Porosity defects are the main cause of the fatigue failures of directed energy deposited Ti-6Al-4V. Porosity defect-induced cracks initiate in pseudo-vacuum and switch to air upon reaching the specimen surface. In view of this failure characteristic, this study establishes a fatigue life prediction approach based on the fracture mechanics framework considering crack growth environment. Firstly, high cycle fatigue tests at room temperature under atmospheric conditions and fracture surface observation were carried out to obtain the fatigue lives, key defect parameters and crack initiation types. Then, long and small crack growth tests in air were conducted, and crack growth rate curve in air was fitted by the Hartman-Schijve equation variant. Next, the crack growth data in the fracture surfaces were retrieved by the marker load method, which were compared with the crack growth data in vacuum in the existing literature to determine the long crack growth data in pseudo-vacuum. Small crack growth data in pseudo-vacuum were determined by inverse method. And thus, the crack growth rate curve in pseudo-vacuum was established. Finally, a crack length criterion for environmental transition was proposed to segmentally predict the fatigue lives which were compared with the experimental lives to verify the effectiveness of the approach.

中文翻译：

---

考虑裂纹扩展环境的定向能沉积 Ti-6Al-4V 中孔隙缺陷引起的失效的疲劳寿命预测方法

孔隙缺陷是定向能沉积Ti-6Al-4V疲劳失效的主要原因。孔隙缺陷引起的裂纹在伪真空中开始，并在到达样品表面后转变为空气。针对这种失效特征，本研究建立了一种基于考虑裂纹扩展环境的断裂力学框架的疲劳寿命预测方法。首先，进行大气条件下的室温高周疲劳试验和断口观察，获得疲劳寿命、关键缺陷参数和裂纹萌生类型。然后进行空气中长、小裂纹扩展试验，并利用Hartman-Schijve方程变体拟合空气中裂纹扩展速率曲线。接下来，通过标记载荷法检索断口裂纹扩展数据，与现有文献中真空裂纹扩展数据进行比较，确定拟真空下长裂纹扩展数据。伪真空中小裂纹扩展数据采用反演法测定。由此建立了拟真空裂纹扩展速率曲线。最后，提出了环境转变裂纹长度准则来分段预测疲劳寿命，并与实验寿命进行比较，验证了该方法的有效性。