Parametric Modeling And Analysis On Composite Adhsively Bonded Joints

Composite adhesively bonding technique has been becoming widely applied in aerospace and other engineering fields.The mechanical properties of the bonding structure are closely related to the reliability of the overlapping area.Therefore,it is particularly important to establish an efficient and accurate theoretical analysis framework from the static to the dynamic mechanical behaviors of the joining structures.Aiming at this goal,the following works have been carried out.(1)A research was conducted to analyze the static mechanical behaviors of the adhesively bonded double-lap joints with stepped outer adherend,subjected to tensile loading.By extending the typical shear-lag model,the explicit solutions for adhesive interfacial shear and normal stresses are derived,with considerations of shear deformation in the adherends and elastic-perfect plastic behavior in the adhesive layer.Applying the proposed analytical model,one can quickly obtain the joint adhesive interfacial stress distributions under various loadings.For the joints with different materials and geometric parameters,a comparative analysis of the results of the finite element analysis and that from the proposed model was carried out.The results show that the model can efficiently and accurately describe the adhesive stress distribution of the stepped double-lap joint.Furthermore,the present method is applicable to implement parametric analysis on the adhesively bonded double-lap joint,finding the correlation between the geometrical and material parameters such as the adhesive thickness,the lap length with the mechanical properties of the joint under tensile loading.(2)An experimental study on the tensile load carrying capacity of a typical composite bonded repair structure was carried out.First of all,the component-level experiments including composite double-lap joints,stepped scarf joints and tapered scarf joints were conducted under unidirectional tensile loads.The properties including tensile stiffness,strength,failure modes and strain monitoring on critical points of specimen surface were investigated one by one,to assess the effects of the bonding area on the properties of double-lap joints,step quantity and the addition of extra plies on that of stepped-scarf joints,scarf angle and addition of extra plies on that of tapered-scarf joints.Then the structural-level plate repair experiment was conducted,finding that component-level bonding structure tests can be equivalent to the corresponding structure-level bonding structure tests to some extent.(3)Following the existing analytical framework of double-lap joint,the analytical models for adhesively bonded stepped-scarf joint and tapered-scarf joint were further derived,and hence a user-friendly computerized analysis and design tool was developed.Long-overlapping and short-overlapping double lap joint tests were conducted to estimate the critical parameters of the adhesive layer including elastic shear stress limit and plastic shear strain limit.These two estimated parameters together with other material and geometric parameters of the repaired structure were input into the developed software to predict the tensile stiffness and strength of those two types of composite stepped-scarf repaired specimens and two types of composite tapered-scarf repaired specimens,which afterwards were compared to those corresponding test results.The comparison results show that the analytical model can accurately predict the tensile properties(including tensile stiffness,tensile strength and failure mode)of stepped-scarf repaired joints with various steps and extra ply quantity,as well as tapered-scarf repaired joints with different scarf angle and extra ply quantity.(4)An investigation was carried out to study the mechanical characteristics of double lap joints subjected to harmonic axial load.On the basis of the existing framework in the prior works,a modified analytical model with closed-form solution was proposed to analyze the dynamic responses of adhesive interfacial shear and peel stresses subjected to harmonic axial load.By involving the inertia effect of the joint,the expected governing differential equations can be established,and the expressions of adhesive interfacial shear and peeling stresses can hence be presented.The proposed analytical model was validated by the corresponding finite element analysis under four different loading conditions.The parametric study based on the proposed model was further implemented to assess the effect of some geometric parameters and loading conditions on the dynamic response of adhesive interfacial stresses.(5)To study the vibration behaviors of adhesively bonded lap joints,a layerwise plate finite element method was proposed to model the lap joint,which can easily transform a spatial finite element modeling of such multilayer structures to a 2D type structure.According to the joint structure,some additional virtual boundary conditions are imposed to make the lap joint equivalent to a special sandwich plate,by which the prosed layerwise finite element can be derived.The applicability of the present modeling method was verified through comparison with the numerical data obtained by traditional solid finite element.Finally,a parametric study following the proposed method was also conducted to evaluate the effect of the adhesive loss factor,the overlap area,the adhesive thickness,the step height ratio and step number on the modal frequency and its corresponding modal loss factor.