超声波塑料焊接机理

采用数值仿真和试验研究了超声波塑料焊接过程中不同特征温度段的产热机理.利用有限元法(FEM)对聚甲基丙烯酸甲酯(PMMA)材料超声波焊接过程中的粘弹性热以及摩擦热进行了计算.基于计算结果,提出了摩擦热是焊接过程的启动热源,粘弹热是焊接过程主要热源的观点.制备了相应的试件并搭建测温系统对焊接过程进行测温试验,试验结果验证了仿真结果的正确性.对焊接过程中的产热机理给出了更清晰的解释,有助于超声波塑料焊接技术进一步在精密焊接领域的应用.     

热塑性塑料超声波焊接的热力过程数值模拟

在建立热塑性塑料的粘弹性力学模型和传热模型的基础上,根据应力和粘度模型计算粘弹性热能可以准确地计算粘弹性热。通过开发超声波塑料焊接的有限元数值模拟程序研究了焊接瞬间的热力行为,数值模拟结果表明,平板件焊接时受压区域整体温度有不同程度的升高,焊接首先发生在焊接面的外围区域,平板件焊接实验验证了数值模拟的准确性。数值算例还研究了初始温度、焊接压力、超声波频率和超声波振幅等焊接工艺参数对焊接过程的影响,有效的焊接必须高于临界初始温度,提高超声波频率和振幅可显著提高温度升高的速度与幅度。     

高填充因子微透镜阵列热压印保真度研究

介绍了压印聚合物材料在不同温度下的力学状态,运用粘弹性材料模型解释了压印材料PMMA在玻璃化温度附近的力学性质及PMMA冷却到玻璃化温度以下时的材料模型。在热压印过程中,PMMA的应力发生了复杂的变化。压印产生的应力在保压阶段不断松弛,随着温度的降低不仅会产生热应力,同时应力松弛时间也会变长,脱模时候的应力状态是由压印、冷却以及材料多种因素决定的。通过粘弹性材料模型分析热压印过程中PMMA的应力变化,运用响应面法拟合应力与工艺参数的数学模型,分析温度对PMMA应力的影响。     

超声波塑料焊接临界温度场分析

采用多场耦合模型对超声波塑料焊接过程中的摩擦热和粘弹热进行了数值计算,并分析了超声振幅对焊接区温度场的影响. 基于计算结果,提出了超声波焊接过程存在临界振幅,当超声振幅低于该临界振幅时,不管焊接时间多长,都不能形成熔融焊接区. 制备了相应的试件并搭建测温系统对焊接过程进行测温试验,试验结果与数值计算结果具有较高一致性,进一步对临界振幅附近的临界温度场进行了分析和探讨. 研究临界温度场,有助于解决超声波塑料焊接过程中温度场难以控制的难题,同时为超声波塑料器件非熔融连接方法奠定基础.     

GMAW焊接X5CrNi1810不锈钢温度场和焊接变形的有限元预测

基于焊接热弹塑性有限元分析理论,采用焊接专用有限元分析软件对X5CrNi1810不锈钢口字型多焊缝焊接结构GMAW焊接过程进行数值仿真.计算时,考虑材料热物理、力学性能随温度的变化及相变潜热对温度的影响.对仿真结果进行了理论分析,预测了温度场和焊接变形的分布.     

6061铝合金超声波点焊温度场数值模拟及试验

针对超声波焊接过程中温度演化过程监测存在的困难,考虑焊接过程中塑性变形产热和高频摩擦产热,建立了三维超声波焊接热-结构耦合Ansys有限元模型,模拟了6061铝合金超声波金属焊接过程,计算了不同焊接参数下的温度场,用细丝热电偶测温试验验证了焊接温度.结果表明,焊接过程中焊接区域最高温度模拟值与试验值误差在5%以内,表明了模型的准确性;温度最高处位于焊接区域中心位置,高温区随焊接时间的增大而增大;超声波金属焊接过程中,温度场主要受焊接压力及焊接时间的影响.     

9Ni钢焊接温度场的数值模拟

采用SYSWELD有限元计算软件,时9Ni钢多道多层焊的焊接温度场进行了数值模拟,并实现了焊接材料逐步填充的模拟,计算结果与试验结果非常一致.利用一次回归正交试验,研究焊接热输入和层间温度对焊接热影响区的热循环的影响.结果表明,焊后冷却时间t8/3和高温停留时间tH随着焊接热输入和层间温度的增加而增大.焊接热输入对t8/3和tH的影响较为显著,根据焊接生产实际条件,应采用小的焊接热输入.     

