A new Fe-based metallic glass with composition Fe 76 B 12 Si 9 Y 3 (at. %) is found to have extra... more A new Fe-based metallic glass with composition Fe 76 B 12 Si 9 Y 3 (at. %) is found to have extraordinary degradation efficiency towards methyl orange (MO, C14H14N3SO3) in strong acidic and near neutral environments compared to crystalline zero-valent iron (ZVI) powders and other Fe-based metallic glasses. The influence of temperature (294–328 K) on the degradation reaction rate was measured using ball-milled metallic glass powders revealing a low thermal activation energy barrier of 22.6 kJ/mol. The excellent properties are mainly attributed to the heterogeneous structure consisting of local Fe-rich and Fe-poor atomic clusters, rather than the large specific surface and strong residual stress in the powders. The metallic glass powders can sustain almost unchanged degradation efficiency after 13 cycles at room temperature, while a drop in degradation efficiency with further cycles is attributed to visible surface oxidation. Triple quadrupole mass spectrometry analysis conducted during the reaction was used to elucidate the underlying degradation mechanism. The present findings may provide a new, highly efficient and low cost commercial method for azo dye wastewater treatment. Azo dyes represent about one-half of the dyes used in the textile industry. It has been reported that about 20% of dyestuff is discharged directly into the environment by textile factories 1,2. However, azo dyes are very difficult to decompose and often result in the pollution of water and irrevocable environmental problems 3. New and effective treatment methods are needed for degrading azo dyes in order to reduce, or preferably eliminate, the amount of toxic and/or carcinogenic azo dye in wastewater. Traditional azo dye wastewater treatment methods usually filter or separate the organic chemicals through physical methods rather than decomposing the azo dye 4. However, these methods cannot sufficiently eliminate azo dyes and their environmental impacts. Furthermore, the chemical and biological methods currently available are often more expensive and display even lower efficiency 5,6 , so their applications are largely restricted. The removal of pollutants in water by means of a reduction reaction using zero-valent metals has been developed recently as a new treatment technology. The most commonly used metal is zero-valent iron, which exhibits low cost, simple operation, and high efficiency 7–9. However, ZVI is easily oxidized (rust) and its degradation efficiency decays rapidly. Recently, Fe-based amorphous powders and metallic glass composites have been developed that demonstrate higher efficiency in the degradation of azo dyes and other organic contaminants compared to conventional iron powders 10–12. Metallic glasses have attracted considerable attention over the past three decades due to their unique physical, mechanical and chemical properties 13,14. In particular, their good chemical and catalytic properties have been highlighted due to the far-from-equilibrium structures and high-energy states of the metallic glasses 15–18. A long-range disordered atomic configuration gives metallic glasses a complex atomic and electronic structure, which is distinct from their crystalline counterparts and can contribute to excellent chemical and catalytic properties 18. A shortcoming for using metallic glasses in applications requiring components with large physical dimensions are the size restrictions imposed by the glass formation ability 13. However, for functional applications, such as dye degradation, powder samples can be prepared easily and there is little concern about any processing restrictions. While there has also been interest in using Mg-based metallic glass and metallic glass composite powders for degrading azo dyes 19–21 , Fe-based metallic glasses represent a lower cost alternative and are thus most attractive for commercialization. Furthermore, in comparison to commercially pure ZVI, Fe-based metallic glasses
Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughnes... more Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughness that cannot be achieved by traditional metals, making them attractive for load bearing, mechanical engineering applications. Furthermore, recent research on BMGs has shown that many early perceived shortcomings, such as apparent brittleness or poor fatigue resistance, are not as big of a problem as once thought. The purpose of this paper is to review some of the unique mechanical characteristics of BMGs along with some of the strategies that may be used to improve or exploit their characteristics so that BMGs may be used as structural materials in engineering applications.
Bulk metallic glasses (BMGs) have been reported to have large variability in mode I fracture toug... more Bulk metallic glasses (BMGs) have been reported to have large variability in mode I fracture toughness. In this study, alternating soft and hard regions were created in a Zr52.5Ti5Cu18Ni14.5Al10 (at%) BMG via mechanical imprinting at room temperature. While only 50% of as-cast samples demonstrated plastic deformation during mode I fracture tests, 100% of imprinted samples demonstrated measurable plasticity and the scatter in measured mode I fracture toughness was significantly reduced. Mechanical treatments show promise for improving the fracture reliability of BMGs.
Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic... more Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic glass (BMG) single edge notch bend samples. The results show a 54% improved fracture toughness (KJ = 55.2 vs. 85.0 MPa√m) is obtained by cold rolling to ~2.5% true strain. The precrack patterns and fractography were also analyzed to evaluate the influence of cold rolling on the fracture process and also the validity of the mode I fracture toughness results. Cold rolling was found to promote numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to the improved toughness. The influence of various precrack morphologies on the fracture toughness results are discussed. It was found that it is difficult to produce perfect standardized precracks in BMGs with heterogeneous glassy structures induced by cold rolling and this issue needs further evaluation if fracture toughness testing standards for BMGs are to be established.
Mo-Si-B silicides consisting of the phases -Mo (Mo solid solution), MoSi, and MoSiB have melting ... more Mo-Si-B silicides consisting of the phases -Mo (Mo solid solution), MoSi, and MoSiB have melting points on the order of 2000 C and have potential as ultra-high temperature structural materials. Mo-Si-B alloys can be processed such that the -Mo is present in the form of isolated particles in a silicide matrix, or as a continuous matrix 'cementing' individual silicide particles together. The latter microstructure is similar to that of WC-Co hard metals. This paper focuses on the relationship between the topology as well as scale of the microstructure of Mo-MoSi-MoSiB alloys, and their creep strength and fracture toughness. For example, the creep strength of Mo-Si-B alloys is improved by reducing the -Mo volume fraction and by making the -Mo phase discontinuous. The fracture toughness is improved by increasing the -Mo volume fraction and by making the -Mo phase continuous. Room temperature stress intensity factors as high as 21 MPa m¹² were obtained. The room temperature frac...
ABSTRACT The fatigue crack growth behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass (BMG) wa... more ABSTRACT The fatigue crack growth behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass (BMG) was measured in an inert dry N2 environment. When compared to results measured in ambient air, the present results showed a higher fatigue threshold and lower crack growth rates in the near-threshold region. Such results reveal that there is a significant effect of ambient air in accelerating fatigue crack growth for this BMG.
Journal of the Mechanical Behavior of Biomedical Materials, 2010
A new technique using contrast enhanced micro-computed tomography (micro-CT) was developed to imp... more A new technique using contrast enhanced micro-computed tomography (micro-CT) was developed to improve the ability to detect dentinal cracks in teeth and assess associated risks to oral health. Extracted, whole human molars that exhibited visual evidence of external cracks following extraction and machined, partially fractured elephant dentin specimens were labeled by BaSO 4 precipitation and imaged by micro-CT. Contrastenhanced micro-CT was demonstrated in vitro to enable non-destructive, 3-D imaging of the presence, morphology and spatial location of dentinal cracks in whole human molars and machined specimens. BaSO 4 staining provided enhanced contrast for the detection of cracks that could not be detected prior to staining. Backscattered SEM micrographs showed that BaSO 4 was precipitated on the surfaces of dentinal cracks and within adjacent tubules.
The role of microstructure in affecting the fatigue crack growth resistance of grain bridging sil... more The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO-RE 2 O 3 and results were compared with a commercial Al 2 O 3 -Y 2 O 3 -doped material. Decreasing stress intensity range (DK) fatigue tests were conducted using compact-tension specimens to measure steady-state fatigue crack growth rates. Specimens doped with MgO-RE 2 O 3 additives showed a significantly higher resistance to crack growth than those with Al 2 O 3 -Y 2 O 3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO-RE 2 O 3 -doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R-curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO-RE 2 O 3 -doped silicon nitrides displayed similar steady-state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.
Design should begin by identifying a human or societal need -a problem worth solving -and then fu... more Design should begin by identifying a human or societal need -a problem worth solving -and then fulfill that need by tailoring the technology to the specific, relevant human factors.
Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there ha... more Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods for predicting small crack fatigue behavior. In this paper, small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and it was used to 1) determine the bridging stress profile for the material and 2) make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, is has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.
