behaviour into five different classes using the experimental AK, h-R data. The characteristic feature of each class is discussed, and the supporting examples of materials behaviour are provided. This classification could provide a basis for understanding the synergistic effects of mechanical and chemical driving forces and microstructure contributing to fatigue crack growth. High-strength low-alloy steels and INg05XL aluminium alloy are discussed. Graphs, 39 refs.
Fracture characteristics of Ti-6AI-4V and Ti-5AI-2.5Fe with refined microstructure using hydrogen Niinomi, M., gobayashi, T., Gong, B., Ohyabu, Y. and Toriyama, O. MetalL Mater. Trans. A (1995) 26A (5), 1141-1151 The hydrogenation behaviour of Ti-6A1--4V, with the starting microstructures of coarse equiaxed a and coarse Widmanstatten a, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature and hydrogen flowrate. The phases presented in the alloy after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of thermochemical processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti--6A1--4V and Ti-5A1-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6A1--4V with the starting microstmcture of coarse equiaxed t~ at 1023 K, just below hydrogen-saturated /3 (denoted /3(H)) transus temperature, produces a microstructure of ct, orthobombic martensite (denoted og'(H)) and /3(H). Hydrogenation at 1123K, >/3(H) transus, results in a microstrueture of a"(H) and /3(H) in a fine mixture of a and /3 during dehydrogenation. An alternative TCP method is
The subsequent recrystallization technique was used to study the process of local damage accumulation around a notch under conditions of low-cycle fatigue. A 0.8 in compact tension specimen of 304 stainless steel with a notch radius of 1 mm was used. The accumulated plastic zone around notch increases with the number of cycles, N. The accumulated plastic strain within the zone also increases with N, producing the strain gradient (damage gradient). A fatigue crack initiates when the accumulated plastic strain at the notch root reaches a critical value equal to the fracture strain of the material: that is, when the accumulated plastic work at the crack initiation site becomes critical. The fatigue crack emanating from a notch root grows through the pre-existing damaged zone. It is shown that this local damage accumulation approach can explain the fast growth of a short crack from a notch. Graphs, photomicrographs, 29 refs.
The effect of an overload on the rate of fatigue crack propagation under plane stress conditions Bao, H. and McEvily, A.J. MetalL Mater. Trans. A (1995) 26A (7), 1725-1733 It has been shown that the retardation in the rate of fatigue crack growth following an overload is largely the result of surface-related, plane stress plastic deformation. In the present article, in order to isolate the plane stress behaviour, the effect of an overload on the subsequent rate of fatigue crack growth of 0.3 mm thick specimens of 9Cr-lMo steel has been investigated and compared with results obtained using 6.35 mm thick specimens. It was found that for the 0.3 mm thickness, as with thicker specimens, two opening load levels were associated with plane stress deformation in the overload plastic zone, and this opening process is more clearly observed with thin than with thicker specimens. Based upon the determined level of the upper opening load, a semi-empirical analysis is developed for calculating the number of delay cycles due to an overload as a function of thickness. Photomicrographs, graphs, 7 refs.
An investigation of the fatigue and fracture behaviour of manganesecontaining ganuna titanium aluminides Soboyejo, W.O., Mercer, C., Lou, K. and Heath, S. Metall. Mater. Trans. A (1995) 26A (9), 2275-2291 The fatigue and fracture mechanisms in Ti-48Al-xMn (x = 1.4-2.0 at.%) gamma-based titanium aluminide allloys are elucidated. Unlike most gamma alloys, which fail predominantly by transgranular fracture at room temperature, fracture in ternary Ti-48Al-xMn alloys is shown to occur mainly by intergranular failure. The incidence of intergranular failure increased with increasing annealing duration and temperature. Intergranular fracture is shown to occur as a result of the segregation of Mn to equiaxed and interlamellar boundaries. Annealing either above or below the eutectoid temperature results in the precipitation of alpha 2 particles. The reduction in the strength and toughness of ternary Mn-containing alloys is attributed to the combined effects of segregation and alpha 2 precipitation. A micromechanics framework is presented for the assessment of twin toughening mechanisms under monotonic and cyclic loading. Photomicrographs, 40 refs.
