基于神经网络的选区激光熔化残余应力预测

当前对选区激光熔化产生的残余应力预测方法主要为数值模拟,但由于设备、环境、粉末等因素差异性较大,且具有较大不确定性,很难建立符合实际情况的数值模拟模型。利用神经网络在预测多变量、复杂线性信息处理方面能力强的特点,建立适用于预测316L不锈钢粉末选区激光熔化残余应力的模型。使用选区激光熔化技术打印相当数量的不同工艺参数的试样,采用超声波检测其内部残余应力作为神经网络的训练样本,并使用这些样本对神经网络模型进行训练,获得具有预测功能的神经网络,将验证样本的工艺参数输入神经网络,计算出预测的残余应力值,与实际检测值进行对比。实验结果表明,预测值与实际测量值偏差较小,验证了所提方法的有效性。采用神经网络预测残余应力的方法,可以快速确定不同选区激光熔化工艺参数对应的残余应力,避免设置残余应力较高的工艺参数,有效缩短制备高质量工件试样的周期,降低成本。     

基于ANSYS的T型焊接接头温度场数值模拟

准确计算瞬时焊接温度场是控制焊接应力及变形的先决条件。本文采用高斯热源模型,基于ANSYS分析软件,通过参数化语言APDL对T型焊接接头温度场进行数值模拟。结果表明,所得焊接温度场的变化过程及分布规律符合焊接工艺实际,这为控制焊接残余应力及变形,进一步优化焊接结构及焊接顺序提供了基础。     

未来聚变工程实验堆真空室壳体成型模拟与实验研究

参照ITER真空室成型工艺,对未来聚变堆真空室壳体各分片进行成型数值模拟,探索成型温度、脱模温度对成型工件减薄量、回弹量和残余应力的影响。并通过成型实验对数值模拟结果进行了检验,二者结果一致,所用数值模型可用于后续模具设计。     

激光冲击7075-T6铝合金焊缝的残余应力场数值模拟

针对激光冲击强化铝合金焊缝过程中存在多场耦合导致残余应力变化幅度大、实时表征难的问题,采用ABAQUS非线性有限元软件建立了激光冲击强化7075-T6铝合金板材焊接件的有限元模型,模拟计算焊缝在激光冲击强化前后的残余应力场分布,重点分析了激光冲击强化对焊缝的横、纵向残余应力场的影响规律,并对焊缝的激光冲击强化工艺参数进行优化。研究结果表明,焊缝表面为残余拉应力场并且分布不均,激光冲击强化使得焊缝的拉应力状态转变为高幅压应力状态;优化激光冲击强化工艺参数(如激光能量、激光光斑尺寸、搭接率),可以明显改善焊缝区域和热影响区的残余应力分布;当激光光斑搭接率为50%至70%时,残余应力分布趋于均匀,可以有效消除"残余应力洞"现象。     

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参照ITER真空室成型工艺,对未来聚变堆真空室壳体各分片进行成型数值模拟,探索成型温度、脱模温度对成型工件减薄量、回弹量和残余应力的影响.并通过成型实验对数值模拟结果进行了检验,二者结果一致,所用数值模型可用于后续模具设计.     

聚变实验堆真空室壳体成型残余应力分析

为了了解聚变实验堆真空室壳体表面残余应力的分布以及退火工艺对残余应力的影响,通过模拟分析和实验检测两种方式对不锈钢316LN冷压曲面和热压曲面残余应力进行研究,获得退火前后曲面表面残余应力的大小,得到冷压曲面和热压曲面残余应力的分布以及退火工艺对残余应力分布的影响。研究结果为分析成型工艺提供数据支撑,对中国聚变工程实验堆真空室的研究与制造具有重要意义。     

聚变实验堆真空室壳体成型残余应力分析

为了了解聚变实验堆真空室壳体表面残余应力的分布以及退火工艺对残余应力的影响，通过模拟分析和实验检测两种方式对不锈钢316LN冷压曲面和热压曲面残余应力进行研究，获得退火前后曲面表面残余应力的大小，得到冷压曲面和热压曲面残余应力的分布以及退火工艺对残余应力分布的影响。研究结果为分析成型工艺提供数据支撑，对中国聚变工程实验堆真空室的研究与制造具有重要意义。     

Nb_3Sn超导股线残余应力分析

Nb3Sn超导股线在900K形成,而工作环境是在4.2K,从高温到低温会在Nb3Sn超导股线内部产生残余应力与应变,Nb3Sn超导股线对应力应变十分敏感,过大的应力会使股线的性能严重退化。通过理论计算和数值模拟的方法来计算Nb3Sn超导股线的残余应力与应变。     

利用X射线衍射研究聚晶金刚石复合片的残余应力(英文)

