Aging decreases the human femur's fatigue resistance, impact energy absorption, and the ability t... more Aging decreases the human femur's fatigue resistance, impact energy absorption, and the ability to withstand load. Changes in the osteocyte distribution and in their elemental composition might be involved in age-related bone impairment. To address this question, we carried out a histomorphometric assessment of the osteocyte lacunar distribution in the periosteal and endosteal human femoral cortexes of 16 female and 16 male donors with regard to age-and sex-related bone remodeling. Measurements of the bone mineral density distribution by quantitative backscattered electron imaging and energy dispersive X-ray analysis were taken to evaluate the osteocyte lacunar mineral composition and characteristics. Age-dependent decreases in the total osteocyte lacunar number were measured in all of the cases. This change signifies a risk for the bone's safety. Cortical subdivision into periosteal and endosteal regions of interest emphasized that, in both sexes, primarily the endosteal cortex is affected by age-dependent reduction in number of osteocyte lacunae, whereas the periosteal compartment showed a less pronounced osteocyte lacunar deficiency. In aged bone, osteocyte lacunae showed an increased amount of hypermineralized calcium phosphate occlusions in comparison with younger cases. With respect to Frost's early delineation of micropetrosis, our microanalyses revealed that the osteocyte lacunae are subject to hypermineralization. Intralacunar hypermineralization accompanied by a decrease in total osteocyte lacunar density may contribute to failure or delayed bone repair in aging bone. A decreased osteocyte lacunar density may cause deteriorations in the canalicular fluid flow and reduce the detection of microdamage, which counteracts the bone's structural integrity, while hypermineralized osteocyte lacunae may increase bone brittleness and render the bone fragile.
Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree tha... more Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree than predicted from change in BMD in women with postmenopausal osteoporosis. We hypothesize that bone active agents improve bone strength in osteoporotic bone by altering different material properties of the bone. Eighteen-month-old female Fischer rats were ovariectomized (OVX) or sham-operated and left untreated for 60 days to induce osteopenia before they were treated with single doses of either risedronate (500 mg/kg, IV), zoledronic acid (100 mg/kg, IV), raloxifene (2 mg/kg, PO, three times per week), hPTH(1-34) (25 mg/kg, SC, three times per week), or vehicle (NS; 1 ml/kg, three times per week). Groups of animals were killed after days 60 and 180 of treatment, and either the proximal tibial metaphysis or lumbar vertebral body were studied. Bone volume and architecture were assessed by mCT and histomorphometry. Measurements of bone quality included the degree of bone mineralization (DBM), localized elastic modulus, bone turnover by histomorphometry, compression testing of the LVB, and three-point bending testing of the femur. The trabecular bone volume, DBM, elastic modulus, and compressive bone strength were all significantly lower at day 60 post-OVX (pretreatment, day 0 study) than at baseline. After 60 days of all of the bone active treatments, bone mass and material measurements agent were restored. However, after 180 days of treatment, the OVX + PTH group further increased BV/TV (+30% from day 60, p < 0.05 within group and between groups). In addition, after 180 days of treatment, there was more highly mineralized cortical and trabecular bone and increased cortical bone size and whole bone strength in OVX + PTH compared with other OVX + antiresorptives. Treatment of estrogen-deficient aged rats with either antiresorptive agents or PTH rapidly improved many aspects of bone quality including microarchitecture, bone mineralization, turnover, and bone strength. However, prolonged treatment for 180 days with PTH resulted in additional gains in bone quality and bone strength, suggesting that the maximal gains in bone strength in cortical and trabecular bone sites may require a longer treatment period with PTH.
