Study On Failure Criterion Of Body Bonding Structure In Full Service Temperature Range After Aging

Whether it is a traditional fuel vehicle or a new energy vehicle,lightweight is always the goal of sustainable development in the automotive industry.By reducing the weight of body structure,the purpose of energy saving and emission reduction can be achieved.The development of multi-material body provides an important technical route for body lightweight.With the wide application of new materials such as fiber reinforced composite materials,engineering plastics and lightweight alloys,the connection of dissimilar materials has become an urgent problem to be solved.Bonding technology can make up for the deficiency of traditional connection methods such as welding and bolting,and solve the connection problems caused by different mechanical properties between different materials.It has the advantages of electrochemical corrosion resistance,uniform stress distribution and good fatigue resistance.The development of bonding technology can provide technical guarantee for the wide application of new materials in the body.For the study of the failure criterion of bonded structures,our team proposed a failure judgment criterion based on the ratio of the second invariant and the third invariant of deviatoric stress,which preliminarily solved the problem of checking the strength of bonded structures under complex stress conditions.However,after further research,it is found that this criterion still has shortcomings: it only has relatively accurate failure prediction for unaged bonded joints at room temperature.For aged bonded joints and bonded joints at different temperatures,the accuracy of this failure criterion is significantly reduced.Therefore,in order to more accurately predict the failure of the bonding structure under actual service conditions,this paper selects Sikaflex-265 adhesive and aluminum-aluminum bonding specimen as the research object,and based on the conclusions of our team,the failure criterion is proposed improvement,extended research on failure criteria around aging and temperature,and explored the applicability of failure criteria in the full service temperature range after accelerated aging.The contents are as follows:（1）Improve the failure criterion of elastic bonding structure under non-aging and normal temperature.Extract the stress components in the failure area.The stress components are processed by means of ratio method and power reduction method.According to the requirements that the equivalent stress should meet,the combined stress that meets the requirements is obtained.The equivalent stress is embedded in the simulation model,and the test results and simulation results are compared and analyzed.The results are as follows: the error between the simulation failure load and the test failure load is slightly improved compared with the original failure criterion,and the simulation failure process is basically the same as the test failure process.This proves that the failure criterion established according to this equivalent stress can more accurately predict the failure of the elastic bonding structure under normal temperature and unaged state.（2）The influence of aging on failure criteria of bonded structures is studied.The dumbbell-shaped specimens are subjected to different cycles of aging tests,and it is found that the longer the aging cycle,the smaller the elastic modulus of the adhesive and the larger the Poisson’s ratio.Through finite element simulation of bonded joints aged in different periods,the equivalent stress value of the failure area is obtained and embedded in the simulation model.The calculation results show that the equivalent stress value gradually decreases with the prolonging of the aging period.Compared with the unimproved failure criterion,the error between the simulation results and the test results of the improved failure criterion is significantly reduced,which indicates that the established failure criterion can check the strength of the elastic bonded joint after aging.（3）On the basis of 120 cycles of damp-heat cyclic aging,the effect of temperature on the failure criterion of the bonded structure is studied.Tensile tests at different temperatures are carried out on dumbbell-shaped specimens after 120 cycles of dampheat cyclic aging.It is concluded that with the increase of temperature,the elastic modulus of the adhesive gradually decreases,and the Poisson’s ratio increases gradually.Through the simulation analysis of the bonded joints at different temperatures,it is found that the equivalent stress value gradually decreases with the increase of temperature;The equivalent stress is embedded in the simulation model,and it is found that the error between the simulation results and the test results of the improved failure criterion at different temperatures is significantly reduced compared with the unimproved failure criterion.This proves that the established failure criterion can check the strength of the elastic bonded joints in the full service temperature range of the vehicle body.The failure criterion established in this paper can provide a theoretical basis for the application of the bonding structure in practical engineering.