铝合金／钛合金超声波点焊热-机耦合分析

超声波焊接技术是一种通过施加压力和高频剪切运动来实现材料固态连接的方法。利用超声波焊接技术进行金属间的焊接能在低于金属熔点的温度下实现良好的冶金连接。因此可以应用于冶金性能差异较大的异种金属的焊接。本文针对超声波焊接过程中温度瞬间变化的特点，利用超声波焊接界面反应机理的研究结果，建立了超声波焊接铝钛异种合金的二维有限元模型。通过有限元数值模拟的方法得到了超声波焊接过程中瞬态温度和应力分布。为超声波焊接工艺的优化和制定提供了理论依据。     

小尺寸环形焊缝对接焊管焊接变形控制研究

针对环形焊缝焊管接头焊接时焊接局部升温快,焊接变形高,残余应力高等问题,采用有限元法对焊接过程的变形和残余应力研究.分析常用结构尺寸、材料、焊接温度下焊管热变形及残余应力分布情况;基于热变形计算不同尺寸下焊管预弯曲范围,降低焊接过程中的变形.同时对数值仿真模型进行试验对比验证.分析发现,焊缝两侧焊管的残余应力和变形分布...     

塑料压力管道热板焊接接头瞬时应力有限元分析

基于热粘弹性积分型本构关系,考虑材料性能依赖于温度变化及相变潜热的影响,利用有限元软件ANSYS热-力耦合及载荷步功能模拟结晶型高密度聚乙烯塑料压力管道热板焊接过程.并对焊接接头的应力分布进行有限元分析,得到了环向、轴向及径向瞬态应力分布规律.采用盲孔法和锯切法测量焊后残余应力,实测结果与数值分析基本吻合.     

Relaxation Behavior of Acrylic Latex Polymer Modified by Water-Soluble Dye. Freezing–Thawing Mode

The relaxation behavior of a styrene–acrylic latex polymer with a glass-transition temperature of ≤5°C and polymer composites on its basis modified by an indanthrene-type water-soluble dye was analyzed using the method of dynamic mechanical relaxation spectroscopy. The change in the intensity of maximum of dissipative losses of α-relaxation and also of the μ-process is found in polymer films in the negative-temperature range as dependent on the duration and number of their preliminary freezing–thawing cycles at?30°C taking into account the modifier concentration, which is related to the change in Young’s modulus (modulus defect). Theoretical analysis is performed involving phenomenological model concepts as to the relationship between the vibration-process frequency and Young’s modulus of the polymer material.     

Influence of heat treatment on the internal friction of 2D-C/SiC composites

The internal friction of two-dimensional C/SiC composites (2D-C/SiC) was investigated in air by means of three-point bending forced vibration, compared with the specimens after heat treatment. The result shows that the effects of PyC interface between carbon fibers and SiC matrix play an important role on the internal friction characters of 2D-C/SiC composites, which leads to some unique internal friction phenomena in 2D-C/SiC composites. After the heat treatment, the PyC interface produces damage that make internal friction increased and dynamic modulus decreased. The heat treatment makes the internal friction of 2D-C/SiC composites more insensitive to temperature and frequency, and more sensitive to amplitude.     

A new method of virtual material hypothesis-based dynamic modeling on fixed joint interface in machine tools

The dynamic characteristics of joint interfaces affect the dynamic behaviors of a whole machine tool structure notably. An analytic method of virtual material hypothesis-based dynamic modeling on fixed joint interface in machine tools was conducted so as to improve the modeling accuracy of whole machine tools. The microcontact part of two contact surfaces in fixed joint interface was assumed as a virtual isotropic material, which is rigidly connected with two components composing fixed joint interface. The interaction between normal and tangential characteristics of fixed interface was taken into account, a set of analytic solutions of elastic modulus, shear modulus, Poisson ratio and density was deduced from virtual material by adopting Hertz contact theory and fractal theory. Using the finite element structural dynamic modeling approach in existence, when some parameters of material’s elastic modulus, Poisson ratio, density, etc. are known, the dynamic model for virtual material composing a component could be established; therefore, the dynamic model for whole structure, including joint interface, would be obtained. The theoretical mode shapes were compared with the experimental ones (qualitative comparison of similar mode shape and quantitative comparison of the corresponding natural frequency). The comparison results show that the theoretical mode shapes are in excellent agreement with the experimental ones. The relative errors between the theoretical natural frequencies and the experimental ones are less than 9%. The good agreements of theoretical results with experimental ones confirm analytic solutions for virtual material’s parameters. The present solutions would be useful to the precise dynamic modeling of fixed joint interfaces in CNC machine tools in practice.     