通过x射线衍射方法研究了聚晶金刚石复合片中金刚石层内的宏观残余应力以及微观残余应力.宏观残余应力通过测量x射线衍射的峰值移动可以计算出来,微观残余应力由x射线衍射的峰形宽化决定.本文研究结果表明:我们研究的三种型号聚晶金刚石复合片中存在着宏观残余压应力和微观残余张应力,并且发现宏观残余压应力和微观残余张应力的大小随着金刚石平均粒径的增大而减小,根据x射线衍射研究表明这可能是由于随着金刚石粒径的增大,聚晶金刚石复合片中金刚石相的减少而非金刚石相的增加造成的.x射线衍射技术可以非破坏性的测量宏观和微观残余应力的大小,因此,这是一种很好的检测聚晶金刚石复合片中残余应力的方法,并可以通过应力的大小来优化聚晶金刚石复合片的最佳组分和工艺,同时也可以给出聚晶金刚石复合片的最佳工作条件.     

SQUID结合涡旋电流法用于铍材残余应力检测的可行性

涡旋电流法是材料无损检测常用方法之一，近年国际上多个研究小组对用于金属材料内残余应力检测开展了研究，涡旋电流法被认为是无损检测确定材料内残余应力的潜在方案。文中从实验和电磁场数值模拟两方面对此问题开展研究。     

Relation between residual stresses and microstructure evolution in Al–Si alloys based on different casting parameters

Most designers take into consideration the stresses that act on a material but despite safety considerations, failure may occur due to other factors that were neglected in their design. These factors can be a pre-existing flaw, microstructure deficiency or the presence of residual stresses. Depending on the stress type, residual stresses combined with applied stresses can aid or hinder failures. Consequently, reducing the amount of residual stress can have a promising effect on the life of the component. Different casting parameters can change the microstructure and residual stresses of castings. In this research, the relation between residual stresses and microstructure evolution under the influence of different casting parameters was investigated, using both Al–Si–Mg (Al-356) and Al–Si–Mg–Cu (Al-319) alloys. Solidification rate, quenching rate, aging temperature and aging time were the main parameters considered for this study. The results indicate that the magnitude of the residual stresses increases with increasing solidification rate and quenching rate. Also, the residual stress relieving is proportional to the aging temperature     

Drawing parameters optimization for birefringence reduction in optical fibers

The impact of the drawing process on the stress-induced birefringence in optical fiber is experimentally analyzed. By means of the photoelastic tomographic technique, residual internal stresses are measured in fiber samples drawn under different conditions of temperature, drawing tension and speed. From measured residual stress distributions, the stress-induced birefringence and beatlength are mathematically derived and compared, proving that an effective birefringence reduction can be obtained by a proper choice of drawing parameters.     

Residual stress of physical vapor-deposited polycrystalline multilayers

An extended one-dimensional stress model for the deposition of multilayer films is built based on the existing stress model by considering the influence of deposition conditions. Both thermal stress and intrinsic stress are considered to constitute the final residual stress in the model. The deposition process conditions such as deposition temperature, oxygen pressure, and film growth rate are correlated to the full stress model to analyze the final residual stress distribution, and thus the deformation of the deposited multilayer system under different process conditions. Also, the model is numerically realized with in-house built code. A deposition of Ag-Cu multilayer system is simulated with the as-built extended stress model, and the final residual stresses under different deposition conditions are discussed with part of the results compared with experiment from other literature.     

On the delamination and crack formation in a thin wall fabricated using laser solid freeform fabrication process: An experimental–numerical investigation

Parts fabricated using laser solid freeform fabrication (LSFF) are subject to thermal stresses due to the layer-by-layer material deposition and the temperature distribution characteristic throughout the process domain. The thermal stress patterns and intensity contribute significantly to potential delamination and crack formation. In this paper, the temperature distribution and stress field induced during the multilayer LSFF process, and their correlation with delamination and crack formation are studied. This is performed by a numerical and experimental investigation in the fabrication of a thin wall of 304L stainless steel. For time-dependent predictions on the locations of maximum temperatures and thermal stresses and their patterns, a three-dimensional (3D) transient finite element model is employed to simulate the process, including the geometry of the deposited materials as well as coupled temperature and stress distributions across the process domain. The experimental results are used to verify the numerical results as well as to investigate the correlation between the numerical results and micro-crack formations across the fabricated parts. The experiments are conducted with the same process parameters used in the numerical analyses using a 1 kW Nd:YAG pulsed laser. The trend of numerical and experimental results reveals that by preheating the substrate prior to the fabrication process, it is possible to substantially reduce the micro-cracks formed across the part. To demonstrate the feasibility of preheating on the reduction of micro-cracks, several simulations and experiments are performed in which a crack-free result is obtained when the substrate is preheated to 800 K. For this case, 22% reduction in thermal stresses is obtained throughout the process domain.     