The crack-tip mechanics and growth rates of small fatigue cracks in ASTROLOY Davidson, D.L and Hudak, S.J. Jr. Metall. Mater. Trans. A (1995) 26A (9), 2247-2257 The micromechanics of naturally initiated small fatigue cracks in coarse- and fine-grained nickel-based ASTROLOY have been studied at ambient and elevated temperature using a cyclic loading stage for the scanning electron microscope (SEM) and the stereo-imaging technique. The objective of these exper-
Abstracts
torsional moments. General aspects of the proof of fatigue strength and stress analysis are discussed first. Finite element analyses have been done for the whole range of practically important geometries. The results for flush and protruded nozzles are given as stress concentration factors (SCF) representing the maximum yon Mises stress in the intersection region referred to an appropriate nominal stress. Comparisons are made with the curves given in BS 5500 and with analytical results. Graphs, 9 refs.
strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti~tAI-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5AI-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in improvement in fatigue strength in both Ti-6A1-4V and Ti-5A1-2.5Fe. Graphs, photomicrographs, diffraction patterns, 17 refs.
Effects of iron and calcium on impact fatigue characteristics of AC2B-T6 aluminium casting alloys Kobayashi, T., Niinomi, M., Harata, T., Shimomura, Y. and lto, T. J. Jpn. Inst. Light Met. (1995) 45 (2), 88-94 (in Japanese)
High-temperature low-cycle fatigue and lifetime prediction of Ti-24AIl l N b alloy Malakondaiah, G. and Nicholas, T. MetalL Mater. Trans. A (1995) 26A (5), 1113-1121
The effects of iron and calcium additions on impact fatigue characteristics were investigated, comparing with their effects on the usual fatigue characteristics in AC2B-T6 aluminium casting alloys. Impact fatigue strength in the low-cycle life region of a low Fe-content sample is greater than that for high-Fe content. This trend is, however, exactly opposite to the trend in the high-cycle life region. Both impact and usual fatigue strengths, especially in the high-cycle life region, increased by refining and spheroidizing of eutectic silicon particles and refining of AI-Fe-based intermetallic compounds by adding small amount of Ca. They were confirmed by fractographic observation of fatigue cracks around eutectic Si particles and AI-Fe system intermetallic compounds on the fatigue fracture surface. The usual fatigue strength is greater than the impact fatigue strength when fatigue strength is characterized by the cumulative duration time of loading. Graphs, photomicrographs, 11 refs.
The influence of hold time on low-cycle fatigue (LCF) of Ti-24Al-I INb was studied at 650°C. At 0.167 Hz, the alloy exhibits cyclic hardening at all strain levels studied, and obeys the well-known Manson-Coffin behaviour. A 100 s hold at peak tensile or compressive strain at +-0.6% strain has no observable effect on cycles to failure. For hold times at +-0.5% strain, however, the fatigue lives are nearly halved, and specimens show secondary cracking normal to the stress axis. The increase in inelastic strain as a result of hold time appears to be primarily responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with predictions. For the range of test conditions employed, the total and the tensile hysteretic energy per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction. Graphs, photomicrographs, 18 refs.
Rational assessment of flexural fatigue characteristics of ferrocement for reliable design Singh, G. and Guang, J.X. Cem. Concr. Compos. (1995) 17 (1), 47-55
Local fatigue damage accumulation around notch attending crack initiation Iino, Y. Metall. Mater Trans. A (1995) 26A (6), 1419-1430
The shortcoming of a design method based on a stress-life (S-N) plot is pointed out and a new method based on a probability-stress-life (P-S-N) plot is proposed. Rectangular stress distribution is shown to be better for predicting steel stress when designing ferrocement against fatigue by using the P - S - N relationship of wire tested in air. The inclusion of run-outs in regression analysis is found to lead to more reliable P - S - N relationships.
Classification of fatigue crack growth behavior Vasudevan, A.K. and Sadananda, K. Metall. Mater. Trans. A (1995) 26A (5), 1221-1234 A self-consistent theory has been developed to account for the variation in fatigue crack growth rates with load ratio, R, without reference to crack closure concepts. The theory states that: (a) for an unambiguous description of cyclic damage, two loading parameters are required; (b) consequently, there are two thresholds corresponding to each parameter that must be satisfied for a crack to grow; (c) these two thresholds are intrinsic and are independent of specimen geometry; (d) a fundamental threshold curve can be developed that is independent of test methods defining these two thresholds from the asymptotic values; and (e) the two thresholds vary with the degree of slip planarity, microstructure, and environment. Based on these new concepts, we have classified the entire fatigue crack growth behaviour into five different classes using the experimental AK, h-R data. The characteristic feature of each class is discussed, and the supporting examples of materials behaviour are provided. This classification could provide a basis for understanding the synergistic effects of mechanical and chemical driving forces and microstructure contributing to fatigue crack growth. High-strength low-alloy steels and INg05XL aluminium alloy are discussed. Graphs, 39 refs.