Early studies suggested there was a severe problem with the fatigue resistance of some bulk metal... more Early studies suggested there was a severe problem with the fatigue resistance of some bulk metallic glasses (BMGs) and BMG matrix composites, while more recent studies begin to demonstrate a wide variety of fatigue behaviors may be possible for both fully amorphous BMGs and their composites. However, in order to truly understand and control the fatigue behavior of these materials, the role of such factors as thermomechanical processing, the corresponding glass structure, environment, and defects must be understood. Additionally, it is important to understand how these factors relate to the mechanisms of fatigue. This article reviews the current understanding in this regard, and identifies some of the challenges for the future development of fatigue-resistant BMG-based materials.
A method for determining worst-case cyclic fatigue thresholds in grain-bridging ceramics by quant... more A method for determining worst-case cyclic fatigue thresholds in grain-bridging ceramics by quantifying the role of bridging is demonstrated for a model alumina. Crack-growth properties for both long and short (< 2 mm) cracks emanating from machined notches (root radii, (rho) ~; 15 -150 (mu)m) were investigated. When compared as a function of the applied stress-intensity range (delta K), growth rates (da/dN) were far higher and fatigue thresholds (Delta)KTH were markedly lower with short cracks, with growth being observable at the lowest driving forces for short cracks emanating from razor micronotches ((rho)is approximately equal to 15 (mu)m). For growth rates < 10-8 m/cycle, da/dN vs. (delta)K data for short cracks merged with the steady-state data for long cracks after ~;2 mm of extension.
Gamma-based TiAl intermetallic alloys have received considerable attention recently as candidate ... more Gamma-based TiAl intermetallic alloys have received considerable attention recently as candidate materials for high-temperature aerospace applications. Two classes of microstructure have been prominent in the two-phase (γ + αâ) alloys: a lamellar structure consisting of lamellar colonies containing alternating γ and αâ grains. In general, duplex structures display better elongation and strength, whereas lamellar structures show better toughness and fatigue
Two major mechanisms that could potentially be responsible for toughening in mineralized tissues,... more Two major mechanisms that could potentially be responsible for toughening in mineralized tissues, such as bone and dentin, have been identified—microcracking and crack bridging. While evidence has been reported for both mechanisms, there has been no consensus thus far on which mechanism plays the dominant role in toughening these materials. In the present study, we seek to present definitive experimental evidence supporting crack bridging, rather than microcracking, as the most significant mechanism of toughening in cortical bone and dentin. In vitro fracture toughness experiments were conducted to measure the variation of the fracture resistance with crack extension (resistance- curve (R-curve) behavior) for both materials with special attention paid to changes in the sample compliance. Because these two toughening mechanisms induce opposite effects on the sample compliance, such experiments allow for the definitive determination of the dominant toughening mechanism, which in the p...
The development of a mechanistic understanding of the increase in fracture risk in human bone wit... more The development of a mechanistic understanding of the increase in fracture risk in human bone with age is essential to public health. This represents a challenge for fracture mechanics as bone has a complex, hierarchical structure with characteristic features ranging from nanometer to macroscopic dimensions, and is thus much more complex than most engineering materials. In this study, we review ex vivo fracture experiments which quantitatively assess the effect of age on human cortical bone in the proximal-distal orientation, i.e. along the long axis of the bone. Specifically, cortical bone is seen to exhibit rising crack-growth resistance with crack extension; the toughness is consequently evaluated in terms of R-curves, measured in bones taken from a wide range of age groups (34-99 years). Both crack-initiation and crack-growth toughnesses were determined and were found to deteriorate with age; the initiation toughness decreases some 40% over the 65 years of ageing, while growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular involving crack bridging in the wake of the crack. This explanation is supported by an examination of the micro-/nanostructural changes accompanying the process of ageing, performed using deep-UV Raman spectroscopy, computed X-ray tomography and optical/ electron microscopy.
