基于数字图像相关技术的SLM-IN718细观表面裂纹扩展机制

Mesoscopic Surface Crack Propagation Mechanism of SLM-IN718 Based on Digital Image Correlation Technology

摘要:

开展了室温下不同应力比(R = 0.1及R = -1)的原位疲劳试验,并结合数字图像相关技术(DIC)分析了选区激光熔化IN718镍基高温合金(SLM-IN718)的细观超高周表面裂纹扩展机制。结果表明：首先,DIC分析表明SLM-IN718受载时裂纹尖端出现了类似于蝴蝶形的塑性应变区,这与采用Von Mises屈服准则相一致；其次,分析裂尖前方应变场特征、位移场特征,确定SLM-IN718存在裂纹闭合效应,并对其裂纹闭合效应进行评估,对SLM-IN718而言,R = 0.1与R = 0条件下载荷分别达到最大载荷的53%与29%时裂纹张开；此外,建立了虑及裂纹闭合效应的裂尖塑性区尺寸评估模型,计算值与实测值一致性较好；最后,结合DIC分析结果探讨了SLM-IN718低应力条件下的表面裂纹扩展机制。

Abstract:

Combined with digital image correlation (DIC) technique, in-situ fatigue tests with different stress ratios (R = 0.1 and R = -1) at room temperature were carried out to analyze the mesoscopic surface crack propagation mechanism of selected laser melted Ni-based superalloy (SLM-IN718) in the very high cycle fatigue regime. The results show that: firstly, the DIC analysis results indicated that a plastic strain zone, similar to a butterfly shape, appeared at the crack tip when SLM-IN718 was loaded, which is consistent with the adoption of the Von Mises yield criterion; Secondly, the strain field characteristics and displacement field characteristics in front of the crack tip were analyzed to determine the existence of crack closure effect in SLM-IN718. The crack closure effect was also evaluated, i.e., for SLM-IN718, crack opened when the load reached 53% and 29% of the maximum load under R = 0.1 and R = 0, respectively; In addition, a considered the crack closure effect model was developed to evaluate the size of the plastic zone in front of the crack tip. The calculated values were in good agreement with the measured values; Finally, the surface crack propagation mechanism of SLM-IN718 under low stress conditions was suggested based on DIC analysis results.