Low friction coefficient (approximately tan1°) of subaqueous debris flow in rotating flume tests and its mechanism

To reproduce a subaqueous debris flow with a friction coefficient of approximately tan1° in field, material with a huge volume may be involved. In this research, a steady subaqueous debris flow with a friction coefficient of approximately tan1° was produced by using the rotating flume experiment apparatus (Debris Flow Maker, DFM) with only 40 g of silicon powder. Subsequently, the mechanism of the low friction coefficient was interpreted with the hypothesis of dense agglomeration hitting (DAH). DAH means hitting of the dense agglomeration of dense flow on the flume's bottom plane. The following issues on DAH are considered: (I) the high velocity of the flume's bottom plane induces a strong hitting of particles; (II) the hitting elevates the liquid pressure near the boundary between the agglomeration and the flume's bottom plane; (III) the energy loss due to collision inside the particle agglomeration is limited. The experimental results provide an alternative method of producing subaqueous debris flow and an alternative measurement method of its friction coefficient; the hypothesis provides an alternative interpretation on the mechanism of low friction coefficient of subaqueous debris flow.     

Modified collaborative optimization for feasibility problem of final solution

This paper focuses on the feasibility problem of the final solution obtained by the relaxation collaborative optimization method. First, a mixed relaxation (MR) method is developed to enhance the possible premature convergence and non-solution problems of the variable relaxation method. Then the feasibility problem of the final solution is analyzed, that is the optimal solution obtained from system level may not satisfy the constraints of original optimization problem due to the inconsistency between system level and subsystem level for shared variables. A method of modified collaborative optimization (MCO) is put forward to solve this problem. In this method, the impact of the inconsistency is considered and supplemented in the subsystem-level constraints in order to approximate the original constraints as much as possible. At last, based on the MR method, the feasibility problem of the final solution is illustrated by three examples, and the effectiveness and accuracy of MCO are demonstrated by the optimization results.     

Reduced Annealing Time and Enhanced Magnetocaloric Effect of La(Fe,Al)13 Alloy by La-nonstoichiometry and Si-doping

LaFe_13-xM_x (M = Si, Al) alloys are promising for use in magnetic refrigeration. However, they require long annealing time (30 days) in order to optimize the magnetocaloric properties. Research has shown that the addition of extra La in off-stoichiometric alloys can greatly shorten the annealing time. Therefore, the purpose of this study is to investigate the influence of the extra addition of La on the annealing properties of a new off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x (x = 0, 0.1, 0.4) alloys. It was demonstrated that after a 36h annealing time, a large volume fraction of 1:13 magnetocaloric phase was obtained for all alloys. Further microstructural analysis of the off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x alloys revealed a facet-like grain morphology. The La_1.7Fe_11.6Al_1.4 and La_1.7Fe_11.6Al₁Si_0.4 alloys were shown to contain large 1:13 phase precipitates separated in a La-rich matrix, while the La_1.7Fe_11.6Al_1.3Si_0.1 alloy had a continuous 1:13 phase matrix with a fine dispersion of La-rich precipitates throughout. When the magnetic field varied between 0 and 2 T, the corresponding magnetic entropy change and relative cooling capacity for the La_1.7Fe_11.6Al_1.3Si_0.1 specimen were determined as 4.58 J/kg K and 173.6 J/kg, respectively. More importantly, the La_1.7Fe_11.6Al_1.3Si_0.1 alloy displayed only a slight volume change when the meta-magnetic phase transition occurred, which is promising for cyclic use.     

Dispersion coefficients for ideal bubbly flow in truly vertical bubble columns

A novel moving-boundary technique using acid-base neutralization was applied to truly vertical bubble columns to find axial dispersion coefficient. In addition, the effect of slight vertical misalignment (0.25 and 0.5°) was measured and showed increases in dispersion coefficient as much as two orders of magnitude. Using a punctured rubber membrane as a sparger, it was observed that ideal bubbly flow was maintained up to superficial gas velocities of around 4 cm/s for a column 14 cm in diameter. Photographic evidence along with the moving-boundary measurements suggest the proper turbulence length scale is the diameter of the bubble. The Baird-Rice isotropic turbulence model, suitably modified to account for changing length scales, produced an excellent fit of the bubbly-regime data.     

Impact response of rectangular and square stiffened plates supported on two opposite edges

Experimental drop weight impact tests have been performed to examine the dynamic response of small-scale stiffened plates struck laterally by a mass with a spherical indenter. The laboratory results are compared with numerical simulations. The plates stiffened with a flat bar or L profile are supported at two opposite edges and impacted at different velocities and locations along the span. The impact scenarios could represent incidents in marine structures, such as load actions due to dropped objects on decks. The experiments are conducted using a fully instrumented impact testing machine. The obtained force–displacement responses show a good agreement with the simulations performed by the LS-DYNA finite element solver. The finite element model includes defining the experimental boundary conditions so as to simulate small axial displacements of the specimen at the supports. This representation can be used to analyze the structural crashworthiness of similar marine structures under collision scenarios. The strain hardening of the material is defined using experimental data of quasi-static tension tests and the strain rate sensitivity is evaluated using standard coefficients of the Cowper–Symonds constitutive model. The results show that the plastic response of the specimens is highly sensitive to the amount of restraint provided at the supports. Furthermore, it is found that in most of the specimens the contribution of the stiffeners to the impact response is insignificant, since the ends of the stiffener are free at the unsupported edges and the specimens experience small axial displacements at the supports.     