The role of metal layer thickness and resultant plastic constraint in the metal layer during the ... more The role of metal layer thickness and resultant plastic constraint in the metal layer during the failure of metal/ceramic layered structures is examined under cyclic and static loading conditions. Crack-growth experiments were conducted on sandwich specimens consisting of 99.999% pure aluminum layers bonded between 99.5% pure polycrystalline alumina with the metal layer thickness varying from 5 to 100 lm. Under cyclic loading, crack growth occurred primarily at the interface separating the two materials; additionally, stable fatigue cracks deviated into the alumina for thin-layered samples at high driving forces. Under monotonically increasing loads, the fracture toughness increased with Al layer thickness, whereas under cyclic loads the threshold driving force for crack growth conversely decreased with increasing layer thickness. Under static loading in a moist environment, interfacial crack growth was never observed at measurable rates (P10 À9 m/s) for driving forces up to 200 J/m 2 ; however, for thin-layered samples, subcritical cracks did deviate off the interface and grow, sometimes stably, into the alumina. Trends in crack-growth rates and crack trajectories are examined in terms of the level of constraint, loading conditions and environmental influences.
The aging-related deterioration of the fracture properties of bone, coupled with higher life expe... more The aging-related deterioration of the fracture properties of bone, coupled with higher life expectancy, is responsible for increasing incidence of bone fracture in the elderly; consequently, an understanding of how these fracture properties degrade with age is essential. In this study, ex vivo fracture experiments have been performed to quantitatively assess the effect of age on human cortical bone in the proximaldistal orientation, i.e., longitudinally along the osteons. Because cortical bone exhibits rising crack-growth resistance with crack extension, the toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from wide range of age groups (34 -99 years). Using this approach, both the crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases some 40% over six decades from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular, involving crack bridging in the wake of the crack. An examination of the micro-/nano-structural changes accompanying the process of aging, using optical microscopy, X-ray tomography, nanoindentation and Raman spectroscopy, is shown to support such observations. Published by Elsevier B.V.
Journal of the Mechanical Behavior of Biomedical Materials, 2009
Indentation techniques for assessing fracture toughness are attractive due to the simplicity and ... more Indentation techniques for assessing fracture toughness are attractive due to the simplicity and expediency of experiments, and because they potentially allow the characterization of both local and bulk fracture properties. Unfortunately, rarely have such techniques been proven to give accurate fracture toughness values. This is a concern, as such techniques are seeing increasing usage in the study of biomaterials and biological hard tissues.
A new Fe-based metallic glass with composition Fe 76 B 12 Si 9 Y 3 (at. %) is found to have extra... more A new Fe-based metallic glass with composition Fe 76 B 12 Si 9 Y 3 (at. %) is found to have extraordinary degradation efficiency towards methyl orange (MO, C14H14N3SO3) in strong acidic and near neutral environments compared to crystalline zero-valent iron (ZVI) powders and other Fe-based metallic glasses. The influence of temperature (294–328 K) on the degradation reaction rate was measured using ball-milled metallic glass powders revealing a low thermal activation energy barrier of 22.6 kJ/mol. The excellent properties are mainly attributed to the heterogeneous structure consisting of local Fe-rich and Fe-poor atomic clusters, rather than the large specific surface and strong residual stress in the powders. The metallic glass powders can sustain almost unchanged degradation efficiency after 13 cycles at room temperature, while a drop in degradation efficiency with further cycles is attributed to visible surface oxidation. Triple quadrupole mass spectrometry analysis conducted during the reaction was used to elucidate the underlying degradation mechanism. The present findings may provide a new, highly efficient and low cost commercial method for azo dye wastewater treatment. Azo dyes represent about one-half of the dyes used in the textile industry. It has been reported that about 20% of dyestuff is discharged directly into the environment by textile factories 1,2. However, azo dyes are very difficult to decompose and often result in the pollution of water and irrevocable environmental problems 3. New and effective treatment methods are needed for degrading azo dyes in order to reduce, or preferably eliminate, the amount of toxic and/or carcinogenic azo dye in wastewater. Traditional azo dye wastewater treatment methods usually filter or separate the organic chemicals through physical methods rather than decomposing the azo dye 4. However, these methods cannot sufficiently eliminate azo dyes and their environmental impacts. Furthermore, the chemical and biological methods currently available are often more expensive and display even lower efficiency 5,6 , so their applications are largely restricted. The removal of pollutants in water by means of a reduction reaction using zero-valent metals has been developed recently as a new treatment technology. The