Fatigue Fracture Characteristic of Aluminum Alloy Drill Pipes after Rotating Bending

Rotary bending fatigue tests were conducted on aluminum alloy drill pipes under different stress amplitude firstly, and then the micro fracture characteristic of the fatigue samples under high and low stress was observed by scanning electron microscope. The results show that:under the low stress of 170 MPa, the fracture surface was mainly divided into three zones as the crack initiation zone, steady crack propagation zone and instantaneous fracture zone; the fatigue cracks initiated on phase boundary of subsurface, and the crack stable propagation zone was large and the area was more than 60%; while under the high stress of 290 MPa, multi crack sources appeared on fracture surface, and the micro cracks formed on surface or subsurface, and the crack propagation zone was only 15%~30%. For the same sample, with the propagation of fatigue cracks, the fatigue glide steps on the fracture surface broadened and the fatigue strips got clear. For different samples, with increase of the stress, crack propagation rate increased and the fatigue glide strip space broadened, and the secondary fatigue cracks were easier to appear. The area of crack propagation zone decreased evidently and the corresponding fatigue life dramatically reduced.