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John G. Skedros - One of the best experts on this subject based on the ideXlab platform.
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utility of osteon circularity for determining species and interpreting load history in primates and nonprimates
American Journal of Physical Anthropology, 2017Co-Authors: Kendra E. Keenan, Chad S Mears, John G. SkedrosAbstract:Objectives Histomorphological analyses of bones are used to estimate an individual's chronological age, interpret a bone's load history, and differentiate species. Among various histomorphological characteristics that can influence mechanical properties of cortical bone, secondary osteon (Haversian system) population density and predominant collagen fiber orientation are particularly important. Cross-sectional shape characteristics of secondary osteons (On.Cr = osteon circularity, On.El = osteon ellipticality) are considered helpful in these contexts, but more robust proof is needed. We sought to determine if variations in osteon shape characteristics are sufficient for accurately differentiating species, load-complexity categories, and regional habitual Strain-Mode distributions (e.g., tension vs. compression regions). Materials and Methods Circularly polarized light images were obtained from 100-micron transverse sections from diaphyses of adult deer calcanei; sheep calcanei, radii, and tibiae; equine calcanei, radii, and third metacarpals (MC3s); chimpanzee femora; and human femora and fibulae. Osteon cross-sectional area (On.Ar), On.Cr, and On.El were quantified indiscriminately and in the contexts of load-complexity and regional Strain-Mode distributions. Results On.Cr and On.El, when examined independently in terms of all data, or mean (nested) data, for each bone, exceeded 80% accuracy in the inter-species comparisons only with respect to distinguishing humans from nonhumans. Correct classification among the nonhuman species was <70%. When On.Cr and On.El were coupled together and with On.Ar in discriminant function analyses (nested and unnested data) there were high misclassifications in all but human vs. nonhuman comparisons. Discussion Frequent misclassifications in nonhuman comparisons might reflect influences of habitual load complexity and/or Strain-Mode distributions, or other factors not accounted for by these two considerations.
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secondary osteon size and collagen lamellar organization osteon morphotypes are not coupled but potentially adapt independently for local Strain Mode or magnitude
Journal of Structural Biology, 2013Co-Authors: John G. Skedros, Kendra E. Keenan, Tyler J. Williams, Casey J. KiserAbstract:In bone, matrix slippage that occurs at cement lines of secondary osteons during loading is an important toughening mechanism. Toughness can also be enhanced by modifications in osteon cross-sectional size (diameter) for specific load environments; for example, smaller osteons in more highly Strained "compression" regions vs. larger osteons in less Strained "tension" regions. Additional osteon characteristics that enhance toughness are distinctive variations in collagen/lamellar organization (i.e., "osteon morphotypes"). Interactions might exist between osteon diameter and morphotype that represent adaptations for resisting deleterious shear stresses that occur at the cement line. This may be why osteons often have a peripheral ring (or "hoop") of highly oblique/transverse collagen. We hypothesized that well developed/distinct "hoops" are compensatory adaptations in cases where increased osteon diameter is mechanically advantageous (e.g., larger osteons in "tension" regions would have well developed/distinct "hoops" in order to resist deleterious consequences of co-existing localized shear stresses). We tested this hypothesis by determining if there are correlations between osteon diameters and strongly hooped morphotypes in "tension", "compression", and "neutral axis" regions of femora (chimpanzees, humans), radii (horse, sheep) and calcanei (horse, deer). The results reject the hypothesis-larger osteons are not associated with well developed/distinct "hoops", even in "tension regions" where the effect was expected to be obvious. Although osteon diameter and morphotype are not coupled, osteon diameters seem to be associated with increased Strain magnitudes in some cases, but this is inconsistent. By contrast, osteon morphotypes are more strongly correlated with the distribution of tension and compression.
