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Effects of small prestrain on the fatigue behaviour of spheroidal pearlite steel
High-temperature low-cycle fatigue behavior of a nimonic PE-16 superalIoy - correlation with deformation and fracture. Valsan, M., Parameswaran, P., Rao, K. B. S., Vijayalakshmi, M., Mannah, S. L. and Shastry, D. H. MetalL Trans. A June 1992 23A (6) 1751-1761 Low-cycle fatigue (LCF) responses of nimonic PE-16for various prior microstructures and strain amplitudes have been evaluated and the fatigue behaviour has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed st 923 K on samples possessing three different prior miorostructures: alloy A in solution-annealed condition (free of ~' and carbides), alloy B with double ageing treatment (spherical ~/ of 18 nm diameter and M 23 C6), and alloy C with another double ageing treatment (~' of size 35 nm, MC and M 23 C6). All three microstructures exhibited an initial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudes vs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behaviour in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testing. Fresh precipitation of fine ~,' was confirmed by the development of "mottled contrast' in alloy C. Evidence for the shearing of the ordered ?' precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to a reduction in the size of the ~' and consequent softening. Coarser ~' precipitates were associated with Orowan loops. The observed fatigue behaviour has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples. Numeric data, photomicrographs, graphs. 44 refs.
An investigation of the effects of microstructure on the fatigue and fracture behavior of ~z + ~ forged T i - 2 4 A I - 1 1 N b . Soboyejo, W.O. Metall. Trans. A June 1992 23A (6) 1737-1750 The results of a recent study of the effects of Widmanstatten and basket weave microstructures on the fracture toughness and fatigue crack growth behaviour of Ti-24Ai-11Nb are reported. Intrinsic and extrinsic toughening components due to crack blunting and bridging by the ~ phase, crack deflection, and microcracking are computed from existing crack-tip shielding models. Predictions of fracture toughness and fatigue thresholds obtained by the superposition of extrinsic toughening components are compared with measured values obtained from compression precracked single'edge notch (SEN) bend specimens. The results indicate that the continuity of the 13 matrix between the ~2 laths is important for the effectiveness of crack-tip blunting mechanisms. Widmanstatten microstructures obtained by annealing solely in the ~2 + 13 phase field are shown to promote crack deflection and unstable room-temperature fatigue crack growth rates, ie crack growth rates that increase after further thermal exposure in the ~2 + 13phase field. Basket weave microstructures produced by two-stage annealing (TSA) in the ~ and ~2 + 13phase fields are shown to promote crack bifurcation and deflection and significant improvements in fatigue crack growth resistance when the ~ anneal is followed by a furnace-cool. The significant role of microcracking in the fatigue propagation mechanism is highlighted. The micromechanisms of fatigue and fracture are also discussed for the microstructures examined. Photomicrographs, graphs. 42 refs.
Fatigue crack growth in 5083 aluminum alloy under varying loading conditions. Takaai, T. and Nakayama, Y. J. Jpn Inst. Met. Jan. 1992 42 (1) 45-51 (in Japanese) Fatigue crack growth in a 5083 AI alloy specimen was studied experimentally using compact tension specimens. Results are summarized as follows. At a constant load amplitude, the log-log plot of crack growth rate (da/dN) vs stress intensity factor range (~'Keff) becomes linear in the intermediate A.K range. Fatigue crack growth is retarded by the overloading and the crack growth rate is depressed with increase in the cycle numbers of overloading. Crack growth rate depends on the number of overloading cycles in view of the crack opening ratio. The overloading results in the closure of the crack, the area of which increases with increase in the number of overloading cycles. Photomicrographs, graphs. 10 refs.
Fatigue in single crystal nickel superalloys. Technical progress report, 16 September-15 December 1991. AD-A244 8 1 5 / 7 / X A B 1991 pp 13 The behaviour of single-crystal (SC) airfoil materials is investigated. The fatigue initiation processes in SC materials are significantly more complicated and involved than fatigue initiation and subsequent behaviour of a (single) macrocrack in conventional, isotropic materials. To understand these differences it is helpful to review the evolution of high-temperature airfoils. Modern gas turbine flight propulsion systems employ single-crystal materials for turbine airfoil applications because of their superior performance in resisting creep, oxidation, and thermal mechanical fatigue (TMF). These properties have been achieved by composition and alloying, and by appropriate crystal orientation and associated anisotropy.
