The Experts below are selected from a list of 285 Experts worldwide ranked by ideXlab platform
John R. Tyrer - One of the best experts on this subject based on the ideXlab platform.
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Nd:YAG laser drilling of 8.3 mm thick partially stabilized tetragonal zirconia-control of recast layer Microcracking using localized heating techniques
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the level of recast layer Microcracking. A method for the numerical characterization of Microcracking was developed for this work and was based upon SEM image processing. The use of a high temperature furnace to heat substrates before and after laser processing reduced the level of recast layer Microcracking by half, when compared to the ambient temperature process. However, sufficient Microcracking was present to cause substrate failure. An unconventional plasma heating technique was developed and applied for the reduction of this cracking. For laser drilling at 1300 °C, the addition of localized plasma heating gave a further 14% mean reduction in recast layer Microcracking. This advanced drilling technique also produced a 12% increase in mean hole diameter when compared to the furnace heating method.The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the l...
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nd yag laser drilling of 8 3 mm thick partially stabilized tetragonal zirconia control of recast layer Microcracking using localized heating techniques
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the l...
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Nd:YAG laser cutting and drilling of PSTZ—Influence of substrate heating temperature on recast layer Microcracking
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. Recast layer Microcracking is the Achilles heel of laser processing of ceramics. Techniques for the reduction of the thermal shear caused by laser beam interaction have been investigated. A method for the numerical characterization of Microcracking was developed for this work and was based upon scanning electron microscopy image processing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of the partially stabilized tetragonal zirconia (PSTZ) substrates to high temperatures before and after laser processing was found to reduce the thermal gradients that cause Microcracking. Holes with a mean diameter of 679 μm were percussion drilled through the 8.3 mm thick substrates in 0.75 s, and had limited tapering (
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nd yag laser cutting and drilling of pstz influence of substrate heating temperature on recast layer Microcracking
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. Recast layer Microcracking is the Achilles heel of laser processing of ceramics. Techniques for the reduction of the thermal shear caused by laser beam interaction have been investigated. A method for the numerical characterization of Microcracking was developed for this work and was based upon scanning electron microscopy image processing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of the partially stabilized tetragonal zirconia (PSTZ) substrates to high temperatures before and after laser processing was found to reduce the thermal gradients that cause Microcracking. Holes with a mean diameter of 679 μm were percussion drilled through the 8.3 mm thick substrates in 0.75 s, and had limited tapering (<150 μm). Single pass, full...
J D Currey - One of the best experts on this subject based on the ideXlab platform.
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Microcracking damage and the fracture process in relation to strain rate in human cortical bone tensile failure
Journal of Biomechanics, 2008Co-Authors: Peter Zioupos, Ulrich Hansen, J D CurreyAbstract:Abstract It is difficult to define the ‘physiological’ mechanical properties of bone. Traumatic failures in-vivo are more likely to be orders of magnitude faster than the quasistatic tests usually employed in-vitro. We have reported recently [Hansen, U., Zioupos, P., Simpson, R., Currey, J.D., Hynd, D., 2008. The effect of strain rate on the mechanical properties of human cortical bone. Journal of Biomechanical Engineering/Transactions of the ASME 130, 011011-1-8] results from tests on specimens of human femoral cortical bone loaded in tension at strain rates ( e ˙ ) ranging from low (0.08 s−1) to high (18 s−1). Across this strain rate range the modulus of elasticity generally increased, stress at yield and failure and strain at failure decreased for rates higher than 1 s−1, while strain at yield was invariant for most strain rates and only decreased at rates higher than 10 s−1. The results showed that strain rate has a stronger effect on post-yield deformation than on initiation of macroscopic yielding. In general, specimens loaded at high strain rates were brittle, while those loaded at low strain rates were much tougher. Here, a post-test examination of the Microcracking damage reveals that Microcracking was inversely related to the strain rate. Specimens loaded at low strain rates showed considerable post-yield strain and also much more Microcracking. Partial correlation and regression analysis suggested that the development of post-yield strain was a function of the amount of Microcracking incurred (the cause), rather than being a direct result of the strain rate (the excitation). Presumably low strain rates allow time for Microcracking to develop, which increases the compliance of the specimen, making them tougher. This behaviour confirms a more general rule that the degree to which bone is brittle or tough depends on the amount of Microcracking damage it is able to sustain. More importantly, the key to bone toughness is its ability to avoid a ductile-to-brittle transition for as long as possible during the deformation. The key to bone's brittleness, on the other hand, is the strain and damage localisation early on in the process, which leads to low post-yield strains and low-energy absorption to failure.
