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Measurement of crack tip strains using neutron diffraction
Fatigue Abstracts
265
crack surface roughness in the coarser-grained alloy than in the finer-grained alloy. Graphs, photomicrographs, 32 ref.
the influence of a macrocrack population on the BN signal will be discussed. Photomicrographs, 43 ref.
Notch effect on two-level cumulative low-cycle fatigue life under different biaxial loading mode sequences Yip, M.-C. and Jen, E-M. Fatigue Fraet. Eng. Mater. Struct. (1995) 18 (1 I), 1323-1332
Fatigue of nanocrystalline copper Witney, A.B., Sanders, P.G., Weertman, J.R. and Eastman, J.A. Scr. MetalL Mater. (15 Dec. 1995) 33 (12), 2025-2030
Two-level cumulative low-cycle fatigue lives of AISI 316 stainless steel notched specimens with different hiaxial loading mode sequences are experimentally analysed in this paper. Forty-eight cases were conducted in the experimental programme by considering the loading level sequence effect, the biaxiality of two levels, and the cycle ratios. Results show that the interlock effect caused by the characteristic fracture surfaces of the different biaxial states is beneficial to the cumulative fatigue lives. On the other hand, tensile loading of the second level will accelerate the opening of cracks and decrease fatigue strength. Miner's rule predicts most fatigue lives within 30% error hands, and the loading level sequence effect was not found in this research involving complex fracture modes. Fractography of specimens in the cumulative fatigue tests is reported and discussed in this paper. Graphs, photomicrographs, 17 ref.
A fatigue-crack-growth-based analysis of two-step corrosion fatigue tests lshihara, S., MeEvil~,, A.J. and Shiozawa, K. Fatigue Fruct. Eng. Mater. Struct. (1995) 18 (ll), 1311-1321 A method for the analysis of two-step fatigue level sequences is proposed and compared with experimental results. Two-step loading tests of the aluminum alloy 2017-T4 in 3% sodium chloride solution have been carried out in conjunction with a replica technique used to monitor the growth of fatigue cracks. Fatigue cracks were nucleated at corrosion pits 10-15 /zm in size, and crack growth rather than crack initiation was found to take up the major portion of the fatigue lifetime in these tests. The results could therefore be analyzed on the basis of the following constitutive relation for fatigue crack growth dN da = A(AK,,. - AK~,,H)2. This analysis was simplified since the influence of transients in the crack growth rate induced upon change in load level was found to be minimal. The approach provides a rational basis for dealing with load-sequence effects. Graphs, 30 ref.
Fraetographie analyses of fatigue crack growth in D26T alloy subjected to biaxial cyclic loads at various R-ratios Shanyavsky, A.A., Orlov, E.F. and Koronov, M.Z. Fatigue Fract. Eng. Mater. Struct. (1995) 18 (11), 1263-1276 Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of DI6T aluminium alloy under biaxial tension and tensioncompression. In the range of the biaxial load ratios, A, from -1 to +1.5, and in the range of R-ratios 0.05~).8, fatigue striation formation took place over a crack growth rate near to 4 x 10-8 m cycle-L The striation spacing and the crack growth rate decrease as the ratios A and R increase. The ratio between the increment of crack growth, &ddN, and the striation spacing, 6, is approx 1:1 when da/dN is > 4 x 10-~ m cycle ~. The relationship between the number of cycles from the beginning of a test up to the growth rate of 2 . 1 4 × l0 -7 m cycle J (No), and the crack growth period, N~,, from when the crack initiates up to the instant when that growth rate is reached, was determined for different A and R-ratios. The value of Nd increases as the stress ratio A is increased. Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method involving an equivalent stress intensity factor K~ = KfF(A, R). The values of F(A, R) for the growth rate (F(A, R)~) and for the striation spacing (F(A, R)~) were determined and compared. The fatigue crack growth period, Npt, applicable to the stage of fatigue striation formation, (predicted by using both of the F(A, R) values) is correlated with the experimental data, and the error is of the order of 15%. 19 ref.
