MRC Laboratory of Molecular Biology
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GMCs stabilized/solidified Pb/Zn contaminated soil under different curing temperature: Physical and microstructural properties
No full textStabilization/Solidification (S/S) has been widely used in soil remediation to both improve physical properties and immobilize extensive contaminants. GGBS (granulated ground blast furnace slag)-MgO-CaO (GMCs) was used to treat Pb/Zn contaminated soil. The physical and microstructural characteristics of stabilized/solidified contaminated soil were investigated in this study. Microstructural analysis showed that the main hydration products of GMC treated contaminated soil were C–S–H and hydrotalcite like gels (Ht), which dominated the physical strength of S/S soil. The unconfined compressive strength (UCS) and the leachability of GMC treated contaminated soil were improved with the increase in GMC proportion (5%–15%), curing time (7 days and 28 days) and temperature (5 °C, 21 °C and 45 °C) due to the enhanced hydration. The compressive strengths of the majority mixes met the US EPA criterion (0.35 MPa). The strength of S/S soils was less affected by the increase of curing temperature after a longer curing period (28 days). According to the XRD and SEM results, both Pb and Zn in S/S contaminated soil could be immobilized by the precipitation and the adsorption on the surface of calcium silicate hydrate (C–S–H). Zn can also be incorporated into the structure of C–S–H and Ht. The addition of Pb/Zn decreased the physical strength in the order of: Pb(5000 mg/kg)>Pb(10000 mg/kg)>Zn/Pb(5000 mg/kg)>Pb(20000 mg/kg)
Mode II Fracture of an Elastic-Plastic Sandwich Layer
No full textThe shear strength of a pre-cracked sandwich layer is predicted, assuming that the layer is linear elastic or elastic-plastic, with yielding characterized either by the J2 plasticity theory or by a strip-yield model. The substrates are elastic and of dissimilar modulus to that of the layer. Two geometries are analyzed: (i) a semi-infinite crack in a sandwich layer, subjected to a remote mode II K-field and (ii) a center-cracked sandwich plate of finite width under remote shear stress. For the semi-infinite crack, the near-tip stress field is determined as a function of elastic mismatch, and crack tip plasticity is either prevented (the elastic case) or duly accounted for (the elastic-plastic case). Analytical and numerical solutions are then obtained for the center-cracked sandwich plate of the finite width. First, a mode II K-calibration is obtained for a finite crack in the elastic sandwich layer. Second, the analysis is extended to account for crack tip plasticity via a mode II strip-yield model of finite strength and finite toughness. The analytical predictions are verified by finite element simulations, and a failure map is constructed in terms of specimen geometry and crack length
Hydrogen induced fast-fracture
No full textOne of the recurring anomalies in the hydrogen induced fracture of high strength steels is the apparent disconnect between their toughness and uniaxial tensile strength in identical hydrogen environments. Here we propose, supported by detailed atomistic and continuum calculations, that unlike macroscopic toughness, hydrogen-mediated tensile failure is a result of a fast-fracture mechanism. Specifically, we show that failure originates from the fast propagation of cleavage cracks that initiate from cavities that form around inclusions such as carbide particles. The failure process occurs in two stages. In stage-A, hydrides rapidly form around the roots of stressed notches on the cavity surfaces with hydrogen fed from the hydrogen gas within the cavity. These hydrides promote cleavage fracture with the cracks propagating at >100ms−1 until the hydrogen gas in the cavity is exhausted. Predictions of this hydrogen-assisted crack growth mechanism are supported by atomistic calculations of binding energies, mobility barriers and molecular dynamics calculations of the fracture process. Typically, cracks grow by less than 1 μm via this hydrogen-assisted mechanism and thus insufficient to cause macroscopic fracture of the specimen. However, this stage is then followed by a stage-B process where these fast propagating cracks can continue to grow, now in the absence of hydrogen supply, given an appropriate level of remote tensile stress. This is surprising because the fracture energy is now that of Fe in the absence of H and cleavage fracture requires opening tractions on the order of 15 GPa to be generated. Thus, fracture is usually precluded due to plasticity around the crack-tip. Here we show via macroscopic continuum