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interpreting cortical bone adaptation and load history by quantifying osteon morphotypes in circularly polarized light images
Bone, 2009Co-Authors: John G. Skedros, Casey J. Kiser, Shaun D Mendenhall, Howard WinetAbstract:Birefringence variations in circularly polarized light (CPL) images of thin plane-parallel sections of cortical bone can be used to quantify regional differences in predominant collagen fiber orientation (CFO). Using CPL images of equine third metacarpals (MC3s), R.B. Martin, V.A. Gibson, S.M. Stover, J.C. Gibeling, and L.V. Griffin. (40) described six secondary osteon variants ('morphotypes') and suggested that differences in their regional prevalence affect fatigue resistance and toughness. They devised a numerical osteon morphotype score (MTS) for quantifying regional differences in osteon morphotypes. We have observed that a modification of this score could significantly improve its use for interpreting load history. We hypothesized that our modified osteon MTS would more accurately reveal differences in osteon MTSs between opposing "tension" and "compression" cortices of diaphyses of habitually bent bones. This was tested using CPL images in transverse sections of calcanei from sheep, deer, and horses, and radii from sheep and horses. Equine MC3s and sheep tibiae were examined as controls because they experience comparatively greater load complexity that, because of increased prevalence of torsion/shear, would not require regional mechanical enhancements provided by different osteon morphotypes. Predominant CFO, which can reliably reflect adaptation for a regionally prevalent Strain Mode, was quantified as mean gray levels from birefringence of entire images (excluding pore spaces) in anterior, posterior, medial, and lateral cortices. Results showed that, in contrast to the original scoring scheme of Martin et al., the modified scheme revealed significant anterior/posterior differences in osteon MTSs in nearly all "tension/compression" bones (p<0.0001), but not in equine MC3s (p=0.30) and sheep tibiae (p=0.35). Among habitually bent bones, sheep radii were the exception; relatively lower osteon populations and the birefringence of the primary bone contributed to this result. Correlations between osteon MTSs using the scoring scheme of Martin et al. with CFO data from all regions of each bone invariably demonstrated weak-to-Moderate negative correlations. This contrasts with typically high positive correlations between modified osteon MTSs and regional CFO. These results show that the modified osteon MTS can be a strong correlate of predominant CFO and of the non-uniform Strain distribution produced by habitual bending.
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the influence of collagen fiber orientation and other histocompositional characteristics on the mechanical properties of equine cortical bone
The Journal of Experimental Biology, 2006Co-Authors: John G. Skedros, Roy D Bloebaum, Michael R Dayton, Christian L Sybrowsky, Kent N BachusAbstract:SUMMARY This study examined relative influences of predominant collagen fiber orientation (CFO), mineralization (% ash), and other microstructural characteristics on the mechanical properties of equine cortical bone. Using Strain-Mode-specific (S-M-S) testing (compression testing of bone habitually loaded in compression; tension testing of bone habitually loaded in tension), the relative mechanical importance of CFO and other material characteristics were examined in equine third metacarpals (MC3s). This Model was chosen since it had a consistent non-uniform Strain distribution estimated by finite element analysis (FEA) near mid-diaphysis of a thoroughbred horse, net tension in the dorsal/lateral cortices and net compression in the palmar/medial cortices. Bone specimens from regions habitually loaded in tension or compression were: (1) tested to failure in both axial compression and tension in order to contrast S-M-S vs non-S-M-S behavior, and (2) analyzed for CFO, % ash, porosity, fractional area of secondary osteonal bone, osteon cross-sectional area, and population densities of secondary osteons and osteocyte lacunae. Multivariate multiple regression analyses revealed that in S-M-S compression testing, CFO most strongly influenced total energy (pre-yield elastic energy plus post-yield plastic energy); in S-M-S tension testing CFO most strongly influenced post-yield energy and total energy. CFO was less important in explaining S-M-S elastic modulus, and yield and ultimate stress. Therefore, in S-M-S loading CFO appears to be important in influencing energy absorption, whereas the other characteristics have a more dominant influence in elastic modulus, pre-yield behavior and strength. These data generally support the hypothesis that differentially affecting S-M-S energy absorption may be an important consequence of regional histocompositional heterogeneity in the equine MC3. Data inconsistent with the hypothesis, including the lack of highly longitudinal collagen in the dorsal-lateral `tension9 region, paradoxical histologic organization in some locations, and lack of significantly improved S-M-S properties in some locations, might reflect the absence of a similar habitual Strain distribution in all bones. An alternative Strain distribution based on in vivo Strain measurements, without FEA, on non-Thoroughbreds showing net compression along the dorsal-palmar axis might be more characteristic of the habitual loading of some of the bones that we examined. In turn, some inconsistencies might also reflect the complex torsion/bending loading regime that the MC3 sustains when the animal undergoes a variety of gaits and activities, which may be representative of only a portion of our animals, again reflecting the possibility that not all of the bones examined had similar habitual loading histories.
