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金属/金属粘结体系的强韧和失效机制研究
英文题名Study on strength, toughness and failure mechanism of the metal/metal bonding system
李景传
导师魏悦广
2018-05-20
学位授予单位中国科学院大学
学位授予地点北京
学位类别博士
学位专业固体力学
关键词金属粘结 胶层厚度 斜接角度 失效强度 界面能
英文摘要

粘结比传统的连接方法有众多的优点,被广泛应用于飞机、运载火箭、卫星和导弹等重要领域中。粘结能够实现高强度的结合作用,有研究表明,胶粘剂在粘结体系中的强度能达到本身拉伸或剪切强度的5倍之高。对粘结体系强韧和失效机制的研究,包括尺度效应的实验观测及表征,既是学术前沿,又是航空航天等高科技领域的重要需求问题。本文以金属/金属斜接粘结体系为研究对象,主要针对体系的强度和韧性、界面失效机制、失效强度和界面断裂能的尺度效应、胶层中的应力分布等问题开展了系统的实验及数值模拟和理论模型研究。主要研究工作和取得的成果如下:

(1)本文在前人工作的基础上,为了消除矩形截面试样棱角应力奇异性效应,特别设计了铝合金圆棒搭配两种不同种类胶粘剂的试样,通过各种不同斜接角度和胶层厚度的组合,系统地进行了实验研究。研究发现硅橡胶试样主要表现出韧性破坏的特征,环氧树脂试样主要表现出脆性破坏的特征。薄胶层对应的接头失效载荷更高,厚胶层对应的接头失效载荷更低,失效载荷对于胶层厚度具有明显的尺度效应。同样胶层厚度的试样单位面积失效荷载近似相等。硅橡胶试样都发生胶内破坏,由剪切失稳引起的波纹状结构均匀地分布于整个粘结区域,环氧树脂试样主要发生混合破坏。

(2)为了对圆棒斜接试样进行力学机制的分解,我们认为斜接接头受轴向拉伸荷载作用时, 胶层内的应力状态可近似被看作是垂直于粘结界面的单向拉伸和平行于界面的简单剪切的复合。通过引入平均应力和平均应变等概念,得到强度及失效面。对于给定的胶层厚度,不同粘结界面角度试样对应的平均失效应力近似在同一圆弧上,其半径随着胶层厚度增加而减小。这为金属/胶层粘结体系的强度预测提供了可方便应用的强度破坏准则。硅橡胶或环氧树脂斜接接头的强度是其对应胶粘剂标准试样最大拉伸或剪切强度的2~5倍,由于受金属被粘物的约束作用,胶层在铝合金之间发挥出比本身更强的承载能力。粘结界面的断裂能和体系的能量释放率都随着胶层厚度的增加而增加,随着粘结界面角度的增加而增加。失效强度和界面断裂能当胶层厚度在百微米量级时,表现出强烈的尺度效应。                  

(3)为了验证圆截面试样选取的合理性以及观察胶层中的应力分布,我们用商业有限元软件对对接接头和斜接接头进行了有限元模拟。用轴对称单元模拟的对接接头结果和用六面体单元模拟的斜接接头结果之间自成规律性,并且和实验结果吻合。模拟发现对接接头在界面边缘处有应力集中,胶层对称面外表面和胶层中间区域Mises应力值最小。发现在胶层外表面界面附近有应力的跳跃,应力跳跃的影响范围非常小(约为试样直径的1%)。斜接接头容易在粘结椭圆面长轴端部附近发生应力集中现象。斜接角度30°的模拟,粘结面上拉伸荷载的作用大于剪切荷载;斜接角度60°的模型,粘结面上剪切荷载的作用大于拉伸荷载,两个模拟应力集中的区域不同。两种模拟界面边缘处应力集中现象都不太明显,和矩形截面试样相比,有明显的优化。另外,试样中间区域应力值较小,实验中在胶层内放置铜丝控制厚度的做法,不会引起大的误差。

(4)建立了简单的理论模型,运用能量关系得到载荷-位移曲线和应力-应变曲线斜率的表达式。根据界面断裂能和能量释放率的数值关系,可以得到硅橡胶试样的模型预测斜率值,和实验结果高度吻合。环氧树脂试样只有很小一部分能量用于界面断裂,模型不能直接给出斜率结果。