crack growth calculations in a rate dependent elastic-plastic solid with fracture modelled using a cohesive zone that cleavage is possible if the crack propagates fast enough. This is because strain-rates at the tips of fast propagating cracks are sufficiently high for the drag on the motion of dislocations resulting from phonon scattering to limit plasticity. This combined atomistic/continuum model is used to explain a host of well-established experimental observations including (but not limited to): (i) insensitivity of the strength to the concentration of trapped hydrogen; (ii) the extensive microcracking in addition to the final cleavage fracture event and (iii) the higher susceptibility of high strength steels to hydrogen embrittlement. Importantly, we also show that the stage-A hydrogen-assisted fracture process only occurs in certain crystallographic orientations with crack-tip plasticity processes, such as twinning, blunting cracks in other orientations. This inhibits the fast-fracture mechanism in a macroscopic toughness on a polycrystalline material and thus explains the apparent contradiction between the hydrogen-assisted macroscopic toughness and tensile strength of steels
Sensitivity of the rayleigh criterion in thermoacoustics
No full textThermoacoustic instabilities are one of the most challenging problems faced by gas turbine and rocket motor manufacturers. The key instability mechanism is described by the Rayleigh criterion. The Rayleigh criterion does not directly show how to alter a system to make it more stable. This is the objective of sensitivity analysis. Because thermoacoustic systems have many design parameters, adjoint sensitivity analysis has been proposed to obtain all the sensitivities with one extra calculation. Although adjoint sensitivity analysis can be carried out in both the time and the frequency domain, the frequency domain is more natural for a linear analysis. Perhaps surprisingly, the Rayleigh criterion has not yet been rigorously derived and comprehensively interpreted in the frequency domain. The contribution of this theoretical paper is threefold. First, the Rayleigh criterion is interpreted in the frequency domain with integral formulae for the complex eigenvalue. Second, the first variation of the Rayleigh criterion is calculated both in the time and frequency domain, both with and without Lagrange multipliers (adjoint variables). The Lagrange multipliers are physically related to the system's observables. Third, an adjoint Rayleigh criterion is proposed. The paper also points out that the conclusions of Juniper (Phys. Rev. Fluids, vol. 3, 2018, 110509) apply to the first variation of the Rayleigh criterion, not to the Rayleigh criterion itself. The mathematical relations of this paper can be used to compute sensitivities directly from measurable quantities to enable optimal design
Measurement of the times for pyrolysis and the thermal diffusivity of a pyrolysing particle of wood and also of the resulting char
No full textCubes and spheres of spruce wood have been prepared, with a fine thermocouple inserted to measure the temperature at their centre. Individual particles were immersed rapidly in a bed of sand (mean size ∼0.2 mm), which was fluidised by nitrogen and held at a fixed temperature up to 700 °C. The rising temperature measured at a particle's centre yielded the effective value of the particle's thermal diffusivity. The temperature response showed evidence of at least two endothermic decomposition reactions, which corresponded to the pyrolysis of fine particles of the wood in a thermogravimetric analyser (TGA). However, the wood undergoing thermal decomposition in a fluidised bed at 500 °C revealed at least one exothermic step at the very end of heating. After being heated this way in a hot bed fluidised by nitrogen, the particles of char formed by spruce wood had very much the same size and shape as the original piece of wood before being heated. These new particles of “char”, formed whilst being heated in a hot fluidised bed, were cooled in a stream of nitrogen and returned to the fluidised bed for re-heating without any complications from pyrolysis. The rise in the char's central temperature with time gave an unambiguous value for the thermal diffusivity of the char. It is clear that volatile matter leaving a particle of wood reduced the rate of heat transfer between a hot fluidised bed and the centre of a devolatilising particle. Also, the time for complete pyrolysis was proportional to the square of the characteristic size (r0) of the spruce being heated. In addition, the time for pyrolysis was proportional to Tbed3±1, so that for a cube of spruce tpyr = 2.9 ± 0.3 × 1015 r02/Tbed3 in seconds. Photographic evidence confirmed that devolatilisation of particles of spruce larger than ≈ 2 mm in a fluidised bed follows a shrinking core model and is accordingly controlled by internal heat transfer