Effect of current on fatigue life of 60Sn-4OPb solder. Vaynarn, X., Fine, M. E. and Jeannotte, D.A. Scr. Metal/. Mater. 15 Mar. 1992 26 (6) 9 9 9 - 1 0 0 0 The effect on an electrical current on the fatigue life of 60Sn-40Pb solder was determined in tension-tension fatigue tests. Although there was considerable scatter, the fatigue life in tests with current was lower than in tests without current. The effect of current appeared to be more pronounced at lower strain ranges and also in tests with tensile hold time. Tensile and stress relaxation at constant strain rate tests were also performed but no effects of currant on stresses during these tests were recorded. Graphs. 1 ref.
158
Intrinsic stage I crack propagation in A l - Z n - M g single crystals. Petit, J., Kosche, K. and Gudladt, H.J. Scr. MetalL Mater. 1 Apr. 1992 26 (7) 1049-1054 Fatigue tests have been performed on single- and potycrystals of an AI-4.5 wt.% Zn-l.25wt.% Mg alloy in ambient air and vacuum (including crack closure measurements) to determine if stage I propagation is associated with an environmentally assisted mechanism, Based on observations made during testing, three main conclusions were drawn: the intrinsic stage I regime is a great deal faster than intrinsic stage II for the same loading condition; the shielding effect related to stage I propagation is much more important than expected from a comparison to stage II; and the propagation of a small surface crack growing in stage I regime must be compared with the intrinsic stage I in single crystals instead of stage II or stage I in polycrystals. Photomicrographs, graphs. 11 refs.
Sources of acoustic emission during fatigue of Ti-6AI-4V: effect of microstructure. Kohn, D.H., Ducheyne, P. and Awerbuch, J. J. Mater. Sci. 15 Mar. 1992 27 (6) 1633-1641 The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6AI-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6AI-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous c~-13interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lameHar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high-level intensities were also generated at stresses other than the peak stress. This is because in Ti alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension. Photomicrographs, graphs. 21 refs.
A criterion for fatigue damage under combined bending and torsion based on the local atre=s-strain method. Nie, H., Qiao, X. and Fan, W. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 2 5 - 2 2 7 Prediction of fatigue crack initiation life by the local stress-strain method has found wide application in fatigue analysis. This method seems to be satisfactory, especially for low/medium cycle life prediction used for landing gears. The local stress-strain method for fatigue life prediction under combined bending and torsion is examined. The material used was a 30CrMnSiNi-2A alloy steel, whose properties are close to AISI-4340. 4 refs.
Effects of small prestrain on the fatigue behaviour of spheroidal pearlite steel. Na=qase, Y. and Kanri, 7".
Fatigue/-ract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 1 3 - 2 2 3 To clarify the effect of a small prestrain on the fatigue strength of a carbon steel, the fatigue behaviour of plain specimens of a eutectoid steel with spheroidal pearlite, previously subjected to a tensile prestrain, is investigated and compared with that of annealed and strain-aged specimens. It is found that a small prestrain of a few percent does not decrease the fatigue limit nor the crack initiation or propagation lives. Fatigue slip initiation, however, is accelerated by the small prestrain. It is also observed that cyclic strain ageing does not occur. From these results, it is inferred that dislocations are unpinned by the small prestrain and this causes fatigue slip to occur more readily, but the fatigue behaviour following slip initiation is not affected because the spheroidal cementite particles act as obstacles to further slip deformation and Stage I crack propagation. Graphs, photomicrographs. 14 refs.
Fracture mechanics of physically short cracks. Vasjutin, A.N. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 0 3 - 2 1 2 On the basis of an energy analysis, a generalized fracture criterion is derived for a body with crack-like defects of various size. Using the criterion an expression for the fatigue crack growth rate is proposed. Observed and predicted results of short fatigue crack propagation rate are presented and discussed, Graphs. 8 refs.
, ~ / ~ : which formula? Finney, J.M. and Deirmendjian, G. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 151-158 Several equations are examined which were developed to eliminate the R effect in fatigue crack growth rate by using the concept of a ~Keff, Their performance was objectively rated by using a limited set of growth rate data with the result that an early equation by Schijve proved the best. However, relatively small variations in growth rates markedly affected the order of rating the equations, and none of them appears to include the variables necessary to cope with a wide range of R values and growth rates. The usefulness of an effective AK in predicting fatigue crack growth is thus brought into question. Materials discussed include 2214. Graphs. 15 refs.