飞机金属结构防护层在多因素耦合作用下损伤行为与失效预测模型

模拟飞机中央翼、平尾大轴等部位的基体及其防护层，在紫外、热冲击、低温疲劳、盐雾多因素耦合作用下开展试验，对不同试样的损伤形貌和电化学参量随着试验周期的演变规律进行统计分析。结果表明:损伤主要为颜色变化和鼓泡，损伤形貌的变化与低频下阻抗模值存在一定的对应关系；给出不同失效阶段低频电化学阻抗模值范围以及对应的失效形貌。对国内外防护层失效预测模型进行调研分析，防护层失效预测模型主要有阻抗模值数据处理法、老化动力学模型。分析现有模型存在的局限性，在此基础上，对老化动力学模型进行改进，提出全频率老化动力学模型，并新建立神经元网络失效预测模型。利用铝合金、钢及其防护层在不同实验条件下的实验结果对各模型的预测结果进行比对分析。     

半连续铸造铝合金圆锭冷却过程传热研究

通过试验法测定了铸造过程中φ100 mm的铝合金铸锭近表面的动态温度,采用逆向法计算出其水冷段的换热系数.结果表明,随着铸锭表面温度的降低,换热系数逐渐增大;在温度由400℃降至130℃的过程中,换热系数急剧增大,在130℃左右时达到最大,其最大值大约为23000 W/(m·K);当温度继续降低时,铸锭表面换热系数迅速减小.最后分析了换热系数变化的原因.     

连续运动薄板横向磁通感应加热耦合场分析新方法

通过沿薄板运动反方向以单元格的形式平移薄板中的热源来取代薄板的连续运动，使得温度场的每次计算可以采用相同的热源分布，无需重新计算涡流场，大大减少了横向磁通感应加热耦合场数值仿真的时间。利用有限元数值计算结果作为神经网络的训练样本，分别建立了横向磁通感应加热装置出口处平均温度及其平均相对误差的预测模型。检验样本的结果表明所训练的神经网络具有很高的准确性。利用神经网络分析了频率和电流对平均温度及其平均相对误差的影响，并应用模拟退火法对频率和电流进行了优化以获得给定温度下的最小平均相对误差。     

超声波检测混凝土结构抗冻性的方法研究

依据固体结构动弹模与表面波速度的关系，采用超声波检测混凝土冻融试件的动弹模，并与共振法测出的动弹模进行对比，总结出其内在相关规律，建立起预报混凝土动弹模的回归主程，由测得的波速计算出相应的动弹模，再利用动弹模之比评估出混凝土结构的抗冻性。初步试验和研究表明，此方法研究思路切实可行，符合混凝土冻融特性的基本变化规律，能够实现从度件测试向结构检测的发展，可供混凝土耐久性检测工程参考。     

Cr-Mo钢管子局部焊后热处理加热宽度准则的确定

采用粘弹塑性有限元方法从应力释放角度讨论了Cr-Mo钢局部热处理下各种优化加热条件.研究表明,局部热处理过程产生的瞬态和残余应力受蠕变性能和温度关系弹性模量等因素的影响.当加热宽度足够大时,局部热处理后的应力与炉中热处理的应力接近.通过与现有标准的比较,从应力释放角度位于焊缝中心5√Rt的加热宽度较为合理.提出了考虑焊接冷却过程Cr-Mo低温膨胀现象的相变模型.给出了焊接残余应力与局部焊后热处理应力的试验结果与模拟结果的对比.     

Microwave processing of adhesive joints using a temperature controlled feedback system

Previous experiments have shown that microwave curing of epoxies reduces the curing time and could lead to improvements in the mechanical properties of the cured epoxies. Precise temperature control is important as inadequate control may lead to thermal “run-away”, resulting in localized material damage. Consequently, the main objective of the present work is to use temperature for controlling the intensity of microwave radiation during the adhesive curing process. Using microwave energy delivered through a slotted waveguide at a frequency of 2.45 GHz, and with the power varied between 300 and 600 W, three engineering thermoplastics or adherends were joined by curing two types of “two-pack” epoxy adhesives. Subsequently, the bond strengths of the microwave cured samples were compared to those of the specimens cured in ambient conditions by subjecting the joined samples to shear loading. The results obtained indicate that the samples cured with microwave under a temperature feedback control protocol achieved higher bond strengths after curing for only a fraction of the time required for ambient curing. It can therefore be inferred that, with an adequate temperature control strategy, increased efficiency of curing, higher quality of adhesive joint, and improved bond strength can be achieved when joining thermoplastics.