Depercolation threshold of porosity in model cement: approach by morphological evolution during hydration

The depercolation threshold of porosity is an important parameter to assess the permeability of cement-based materials. The depercolation threshold is usually defined as the porosity whereby the volume fraction of connected pores in the cement paste decreases to zero. In this paper, the depercolation threshold is defined and determined with respect to the morphological development of pore space during hydration. The morphology of solid phase and pore structure is studied on model cement simulated by the SPACE system, using stereological theory. The influences of particle size distribution and water to cement ratio (w/c) on the depercolation threshold of porosity are discussed. It is found that particle size distribution of cement has significant influence on the depercolation threshold of porosity. The depercolation threshold is higher for finer cement system. However, the influence of w/c on the depercolation threshold of porosity is negligible. For a model cement of moderate fineness, depercolation is not possible at a relatively high w/c (say, 0.6), because the porosity of cement paste remains above the depercolation threshold even at complete hydration.     

Influence of second phases on fatigue crack growth behavior of nickel aluminum bronze

The influence of phases with different morphology and mechanical properties on fatigue crack growth behavior in nickel aluminum bronze (NAB) has been investigated. Annealing at 675 °C and normalizing at 920 °C heat treatments were used to produce different morphologies and fractions of second phases. This analysis shows that the coarse dendritic κ_II particles and κ_III lamellae as hard brittle phases in as-cast and annealed NAB have an accelerative effect on the fatigue crack propagation where by cracks propagate through α and κ_II/κ_III interface. Fatigue cracks in normalized NAB prefer to propagate through the ductile α grains, form fatigue striations and have the lowest crack growth rate. The uniformly distributed, fine κ_IV precipitates in the α grains improves fatigue crack growth resistance. This work identifies the role of NAB second phases on propagation of fatigue cracks, and provides suitable heat treatment for improving fatigue crack resistance in terms of controlling second phase type, distribution and percentage.     

Investigation on the cracking behavior of concrete cover induced by corner located rebar corrosion

Non-uniform corrosion of reinforcement causes concrete cracking in chloride contaminated RC structures. Due to the special boundary conditions, the concrete cover with corner located rebar is often subjected to the attack of chloride ions in a marine environment from two directions, and thus the corresponding non-uniform corrosion distribution should be different from the one for side-located rebar. The aim of the work is to explore the effect of corner located rebar corrosion on the cracking of cover concrete. For corner located rebar, an improved non-uniform corrosion distribution model was established based on the analysis results of two-dimensional chloride diffusion in concrete. Considering the heterogeneities of concrete, a meso-scale mechanical model and method for the study on the failure behavior of concrete cover was built. In the analysis model and method, the non-uniform radial displacement distribution was adopted to simulate the corrosion expansion behavior of the rebar. The cracking of concrete cover with corner located rebar was simulated and studied. The present approach was verified by the available experimental observations. The influences of concrete heterogeneity, corrosion distribution types, rebar diameter and concrete cover thickness on the failure patterns of concrete cover and the expansive pressure were investigated. The simulation results indicate that the developed approach can well describe the cracking behavior of cover concrete and the corrosion-expansion behavior of steel rebar.     

An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

An experimental study of toluene pyrolysis (1.24 vol.% toluene in argon) was performed at low pressure (1.33 kPa) in the temperature range of 1200–1800 K. The pyrolysis process was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). Species up to m/z = 202 (C₁₆H₁₀), containing many radicals (CH₃, C₃H₃, C₅H₃, C₅H₅, C₇H₅, C₇H₇, C₉H₇, C₁₁H₇ and C₁₃H₉) and isomers, such as C₃H₄ (propyne and allene), C₄H₄ (vinylacetylene and 1,2,3-butatriene), C₅H₅ (cyclopentadienyl radical and pent-1-en-4-yn-3-yl radical), C₆H₄ (3-hexene-1,5-diyne and benzyne), C₆H₆ (benzene and fulvene), C₇H₈ (toluene and 5-methylene-1,3-cyclohexadiene) and so on, were identified from near-threshold measurements of photoionization mass spectra, and the mole fraction profiles of the pyrolysis products were evaluated from measurements of temperature scan. Experimental results indicate that the reaction C₇H₈ → C₇H₇ and the subsequent reactions are dominant at comparatively low temperature (<1440 K), while the reaction C₇H₈ → C₆H₅ and subsequent reactions gradually become competitive and important with increasing temperature. Furthermore the barriers of the decomposition pathways of toluene and benzyl radical determined by quantum mechanical calculation are in good agreement with the initial formation temperatures of the species. Based on the mole fractions and formation temperatures of the detected pyrolysis species, a simple reaction network is deduced. At relatively high temperatures, H-abstraction is prevalent and the mole fraction of C₂H₂ is so high that many aromatics are formed through the hydrogen-abstraction/C₂H₂-addition (HACA) mechanism. Moreover the reactions of benzyl with toluene/benzyl/phenyl/propargyl radicals to directly produce larger aromatics should play an influential role in PAH formation. Meanwhile the five-member-ring recombination mechanism also plays an indispensable role in the aromatics growth, as cyclopentadienyl radical (C₅H₅) was determined to be a major product of the decomposition of toluene.