Critical natural frequency: an improved empirical effectiveness criterion in vibration stress relief of rectangle welded plates

Decreasing of natural frequency of the treated structure is the most frequently used empirical effectiveness criteria in vibration stress relief (VSR). However, dependability and reliability of this criteria is still far from sufficient. In this study, a covert negative treatment phenomenon was investigated, i.e. natural frequency of welded structures decreased after VSR but residual stress in one direction increased. Relationship between natural frequency and residual stresses was studied by mathematical deduction and finite element method. “Natural Frequency Function” and “Natural Frequency Surface (NFS)” was proposed to describe that relationship. “Critical Natural Frequency” (CNF) was proposed to depict possible situations after VSR. A quantitative natural frequency criterion for VSR effectiveness estimation was proposed

Cost-effective online trending topic detection and popularity prediction in microblogging

Identifying topic trends on microblogging services such as Twitter and estimating those topics’ future popularity have great academic and business value, especially when the operations can be done in real time. For any third party, however, capturing and processing such huge volumes of real-time data in microblogs are almost infeasible tasks, as there always exist API (Application Program Interface) request limits, monitoring and computing budgets, as well as timeliness requirements. To deal with these challenges, we propose a cost-effective system framework with algorithms that can automatically select a subset of representative users in microblogging networks in offline, under given cost constraints. Then the proposed system can online monitor and utilize only these selected users’ real-time microposts to detect the overall trending topics and predict their future popularity among the whole microblogging network. Therefore, our proposed system framework is practical for real-time usage as it avoids the high cost in capturing and processing full real-time data, while not compromising detection and prediction performance under given cost constraints. Experiments with real microblogs dataset show that by tracking only 500 users out of 0.6 million users and processing no more than 30,000 microposts daily, about 92% trending topics could be detected and predicted by the proposed system and, on average, more than 10 hours earlier than they appear in official trends lists

Critical Velocities for High Speed Particle Deposition in Kinetic Spraying

During kinetic spraying (or cold gas dynamic spraying), small solid particle can be deposited onto the substrate by a supersonic velocity impaction. A critical velocity of flying particle was found for the minimum deposition speed. Generally, it was thought that higher impact velocity caused higher deposition efficiency. However, some different phenomena were observed during spraying processing in laboratory. In this study, individual particle impact tests have been carried out to probe the effect of impact velocity. Instead of deposition efficiency (DE) of coatings, ratio of bonds, which was defined as the number fraction of attached particles to total impact particles (Craters + Bonds) in unit area of impact surface, was measured in our test. A maximum velocity for particle deposition was observed, because of which a high fraction of rebounded particles occurred at a high impact velocity. A model considering the adhesion and rebound energy was built up to estimate the particle/substrate interaction. The particle deposition behavior was a result of the competition between adhesion and rebound energies during the impact process. Only when the adhesion energy was higher than the rebound energy, the impacting particles could be attached onto the substrate

The micro response mechanisms of foamed polymer rehabilitation material under compression: From a closed cell view

Non-aqueous reactive foamed polyurethanes are widely used in the non-excavation rehabilitation of infrastructures and fundamental engineering. However, the effect of a microscopic cell on the understanding of micro-mechanical properties of a polyurethane is not clearly stated. In this work, a molecular model of a polyurethane closed cell is established corresponding to the field emission scanning electron microscopy results. Molecular dynamics simulations were subsequently employed to investigate the effect of a closed cell on the micro-mechanical response of polyurethane during compression. Based on the layered non-affine displacement and internal structure variables, we find that the existence of cells reduces changes in the polymer chain structure, i.e., the bond stretching, bond angle bending, and dihedral angle rotation. This makes slippage of the polymer chain more violent, and the farther away from the centre of the cell, the greater the level of the polymer chain slippage. In other words, chain slippage plays a key role in reducing the mechanical properties of a polymer. What’ s more, it is found that the number of hydrogen bonds in the cell is significantly less than that in the elastomer, which also could reduce the mechanical properties of foamed polyurethane. This study provides an efficient route for studying the micro-mechanical characterization of foamed polyurethane

