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The use of SPATE to measure residual stresses and fatigue crack growth
Fracture mechanisms and life assessment under high-strein biaxial cyclic loading of Type 304 stainless steel. Nitta, A., Ogata, T. and Kuwabara, K. Fatigue Fract. Eng. Mater. Struct. 1989 12, (2), 77-92 In order to investigate the elevated-temperature low-cycle fatigue characteristics of Type 304 stainless steel under biaxial loading, strain-controlled push-pull and torsional fatigue tests were conducted at 550°C under proportional and nonproportional loading conditions with phase differences of 0, 1T/6, -~/4, ~/3 and w/2 between the applied sinusoidal axial and torsional strains. In the case of proportional loading, the fracture mode was found, from fractographic observations, to be classified into either Mode I or Mode II, while the fractures were of mixed mode under non-proportional loading. However, fatigue lives under both the loading conditions could be correlated well in this study with the equivalent shear strain range, ~ , defined by extending the Gamma-plane theory of Brown and Miller. The fatigue life under non-proportional loading could also be predicted by applying a strain-energy-based approach. Graphs. Photomicrographs. 20 refs.
The influence of test variables on the fatigue crack growth threshold. Priddle, E.K. Fatigue Fract. Eng. Mater. Struct. 1989 12, (4), 333-345 A microcomputer controlled fatigue crack growth and threshold testing system has been used to investigate the influence of test variables on the measured values of •~Kth, the threshold for fatigue crack growth, using a C-Mn steel (eg EN3A). The work has examined: (1) the influence of crack length and test management; (2) the basic material scatter from repeated testing; (3) the effect of unloading rate C where C = (1/AK)(dAK/da); (4) the effect of step unloading; and (5) the influence of minimum stress intensity factor, Krnin. Comparisons have been made between the results of this computer controlled work and those published previously but made using a manual load shedding technique. The results of ~'Kth and fatigue crack growth rates are in general agreement with previous data and confirm the Kmin dependence of AKth and da/dn. The value of ~Kth is shown to be generally independent of the other test variables for a wide range of conditions and is reproducible with a low degree of scatter. Graphs. 18 refs.
paid to crack growth curve transitions in the near-threshold regime. These transitions correspond to monotonic or cyclic plane strain plastic zone dimensions becoming equal to characteristic microstructural dimensions. Changes in fracture surface topography are associated with the transitions also. From the observations an explanation of the shape of the low stress intensity fatigue crack growth curve and the transitions is given.
Fatigue crack initiation associated with inclusions in a low-carbon steel. Singh, V.K. J. Mater. Sci. Left. Apr. 1989 8, (4), 427-429 The chemical composition of the relatively ductile steel is Fe-0.14C-0.51Mn-0.16Si-0.0285S-0.026P. Rotating bending fatigue tests were carried out at a frequency of 50 Hz in air at room temperature. Although crack initiation occasionally occurred at inclusions, many crack initiation sites were not associated with inclusions. The occurrence of cleavage facets in some areas of the fracture surfaces in the neighborhood of inclusions is an indication of embrittlement which may have resulted from impurity segregation. Although it is generally accepted that inclusions are more deleterious to fatigue performance of the harder microstructures, the investigation indicates that softer microstructures are vulnerable, perhaps to a lesser extent, to such damaging effects. Graphs. Photomicrographs. 11 refs.
Fatigue crack closure in the near-threshold level in a high-strength lowalloy structural steel. Dhar, S. Theor. Appl. Fract. Mech. Apr. 1989 11, (1), 21-25 Fatigue crack closure is measured by a back face strain gauge for a high-strength structural steel SANH-55. In the near-threshold region, Kop data are found to be consistent with either oxide-induced closure or roughness-induced closure, In the linear regime of the Paris plot, Kop is roughly constant, independent of ~K and the R-ratio. The contribution of the oxide-induced crack closure can be deleted by Kmax constant and ~K decreasing tests in near threshold, in which case plasticity-induced crack closure plays an important role. Graphs. 7 refs,
I n i t i a t i o n and propagation of short fatigue cracks in a weld metal. Hong,
Y., Lu, Y. and Zheng, Z. Fatigue Fract. Eng. Mater. Sfruct. 1989 12, (4), 323-331 Fatigue tests were performed using a purpose designed triangular shaped specimen (eg low carbon steel) to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage, resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed. Photomicrographs. 7 refs.