most commonly used metal is zero-valent iron, which exhibits low cost, simple operation, and high efficiency 7–9. However, ZVI is easily oxidized (rust) and its degradation efficiency decays rapidly. Recently, Fe-based amorphous powders and metallic glass composites have been developed that demonstrate higher efficiency in the degradation of azo dyes and other organic contaminants compared to conventional iron powders 10–12. Metallic glasses have attracted considerable attention over the past three decades due to their unique physical, mechanical and chemical properties 13,14. In particular, their good chemical and catalytic properties have been highlighted due to the far-from-equilibrium structures and high-energy states of the metallic glasses 15–18. A long-range disordered atomic configuration gives metallic glasses a complex atomic and electronic structure, which is distinct from their crystalline counterparts and can contribute to excellent chemical and catalytic properties 18. A shortcoming for using metallic glasses in applications requiring components with large physical dimensions are the size restrictions imposed by the glass formation ability 13. However, for functional applications, such as dye degradation, powder samples can be prepared easily and there is little concern about any processing restrictions. While there has also been interest in using Mg-based metallic glass and metallic glass composite powders for degrading azo dyes 19–21 , Fe-based metallic glasses represent a lower cost alternative and are thus most attractive for commercialization. Furthermore, in comparison to commercially pure ZVI, Fe-based metallic glasses
Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughnes... more Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughness that cannot be achieved by traditional metals, making them attractive for load bearing, mechanical engineering applications. Furthermore, recent research on BMGs has shown that many early perceived shortcomings, such as apparent brittleness or poor fatigue resistance, are not as big of a problem as once thought. The purpose of this paper is to review some of the unique mechanical characteristics of BMGs along with some of the strategies that may be used to improve or exploit their characteristics so that BMGs may be used as structural materials in engineering applications.
Bulk metallic glasses (BMGs) have been reported to have large variability in mode I fracture toug... more Bulk metallic glasses (BMGs) have been reported to have large variability in mode I fracture toughness. In this study, alternating soft and hard regions were created in a Zr52.5Ti5Cu18Ni14.5Al10 (at%) BMG via mechanical imprinting at room temperature. While only 50% of as-cast samples demonstrated plastic deformation during mode I fracture tests, 100% of imprinted samples demonstrated measurable plasticity and the scatter in measured mode I fracture toughness was significantly reduced. Mechanical treatments show promise for improving the fracture reliability of BMGs.
Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic... more Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic glass (BMG) single edge notch bend samples. The results show a 54% improved fracture toughness (KJ = 55.2 vs. 85.0 MPa√m) is obtained by cold rolling to ~2.5% true strain. The precrack patterns and fractography were also analyzed to evaluate the influence of cold rolling on the fracture process and also the validity of the mode I fracture toughness results. Cold rolling was found to promote numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to the improved toughness. The influence of various precrack morphologies on the fracture toughness results are discussed. It was found that it is difficult to produce perfect standardized precracks in BMGs with heterogeneous glassy structures induced by cold rolling and this issue needs further evaluation if fracture toughness testing standards for BMGs are to be established.
Mo-Si-B silicides consisting of the phases -Mo (Mo solid solution), MoSi, and MoSiB have melting ... more Mo-Si-B silicides consisting of the phases -Mo (Mo solid solution), MoSi, and MoSiB have melting points on the order of 2000 C and have potential as ultra-high temperature structural materials. Mo-Si-B alloys can be processed such that the -Mo is present in the form of isolated particles in a silicide matrix, or as a continuous matrix 'cementing' individual silicide particles together. The latter microstructure is similar to that of WC-Co hard metals. This paper focuses on the relationship between the topology as well as scale of the microstructure of Mo-MoSi-MoSiB alloys, and their creep strength and fracture toughness. For example, the creep strength of Mo-Si-B alloys is improved by reducing the -Mo volume fraction and by making the -Mo phase discontinuous. The fracture toughness is improved by increasing the -Mo volume fraction and by making the -Mo phase continuous. Room temperature stress intensity factors as high as 21 MPa m¹² were obtained. The room temperature frac...
ABSTRACT The fatigue crack growth behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass (BMG) wa... more ABSTRACT The fatigue crack growth behavior of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass (BMG) was measured in an inert dry N2 environment. When compared to results measured in ambient air, the present results showed a higher fatigue threshold and lower crack growth rates in the near-threshold region. Such results reveal that there is a significant effect of ambient air in accelerating fatigue crack growth for this BMG.
