Fracture in metallic glasses: mechanics and mechanisms

Significant progress in understanding the mechanical behavior of metallic glasses (MGs) was made over the past decade, particularly on mechanisms of plastic deformation. However, recent research thrust has been on exploring the mechanics and physics of fracture. MGs can be very brittle with K-Ic values similar to silicate glasses and ceramics or very tough with K-Ic akin to high toughness crystalline metals. Even the tough MGs can become brittle with structural relaxation following annealing at temperatures close to glass transition temperature (T-g). Detailed experimental studies coupled with complementary numerical simulations of the recent past have provided insights on the micromechanisms of failure as well as nature of crack tip fields, and established the governing fracture criteria for ductile and brittle glasses. In this paper, the above advances are reviewed and outstanding issues in the context of fracture of amorphous alloys that need to be resolved are identified.

On the variability in fracture toughness of `ductile' bulk metallic glasses

The mode I fracture toughness, K-Ic, of ductile bulk metallic glasses (BMGs) exhibits a high degree of specimen-to-specimen variability. By conducting fracture experiments in modes I and II, we demonstrate that the observed high variability in mode I, vis-a-vis mode II, is a result of highly variable propensity for the conversion of shear bands into cracks in mode I whereas in mode II, crack growth direction is fixed. Thus, the measured variability in K-Ic is intrinsic to the nature of BMGs. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Magnetic field assisted stem cell differentiation - role of substrate magnetization in osteogenesis

Among the multiple modulatory physical cues explored to regulate cellular processes, the potential of magneto-responsive substrates in magnetic field stimulated stem cell differentiation is still unperceived. In this regard, the present work demonstrates how an external magnetic field can be applied to direct stem cell differentiation towards osteogenic commitment. A new culture methodology involving periodic delivery of 100 mT static magnetic field (SMF) in combination with HA-Fe3O4 magnetic substrates possessing a varying degree of substrate magnetization was designed for the study. The results demonstrate that an appropriate combination of weakly ferromagnetic substrates and SMF exposure enhanced cell viability, DNA synthesis and caused an early switchover to osteogenic lineage as supported by Runx2 immunocytochemistry and ALP expression. However, the mRNA expression profile of early osteogenic markers (Runx2, ALP, Col IA) was comparable despite varying substrate magnetic properties (diamagnetic to ferromagnetic). On the contrary, a remarkable upregulation of late bone development markers (OCN and OPN) was explicitly detected on weak and strongly ferromagnetic substrates. Furthermore, SMF induced matrix mineralization with elevated calcium deposition on similar substrates, even in the absence of osteogenic supplements. More specifically, the role of SMF in increasing intracellular calcium levels and in inducing cell cycle arrest at G0/G1 phase was elucidated as the major molecular event triggering osteogenic differentiation. Taken together, the above results demonstrate the competence of magnetic stimuli in combination with magneto-responsive biomaterials as a potential strategy for stem cell based bone tissue engineering.

Optimization of clamped beam geometry for fracture toughness testing of micron-scale samples

Fracture toughness measurements at the small scale have gained prominence over the years due to the continuing miniaturization of structural systems. Measurements carried out on bulk materials cannot be extrapolated to smaller length scales either due to the complexity of the microstructure or due to the size and geometric effect. Many new geometries have been proposed for fracture property measurements at small-length scales depending on the material behaviour and the type of device used in service. In situ testing provides the necessary environment to observe fracture at these length scales so as to determine the actual failure mechanism in these systems. In this paper, several improvements are incorporated to a previously proposed geometry of bending a doubly clamped beam for fracture toughness measurements. Both monotonic and cyclic loading conditions have been imposed on the beam to study R-curve and fatigue effects. In addition to the advantages that in situ SEM-based testing offers in such tests, FEM has been used as a simulation tool to replace cumbersome and expensive experiments to optimize the geometry. A description of all the improvements made to this specific geometry of clamped beam bending to make a variety of fracture property measurements is given in this paper.

Fracture behavior of magnesium alloys - Role of tensile twinning

In this work, Mode-I fracture experiments are conducted using notched compact tension specimens machined from a rolled AZ31 Mg alloy plate having near-basal texture with load applied along rolling direction (RD) and transverse direction (TD). Moderately high notched fracture toughness of J(C) similar to 46 N/mm is obtained in both RD and TD specimens. Fracture surface shows crack tunneling at specimen mid-thickness and extensive shear lips near the free surface. Dimples are observed from SEM fractographs suggesting ductile fracture. EBSD analysis shows profuse tensile twinning in the ligament ahead of the notch. It is shown that tensile twinning plays a dual role in enhancing the toughness in the notched fracture specimens with reduced triaxiality. It provides significant dissipation in the background plastic zone and imparts hardening to the material surrounding the fracture process zone via operation of several mechanisms which retards micro-void growth and coalescence. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