Extending the fatigue life of multi-layer metal joints Finney, J.M. and Evans, R.L. Fatigue Fract. Eng. Mater. Struct. (1995) 18 (11), 1231-1247 The influences of both hole cold expansion and interference-fit fasteners for extending the fatigue life of multi-layer aluminium alloy joint specimens under variable-amplitude loading have been examined experimentally. Improvements in fatigue life were markedly dependent on the degree of load transfer in the specimen joint. Secondary bending in the joint also had a major influence, reducing the effectiveness of these life improvement techniques. Depending on the joint configuration, improvements in fatigue life ranged from nothing at all to a factor of approx 40. Fretting appeared to be involved in the initiation of all cracks, and where this occurred on fraying surfaces only small, if any, improvements in fatigue life were obtained. Graphs, photomicrographs, 17 ref.
Barkhausen noise in mechanically fatigued nickel single crystals Buque, C., Tirscher, W. and Blochwitz, C. Z Memllkd. (Oct. 1995) 86 (10), 67[~581 Mechanically fatigued nickel single crystals have been investigated by magnetic Barkhausen noise (BN). The distribution of the BN signal along the specimen surface will he correlated with strain localization, near-surface dislocation structures (persistent slip bands (PSB) and matrix), which could he imaged by scanning electron microscopy channeling effect, and microcrack populations. Taking into account the magneto-elastic coupling, conclusions are possible concerning the internal stresses of PSB and matrix structures. The experimental results agree with internal stresses of a macroscopic long-range scale. Further,
Little is known about the fatigue properties of nanocrystalline materials or the effect of prolonged cycling on their internal structure. For example, it could be speculated that continuous deformation would lead to extensive grain growth. An initial set of fatigue experiments have been carried out on nanocrystalline Cu, with the primary goal of examining the stability of the internal structure under cycling. In addition, external surfaces were examined for possible clues to deformation mechanisms. Graphs, photomicrographs, 9 ref.
Fatigue crack growth at arbitrary angles to bimaterial interfaces Suresh, S., Sugimura, Y. and Grand&, L. Set. Metall. Mater. (1995) 33 (12), 2007-2012 The fracture characteristics of monotonically loaded cracks located parallel to, in the vicinity of, and at arbitrary angles to the interface between two elastic solids have been the subject of considerable theoretical research in the literature. The present experimental work on steel-steel bimaterials clearly demonstrates that, for essentially the same elastic properties, the plastic mismatch between the constituent layers of a bilayer composite can dictate whether a fatigue crack approaching the interface at various angles continues to advance through the interface or arrests prior to penetrating the interface. The former crack arrest phenomenon occurs when the fatigue crack approaches the interface from the plastically weaker material, whereas continued crack growth results when the crack approaches the interface from the plastically stronger material. The observed experimental trends are qualitatively consistent with the numerical predictions of crack-tip shielding or amplification from the interaction of the near-tip plastic zone with an interface oriented normal to the plane of the crack. When the fatigue crack is initiated parallel to the interface, it deviates from the strongly bonded interface to advance in one of the two steels. This trend appears to experimentally confirm predictions of the possible evolution of strong mode mixity ahead of interface cracks subjected to cyclic tensile loading. Graphs, photomicrographs, 12 ref.
Stacking faults in fatigued titanium single crystals Tan, X.-L and Gu, H.-C. Scr. MetalL Mater. (1995) 33 (12), 1977-1980 It is generally accepted that the mechanical behaviour and interior dislocation substructure of metals and alloys are functions of the stacking fault energies (SFEs). The higher the SFE, the more difficult it is to split a dislocation into two partial dislocations separated by a stacking fault, whereas materials with high SFE allow easy cross-slip of screw dislocations. Titanium is known to have a high SFE. Akhtar and Teghtsoonian estimated the SFE on the prismatic planes of Ti as 0.145 J m : . DeCracy et al. measured the core extension of the edge dislocation in the prismatic planes with high resolution electron microscopy and deduced the SFE to be of the order of 0.150 J m ~. Even though stacking faults in Ti have been observed experimentally, there is still doubt on them due to the high values of SFE. The purpose of the present study is to obtain further information on stacking faults in fatigued Ti single crystals and to discuss the relation between stacking faults and twinning. Photomicrographs, spectra, 8 ref.
