Study on fatigue behavior of adhesive-bonded butt joint (Effect of unbonded defects at adhesive/adherend interface on fatigue strength)

As a part of our ongoing study to investigate the effect of adhesive defects on fatigue strength of adhesive-bonded joint, a series of fatigue tests were carried out by using adhesive-bonded butt joints with unbonded layer at adhesive/adherend interface under the push-pull fatigue load condition of stress ratio R=-1.0.The specimens used possessed two unbonded portions of nail shape along the adhesive/adherend interface, which were located at the opposite sides of the specimen. To characterize the defect size quantitatively, it was defined by the following two quantities; the unbonded width which was the same as the defect length exposed to the free surface of the specimen, 2r, and the unbonded depth from the free surface, l.The fatigue strength of the adhesive-bonded joint specimens with two kinds of unbonded width (2r=3, 5mm) was discussed in connection with the unbonded depth l. The main results obtained in this study are as follows:(1) For the specimen with 2r=3mm, the fatigue strength decreased with increasing l in the low stress cycle range. But, the effect of l on the fatigue strength diminished with increasing stress cycles.(2) For the specimen with 2r=5mm, the fatigue strength decreased with increasing l irrespective of the number of stress cycles.