|英文题名||Study on strength, toughness and failure mechanism of the metal/metal bonding system|
|关键词||金属粘结 胶层厚度 斜接角度 失效强度 界面能|
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.
|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.|