Experimental Study On Mechanical Properties Of Adhesive Joints With Dissimilar Materials(CFRP/Metal) After Transverse Impact

With expanding use of multi-materials in vehicle components for attaining light-weighting structures,growing interests have been refilled in the adhesive bonding technique attributable to its compelling advantages to bond dissimilar materials,especially for carbon fiber reinforced plastic(CFRP)composite and metals.A significant issue related to the bonding of dissimilar materials is,nevertheless,that there are substantial differences between adherend material properties,which often lead to different structural and fracture responses.Many related investigators have done a lot of researches on the mechanical proeprties of the adhesive joints,however,the fracture characteristics of adhesive joints with dissimilar materials have not been fully revealed.Moreover,in real life however,vehicle could experience dynamic damage due to traffic accident,hail impact or falling objects in service.Such pre-mature damage could to a certain extent sacrifice safety performance,especially in the critical areas of connection.It is thus important to evaluate the residual performance of the joints after pre-impact.To further explore the mechanical characteristics of the adhesive joints with dissimilar materials after transverse impact,a series of innovative experiments were carried out,the specific contents of this paper are as follows:(1)The effects of adherend thickness and adherend material types(e.g.Q235 steel,5182 aluminum,CFRP composite)on the fracture behavior of the joints.A digital image correlation(DIC)technique was used to capture full-field,out-of-plane deformation of the adherend,the strain distribution and strain evolution along the bondline so that the fracture process can be characterized visually.The results divulged that the joint strength increased with increasing adherend thickness;and the sensitivity of adherend thickness on the joint strength depended on the joint material types.Adherend yield strength determined the joint strength,fracture state and failure modes when the failure occurred with yielding in the adherend.The fracture processes and strain evolutions in the bondline were found to be symmetric for the joints with the same adherends,while the maximum strain and crack appeared first on the lap end of the lower yield strength adherend for the joints with dissimilar materials.(2)The CFRP/Al adhesive joints were experimentally investigated for their failure mechanisms after low velocity transverse impact.The effects of impact surface and impact energy on the residual characteristics of the CFRP/Al adhesive bonded joints were explored.The results divulged that CFRP,as an impact surface,could generate better structural integrity and is able to reduce the loss of joint strength in comparison with the aluminum pre-impact surface.The mechanism for alternating residual tensile strength resulting from transverse pre-impact load was the combined effect of both damage and mechanical interlocking.The residual tensile strength and failure displacement decreased with increasing pre-impact energy except for the joint strength impacted onto the aluminum surface at 10 J attributable to the prominent mechanical interlocking effect.(3)The dynamic residual properties of CFRP/Al single-lap adhesive joints at different strain rates after transverse pre-impact were experimentally investigated.The tests were conducted at four different loading velocities of 0m/s,0.1m/s,2.5m/s and 5m/s,respectively.The experimental tests show that the CFRP/Al adhesive joints presented significant rate dependence and the joints strength showed a rising tendency with increasing strain rate.The failure modes for the adhesive joints gradually transformed from the cohesive failure and adhesive failure modes to the cohesive failure and fiber-tear failure modes with increasing strain rate.The results of two-way analysis of variance(ANOVA)showed that the effect of axial strain rate and transverse pre-impact were the two main factors to affect the residual strength of joints,but the interaction was not strong.(4)The fatigue properties of CFRP/Al single-lap adhesive joints after transverse pre-impact were experimentally investigated.The results show that the fatigue life decrease with the impact energy but the fatigue life of the joint underwent 20 J was more than 10 J.The deformation and stiffeness degradation of the joints increased with the increasing of the fatigue cycles.With the increase of impact energy,the joint deformation rate and stiffness degradation rate increased obviously.Adhesive failure mode was the main failure mode for all the joints underwent the fatigue load.Thus,it's a good way for improving the fatigue life through surface modification of adherend materials.