Formation Mechanism of Specific Fracture Surface Region in the Sub-Surface Fracture of Titanium Alloy

Abstract:

In Ti–6Al–4V alloy, fatigue properties have been widely investigated, and the origin of fatigue fracture is usually at the surface in the high stress and lower fatigue life region, whereas in low stress and longer fatigue lifetimes origins are generally sub-surface in nature. Very high cycle fatigue tests were conducted, and observation of fracture surfaces revealed that a unique fine concave and convex agglutinate (hereinafter called Granular Region) formed on the fracture surface of sub-surface fractures. The granular region was not observed on the fracture surface of surface fractures. To clarify the formation mechanism and process of forming the granular region, which is a unique phenomenon in the very high cycle fatigue, fatigue tests using specimens with an artificial surface defect were conducted in air and vacuum. The fatigue tests were based on the idea that the environment around a sub-surface fatigue crack is a vacuum-like environment. During the tests, fracture surfaces were intentionally contacted in air and vacuum under different loading conditions. Fracture surface observations revealed that repeated contact of the fracture surfaces and a vacuum environment are necessary for the formation of the granular region. A mechanism for the formation of the granular region will be proposed.

• Periodical: Advanced Materials Research (Volumes 891-892)
• Pages: 1436-1441
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.891-892.1436
• Online since: March 2014
• Authors: Hiroyuki Oguma*, Takashi Nakamura
• Keywords: Fracture Surface, Granular Region, Sub-Surface Fracture, Titanium Alloy, Vacuum Environment, Very High Cycle Fatigue (VHCF)

References

[1] H. Oguma, T. Nakamura, Fatigue crack propagation properties of Ti–6Al–4V in vacuum environments, Int. J. of Fatigue 50 (2013) 89–93.

DOI: 10.1016/j.ijfatigue.2012.02.012

[2] H. Oguma and T. Nakamura, The Effect of Stress Ratios on Very High Cycle Properties of Ti-6Al-4V, Key Engineering Materials 261-263 (2004) 1227-1232.

DOI: 10.4028/www.scientific.net/kem.261-263.1227

[3] M. Ueno, T. Nakamura, K. Ookubo and S. Watanabe, A Development of Cyclic Press Method for Nano-Structural Refinement of Metal Surface, Proc. of M&M2012 Annual meeting of Materials and Mechanics Div., The Japan Society of Mechanical Engineers (JSME) CD-ROM (2012).