Research On Interface Failure Mechanisms Of Composite Adhesive Joints Based On The Cohesive Model

Compared with the traditional fastener connection method,the adhesive bonding method shows favaroble application prospect in fields of the assembly,manufacture and repair of composite structures due to advantages of reducing weight and stress concentration,and guaranteeing structural integrity.However,the interface stress state and fracture failure mechanism of the adhesive structures are complicated.Affected by many factors such as the adhesive type,the processing parameters,the size of the adhesive layer as well as the load and service environments,a large challenge to the structural design and performance evaluation emerges.Based on the elastoplastic fracture mechanics,the cohesive theory has been widely used in the field of adhesive jointing.It shows the advantages of predicting both the crack initiation and propagation,as a theoretical frontier of the interface mechanics.According to the shape of the traction-displacement jump relationship function,the regular cohesive models include the bilinear cohesive model,the exponential cohesive model and the trapezoidal model.Combined with finite element analysis,these models are widely applied to the interface failure analysis of brittle and ductile adhesive joints.Numerical analysis shows that the zero-thickness cohesive element is most robust for predicting the failure mechanisms of adhesive structures,which shows superior computational efficiency and numerical performance.However,there are still two problems for studying the interface failure mechanism of adhesive structures:First,for the widely used brittle adhesive structures,the zero-thickness cohesive model can elucidate the interface failure mechanism,but cannot predict the failure mechanism of the adhesive layer of ductile adhesive structures;Second,because the brittle adhesive belongs to polymer materials,the failure mechanism of materials is still related to time even if under the regular load and temperature environments.It is impossible to truly predict the rate-dependent interface failure mechanism by the zero-thickness cohesive model.Based on the cohesive model and finite element analysis,the interface failure mechanism of composite adhesive structures is studied in this paper.The main research contents include three parts:First,based on the bilinear and exponential cohesive models,the failure mechanisms of the single lap adhesive joint are studied by the implicit finite element analysis,and the influence of the cohesive strength and the shape of cohesive model on the interface failure mechanisms are discussed.The viscous parameters are introduced to improve the convergence due to cohesive softening,which provides some suggestions and references for the sound selection of cohesive parameters and viscous parameters;Second,all interface failure mechanisms of the ductile adhesive structures are considered.Based on the explicit finite element analysis,the competitive relationship between the interface debonding and the void evolution of adhesive layer is studied,in which the bilinear cohesive model is used to predict the interface failure between the adhesive layer and the solid layer,and the finite deformation Gurson model is used to predict the failure behavior of the adhesive layer with finite thickness.The effects of the void parameters,the cohesive strength and the thickness of the adhesive layer on the failure mechanisms of the adhesive interface are discussed;Finally,based on the bilinear cohesive model and the Schapery viscoelastic theory,an integral viscoelastic cohesive model is developed.By using the pattern search algorithm and combining with the explicit finite element analysis and the tensile tests of single and double lap adhesive joints,a parameter inversion method for the viscoelastic cohesive model is developed.Combined with super high performance computing,the ABAQUS-PYTHON script language,the explicit finite element module and the MATLAB software are employed to implement the parameter inversion method.The effects of different mesh densities,cohesive strengths and loading rates are also discussed.The research above provides theoretical and technical supports for in-depth analysis and prediction of the failure mechanisms of ture adhesive structures under different loads and environments.The originality of this paper includes two parts:First,for the ductile adhesive structure,the failure of the finite-thickness adhesive layer and the interface debonding between the interface layer and the solid layer are fully considered,while the current research at home and abroad focused only on the analysis.and application of the zero-thickness cohesive model;Second,some scholars developed viscoelastic cohesive models by introducing the series and parallel combination between several springs and dashpots,but most of them are in differential forms.This paper develops the integral-form viscoelastic cohesive model and the parameter inversion method.Although these viscoelastic models are essentially equivalent,the physical meanings are different.The integral viscoelastic cohesive model represents the time delay effect between interface traction and displacement jump more distinctly.