Experimental and Numerical Analyses on Mixing Uniformity of Water and Saline in Pipe Flow

Liquid—liquid mixing is commonly observed in many applications such as the chlorination of water supplies and the agricultural fertigation. In order to study the mixing law of water-chlorine or water-fertilizer in a turbulent pipeline, saline was selected as a tracer injected into the pipeline. In this paper, the computational fluid dynamics (CFD) software was employed to study flow fields in water-saline pipelines. Four variates (mixing ratio "i"δ"/i", pipe diameter "i"D"/i", volume flow rate in the main pipe "i"Q, "/i"saline density "i"ρ"sub"s"/sub""/i") were considered to investigate the effects of multiple variates on mixing uniformity. The coefficient of variation (COV) was selected as the evaluation index of mixing uniformity, effective mixing length ("i"L"sub"EML"/sub""/i", the distance from the saline inlet to the fully mixed position) was chosen to quantitatively analyze the fully mixed position of water and saline in pipelines. The results of this numerical model agree well with experimental measurements and it shows that this model can effectively predict the concentration field of water and saline in the pipeline. Based on the experimental and simulated results, it was found that for the fixed mixing ratio, saline density and volume flow rate, the values of "i"L"sub"EML"/sub""/i" increased significantly with increasing pipe diameters. Furthermore, dimensional analysis (D-A) was adopted to examine the influences of the four variates on "i"L"sub"EML"/sub""/i", and their correlation coefficient of the curve-fitting equation was calculated to be 0.996. Document type: Articl

A combined computational and experimental study on vibration stress relief for large welded DH36 steel tube

Vibration stress relief (VSR) is an effective and economic method for reducing residual stress in various welding components. Compared with other methods, it costs much less time and energy. In this study, finite element method (FEM) was used to assist VSR treatment of large DH36 steel welded tube by determining the 1st order vibration mode of the tube and the natural frequency of the tube in a hypothetic zero-stress state. According to the computational results, proper vibration exciting assembling and excitation strategy was selected. An effectiveness index, η, for fast and quantitative estimation of the residual stress decrease rate was proposed. η is determined by 1st order natural frequencies of the tube in three states, i.e. as-welded, VSR treated and a hypothetic zero-stress state. η in this study was 49.8 %, meeting well with the experimentally measured residual stress decrease rate, ~50 %. Thus, the validity of the effectiveness index was verified. This study provides a novel method for analysis of VSR effectiveness. In comparison, maximum residual stress was reduced by 20-57 % when traditional local post weld heat treatment (PWHT) was used. This indicates that VSR is a good stress relief method for DH36 welded structures

The various substrates of Usnea aurantiaco-atra and its algal sources in the Fildes Peninsula, Antarctica

The lichen species Usnea aurantiaco-atra (Jacq.) Bory is the most dominant vegetation on the Fildes Peninsula, Antarctica. Most individuals grow on rocks, and some are found with mosses. During the 27th and 28th Chinese National Antarctic Research expeditions of the Great Wall Station, U. aurantiaco-atra was observed growing on the lichen thallus of Umbilicaria antarctica Frey & I.M. Lamb, or on wood, which indicated that Usnea aurantiaco-atra could grow on various substrates. The diversities of the symbionts in U. aurantiaco-atra collected in the Fildes Peninsula were investigated using ITS rDNA sequences. The results showed that the sequences from mycobionts of U. aurantiaco-atra growing on various substrates did not exhibit significant differences. All photobionts in this lichen species were the green algae Trebouxia jamesii (Hildreth & Ahmadjian) Gärtner. The identical sequences from the photobionts of both Umbilicaria antarctica and Usnea aurantiaco-atra indicated there was an algae pool in this area and different mycobionts could obtain their algal partners from this pool. The variety of substrates for U. aurantiaco-atra suggested its photobiont could be obtained from a mature lichen thallus by vegetative propagation; from other lichen thalli (e.g. Umbilicaria antarctica); or from the surroundings. This study will promote understanding of the distribution of photobionts and the process of lichenization