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the effects of damage and Microcracking on the impact strength of bone
Journal of Biomechanics, 2000Co-Authors: Gwendolen C Reilly, J D CurreyAbstract:Abstract Microcracking has been shown to occur when bone is `damaged’ as shown by a loss of stiffness. The effect on bone's toughness of the types of damage produced at low losses of stiffness are not known. We loaded bovine bone specimens in bending and tension to stiffness losses of up to 27%, and examined the Microcracking produced. The tensile specimens had diffuse arrays of microcracks of 2–20 μm in length, characteristic of tensile loading, on all surfaces. The bending specimens showed tensile Microcracking on the tensile surface and characteristic long, straight, cross-hatched compression cracks on the compressive surface. Specimens were then broken in impact. Those that had been damaged in bending were divided into two groups, in one group the part of the specimen which had undergone compression damage was placed in tension, and in the other group the tensile damage was placed in tension. Tensile damage loaded in tension did not reduce the bone's energy-absorbing ability in impact until a modulus reduction of over 20%. However compression damage loaded in tension did severely reduce the bone's energy absorption capabilities (by an average of about 40%).
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the development of Microcracking and failure in bone depends on the loading mode to which it is adapted
The Journal of Experimental Biology, 1999Co-Authors: Gwendolen C Reilly, J D CurreyAbstract:During locomotion, the anterior cortex of the equine radius is loaded predominantly in tension, the posterior predominantly in compression. The anterior cortex is relatively strong in tension, the posterior in compression. We investigated the pattern of failure of specimens from the two cortices using laser scanning confocal microscopy. All specimens were loaded in four-point bending to increasingly higher loads. We quantified the amount of diffuse Microcracking on the tensile side of these specimens by observing the amount of light emitted under laser illumination. The amount of light emitted agreed well with subjective estimates of the amount of Microcracking. Tensile microcracks first appeared at a strain of approximately 0.004, and all specimens showed considerable growth in microcrack density once the tensile strain had passed approximately 0.008. In specimens from the posterior cortex, there was little compressive Microcracking, and such cracks as were present were small and diffuse. These specimens failed on the tensile side first. In specimens from the anterior cortex, compression cracks were more numerous, longer and less diffuse, and specimens failed initially in compression. The patterns of failure in the bone tissues of the two cortices are what would be expected assuming they were adapted to the mode of loading to which they are usually subjected.
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the extent of Microcracking and the morphology of microcracks in damaged bone
Journal of Materials Science, 1994Co-Authors: Peter Zioupos, J D CurreyAbstract:Strain-induced damage in bovine laminar bone has been examined using laser scanning confocal microscopy (LSCM). The specimens were loaded in a fluorescein solution, which penetrated the newly formed cracks in the specimen. The Microcracking, and the larger cracking, induced by strain were very clearly visible. The Microcracking occurred diffusely in regions of high strain (stress), but was particularly obvious in the vicinity of large machined stressconcentrators. The Microcracking could be shown not to be artefactual, that is, it was produced by strain, and not by specimen preparation. The Microcracking interacted with the structure of the bone, often having a wavy appearance related to the histology. Microcracks seemed to be particularly associated with the most highly mineralized parts of the bone. LSCM is a technique holding great promise for the investigation of the initiation and development of damage in mineralized hard tissues, and other translucent materials.
Stephen Sebesta - One of the best experts on this subject based on the ideXlab platform.
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use of Microcracking to reduce shrinkage cracking in cement treated bases
Transportation Research Record, 2005Co-Authors: Stephen SebestaAbstract:Shrinkage cracking occurs in cement-treated bases because of desiccation and cement hydration; eventually these cracks start to reflect through the pavement surfacing. Although initially considered cosmetic, these cracks open the pavement to water infiltration and increase the likelihood of accelerated pavement distress. Numerous options exist for minimizing the amount of reflective cracks that appear; Microcracking is a promising approach. The Microcracking concept can be defined as the application of several vibratory roller passes to the cement-treated base at a short curing stage, typically after 1 to 3 days, to create a fine network of cracks. In addition to the microcracked test sites, the contractor constructed moist-cured, dry-cured, and asphalt membrane–cured sites for comparison. Researchers used falling weight deflectometer (FWD) tests to control the Microcracking process, periodic crack surveys to monitor crack performance, and FWD tests through time to track base moduli. Microcracking proved quite effective at reducing shrinkage cracking problems in the base; applying the procedure with three passes of the roller after 2 to 3 days of curing resulted in the best performance. In addition, researchers observed that, without Microcracking, excessively high cement contents resulted in problematic cracking in the base even if they were cured according to good construction practice. Microcracking did not result in pavement damage or diminished in-service modulus; thus, Microcracking should be considered a viable and inexpensive option to incorporate shrinkage crack control into the construction of cement-treated bases.
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part 1 cementitious chemical and mechanical stabilization use of Microcracking to reduce shrinkage cracking in cement treated bases
Transportation Research Record, 2005Co-Authors: Stephen SebestaAbstract:Shrinkage cracking occurs in cement-treated bases because of desiccation and cement hydration; eventually these cracks start to reflect through the pavement surfacing. Although initially considered cosmetic, these cracks open the pavement to water infiltration and increase the likelihood of accelerated pavement distress. Numerous options exist for minimizing the amount of reflective cracks that appear; Microcracking is a promising approach. The Microcracking concept can be defined as the application of several vibratory roller passes to the cement-treated base at a short curing stage, typically after 1 to 3 days, to create a fine network of cracks. In addition to the microcracked test sites, the contractor constructed moist-cured, dry-cured, and asphalt membrane-cured sites for comparison. Researchers used falling weight deflectometer (FWD) tests to control the Microcracking process, periodic crack surveys to monitor crack performance, and FWD tests through time to track base moduli. Microcracking proved ...
