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The results of research initiated in the early 1980s led to the replacement of plasticity-based design guidelines for the load-carrying capacity of headed anchors embedded in concrete with those developed using fracture mechanics. While provisions are available in the design codes that account for the presence of tensile fields causing concrete cracking, no provisions are available for anchors embedded in prestressed concrete. This paper presents the results of linear elastic fracture mechanics (LEFM) analyses and of a preliminary experimental investigation of the progressive failure of headed anchors embedded in a concrete matrix under compressive or tensile prestress. The model predicts an increase (decrease) in load-carrying capacity and ductility with increasing compressive (tensile) prestress. It is shown that despite neglecting the dependence on size of concrete fracture toughness, LEFM predicts with remarkable accuracy the functional dependence of the ultimate capacity on prestress. DOI: 10.1061/(ASCE)EM.1943-7889.0000120 CE Database subject headings: Anchors; Axisymmetry; Finite element method; Prestressed concrete; Cracking; Tensile strength. Author keywords: Headed anchors.

Author(s): S. Kamat [1]; X. Su [1]; R. Ballarini [2]; A. H. Heuer (corresponding author) [1] Natural composite materials are renowned for their mechanical strength and toughness: despite being highly [...]

A high-hardness diamond-based composite was synthesized by spark plasma sintering (SPS) under 100 MPa, using SiC-coated diamond powder prepared via chemical vapor deposition (CVD). SiC layers 20-40 nm were uniformly deposited on diamond powders by a rotary CVD technique. The SiC-coated diamond powder was consolidated with SiO [ABSTRACT FROM AUTHOR]

We apply first-principles molecular dynamics to investigate the order-disorder transition in the oxygen sublattice, observed experimentally in thoria. The mean-square displacements were calculated at high temperatures (2700 K-3660 K) and we demonstrate that oxygen atoms become mobile at 3000 K, while they stay confined at 2700 K. The rate of diffusion of oxygen atoms was calculated while for thorium atoms the mean-square amplitudes of vibrations were evaluated. Thoria is predicted to exhibit a higher increase in thermal expansion with temperatures in the premelting region, similar to urania. [ABSTRACT FROM AUTHOR]

Microelectromechanical Systems (MEMS) devices typically need to be designed against a very low failure probability, which is on the order of [ABSTRACT FROM AUTHOR]

Ceramic composites found in nature, such as bone, nacre, and sponge spicule, often provide an effective resolution to a well-known conflict between materials' strength and toughness. This arises, on the one hand, from their high ceramic content that ensures high strength of the material. On the other hand, various pathways are provided for stress dissipation, and thus toughness, due to their intricate hierarchical architectures. Such pathways include crack bridging, crack deflection, and delamination in the case of layered structures. On the basis of these inspiring ideas, we attempted here to create simultaneously strong and tough laminated alumina composite with high ceramic content. Composites were prepared from high-grade commercial alumina with spin-coated interlayers of ductile polymers ( PMMA and PVA). The specimens' ultimate properties (strength, fracture toughness, and work of fracture) were measured by a four-point bending method. In some cases, fracture toughness of the composites was increased by up to an order of magnitude, reminiscent of the natural layered composites. It is proposed that this increase may be attributed to an interlocking mechanism, often encountered in biological composites. The significance of sample architecture and the role of the interfacial and bulk properties of the interlayer material are discussed. [ABSTRACT FROM AUTHOR]

The crack bridging behaviors of silicon nitride (Si3N4) ceramics based on the advancing cracks have been evaluated at cryogenic temperatures in contrast with those happened at ambient temperatures. R-Curve behavior of Si3N4 ceramics was determined to compare the cumulative effect of bridging at 77 and 293 K. The detailed changes in crack morphologies and crack opening displacement profiles at different temperatures were investigated. The bridging stress maps around a bridging ligament were recorded by in situ Raman spectroscopy. It was found that the highest tensile stress measured at 77 K (~1.0 GPa) is much higher than that at 293 K (~0.7 GPa). The experimental results suggested that Si3N4 ceramics at cryogenic temperatures possessed a higher probability of crack bridging with a higher closing force, which could make them a promising candidate for cryogenic applications. [ABSTRACT FROM AUTHOR]