The majority of fracture mechanics studies on the toughness of bone have been performed under ten... more The majority of fracture mechanics studies on the toughness of bone have been performed under tensile loading. However, it has recently been shown that the toughness of human cortical bone in the transverse (breaking) orientation is actually much lower in shear (mode II) than in tension (mode I); a fact that is physiologically relevant as in vivo bone is invariably loaded multiaxially. Since bone is a material that derives its fracture resistance primarily during crack growth through extrinsic toughening mechanisms, such as crack deflection and bridging, evaluation of its toughness is best achieved through measurements of the crack-resistance or R-curve, which describes the fracture toughness as a function of crack extension. Accordingly, in this study, we attempt to measure for the first time the R-curve fracture toughness of human cortical bone under physiologically relevant mixed-mode loading conditions. We show that the resulting mixed-mode (mode I + II) toughness depends strongly on the crack trajectory and is the result of the competition between the paths of maximum mechanical driving force and "weakest" microstructural resistance.
Dislocation activities are crucial in facilitating plastic deformation, even in metals that are p... more Dislocation activities are crucial in facilitating plastic deformation, even in metals that are prone to deformation twinning. We have discovered a novel type of proli c dislocation sources, which reside on nano-sized ridges along the borders between different twin variants in low stacking-fault energy (SFE) materials. These sources act as dislocation generators that promote dislocation interaction and accumulation, spreading plastic strain and leading to robust strain hardening at the early stages of plastic deformation. Molecular dynamic simulations indicate that the formation of nano-sized ridge-twin structures is energetically favorable at the junctions between multiple twins, explaining why such structures are ubiquitous in low-SFE materials. Decreasing the SFE can signi cantly increase the population of ridge-twin boundaries, facilitating strain hardening to sustain the stability of plastic ow. These ndings provide new insights into the origin of dislocation plasticity and the high early-stage strain hardening rate in low-SFE materials.
A study has been made of the mechanics and mechanisms of fati9ue crack propagation in a commercia... more A study has been made of the mechanics and mechanisms of fati9ue crack propagation in a commercial plate of aluminum-lithium alloy 2090-TBE41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-small (2 to 1000 um) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventiona 1 long (~5 nm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at 6K levels as low as 0.7 MP arm, far be 1 ow the long crack thresho 1 d 6KTH. The i r behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack resu 1 ts wi th long crack growth rates plotted in terms of 6K e ff (i .e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of 1 arge and sma 11 cracks wi th mi.crostructura 1 features, are briefly discussed.
Quantitative assessment of the strength and toughness of bone has become an integral part of many... more Quantitative assessment of the strength and toughness of bone has become an integral part of many biological and bioengineering studies on the structural properties of bone and their degradation due to aging, disease and therapeutic treatment. Whereas the biomechanical techniques for characterizing bone strength are well documented, few studies have focused on the theory, methodology, and various experimental procedures for evaluating the fracture toughness of bone, i.e., its resistance to fracture, with particular reference to whole bone testing in small animal studies. In this tutorial, we consider the many techniques for evaluating toughness and assess their specific relevance and application to the mechanical testing of small animal bones. Parallel experimental studies on wild-type rat and mouse femurs are used to evaluate the utility of these techniques and specifically to determine the coefficient of variation of the measured toughness values.
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercia... more A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-TBE41. In Part I, the crack growth and crack shielding behavior of 1 ong (~5 mm) through-th i ckness cracks is exami ned as a funct ion of plate or i entat i on and load ratio, and resu 1 ts compared to traditional high strength aluminum alloys. It is shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range) than in traditional alloys, a 1 though beha v i or is strong 1 y ani sotropi c. Di fferences ingrowth rates of up to 4 orders of magnitude are observed between the L-T, T-Land T-S orientations, which show the best crack growth resistance, and the S-L, S-T and L+45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the ro 1 e of the crack path morpho logy in i nduc i ng crack deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane (e.g., L-T, etc.) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest growth rates, fat i gue fractures para 11 e 1 to the ro 11 i ng plane (e.g., S-L, etc.) occur by an intergranu1ar, delamination-type separation, with much lower shielding levels to give the fastest growth rates. The imp 1 ications of such "extrinsic toughening" effects on the fracture and fatigue properties of aluminum-lithium alloys are discussed in detai 1.
