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Correlation of fatigue crack growth behavior with crack closure in peened specimens
Fatigue Abstracts A procedure to minimize error in fatigue crack growth testing Oore, M. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 905-915 A concept for minimizing the error associated with the K gradient and the interval of crack length measurement in fatigue crack growth testing has been extended to include crack configurations, where crack size is significant relative to specimen width. This was used to develop a procedure for deriving the da/dn vs A K relationship from crack growth data. The procedure was applied to several test data, utilizing six measurement points of length vs cycles data and experimenting with various sizes of crack measurement interval (up to 85% of crack size). The accuracy of the derived da/dn vs AK data was examined by comparison with the baseline data curves. The results were within a few percent of the actual test data and were usually more accurate than those obtained by secant methods. Increasing the number of data points used in the analysis generally improved the accuracy of the present procedure more than it did with the standard secant method results. 7075 and 2024 are mentioned, as is Ti-6A1-4V. Graphs, 8 ref.
Dislocation structures near the fatigue crack tip in copper Tong, Z.-X. and Bailon, J.-P. Fatigue Fract. Engng. Mater. Struct. (July-August 1995) 18 (7-8), 847-859 The dislocation structures formed in the plastic wake of a long fatigue crack have been characterized by TEM for polycrystalline copper tested in air and in vacuum. In order of increasing distance D from the fracture surface, typical structures are cells, walls, PSBs and veins, and finally loop patches and dislocation tangles. The outer boundaries of these different regions depend on the stress intensity factor range A K. By comparison with the dislocation structures developed in LCF specimens fatigued at a constant strain amplitude, the structures in the plastic wake appear greatly distorted and more than one set of slip systems are often found to be activated. This feature is the consequence of the complex stress history occurring in a unit volume of material located at a distance D from the fracture surface. Due to this fact, the plastic strain gradient deduced from the size of dislocation cells can only be approximated. The dislocation structures formed near the fatigue crack tip in vacuum appear to be somewhat different from those formed in air; the cell boundaries are much denser, sharper and straighter, the misorientations betwen them are more marked and, at a given distance D and for a given applied A K, the cell size is also smaller than that formed in air. These effects are the consequence of the larger number of cycles that a unit volume of material located at a given distance D can sustain during fatigue cracking in vacuum. Graphs, photomicrographs, 27 ref.
A new approach to fatigue threshold Wu, X.J. and Wallace, W. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 833-845 Conventionally, the fatigue threshold of a long-crack is obtained by load shedding using a constant normalized K-gradient, as recommended by ASTM E-647. However, this load shedding procedure often causes the crack opening displacement to decrease with increasing crack length, which may trigger crack closure. In this study, load shedding tests were conducted in load control following a power-law load shedding schedule such that the crack opening displacement was kept at a relatively constant level. Using this new testing procedure, it is shown that crack closure is not always as high as that associated with the ASTM recommended procedure at a given A K. Comparisons of fatigue crack growth rates under identical testing parameters, but with different closure levels produced by the two load shedding procedures, have been made for several structural alloys. The extrinsic shielding of the crack tip zone via crack closure has also been examined using an energy approach for these alloys. On the basis of these analyses, the true effective stress intensity range is evaluated for fatigue crack propagation and the role of crack closure in creating a fatigue threshold is re-assessed. 7075, 8090, 316SS, and IN718 are mentioned. Graphs, photomicrographs, 30 ref.
A probabilistic model for initiation and propagation of surface cracks in welded joints Lecsek, R.L., Yee, R., Lambert, S.B. and Burns, D.J. Fatigue Fract. Engng. Mater. Struct. (July-August 1995) 18 (7-8), 821-831 Monte Carlo simulations of fatigue in welded joints have been performed using an approach which combines a model for initiation with a multiple surface crack propagation model. The results have been compared with experiments on T-plate welded joints in which the initiation and propagation of surface cracks were monitored using potential drop techniques. Predictions of initiation life using a local strain approach were conservative. Despite this underprediction of initiation life, predictions of total fatigue life were very good as a consequence of accurate simulations of propagation life and the fact that the initiation represented on average only 12 to 22% of total life, depending on stress level. The initiation model considered variability due to the local weld toe angle and radius, and material strain-life behaviour. The only variability considered in the propagation analysis was the position and timing of initiation events, which leads to variability in coalescence. The underprediction of the variability in propagation and total lives was attributed to the underprediction of initiation life and the fact that out-of-plane coalescence effects were ignored. Graphs, 15 ref.
