Prediction of welding stresses in WIC test and its application in pipelines

In the present study, the Welding Institute of Canada (WIC) restraint test was used to simulate the restraint conditions of full-scale girth welds on energy pipelines to ascertain the influence of welding process parameters on welding stresses. Finite element models are developed, and validated with neutron diffraction measurements, to evaluate the welding stresses for under-matched, matched and over-matched welds. The effects of heat input, wall thickness and variable restraint lengths of WIC sample are systematically investigated. As a practical outcome, this work can help in selection of the appropriate restraint length for WIC tests to simulate the specified stress conditions in the pipeline, and, ultimately, reduce the risk of Hydrogen Assisted Cold Cracking (HACC) in high strength low alloy.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

轴肩结构对搅拌摩擦焊过程中材料流动的影响

考虑材料参数随温度的变化关系以及搅拌头的实际结构形式,利用ANSYS FLUENT软件对搅拌摩擦焊过程中材料的流动行为进行了数值分析,研究了轴肩结构分别为平面、内凹与同心圆时的材料流动规律.研究表明,当轴肩结构发生变化时,焊件表面及内部的材料流动趋势基本相同;靠近焊件表面的材料流动速度在轴肩结构为同心圆时最大,在轴肩结构为平面时最小.从避免搅拌摩擦焊根部缺陷的角度看,同心圆轴肩的搅拌头优于内凹轴肩,这一规律得到了试验证实.     

Effects of welding speed on the multiscale residual stresses in frictionstir welded metal matrix composites

The effects of welding speed on the macroscopic and microscopic residual stresses(RSes) in friction stir welded 17 vol.% SiCp/2009 Al-T4 composite plates were studied via neutron diffraction and an improved decoupled hierarchical multiscale modeling methods. Measurements showed that the macroscopic and total RSes had the largest variations in the longitudinal direction(LD). Increasing the welding speed led to higher values of measured LD macroscopic and total RSes in the matrix. The welding speed also significantly influenced the distributions and magnitudes of the microscopic RSes. The RSes were predicted via an improved hierarchical multiscale model, which includes a constant coefficient of friction based thermal model. The RSes in the composite plates before friction stir welding(FSW) were computed and then set as the initial states of the FSW process during modeling. This improved decoupled multiscale model provided improved predictions of the temperature and RSes compared with our previous model.     

Numerical simulation of the liquating behavior of niobium carbide in heat-affected-zone during welding of a superalloy

In order to predict the propensity of a superalloy to heat-affected-zone (HAZ) liquation cracking, Visual FORTRAN procedures were developed based on a heat transfer and mass diffusion model and the constitutional liquation of precipitate (NbC) at grain boundaries was simulated numerically. The results show that with the increase in the rate of welding thermal cycle, the solid dissolution of precipitate prior to liquation decreases and the thickness of liquid film produced by constitutional liquation increases. Higher heating rate inhibits the further melting of adjacent matrix and the solidification of liquid by liquid-to-γ mode in the subsequent thermal cycle. As a result, the residual liquid film still maintains a great thickness at the moment when temperature is down to the eutectic point, which will promote HAZ micro-fissuring. Finally, hot ductility tests on a low-expansion superalloy were performed to verify indirectly the conclusions drawn from the numerical simulation.     

Numerical investigation of droplet impact on the welding pool in gas metal arc welding

A transient three‐dimensional model that describes physical phenomena inside a welding pool during gas–metal arc welding process is presented. The model considers such phenomena as heat‐mass transfer, electromagnetics, hydrodynamic processes and deformation of the weld pool free surface. The fluid flow in the weld pool is induced due to the presence of the mechanical impact of the droplets, thermo‐capillary surface tension, thermal buoyancy and electromagnetic forces. The weld pool surface deformation is calculated by considering arc pressure and droplet impact force. A comparative analysis of the impact of the electric current of the welding arc and different force factors causing the motion of liquid metal in the weld pool on the shape of the welded seam was carried out and discussed.     

Magnetism-current inverse image reconstruction of the nugget in aluminum alloy spot welding based on ART algorithm

In this work, the computer tomography (CT) theory and its reconstruction algorithm were used to deal with the magnetism-current inverse problem in the resistance spot welding (RSW). At first, the magnetic fields around the nugget were detected. Then, the current distribution of the nugget section was calculated by reconstruction algorithm. At last, we changed the current distribution data into a graph using Matlab. The inversed graph of the nugget-section current distribution in the inverted RSW can be achieved, and by this graph the details of the nugget can be observed directly, which can help evaluate the joint quality.     

