Methods And Mechanisms Of Toughening Of Adhesively Bonded Joints

With various kinds of lightweight materials and well-designed structures introduced into vehicle design and manufacturing,the adhesive bonding,as a joining method,is showing its advantages in many aspects.However,the mechanical performance of the adhesive bonding is significantly affected by the loading states.Compared with its superior toughness under shear loading,the relatively lower toughness under tensile loading of adhesive bonding compromises its performance under complex loading.An experimental/analytical/numerical method is applied to investigate the fracture behaviors of the adhesively bonded joints under various loading states.Two methods are proposed to suppress the crack propagation under mode I fracture.The strength and toughness of the adhesive bonding are greatly improved in experiments,and the toughening mechanisms related are studied.The joint strength and energy absorption during the fracture are obtained by Arcan experiments,which enable the fracture tests of adhesively bonded joints under various loading states.The crack initiation positions,propagating paths and topographies of the fractured surfaces are observed in tests,which show strong correlations to the loading states.Double cantilever beam tests are carried out to determine the fracture parameters under mode I fracture.The stick-slip crack propagation behavior occurs when some certain adhesives applied to fabricate the joints.The mechanism behind is provided in mechanical aspect by establishing an analytical model and the material science aspect by the observed fractured surface topographies.Numerical simulations are conducted with a novel technique to mimic the stick-slip crack propagation behavior in DCB tests.The crack propagation behaviors are believed to have relationship with the joint strength and toughness.Two toughening approaches,distributed local reinforcement in the adhesive layer and patterned bonding surfaces,are proposed to suppress the crack propagation.Simulations with the cohesive zone model,representing the adhesive layer,are performed to validate the proposed approaches.The laser ablation in pulsed mode is applied to fabricate the patterned surfaces.A nonlinear relationship between laser parameters and surface amplitude parameter is established and an optimum surface topography is achieved,which provides the best strength and toughness of the adhesively bonded joints.The solder balls are mixed into the adhesive layer to form the local reinforcement,and is found to be able to inhibit the stick-slip crack propagation effectively in double cantilever beam tests.A heterogeneous representative volume element of the solder reinforced adhesively bonded joint in meso-scale is established with accurate mechanical properties of its components and periodic boundary conditions.A multi-scale simulation strategy is developed.The cohesive law,i.e.the traction-separation curve,applied in macro-models can be extracted from the representative volume element.Effects of the mechanical properties of the interfaces between components on the joint performances are investigated by the meso-models.The toughening mechanism of the solder reinforced adhesive bonding is further explained.The simulation work guides the selection of solder balls,the design the interfaces and the optimization of the process window.