509
Correlation of fatigue crack growth behavior with crack closure in peened specimens Zhu, X.Y. and Shaw, W.J.D. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 811-820 Various residual stress conditions were introduced into the surface of a 7075T6 aluminium alloy by shot peening. This resulted in a considerable alteration in subsequent fatigue crack growth behavior. A systematic investigation into the effects of a variety of conditions on fatigue crack propagation behavior was conducted. The degree of shot peening was established using the Almen scale and calibration strips. Changes occurred in crack re-initiation lives, crack growth rate and hardness across the thickness profile. Residual stresses were introduced by various peening operations (such as different peening positions and loading specimens prior to and during peening). By measuring the crack opening level of peened specimens, changes in the fatigue crack closure effect with respect to the crack extension in CT specimens are reported. The relations of re-initiation life vs crack opening level and Almen levels, and of the crack closure effect vs Almen scales, were addressed experimentally. It was found that the improvement in re-initiation life and the retarding effect of crack growth, both influenced by the Almen scales, are associated directly with the elevation of crack opening level, particularly at the stage of crack re-initiation. Graphs, photomicrographs, 21 ref.
Microstructural effects on the small fatigue crack behavior of an aluminum alloy plate Zabett, A. and Plumtree, A. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 801-809 A study has been conducted on the initiation and growth of fatigue cracks in the three principal directions of an aluminium alloy 2024-T351 plate tested under stress control (R, minimum to maximum stress =-1). Early and multiple fatigue crack nucleation from broken A17Cu2Fe second phase particles resulted in shorter lives for the longitudinal direction specimens in the medium to long life regime. Although fatigue cracks nucleated in large surface grains, rather than at broken particles, the lives of the short-transverse direction specimens were marginally longer. Cracks also nucleated in large surface grains in the transverse direction specimens, yet the average fatigue lives were about twice as long. This was the consequence of wider slip bands and fewer initial microcracks. Graphs, photomicrographs, 21 ref.
Effect of re-coldworking on fatigue life enhancement of a fastener hole Bernard, M., Bui-Quoc, T. and Burlat, M. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 765-775 The aim is to assess the additional fatigue life enhancement obtained by coldworking a previously cold expanded plate hole. Two different methods of performing the second coldworking were considered, i.e. moving the mandrel in the same direction as for the first coldworking or moving it in the opposite direction. A three-dimensional finite element analysis for establishing the residual stress field induced by two successive coldworkings (5.58% then 4.8%) was carried out. A17475-T7351 specimens with a central hole were cold expanded at 5.58%, subjected to cyclic loading at constant amplitude for a predetermined life fraction (on the basis of 5.58% cold expanded hole fatigue life) and then re-coldworked at 4.8%. After this rework, the specimens were again subjected to the same fatigue loading conditions until failure. During cycling, the fatigue crack extension was monitored using a video-camera in order to determine the coldworking effect on both the initiation period and the propagation life. The fatigue test results have shown that a second coldworking may enhance the fatigue life of an already coldworked hole. For a given cyclic loading level, the beneficial effect depends upon the applied life fraction before reworking. The direction of the mandrel movement during the second coldworking has no noticeable influence on the additional fatigue life improvement. These experimental results have been analysed with reference to the calculated residual stress field. Graphs, 12 ref.
Mean stress effects in muitiaxial fatigue Glinka, G., Wang, G. and Plumtree, A. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 755-764 A multiaxial fatigue strain energy density parameter has been formulated which normalizes fatigue data obtained under a variety of mean stress levels and loading combinations. This parameter represents that proportion of the overall strain energy contributed by the stresses and strains on the critical or fracture plane. It is shown that multiaxial fatigue life data may be accurately correlated by applying this parameter to the experimental results of Inconel 718 alloy subjected to a variety of mean normal and shear stress levels, and to SAE 1045 steel tested under tension, torsion and simultaneous tension and torsion. Graphs, 17 ref.