Measurement of crack tip strains using neutron diffraction Smith, J., Bassim, M.N., Liu, C.D. and Holden, 7~M. Eng. Fract. Mech. (1995) 52 (5), 843-851 Residual elastic strains and plastic zones near fatigue cracks and monotonically loaded fatigue cracks in compact tension specimens of AISI 4340 steel were measured using neutron diffraction. Compressive residual strain fields were observed close to the crack tip. Further from the crack tip the residual strains were tensile, reaching a maximum within 5 mm from the crack. Mode l stress intensity factors required to compensate for the residual compressive crack tip strains were approximated using crack tip profiles estimated from stretch zone measurements. Diffraction peak broadening indicating plastic deformation was observed within regions up to 2 mm from the crack tips. Estimation of the plastic zone sizes from residual strain distributions and peak broadening measurements agrees well with theoretical predictions. Graphs, 14 ref.
Deflection fatigue of cobalt-chromium, titanium, and gold alloy cast alloy cast denture clasp Vallittu, P.K. and Kokkoneu, K. J. Prasthet. Dent. 11995) 74 (4), 412-419 The aim was to determine the fatigue resistance of the cast clasps of removable partial dentures. The different commercial types of metals used included five cobalt--chromium alloys, pure titanium, one Ti alloy (Ti~AI~4V) and one gold alloy (type IV) that was either unhardened or age-hardened (n = 5 per group). The test method used was a constant-deflection fatigue test in which the force required to deflect the clasp for 0.6 mm and the number of loading cycles required to fracture the clasp were determined. The fatigue fracture surface of the clasps was examined with the scanning electron microscope. The results revealed that a fatigue fracture occurred in the Co-Cr clasp after approx 25 000 loading cycles, in the pure Ti clasps after 4500 loading cycles, in the Ti alloy clasp after 20000 loading cycles, and in the Au alloy clasp after 21 000 loading cycles. The means differed significantly (p < 0.001). Activation of the clasp by bending it 0.5 mm increased the fatigue resistance of the Co~Sr alloy and Au alloy clasps but decreased the fatigue resistance
265
crack surface roughness in the coarser-grained alloy than in the finer-grained alloy. Graphs, photomicrographs, 32 ref.
the influence of a macrocrack population on the BN signal will be discussed. Photomicrographs, 43 ref.
Notch effect on two-level cumulative low-cycle fatigue life under different biaxial loading mode sequences Yip, M.-C. and Jen, E-M. Fatigue Fraet. Eng. Mater. Struct. (1995) 18 (1 I), 1323-1332
Fatigue of nanocrystalline copper Witney, A.B., Sanders, P.G., Weertman, J.R. and Eastman, J.A. Scr. MetalL Mater. (15 Dec. 1995) 33 (12), 2025-2030
Two-level cumulative low-cycle fatigue lives of AISI 316 stainless steel notched specimens with different hiaxial loading mode sequences are experimentally analysed in this paper. Forty-eight cases were conducted in the experimental programme by considering the loading level sequence effect, the biaxiality of two levels, and the cycle ratios. Results show that the interlock effect caused by the characteristic fracture surfaces of the different biaxial states is beneficial to the cumulative fatigue lives. On the other hand, tensile loading of the second level will accelerate the opening of cracks and decrease fatigue strength. Miner's rule predicts most fatigue lives within 30% error hands, and the loading level sequence effect was not found in this research involving complex fracture modes. Fractography of specimens in the cumulative fatigue tests is reported and discussed in this paper. Graphs, photomicrographs, 17 ref.
A fatigue-crack-growth-based analysis of two-step corrosion fatigue tests lshihara, S., MeEvil~,, A.J. and Shiozawa, K. Fatigue Fruct. Eng. Mater. Struct. (1995) 18 (ll), 1311-1321 A method for the analysis of two-step fatigue level sequences is proposed and compared with experimental results. Two-step loading tests of the aluminum alloy 2017-T4 in 3% sodium chloride solution have been carried out in conjunction with a replica technique used to monitor the growth of fatigue cracks. Fatigue cracks were nucleated at corrosion pits 10-15 /zm in size, and crack growth rather than crack initiation was found to take up the major portion of the fatigue lifetime in these tests. The results could therefore be analyzed on the basis of the following constitutive relation for fatigue crack growth dN da = A(AK,,. - AK~,,H)2. This analysis was simplified since the influence of transients in the crack growth rate induced upon change in load level was found to be minimal. The approach provides a rational basis for dealing with load-sequence effects. Graphs, 30 ref.
