Crack initiation and early growth behavior of TC4 titanium alloy under high cycle fatigue and very high cycle fatigue
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The crack initiation and early growth behaviors of a TC4 titanium alloy under high cycle fatigue and very high cycle fatigue were experimentally investigated. The results show that it exhibits the duplex S–N curve characteristics associated with surface and interior failures at a stress ratio of 0.1, while it represents the similar S–N curve characteristics only related to surface failure at a stress ratio of 1. The interior failure is accompanied with the occurrence of facets, granular bright facets (GBFs), and fisheye. Slip-like patterns are observable on the facets easily formed under positive stress ratio. The interior failure process is characterized as (i) occurrence of slip lines on partial a grains under cyclic loading, (ii) initiation and growth of microcracks within some a grains, (iii) coalescence of microcracks and formation of GBF, (iv) stable long crack growth within fisheye, (v) unstable crack growth outside fisheye, and (vi) final momentary fracture.
I. INTRODUCTION
Very high cycle fatigue (VHCF) that is generally recognized as the field of fatigue damage in the life regime larger than 107 cycles has become a subject of growing interest in recent years.1–3 This is motivated from both fundamental and practical considerations. First, unexpected fatigue failures in the VHCF regime are continuously reported in metals, such as titanium alloys,4,5 aluminum alloys,6,7 high strength steels,8,9 cast iron,10 etc., that are assumed to be operating at safe stresses below the traditional fatigue limit corresponding to around 107 cycles. Then, more and more components or parts have to be subjected to cyclic load, and the actual number of loading cycles is much larger than 107 cycles in practical engineering applications.11,12 Thus, these requirements motivate the substantial efforts and thinking, concerning the fundamental fatigue mechanisms of metals in the VHCF regime. The VHCF of metals is always accompanied by the change of failure mode from the surface failure at higher stress level to the subsurface or interior failure at lower stress level.1–5,8–12 Just because of the change in the failure mode, metals sometimes can represent peculiar S–N curve characteristics such as “duplex S–N curve characteristics” or “step-wise S–N curve characteristics”13,14 in the whole life regime involving low cycle, high cycle, and very high cycle. Also, studies have shown that not only face-centered cubic metals6,7 but also some Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.476
body-centered cubic metals5,8,9,13,14 show a decreasing fatigue strength in the VHCF regime. This means that the traditional standards and criteria about body-centered cubic metals often having a fatigue limit after about 107 cycles are inexact. Although the essence of this special S–N curve property in the VHCF regime is not yet well understood, it can conclude that a number of factors such as the size,15 type and distribution16 of metallurgical defects or discontinuities, th
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