High-performance, fast-growing natural materials with sustainable and functional features current... more High-performance, fast-growing natural materials with sustainable and functional features currently arouse significant attention. Here, facile processing, involving delignification, in situ hydrothermal synthesis of TiO2 and pressure densification, is employed to transform natural bamboo into a high-performance structural material. The resulting TiO2-decorated densified bamboo exhibits high flexural strength and elastic stiffness, with both properties more than double that of natural bamboo. Real-time acoustic emission reveals the key role of the TiO2 nanoparticles in enhancing the flexural properties. The introduction of nanoscale TiO2 is found to markedly increase the degree of oxidation and the formation of hydrogen bonds in bamboo materials, leading to extensive interfacial failure between the microfibers, a micro-fibrillation process that results in substantial energy consumption and high fracture resistance. This work furthers the strategy of the synthetic reinforcement of fas...
In many concentrated alloys of current interest, the observation of diffuse superlattice intensit... more In many concentrated alloys of current interest, the observation of diffuse superlattice intensities by transmission electron microscopy has been attributed to chemical short-range order. We briefly review these findings and comment on the plausibility of widespread interpretations, noting the absence of expected peaks, conflicts with theoretical predictions, and the possibility of alternative explanations.
Dislocation activities play an important role in mediating plastic deformation, even in metals th... more Dislocation activities play an important role in mediating plastic deformation, even in metals that are prone to deformation twinning. Combining multi-scale and in situ electron microscope characterizations, here we report a discovery of a unique type of dislocation sources that are particularly fertile in low stacking-fault energy materials, including CrCoNi-based high-entropy alloys and twinning-induced plasticity steel. These sources reside on nano-sized ridges, which form strings along the borders between different twin variants to accommodate the multiple twinning relationships. Upon plastic deformation, such ridge-twin structures act as an effective dislocation generator, from which dislocations are emitted into the coherent twin boundaries or cross slip into the interior of grains. At larger strain, the incoherent boundaries of the nano-sized ridge-twins emit partial dislocations to mediate deformation twinning as well. Molecular dynamic simulations indicate that the formation of nano-sized ridge-twin structures is
Traditional metallic alloys are mixtures of elements where the atoms of minority species tend to ... more Traditional metallic alloys are mixtures of elements where the atoms of minority species tend to distribute randomly if they are below their solubility limit, or lead to the formation of secondary phases if they are above it. Recently, the concept of medium/high entropy alloys (MEA/HEA) has expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements that have been shown to display enhanced mechanical properties. However, the question has remained as to how random these solid solutions actually are, with the influence of chemical short-range order (SRO) suggested in computational simulations but not seen experimentally. Here we report the first direct observation of SRO in the CrCoNi MEA using high resolution and energy-filtered transmission electron microscopy. Increasing amounts of SRO give rise to both higher stacking fault energy and hardness. These discoveries suggest that the degree of chemical ordering at the nanometer scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of MEA/HEAs.
Heterostructured materials are an emerging class of materials with superior performances that are... more Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones with dramatic (> 100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. The heterostructured materials field explores heterostructures to control defect distributions, long-range internal stresses, and nonlinear inter-zone interactions for unprecedented performances. This paper is aimed to provide perspectives on this novel field, describe the state-of-the-art of heterostructured materials, and identify and discuss key issues that deserve additional studies. IMPACT STATEMENT This paper delineates heterostructured materials, which are emerging as a new class of materials with unprecedented properties, new materials science and economic industrial production.
Strong and tough steel Ultrahard materials often do not have similarly impressive fracture toughn... more Strong and tough steel Ultrahard materials often do not have similarly impressive fracture toughness. Liu et al. discovered a processing route for medium manganese steel alloy in which ultrahigh strength accompanies high fracture toughness. The steel relies on both transformation-induced plasticity and delamination toughening to boost the fracture properties. The steel is composed of less expensive elements, making it a potentially inexpensive material attractive for structural applications. Science , this issue p. 1347
Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and... more Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and low density. It is found in various tissues such as hairs, feathers, horns, and hooves with various functionalities. For instance, horn keratin absorbs a large amount of energy during intraspecific fights. Keratinized tissues are permanent tissues because of their basic composition consisting of dead keratinized cells that are not able to remodel or regrow once broken or damaged. The lack of a self‐healing mechanism presents a problem for horns, as they are under continued high risk from mechanical damage. In the present work, it is shown for the first time that a combination of material architecture and a water‐assisted recovery mechanism, in the horn of bighorn sheep, endows them with shape and mechanical property recoverability after being subjected to severe compressive loading. Moreover, the effect of hydration is unraveled, on the material molecular structure and mechanical behavi...