Fraetographie analyses of fatigue crack growth in D26T alloy subjected to biaxial cyclic loads at various R-ratios Shanyavsky, A.A., Orlov, E.F. and Koronov, M.Z. Fatigue Fract. Eng. Mater. Struct. (1995) 18 (11), 1263-1276 Fractographic peculiarities of fatigue crack development are studied in cruciform specimens of DI6T aluminium alloy under biaxial tension and tensioncompression. In the range of the biaxial load ratios, A, from -1 to +1.5, and in the range of R-ratios 0.05~).8, fatigue striation formation took place over a crack growth rate near to 4 x 10-8 m cycle-L The striation spacing and the crack growth rate decrease as the ratios A and R increase. The ratio between the increment of crack growth, &ddN, and the striation spacing, 6, is approx 1:1 when da/dN is > 4 x 10-~ m cycle ~. The relationship between the number of cycles from the beginning of a test up to the growth rate of 2 . 1 4 × l0 -7 m cycle J (No), and the crack growth period, N~,, from when the crack initiates up to the instant when that growth rate is reached, was determined for different A and R-ratios. The value of Nd increases as the stress ratio A is increased. Cycle loading parameters must be taken into account in order to describe the crack growth period when using a unified method involving an equivalent stress intensity factor K~ = KfF(A, R). The values of F(A, R) for the growth rate (F(A, R)~) and for the striation spacing (F(A, R)~) were determined and compared. The fatigue crack growth period, Npt, applicable to the stage of fatigue striation formation, (predicted by using both of the F(A, R) values) is correlated with the experimental data, and the error is of the order of 15%. 19 ref.
Extending the fatigue life of multi-layer metal joints Finney, J.M. and Evans, R.L. Fatigue Fract. Eng. Mater. Struct. (1995) 18 (11), 1231-1247 The influences of both hole cold expansion and interference-fit fasteners for extending the fatigue life of multi-layer aluminium alloy joint specimens under variable-amplitude loading have been examined experimentally. Improvements in fatigue life were markedly dependent on the degree of load transfer in the specimen joint. Secondary bending in the joint also had a major influence, reducing the effectiveness of these life improvement techniques. Depending on the joint configuration, improvements in fatigue life ranged from nothing at all to a factor of approx 40. Fretting appeared to be involved in the initiation of all cracks, and where this occurred on fraying surfaces only small, if any, improvements in fatigue life were obtained. Graphs, photomicrographs, 17 ref.
Barkhausen noise in mechanically fatigued nickel single crystals Buque, C., Tirscher, W. and Blochwitz, C. Z Memllkd. (Oct. 1995) 86 (10), 67[~581 Mechanically fatigued nickel single crystals have been investigated by magnetic Barkhausen noise (BN). The distribution of the BN signal along the specimen surface will he correlated with strain localization, near-surface dislocation structures (persistent slip bands (PSB) and matrix), which could he imaged by scanning electron microscopy channeling effect, and microcrack populations. Taking into account the magneto-elastic coupling, conclusions are possible concerning the internal stresses of PSB and matrix structures. The experimental results agree with internal stresses of a macroscopic long-range scale. Further,
Little is known about the fatigue properties of nanocrystalline materials or the effect of prolonged cycling on their internal structure. For example, it could be speculated that continuous deformation would lead to extensive grain growth. An initial set of fatigue experiments have been carried out on nanocrystalline Cu, with the primary goal of examining the stability of the internal structure under cycling. In addition, external surfaces were examined for possible clues to deformation mechanisms. Graphs, photomicrographs, 9 ref.