Journal of the Mechanical Behavior of Biomedical Materials, 2010
A new technique using contrast enhanced micro-computed tomography (micro-CT) was developed to imp... more A new technique using contrast enhanced micro-computed tomography (micro-CT) was developed to improve the ability to detect dentinal cracks in teeth and assess associated risks to oral health. Extracted, whole human molars that exhibited visual evidence of external cracks following extraction and machined, partially fractured elephant dentin specimens were labeled by BaSO 4 precipitation and imaged by micro-CT. Contrastenhanced micro-CT was demonstrated in vitro to enable non-destructive, 3-D imaging of the presence, morphology and spatial location of dentinal cracks in whole human molars and machined specimens. BaSO 4 staining provided enhanced contrast for the detection of cracks that could not be detected prior to staining. Backscattered SEM micrographs showed that BaSO 4 was precipitated on the surfaces of dentinal cracks and within adjacent tubules.
The role of microstructure in affecting the fatigue crack growth resistance of grain bridging sil... more The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO-RE 2 O 3 and results were compared with a commercial Al 2 O 3 -Y 2 O 3 -doped material. Decreasing stress intensity range (DK) fatigue tests were conducted using compact-tension specimens to measure steady-state fatigue crack growth rates. Specimens doped with MgO-RE 2 O 3 additives showed a significantly higher resistance to crack growth than those with Al 2 O 3 -Y 2 O 3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO-RE 2 O 3 -doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R-curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO-RE 2 O 3 -doped silicon nitrides displayed similar steady-state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.
Design should begin by identifying a human or societal need -a problem worth solving -and then fu... more Design should begin by identifying a human or societal need -a problem worth solving -and then fulfill that need by tailoring the technology to the specific, relevant human factors.
Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there ha... more Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods for predicting small crack fatigue behavior. In this paper, small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and it was used to 1) determine the bridging stress profile for the material and 2) make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, is has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.
Early studies suggested there was a severe problem with the fatigue resistance of some bulk metal... more Early studies suggested there was a severe problem with the fatigue resistance of some bulk metallic glasses (BMGs) and BMG matrix composites, while more recent studies begin to demonstrate a wide variety of fatigue behaviors may be possible for both fully amorphous BMGs and their composites. However, in order to truly understand and control the fatigue behavior of these materials, the role of such factors as thermomechanical processing, the corresponding glass structure, environment, and defects must be understood. Additionally, it is important to understand how these factors relate to the mechanisms of fatigue. This article reviews the current understanding in this regard, and identifies some of the challenges for the future development of fatigue-resistant BMG-based materials.
A method for determining worst-case cyclic fatigue thresholds in grain-bridging ceramics by quant... more A method for determining worst-case cyclic fatigue thresholds in grain-bridging ceramics by quantifying the role of bridging is demonstrated for a model alumina. Crack-growth properties for both long and short (< 2 mm) cracks emanating from machined notches (root radii, (rho) ~; 15 -150 (mu)m) were investigated. When compared as a function of the applied stress-intensity range (delta K), growth rates (da/dN) were far higher and fatigue thresholds (Delta)KTH were markedly lower with short cracks, with growth being observable at the lowest driving forces for short cracks emanating from razor micronotches ((rho)is approximately equal to 15 (mu)m). For growth rates < 10-8 m/cycle, da/dN vs. (delta)K data for short cracks merged with the steady-state data for long cracks after ~;2 mm of extension.
Gamma-based TiAl intermetallic alloys have received considerable attention recently as candidate ... more Gamma-based TiAl intermetallic alloys have received considerable attention recently as candidate materials for high-temperature aerospace applications. Two classes of microstructure have been prominent in the two-phase (γ + αâ) alloys: a lamellar structure consisting of lamellar colonies containing alternating γ and αâ grains. In general, duplex structures display better elongation and strength, whereas lamellar structures show better toughness and fatigue
Two major mechanisms that could potentially be responsible for toughening in mineralized tissues,... more Two major mechanisms that could potentially be responsible for toughening in mineralized tissues, such as bone and dentin, have been identified—microcracking and crack bridging. While evidence has been reported for both mechanisms, there has been no consensus thus far on which mechanism plays the dominant role in toughening these materials. In the present study, we seek to present definitive experimental evidence supporting crack bridging, rather than microcracking, as the most significant mechanism of toughening in cortical bone and dentin. In vitro fracture toughness experiments were conducted to measure the variation of the fracture resistance with crack extension (resistance- curve (R-curve) behavior) for both materials with special attention paid to changes in the sample compliance. Because these two toughening mechanisms induce opposite effects on the sample compliance, such experiments allow for the definitive determination of the dominant toughening mechanism, which in the p...