Study on fatigue threshold behaviour and fatigue crack propagation in a cast C o - C r - M o a l l o y used for surgical implants. Zhuang, L.Z. and Langer, E.W. Fatigue Fract. Eng. Mater. Struct. 1989 12, (4), 283-293 Fatigue crack propagation rates (da/dN) and fatigue crack thresholds (~,K~h) have been studied in a cast Co-Cr-Mo alloy used for surgical implants with various grain sizes. Results for materials with average grain sizes of approx 400 and 60 p-m respectively are presented. Threshold values close to 10-15 MPa ~/m have been measured, with decreasing values observed on increasing the grain size. Similar effects of grain size are found on the crack propagation behaviour at higher growth rates, where a coarse grain size material shows a higher crack growth rate than a fine grain size material at the same ~K levels. The effects of microstructure on fatigue properties of the cast Co-Cr-Mo alloy are caused not only by grain size variation but are also attributed to the microstructural differences: a coarse-grained material with a directionally grown dendritic structure vs a fine-grained material with an equiaxed grain structure. Graphs. Photomicrographs. 43 refs.
The use of SPATE to measure residual stresses and fatigue crack l r o w t h . Leaity, G.P. and Smith, R.A. tigue Fract. Eng. Mater. Struct. 1989 12, (4), 271-282
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SPATE was used to monitor thermoemissions during fatigue crack propagation tests conducted on centre-cracked tension {CCT) specimens (eg SS4360Gr.50B) at different mean stress levels, and on CCT specimens containing compressive residual stresses. The results obtained showed the SPATE stress intensity factor (SIF) predictions to correlate with the applied SIF range rather than the effective SIF range, thus demonstrating that SPATE was unable to account for the effects of mean stress and residual stress on fatigue crack growth. Further, although SPATE can be used to predict fatigue crack growth rates faster than 10 8 m/cycle, in the region of low growth rates slower than this, the prediction of growth rates became impractical. Even at maximum resolution and scan time, the technique was unable to detect the changes in thermoemissions caused by such small and slow crack tip advances. Graphs. 9 refs.
Fatigue evolution o f small t w o - and three-dimensional cracks under variable and constant a m p l i t u d e loading. Final report,
Ecole National Superieure des Mines de Paris Report No N89-14471/1/ XAB 9 Sept. 1988 (in French) Load tests were carried out using two AI alloys with different tensile behaviour, 2124T351 and 7475T7351. Loading was either monotonic or including overloading. The parameters governing crack propagation are identified as related to the opening k value. The difficulties related to the practical determination of this parameter are discussed. The effects related to surcharges are attributed to the plasticity of the materials.
Low stress intensity fatigue crack growth in 2024-T3 and T351. Wanhill, R.J.H. National Aerospace Laboratory (Netherlands) Report No PB89-146013/ XAB 30 Mar. 1987 Constant amplitude fatigue crack growth properties of five batches of AI alloy 2024 in the naturally aged T3 and T351 conditions are compared. Particular attention is
454
A comparison of fatigue crack growth of three alloy steels at elevated temperature. Cortie, M.B. and Garrett, G.G. Theor. AppL Fract. Mech. Apt. 1989 11, (1), 9-19 Fatigue cracks were grown in alloy steels by the use of a triangular loading waveform with a frequency of 1 Hz and an R-ratio of 0,1, and the crack growth rates were determined. The steel alloys Fe-ICr-0.hMo and Fe-l.hMn-0.8Ni-0.hMo (SA508) were tested at 450°C, and Fe-lCr-0.hMo and Fe-0.5Cr-0,5Mo-0.25V at 550°C, Statistical treatment of the results indicated that, over the range of alternating stress intensities applied, the fatigue crack growth rates were very similar for each pair of alloys tested. The use of the Paris law to describe the rates of fatigue crack growth would have led to an erroneous conclusion in this regard, and a sigmoidal descriptive model is therefore proposed for the growth of fatigue cracks. With the use of a microcomputer, this model is not much more difficult to apply than the Paris law, and gives a far better description of the results. Finally, a statistical method for the comparison of sets of data on fatigue crack growth is presented and demonstrated. Graphs. Photomicrographs. 31 refs.
