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R O Ritchie - One of the best experts on this subject based on the ideXlab platform.
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the true toughness of human cortical bone measured with realistically Short Cracks
Nature Materials, 2008Co-Authors: Kurt J Koester, R O Ritchie, J W AgerAbstract:Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behaviour of long Cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behaviour of human cortical bone that accurately assesses its toughness at appropriate size scales. Here, we use in situ mechanical testing to examine how physiologically pertinent Short (<600 μm) Cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 μm of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction owing to major crack deflections/twists, principally at cement sheaths. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where Cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when Cracks become several millimetres in length that bridging mechanisms can fully develop leading to the (larger-crack) toughnesses generally quoted for bone. The toughness of human bone is difficult to measure, as it is more difficult to break than to split. It is now shown that in the transverse orientation, relevant for breaking, bone is much tougher than previously thought owing to a surprising increase in toughness during the growth of small Cracks.
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how tough is human cortical bone in situ measurements on realistically Short Cracks
Lawrence Berkeley National Laboratory, 2008Co-Authors: J W Ager, Kurt J Koester, R O RitchieAbstract:Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behavior of long Cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behavior of human cortical bone which accurately assesses its toughness at appropriate size-scales. Here we use in-situ mechanical testing in the scanning electron microscope and x-ray computed tomography to examine how physiologically-pertinent Short (<600 mu m) Cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 mu m of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction due to major crack deflections/twists principally at cement sheathes. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where Cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when Cracks become several millimeters in length that bridging mechanisms can develop leading to the (larger-crack) toughnesses generally quoted for bone.
John Martin - One of the best experts on this subject based on the ideXlab platform.
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Measuring Short Cracks by Time-Resolved Acoustic Microscopy
Advances in Acoustic Microscopy, 1995Co-Authors: Daniel Knauss, Tongguang Zhai, G. A. D. Briggs, John MartinAbstract:Detecting defects, for example Cracks, (1) is important in predicting the lifetime of a material. The growth behavior of Short Cracks(2) plays an essential role in the lifetime of a component, since the lifetime is mainly controlled by the time required for a crack to grow from a certain initial size to about 1 millimeter. Cracks are defined as Short when for example the crack length is small compared with the microstructure of the specimen or when the crack is simply Shorter than ≈0.5 mm.(3) The growth of Short surface breaking Cracks can be measured by light microscopy (LM) or scanning electron microscopy (SEM). A common method of studying Short Cracks is a replica technique based on taking several plastic replicas at various stages of crack growth and subsequently examining these replicas with LM or SEM.(4,5) The disadvantage of these techniques however, is that they give information about crack development only on the surface of the specimen, so that the depth of the crack has to be determined indirectly by assuming the shape of the crack. For long Cracks this may be appropriate because a local change in propagation direction does not alter the overall crack geometry on which the driving force of the crack depends.(6–8) However for Short Cracks, a change in propagation direction can alter the crack geometry significantly and thus change the driving force for the crack propagation. If its size is comparable with the microstructure of the material, a deflection of the crack at a grain boundary can alter the overall crack geometry. To understand the behavior of Short Cracks, it is therefore necessary to measure their three-dimensional growth. This can be achieved by using acoustic waves, which can penetrate into the material. In this way the crack depth can be measured directly.
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Depth Measurements of Short Cracks in Perspex with the Scanning Acoustic Microscope
Materials Characterization, 1993Co-Authors: Tongguang Zhai, D. D. Bennink, Daniel Knauss, G. A. D. Briggs, John MartinAbstract:The geometry, such as surface length, depth, shape and orientation, is one of the key factors that dominate the propagation behaviour of Short fatigue Cracks. Thus, for a quantitative understanding of Short crack growth behaviour, it is necessary to monitor the crack geometry throughout the fatigue test. However, a technique for the direct measurement of Short crack geometry has been lacking, which, together with the difficulty of Short crack detection, is one of the major reasons for the slow progress taking place in the study of Short fatigue Cracks. The growth of Short fatigue Cracks is commonly monitored only in its surface length with an optical or scanning electron microscope. The depth determination for the crack is then usually made on the basis of an empirical estimation from the crack surface length. For instance, the penny-shape or semi-elliptical estimation by assuming the profile of the crack as a semi-circle infers the depth of a Short crack to be half of its surface length1,2. Obviously, such an estimation cannot usually provide reliable data on the depth of Short Cracks in many materials, because of the strong dependence of the Short fatigue crack shape on local microstructural characteristics and environment.
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Depth measurement of Short Cracks with an acoustic microscope
Journal of Materials Science, 1993Co-Authors: Daniel Knauss, D. D. Bennink, Tongguang Zhai, G. A. D. Briggs, John MartinAbstract:The depth of Short Cracks (70–200 μm surface length) has been measured with an acoustic microscope by utilizing the nondestructive time-of-flight diffraction technique (TOFD). The depth measurements were first carried out in the transparent polymer polystyrene, thus allowing a comparison between the acoustical values and direct optical measurements: the agreement in the results was better than 95%. The depth of a 70 μm long crack in an aluminium alloy was then measured, demonstrating the application of the technique to metals.
