Effects Of Temperature And Humidity Aging On The Static Failure Of Adhesively Bonded CFRP/Aluminium Alloy Joints For Automotive Applications

The two major challenges of automotive industry are energy crisis and environmental contamination.One of the effective solutions is to use all kinds of lightweight materials,such as CFRP?Carbon Fiber Reinforced Plastics?,aluminum alloy and high-strength steel,to achieve lightweight.The application of multi-materials can reduce weight and save costs,but it also brings up the question of connection of dissimilar materials.The traditional joints?such as bolts and rivets?always destroy the integrity of the structures,resulting in the reduce of load capacity and early failure,especially for the CFRP composites.While adhesive joints can not only ensure the structural integrity and uniform stress distribution,but also connect the complex structures.The adhesive and resin matrix of CFRP in adhesively bonded CFRP/aluminum alloy joints are polymer materials,which are sensitive to different service temperatures.Besides,they may degrade when subjected to long-term temperature and humidity,leading to the decrease of mechanical properties.Therefore,service temperature and degradation are the two major factors that affect the performance of adhesive and CFRP,the combined action of which determines the performance of adhesively bonded CFRP/aluminum alloy 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 considered the effect of adhesive and CFRP on the static failure of adhesively bonded CFRP/aluminum alloy joints,and studied the effect of service temperature and degradation caused by temperature,humidity and load on the ageing mechanism and rule of adhesively bonded CFRP/aluminum alloy joints.Then a failure prediction method based on the initial failure criterion in cohesive zone model?CZM?and a rapid evaluation method of adhesive strength based on submodel technology were proposed.The details of this paper are divided into six parts as follows:?1?Seven service temperatures between-40°C and 80°C were selected to test the static failure of bulk specimens of adhesive and adhesively bonded CFRP/aluminum alloy joints under different stress states.Glass transition temperature?T_g?of adhesive and CFRP from DSC?Differential Scanning Calorimeter?analysis,failure strength and failure modes of adhesive joints were used to analyze the failure mechanism of adhesively bonded CFRP/aluminum alloy joints at different temperatures.The effect of adhesive and CFRP on the adhesively bonded CFRP/aluminum alloy joints were also obtained.?2?Three representative temperature environments?high temperature,low temperature,temperature cycling between high and low?were selected to study the effect of temperature degradation on the adhesive,CFRP and adhesively bonded CFRP/aluminum alloy joints.FTIR?Fourier Transform infrared spectroscopy?and DSC were used to analyze the variations of composition and T_g.Mechanical performances of bulk specimens of adhesive and CFRP were also tested before and after temperature degradation.The influence mechanism and rules of adhesive and CFRP on the failure strength and failure modes of adhesively bonded CFRP/aluminum alloy joints subjected to different temperature environments were obtained.?3?Three representative temperature and humidity environments?immersed in water at room temperature,high temperature and high humidity,hygrothermal cycling?were selected to investigate the single effect of humidity,and coupling effect of temperature and humidity on the adhesive,CFRP and dhesively bonded CFRP/aluminum alloy joints.The degradation mechanism of adhesive and CFRP were studied by chemical analysis?FTIR and DSC?and mechanical tests.The influence rules of adhesive and CFRP on the failure strength and failure modes of adhesively bonded CFRP/aluminum alloy joints subjected to different temperature and humidity environments were analyzed.?4?To investigate the effect of static load on the accelerated degradation of adhesive joints subjected to different environments,the coupling effect of static load and seven environments?room temperature,high temperature,low temperature,temperature cycling between high and low,immersed in water at room temperature,high temperature and high humidity,hygrothermal cycling?on the failure strength and failure modes of adhesively bonded CFRP/aluminum alloy joints were studied.SEM?Scanning Electron Microscope?were also used to observe the microstructure changes of adhesive,resin matrix and fiber/matrix interface of CFRP.Direct analysis and analysis of variance?ANOVA?were applied to investigate the single and coupling effect of different environments,aging time and static loads on the influence rule and degree of adhesively bonded CFRP/aluminum alloy joints.?5?Take the failure prediction of adhesively bonded CFRP/aluminum alloy joints under different stress states at different temperatures as an example to show the prediction method of failure based on the the initial failure criterion in CZM.A response surface of quadric stress criteria over the service temperature of automobiles was built according to the failure loads to be used as initial failure criterion of cohesive zone element.Temperature impact factor T_em was defined to calculate the parameters of cohesive element at different temperatures,which were introduced into CZM to conduct simulation analysis.The stress distributions and failure processes of adhesive joints under different stress states were analyzed and the validity of the prediction method was also verified by experimental tests.?6?Border displacement interpolating methods of submodel based on triangle area and RS mapping of quadrangle were proposed and proved by simulation analysis.The interpolation precision and boundary effect of the two interpolating methods were compared and analyzed.On the basis of submodel and quadric stress criteria,a rapid evaluation method of adhesive strength was proposed and applied to the strength evaluation of side-window of high speed EMU?Electric Multiple Units?.The reliability and high efficiency of the rapid evaluation method was found by comparing with the traditional method.The entire process of the rapid evaluation method was implemented through FORTRAN programs.