The equine hoof has been considered as an efficient energy absorption layer that protects the ske... more The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60% compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure.
Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetti... more Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetting polymeric matrices is of significant interest to a broad range of applications. In the present work we report a multi-scale approach to synergistically toughen composites by combining nano-and macro-scale reinforcements inspired by natural composite materials. Carbon reinforcements with two different length scales are used: nano-scale carbon nanofibres (~100 nm diameter) and macro-scale carbon z-pins (~280 µm diameter) to reinforce continuous carbon-fibre composites in the through-thickness direction. The resultant composite, featuring three-dimensional reinforcement architecture, possesses triple toughening mechanisms at three different scales, thus yielding a synergistic effect. At the nano-scale, the carbon nanofibres alone promote high mode I delamination resistance (~70% increase in interlaminar fracture energy) by multiple intrinsic and extrinsic toughening processes around the crack tip. The macro-size carbon z-pins, together with the crossover continuous fibres, promote a strong extrinsic toughening mechanism (~200% increase in the interlaminar fracture energy) behind the crack tip and over a larger length-scale via both the z-pins and crossover fibres bridging the crack faces. When used concurrently, the nanofillers and z-pins promote a higher toughness under quasi-static loading (~400% increase in fracture energy) than when used separately due to a multiplicative effect from the interplay between intrinsic and extrinsic toughening processes operative ahead of, and behind, the crack tip. Under mode I interlaminar cyclic-fatigue loading, the multiscale laminates show a strong improvement in resistance against fatigue delamination growth. Similar to the synergistic increase in fracture energy, a greater increase in the delamination fatigue resistance occurs when both are active together. However, the results indicate that the synergistic effect of the multi-scale toughening is statistically significant under quasi-static loading but not under fatigue loading. A very small reduction (~2%) in the tensile strength is observed for the multi-scale reinforced laminates.
Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA, Jan 18, 2015
This study was to determine if antibody against sclerostin (Scl-Ab) could prevent glucocorticoid ... more This study was to determine if antibody against sclerostin (Scl-Ab) could prevent glucocorticoid (GC)-induced osteoporosis in mice. We found that Scl-Ab prevented GC-induced reduction in bone mass and bone strength and that the anabolic effects of Scl-Ab might be partially achieved through the preservation of osteoblast activity through autophagy. Glucocorticoids (GCs) inhibit bone formation by altering osteoblast and osteocyte cell activity and lifespan. A monoclonal antibody against sclerostin, Scl-Ab, increased bone mass in both preclinical animal and clinical studies in subjects with low bone mass. The objectives of this study were to determine if treatment with the Scl-Ab could prevent loss of bone mass and strength in a mouse model of GC excess and to elucidate if Scl-Ab modulated bone cell activity through autophagy. We generated reporter mice that globally expressed dsRed fused to LC3, a protein marker for autophagosomes, and evaluated the dose-dependent effects of GCs (0, 0...
Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is ce... more Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is central to their application as structural materials. Materials discovery efforts for HEAs are therefore impeded by an incomplete understanding of the chemical factors governing this balance. Through first-principles calculations, this study explores factors governing intrinsic ductility of a crucial subset of HEAs-those with a body-centered cubic (bcc) crystal structure. Analyses of three sets of bcc HEAs comprising nine different compositions reveal that alloy chemistry profoundly influences screw dislocation core structure, dislocation vibrational properties, and intrinsic ductility parameters derived from unstable stacking fault and surface energies. Key features in the electronic structure are identified that correlate with these properties: the fraction of occupied bonding states and bimodality of the d-orbital density of states. The findings enhance the fundamental understanding of the origins of intrinsic ductility and establish an electronic structure-based framework for computationally accelerated materials discovery and design.