Monitoring of residual stresses in injection-molded plastics with holographic interferometry

Residual stresses are often trapped in injection-molded plastic parts due to the rapid cooling of the material in this manufacturing process. These stresses are a common source of failure in plastic components in automobiles, appliances and computers and are difficult to measure with conventional residual-stress experimental methods. Real-time holographic interferometry appears to be a viable technique to identify and monitor these stresses in plastic parts. In this investigation, holographic interferometry was used to monitor the relaxation of residual stresses in the plastic-molded actuator arm of a computer hard drive. In the first phase of this study, the relaxation of these residual stresses as a function of temperature was observed. In the second phase, the time to completely relax the residual stresses in the plastic part at an elevated temperature, the annealing temperature, was determined. In the third phase of this investigation, the rate of relaxation of these residual stresses as a function of time at various operating temperatures, was studied. Based on the results of this study, holographic interferometry appears to be a powerful research tool in the study of residual stresses in plastic parts. It also has the potential to be a practical tool for the inspection of manufactured plastic parts for the presence of residual stress.     

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对Nb3Sn超导线低温下残余应力进行了分析。通过分析复合材料产生热应力的机理,给出弹性假设下计算残余应变的公式。用有限元分析计算了超导线制造到降温过程中各组分的应力变化情况,得到Nb3Sn超导丝在低温下的残余应变,模拟了不锈钢管对超导线残余应力的影响。     

Laser cutting of holes in thick sheet metals: Development of stress field

Laser cutting of hole in a mild steel thick sheet metal is investigated. Temperature and stress fields developed around the cutting section are simulated using the finite element method. An experimental is carried out accommodating the simulation parameters. The residual stress developed in the cutting section is measured using the XRD technique and findings are compared with the predictions. Optical microscopy and SEM are carried out to examine the morphological changes in the cutting sections. It is found that temperature decays sharply in the region of the laser heat source, which results in high temperature gradient in this region. This causes the development of high stress levels around the cut edges. The residual stresses predicted are in agreement with the measured results.     

Orientation dependent size effects in thermal buckling and post-buckling of nanoplates with cubic anisotropy

In this work, a continuum model is presented for size and orientation dependent thermal buckling and post-buckling of anisotropic nanoplates considering surface and bulk residual stresses. The model with von-Karman nonlinear strains and material cubic anisotropy of single crystals contains two parameters that reflect the orientation effects. Using Ritz method, closed form solutions are given for buckling temperature and post-buckling deflections. Regarding self-instability states of nanoplates and their recovering at higher temperatures, an experiment is discussed based on low pressurized membranes to verify the predictions. For simply supported nanoplates, the size effects are lowest when they are aligned in [100] direction. When the edges get clamped, the orientation dependence is ignorable and the behavior becomes symmetric about [510] axis. The surface residual stress makes drastic increase in buckling temperature of thinner nanoplates for which a minimum thickness is pointed to stay far from material softening at higher temperatures. Deflection of [100]-oriented buckled nanoplates is higher than [110] ones but this reverses at higher temperatures. The results for long nanoplates show that the buckling mode numbers are changed by orientation which is verified by FEM.     

Effects of processing parameters on fatigue properties of LY2 Al alloy subjected to laser shock processing

We address the effects of processing parameters on residual stresses and fatigue properties of LY2 Al alloy by laser shock processing (LSP). Results show that compressive residual stresses are generated near the surface of samples due to LSP. The maximum compressive residual stress at the surface by two LSP impacts on one side is higher than that by one LSP impact. The maximum value of tensile residual stress is found at the mid-plane of samples subjected to two-sided LSP. Compared with fatigue lives of samples treated by single-sided LSP, lives of those treated by two-sided LSP are lower. However, these are higher than untreated ones.     

Combined implementing the hole-drilling method and reflection hologram interferometry for residual stresses determination in cylindrical shells and tubes

Main features inherent in simplified approach to residual stresses determination in cylindrical shells and tubes, external diameter of which is not less than 60 mm, by combing the hole-drilling method and reflection hologram interferometry are discussed in detail. Initial experimental information in a form of hole diameter increments in principal stress directions is derived from high-quality reflection holograms recorded near cylindrical objects of intermediate curvature value. Converting measured parameters into required stress values is based on the transition model that corresponds to plane stress conditions of pure membrane type. The technique developed is capable of determining residual stress component values within 5% accuracy in an absence of stress gradients over the probe hole diameter when a type of residual stress field corresponds to the transition model adopted. The accuracy analysis involved is based on matrix formulation of conventionally direct problem and an assumption on a pure membrane character of residual stress field under study for thin-walled shell. Required error estimations in a case of inspecting thick-walled cylindrical tube are obtained by combining the above-mentioned approach and an analogy of reconstructed fringe patterns with actual and artificial interferograms, which follow from drilling blind hole of the same geometrical parameters in thick-walled plates. Experimental verification of the developed approach is founded upon a determination of actual stresses in thin-walled cylindrical shell and obtaining residual stress distributions at the proximity of welded joint in thick-walled cylindrical tube.