Increasing the fatigue life of cracked fastener holes using bonded repairs. Heller, M., Hill, T.G., Williams, J.F. and Jones, R. Theor. Appl. FracL Mech. Apr. 1989 11, (1), 1-8 Recent experimental work has shown that the use of a suitable bonded insert can significantly reduce the rate of fatigue crack propagation for initially uncracked geometries, and that this approach may be superior to other life enhancement schemes. Some initial experimental work on three-dimensional cracked geometries was also reported which demonstrated that significant fatigue life extension could be achieved for this type of problem. The fatigue testing of specimens of AI base alloy 2024 containing three-dimensional cracks at the edge of a fastener hole, repaired with a bonded sleeve and/or a bonded composite patch is presented. It is shown that significant increases in fatigue life can be achieved using a bonded sleeve alone. However, the additional use of an externally bonded patch produces an increase in the fatigue life of the specimens of between one to two orders of magnitude. Graphs. 9 refs.
P-Nf diagram and fatigue-creck initiation and propagation of notched 5083-0 a l u m i n i u m alloy. M u r a m a t s u , K J. J p n /nst. Light Met. Feb. 1989 39, (2), 116-122 (in Japanese) Fatigue test was performed by using rotating-bending machines on the notched 5083-0 extruded alloy to investigate the relation between the P-Nf diagram and the fracture surface morphology in crack initiation and propagation. The relation between the length Li (turn) of the crack initiation and the stress amplitude S (kgf/mm 2) can be written in the form L i - 28.8/S2. When the length of the microcrack which propagates at the notch root surface ranges Li and the crack front forms a semiellipse in which the long axis is Li and the aspect ratio is approx 0.3, the microcrack becomes a crack. The fracture surface morphology in crack initiation is classified into three types, and is related to the P-Nf diagram. When two or more main cracks originate at the notch root surface, the fatigue tife is controlled by crack propagation. When only one main crack originates, or when the main crack originates in the subsurface region, the fatigue life is controlled by crack initiation. Graphs. Photomicrographs. 7 refs.
The influence on fatigue behaviour of a cellular structure formed during nitriding. De~in, G., Zhiwei, Y., Juncai, S. and Lieyu, Y. Scr. Metal/. Apr. 1989 23, (4), 569-571 (in French) An Fe-base alloy containing 25% Ni, 15% Cr, 1.3% Mo and 1.65% Ti was solution treated, aged and then nitrided. Specimens were incrementally loaded to failure. The microstructures showed a nitrogen rich layer up to 50 i~m deep and a cellular transition layer. This transition layer does not modify the fatigue behaviour of the alloy. Photomicrographs. Graphs. 5 refs.
Int J Fatigue November 1989
The influence of test variables on the fatigue crack growth threshold. Priddle, E.K. Fatigue Fract. Eng. Mater. Struct. 1989 12, (4), 333-345 A microcomputer controlled fatigue crack growth and threshold testing system has been used to investigate the influence of test variables on the measured values of •~Kth, the threshold for fatigue crack growth, using a C-Mn steel (eg EN3A). The work has examined: (1) the influence of crack length and test management; (2) the basic material scatter from repeated testing; (3) the effect of unloading rate C where C = (1/AK)(dAK/da); (4) the effect of step unloading; and (5) the influence of minimum stress intensity factor, Krnin. Comparisons have been made between the results of this computer controlled work and those published previously but made using a manual load shedding technique. The results of ~'Kth and fatigue crack growth rates are in general agreement with previous data and confirm the Kmin dependence of AKth and da/dn. The value of ~Kth is shown to be generally independent of the other test variables for a wide range of conditions and is reproducible with a low degree of scatter. Graphs. 18 refs.
paid to crack growth curve transitions in the near-threshold regime. These transitions correspond to monotonic or cyclic plane strain plastic zone dimensions becoming equal to characteristic microstructural dimensions. Changes in fracture surface topography are associated with the transitions also. From the observations an explanation of the shape of the low stress intensity fatigue crack growth curve and the transitions is given.