Aging decreases the human femur's fatigue resistance, impact energy absorption, and the ability t... more Aging decreases the human femur's fatigue resistance, impact energy absorption, and the ability to withstand load. Changes in the osteocyte distribution and in their elemental composition might be involved in age-related bone impairment. To address this question, we carried out a histomorphometric assessment of the osteocyte lacunar distribution in the periosteal and endosteal human femoral cortexes of 16 female and 16 male donors with regard to age-and sex-related bone remodeling. Measurements of the bone mineral density distribution by quantitative backscattered electron imaging and energy dispersive X-ray analysis were taken to evaluate the osteocyte lacunar mineral composition and characteristics. Age-dependent decreases in the total osteocyte lacunar number were measured in all of the cases. This change signifies a risk for the bone's safety. Cortical subdivision into periosteal and endosteal regions of interest emphasized that, in both sexes, primarily the endosteal cortex is affected by age-dependent reduction in number of osteocyte lacunae, whereas the periosteal compartment showed a less pronounced osteocyte lacunar deficiency. In aged bone, osteocyte lacunae showed an increased amount of hypermineralized calcium phosphate occlusions in comparison with younger cases. With respect to Frost's early delineation of micropetrosis, our microanalyses revealed that the osteocyte lacunae are subject to hypermineralization. Intralacunar hypermineralization accompanied by a decrease in total osteocyte lacunar density may contribute to failure or delayed bone repair in aging bone. A decreased osteocyte lacunar density may cause deteriorations in the canalicular fluid flow and reduce the detection of microdamage, which counteracts the bone's structural integrity, while hypermineralized osteocyte lacunae may increase bone brittleness and render the bone fragile.
Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree tha... more Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree than predicted from change in BMD in women with postmenopausal osteoporosis. We hypothesize that bone active agents improve bone strength in osteoporotic bone by altering different material properties of the bone. Eighteen-month-old female Fischer rats were ovariectomized (OVX) or sham-operated and left untreated for 60 days to induce osteopenia before they were treated with single doses of either risedronate (500 mg/kg, IV), zoledronic acid (100 mg/kg, IV), raloxifene (2 mg/kg, PO, three times per week), hPTH(1-34) (25 mg/kg, SC, three times per week), or vehicle (NS; 1 ml/kg, three times per week). Groups of animals were killed after days 60 and 180 of treatment, and either the proximal tibial metaphysis or lumbar vertebral body were studied. Bone volume and architecture were assessed by mCT and histomorphometry. Measurements of bone quality included the degree of bone mineralization (DBM), localized elastic modulus, bone turnover by histomorphometry, compression testing of the LVB, and three-point bending testing of the femur. The trabecular bone volume, DBM, elastic modulus, and compressive bone strength were all significantly lower at day 60 post-OVX (pretreatment, day 0 study) than at baseline. After 60 days of all of the bone active treatments, bone mass and material measurements agent were restored. However, after 180 days of treatment, the OVX + PTH group further increased BV/TV (+30% from day 60, p < 0.05 within group and between groups). In addition, after 180 days of treatment, there was more highly mineralized cortical and trabecular bone and increased cortical bone size and whole bone strength in OVX + PTH compared with other OVX + antiresorptives. Treatment of estrogen-deficient aged rats with either antiresorptive agents or PTH rapidly improved many aspects of bone quality including microarchitecture, bone mineralization, turnover, and bone strength. However, prolonged treatment for 180 days with PTH resulted in additional gains in bone quality and bone strength, suggesting that the maximal gains in bone strength in cortical and trabecular bone sites may require a longer treatment period with PTH.
The majority of fracture mechanics studies on the toughness of bone have been performed under ten... more The majority of fracture mechanics studies on the toughness of bone have been performed under tensile loading. However, it has recently been shown that the toughness of human cortical bone in the transverse (breaking) orientation is actually much lower in shear (mode II) than in tension (mode I); a fact that is physiologically relevant as in vivo bone is invariably loaded multiaxially. Since bone is a material that derives its fracture resistance primarily during crack growth through extrinsic toughening mechanisms, such as crack deflection and bridging, evaluation of its toughness is best achieved through measurements of the crack-resistance or R-curve, which describes the fracture toughness as a function of crack extension. Accordingly, in this study, we attempt to measure for the first time the R-curve fracture toughness of human cortical bone under physiologically relevant mixed-mode loading conditions. We show that the resulting mixed-mode (mode I + II) toughness depends strongly on the crack trajectory and is the result of the competition between the paths of maximum mechanical driving force and "weakest" microstructural resistance.
