Theoretical And Numerical Investigation On The Residual Thermal Stress Distribution In Single Lap Joint

The present study adopted the finite element method; the single-lap joint and its overlapping zone were the main research objects. The main purpose of the study is to find out ways to optimize the distribution of the joint stress and improve the joint strength. After investigating the residual thermal stress distribution, the main factors influencing the residual thermal stress and the adjustment measures were found, the main research results are as follows:1) The theoretical calculation formula of the thermal stress distribution in overlapping zone of the single lap joint was derived based on the metallic plate symmetrically repaired with composite patches (Baker model). Then the mathematical model for residual stress was established and the analytical solution formula was also derived based on this model.2) In this paper, the residual thermal stress finite element method was summarized and the finite element model of the residual thermal stress was also obtained. When the temperature dropping from the curing temperature (80℃) to the room temperature (20℃), the value of residual thermal stress distribution (axial stress) in the adhesive layer was 5.5Mpa with the direct-coupled method of ANSYS, it is coincide with the theoretical values (5.53 Mpa).3) Adopting the monte-carlo method of the probability design system (PDS), among the main factors influencing the joint properties, the most significant factors influencing the residual thermal stress were confirmed to be: 1) the process of temperature rising and falling; 2) the temperature difference; 3) the elastic modulus of the adhesive; 4) the dissimilar adherend and; 5) the interface roughness of adherend.4) The present study explored the effect of the temperature gradient on the residual thermal stress distribution in the overlapping zone by numerical simulation. The results showed that when the temperature rising, the residual thermal stress in the adhesive layer was a compressive stress, while the stress was a tensile stress when the temperature falls. The value of tensile stress was bigger than the compressive stress when the temperature changes within the same amplitude. The effect of the residual thermal stress on the strength of the adhesive bonded joints was evident, especially in the cooling process.5) The present research studied the effect of different levels of temperature difference on the residual thermal stress distribution in the overlapping zone. The results indicated that the changing of temperature exert an evident effect on the residual thermal stress. The greater the temperature difference is, the bigger the residual thermal stress will be. By comparing the ratio of the residual thermal stress and temperature difference with the result of FEM, the theoretical formula was obtained. In order to reduce the residual thermal stress to improve the joints, it is suggested by the present study that the suitable curing temperature and reduce temperature difference should be taken into consideration.6) This study also investigated the effect of the elastic modulus of adhesive on the residual thermal stress in the adhesive bonded joints. The results showed that the effect of the elastic modulus on the residual stress distribution was evident. When the elastic modulus of the adhesive was increased, the residual thermal stress in the middle of adhesive layer will also increase. At the free-end of overlapping zone, the stress will also increase. It is suggested that when using the adhesive bonded joints in the environment where the temperature change severely, the lower elastic modulus of adhesive should be used in order to reduce the residual thermal stress.7) In this paper, the heterogeneous adherend glue layer and the distribution of the interfacial residual thermal stress were investigated then the following conclusion was made: when the same materials were used in the upper and lower adherend, the value of the residual thermal stress was mainly decided by the linear expansion coefficient difference between the adhesive and adherend. At thermal loading conditions, the shrinkage of upper and lower adherents was inconsistency in the joints made of dissimilar adherend. The upper and lower adherend restricted the deformation of each other through the adhesive layer, resulting in the residual thermal stress increased in the adhesive bonded joints.8) Changing the interface amplitude (A) and single-peak spacing (S) of the adherend, then analyzing the different morphology of the adherend on the residual stress distribution, the results showed that the effect of interface morphology on the thermal stress distribution was evident. The protruding part of the sine wave-shaped interface adherend had a greater peak stress; it is a more dangerous area in the system. The amplitude (A) affected the filling glue between the adherend. The thick adhesive layer became thinner and thinner with the decreasing of the adherend interface amplitude A, which caused the increasing residual thermal stress. Comparing the effect of amplitude and the single-peak spacing on thermal residual stress, it can be known that the former exerts a greater influence on thermal residual stress than the latter. Referring to the thermal barrier coating system of the thermal stress method, smoothing the tip of the bulge point can effectively improve the joints in the distribution of residual thermal stress and avoid stress concentration.9) After examining the influence of the five different kinds of adhesive bonded jointss on the residual thermal stress distribution under different restriction conditions, the following conclusions can be drawn: When using, preparing and solidifying the adhesive bonded jointss, the effect on the residual thermal stress of the adhesive bonded jointss is tremendous under different boundary restriction conditions. When the boundary restriction condition is different, the distortion of the adhesive bonded jointss under thermal loading conditions will be influenced to some extent, accordingly the residual thermal stress will appear in the joints. During the process of the temperature changing, the degree of freedom is smaller, the more effectively the thermal stress caused by the temperature changing will release, the residual thermal stress in the joints will be smaller as well.10) The materials in different layers of the adhesive bonded jointss possess different thermal physical properties and different material properties; therefore the thermal stress is generated in the thermal cycling process. The following conclusion can be drawn based on this: the residual thermal stress in the glue layer will change with the temperature cycling; in the meantime the adhesives are in an elastic deformation stage. In this particular stage the strain relationship of the thermal stress can be expressed by the following formula:εx =9 .91393 E -7 + 2.88542 E 9σx.11) The paper also examined the influence of the residual thermal stress and the operation stress superposition on the stress distribution of the adhesive bonded jointss, analyzed the symbol, the direction and the distribution of the residual tress and then estimated their effects on the joint strength. The results indicate that the adhesive bonded joints will generate residual tensile stress when the temperature falls. This residual tensile stress will superpose with the tensile stress generated by the tensile load withstanding by the joint, which will enlarge the stress value of the joint, therefore it exert great influence on the bearing capacity of the joint. The peak stress concentration at the end part of the joint will increase from 7.48Mpa to 12.2Mpa. The axial stress in the middle part of the overlapping zone will also increase from 0.2Mpa to 5.5Mpa.