Fracture characteristics of Ti-6AI-4V and Ti-5AI-2.5Fe with refined microstructure using hydrogen Niinomi, M., gobayashi, T., Gong, B., Ohyabu, Y. and Toriyama, O. MetalL Mater. Trans. A (1995) 26A (5), 1141-1151 The hydrogenation behaviour of Ti-6A1--4V, with the starting microstructures of coarse equiaxed a and coarse Widmanstatten a, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature and hydrogen flowrate. The phases presented in the alloy after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of thermochemical processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti--6A1--4V and Ti-5A1-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6A1--4V with the starting microstmcture of coarse equiaxed t~ at 1023 K, just below hydrogen-saturated /3 (denoted /3(H)) transus temperature, produces a microstructure of ct, orthobombic martensite (denoted og'(H)) and /3(H). Hydrogenation at 1123K, >/3(H) transus, results in a microstrueture of a"(H) and /3(H) in a fine mixture of a and /3 during dehydrogenation. An alternative TCP method is
The subsequent recrystallization technique was used to study the process of local damage accumulation around a notch under conditions of low-cycle fatigue. A 0.8 in compact tension specimen of 304 stainless steel with a notch radius of 1 mm was used. The accumulated plastic zone around notch increases with the number of cycles, N. The accumulated plastic strain within the zone also increases with N, producing the strain gradient (damage gradient). A fatigue crack initiates when the accumulated plastic strain at the notch root reaches a critical value equal to the fracture strain of the material: that is, when the accumulated plastic work at the crack initiation site becomes critical. The fatigue crack emanating from a notch root grows through the pre-existing damaged zone. It is shown that this local damage accumulation approach can explain the fast growth of a short crack from a notch. Graphs, photomicrographs, 29 refs.
The effect of an overload on the rate of fatigue crack propagation under plane stress conditions Bao, H. and McEvily, A.J. MetalL Mater. Trans. A (1995) 26A (7), 1725-1733 It has been shown that the retardation in the rate of fatigue crack growth following an overload is largely the result of surface-related, plane stress plastic deformation. In the present article, in order to isolate the plane stress behaviour, the effect of an overload on the subsequent rate of fatigue crack growth of 0.3 mm thick specimens of 9Cr-lMo steel has been investigated and compared with results obtained using 6.35 mm thick specimens. It was found that for the 0.3 mm thickness, as with thicker specimens, two opening load levels were associated with plane stress deformation in the overload plastic zone, and this opening process is more clearly observed with thin than with thicker specimens. Based upon the determined level of the upper opening load, a semi-empirical analysis is developed for calculating the number of delay cycles due to an overload as a function of thickness. Photomicrographs, graphs, 7 refs.
An investigation of the fatigue and fracture behaviour of manganesecontaining ganuna titanium aluminides Soboyejo, W.O., Mercer, C., Lou, K. and Heath, S. Metall. Mater. Trans. A (1995) 26A (9), 2275-2291 The fatigue and fracture mechanisms in Ti-48Al-xMn (x = 1.4-2.0 at.%) gamma-based titanium aluminide allloys are elucidated. Unlike most gamma alloys, which fail predominantly by transgranular fracture at room temperature, fracture in ternary Ti-48Al-xMn alloys is shown to occur mainly by intergranular failure. The incidence of intergranular failure increased with increasing annealing duration and temperature. Intergranular fracture is shown to occur as a result of the segregation of Mn to equiaxed and interlamellar boundaries. Annealing either above or below the eutectoid temperature results in the precipitation of alpha 2 particles. The reduction in the strength and toughness of ternary Mn-containing alloys is attributed to the combined effects of segregation and alpha 2 precipitation. A micromechanics framework is presented for the assessment of twin toughening mechanisms under monotonic and cyclic loading. Photomicrographs, 40 refs.
The crack-tip mechanics and growth rates of small fatigue cracks in ASTROLOY Davidson, D.L and Hudak, S.J. Jr. Metall. Mater. Trans. A (1995) 26A (9), 2247-2257 The micromechanics of naturally initiated small fatigue cracks in coarse- and fine-grained nickel-based ASTROLOY have been studied at ambient and elevated temperature using a cyclic loading stage for the scanning electron microscope (SEM) and the stereo-imaging technique. The objective of these exper-