基于振动疲劳试验的复合材料螺栓连接预紧力松弛特性

在强迫弯曲振动试验的基础上,建立了基于模态参数(共振频率和阻尼比)表征螺栓连接结构动态性能的分析方法和试验测试手段;通过施加不同初始预紧力和激振频率,探究碳纤维/环氧复合材料螺栓连接预紧力松弛的时变行为及其影响因素。结果表明:在10h振动疲劳过程中,螺栓初始预紧力越小,激振频率越大,连接件预紧力松弛程度越大;振动疲劳损伤会导致连接结构刚度衰退、阻尼增加;复合材料螺栓连接松弛受到材料黏弹性以及界面摩擦的共同影响,其中约50%的松弛是由复合材料黏弹性效应引起的。      

微动损伤对复合材料螺栓连接预紧力松弛的影响

开展了横向循环载荷下复合材料螺栓连接件预紧力松弛试验,探究了材料疲劳损伤与接触面微动磨损联合作用下螺栓连接预紧力松弛过程。基于有限元软件ABAQUS,采用Archard磨损模型和ALE（Arbitrary Lagrangian-Eulerian）自适应网格技术,编制了适用于连接结构微动磨损的UMESHMOTION子程序,建立了分析连接支承面微动磨损的计算模型;利用子程序UMAT编制了Shokrieh和Lessard提出的疲劳累积损伤定量分析程序。在此基础上分析了复合材料疲劳损伤、螺孔伸长及接触磨损的耦合作用下预紧力随循环周次变化的机制,与实验结果进行对比,验证了所提出的预测方法的合理性、有效性。      

复合材料螺栓连接松弛的弹-黏塑性分析方法

预紧力松弛是影响复合材料螺栓连接结构耐久性的主要原因之一。本文重点讨论导致预紧力松弛的材料蠕变与粗糙表面接触蠕变的相互影响。内容包括建立了以弹-黏塑性理论为基础的复合材料蠕变本构模型,并结合考虑粗糙表面的分形接触理论,将其推广到与时间相关的弹-黏塑性接触问题。数值结果与实验结果对比表明,考虑粗糙表面接触效应时,计算误差从2.87%~4.37%降至0.04%~0.5%,预测准确性有显著提高。表面分形参数D和G的讨论结果表明,接触表面越粗糙,预紧力越容易松弛。这对工程上通过控制表面形貌参数来改善表面接触性质具有指导意义。     

复合材料螺栓连接预紧力松弛的改进预测模型

该文研究了复合材料构件螺栓连接预紧力的长期性能预测。在作者前期工作的基础上,建立并完善了基于蠕变全应变理论的螺栓连接预紧力松弛预测模型。主要改进包括各蠕变阶段表达式转换为分段形式和修正时-温转换因子的作用方式两方面。采用时-温等效原理获得的长时加速表征实验数据对模型进行验证。结果表明,本模型能较好地描述预紧力长期特性,且预测效果比现有的Shivakumar-Crews模型及Hook-Norton模型更好,从而证明了该模型的准确性和可靠性。     

室温下C/SiC复合材料螺纹紧固件的拧紧特性

探讨了C/SiC陶瓷基复合材料螺纹紧固件在室温下的拧紧特性。试验测试了紧固件在拧紧和拧松过程中, 力矩与预紧力两者的对应关系, 记录了预紧力在短时、 长时内的减小比率, 并用显微镜观察了螺纹面的磨损情况。另外, 分析了材料非线性拉伸行为对拧紧状态所造成的影响。结果表明: 拧紧、 拧松力矩与预紧力之间近似呈线性关系, 螺纹面和支承面的平均摩擦系数分别为0.52和0.46; 随拧紧力矩增大, 螺纹面产生一定程度磨损, 螺纹之间的相互嵌入作用减弱, 因此预紧力在拧紧后的降幅减小, 稳定性提高; 对螺栓进行适当的预拉伸处理, 提高材料的弹性极限后, 可提高紧固件的抗松弛能力。     