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Time-resolved acoustic microscopy of Short Cracks
1993 Proceedings IEEE Ultrasonics Symposium, 1993Co-Authors: Daniel Knauss, Tongguang Zhai, G. A. D. Briggs, John MartinAbstract:Time-resolved acoustic microscopy has been used to measure the 3-dimensional profile of Short Cracks (70-200 mm surface length). In order to establish this technique the depth of Short Cracks in transparent materials were determined. This allows a comparison between the acoustic and direct optical measurements. Subsequently the depth of Cracks and the growth of Short fatigue Cracks in Al-alloy were measured
R Pippan - One of the best experts on this subject based on the ideXlab platform.
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the effect of single overloads in tension and compression on the fatigue crack propagation behaviour of Short Cracks
International Journal of Fatigue, 2016Co-Authors: Xiang Zhou, Hanspeter Gaenser, R PippanAbstract:Abstract The crack propagation behaviour in cast quenched and tempered steel after one overload cycle in tension as well as in compression on Short Cracks is investigated in deep notched specimens. The overload cycle exhibits a significant influence on the fatigue life endurance, due to the formation of an overload plastic zones in front of the crack tip. The crack propagation after overload cycles is investigated by inspection of the fatigue threshold R-curve and fatigue crack propagation rate. Tension overload increased the long crack threshold and reduced the R-curve effect, whereas overloads in compression reduced the crack growth resistance and shifted the threshold value to larger crack extension. A simple FE simulation was also performed to investigate the variation in the contribution of plasticity induced crack closure during crack propagation after the overload. Macroscopic mechanistic and dislocation models are introduced to explain the results obtained.
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modified nasgro equation for Short Cracks and application to the fitness for purpose assessment of surface treated components
Procedia Materials Science, 2014Co-Authors: Jurgen Maierhofer, R Pippan, Hanspeter GanserAbstract:Abstract Fatigue Cracks in cyclically loaded components usually initiate in areas of high stress concentration. Such stress concentrations do not only occur due to the geometrical design of a component, but also near material inhomogeneities or Foreign Object Damage (FOD). In the context of a fitness-for-purpose (FFP) assessment, it is important to estimate the fatigue crack growth rate as accurately as possible, e.g., for estimating inspection intervals for public transportation. However, the crack growth rate depends not only on the applied load and crack length, but also on residual stress fields introduced intentionally (surface treatment) or unintentionally (FOD or inappropriate handling). In this work, an analytical model for describing the fatigue crack growth rate of Short Cracks (provided the conditions of LEFM are fulfilled) as well as of long Cracks is developed. Also the influence of residual stresses on the crack growth behavior is investigated. This permits to assess the combined influence of load stresses and residual stresses together with the build-up of crack closure, and leads to a simple but effective modification of the NASGRO equation for fatigue crack growth. The approach is validated experimentally, and its application to the fitness-for-purpose assessment of surface-treated components is discussed.
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Short Cracks initiated in al 6013 t6 with the focused ion beam fib technology
International Journal of Fatigue, 2007Co-Authors: A Tesch, R Pippan, Karlheinz Trautmann, H DokerAbstract:Abstract The fatigue crack growth behaviour of Short corner Cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner Cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long Cracks. Corner Cracks were introduced into Short crack specimens, similar to M(T) – specimens, at one side of a hole (O = 4.8 mm) by cyclic compression (R = 20). The preCracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with Short corner Cracks were compared with the long fatigue crack growth test data. The Short Cracks grew at ΔK-values below the threshold for long Cracks at the same stress ratio. They also grew faster than long Cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant Kmax-tests with long Cracks that gives a good and conservative estimation of the Short crack growth rates.
W Skrotzki - One of the best experts on this subject based on the ideXlab platform.
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initiation and propagation of Short Cracks in austenitic ferritic duplex steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2013Co-Authors: M Scharnweber, W. Tirschler, C G Oertel, W SkrotzkiAbstract:Abstract Austenitic–ferritic duplex stainless steels are widely used for applications, where both excellent strength and high corrosion resistance are required. In the current study, high cycle fatigue experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462 in order to investigate Short crack initiation and propagation. It is shown that the most frequent crack initiation sites are grain boundaries between ferritic grains followed by slip bands in austenite and phase boundaries. Taking into account the elastic anisotropy of the austenitic phase, the surprisingly high fraction of Cracks formed along slip bands in austenite can be explained with the texture and microstructure of the material. Analyzing the correlation between crack propagation rate and crack length, it is found that in both phases, the transition crack length between the regimes of microstructurally and mechanically Short Cracks correlates with the mean grain elongation under the average angle between crack path and loading axis. Additionally, the dependence of the crack propagation rate on the distance between crack tip and opposing grain or phase boundary, i.e. the barrier effect of these boundaries for Short crack propagation, could be measured. The results obtained expand the knowledge about Short crack behavior in the investigated material, which is a key parameter for the lifetime in the high cycle fatigue regime.