EXPERIMENTAL STUDIES ON AIR-ASSISTED ATOMIZATION OF JATROPHA PURE PLANT OIL

The present study focuses on exploring air-assisted atomization strategies for effective atomization of high-viscosity biofuels, such as pure plant oils (PPOs). The first part of the study concerns application of a novel air-assisted impinging jet atomization for continuous spray applications, and the second part concerns transient spray applications. The particle/droplet imaging analysis (PDIA) technique along with direct imaging methods are used for the purpose of spray characterization. In the first part, effective atomization of Jatropha PPO is demonstrated at gas-to-liquid ratios (GLRs) on the order 0.1. The effect of liquid and gas flow rates on the spray characteristics is evaluated, and results indicate a Sauter mean diameter (SMD) of 50 mu m is achieved with GLRs as low as 0.05. In the second part of the study, a commercially available air-assisted transient atomizer is evaluated using Jatropha PPO. The effect of the pressure difference across the air injector and ambient gas pressure on liquid spray characteristics is studied. The results indicate that it is possible to achieve the same level of atomization of Jatropha as diesel fuel by operating the atomizer at a higher pressure difference. Specifically, a SMD of 44 mu m is obtained for the Jatropha oil using injection pressures of <1 MPa. A further interesting observation associated with this injector is the near constancy of a nondimensional spray penetration rate for the Jatropha oil spray.

An enolato-bridged dinuclear Cu(II) complex with a coumarin-assisted precursor: a spectral, magnetic and biological study

A new, phenoxo-bridged Cu-II dinuclear complex Cu-2(L)(2)(DMF)(2)] (1) has been obtained by employing the coumarin-assisted tridentate precursor, H2L, benzoic acid(7-hydroxy-4-methyl-2-oxo-2H-chromen-8-ylmethylene)-hydrazide]. Complex 1 has been systematically characterized by FTIR, UV-Vis, fluorescence and PR spectrometry. The single crystal X-ray diffraction analysis of 1 shows that the geometry around each copper ion is square pyramidal, comprising two enolato oxygen atoms belonging to different ligands (which assemble the dimer bridging the two metal centers), one imine-N and one phenolic-O atoms of the Schiff base and one oxygen atom from the DMF molecule. The temperature dependent magnetic interpretation agrees with the existence of weak ferromagnetic interactions between the bridging dinuclear Cu(II) ions. Both the ligand and complex 1 exhibit anti-mycobacterial activity and considerable efficacy towards M. tuberculosis H37Rv ATCC 27294 and M. tuberculosis H37Ra ATCC 25177 strains. The cytotoxicity study on human adenocarcinoma cell lines (MCF7) suggests that the ligand and complex 1 have potential anticancer properties. Molecular docking of H2L with the enoyl acyl carrier protein reductase of M. tuberculosis H37R(v) (PDB ID: 4U0K) is examined and the best docked pose of H2L shows one hydrogen bond with Thr196 (1.99 angstrom).

A thermodynamic correlation between damage and fracture as applied to concrete fatigue

Fatigue damage in concrete is characterized by the simultaneous presence of micro and macrocracks. The theory of fracture mechanics conveniently handles the propagation of macrocracks, whereas damage mechanics precisely describes the state of microcracking. This paper provides a platform to correlate fracture mechanics and damage mechanics theories through an energy equivalence within a thermodynamic framework by equating the energy dissipated according to each theory. Through this correlation, damage corresponding to a given crack length could be obtained, and alternatively a discrete crack could be transformed into an equivalent damage zone. The results are validated using available experimental data on concrete fatigue including stiffness degradation and acoustic emission. (C) 2015 Elsevier Ltd. All rights reserved.

A thermodynamic correlation between damage and fracture as applied to concrete fatigue

Fatigue damage in concrete is characterized by the simultaneous presence of micro and macrocracics. The theory of fracture mechanics conveniently handles the propagation of macrocracks, whereas damage mechanics precisely describes the state of microcracking. This paper provides a platform to correlate fracture mechanics and damage mechanics theories through an energy equivalence within a thermodynamic framework by equating the energy dissipated according to each theory. Through this correlation, damage corresponding to a given crack length could be obtained, and alternatively a discrete crack could be transformed into an equivalent damage zone. The results are validated using available experimental data on concrete fatigue including stiffness degradation and acoustic emission. (C) 2015 Elsevier Ltd. All rights reserved.

Gold catalyzed intramolecular hydroalkoxylation assisted ring opening of furans to the corresponding saturated gamma-keto esters

A facile ring opening of furans in furyl propargyl alcohols to the corresponding saturated gamma-keto esters is observed in the gold(III) chloride catalyzed reaction with MeOH. It is found that the ring opening of furan is driven by the intramolecular hydroalkoxylation. Mitigating the intramolecular hydroalkoxylation led to the expected conjugated enyne resulting from the dehydration. (C) 2015 Elsevier Ltd. All rights reserved.

