Research On Improvement Of Experimental Failure Strength Calculation Method For Adhesive Joints

With the wide application of lightweight materials such as composites in vehicles,the adhesive technology has been used more and more by its uniform stress distribution,light weight and high strength.However,due to the insufficient understanding of the mechanical properties of adhesive joints,the application of adhesives in many key structures has been limited.As the weak link of adhesive structures,whether can have an accurate understanding of its strength is the key to realize the lightweight of body by using lightweight materials.At present,the strength of the adhesive joints is mainly obtained by the specimen experiment,and in the adhesive experiment,the nominal stress that obtained by dividing the adhesion load by the adhesion area can act as the failure strength.But,there are two problems in evaluating the strength by using nominal stress.First,the calculation of nominal stress needs to meet the premise of uniform bearing on the adhesive surface.Actuallly,in the adhesive speciments,due to the difference of relative stiffness between the adhesive and substrate,the stress distribution on the adhesive layer is disuniform,which makes the nominal stress can not really reflect the actual strength of the adhesive structures.Second,through the adhesive experiment can only obtain a stress along the loading direction,but there are six normal and shear stress components in the actual adhesive layer.A stress component represented by nominal stress may not be the main cause of failure.Therefore,it is not accurate enough to use the nominal stress to evaluate the strength of the adhesive structure.This paper mainly focuses on the problems existing in the evaluation of adhesive strength.The stress state of the adhesive layer was analyzed by using the method of combining experiment and simulation with the main research object of Sikaflex-265/aluminum alloy butt joints.The aim is to improve the accuracy of strength evaluation in the experimental method of adhesive joints,making it closer to the true strength of adhesive structures.In order to solve the problem of nominal stress uniformity,on the one hand,the structure form of adhesive joints were optimized,and the accuracy of the nominal stress for the strength evaluation of the adhesive joints were improved by improving the uniformity of the stress distribution of the adhesive layer.It was found that when the boundary of the adhesive layer adopts the design form of inner concave,it can not only improve the uniformity of the stress distribution,but also can improve the average value of each stress.Besides,when the adhesive layer boundary adopts the design form of external convex,it can reduce the stress component inside the adhesive layer and make the stress component more simple.On the other hand,using simulated stress calculated by the area weighted average method to evaluate the strength of adhesive structures,which can consider the influence of disuniform stress distribution on the adhesive layer,and further improve the accuracy of the strength evaluation of the adhesive joints.In order to solve the problem that the stress components inside the adhesive layer are not single,the equivalent stress was introduced to improve the strength evaluation method of adhesive specimens.It can not only solve the problem that the existing evaluation method can not truly reflect the actual strength,but also can prove that the theory that the adhesive strength will decrease with the increase of the adhesive thickness has been inaccurate because of the unreasonable application of the strength evaluation method of adhesive specimens.By using the improved method to evaluate the strength of adhesive specimens,it can be found that the adhesive thickness does not cause the change of the strength.The tensile fracture load obtained by the adhesive speciment with different thickness was different,because the adhesive thickness affects the stress state inside the adhesive layer,which leads to the equivalent stress with large bonding thickness is more closer to the strength limit.Therefore,it is more likely to cause damage.In addition,by embedding the equivalent stress through the subroutine into the ABAQUS software,it is found that for the adhesive speciments with different thickness,the failure load obtained by the simulation analysis is compared with that obtained by the experiment,and the average error is less than 10%.What's more,the failure position of the element obtained by simulation is more consistent with the experimental phenomenon,which verified that the improved method of adhesive joint strength proposed in this paper is reasonable.