Dislocation activities are crucial in facilitating plastic deformation, even in metals that are p... more Dislocation activities are crucial in facilitating plastic deformation, even in metals that are prone to deformation twinning. We have discovered a novel type of proli c dislocation sources, which reside on nano-sized ridges along the borders between different twin variants in low stacking-fault energy (SFE) materials. These sources act as dislocation generators that promote dislocation interaction and accumulation, spreading plastic strain and leading to robust strain hardening at the early stages of plastic deformation. Molecular dynamic simulations indicate that the formation of nano-sized ridge-twin structures is energetically favorable at the junctions between multiple twins, explaining why such structures are ubiquitous in low-SFE materials. Decreasing the SFE can signi cantly increase the population of ridge-twin boundaries, facilitating strain hardening to sustain the stability of plastic ow. These ndings provide new insights into the origin of dislocation plasticity and the high early-stage strain hardening rate in low-SFE materials.
A study has been made of the mechanics and mechanisms of fati9ue crack propagation in a commercia... more A study has been made of the mechanics and mechanisms of fati9ue crack propagation in a commercial plate of aluminum-lithium alloy 2090-TBE41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-small (2 to 1000 um) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventiona 1 long (~5 nm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at 6K levels as low as 0.7 MP arm, far be 1 ow the long crack thresho 1 d 6KTH. The i r behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack resu 1 ts wi th long crack growth rates plotted in terms of 6K e ff (i .e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of 1 arge and sma 11 cracks wi th mi.crostructura 1 features, are briefly discussed.
Quantitative assessment of the strength and toughness of bone has become an integral part of many... more Quantitative assessment of the strength and toughness of bone has become an integral part of many biological and bioengineering studies on the structural properties of bone and their degradation due to aging, disease and therapeutic treatment. Whereas the biomechanical techniques for characterizing bone strength are well documented, few studies have focused on the theory, methodology, and various experimental procedures for evaluating the fracture toughness of bone, i.e., its resistance to fracture, with particular reference to whole bone testing in small animal studies. In this tutorial, we consider the many techniques for evaluating toughness and assess their specific relevance and application to the mechanical testing of small animal bones. Parallel experimental studies on wild-type rat and mouse femurs are used to evaluate the utility of these techniques and specifically to determine the coefficient of variation of the measured toughness values.
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercia... more A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-TBE41. In Part I, the crack growth and crack shielding behavior of 1 ong (~5 mm) through-th i ckness cracks is exami ned as a funct ion of plate or i entat i on and load ratio, and resu 1 ts compared to traditional high strength aluminum alloys. It is shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range) than in traditional alloys, a 1 though beha v i or is strong 1 y ani sotropi c. Di fferences ingrowth rates of up to 4 orders of magnitude are observed between the L-T, T-Land T-S orientations, which show the best crack growth resistance, and the S-L, S-T and L+45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the ro 1 e of the crack path morpho logy in i nduc i ng crack deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane (e.g., L-T, etc.) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest growth rates, fat i gue fractures para 11 e 1 to the ro 11 i ng plane (e.g., S-L, etc.) occur by an intergranu1ar, delamination-type separation, with much lower shielding levels to give the fastest growth rates. The imp 1 ications of such "extrinsic toughening" effects on the fracture and fatigue properties of aluminum-lithium alloys are discussed in detai 1.
