| With the continuous development of the automotive industry,the problems of energy crisis and environmental contamination are becoming more serious.One of the effective ways to solve the current problems is to achieve the lightweight of automotive through the mixed use of multi-material.Lightweight materials such as CFRP(Carbon Fiber Reinforced Plastic),aluminum alloy,magnesium alloy and high-strength steel are widely used in the automotive industry due to their great lightweight effect.The multi-material car body design can achieve lightweight while saving cost,however,it also brings up the issue of heterogeneous materials connection.Achieving effective connection between different materials is one of the key technologies in the design of multi-material automotive structures.Adhesive structure can provide a certain connection strength without damaging the substrate and play an important role in the connection of heterogeneous material,especially those involved CFRP.Due to the polymer nature of adhesive and composite resin matrix,they are sensitive to different service temperature.With the changes of environment temperature,their mechanical properties may also vary.Meanwhile,the mechanical performance of adhesively bonded structures may degrade when subjected to long-term hygrothermal environment.Besides the influence of external environment,the automotive adhesively bonded structures also subjected to alternating loads for a long time,which may cause fatigue failure during service.In addition,under continuous alternating load,the performance of the adhesively bonded structures may degrade to some extent.Therefore,to provide an effective method for predicting the failure of adhesive structures,it is essential to study the individual or coupled effects of temperature,humidity,and alternating load alone or coupled on the mechanical performance of composite adhesively bonded joints.Based on the project ‘Research on the Prediction Method for Aging Life of Adhesive Structures Oriented to Advanced Material Automotive Body(NO.51775230)’ supported by the NSFC(Natural Science Foundation of China),this paper focused on the aging laws and failure mechanism of the adhesively bonded joints performance when subjected to temperature,humidity,and alternating load individual or coupled conditions.The inner relation between single factor and multi-factor coupling was revealed and the coupling mechanism was analyzed.A method for predicting the failure of adhesively bonded structures in a complicated stress state based on the adhesive chemical properties and cohesive zone model was established.The details of this paper were divided into six parts as follows:(1)According to the practical application requirements of automotive,epoxy adhesive Araldite~? 2015 was selected to fabricate the adhesively bonded CFRP-aluminum alloy butt joints.Combined the quasi-static tensile strength tests of the joints,CLSM(Confocal Laser Scanning Microscope)and SEM(Scanning Electron Microscope,SEM)analysis,the influence of CFRP surface roughness on the strength of the adhesively bonded joints was studied.Special test equipment was designed and processed to test the alternating load aging behavior of adhesively bonded joints under different load levels.The effect mechanism of the alternating load on the performance of joints was revealed by analyzing the failure modes and failure strength degradation laws of adhesively bonded joints.(2)Five temperature measurement points were selected in the service temperature range(-40~80 °C)of the automotive adhesive structures and the quasi-static tensile tests of the adhesively bonded joints at different environment temperature were performed to study the variation of joint performance with temperature.Then the thermal aging tests of adhesively bonded joints were also carried out at different temperature,and the aging mechanisms under different temperature of adhesive and CFRP were analyzed by FTIR(Fourier Transform Infrared Spectroscopy)and DSC(Differential Scanning Calorimeter).To investing the coupling aging law,the failure strength of the adhesively bonded joints exposure to coupled temperature and alternating load was tested and the corresponding failure mode was analyzed.Direct analysis and variance analysis methods were adopted to determine the influence laws of temperature and alternating load in coupling conditions and the corresponding influence degree.(3)The CFRP-aluminum alloy adhesively bonded joints were subjected to accelerated aging tests under three conditions: temperature cycling,high temperature and humidity,hygrothermal cycling.Based on the FTIR and DSC results of CFRP and adhesive,as well as the analysis of failure modes,the influence mechanism of the different conditions on the joint performance was revealed.Further the alternating load was introduced in aging conditions,and the failure strength of the joints exposed to three different environment conditions coupled with their corresponding alternating load was tested respectively.Combined with the analysis of failure modes,the aging laws of joint was revealed.Variance analysis was adopted to study the coupled aging mechanisms of environment and alternating load on the performance of joints.(4)Focused on the influence of environment temperature and humidity on the fatigue performance of adhesively bonded joints,the static tensile tests and fatigue tests of CFRP-aluminum alloy butt joints that subjected to different temperature and hygrothermal aging time were performed.By analyzing the effects of the two mentioned environmental factors on the fatigue performance of joints,the relation function of stress-temperature-fatigue cycles and stress-aging time-fatigue cycles were established respectively to reveal the fatigue performance of the environment on joints.Therefore,it can provide a reference for the fatigue performance prediction of adhesively bonded joints with different temperature / hygrothermal aging time.(5)Through the hygrothermal aging tests of the CFRP-aluminum alloy butt and shear adhesively bonded joints,the function relation curve of joint failure strength and the absorbance of adhesive functional groups with aging time were obtained respectively.According to the correlation analysis and the basic transformation of the curve,the quantitative relationship between the characteristic functional groups and the failure strength of joint was established.The weighted combination of each functional group was used to establish a rapid method for predicting the failure strength of the hygrothermally degraded adhesively bonded joints based on the chemical properties of the adhesive.Then the alternating load was further introduced in aging conditions and a prediction method of joint failure strength based on adhesive chemical properties was explored,which was suitable for the condition of alternating loads and hygrothermal coupling aging.(6)Cohesive zone model of the adhesively bonded joints based on the bilinear traction-separation law was established.The degradation factors were determined according to the prediction function of joint failure strength based on adhesive chemical characteristics under single stress state.The parameters of CZM were modified to to realize the performance prediction of single lap joint after aging.Thus,a method for predicting the failure of adhesively bonded joints under complicated stress state was further established,which based on adhesive chemical properties and cohesive zone model.The initial failure criteria of joints were established based on results of Arcan tests.The VUMAT subroutine was coded and then the process of the joint failure prediction was embedded into the finite element analysis software ABAQUS~?.The failure process of adhesive joints was calculated automatically and the influence of different initial failure criterion on the results was compared. |
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