Fatigue crack growth at arbitrary angles to bimaterial interfaces Suresh, S., Sugimura, Y. and Grand&, L. Set. Metall. Mater. (1995) 33 (12), 2007-2012 The fracture characteristics of monotonically loaded cracks located parallel to, in the vicinity of, and at arbitrary angles to the interface between two elastic solids have been the subject of considerable theoretical research in the literature. The present experimental work on steel-steel bimaterials clearly demonstrates that, for essentially the same elastic properties, the plastic mismatch between the constituent layers of a bilayer composite can dictate whether a fatigue crack approaching the interface at various angles continues to advance through the interface or arrests prior to penetrating the interface. The former crack arrest phenomenon occurs when the fatigue crack approaches the interface from the plastically weaker material, whereas continued crack growth results when the crack approaches the interface from the plastically stronger material. The observed experimental trends are qualitatively consistent with the numerical predictions of crack-tip shielding or amplification from the interaction of the near-tip plastic zone with an interface oriented normal to the plane of the crack. When the fatigue crack is initiated parallel to the interface, it deviates from the strongly bonded interface to advance in one of the two steels. This trend appears to experimentally confirm predictions of the possible evolution of strong mode mixity ahead of interface cracks subjected to cyclic tensile loading. Graphs, photomicrographs, 12 ref.
Stacking faults in fatigued titanium single crystals Tan, X.-L and Gu, H.-C. Scr. MetalL Mater. (1995) 33 (12), 1977-1980 It is generally accepted that the mechanical behaviour and interior dislocation substructure of metals and alloys are functions of the stacking fault energies (SFEs). The higher the SFE, the more difficult it is to split a dislocation into two partial dislocations separated by a stacking fault, whereas materials with high SFE allow easy cross-slip of screw dislocations. Titanium is known to have a high SFE. Akhtar and Teghtsoonian estimated the SFE on the prismatic planes of Ti as 0.145 J m : . DeCracy et al. measured the core extension of the edge dislocation in the prismatic planes with high resolution electron microscopy and deduced the SFE to be of the order of 0.150 J m ~. Even though stacking faults in Ti have been observed experimentally, there is still doubt on them due to the high values of SFE. The purpose of the present study is to obtain further information on stacking faults in fatigued Ti single crystals and to discuss the relation between stacking faults and twinning. Photomicrographs, spectra, 8 ref.
Measurement of crack tip strains using neutron diffraction Smith, J., Bassim, M.N., Liu, C.D. and Holden, 7~M. Eng. Fract. Mech. (1995) 52 (5), 843-851 Residual elastic strains and plastic zones near fatigue cracks and monotonically loaded fatigue cracks in compact tension specimens of AISI 4340 steel were measured using neutron diffraction. Compressive residual strain fields were observed close to the crack tip. Further from the crack tip the residual strains were tensile, reaching a maximum within 5 mm from the crack. Mode l stress intensity factors required to compensate for the residual compressive crack tip strains were approximated using crack tip profiles estimated from stretch zone measurements. Diffraction peak broadening indicating plastic deformation was observed within regions up to 2 mm from the crack tips. Estimation of the plastic zone sizes from residual strain distributions and peak broadening measurements agrees well with theoretical predictions. Graphs, 14 ref.
Deflection fatigue of cobalt-chromium, titanium, and gold alloy cast alloy cast denture clasp Vallittu, P.K. and Kokkoneu, K. J. Prasthet. Dent. 11995) 74 (4), 412-419 The aim was to determine the fatigue resistance of the cast clasps of removable partial dentures. The different commercial types of metals used included five cobalt--chromium alloys, pure titanium, one Ti alloy (Ti~AI~4V) and one gold alloy (type IV) that was either unhardened or age-hardened (n = 5 per group). The test method used was a constant-deflection fatigue test in which the force required to deflect the clasp for 0.6 mm and the number of loading cycles required to fracture the clasp were determined. The fatigue fracture surface of the clasps was examined with the scanning electron microscope. The results revealed that a fatigue fracture occurred in the Co-Cr clasp after approx 25 000 loading cycles, in the pure Ti clasps after 4500 loading cycles, in the Ti alloy clasp after 20000 loading cycles, and in the Au alloy clasp after 21 000 loading cycles. The means differed significantly (p < 0.001). Activation of the clasp by bending it 0.5 mm increased the fatigue resistance of the Co~Sr alloy and Au alloy clasps but decreased the fatigue resistance