Host: The Japan Society of Mechanical Engineers
Name : [in Japanese]
Date : December 22, 2018 - December 24, 2018
Titanium alloys have been used for jet engine parts because of their excellent strength-to-weight ratio. However, the phenomenon of dwell sensitive fatigue in titanium alloys have been a concern to the aviation industries for several decades. A significant amount of research has been conducted during the intervening years to understand the fundamental deformation mechanisms controlling dwell behavior. In this work, we performed the fatigue test under load control with a triangular waveform, which includes 10-min displacement holding at the peak load for only one cycle at ΔK = 25, 30, 40, 50 MPa√m, namely stress holding of four times. From the observation of fracture surface with a focus on striation, we proposed that the dwell behavior is controlled by two mechanisms: (1) increase of crack growth rate during displacement holding (2) increase of crack growth rate in several cycles after displacement holding. In terms of mechanism (1), we obtained digital image correlation-based strain map of in-situ fatigue test to investigate the relationship of crack growth and crack tip deformation. We suggested that mechanism (1) originates from the blunting of crack tip during displacement holding. Moreover, we observed a planar array of dislocations in the failed specimen by electron channeling contrast imaging. This dislocation structure is important to understand why titanium alloys are sensitive for dwell fatigue. In terms of mechanism (2), we assumed that the reduction of crack closure effect or softening of the material causes the accelerated crack growth in several cycles after displacement holding.
