Effect Of Adhesive Bondline Thickness Under Salt Solution On The Single Lap Joint Bonding Performance

With the development of vehicle lightweight,structural adhesive bonding technology is widely used as an important connection between dissimilar materials.The actual application environment of the automobile is complex,and the bonded joints in vehicle body may have ageing phenomena like strength degradation in long-term corrosion environment such as salt,acid,alkali,and damp heat.At present,the effect of different adhesive bondline thickness on the bonding performance of unaged joints has been improved,but the effect during ageing remains to be further studied.In this study,the effect of the bondline thickness on the bonding performance of the single lap joint(SLJ)while under salt water ageing were studied by using different ageing time and different bondline thickness as the double variables.Firstly,the Araldite 2015 adhesive specimens were made,the moisture uptake weight and the hygroscopic expansion of adhesive specimens under the ageing environment of 30 ° C and 10% NaCl solution were measured.The Fickian moisture diffusion theory was used to fit the moisture uptake curve,and the diffusion coefficient was obtained.At the same time,the quasi-static tensile test of the adhesive specimens with different ageing time was carried out,and the strength degradation curve of the adhesive specimens with ageing time was measured.Besides,the mechanical properties such as elastic modulus and flexural modulus of carbon fiber substrate and 6061 aluminum alloy substrate were measured by quasi-static tensile test and three-point bending test,and the SLJs based on carbon fiber and aluminum alloy substrates with different bondline thickness were prepared.Then,quasi-static tensile tests were carried out on SLJs with different bondline thicknesses of 0.2mm,0.4mm and 0.6mm and also different ageing time of 0 days,55 days,110 days and 165 days.The Digital Image Correlation(DIC)method was used to analyze the strain of bonding area,and the failure interface of joints of different experimental groups was observed and the failure type was screened.Experimental results show that the initial failure strength of unaged joint decreases with the increase of the bondline thickness,and the bondline thickness also affects the uniformity of the strain distribution in the joint bonding area,the strain of thin bondline joint is lager and concentrated at the end of bonding area,and vice versa.During the ageing process,as the bondline thickness increases,the damage of salt to the adhesive layer and the bondinginterface becomes larger,with the interface-cohesive mixed failure appear in large numbers,resulting in a decrease of SLJ failure strength.In summary,the increased bondline thickness promotes the degradation of the SLJ performance in the salt water ageing environment,while the SLJ with a smaller bondline thickness is stronger and has a better salt environment durability.