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dissociation of mineral and collagen orientations may differentially adapt compact bone for regional loading environments results from acoustic velocity measurements in deer calcanei
Bone, 2006Co-Authors: John G. Skedros, Scott M Sorenson, Yuichi Takano, Charles H TurnerAbstract:Abstract In limb bone diaphyses, it is hypothesized that collagen and extra-fibrillar mineral are aligned differently in relatively simple loading conditions (e.g., habitual longitudinal compression) when compared to complex or potentially deleterious Strain environments (e.g., habitual shear or tension). These putative differences in collagen/mineral organization might be adaptations that enhance toughness and fatigue resistance by controlling the direction of microdamage propagation. This study examined relationships between the non-uniform Strain distribution of wild deer calcanei and elastic anisotropy of cortical bone specimens in three preparations: (1) demineralized (collagen only), (2) deproteinized (mineral only), and (3) untreated. Using simulated functional loading, the following Strain data were obtained from the dorsal “compression”, plantar “tension”, and medial and lateral (“neutral axis”) cortices of one calcaneus of each of seven pairs: (1) peak Strain magnitude, (2) prevalent/predominant Strain Mode (compression, tension, shear), and (3) principal Strain orientation with respect to the bone’s long axis. In the contralateral calcanei, elastic anisotropy ratios (ARs) were calculated using acoustic velocity (longitudinal and transverse) measurements from a pair of orthogonally sliced specimens (representing each of three preparation types) from each cortex. In a separate set of seven adult calcanei, predominant collagen fiber orientation (CFO) was measured using circularly polarized light (CPL) in the four cortical locations. Results showed that, in general, elastic anisotropy was significant in each region, with ARs being significantly different from isotropy (where AR = 1.0). Compared to CFO, mineral orientation more strongly influenced this anisotropy, which was most notable in the plantar “tension” cortex. High correlations ( r values from −0.675 to −0.734, P r values ∼−0.750). The habitual compression, tension, and shear (neutral axis) regions also had different collagen/mineral organizations, which may be important in accommodating the well-known disparity in the mechanical properties of bone in these loading Modes.
A.k. Ghosh - One of the best experts on this subject based on the ideXlab platform.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading – Part II: Compact tension and middle tension specimens
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in a weld or in the regions near the weld. In part-I, the problem of a stationary crack lying at the centre of a weld in a pure bending specimen SE(PB) was analysed. The detailed structure of the global plastic fields for a deep crack, under fully plastic condition, was presented. Aspects related to the state of stress at the base-weld interface were discussed. To enhance our understanding of the weld strength mismatch effects, other commonly used fracture specimens, that is, compact tension C(T) and middle tension M(T) specimens having a weld centre crack were analysed in the present investigation. The influence of weld mismatch on the structure of global stress fields (leading to plastic yielding of the ligament) as well as on the crack tip conStraint was studied. It is demonstrated that, when a crack is postulated at the centre of a weld, a family of stress fields proposed in part-I for a SE(PB) specimen is applicable to a C(T) specimen also. The studies performed in this article, along with part-I, have established that in comparison to slip line field analysis, the modified upper bound theorem is simple and more general. It is applicable to macroscopically homogeneous materials and can also account for weld mismatch effects.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading—Part I: Pure bending specimen
International Journal of Mechanical Sciences, 2012Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in or the regions near the weld. Interfacial cracks under elastic as well as in elastic–plastic conditions have already been extensively discussed in literature. However, the problem of crack lying at the centre of weld is less understood. Though several detailed numerical studies have been performed to investigate the influence of weld strength mismatch on crack-tip stress fields till date, however, the detailed insight of the structure of stress fields under large scale plasticity is still lacking. The present article is intended to bridge that gap. In this work, detailed structure of the global plastic fields which occur in a deeply cracked (a/W>0.3) mismatch welded pure bending specimen, under fully plastic condition, is presented. Aspects related to the state of stress at the interface of two materials are discussed. It is shown that a family of five fields proposed in this work is adequate to cover all practical cases of weld mismatch. Proposed fields were confirmed by detailed full-field finite element analyses. Excellent agreement is observed between the proposed theoretical solutions and the numerical results.
