A validated thermal model of bead-on-plate welding

In this paper, finite element model is used to carry out thermal analysis of bead-on-plate welding. The model followed the proposed five step strategies which were then built into a model to obtain temperature history at the positions of thermocouples. Temperature field was also evaluated by comparing predicted weld bead with the actual weld bead. Using these proposed strategies, well matched temperature histories and temperature field have been obtained.     

Chemorheological response of elastomers at elevated temperatures: Experiments and simulations

An experimental study and a method for simulating the constitutive response of elastomers at temperatures in the chemorheological range (90-150 °C for natural rubber) are presented. A comprehensive set of uniaxial experiments for a variety of prescribed temperature histories is performed on natural rubber specimens that exhibit finite elasticity, entropic stiffening with temperature, viscoelasticity, scission, and oxygen diffusion/reaction effects. The simulation approach is based on a multi-network framework for finite elasticity, isothermal incompressibility, thermal expansion, and temperature-induced degradation. The model extends previous work to account for kinetics of scission for arbitrary time-varying temperature histories and incorporates the effects of viscoelastic relaxation and diffusion-limited oxidative scission. The model is calibrated to experiments performed on a commercially-available filled natural rubber material, and numerical simulations are compared favorably to experiments for a variety of temperature histories.     

Effect of porosity and the inlet heat transfer fluid temperature variation on the performance of cool thermal energy storage system

This paper discusses the results of numerical and experimental study of an encapsulated cool thermal energy storage system. The storage system is a cylindrical storage tank filled with phase change material encapsulated in spherical container, placed in a refrigeration loop. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid and the phase change material at any axial location during the charging period. The present analysis aims at studying the influence of the inlet heat transfer fluid temperature and porosity on system performance. An experimental setup was designed and constructed to conduct the experiments. The results of the model were validated by comparison with experimental results of temperature profiles for different inlet heat transfer fluid temperatures and porosity. The results are in good agreement with the experimental results. The results reported are much useful for designing cool thermal energy storage systems.     

An examination of the competition between bulk behavior and interfacial behavior of ceramics subjected to dynamic pressure-shear loading

A grain level micromechanical analysis of ceramic microstructures subjected to dynamic compression-shear loading conditions is presented. The investigation consists of a combined experimental/numerical approach in which bulk and surface properties are examined by means of pressure-shear impact experiments for a ceramic plate in contact with steel plates. The model for the ceramic microstructure accounts for heterogeneities and randomness in grain orientation and composition. A cohesive zone model is included to capture inter-granular microcrack initiation and evolution as a natural outcome of the calculated material response. Surface roughness is also included in the analysis to capture the time-dependent frictional behavior of the various interfaces. The model for the steel anvil plate accounts for visco-plasticity, thermal softening and strain hardening. Representative volume elements of ceramic microstructure and anvil plates are considered to account for features observed in real experiments. Pressure-shear impact velocity histories are used not only to identify inelasticity, but also to determine dominant failure modes. Bridging between micro- and macroscales is achieved by using the developed model. Simulated velocity histories have been found to be in a good agreement with the experimental observations when bulk and surface features are included in the analysis. However, it is demonstrated that information gathered from these experiments is not sufficient to determine the mechanical behavior of the brittle material. Instead, the velocity histories provide important information on the time-dependent frictional behavior of the specimen-anvil interfaces.     

THERMOMECHANICAL RESPONSE OF SMA FIBER COMPOSITES WITH NON-UNIFORM TEMPERATURE DISTRIBUTION

A micromechanics method based on the High-Order-Theory developed by Aboudi et al. is used to predict the thermomechanical response of composites reinforced by shape memory alloy (SMA) fibers, and the non-uniform thermal distribution in composite arising from the process of heating or cooling is considered. The numerical development based on this model was coded to predict the thermomechanical response of shape memory alloy fiber/elastomer matrix composite subjected to thermal cycle loading. When the composite is heated, two heating ways, thermal gradients and heat source by passing an electric current through the SMA fibers are imposed on the composite respectively. Upon cooling, the first thermal boundary condition and the second thermal boundary condition are subjected to the composite respectively. A series of stress distributions and temperature distributions for different instants are calculated to reveal the interaction between the SMA material and matrix. It is useful to analyze and design the SMA actuator driven by heat source or the surface temperature.     