Fatigue life predictions for notched AI-2.5Mg alloy in corrosive environment Gasem, Z. and Khan, Z. Mater. Sci. Technol. (1995) 11 (2), 159-162 The use of empirically developed procedures in fatigue life predictions is current practice for notched members under the joint action of mechanical loads and a corrosive environment. A simple model has been investigated in which the total fatigue life of a notched member is considered to be the sum of the crack initiation and crack propagation lives. The local strain approach has been employed for the crack initiation life estimates. A linear elastic fracture mechanics concept using the Paris relationship was used for fatigue crack propagation life estimates. It is shown that the influence of corrosive environment on fatigue life can be incorporated by determining the relevant
Dislocation structures near the fatigue crack tip in copper Tong, Z.-X. and Bailon, J.-P. Fatigue Fract. Engng. Mater. Struct. (July-August 1995) 18 (7-8), 847-859 The dislocation structures formed in the plastic wake of a long fatigue crack have been characterized by TEM for polycrystalline copper tested in air and in vacuum. In order of increasing distance D from the fracture surface, typical structures are cells, walls, PSBs and veins, and finally loop patches and dislocation tangles. The outer boundaries of these different regions depend on the stress intensity factor range A K. By comparison with the dislocation structures developed in LCF specimens fatigued at a constant strain amplitude, the structures in the plastic wake appear greatly distorted and more than one set of slip systems are often found to be activated. This feature is the consequence of the complex stress history occurring in a unit volume of material located at a distance D from the fracture surface. Due to this fact, the plastic strain gradient deduced from the size of dislocation cells can only be approximated. The dislocation structures formed near the fatigue crack tip in vacuum appear to be somewhat different from those formed in air; the cell boundaries are much denser, sharper and straighter, the misorientations betwen them are more marked and, at a given distance D and for a given applied A K, the cell size is also smaller than that formed in air. These effects are the consequence of the larger number of cycles that a unit volume of material located at a given distance D can sustain during fatigue cracking in vacuum. Graphs, photomicrographs, 27 ref.
A new approach to fatigue threshold Wu, X.J. and Wallace, W. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 833-845 Conventionally, the fatigue threshold of a long-crack is obtained by load shedding using a constant normalized K-gradient, as recommended by ASTM E-647. However, this load shedding procedure often causes the crack opening displacement to decrease with increasing crack length, which may trigger crack closure. In this study, load shedding tests were conducted in load control following a power-law load shedding schedule such that the crack opening displacement was kept at a relatively constant level. Using this new testing procedure, it is shown that crack closure is not always as high as that associated with the ASTM recommended procedure at a given A K. Comparisons of fatigue crack growth rates under identical testing parameters, but with different closure levels produced by the two load shedding procedures, have been made for several structural alloys. The extrinsic shielding of the crack tip zone via crack closure has also been examined using an energy approach for these alloys. On the basis of these analyses, the true effective stress intensity range is evaluated for fatigue crack propagation and the role of crack closure in creating a fatigue threshold is re-assessed. 7075, 8090, 316SS, and IN718 are mentioned. Graphs, photomicrographs, 30 ref.
A probabilistic model for initiation and propagation of surface cracks in welded joints Lecsek, R.L., Yee, R., Lambert, S.B. and Burns, D.J. Fatigue Fract. Engng. Mater. Struct. (July-August 1995) 18 (7-8), 821-831 Monte Carlo simulations of fatigue in welded joints have been performed using an approach which combines a model for initiation with a multiple surface crack propagation model. The results have been compared with experiments on T-plate welded joints in which the initiation and propagation of surface cracks were monitored using potential drop techniques. Predictions of initiation life using a local strain approach were conservative. Despite this underprediction of initiation life, predictions of total fatigue life were very good as a consequence of accurate simulations of propagation life and the fact that the initiation represented on average only 12 to 22% of total life, depending on stress level. The initiation model considered variability due to the local weld toe angle and radius, and material strain-life behaviour. The only variability considered in the propagation analysis was the position and timing of initiation events, which leads to variability in coalescence. The underprediction of the variability in propagation and total lives was attributed to the underprediction of initiation life and the fact that out-of-plane coalescence effects were ignored. Graphs, 15 ref.