High-performance, fast-growing natural materials with sustainable and functional features current... more High-performance, fast-growing natural materials with sustainable and functional features currently arouse significant attention. Here, facile processing, involving delignification, in situ hydrothermal synthesis of TiO2 and pressure densification, is employed to transform natural bamboo into a high-performance structural material. The resulting TiO2-decorated densified bamboo exhibits high flexural strength and elastic stiffness, with both properties more than double that of natural bamboo. Real-time acoustic emission reveals the key role of the TiO2 nanoparticles in enhancing the flexural properties. The introduction of nanoscale TiO2 is found to markedly increase the degree of oxidation and the formation of hydrogen bonds in bamboo materials, leading to extensive interfacial failure between the microfibers, a micro-fibrillation process that results in substantial energy consumption and high fracture resistance. This work furthers the strategy of the synthetic reinforcement of fas...
In many concentrated alloys of current interest, the observation of diffuse superlattice intensit... more In many concentrated alloys of current interest, the observation of diffuse superlattice intensities by transmission electron microscopy has been attributed to chemical short-range order. We briefly review these findings and comment on the plausibility of widespread interpretations, noting the absence of expected peaks, conflicts with theoretical predictions, and the possibility of alternative explanations.
Dislocation activities play an important role in mediating plastic deformation, even in metals th... more Dislocation activities play an important role in mediating plastic deformation, even in metals that are prone to deformation twinning. Combining multi-scale and in situ electron microscope characterizations, here we report a discovery of a unique type of dislocation sources that are particularly fertile in low stacking-fault energy materials, including CrCoNi-based high-entropy alloys and twinning-induced plasticity steel. These sources reside on nano-sized ridges, which form strings along the borders between different twin variants to accommodate the multiple twinning relationships. Upon plastic deformation, such ridge-twin structures act as an effective dislocation generator, from which dislocations are emitted into the coherent twin boundaries or cross slip into the interior of grains. At larger strain, the incoherent boundaries of the nano-sized ridge-twins emit partial dislocations to mediate deformation twinning as well. Molecular dynamic simulations indicate that the formation of nano-sized ridge-twin structures is
Traditional metallic alloys are mixtures of elements where the atoms of minority species tend to ... more Traditional metallic alloys are mixtures of elements where the atoms of minority species tend to distribute randomly if they are below their solubility limit, or lead to the formation of secondary phases if they are above it. Recently, the concept of medium/high entropy alloys (MEA/HEA) has expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements that have been shown to display enhanced mechanical properties. However, the question has remained as to how random these solid solutions actually are, with the influence of chemical short-range order (SRO) suggested in computational simulations but not seen experimentally. Here we report the first direct observation of SRO in the CrCoNi MEA using high resolution and energy-filtered transmission electron microscopy. Increasing amounts of SRO give rise to both higher stacking fault energy and hardness. These discoveries suggest that the degree of chemical ordering at the nanometer scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of MEA/HEAs.
Heterostructured materials are an emerging class of materials with superior performances that are... more Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones with dramatic (> 100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. The heterostructured materials field explores heterostructures to control defect distributions, long-range internal stresses, and nonlinear inter-zone interactions for unprecedented performances. This paper is aimed to provide perspectives on this novel field, describe the state-of-the-art of heterostructured materials, and identify and discuss key issues that deserve additional studies. IMPACT STATEMENT This paper delineates heterostructured materials, which are emerging as a new class of materials with unprecedented properties, new materials science and economic industrial production.
Strong and tough steel Ultrahard materials often do not have similarly impressive fracture toughn... more Strong and tough steel Ultrahard materials often do not have similarly impressive fracture toughness. Liu et al. discovered a processing route for medium manganese steel alloy in which ultrahigh strength accompanies high fracture toughness. The steel relies on both transformation-induced plasticity and delamination toughening to boost the fracture properties. The steel is composed of less expensive elements, making it a potentially inexpensive material attractive for structural applications. Science , this issue p. 1347
Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and... more Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and low density. It is found in various tissues such as hairs, feathers, horns, and hooves with various functionalities. For instance, horn keratin absorbs a large amount of energy during intraspecific fights. Keratinized tissues are permanent tissues because of their basic composition consisting of dead keratinized cells that are not able to remodel or regrow once broken or damaged. The lack of a self‐healing mechanism presents a problem for horns, as they are under continued high risk from mechanical damage. In the present work, it is shown for the first time that a combination of material architecture and a water‐assisted recovery mechanism, in the horn of bighorn sheep, endows them with shape and mechanical property recoverability after being subjected to severe compressive loading. Moreover, the effect of hydration is unraveled, on the material molecular structure and mechanical behavi...