随机激励下受热翼面的模态频率特性试验研究

现代高速飞行器结构热模态频率特性试验研究,对这类飞行器设计校核和飞行安全具有重要意义。根据飞行过程中遭受的气动加热特性设计了瞬态热环境模拟系统,同时,根据高温环境的特点对测试中的激励和测量方式进行了重新设计,成功地将普通激振器应用于高温结构模态试验,最终将热环境模拟系统与振动测试系统组合,形成一套考虑瞬态热影响的热模态试验系统,实现了瞬态热环境下结构模态的地面测试。对一个切尖三角翼测量了各个加热区的温度随加热时间的变化,验证了加热温度控制的精确性;在纯随机激励下对测得的激励和振动响应信号采用短时傅里叶变换(Short Time Fourier Transformation,STFT)进行时变模态参数辨识,获得了前四阶模态频率随加热时间的变化,并与结构有限元数值计算结果进行了比较,试验与计算结果吻合得很好,验证了该试验方法对热模态测试问题的有效性和准确性。通过分别对瞬态和稳态热环境下结构模态频率试验和计算结果的分析,探讨了结构瞬态温度场对模态频率影响的机理,揭示了结构内部存在的热应力和材料属性的变化,是决定模态频率随加热时间变化趋势的内在原因。     

Comprehensive numerical analysis of a three-pass bead-in-slot weld and its critical validation using neutron and synchrotron diffraction residual stress measurements

The current paper presents a finite element simulation of the residual stress field associated with a three pass slot weld in an AISI 316LN austenitic stainless steel plate. The simulation is split into uncoupled thermal and mechanical analyses which enable a computationally less expensive solution. A dedicated welding heat source modelling tool is employed to calibrate the ellipsoidal Gaussian volumetric heat source by making use of extensive thermocouple measurements and metallographic analyses made during and after welding. The mechanical analysis employs the Lemaitre–Chaboche mixed hardening model. This captures the cyclic mechanical response which a material undergoes during the thermo-mechanical cycles imposed by the welding process. A close examination of the material behaviour at various locations in the sample during the welding process, clearly demonstrates the importance of defining the correct hardening and high temperature softening behaviour. The simulation is validated by two independent diffraction techniques. The well-established neutron diffraction technique and a very novel spiral slit X-ray synchrotron technique were used to measure the residual stress–strain field associated with the three-pass weld. The comparison between the model and the experiment reveals close agreement with no adjustable parameters and clearly validates the used modelling procedure.     

Measurement of Creep Deformation across Welds in 316H Stainless Steel Using Digital Image Correlation

Spatially resolved measurement of creep deformation across weldments at high temperature cannot be achieved using standard extensometry approaches. In this investigation, a Digital Image Correlation (DIC) based system has been developed for long-term high-temperature creep strain measurement in order to characterise the material deformation behaviour of separate regions of a multi-pass weld. The optical system was sufficiently stable to allow a sequence of photographs to be taken suitable for DIC analysis of creep specimens tested at a temperature of 545 °C for over 2000 h. The images were analysed to produce local creep deformation curves from two cross-weld samples cut from contrasting regions of a multi-pass V-groove weld joining thick-section AISI Type 316H austenitic stainless steel. It is shown that for this weld, the root pass is the weakest region of the structure in creep, most likely due to the large number of thermal cycles it has experienced during the fabrication process. The DIC based measurement method offers improved spatial resolution over conventional methods and greatly reduces the amount of material required for creep characterisation of weldments.     

U71Mn钢轨气压焊焊接接头滚动磨损与损伤性能研究

研究了U71Mn钢轨气压焊焊接接头上各区域的组织与性能,并利用MMS-2A轮轨滚动磨损与接触疲劳试验机对焊接接头材料进行试验,分析了各区域的磨损与损伤特性. 结果表明:焊接接头组织为珠光体,但晶粒大小及渗碳体形态和大小存在差异. 焊缝中珠光体晶粒较小,渗碳体呈细小片层状及细小颗粒状,因此硬度高且塑性变形能力强. 在1×105和2×105循环次数时耐磨性优于母材,在3×105循环次数时,焊缝磨损量大于母材磨损量,且焊缝表面损伤较母材严重. 在焊缝两侧各有1个区域(软化区),组织为粒状珠光体和少量片层状珠光体,颗粒大小和片层厚度不均匀,硬度较小,磨损量较大,塑性变形层较厚,表面损伤最严重.      

