Evaluation of fatigue characteristics for adhesively-bonded composite stepped lap joint

The stepped lap joint for composite structure were manufactured and tested under static and fatigue tensile load. With test results, it was found that crack was initiated at the end of overlap and propagated through the delamination of composite adherend. The average tensile load of the joint increased with the increasing of the number of step and edge angle of adherend, while the peak shear stress at the end of overlap decreased. But the average tensile load did not increase and was converged when the ratio value of joint length against the number of step was less than 6. The fatigue endurance limit of stepped joint was found 30% of static tensile load regardless with the geometry of specimen but the fatigue life of each load level increased significantly with the increasing of the number of step to exceed 6, the ratio value of joint length against the number of step, while the fatigue life decreased significantly when the ratio was less than 6. The ratio 6 was the critical value to be able to increase the static tensile load effectively with the increasing of step number for each joint length. The critical crack propagation distance was observed 5–7 mm from the end of overlap.