Ancilla-assisted measurements on quantum ensembles: general protocols and applications in NMR quantum information processing

Quantum ensembles form easily accessible architectures for studying various phenomena in quantum physics, quantum information science and spectroscopy. Here we review some recent protocols for measurements in quantum ensembles by utilizing ancillary systems. We also illustrate these protocols experimentally via nuclear magnetic resonance techniques. In particular, we shall review noninvasive measurements, extracting expectation values of various operators, characterizations of quantum states and quantum processes, and finally quantum noise engineering.

Detection and estimation of liquid flow through a pipe in a tissue-like object with ultrasound-assisted diffuse correlation spectroscopy

Using coherent light interrogating a turbid object perturbed by a focused ultrasound (US) beam, we demonstrate localized measurement of dynamics in the focal region, termed the region-of-interest (ROI), from the decay of the modulation in intensity autocorrelation of light. When the ROI contains a pipe flow, the decay is shown to be sensitive to the average flow velocity from which the mean-squared displacement (MSD) of the scattering centers in the flow can be estimated. While the MSD estimated is seen to be an order of magnitude higher than that obtainable through the usual diffusing wave spectroscopy (DWS) without the US, it is seen to be more accurate as verified by the volume flow estimated from it. It is further observed that, whereas the MSD from the localized measurement grows with time as tau(alpha) with alpha approximate to 1.65, without using the US, a is seen to be much less. Moreover, with the local measurement, this super-diffusive nature of the pipe flow is seen to persist longer, i.e., over a wider range of initial tau, than with the unassisted DWS. The reason for the super-diffusivity of flow, i.e., alpha < 2, in the ROI is the presence of a fluctuating (thermodynamically nonequilibrium) component in the dynamics induced by the US forcing. Beyond this initial range, both methods measure MSDs that rise linearly with time, indicating that ballistic and near-ballistic photons hardly capture anything beyond the background Brownian motion. (C) 2015 Optical Society of America

Twinning induced enhancement of fracture toughness in ultrafine grained Hydroxyapatite-Calcium Titanate composites

Despite being highly bioactive and biocompatible, the limitations of monolithic hydroxyapatite (HA) include extremely low fracture toughness, poor electrical conductivity. While addressing these issues, the present study demonstrates how CaTiO3 (CT) addition to HA can be utilized to obtain a combination of long crack fracture toughness (1.7 MPa m(1/2) SEVNB technique) and flexural strength of 98-155 MPa (3-point bending) and a moderate tensile strength (diametral compression) of 17-36 MPa. The enhancement in fracture resistance in spark plasma sintered HA-CT composites has been explained in reference to the observed twin morphology. TEM reveals the presence of twins in CT grains due to 1800 rotation about 101]. The measured properties along with our earlier reports on biocompatibility and electrical properties make HA-CT suitable for bone tissue engineering applications. When compared with other competing HA-based biocomposites, HA-CT composites are found to have a better combination of properties useful for medium load bearing implant applications. (C) 2015 Elsevier Ltd. All rights reserved.

Fracture Testing at Small-Length Scales: From Plasticity in Si to Brittleness in Pt

The field of micro-/nano-mechanics of materials has been driven, on the one hand by the development of ever smaller structures in devices, and, on the other, by the need to map property variations in large systems that are microstructurally graded. Observations of `smaller is stronger' have also brought in questions of accompanying fracture property changes in the materials. In the wake of scattered articles on micro-scale fracture testing of various material classes, this review attempts to provide a holistic picture of the current state of the art. In the process, various reliable micro-scale geometries are shown, challenges with respect to instrumentation to probe ever smaller length scales are discussed and examples from recent literature are put together to exhibit the expanse of unusual fracture response of materials, from ductility in Si to brittleness in Pt. Outstanding issues related to fracture mechanics of small structures are critically examined for plausible solutions.

In-situ study of microscale fracture of diffusion aluminide bond coats: Effect of platinum

The influence of Pt layer thickness on the fracture behavior of PtNiAl bond coats was studied in situ using clamped micro-beam bend tests inside a scanning electron microscope (SEM). Clamped beam bending is a fairly well established micro-scale fracture test geometry that has been previously used in determination of fracture toughness of Si and PtNiAl bond coats. The increasing amount of Pt in the bond coat matrix was accompanied by several other microstructural changes such as an increase in the volume fraction of alpha-Cr precipitate particles in the coating as well as a marginal decrease in the grain size of the matrix. In addition, Pt alters the defect chemistry of the B2-NiAl structure, directly affecting its elastic properties. A strong correlation was found between the fracture toughness and the initial Pt layer thickness associated with the bond coat. As the Pt layer thickness was increased from 0 to 5 mu m, resulting in increasing Pt concentration from 0 to 14.2 at.% in the B2-NiAl matrix and changing alpha-Cr precipitate fraction, the initiation fracture toughness (K-IC) was seen to rise from 6.4 to 8.5 MPa.m(1/2). R-curve behavior was observed in these coatings, with K-IC doubling for a crack propagation length of 2.5 mu m. The reasons for the toughening are analyzed to be a combination of material's microstructure (crack kinking and bridging due to the precipitates) as well as size effects, as the crack approaches closer to the free surface in a micro-scale sample.