石油钻机盘式刹车副材料选配的实验研究

在 MMW- 1型摩擦磨损试验机上 ,通过变温摩擦磨损特性试验 ,对优选出的 2种刹车盘表面堆焊材料和研制出的 2种刹车块材料进行了选配 ,并对各刹车副的摩擦学性能进行了分析与评价 .结果表明 ,摩擦偶件对刹车副材料的摩擦学性能有一定的影响 ,因此在进行材料的选配试验时应考虑偶件的匹配问题 .优选出的 2种表面堆焊材料与所研制的 2种刹车块材料组成摩擦副时具有较高的平均摩擦系数和热摩擦系数以及优良的高温耐磨性能 ,其中第二种表面堆焊材料与所研制的第二种无石棉刹车块材料组成摩擦副时的摩擦学性能指标最优 ,可以作为选配出的最优刹车副材料     

Sensitivity analysis of the thermomechanical response of welded joints

A computational procedure is presented for evaluating the sensitivity coefficients of the thermomechanical response of welded structures. Uncoupled thermomechanical analysis, with transient thermal analysis and quasi-static mechanical analysis, is performed. A rate independent, small deformation thermo-elasto-plastic material model with temperature-dependent material properties is adopted in the study. The temperature field is assumed to be independent of the stresses and strains. The heat transfer equations emanating from a finite element semi-discretization are integrated using an implicit backward difference scheme to generate the time history of the temperatures. The mechanical response during welding is then calculated by solving a generalized plane strain problem. First- and second-order sensitivity coefficients of the thermal and mechanical response quantities (derivatives with respect to various thermomechanical parameters) are evaluated using a direct differentiation approach in conjunction with an automatic differentiation software facility. Numerical results are presented for a double fillet conventional welding of a stiffener and a base plate made of stainless steel AL-6XN material. Time histories of the response and sensitivity coefficients, and their spatial distributions at selected times are presented.     

A study of mechanical properties of pure and nitrogen-doped ultrananocrystalline diamond films under various loading conditions

To better understand the mechanical responses of ultrananocrystalline diamond (UNCD) under various loading conditions, a numerical study is performed to investigate the size, loading rate and temperature effects on the material properties of pure and nitrogen-doped UNCD films. Since the UNCD growth mechanism is not completely understood yet, a simple procedure by combining kinetic Monte Carlo and molecular dynamics (MD) methods is developed to form a polycrystalline UNCD block with an artificial grain boundary (GB). By randomly inserting different numbers of nitrogen (N) atoms into the GB of the resulting polycrystalline UNCD films, N-doped UNCD films can be formed. The responses of the simulated pure and N-doped UNCD films with various grain sizes are then investigated by applying displacement-controlled tensile loading under different rates and temperatures in the MD simulations. The simulation results presented in this paper provide a better understanding of the combined size, rate and thermal effects on the material responses of pure and N-doped UNCD films.     

A new strategy for the numerical modeling of a weld pool

Welding processes involve high temperatures and metallurgical and mechanical consequences that must be controlled. For this purpose, numerical simulations have been developed to study the effects of the process on the final structure. During the welding process, the material undergoes thermal cycles that can generate different physical phenomena, like phase changes, microstructure changes and residual stresses and distortions. But the accurate simulation of transient temperature distributions in the part needs to carefully take account of the fluid flow in the weld pool. The aim of this paper is thus to propose a new approach for such a simulation taking account of surface tension effects (including both the “curvature effect” and the “Marangoni effect”), buoyancy forces and free surface motion.The proposed approach is validated by two numerical tests from the literature: a sloshing test and a plate subjected to a static heat source. Then, the effects of the fluid flow on temperature distributions are discussed in a hybrid laser/arc welding example.     

Macro-performance evaluation of friction stir welded automotive tailor-welded blank sheets: Part I – Material properties

In order to evaluate the macroscopic performance of friction stir welded automotive tailor-welded blank (TWB) sheets, the hardening behavior, anisotropic yielding properties and forming limit diagram were characterized both for base (material) and weld zones. In order to describe the Bauschinger and transient hardening behaviors as well as permanent softening during reverse loading, the modified Chaboche type combined isotropic–kinematic hardening law was applied. As for anisotropic yielding, the non-quadratic anisotropic yield function, Yld2000-2d, was utilized for base material zones, while isotropy was assumed for weld zones for simplicity. As for weld zones, hardening properties were obtained using the rule of mixture and selectively by direct measurement using sub-sized specimens. Forming limit diagrams were measured for base materials but calculated for weld zones based on Hill’s bifurcation and M–K theories. In this work, four automotive sheets were considered: aluminum alloy 6111-T4, 5083-H18, 5083-O and dual-phase steel DP590 sheets, each having one or two thicknesses. Base sheets with the same and different thicknesses were friction-stir welded for tailor-welded blank (TWB) samples.     

