A fatigue life prediction approach for porosity defect-induced failures in directed energy deposited Ti-6Al-4V considering crack growth environment

Abstract

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.