| At present,the automobile industry is confronted with two challenges of energy crisis and environmental pollution.One of the most effective method is to reduce the weight of automobiles by using new materials.High strength steel,aluminum alloy and carbon fiber reinforced composites offer the possibility of making lightweight cars by virtue of their high strength and lightweight.And the mixed use of various materials is an inevitable trend in the future of automobile development.Therefore,it is one of the key technologies that need to be solved to develop the connection technology of the autobody parts of these materials.The bonding technology as a new kind of structural connection technology can not only realize complex structure shape,different material connection,but also can realize connection without destroying autobody parts structure.The adhesive structure has the characteristics of a large bearing area and uniform stress distribution,and can achieve the lightweight design aim while ensuring the bonding strength.However,there are some disadvantages of bonding technology.The adhesive as polymer material is very sensitive to the environmental changes,so the durability of automotive bonding structure in the course of service is one of the key factors in vehicle design.This paper combines the National Natural Science Foundation of China "Research on Aging Life Prediction Method of Adhesive Structures for New Material Body(51775230)".The effects of temperature,humidity,single factor environmental changes and multi-factor coupling conditions of moisture on the adhesive properties of polyurethane adhesives were studied.The failure mode of aged joints was analyzed,and the effect of aging time on the static strength and fatigue life of joints under high temperature was also discussed.The specific content is as follows:(1)Based on the range of vehicle bonding structure service temperature,by testing the failure strength of joints at-40°C,-20°C,0°C,25°C(RT),40°C,60°C and 80°C respectively,two stress failure criterions at different temperature were obtained.Then,the failure criterion surface function under the condition of temperature field was established to provide a reference for the failure prediction of the bonding structure under different temperature and complex stress conditions.The experimental results showed that the Young's modulus,tensile strength and failure strain of the adhesive material were decreasing with the increase of ambient temperature,while the failure strength and joint stiffness of the bonding joint were decreasing with the increase of temperature.(2)Study on the influence of continuous high temperature aging on joint strength.Under the conditions of 80°C,90°C,80°C loading and 80°C/95%RH,joints had 144 h,288h,432 h,576h and 720 h aging treatment respectively,in order to study the effect of aging time on the failure strength,and the relationship between failure strength and aging time of joints by studying the influence of same aging time and aging environment on the failure strength of joints.The effect of aging time on the glass transition temperature(Tg)of the adhesive was studied by using Differential Scanning Calorimetry(DSC).In addition,the effects of drying time on the failure strength of joints were studied by drying 24 h,48h,72 h,96h and 120 h in the 80°C/95%RH environment with different aging time.The results showed that the failure strength decreased with the increase of the aging time,and the attenuation of the 80°C/95%RH was the most obvious.Under the condition of 80°C loading,with the increase of aging time,the toughness of the adhesive was reduced gradually.Besides,the higher the temperature,the longer the aging time,the more pronounced the toughness of the adhesive decreased.The effect of high temperature loading on the bonding agent was more significant than that of no-load aging.At 80°C/95%RH,the joint toughness increased significantly with the aging time increase.The results of DSC show that the value of Tg increased with the aging time.While with the increase of aging time,the value of Tg decreased gradually in 80°C/95%RH environment.In the 80°C/95%RH environment,the failure strength of joint has the same trend as drying time after treatment with different aging time.With the increase of drying time first and then decrease,after the drying of 72 h,the failure strength of the drying joint is increased to the maximum,then the failure strength of the dry joint is decreased.(3)The influence of humidity on the mechanical properties of the bonding joints.The effect of different humidity conditions(55%,65%,75%,85%,95%,and 99%RH)on joint failure strength at 25(RT),40,60,and 80°C was studied.Effect of humidity on joint failure form was discussed by macroscopically and microscopically analyzing the failure section of the joints.Finally,by using DSC technique to indicate the effect of aging environment on the value of Tg,it can be found that the failure strength of joints from RT to 80°C decreases with the temperature at constant humidity,and the adhesion strength of high temperature 40°C was the most obvious.At different experiment temperatures,the failure strength and the joint stiffness gradually decreased from the 55%~99%RH.95%RH condition was the joint strength attenuation turning point,because the humidity exceeds the 95%RH failure strength had a cliff type descent.There is a linear relationship between joint failure strength and relative humidity.The increase in temperature had no effect on the decrease rate of failure strength.And mathematical statistics results showed that temperature and humidity had a significant effect on joint failure strength.(4)Study on durability and failure analysis of bonded joints under hot and humid cycle coupling environment.The influence of hot and humid coupling cycle on the durability of bonding joint was studied by reference DIN-6701 standard.The strength prediction model and the failure criterion of bonding joint were established by testing the failure strength of different aging cycle joints,and the failure analysis of the adhesives was performed using FTIR,TGA/DSC,EDX and SEM.The results showed that the joint failure strength decreased exponentially with the aging period increased.The failure mode of the joints was changed under hot and humid coupling cycle.The joint failure mode of single lap joints and butt joints was changed from the inner coalescence failure before aging to the mixed failure after aging,while it was changed to interface failure for scarf joints after 60 cycles of aging.The results of FTIR and EDX test showed that the failure strength of the joints was attenuated by the hydrolysis of adhesives in hot and humid environment.(5)Effect of continuous high temperature on the fatigue properties of bonding joints.The failure strength of butt joints and lap joints were tested under 80°C and 90°C conditions,respectively,aging 144 h,288h,432 h,576h and 720 h.By dividing the failure strength into 100%,60%,50%,40%,30% and 20% six grades,the fatigue tests were carried out to obtain the fatigue failure times of the corresponding grade joints.The fatigue life function of joints is established based on the fatigue failure times.Finally,the microscopic appearance of the failure joint was observed by SEM.The results showed that the fatigue life of joints decreased with the increase of aging time at 80°C and 90°C,and the lap joints and butt joints had the same change law.Besides,the fatigue life of aging 144 h,288h,432 h,576h and 720 h in 80°C and 90°C was in accordance with exponential function. |
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