Crack propagation in aluminium gas cylinder neck material at constant load and room temperature. Stark, H.L. and ~brahMin, R.N. Eng. Fract. Mech. Mar. 1992 41 (4) 5 6 9 - 5 7 5 Sustained tensile load tests were carried out at room temperature on small circumferentialty cracked cylindrical specimens taken from the necks of AI gas cylinders. Comparison was made of the resulting time to failure of specimens of AI alloys 6351, 6061 and 6351 with Pb impurity levels > 200 ppm. The effect of superimposing a small number of interruptions to the sustained load was also examined. With a few exceptions the 6061 alloy withstood the sustained loading for significantly longer periods than the 6351 alloy. The presence of Pb substantially reduced the time to failure, as did also the addition of a small number of fatigue cycles. Graphs. 6 refs.
Int J Fatigue March 1993
An investigation of the effects of microstructure on the fatigue and fracture behavior of ~z + ~ forged T i - 2 4 A I - 1 1 N b . Soboyejo, W.O. Metall. Trans. A June 1992 23A (6) 1737-1750 The results of a recent study of the effects of Widmanstatten and basket weave microstructures on the fracture toughness and fatigue crack growth behaviour of Ti-24Ai-11Nb are reported. Intrinsic and extrinsic toughening components due to crack blunting and bridging by the ~ phase, crack deflection, and microcracking are computed from existing crack-tip shielding models. Predictions of fracture toughness and fatigue thresholds obtained by the superposition of extrinsic toughening components are compared with measured values obtained from compression precracked single'edge notch (SEN) bend specimens. The results indicate that the continuity of the 13 matrix between the ~2 laths is important for the effectiveness of crack-tip blunting mechanisms. Widmanstatten microstructures obtained by annealing solely in the ~2 + 13 phase field are shown to promote crack deflection and unstable room-temperature fatigue crack growth rates, ie crack growth rates that increase after further thermal exposure in the ~2 + 13phase field. Basket weave microstructures produced by two-stage annealing (TSA) in the ~ and ~2 + 13phase fields are shown to promote crack bifurcation and deflection and significant improvements in fatigue crack growth resistance when the ~ anneal is followed by a furnace-cool. The significant role of microcracking in the fatigue propagation mechanism is highlighted. The micromechanisms of fatigue and fracture are also discussed for the microstructures examined. Photomicrographs, graphs. 42 refs.
Fatigue crack growth in 5083 aluminum alloy under varying loading conditions. Takaai, T. and Nakayama, Y. J. Jpn Inst. Met. Jan. 1992 42 (1) 45-51 (in Japanese) Fatigue crack growth in a 5083 AI alloy specimen was studied experimentally using compact tension specimens. Results are summarized as follows. At a constant load amplitude, the log-log plot of crack growth rate (da/dN) vs stress intensity factor range (~'Keff) becomes linear in the intermediate A.K range. Fatigue crack growth is retarded by the overloading and the crack growth rate is depressed with increase in the cycle numbers of overloading. Crack growth rate depends on the number of overloading cycles in view of the crack opening ratio. The overloading results in the closure of the crack, the area of which increases with increase in the number of overloading cycles. Photomicrographs, graphs. 10 refs.
Fatigue in single crystal nickel superalloys. Technical progress report, 16 September-15 December 1991. AD-A244 8 1 5 / 7 / X A B 1991 pp 13 The behaviour of single-crystal (SC) airfoil materials is investigated. The fatigue initiation processes in SC materials are significantly more complicated and involved than fatigue initiation and subsequent behaviour of a (single) macrocrack in conventional, isotropic materials. To understand these differences it is helpful to review the evolution of high-temperature airfoils. Modern gas turbine flight propulsion systems employ single-crystal materials for turbine airfoil applications because of their superior performance in resisting creep, oxidation, and thermal mechanical fatigue (TMF). These properties have been achieved by composition and alloying, and by appropriate crystal orientation and associated anisotropy.
Effect of current on fatigue life of 60Sn-4OPb solder. Vaynarn, X., Fine, M. E. and Jeannotte, D.A. Scr. Metal/. Mater. 15 Mar. 1992 26 (6) 9 9 9 - 1 0 0 0 The effect on an electrical current on the fatigue life of 60Sn-40Pb solder was determined in tension-tension fatigue tests. Although there was considerable scatter, the fatigue life in tests with current was lower than in tests without current. The effect of current appeared to be more pronounced at lower strain ranges and also in tests with tensile hold time. Tensile and stress relaxation at constant strain rate tests were also performed but no effects of currant on stresses during these tests were recorded. Graphs. 1 ref.
