Effects Of Geometrical Dimensions On Composite Bonded Joints Featured With Thick Adhesive Layer

Composite is widely used in lightweight structures for it's excellent strength/weight ratio,when applied in large structures,connection becomes essential,and as one of the most important means of jointing,adhesively bond gain less wight to structures,and have good performance in fatigue,air/water tightness and corrosion resistance.Because the composite was firstly applied in the aerospace field,then other industries,most adhesively bonded joint researches are about bondline thickness of 0.1~1mm,but in the wind turbine blade,car and other civilian industries,the adhesive layer thickness can reach 6mm or much more,the widely used classic theory are not applicable for this thick bondline joints.In order to increase the reliability of adhesively jointing,the composite joints with thick bondline must be detailed and systematic studied.In this study,base on thick bondline(1~5mm)composite single-lap joints,and bondline thickness,lap width,lap length,adherend thickness,orientation angles of Ply Adjacent to Adhesive Layer(PAAL)are studied in this research by Finite Element Method(FEM)stress analysis,experiment and Progressive Damage Model(PDM),to study the different failure strength and failure mode between joints with different design factors.Three-dimensional joint FEM model is established to study the stress distribution of joints.Secondly,specimens are made and the the experiment result shows that the failure mode is strongly different with varied dimensions.In addition,a Progressive Damage Model is established by ABAQUS's user subroutine USDFLD,and the failure strength and failure mode of the joint are obtained and predicted by this model.At last,by compareing the experiment,FEM stress analysis and the PDM results,and these following conclusions can be made: with thick bondline,and short lap length(less than 25.4mm),the main failure mode is Adhesion Failure(AF),when the bondline thickness reach 5mm,dominated by strong peel stress,Adhesion and Cohesion Failure with peel(ACFP)shows,and the joint strength decreases with bondline thickness increases;the joint strength and lap width are approximately proportional,but have different failure modes,when the lap width increases,thelateral straining effect in width direction is more obvious,the wide joint shows a different failure mode ACFP with a crescent shape;the increase in lap length will lead to failure mode transform from AF in short lap length into a large area of Cohesion Failure(CF)and Cohesive Substrate Failure(CSF)in longer lap length,and when lap length is longer than 63.5mm,due to the shear stress concentration,specimen begins to appear in large area of composite Delamination Failure(DF);and for the thinner adherend,composite Delamination Failure(DF)area is larger,and joint strength is lower;the shear stress of adhesive layer increases when the PAAL's ply angle decreases to 0°,while the peel stress nearly unchanged;On the contrary,the shear stress and tensile strain of the PAAL are raised when ply angle increases,when designning composite joint,the stacking sequence and the ply angle of PAAL should be considered and optimized.In addition,the PDM in this paper for joints strength prediction has a value of error about 20% with experiments,it's acceptable when applied in engineering.