Numerical Analysis of Effect of Cohesive Parameters on Mixed-Mode Failure of Double-Scarf Adhesive Joint Subjected to Uniaxial Tensile Loadings

In the present study, the effects of cohesive parameters on the mixed-mode failure of double-scarf adhesive joint (DSAJ) subjected to uniaxial tensile loadings were examined and discussed numerically. For DSAJ with no perpendicular or parallel with the external loading direction, complex stress state (mixture of tensile and shear stresses) occurs at the adhesive interface. In addition, adhesive joint failure, which is a gradually process rather than a sudden transition, is accompanied by energy dissipates gradually at the crack tip. Correspondingly, cohesive zone model (CZM) coupled with finite element method (FEM) was implemented to verify the mechanism of crack from initiation to the complete failure. As the constitutive relation of the adhesive layer, the traction-separation (T-S) law determines the interface damage evolution. Additionally, the shape of T-S curves in mode I and mode II are crucially decided by the cohesive strengths and critical fracture energies in each mode, respectively. Firstly, the non-dimensional-normalized form of ultimate tensile loading of DSAJ was obtained using dimensional analysis. Then, three cases of cohesive parameters (case of constant anisotropy extent & case of constant critical fracture energy in each mode & case of constant cohesive strength in each mode) according to the non-dimensional-normalized form of adhesive properties were designed. Two types adhesives (brittle and ductile) were chosen to examine the effects of adhesive properties on the failure of DSAJ in this study. To avoid the influence of the geometries on DSAJ mechanical behaviors, the thickness of the adhesive layer and the scarf angle θ were held constantly, respectively. In numerical calculations, the change trends of the ultimate tensile loading (F_u), the failure energy (E_f) and the damage level (D) corresponding to F_u with respect to the cohesive parameters were discussed. It can be observed the cohesive strengths in mode I and mode II codetermine F_u of DSAJ with unequal rates. Moreover, E_f of DSAJ, which is the necessary energy for the joint failure, is governed by the critical fracture energies in mode I and mode II with different contributions. Besides, it also obtained that the evolutions of D corresponding to F_u of DSAJ with brittle and ductile adhesives are certain different. Generally, D of DSAJ with brittle adhesive is higher and more uneven than that of DSAJ with ductile adhesive. Accordingly, it can be concluded that DSAJ with brittle adhesive has lower ability to distribute the loading over a smaller cohesive zone with less uniform distribution. In addition, the numerical results revealed that with the increment of ratio in each case set in this paper, D of DSAJ does not rise obviously.