I.a. Khan - One of the best experts on this subject based on the ideXlab platform.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading – Part II: Compact tension and middle tension specimens
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in a weld or in the regions near the weld. In part-I, the problem of a stationary crack lying at the centre of a weld in a pure bending specimen SE(PB) was analysed. The detailed structure of the global plastic fields for a deep crack, under fully plastic condition, was presented. Aspects related to the state of stress at the base-weld interface were discussed. To enhance our understanding of the weld strength mismatch effects, other commonly used fracture specimens, that is, compact tension C(T) and middle tension M(T) specimens having a weld centre crack were analysed in the present investigation. The influence of weld mismatch on the structure of global stress fields (leading to plastic yielding of the ligament) as well as on the crack tip conStraint was studied. It is demonstrated that, when a crack is postulated at the centre of a weld, a family of stress fields proposed in part-I for a SE(PB) specimen is applicable to a C(T) specimen also. The studies performed in this article, along with part-I, have established that in comparison to slip line field analysis, the modified upper bound theorem is simple and more general. It is applicable to macroscopically homogeneous materials and can also account for weld mismatch effects.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading—Part I: Pure bending specimen
International Journal of Mechanical Sciences, 2012Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in or the regions near the weld. Interfacial cracks under elastic as well as in elastic–plastic conditions have already been extensively discussed in literature. However, the problem of crack lying at the centre of weld is less understood. Though several detailed numerical studies have been performed to investigate the influence of weld strength mismatch on crack-tip stress fields till date, however, the detailed insight of the structure of stress fields under large scale plasticity is still lacking. The present article is intended to bridge that gap. In this work, detailed structure of the global plastic fields which occur in a deeply cracked (a/W>0.3) mismatch welded pure bending specimen, under fully plastic condition, is presented. Aspects related to the state of stress at the interface of two materials are discussed. It is shown that a family of five fields proposed in this work is adequate to cover all practical cases of weld mismatch. Proposed fields were confirmed by detailed full-field finite element analyses. Excellent agreement is observed between the proposed theoretical solutions and the numerical results.
Van Der Erik Giessen - One of the best experts on this subject based on the ideXlab platform.
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A discrete dislocation analysis of near-threshold fatigue crack growth
Acta Materialia, 2001Co-Authors: Vikram Deshpande, Alan Needleman, Van Der Erik GiessenAbstract:Abstract Analyses of cyclic loading of a plane Strain Mode I crack under small-scale yielding are carried out using discrete dislocation dynamics. The formulation is the same as used to analyze crack growth under monotonic loading conditions, differing only in the remote stress intensity factor being a cyclic function of time. The dislocations are all of edge character and are Modeled as line singularities in an elastic solid. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. Either reversible or irreversible relations are specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading as could occur in a vacuum or in an oxidizing environment, respectively. In accord with experimental data we find that the fatigue threshold Δ K th is weakly dependent on the load ratio R when the reversible cohesive surface is employed. This intrinsic dependence of the threshold on R is an outcome of source limited plasticity at low R values and plastic shakedown at higher R values. On the other hand, Δ K th is seen to decrease approximately linearly with increasing R followed by a plateau when the irreversible cohesive law is used. Our simulations show that in this case the fatigue threshold is dominated by crack closure at low values of R . Calculations illustrating the effects of obstacle density, tensile overloads and slip geometry on cyclic crack growth behavior are also presented.
Jayanta Chattopadhyay - One of the best experts on this subject based on the ideXlab platform.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading – Part II: Compact tension and middle tension specimens
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in a weld or in the regions near the weld. In part-I, the problem of a stationary crack lying at the centre of a weld in a pure bending specimen SE(PB) was analysed. The detailed structure of the global plastic fields for a deep crack, under fully plastic condition, was presented. Aspects related to the state of stress at the base-weld interface were discussed. To enhance our understanding of the weld strength mismatch effects, other commonly used fracture specimens, that is, compact tension C(T) and middle tension M(T) specimens having a weld centre crack were analysed in the present investigation. The influence of weld mismatch on the structure of global stress fields (leading to plastic yielding of the ligament) as well as on the crack tip conStraint was studied. It is demonstrated that, when a crack is postulated at the centre of a weld, a family of stress fields proposed in part-I for a SE(PB) specimen is applicable to a C(T) specimen also. The studies performed in this article, along with part-I, have established that in comparison to slip line field analysis, the modified upper bound theorem is simple and more general. It is applicable to macroscopically homogeneous materials and can also account for weld mismatch effects.
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An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane Strain Mode-I loading—Part I: Pure bending specimen
International Journal of Mechanical Sciences, 2012Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in or the regions near the weld. Interfacial cracks under elastic as well as in elastic–plastic conditions have already been extensively discussed in literature. However, the problem of crack lying at the centre of weld is less understood. Though several detailed numerical studies have been performed to investigate the influence of weld strength mismatch on crack-tip stress fields till date, however, the detailed insight of the structure of stress fields under large scale plasticity is still lacking. The present article is intended to bridge that gap. In this work, detailed structure of the global plastic fields which occur in a deeply cracked (a/W>0.3) mismatch welded pure bending specimen, under fully plastic condition, is presented. Aspects related to the state of stress at the interface of two materials are discussed. It is shown that a family of five fields proposed in this work is adequate to cover all practical cases of weld mismatch. Proposed fields were confirmed by detailed full-field finite element analyses. Excellent agreement is observed between the proposed theoretical solutions and the numerical results.