Fatigue crack initiation associated with inclusions in a low-carbon steel. Singh, V.K. J. Mater. Sci. Left. Apr. 1989 8, (4), 427-429 The chemical composition of the relatively ductile steel is Fe-0.14C-0.51Mn-0.16Si-0.0285S-0.026P. Rotating bending fatigue tests were carried out at a frequency of 50 Hz in air at room temperature. Although crack initiation occasionally occurred at inclusions, many crack initiation sites were not associated with inclusions. The occurrence of cleavage facets in some areas of the fracture surfaces in the neighborhood of inclusions is an indication of embrittlement which may have resulted from impurity segregation. Although it is generally accepted that inclusions are more deleterious to fatigue performance of the harder microstructures, the investigation indicates that softer microstructures are vulnerable, perhaps to a lesser extent, to such damaging effects. Graphs. Photomicrographs. 11 refs.
Fatigue crack closure in the near-threshold level in a high-strength lowalloy structural steel. Dhar, S. Theor. Appl. Fract. Mech. Apr. 1989 11, (1), 21-25 Fatigue crack closure is measured by a back face strain gauge for a high-strength structural steel SANH-55. In the near-threshold region, Kop data are found to be consistent with either oxide-induced closure or roughness-induced closure, In the linear regime of the Paris plot, Kop is roughly constant, independent of ~K and the R-ratio. The contribution of the oxide-induced crack closure can be deleted by Kmax constant and ~K decreasing tests in near threshold, in which case plasticity-induced crack closure plays an important role. Graphs. 7 refs,
I n i t i a t i o n and propagation of short fatigue cracks in a weld metal. Hong,
Y., Lu, Y. and Zheng, Z. Fatigue Fract. Eng. Mater. Sfruct. 1989 12, (4), 323-331 Fatigue tests were performed using a purpose designed triangular shaped specimen (eg low carbon steel) to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage, resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed. Photomicrographs. 7 refs.
Study on fatigue threshold behaviour and fatigue crack propagation in a cast C o - C r - M o a l l o y used for surgical implants. Zhuang, L.Z. and Langer, E.W. Fatigue Fract. Eng. Mater. Struct. 1989 12, (4), 283-293 Fatigue crack propagation rates (da/dN) and fatigue crack thresholds (~,K~h) have been studied in a cast Co-Cr-Mo alloy used for surgical implants with various grain sizes. Results for materials with average grain sizes of approx 400 and 60 p-m respectively are presented. Threshold values close to 10-15 MPa ~/m have been measured, with decreasing values observed on increasing the grain size. Similar effects of grain size are found on the crack propagation behaviour at higher growth rates, where a coarse grain size material shows a higher crack growth rate than a fine grain size material at the same ~K levels. The effects of microstructure on fatigue properties of the cast Co-Cr-Mo alloy are caused not only by grain size variation but are also attributed to the microstructural differences: a coarse-grained material with a directionally grown dendritic structure vs a fine-grained material with an equiaxed grain structure. Graphs. Photomicrographs. 43 refs.
The use of SPATE to measure residual stresses and fatigue crack l r o w t h . Leaity, G.P. and Smith, R.A. tigue Fract. Eng. Mater. Struct. 1989 12, (4), 271-282
~
SPATE was used to monitor thermoemissions during fatigue crack propagation tests conducted on centre-cracked tension {CCT) specimens (eg SS4360Gr.50B) at different mean stress levels, and on CCT specimens containing compressive residual stresses. The results obtained showed the SPATE stress intensity factor (SIF) predictions to correlate with the applied SIF range rather than the effective SIF range, thus demonstrating that SPATE was unable to account for the effects of mean stress and residual stress on fatigue crack growth. Further, although SPATE can be used to predict fatigue crack growth rates faster than 10 8 m/cycle, in the region of low growth rates slower than this, the prediction of growth rates became impractical. Even at maximum resolution and scan time, the technique was unable to detect the changes in thermoemissions caused by such small and slow crack tip advances. Graphs. 9 refs.
Fatigue evolution o f small t w o - and three-dimensional cracks under variable and constant a m p l i t u d e loading. Final report,
Ecole National Superieure des Mines de Paris Report No N89-14471/1/ XAB 9 Sept. 1988 (in French) Load tests were carried out using two AI alloys with different tensile behaviour, 2124T351 and 7475T7351. Loading was either monotonic or including overloading. The parameters governing crack propagation are identified as related to the opening k value. The difficulties related to the practical determination of this parameter are discussed. The effects related to surcharges are attributed to the plasticity of the materials.