509
Correlation of fatigue crack growth behavior with crack closure in peened specimens Zhu, X.Y. and Shaw, W.J.D. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 811-820 Various residual stress conditions were introduced into the surface of a 7075T6 aluminium alloy by shot peening. This resulted in a considerable alteration in subsequent fatigue crack growth behavior. A systematic investigation into the effects of a variety of conditions on fatigue crack propagation behavior was conducted. The degree of shot peening was established using the Almen scale and calibration strips. Changes occurred in crack re-initiation lives, crack growth rate and hardness across the thickness profile. Residual stresses were introduced by various peening operations (such as different peening positions and loading specimens prior to and during peening). By measuring the crack opening level of peened specimens, changes in the fatigue crack closure effect with respect to the crack extension in CT specimens are reported. The relations of re-initiation life vs crack opening level and Almen levels, and of the crack closure effect vs Almen scales, were addressed experimentally. It was found that the improvement in re-initiation life and the retarding effect of crack growth, both influenced by the Almen scales, are associated directly with the elevation of crack opening level, particularly at the stage of crack re-initiation. Graphs, photomicrographs, 21 ref.
Microstructural effects on the small fatigue crack behavior of an aluminum alloy plate Zabett, A. and Plumtree, A. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 801-809 A study has been conducted on the initiation and growth of fatigue cracks in the three principal directions of an aluminium alloy 2024-T351 plate tested under stress control (R, minimum to maximum stress =-1). Early and multiple fatigue crack nucleation from broken A17Cu2Fe second phase particles resulted in shorter lives for the longitudinal direction specimens in the medium to long life regime. Although fatigue cracks nucleated in large surface grains, rather than at broken particles, the lives of the short-transverse direction specimens were marginally longer. Cracks also nucleated in large surface grains in the transverse direction specimens, yet the average fatigue lives were about twice as long. This was the consequence of wider slip bands and fewer initial microcracks. Graphs, photomicrographs, 21 ref.
Effect of re-coldworking on fatigue life enhancement of a fastener hole Bernard, M., Bui-Quoc, T. and Burlat, M. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 765-775 The aim is to assess the additional fatigue life enhancement obtained by coldworking a previously cold expanded plate hole. Two different methods of performing the second coldworking were considered, i.e. moving the mandrel in the same direction as for the first coldworking or moving it in the opposite direction. A three-dimensional finite element analysis for establishing the residual stress field induced by two successive coldworkings (5.58% then 4.8%) was carried out. A17475-T7351 specimens with a central hole were cold expanded at 5.58%, subjected to cyclic loading at constant amplitude for a predetermined life fraction (on the basis of 5.58% cold expanded hole fatigue life) and then re-coldworked at 4.8%. After this rework, the specimens were again subjected to the same fatigue loading conditions until failure. During cycling, the fatigue crack extension was monitored using a video-camera in order to determine the coldworking effect on both the initiation period and the propagation life. The fatigue test results have shown that a second coldworking may enhance the fatigue life of an already coldworked hole. For a given cyclic loading level, the beneficial effect depends upon the applied life fraction before reworking. The direction of the mandrel movement during the second coldworking has no noticeable influence on the additional fatigue life improvement. These experimental results have been analysed with reference to the calculated residual stress field. Graphs, 12 ref.
Mean stress effects in muitiaxial fatigue Glinka, G., Wang, G. and Plumtree, A. Fatigue Fract. Engng. Mater. Struct. (1995) 18 (7-8), 755-764 A multiaxial fatigue strain energy density parameter has been formulated which normalizes fatigue data obtained under a variety of mean stress levels and loading combinations. This parameter represents that proportion of the overall strain energy contributed by the stresses and strains on the critical or fracture plane. It is shown that multiaxial fatigue life data may be accurately correlated by applying this parameter to the experimental results of Inconel 718 alloy subjected to a variety of mean normal and shear stress levels, and to SAE 1045 steel tested under tension, torsion and simultaneous tension and torsion. Graphs, 17 ref.
Fatigue life predictions for notched AI-2.5Mg alloy in corrosive environment Gasem, Z. and Khan, Z. Mater. Sci. Technol. (1995) 11 (2), 159-162 The use of empirically developed procedures in fatigue life predictions is current practice for notched members under the joint action of mechanical loads and a corrosive environment. A simple model has been investigated in which the total fatigue life of a notched member is considered to be the sum of the crack initiation and crack propagation lives. The local strain approach has been employed for the crack initiation life estimates. A linear elastic fracture mechanics concept using the Paris relationship was used for fatigue crack propagation life estimates. It is shown that the influence of corrosive environment on fatigue life can be incorporated by determining the relevant