蜂窝管湿式电除尘器内部流动特性研究

湿式电除尘技术能够有效降低火力发电过程中的烟尘排放浓度。为研究湿式电除尘器在运行过程中喷雾对内部流场的影响,采用计算颗粒流体力学(computational particle fluid dynamics,CPFD)方法对喷嘴位置及喷嘴启用前后除尘器内部流场进行数值模拟,对除尘器内部流场、液滴分布和喷雾轨迹等进行深入分析。结果表明:喷嘴启动时,会提高喷嘴附近烟气流速,同时引起蜂窝管段烟气速度场不均匀;当喷嘴布置在管内时,会加速管内烟气流速,不利于粉尘捕获。研究结果可为实际工程中调整雾化区域提供理论参考依据。     

新风口面积对受限空间内回燃影响的研究

利用火灾动力学模拟软件FDS,以某商场为原型,建立数值模型,模拟不同新风口面积条件下受限空间内部回燃现象,测定火灾过程中初始火源及新风口上方温度、火焰高度,研究新风口面积对回燃现象的影响.模拟结果表明,在通入新鲜空气的180s中,在新风口附近出现持续爆燃;当新风口面积为2m2时,爆燃火焰高度可达5m以上.新风口上部及受限空间内部温度随其面积的增加而提高,因此消防员进入受限空间救援时,应做好个人防护并尽可能远离存在新鲜空气的场所.     

C/C热防护结构弹性波仿真分析方法及损伤对弹性波的影响

热防护结构（Thermal protection structures,TPS）作为可重复使用航天飞行器的关键结构,其安全性和可维护性至关重要。已有的针对可重复使用航天飞行器TPS的结构健康监测的研究不多,特别是C/C、C/SiC等复合材料制造的TPS。针对可重复使用航天飞行器C/C TPS,研究其基于主动弹性波的结构健康监测方法。提出了一种TPS弹性波数值仿真方法,在验证方法有效性的基础上,开展了面向C/C TPS损伤监测的弹性波传播特性研究,从信号时域特征、弹性波传播波场、损伤散射信号、损伤散射信号波场及损伤因子5个方面全面分析了弹性波在C/C TPS中的传播特性及损伤带来的影响。结果表明,当分层损伤边长大于25 mm后,损伤因子剧烈变化,因此采用基于弹性波的结构健康监测方法可以很好地监测C/C TPS内部分层损伤及损伤的扩展情况。      

煤层自燃防治与高温爆破安全技术中的若干力学问题

煤层自燃已经是我国大规模煤矿开采不可回避的问题。在高温环境下的煤矿爆破开采,既涉及到爆破的高温环境也需要改进高温爆破技术,其中认识高温爆破中的工程规律,是解决高温爆破技术的关键。首先对煤层自燃的条件进行了量纲分析,说明了煤层中裂隙对流比热传导更容易诱发煤层自燃;煤体中的破裂场、气流场是煤层自燃的条件,单纯封堵裂缝隔绝空气的方法有短期效果,但地质体因煤层燃烧力学特性改变产生变形和破裂后,会死而复燃。讨论了井工开采诱发煤层自燃的机理以及相关科学问题,对井工开采分析了采空区上方冒落带、破裂带、沉降带与煤层自燃的关系,提出了地表长期封闭覆盖与自适应循环注水相结合的综合灭火方法。进而分析了露天矿开采高边坡稳定性与煤层自燃环境的关系,边坡开挖引起山体变形、开裂,形成破裂场是产生煤层自燃的基本要素。最后,分析了深孔爆破中,爆破技术可靠性中涉及的科学问题,介绍了利用循环水降温的方法。     

Study on stress evolution in SiC particles during crack propagation in cast hybrid metal matrix composites using Raman spectroscopy

The evolution of stress in the SiC particles during crack propagation under monotonic loading in a cast hybrid MMC was investigated by micro Raman spectroscopy. The experiment was carried out in situ in the Raman spectroscopy. Experimental results showed that cracks due to monotonic loading propagated by the debonding of the particle/matrix interface and particle fracture. Secondary cracks those formed in front of the main crack tip coalesced with the main crack in subsequent loading and final failure occurred. A high decrease in stress (several hundreds in MPa) was observed with the interfacial debonding at the interface and with the particle fracture on the particle. Moreover, the critical tensile stresses for particle–matrix interface debonding and particle fracture developed in hybrid MMC were also estimated during the crack propagation.