MULTIAXIAL FATIGUE LIFE EVALUATION OF TUBULAR ADHESIVELY BONDED JOINTS

The only viable method for joining plastic tubes and composite shafts is by bonding them adhesively. These structures are often subjected to complex cyclic loadings. Failure of these tubular joints not only dep ends on the applied loads, but also depends on the tube geometry, mate rial properties of adhesive and tubes, and defects in the joint. The s hear stress distribution in the tubular joints is obtained for joints under axial and torsional loadings using the shear lag model. Under ax ial loading the adhesive is assumed to carry only shear stress and adh erend to carry only axial load. However, the model considers the varia tion of the shear stress across the adhesive thickness. The effect of st void on the maximum shear stress is obtained. A nondimensional thet a(a) parameter is defined and it is shown that the shear stress distri bution not only depends on the value of theta(a) but it also depends o n cross sectional geometry of the tubes. For tubes with equal cross se ctional area, the shear stress distribution along the bonded area is a lmost symmetric. For tubular joints with theta(a) equal or greater tha n 6.7, a centrally 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 dir ection only and neglecting its other deformations. The tubes are assum ed to shear in the axial direction. The analysis considers the variati on of the shear stress across the adhesive thickness. As in the case o f axial loading, a new nondimensional parameter, theta(t), for tubes u nder torsion is defined. The results show that the shear stress in the bonded area not only depends on the theta(t) value, but also depends on the polar moment of inertia, J(1) and J(2), of the tubes. The effec t of annular voids on the shear stress distribution is evaluated for d ifferent void sizes and theta(t) values. The failure locus of adhesive ly bonded tubular specimens under axial, torsional and combined axial and torsional loadings is obtained. Based on these results a damage mo del for the tubular joints under combined axial and torsional cyclic l oading is proposed. It is shown that this model can predict the fatigu e life of the tubular joints reasonably well. (C) 1997 Elsevier Scienc e Limited.