The microstructures and mechanical properties of Clinocardium californiense shell were investigated in relation to the different parts of shell. It is found that the shell can be divided into three parts, that is, the dorsal side, body part, and marginal side based on the variation of microstructures along the longitudinal cross section. Specifically, all areas exhibit a cross-lamellar structure on the dorsal side, and a hierarchical structure comprising three layers including inner (with a cross-lamellar structure), middle (with a complex cross-lamellar structure), and outer (with a prismatic structure) layers was observed on the body part, whereas on the marginal side, the orientation of aragonite sheets shows an obvious deflection. The structural architecture, the dimensions of different-order lamellae in the same kind of structure and the orientation between the indenting direction and aragonite sheet all contribute to the unique mechanical properties of this shell. It is also found that a low value of the ratio of hardness-to-Young's modulus ( H/ E) corresponds to an improved indentation toughness. The middle layer with the densest and most complicated structure on the body part shows the lowest H/ E ratio and the highest hardness and Young's modulus. [ABSTRACT FROM AUTHOR]

Iodoapatite powder prepared by high-energy ball milling is densified by Spark Plasma Sintering to ~96% theoretical density. X-ray Diffraction and First-principle Calculation indicates the sintered phase is iodine-deficient apatite with chemical composition of Pb9.85( VO4)6 I1.7 and iodine confinement over 8 wt%. Thermogravimetric analysis shows the bulk iodoapatite displays exceptionally stability without iodine release until 670°C. The greatly improved iodine confinement can be attributed to the dense matrix upon rapid consolidation from highly activated powders by mechanical attrition. [ABSTRACT FROM AUTHOR]

Failure of surface-mounted multilayer ceramic capacitor ( MLCC) results mainly from bending of the printed circuit board during handling and applications. Cracking of MLCC normally initiates at the junction of the bottom end of termination and the dielectric ceramics and it would cut through dielectric ceramics and electrodes to result in the capacitance loss and failure of MLCC. The purpose of this study was to mitigate the stress concentration and to shift the stress concentration location toward the side end of MLCC to minimize the number of electrodes cut by cracking. To achieve this, effects of the stiffness and the thickness of the solder and the size of MLCC on the location of crack initiation and the fracture load were examined. Nanoindentation was performed to obtain the mechanical properties of the constituents of the system. Finite element analyses were conducted to simulate the stress field in the surface-mounted MLCC subjected to bending and the results were compared to the observed crack initiation locations and the fracture load. The outcome would provide guidelines in mounting and designing of MLCCs to enhance its reliability and lifetime. [ABSTRACT FROM AUTHOR]

The longitudinal and transverse wave velocities of polycrystalline MgO were successfully measured simultaneously and five types of elastic moduli, the Debye temperature, and the Grüneisen parameter were evaluated as functions of temperature from 298 to 1764 K. An aluminum foil was used as a couplant between the MgO specimen and waveguide. Both acoustic waves were successfully propagated into the specimen through the solid and liquid aluminum layers. In particular, no influence of melted aluminum foil on the acoustic wave velocities was observed. With increasing temperature, the Young's, shear, and bulk moduli decreased monotonically and the Grüneisen parameter and Poisson's ratio increased monotonically, which indicated a decrease in the ionic bonding strength. The Debye temperature decreased almost linearly with increasing temperature, which showed the effect of thermal expansion. Although previous investigators concluded that the Grüneisen parameter was almost independent of temperature, our data showed an obvious gradual increase in this parameter with temperature. In addition, our results implied the possibility of an abrupt increase in the Grüneisen parameter at temperatures less than 400 K. The slope of the Lamé parameter-temperature plot was almost zero over the entire investigated temperature range. On the basis of three-dimensional volume-nonpreserving shear, this behavior was interpreted as being caused by interlocking associated with the network of short Mg-O bonds. [ABSTRACT FROM AUTHOR]

Cracks should normally be avoided in the deposition of coatings, but vertical cracks in thermal barrier coatings are engineered to absorb thermo-mechanical stress. Thick lanthanum zirconate coatings were deposited by spray pyrolysis deposition from aqueous nitrate-based precursor solutions, and cracks were formed during decomposition of the nitrate species due to the associated volume change. The crack spacing and crack opening in the deposited coatings were analyzed in terms of thickness, pH of the precursor solution, and deposition and decomposition temperatures and kinetics by thermogravimetry, scanning calorimetry, mass spectroscopy, and electron microscopy. The thickness of the coatings demonstrated the most important effect on the crack pattern. The crack opening and the crack spacing varied linearly with increasing thickness, leading to small delamination at the interface. The cracks were stable after the crystallization of the films by further heat treatment. Knowing the influence of the different parameters, coatings with a designed crack pattern can be deposited. [ABSTRACT FROM AUTHOR]