Low stress intensity fatigue crack growth in 2024-T3 and T351. Wanhill, R.J.H. National Aerospace Laboratory (Netherlands) Report No PB89-146013/ XAB 30 Mar. 1987 Constant amplitude fatigue crack growth properties of five batches of AI alloy 2024 in the naturally aged T3 and T351 conditions are compared. Particular attention is
454
A comparison of fatigue crack growth of three alloy steels at elevated temperature. Cortie, M.B. and Garrett, G.G. Theor. AppL Fract. Mech. Apt. 1989 11, (1), 9-19 Fatigue cracks were grown in alloy steels by the use of a triangular loading waveform with a frequency of 1 Hz and an R-ratio of 0,1, and the crack growth rates were determined. The steel alloys Fe-ICr-0.hMo and Fe-l.hMn-0.8Ni-0.hMo (SA508) were tested at 450°C, and Fe-lCr-0.hMo and Fe-0.5Cr-0,5Mo-0.25V at 550°C, Statistical treatment of the results indicated that, over the range of alternating stress intensities applied, the fatigue crack growth rates were very similar for each pair of alloys tested. The use of the Paris law to describe the rates of fatigue crack growth would have led to an erroneous conclusion in this regard, and a sigmoidal descriptive model is therefore proposed for the growth of fatigue cracks. With the use of a microcomputer, this model is not much more difficult to apply than the Paris law, and gives a far better description of the results. Finally, a statistical method for the comparison of sets of data on fatigue crack growth is presented and demonstrated. Graphs. Photomicrographs. 31 refs.
Increasing the fatigue life of cracked fastener holes using bonded repairs. Heller, M., Hill, T.G., Williams, J.F. and Jones, R. Theor. Appl. FracL Mech. Apr. 1989 11, (1), 1-8 Recent experimental work has shown that the use of a suitable bonded insert can significantly reduce the rate of fatigue crack propagation for initially uncracked geometries, and that this approach may be superior to other life enhancement schemes. Some initial experimental work on three-dimensional cracked geometries was also reported which demonstrated that significant fatigue life extension could be achieved for this type of problem. The fatigue testing of specimens of AI base alloy 2024 containing three-dimensional cracks at the edge of a fastener hole, repaired with a bonded sleeve and/or a bonded composite patch is presented. It is shown that significant increases in fatigue life can be achieved using a bonded sleeve alone. However, the additional use of an externally bonded patch produces an increase in the fatigue life of the specimens of between one to two orders of magnitude. Graphs. 9 refs.
P-Nf diagram and fatigue-creck initiation and propagation of notched 5083-0 a l u m i n i u m alloy. M u r a m a t s u , K J. J p n /nst. Light Met. Feb. 1989 39, (2), 116-122 (in Japanese) Fatigue test was performed by using rotating-bending machines on the notched 5083-0 extruded alloy to investigate the relation between the P-Nf diagram and the fracture surface morphology in crack initiation and propagation. The relation between the length Li (turn) of the crack initiation and the stress amplitude S (kgf/mm 2) can be written in the form L i - 28.8/S2. When the length of the microcrack which propagates at the notch root surface ranges Li and the crack front forms a semiellipse in which the long axis is Li and the aspect ratio is approx 0.3, the microcrack becomes a crack. The fracture surface morphology in crack initiation is classified into three types, and is related to the P-Nf diagram. When two or more main cracks originate at the notch root surface, the fatigue tife is controlled by crack propagation. When only one main crack originates, or when the main crack originates in the subsurface region, the fatigue life is controlled by crack initiation. Graphs. Photomicrographs. 7 refs.
The influence on fatigue behaviour of a cellular structure formed during nitriding. De~in, G., Zhiwei, Y., Juncai, S. and Lieyu, Y. Scr. Metal/. Apr. 1989 23, (4), 569-571 (in French) An Fe-base alloy containing 25% Ni, 15% Cr, 1.3% Mo and 1.65% Ti was solution treated, aged and then nitrided. Specimens were incrementally loaded to failure. The microstructures showed a nitrogen rich layer up to 50 i~m deep and a cellular transition layer. This transition layer does not modify the fatigue behaviour of the alloy. Photomicrographs. Graphs. 5 refs.
Int J Fatigue November 1989