Reliable single grain growth of (RE)BCO bulk superconductors with enhanced superconducting properties
No full textWe review recent progress in the successful and reliable fabrication of large grain (RE)BCO and (RE)BCO-Ag bulk high temperature superconductors by three primary processing methodologies: Top-seeded melt growth (TSMG), top-seeded infiltration growth (TSIG) and a combined processing route comprising both TSMG and TSIG. Several significant and useful modifications have been made to these processing routes over the recent years in order to grow (RE)BCO single grains reliably and to enhance their superconducting properties. With respect to the reliable growth of (RE)BCO single grains, we have further developed the so-called buffer technique for reliable seeding, which includes identifying and providing an appropriate liquid-rich phase at the bottom of pellet. This modification has proved to be critical to the success of the newly-developed fabrication techniques presented here. In addition, we have addressed existing and significant challenges in the context of improving the superconducting properties of the bulk samples by controlling the microstructures of single grain materials (porosity and the RE-211 distribution, in particular) and by suppressing the extent of RE/Ba substitution during grain growth. Finally, we provide a short summary of the potential pathways towards realising practical applications of these technologically important materials in the near future
Centrifuge modelling of structures with oil dampers under seismic loading
No full textExperimental research into the seismic performance of buildings with passive oil dampers has so far been restricted to large-scale testing of frames erected on laboratory shaking tables that ignore the foundation soil below. This simplification of the problem falls short of replicating dynamic soil-structure interaction that would occur in the field. This paper presents the first experimental attempt at utilising high gravity dynamic centrifuge testing to replicate the response of a damped building at a reduced model scale. The paper compares the dynamic response of two similar two-degree-of-freedom model sway frames, one control (bare) frame and one frame equipped with miniature oil dampers, both structures founded on shallow raft foundations in dry dense sand. The miniature oil dampers successfully mitigate floor accelerations, drifts, and storey shear forces in the damped frame with minor modification to the frame stiffness. For strong, near resonance motions, global rocking of the undamped frame associated with physical uplifting of the foundation from the soil surface and subsequent yielding of sand beneath has led to floor acceleration levels, which are comparable to those obtained in the damped building fitted with miniature oil dampers. Assessment of the instrumentation installed on the miniature oil dampers reveals a viscoelastic damper behaviour with a dependency on stroke magnitude and on velocity
A UHF RFID Reader Antenna with Tunable Axial Ratio and Fixed Beamwidth
No full textA novel ultra-high-frequency (UHF) RFID reader antenna is proposed. The antenna has a unique property of being able to change its axial ratio (AR) without affecting its gain, beamwidth or impedance matching performance, enabling the isolated study of the effect of different axial ratios on RFID tag reading
High-Order Compensated Wireless Power Transfer for Dimmable Metal Halide Lamps
No full textA high-order compensated wireless power transfer (WPT) system is proposed and implemented for dimmable metal halide (MH) lamps without using any ignitor or ballast. Possessing high ignition voltage and constant current output, the LCC-LC compensated WPT is identified to be more competent for MH lamps as compared with the four basic compensation networks as well as the double-sided LCC compensation networks. Meanwhile, dimming control and power stabilization are achieved by the phase-shift modulation, and the multilayer multitype coil is newly proposed to enhance the tolerance of varying coupling coefficients. For exemplification, a wireless dimmable MH lamp operating at 400 kHz has been designed and prototyped. Particularly, the system can tolerate coil deviation up to 20 cm and dim the lamp down to 30% of standard illuminance with the merit of acoustic resonance free. Both theoretical and experimental results are given to validate the feasibility of the proposed system
