CT110连续管低周疲劳循环软化行为

Cyclic softening behaviors of CT110 coiled tubing in low-cycle fatigue

对低周疲劳过程中迟滞回线、循环应力响应曲线、循环应力-应变曲线及应变-寿命曲线的变化进行了分析,使用光学显微镜、聚焦离子束、扫描式电子显微镜及透射电子显微镜从疲劳断口的微观组织、裂纹扩展和断口形貌方面研究了CT110连续管的疲劳循环软化行为。结果表明:CT110连续管的循环曲线并未出现前几次循环周次中应力幅升高的情况,低周疲劳过程为稳定的循环软化过程。当应变幅>0.6%时,塑性变形在延性范围内进行,可供塑性变形消耗的抗塑性变形能减少,循环软化迅速进行;而当应变幅<0.6%时,塑性变形在强度范围内进行,CT110连续管基体内有充足的抗塑性应变能消耗,循环软化的饱和阶段较为稳定。进一步微观结构变化表明:CT110连续管疲劳断口具有特征明显的多裂纹源、疲劳辉纹、轮胎花样和韧窝等形貌,在高应变幅下的断口形貌表现出裂纹源区小、扩展区广和瞬断区小,同时在高应变幅下电子衍射多晶环更为致密连续,晶粒更为细小。CT110连续管的疲劳断裂面上多源裂纹萌生时塑性变形量较大,靠近裂纹源区铁素体晶粒更碎化,而远离裂纹源区的晶粒尺寸更接近原始尺寸。

The changes of hysteresis loop,cyclic stress response curves,cyclic stress-strain curves,and strain-life curves during low-cycle fatigue were analyzed.The fatigue cyclic softening behaviors of CT110 coiled tubing were studied by optical microscope,focused ion beam,scanning electron microscope,and transmission electron microscope in terms of microstructure,crack propagation and fracture morphology.The results show that the cyclic curves of CT110 coiled tubing do not show the increase of stress amplitude in the previous cycles,and the low-cycle fatigue process is a stable cyclic softening process.When the strain amplitude is higher than 0.6%,the plastic deformation is carried out in the ductility range,the anti-plastic deformation energy available for plastic deformation consumption is reduced,and the cyclic softening occurs rapidly.When the strain amplitude is lower than 0.6%,the plastic deformation is carried out within the strength range,there is sufficient anti-plastic strain energy consumption in CT110 coiled tubing matrix,and the saturation stage of cyclic softening is relatively stable.Further microstructure changes show that the fatigue fracture of CT110 coiled tubing has obvious features of multiple crack initiations,fatigue striations,tire patterns and dimples.The fracture morphology with high strain amplitude shows that the crack source area is small,the expansion area is wide and the transient fracture area is small.With high strain amplitude,the electron diffraction polycrystalline ring is more compact and continuous,and the grains are smaller.On the fatigue fracture surface of CT110 coiled tubing,the plastic deformation amount is large during the initation of multi-source cracks,and the ferrite grains near the crack initiation zone are more fragmented,while the grain size far away from the crack initiation zone is closer to the original size.