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Bonding has many advantages over traditional connection methods and is widely used in important fields such as aircrafts, launch vehicles, satellites and missiles. Bonding can achieve high-strength bonding. Studies have shown that the strength of the adhesive in the bonding system can reach five times the tensile or shear strength of the adhesive itself. The research on the toughness and failure mechanism of the bonding system, including the experimental observation and characterization of the scale effect, is not only an academic frontier, but also an important demand issue in the aerospace and other high-tech fields. This article takes the metal/metal miter bonding system as the research object, and mainly focuses on the strength and toughness of the system, the interface failure mechanism, the scale effect of the failure strength and interface fracture energy, and the stress distribution in the adhesive layer to carry out systematic experiments and numerical simulation and theoretical model studies. The main research work and achievements are as follows:

(1) In this paper, based on previous work, in order to eliminate the singularity effect of angular stress in rectangular section specimens, we specially designed aluminum alloy round bars with two different types of adhesive samples, and systematically conducted experimental studies through various combinations of scarf angle and adhesive layer thickness. The study found that the silicone rubber samples mainly exhibit the characteristics of ductile failure, and the epoxy samples mainly exhibit brittle fracture characteristics. The corresponding failure load of the joint of the thin adhesive layer is higher, and the corresponding joint failure load of the thick adhesive layer is lower. The failure load has obvious scale effect on the thickness of the adhesive layer. The unit area failure load of the same the adhesive layer thickness is approximately equal. Silicon rubber samples were damaged in the adhesive, and the corrugated structure caused by the shear instability was evenly distributed throughout the bonded area. On the other hand, the epoxy sample was mainly mixed destruction mode.

(2) In order to analysis the mechanical mechanism of the round scarf joint, we believe that when the scarf joint is subjected to an axial tensile load, the stress state within the adhesive layer can be approximated as a composite of uniaxial stretching perpendicular to the bonding interface and simple shear parallel to the interface. By introducing the concepts of average stress and average strain, the strength and failure surface are obtained. For a given adhesive layer thickness, the average failure stress corresponding to different scarf angle specimens is approximately on the same arc, and its radius decreases as the thickness of the adhesive layer increases, which provides a readily applicable strength failure criterion for predicting the strength of a metal/adhesive bonding system. The strength of silicone rubber or epoxy oblique joints is two to five times the maximum tensile or shear strength of the corresponding standard specimens. Due to the binding effect of the metal adherends, the adhesive layer between the aluminum alloy exerts a stronger carrying capacity than the adhesive itself. In addition, both the fracture energy of the bonding interface and the energy release rate of the system increase with the increase of the thickness of the adhesive layer, and increase with the increase of the angle of the bonding interface. Failure intensity and interface fracture energy show a strong scale effect when the thickness of the adhesive layer is on the order of one hundred micrometers.                 

 (3) In order to verify the rationality of the selection of circular section specimens and to observe the stress distribution in the adhesive layer, we used commercial finite element software to perform finite element simulations of butt joints and scarf joints. The results of the butt joint simulated with the axisymmetric element and the scarf joint simulated with the hexahedron element are self-regulating and are consistent with the experimental results. It is found that there is a stress concentration in the butt joint at the edge of the interface, and the Mises stress at the outer surface of the symmetry plane and the middle region of the adhesive layer is the lowest. It was found that there was a jump of stress near the interface of the outer surface of the adhesive layer, and the effect of stress jump was very small (about 1% of the diameter of the sample). Scarf joints are prone to stress concentration near the end of the long axis of the elliptical bonding interface. For the model with a scarf angle of 30 degrees, the effect of tensile load on the bonding interface is greater than the shear load. However, for the model with a scarf angle of 60 degrees, the effect of shear load on the bonding interface is greater than the tensile load. The stress concentration areas of the two models are different. The stress concentration at the edges of the two simulation interfaces is not obvious, and there is a clear optimization compared with the rectangular section specimens. In addition, the stress value in the middle region of the sample is small. The practice of placing copper wire to control the thickness in the adhesive layer during the experiment does not cause large errors.                       

(4) We established a simple theoretical model and the expression of the slope of the load-displacement curve and the slope of the stress-strain curve is obtained using the energy relationship. According to the numerical relationship between interface fracture energy and energy release rate, the model predicted slope value of the silicone rubber sample can be obtained, which is in good agreement with the experimental results. Only a fraction of the energy of the epoxy resin sample is used for interface fractures, and the model cannot directly give a slope result.  

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73959
专题非线性力学国家重点实验室
作者单位中国科学院力学研究所非线性力学国家重点实验室
推荐引用方式
GB/T 7714
李景传. 金属/金属粘结体系的强韧和失效机制研究[D]. 北京. 中国科学院大学,2018.
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