超高温、大热流、非线性气动热环境试验模拟及测试技术研究

超高温、大热流、非线性气动热环境试验模拟技术及相应的极端高温环境力学测试技术,是高超声速飞行器防热材料和结构安全设计中事关研制成败的关键技术。本文介绍了自行研制的可实现高至210℃/s的极快非线性升温速率、能够生成高达2MW/m2的瞬态非线性热流密度、实现高达1500℃超高温氧化热环境的石英灯红外辐射式气动热环境试验模拟系统。基于这一性能优越的超高温气动热环境试验模拟系统,发展了如下超高温热环境力学测试技术:1)提出对环境光变化不敏感的主动成像数字图像相关方法,实现了C/SiC复合材料1550℃高温变形的非接触、全场光学测量;2)发展了1400℃超高温热/力联合试验环境下SiC/SiC复合材料结构的断裂特性试验测试技术。本文还简要介绍了高速巡航导弹翼面结构900℃高温热振联合试验,950℃高温非线性热环境下的蜂窝结构隔热性试验等研究内容。本文所发展的超高温气动热环境试验模拟技术和高温热环境力学测试技术,对航天航空领域高超声速飞行器的研制具有重要的军事工程应用价值。     

磁头/磁盘滑动接触下磁盘温度及热退磁临界条件的研究

采用二维轴对称有限元模型计算磁头/磁盘滑动接触下,铝质磁盘的稳态温度和热应力场以及热退磁临界条件.结果表明:磁盘温度在极短时间内升至摩擦稳态值,然后缓慢线性升高到最终稳态值;经过充分热传导和热交换后磁盘的温度梯度较小,此时磁层内的热应力集中分布于磁盘固定端边缘附近;磁盘的稳态温度和热应力均随速度增大而增大,且载荷越大其值增大越快;热应力小于1.2 GPa时所对应的速度和载荷为安全工况;温升大于373 K时所对应的工况将导致磁盘退磁.     

A two-dimensional simulation of grain structure growth within the substrate and the fusion zone during direct metal deposition

In this paper, a predictive multi-scale model based on a cellular automaton (CA)-finite element (FE) method has been developed to simulate thermal history and microstructure evolution during metal solidification for the Direct Metal Deposition (DMD) process. The macroscopic FE calculation that is validated by thermocouple experiment is developed to simulate the transient temperature field and cooling rate of single layer and multiple layers. In order to integrate the different scales, a CA–FE coupled model is developed to combine with thermal history and simulate grain growth. In the mesoscopic CA model, heterogeneous nucleation sites, grain growth orientation and rate, epitaxial growth, re-melting of pre-existing grains, metal addition, grain competitive growth, and columnar to equiaxed phenomena are simulated. The CA model is able to show the entrapment of neighboring cells and the relationship between undercooling and the grain growth rate. The model predicts the grain size, and the morphological evolution during the solidification phase of the deposition process. The developed “decentered polygon” growth algorithm is appropriate for the non-uniform temperature field. Finally, the single and multiple-layer DMD experiment is conducted to validate the characteristics of grain features in the simulation.     

Reconstruction of the welding thermal cycle and determination of residual stresses from isotherm traces

The earlier method for reconstructing the welding thermal cycle from the dislocation of the temper colors and the cold weld joint boundaries was applied in the case of contact flash welding of rod samples. The possibility of using this method to determine the key parameters of the welding processes such as the approach speed of the welded rods, the temperature at the weld center at the beginning of cooling, and the time in which the characteristic temperature isotherm moves to the largest distance from the weld center, which allow one to reconstruct the temperature distribution curve near the weld at any time after the heating termination, was shown. The reconstructed thermal cycle was used to determine the residual stresses in the weld and in the heat-affected zone.     

Heat transfer and crystallization kinetics during fast cooling of thin polymer films

In this work, the heat transfer phenomena taking place during the cooling of thin films of crystallizable polymers were analyzed. The thermal histories, as recorded during experimental cooling runs carried out at various cooling rates, were compared with the predictions of a general purpose numerical code, which was resulted able to capture all the main features of the process. Thus, the conditions which allow homogeneous cooling (negligible temperature gradient within the sample) or homogeneous cooling history (the same cooling history for all the positions within the sample) were predicted by the simulation code.