The development of a mechanistic understanding of the increase in fracture risk in human bone wit... more The development of a mechanistic understanding of the increase in fracture risk in human bone with age is essential to public health. This represents a challenge for fracture mechanics as bone has a complex, hierarchical structure with characteristic features ranging from nanometer to macroscopic dimensions, and is thus much more complex than most engineering materials. In this study, we review ex vivo fracture experiments which quantitatively assess the effect of age on human cortical bone in the proximal-distal orientation, i.e. along the long axis of the bone. Specifically, cortical bone is seen to exhibit rising crack-growth resistance with crack extension; the toughness is consequently evaluated in terms of R-curves, measured in bones taken from a wide range of age groups (34-99 years). Both crack-initiation and crack-growth toughnesses were determined and were found to deteriorate with age; the initiation toughness decreases some 40% over the 65 years of ageing, while growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular involving crack bridging in the wake of the crack. This explanation is supported by an examination of the micro-/nanostructural changes accompanying the process of ageing, performed using deep-UV Raman spectroscopy, computed X-ray tomography and optical/ electron microscopy.
The role of metal layer thickness and resultant plastic constraint in the metal layer during the ... more The role of metal layer thickness and resultant plastic constraint in the metal layer during the failure of metal/ceramic layered structures is examined under cyclic and static loading conditions. Crack-growth experiments were conducted on sandwich specimens consisting of 99.999% pure aluminum layers bonded between 99.5% pure polycrystalline alumina with the metal layer thickness varying from 5 to 100 lm. Under cyclic loading, crack growth occurred primarily at the interface separating the two materials; additionally, stable fatigue cracks deviated into the alumina for thin-layered samples at high driving forces. Under monotonically increasing loads, the fracture toughness increased with Al layer thickness, whereas under cyclic loads the threshold driving force for crack growth conversely decreased with increasing layer thickness. Under static loading in a moist environment, interfacial crack growth was never observed at measurable rates (P10 À9 m/s) for driving forces up to 200 J/m 2 ; however, for thin-layered samples, subcritical cracks did deviate off the interface and grow, sometimes stably, into the alumina. Trends in crack-growth rates and crack trajectories are examined in terms of the level of constraint, loading conditions and environmental influences.
The aging-related deterioration of the fracture properties of bone, coupled with higher life expe... more The aging-related deterioration of the fracture properties of bone, coupled with higher life expectancy, is responsible for increasing incidence of bone fracture in the elderly; consequently, an understanding of how these fracture properties degrade with age is essential. In this study, ex vivo fracture experiments have been performed to quantitatively assess the effect of age on human cortical bone in the proximaldistal orientation, i.e., longitudinally along the osteons. Because cortical bone exhibits rising crack-growth resistance with crack extension, the toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from wide range of age groups (34 -99 years). Using this approach, both the crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases some 40% over six decades from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular, involving crack bridging in the wake of the crack. An examination of the micro-/nano-structural changes accompanying the process of aging, using optical microscopy, X-ray tomography, nanoindentation and Raman spectroscopy, is shown to support such observations. Published by Elsevier B.V.
Journal of the Mechanical Behavior of Biomedical Materials, 2009
Indentation techniques for assessing fracture toughness are attractive due to the simplicity and ... more Indentation techniques for assessing fracture toughness are attractive due to the simplicity and expediency of experiments, and because they potentially allow the characterization of both local and bulk fracture properties. Unfortunately, rarely have such techniques been proven to give accurate fracture toughness values. This is a concern, as such techniques are seeing increasing usage in the study of biomaterials and biological hard tissues.
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Papers by Jamie J Kruzic