首先对铝合金钻杆进行了不同应力幅值的旋转弯曲疲劳试验，然后采用扫描电镜观察其断口微观形貌特征。结果表明：在170 MPa低应力下，铝合金钻杆的疲劳断口可分为疲劳源区、裂纹扩展区及瞬断区3个区域，微裂纹在试样次表面相界处萌生，裂纹稳态扩展区面积较大，占60%以上；在290 MPa高应力下，铝合金钻杆的疲劳断口上有多个疲劳源区，微裂纹于试样表面或近表面位置形核，裂纹扩展区面积仅占15%～30%。对于同一试样，随着裂纹的扩展，断口上疲劳滑移台阶变宽，疲劳条带逐渐清晰；对于不同试样，随着应力的提高，裂纹扩展速率增大，疲劳条带间距变宽，更易出现二次疲劳裂纹，断口上裂纹扩展区面积明显减小，相应的疲劳寿命也大幅降低。

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.