氮化硅及其复合材料的蠕变

本文评述了氮化硅及其复合材料的蠕变行为；讨论了材料加工工艺，添加剂和增强组元对蠕变速率的影响，以及蠕变机理和蠕变寿命预测方法。     

温度及预紧载荷对氧化石墨烯无石棉垫片蠕变松弛和泄漏率的影响

垫片在高温长期使用过程中发生蠕变松弛导致垫片应力下降,从而影响密封效果。因此,采用理论模型预测垫片的蠕变松弛与确定垫片在高温环境长期使用过程中泄漏率的变化规律非常重要。通过实验测试氧化石墨烯无石棉垫片在不同温度和不同预紧载荷下的垫片应力值,并与根据Burgers模型计算出的垫片应力值进行对比。然后,通过建立泄漏率与使用时间的关系,提出氧化石墨烯无石棉垫片在不同温度和不同预紧载荷下长期使用时的泄漏率计算方法,并计算出该垫片随时间变化的泄漏率变化规律。结果表明：垫片应力的松弛量随着预紧载荷的增加而增加;随着使用时间的增加,垫片的应力松弛变得更加平缓;当预紧载荷相同的情况下温度越高垫片应力下降越明显;Burgers模型对垫片应力的预测值与实验值吻合度很好,最大误差为4.4728%;当温度一定时,氧化石墨烯无石棉垫片计算的泄漏率随着预紧载荷的增加而降低;当预紧载荷相同时,垫片的泄漏率随着温度的增加而增加,且泄漏率均小于密封等级T2的泄漏率2×10^-3 mg·s^-1·mm^-1。研究结果为氧化石墨烯无石棉垫片在高温环境长期使用过程密...     

碳纤维增强树脂基复合材料预紧力单齿接头的静态及疲劳性能试验

通过试验研究了预紧力、齿长和载荷水平对碳纤维增强树脂基复合材料（CFRP）预紧力单齿接头（PTSTC）静态和疲劳性能的影响。试验结果表明:CFRP PTSTC的疲劳极限承载力可以达到静态极限承载力的80%~85%,与螺栓等传统连接方式相比,其疲劳性能具有一定优势;预紧力可以显著改善CFRP单齿接头的静态和疲劳性能;CFRP PTSTC的静态极限承载力随齿长增加而升高,但是在相同载荷水平下增加齿长不一定可以延长接头的疲劳寿命,尤其是在低载荷水平下齿长增加反而会缩短疲劳寿命;在加载初期,PTSTC的疲劳裂纹快速萌生,之后扩展缓慢,在接近破坏前的几次循环中又骤然增大,预紧力可以减缓疲劳损伤的累积速率;疲劳过程中PTSTC的刚度衰退不明显,在前95%疲劳寿命阶段仅下降1%~4%,预紧力也可以减缓刚度的衰退速率。所得研究成果可为复合材料接头抗疲劳设计提供参考依据。      

多壁碳纳米管流变逾渗网络对聚苯乙烯复合材料应力松弛和蠕变的影响

利用流变学方法探讨了多壁碳纳米管(MWCNTs)对聚苯乙烯(PS)/MWCNTs复合材料粘弹行为的影响,根据逾渗理论求出MWCNTs在PS中形成流变逾渗网络的逾渗值,并探讨了该网络的形成对复合材料应力松弛和蠕变的影响。结果表明,MWCNTs能抑制复合材料的应力松弛,增强复合材料抗蠕变能力;当MWCNTs在复合材料中形成流变逾渗网络后,复合材料的应力松弛被进一步地抑制,抗蠕变柔性的能力显著增强。     

木质素磺酸钙/高密度聚乙烯复合材料的力学性能

采用木质素磺酸钙（CL）填充高密度聚乙烯（HDPE）制备CL/HDPE复合材料,利用SEM、DSC、XRD对CL/HDPE复合材料进行表征,并对其强度、蠕变行为及应力松弛等力学性能进行测试。结果表明,CL/HDPE复合材料具有良好的结合界面和热稳定性;CL的加入可以提高CL/HDPE复合材料的弯曲强度,但对其冲击强度会产生不利影响;CL含量的增加有利于提高CL/HDPE复合材料的抗蠕变性能和抗应力松弛能力,而温度的升高会对CL/HDPE复合材料的蠕变行为和应力松弛产生不利影响。      

Mechanics of mechanically fastened joints in polymer–matrix composite structures – A review

As the applications of advanced composite structural materials continue to increase, so does the need to understand the mechanical behavior of mechanically fastened joints in such structures. The most recent and relevant review article on this subject was published more than a decade ago, but it was restricted to stress analysis and strength prediction of mechanically fastened joints in fiber-reinforced plastics. The present article attempts a more comprehensive review of recent literature in the broader area of mechanics of mechanically fastened joints in polymer–matrix composite structures. Since experimental characterization has traditionally played such a fundamental role in such studies, the article begins with a review of relevant mechanical test methods and standards. This is followed by a discussion of the mechanics aspects of design, including joint design methodologies, considerations of the influence of geometric effects, and fastener preload selection. The remaining sections are devoted to failure modes such as bearing failure, failure prediction for both statically and dynamically loaded joints, time-dependent joint preload relaxation, the effects of temperature and moisture on joint strength and failure, and non-destructive evaluation techniques for monitoring the joints. Finally, comments are offered regarding the most important remaining problems in this area, and recommendations for future work.     

