Multiaxial Fatigue Life Evaluation of Tubular Adhesively Bonded Joints

Adhesively bonded plastic tubes and composite shafts are the only viable method for joining these structures. These structures are often subjected to complex cyclic loadings. The failure of these tubular joints not only depends on the applied loads, but it also depends on the tube geometry, material properties of adhesive and tubes, and defects in the joint. The shear stress distribution in the tubular joints is obtained for joints under axial and torsional loadings using the shear lag model. Under axial loading the adhesive is assumed to carry only shear stress and adherend to carry only axial load. However, the model considers the variation of the shear stress across the adhesive thickness. The effect of a void on the maximum shear stress is obtained. A nondimensional θ parameter is defined and it is shown that the shear stress distribution not only depends on the value of θ but it also depends on cross sectional geometry of the tubes. For tubes with equal cross sectional area, the shear stress distribution along the bonded area is almost symmetric. For tubular joints with θ equal or greater than 6.7, a central symmetrical annular void with a size of at least 50% of the overlap length has little effect on the maximum shear stress and thus the failure load. The shear stress under torsional loading is obtained by assuming the adhesive to shear in the circumferential direction only and neglecting its other deformations. The tubes are assumed to shear in the axial direction. The analysis considers the variation of the shear stress across the adhesive thickness. As in the case of the axial loading, a new nondimensional parameter, θ, for tubes under torsion is defined. The results show that the shear stress in the bonded area not only depends on the θ value, but also depends on the polar moment of inertia, J1 and J2 of the tubes. For tubes with equal cross sectional area, J1 and J2 are significantly different, thus a non symmetrical shear stress distribution is developed along the bonded area. The effect of annular voids on the shear stress distribution is evaluated for different void sizes and θ values. The failure locus of adhesively bonded tubular specimens under axial, torsional and combined axial and torsional loadings is obtained. Based on these results a damage model for the tubular joints under combined axial and torsional cyclic loading is proposed. It is shown that this model can predict the fatigue life of the tubular joints reasonably well.