A. J. Murray - One of the best experts on this subject based on the ideXlab platform.
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Nd:YAG laser drilling of 8.3 mm thick partially stabilized tetragonal zirconia-control of recast layer Microcracking using localized heating techniques
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the level of recast layer Microcracking. A method for the numerical characterization of Microcracking was developed for this work and was based upon SEM image processing. The use of a high temperature furnace to heat substrates before and after laser processing reduced the level of recast layer Microcracking by half, when compared to the ambient temperature process. However, sufficient Microcracking was present to cause substrate failure. An unconventional plasma heating technique was developed and applied for the reduction of this cracking. For laser drilling at 1300 °C, the addition of localized plasma heating gave a further 14% mean reduction in recast layer Microcracking. This advanced drilling technique also produced a 12% increase in mean hole diameter when compared to the furnace heating method.The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the l...
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nd yag laser drilling of 8 3 mm thick partially stabilized tetragonal zirconia control of recast layer Microcracking using localized heating techniques
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. However, the performance of ceramic substrates is dependent on the quality of the processed region, with specific reference to the level of Microcracking. The propagation of microcracks into the bulk substrate inevitably leads to component failure. Generation of recast layer Microcracking is the Achilles heel of laser processing of ceramics. The fundamental process of laser radiation interaction with ceramic substrates generates a severe thermal gradient between the recast layer and bulk substrate. This in turn leads to the stresses which cause microcracks to form. Techniques for the reduction of this thermal shear have been investigated. Optimization of Nd:YAG laser drilling enabled repeatable and high quality processing to be undertaken. However, adaptation of these conventional techniques was required to reduce the l...
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Nd:YAG laser cutting and drilling of PSTZ—Influence of substrate heating temperature on recast layer Microcracking
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. Recast layer Microcracking is the Achilles heel of laser processing of ceramics. Techniques for the reduction of the thermal shear caused by laser beam interaction have been investigated. A method for the numerical characterization of Microcracking was developed for this work and was based upon scanning electron microscopy image processing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of the partially stabilized tetragonal zirconia (PSTZ) substrates to high temperatures before and after laser processing was found to reduce the thermal gradients that cause Microcracking. Holes with a mean diameter of 679 μm were percussion drilled through the 8.3 mm thick substrates in 0.75 s, and had limited tapering (
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nd yag laser cutting and drilling of pstz influence of substrate heating temperature on recast layer Microcracking
Journal of Laser Applications, 1999Co-Authors: A. J. Murray, John R. TyrerAbstract:The machining of ceramic components using conventional techniques is slow and expensive due to low yields. High power lasers are capable of machining these materials at far greater speeds. Recast layer Microcracking is the Achilles heel of laser processing of ceramics. Techniques for the reduction of the thermal shear caused by laser beam interaction have been investigated. A method for the numerical characterization of Microcracking was developed for this work and was based upon scanning electron microscopy image processing. Optimization of the pulsed Nd:YAG laser drilling and cutting cycles enabled repeatable, high quality processing to be undertaken. Heating of the partially stabilized tetragonal zirconia (PSTZ) substrates to high temperatures before and after laser processing was found to reduce the thermal gradients that cause Microcracking. Holes with a mean diameter of 679 μm were percussion drilled through the 8.3 mm thick substrates in 0.75 s, and had limited tapering (<150 μm). Single pass, full...
Jun Luo - One of the best experts on this subject based on the ideXlab platform.
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study of transformation induced intergranular Microcracking in tetragonal zirconia polycrystals with the phase field method
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Jingming Zhu, Jun LuoAbstract:Abstract The superior fracture toughness of yttria stabilized tetragonal zirconia polycrystals (TZPs) originates from the stress-induced tetragonal to monoclinic (t-m) phase transformation in the tensile stress field around a propagating crack front. However, spontaneous phase transformation can take place when TZPs are exposed to humid environments, which is accompanied by intergranular Microcracking and results in the loss of mechanical properties. This phenomenon, usually called low temperature degradation (LTD) or aging, is considered as a main drawback of TZPs for their clinical applications such as femoral heads and dental implants. In order to explore the mechanisms behind transformation induced intergranular Microcracking, a coupled phase field model is developed in this paper, which can simulate the t-m phase transformation and intergranular Microcracking simultaneously. The phase field model is validated by studying the microcrack nucleation from a single twin variant with a pure shear eigenstrain. The length of the nucleated microcrack obtained with the phase field modelling matches well with the theoretical prediction. The influence of the dilatational eigenstrain on microcrack nucleation and microcrack nucleation from a monoclinic twin are fully discussed. After that, phase field simulations are conducted to study t-m transformation induced intergranular Microcracking in polycrystalline tetragonal zirconia ceramics. It is found that the width of the martensitic variants and their incidence angles to the grain boundary have significant influences on microcrack nucleation. The microstructure of the martensitic twins and the distribution of the intergranular microcracks are fully discussed. The results are helpful to understand the degradation mechanisms of TZPs after the hydrothermal aging.