A scalable growth process for domain engineered {001}-oriented lead magnesium niobate-lead titanate (PMN-PT) thin films with Lotgering factors of 0.98-1.0, without an orienting perovskite seed layer, is presented. Deposition of a 2-3-nm-thick PbO buffer layer on {111} Pt thin film bottom electrodes, prior to chemical solution deposition of PMN-PT reduces the driving force for Pb diffusion from the PMN-PT to the bottom electrode, and facilitates nucleation of {001}-oriented perovskite grains. Energy dispersive spectroscopy demonstrated that up to 10% of the Pb from a PMN-PT precursor solution may diffuse into the bottom electrode. PMN-PT grains with a mixed {101}/{111} orientation in a matrix of Pb-deficient pyrochlore phase were then promoted near the interface. When this is prevented, phase-pure films with {001} orientation can be achieved. [ABSTRACT FROM AUTHOR]

Freestanding ceramic microbeams were fabricated by inkjet printing of aqueous colloidal suspensions (up to 23 vol% alumina). Their high strength (920 MPa in bending) and stiffness ( E ≈ 400 GPa) is attributed to low porosity (void fraction 0.03) and a paucity of microstructural flaws. A range of ink compositions and beam thicknesses were printed using a commercial inkjet cartridge and isostatically compacted prior to firing. [ABSTRACT FROM AUTHOR]

A PbTiO3 ( PTO) nanodot was formed on an Nb-doped SrTiO3 substrate using dip-pen nanolithography, using a PTO precursor sol. After drying and annealing to remove the solvent contained in the PTO nanodot, this dot had a diameter of about 40 nm. The nanodot was then shattered using an atomic force microscope, producing broken nanoparticles of various sizes, ranging from several nanometers to a few tens of nanometers. Finally, after the crystallization annealing process, ferroelectric PTO nanodots smaller than 10 nm in diameter were successfully fabricated. These small PTO nanodots exhibited good ferroelectric properties, as confirmed using a piezoelectric force microscope. [ABSTRACT FROM AUTHOR]

In motor control studies, the 90% thresholding of variance accounted for (VAF) is the classical way of selecting the number of muscle synergies expressed during a motor task. However, the adoption of an arbitrary cut-off has evident drawbacks. The aim of this work is to describe and validate an algorithm for choosing the optimal number of muscle synergies (ChoOSyn), which can overcome the limitations of VAF-based methods. The proposed algorithm is built considering the following principles: (1) muscle synergies should be highly consistent during the various motor task epochs (i.e., remaining stable in time), (2) muscle synergies should constitute a base with low intra-level similarity (i.e., to obtain information-rich synergies, avoiding redundancy). The algorithm performances were evaluated against traditional approaches (threshold-VAF at 90% and 95%, elbow-VAF and plateau-VAF), using both a simulated dataset and a real dataset of 20 subjects. The performance evaluation was carried out by analyzing muscle synergies extracted from surface electromyographic (sEMG) signals collected during walking tasks lasting 5 min. On the simulated dataset, ChoOSyn showed comparable performances compared to VAF-based methods, while, in the real dataset, it clearly outperformed the other methods, in terms of the fraction of correct classifications, mean error (ME), and root mean square error (RMSE). The proposed approach may be beneficial to standardize the selection of the number of muscle synergies between different research laboratories, independent of arbitrary thresholds.

A brief overview of isolated collagen fibril mechanics testing is followed by presentation of the first results testing fibrils isolated from load-bearing mammalian tendons using a microelectromechanical systems platform. The in vitro modulus (326 ± 112 MPa) and fracture stress (71 ± 23 MPa) are shown to be lower than previously measured on fibrils extracted from sea cucumber dermis and tested with the same technique. Scanning electron microscope images show the fibrils can fail with a mechanism that involves circumferential rupture, whereas the core of the fibril stays at least partially intact.

