Research On The Improvement Of Failure Criterion Of Adhesive Bonded Structure With Thickness-independence Of Adhesive Layer Under Complex Stress

As an important way to realize automobile energy saving and emission reduction,automobile lightweight has always been the goal in the development of the automobile industry.The research on lightweight automobile body is a major content of modern automobile body design and automobile lightweight.The use of lightweight materials such as lightweight alloys and fiber-reinforced composite materials in automobile structures is one of the most effective and direct means to achieve automobile lightweight.Using appropriate materials at appropriate locations is the most cost-effective option.Adhesive connection can solve the connection problem caused by the difference in material properties such as material parameters and mechanical properties between different materials,so it has a wide range of application prospects in automobiles.Our research group has proposed a stress-based failure criterion for the fracture failure of adhesives,through which the failure of any bonding angle can be predicted,however with further research,it has been found that the equivalent failure stress used in this criterion varies linearly in different thicknesses of the adhesive layer.Therefore,the failure criterion can only be used under the premise of the same adhesive layer thickness.In view of the shortcomings of the failure criterion,Sikaflex-265 adhesive was selected to improve the failure criterion under the influence of the thickness of the adhesive layer.The following researches were carried out in this paper:(1)In order to obtain the initial failure load and the location of the initial failure point of the adhesive specimen under different adhesive layer thicknesses,A test plan for adhesive joints with different adhesive layer thicknesses was made.In the experiment,Sikaflex-265 adhesive was selected,three typical bonding angles were selected,and four different adhesive layer thicknesses were selected for each bonding angle,and five adhesive heads of each type were made.The initial failure load and the initial failure point position of the adhesive specimen under different bonding angles and thickness of the adhesive layer were obtained by the quasi-static tensile test of the specimen.(2)In order to obtain the failure stress state of joints with different adhesive layer thicknesses,Quasi-static finite element simulation of adhesive joints with different adhesive layer thicknesses was carried out in this paper based on the experimental results.Finite element models were established according to the size of the bonded specimens in the experiment,and the initial failure loads obtained in the experiment were applied to the models for quasi-static simulation.The basic stresses at the initial failure position of the adhesi ve joint were extracted from the simulation results by python script,and the stress data were extended by the stress formulas and the ratio combining.(3)In order to establish the relationship between the thickness of the adhesive layer and the failure stress of the adhesive layer,and in order to obtain the failure stress parameters that are independent of the thickness of the adhesive layer,this paper studies the failure stress under the influence of the thickness of the adhesive layer on the 90°butt joint.The distribution of stresses in the adhesive layer and its variation with the thickness of the adhesive layer were analyzed,and the stresses were classified and combined according to the target requirements,and the stress combination whose maximum value does not change with the thickness of the adhesive layer is obtained.The obtained stress combinations were embedded in the failure simulation process using VUMAT as the failure criterion.The failure process of the adhesive layer and the failure load error of simulation and test are analyzed to determine whether the failure criterion is appropriate,and finally a stress combination that meets the requirements is obtained.Although it was found in further research that this stress combination is not suitable for other bonding angles,this also proves the feasibility of research on the thickness independent failure criterion.(4)In order to obtain the failure parameters with the thickness independence of the adhesive layer at any bonding angle,this paper conducts a study on the thickness independence of the adhesive layer failure under complex stress conditions.The method of data screening and fitting has been adjusted and improved,and it is no longer limited to stress data,but dimensionless data has been added.According to the target requirements,failure simulation and screening were carried out based on the combined parameters.Finally,the failure parameter that meets the conditions were obtained.Based on this,the failure criteria were improved.Finally,in the case of other bonding angle and other adhesive,the applicability of the improved failure criterion was verified.