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Initiation and propagation of Short Cracks in austenitic–ferritic duplex steel
Materials Science and Engineering: A, 2013Co-Authors: M Scharnweber, W. Tirschler, C G Oertel, W SkrotzkiAbstract:Abstract Austenitic–ferritic duplex stainless steels are widely used for applications, where both excellent strength and high corrosion resistance are required. In the current study, high cycle fatigue experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462 in order to investigate Short crack initiation and propagation. It is shown that the most frequent crack initiation sites are grain boundaries between ferritic grains followed by slip bands in austenite and phase boundaries. Taking into account the elastic anisotropy of the austenitic phase, the surprisingly high fraction of Cracks formed along slip bands in austenite can be explained with the texture and microstructure of the material. Analyzing the correlation between crack propagation rate and crack length, it is found that in both phases, the transition crack length between the regimes of microstructurally and mechanically Short Cracks correlates with the mean grain elongation under the average angle between crack path and loading axis. Additionally, the dependence of the crack propagation rate on the distance between crack tip and opposing grain or phase boundary, i.e. the barrier effect of these boundaries for Short crack propagation, could be measured. The results obtained expand the knowledge about Short crack behavior in the investigated material, which is a key parameter for the lifetime in the high cycle fatigue regime.
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influence of crack length and grain boundaries on the propagation rate of Short Cracks in austenitic stainless steel
Scripta Materialia, 2012Co-Authors: M Scharnweber, W. Tirschler, V Mikulich, S Jacob, C G Oertel, W SkrotzkiAbstract:Cyclic deformation experiments on austenitic stainless steel were performed using different plastic strain amplitudes. The experiments were repeatedly interrupted in order to determine the propagation rate and length of the existing Cracks as well as the corresponding distance between crack tip and opposing grain boundary. The results reveal the transition between the regimes of microstructurally and mechanically Short Cracks as well as the nature of the barrier effect of grain boundaries for crack propagation.
Bernard Chen - One of the best experts on this subject based on the ideXlab platform.
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prediction of fatigue life in aluminium alloy aa7050 t7451 structures in the presence of multiple artificial Short Cracks
Theoretical and Applied Fracture Mechanics, 2015Co-Authors: James Tan, Bernard ChenAbstract:Abstract Although methods for predicting the growth and coalescence of Cracks have been introduced, uncertainties persist in structures with multiple Short Cracks. Quantitative fractographic methods were used to study the way in which multiple Short Cracks in AA7050-T7451 of varying sizes on the same and on different planes grew under low amplitude cyclic loading. The accuracy of existing theoretical methods for predicting the growth and coalescence of multiple Cracks was evaluated. Experimental results corroborate the findings that growth of the ‘largest’ crack dominated the fatigue life of a specimen with very little influence from relatively Shorter Cracks.
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coalescence and growth of two coplanar Short Cracks in aa7050 t7451 aluminium alloys
Engineering Fracture Mechanics, 2013Co-Authors: James Tan, Bernard ChenAbstract:Abstract Quantitative fractography was used to study fatigue test specimens (AA7050-T7451) with Short surface Cracks to determine their interaction and coalescence. Methods for predicting the coalescence of Cracks described by ASME, BSI PD6493 and Iida were compared with the present experimental results. An improved method was developed to predict the coalescence of two coplanar Short Cracks and subsequent growth as a single larger crack by including a period of crack growth deceleration at the point of coalescence as observed in the present experiments, which appeared to be associated with the development of a single, larger semi-elliptical shaped crack (i.e. a coalesced crack).
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a new method for modelling the coalescence and growth of two coplanar Short Cracks of varying lengths in aa7050 t7451 aluminium alloy
International Journal of Fatigue, 2013Co-Authors: James Tan, Bernard ChenAbstract:Abstract The growth, interaction and coalescence of two coplanar Short Cracks of varying lengths in AA7050-T7451 aluminium alloy were studied under low amplitude cyclic loading. Fractographic studies showed that the way in which the fracture surfaces developed was dependent on the relative crack sizes, however interactions between the Cracks did not significantly affect the crack growth rates before the tips of the Cracks touch to form a single crack. Subsequently, the longitudinal growth rates of the crack were retarded for a period which appeared necessary for the newly coalesced crack to form a single semi-elliptical shape before resuming growth rates of a single crack. A new mathematical model was developed to account for this behaviour.