Understanding the viscoelastic behavior of collagenous tissues with complex hierarchical structures requires knowledge of the properties at each structural level. Whole tissues have been studied extensively, but less is known about the mechanical behavior at the submicron, fibrillar level. Using a microelectromechanical systems platform, in vitro coupled creep and stress relaxation tests were performed on collagen fibrils isolated from the sea cucumber dermis. Stress-strain-time data indicate that isolated fibrils exhibit viscoelastic behavior that could be fitted using the Maxwell-Weichert model. The fibrils showed an elastic modulus of 123 ± 46 MPa. The time-dependent behavior was well fit using the two-time-constant Maxwell-Weichert model with a fast time response of 7 ± 2 s and a slow time response of 102 ± 5 s. The fibrillar relaxation time was smaller than literature values for tissue-level relaxation time, suggesting that tissue relaxation is dominated by noncollagenous components (e.g., proteoglycans). Each specimen was tested three times, and the only statistically significant difference found was that the elastic modulus is larger in the first test than in the subsequent two tests, indicating that viscous properties of collagen fibrils are not sensitive to the history of previous tests., (Copyright © 2011 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Mechanical testing of collagenous tissues at different length scales will provide improved understanding of the mechanical behavior of structures such as skin, tendon, and bone, and also guide the development of multiscale mechanical models. Using a microelectromechanical-systems (MEMS) platform, stress-strain response curves up to failure of type I collagen fibril specimens isolated from the dermis of sea cucumbers were obtained in vitro. A majority of the fibril specimens showed brittle fracture. Some displayed linear behavior up to failure, while others displayed some nonlinearity. The fibril specimens showed an elastic modulus of 470 ± 410 MPa, a fracture strength of 230 ± 160 MPa, and a fracture strain of 80% ± 44%. The fibril specimens displayed significantly lower elastic modulus in vitro than previously measured in air. Fracture strength/strain obtained in vitro and in air are both significantly larger than those obtained in vacuo, indicating that the difference arises from the lack of intrafibrillar water molecules produced by vacuum drying. Furthermore, fracture strength/strain of fibril specimens were different from those reported for collagenous tissues of higher hierarchical levels, indicating the importance of obtaining these properties at the fibrillar level for multiscale modeling., (Copyright © 2010 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Collagen, an essential building block of connective tissues, possesses useful mechanical properties due to its hierarchical structure. However, little is known about the mechanical properties of collagen fibril, an intermediate structure between the collagen molecule and connective tissue. Here, we report the results of systematic molecular dynamics simulations to probe the mechanical response of initially unflawed finite size collagen fibrils subjected to uniaxial tension. The observed deformation mechanisms, associated with rupture and sliding of tropocollagen molecules, are strongly influenced by fibril length, width and cross-linking density. Fibrils containing more than approximately 10 molecules along their length and across their width behave as representative volume elements and exhibit brittle fracture. Shorter fibrils experience a more graceful ductile-like failure. An analytical model is constructed and the results of the molecular modelling are used to find curve-fitted expressions for yield stress, yield strain and fracture strain as functions of fibril structural parameters. Our results for the first time elucidate the size dependence of mechanical failure properties of collagen fibrils. The associated molecular deformation mechanisms allow the full power of traditional material and structural engineering theory to be applied to our understanding of the normal and pathological mechanical behaviours of collagenous tissues under load.

Collagen, a molecule consisting of three braided protein helices, is the primary building block of many biological tissues including bone, tendon, cartilage, and skin. Staggered arrays of collagen molecules form fibrils, which arrange into higher-ordered structures such as fibers and fascicles. Because collagen plays a crucial role in determining the mechanical properties of these tissues, significant theoretical research is directed toward developing models of the stiffness, strength, and toughness of collagen molecules and fibrils. Experimental data to guide the development of these models, however, are sparse and limited to small strain response. Using a microelectromechanical systems platform to test partially hydrated collagen fibrils under uniaxial tension, we obtained quantitative, reproducible mechanical measurements of the stress-strain curve of type I collagen fibrils, with diameters ranging from 150-470 nm. The fibrils showed a small strain (epsilon < 0.09) modulus of 0.86 +/- 0.45 GPa. Fibrils tested to strains as high as 100% demonstrated strain softening (sigma(yield) = 0.22 +/- 0.14 GPa; epsilon(yield) = 0.21 +/- 0.13) and strain hardening, time-dependent recoverable residual strain, dehydration-induced embrittlement, and susceptibility to cyclic fatigue. The results suggest that the stress-strain behavior of collagen fibrils is dictated by global characteristic dimensions as well as internal structure.