158
Intrinsic stage I crack propagation in A l - Z n - M g single crystals. Petit, J., Kosche, K. and Gudladt, H.J. Scr. MetalL Mater. 1 Apr. 1992 26 (7) 1049-1054 Fatigue tests have been performed on single- and potycrystals of an AI-4.5 wt.% Zn-l.25wt.% Mg alloy in ambient air and vacuum (including crack closure measurements) to determine if stage I propagation is associated with an environmentally assisted mechanism, Based on observations made during testing, three main conclusions were drawn: the intrinsic stage I regime is a great deal faster than intrinsic stage II for the same loading condition; the shielding effect related to stage I propagation is much more important than expected from a comparison to stage II; and the propagation of a small surface crack growing in stage I regime must be compared with the intrinsic stage I in single crystals instead of stage II or stage I in polycrystals. Photomicrographs, graphs. 11 refs.
Sources of acoustic emission during fatigue of Ti-6AI-4V: effect of microstructure. Kohn, D.H., Ducheyne, P. and Awerbuch, J. J. Mater. Sci. 15 Mar. 1992 27 (6) 1633-1641 The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6AI-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6AI-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous c~-13interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lameHar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high-level intensities were also generated at stresses other than the peak stress. This is because in Ti alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension. Photomicrographs, graphs. 21 refs.
A criterion for fatigue damage under combined bending and torsion based on the local atre=s-strain method. Nie, H., Qiao, X. and Fan, W. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 2 5 - 2 2 7 Prediction of fatigue crack initiation life by the local stress-strain method has found wide application in fatigue analysis. This method seems to be satisfactory, especially for low/medium cycle life prediction used for landing gears. The local stress-strain method for fatigue life prediction under combined bending and torsion is examined. The material used was a 30CrMnSiNi-2A alloy steel, whose properties are close to AISI-4340. 4 refs.
Effects of small prestrain on the fatigue behaviour of spheroidal pearlite steel. Na=qase, Y. and Kanri, 7".
Fatigue/-ract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 1 3 - 2 2 3 To clarify the effect of a small prestrain on the fatigue strength of a carbon steel, the fatigue behaviour of plain specimens of a eutectoid steel with spheroidal pearlite, previously subjected to a tensile prestrain, is investigated and compared with that of annealed and strain-aged specimens. It is found that a small prestrain of a few percent does not decrease the fatigue limit nor the crack initiation or propagation lives. Fatigue slip initiation, however, is accelerated by the small prestrain. It is also observed that cyclic strain ageing does not occur. From these results, it is inferred that dislocations are unpinned by the small prestrain and this causes fatigue slip to occur more readily, but the fatigue behaviour following slip initiation is not affected because the spheroidal cementite particles act as obstacles to further slip deformation and Stage I crack propagation. Graphs, photomicrographs. 14 refs.
Fracture mechanics of physically short cracks. Vasjutin, A.N. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 2 0 3 - 2 1 2 On the basis of an energy analysis, a generalized fracture criterion is derived for a body with crack-like defects of various size. Using the criterion an expression for the fatigue crack growth rate is proposed. Observed and predicted results of short fatigue crack propagation rate are presented and discussed, Graphs. 8 refs.
, ~ / ~ : which formula? Finney, J.M. and Deirmendjian, G. Fatigue Fract. Eng. Mater. Struct. Feb. 1992 15 (2) 151-158 Several equations are examined which were developed to eliminate the R effect in fatigue crack growth rate by using the concept of a ~Keff, Their performance was objectively rated by using a limited set of growth rate data with the result that an early equation by Schijve proved the best. However, relatively small variations in growth rates markedly affected the order of rating the equations, and none of them appears to include the variables necessary to cope with a wide range of R values and growth rates. The usefulness of an effective AK in predicting fatigue crack growth is thus brought into question. Materials discussed include 2214. Graphs. 15 refs.
Crack propagation in aluminium gas cylinder neck material at constant load and room temperature. Stark, H.L. and ~brahMin, R.N. Eng. Fract. Mech. Mar. 1992 41 (4) 5 6 9 - 5 7 5 Sustained tensile load tests were carried out at room temperature on small circumferentialty cracked cylindrical specimens taken from the necks of AI gas cylinders. Comparison was made of the resulting time to failure of specimens of AI alloys 6351, 6061 and 6351 with Pb impurity levels > 200 ppm. The effect of superimposing a small number of interruptions to the sustained load was also examined. With a few exceptions the 6061 alloy withstood the sustained loading for significantly longer periods than the 6351 alloy. The presence of Pb substantially reduced the time to failure, as did also the addition of a small number of fatigue cycles. Graphs. 6 refs.
Int J Fatigue March 1993