Study on creep characterization of nano-sized Ag particle-reinforced Sn–Pb composte solder joints

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the experimental results, the constitutive model on the steady-state creep strain is established by applying a linear curve fitting for the nano-sized Ag particle-reinforced Sn37Pb based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Ag particle-reinforced Sn37Pb based composite solder joints is higher than that of Sn37Pb solder joints. It is expected that the creep resistance of the Ag particle-reinforced Sn37Pb based composite solder joints is superior to that of Sn37Pb solder.     

Prediction of viscoelastic material functions from constant stress- or strain-rate experiments

To predict durability of polymeric structures an information on polymer’s long-term properties in the form of relaxation modulus and/or creep compliance is required. It is well known that determination of relaxation or creep properties from experimental data is an inverse problem, which, due to presence of experimental errors in input data, becomes ill-posed. To find a stable solution using standard integration schemes is practically impossible. In this paper we propose a “hands-on” methodology which bypasses the solution of ill-posed integral equation and allows finding long-term relaxation or creep properties from simple constant strain rate or constant stress-rate experiments performed at different temperatures. The proposed approach can be applied not only for characterization of viscoelastic materials in solid state but can also be used for prediction of time-dependent properties of polymer melts. The paper presents the detailed steps of the proposed method as well as its validation on several simulated and real experimental data. It has been shown that the proposed approach can accurately reconstruct the desired long-term time-dependent properties obtained in traditional way (i.e., from step loading).     

基于应力松弛实验对服役25Cr35Ni型耐热钢的高温性能评估

以服役的25Cr35Ni型钢为对象,研究利用应力松弛实验开展高温性能评估的方法以及对持久性能的预测效果。结果表明:由于高温服役后晶界处碳化物出现网链状和奥氏体基体内二次碳化物明显粗化,25Cr35Ni型耐热钢持久性能降低。通过得到的不同温度和应力的松弛蠕变速率曲线及外推关系,结合松弛蠕变速率-断裂时间关系方程,可以实现由松弛实验及少量蠕变持久实验开展持久寿命评估。与基于高温持久实验的预测结果比较,两者吻合较好。     

Creep property of composite solders reinforced by nano-sized particles

In the present work the creep properties of Sn37Pb and Sn0.7Cu based composite solders with nano-sized metallic Cu, Ag and nano-sized oxide Al₂O₃, TiO₂ reinforcement particles have been studied. First, a series of volume percentages of reinforcements were selected for optimizing the content of particles. Then, the composite solder with optimum volume fraction of the reinforcement particles, corresponding to maximum creep rupture life, is selected for investigating the effect of applied stress level and test temperature on creep rupture life of the composite solder joints. In the creep rupture life test, small single-lap tensile-shear joints were adopted. The results indicate that all the composite solders have improved creep resistance, comparing to the eutectic Sn37Pb solder and the Sn0.7Cu lead-free solder. The creep rupture life of the composite solder joints is first increased with the increase in the volume fraction of reinforcement in the composite solders. Then, the creep rupture life is decreased, as the reinforcement content exceeds a certain value. The creep rupture life of the solder joints is decreased with the increase of applied stress and testing temperature. Moreover, the reinforced efficiency of nano-sized Ag particles is the best in all the tested nano-sized reinforcements for the Sn37Pb based and Sn0.7Cu based composite solders, when the particles contents are in their own optimum content.     

Modification of Sn58Bi solder based on graphite

Abstract

A novel composite solder paste with 0·07 wt-% graphite addition was fabricated. The mechanical properties of matrix solder and composite solder alloys were tested using a kind of miniature joint. The results showed that the tensile strength of the Sn58Bi+graphite composite solder joint declined slightly compared with the Sn58Bi solder joint. However, the elongation of the Sn58Bi+graphite composite solder joint was ～13 times as much as that of the Sn58Bi solder joint. Stress relaxation method, which was used to test the creep performance, was verified by nanoindentation method. The result showed that the stress relaxation method was suitable for creep testing. The creep testing results of Sn58Bi and Sn58Bi+0·07 wt-% graphite solder alloys showed that the anticreep performance of Sn58Bi was improved by the addition of graphite.