The equine hoof has been considered as an efficient energy absorption layer that protects the ske... more The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60% compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure.
Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetti... more Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetting polymeric matrices is of significant interest to a broad range of applications. In the present work we report a multi-scale approach to synergistically toughen composites by combining nano-and macro-scale reinforcements inspired by natural composite materials. Carbon reinforcements with two different length scales are used: nano-scale carbon nanofibres (~100 nm diameter) and macro-scale carbon z-pins (~280 µm diameter) to reinforce continuous carbon-fibre composites in the through-thickness direction. The resultant composite, featuring three-dimensional reinforcement architecture, possesses triple toughening mechanisms at three different scales, thus yielding a synergistic effect. At the nano-scale, the carbon nanofibres alone promote high mode I delamination resistance (~70% increase in interlaminar fracture energy) by multiple intrinsic and extrinsic toughening processes around the crack tip. The macro-size carbon z-pins, together with the crossover continuous fibres, promote a strong extrinsic toughening mechanism (~200% increase in the interlaminar fracture energy) behind the crack tip and over a larger length-scale via both the z-pins and crossover fibres bridging the crack faces. When used concurrently, the nanofillers and z-pins promote a higher toughness under quasi-static loading (~400% increase in fracture energy) than when used separately due to a multiplicative effect from the interplay between intrinsic and extrinsic toughening processes operative ahead of, and behind, the crack tip. Under mode I interlaminar cyclic-fatigue loading, the multiscale laminates show a strong improvement in resistance against fatigue delamination growth. Similar to the synergistic increase in fracture energy, a greater increase in the delamination fatigue resistance occurs when both are active together. However, the results indicate that the synergistic effect of the multi-scale toughening is statistically significant under quasi-static loading but not under fatigue loading. A very small reduction (~2%) in the tensile strength is observed for the multi-scale reinforced laminates.
Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA, Jan 18, 2015
This study was to determine if antibody against sclerostin (Scl-Ab) could prevent glucocorticoid ... more This study was to determine if antibody against sclerostin (Scl-Ab) could prevent glucocorticoid (GC)-induced osteoporosis in mice. We found that Scl-Ab prevented GC-induced reduction in bone mass and bone strength and that the anabolic effects of Scl-Ab might be partially achieved through the preservation of osteoblast activity through autophagy. Glucocorticoids (GCs) inhibit bone formation by altering osteoblast and osteocyte cell activity and lifespan. A monoclonal antibody against sclerostin, Scl-Ab, increased bone mass in both preclinical animal and clinical studies in subjects with low bone mass. The objectives of this study were to determine if treatment with the Scl-Ab could prevent loss of bone mass and strength in a mouse model of GC excess and to elucidate if Scl-Ab modulated bone cell activity through autophagy. We generated reporter mice that globally expressed dsRed fused to LC3, a protein marker for autophagosomes, and evaluated the dose-dependent effects of GCs (0, 0...
Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is ce... more Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is central to their application as structural materials. Materials discovery efforts for HEAs are therefore impeded by an incomplete understanding of the chemical factors governing this balance. Through first-principles calculations, this study explores factors governing intrinsic ductility of a crucial subset of HEAs-those with a body-centered cubic (bcc) crystal structure. Analyses of three sets of bcc HEAs comprising nine different compositions reveal that alloy chemistry profoundly influences screw dislocation core structure, dislocation vibrational properties, and intrinsic ductility parameters derived from unstable stacking fault and surface energies. Key features in the electronic structure are identified that correlate with these properties: the fraction of occupied bonding states and bimodality of the d-orbital density of states. The findings enhance the fundamental understanding of the origins of intrinsic ductility and establish an electronic structure-based framework for computationally accelerated materials discovery and design.
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Papers by Robert O Ritchie