PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 312–319 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

313

2

Nomenclature a

half crack-length hydrogen content

C S √ √ √ f

area

square root of the area of a defect or crack

area 0, FG area 0, CG

square root of the area of a crack formed in the matrix of material FG square root of the area of a crack formed in the matrix of material CG

d a /d N

fatigue crack-growth rate

test frequency Vickers hardness number of cycles

HV

N

N f

number of cycles to failure

R

stress ratio

Δ K

stress intensity factor range

Δ Κ th Δ Κ th,lc

threshold stress intensity factor range

threshold stress intensity factor range of a large crack

σ w

fatigue limit

1. Introduction

The nickel-based superalloy 718, also widely known as Alloy 718 or Inconel 718, is a precipitation-hardened alloy developed for use in a high-temperature environment. One of the crucial factors in determining the strength of this material is its heat treatment condition, since it may lead to variations in microstructural morphologies including grain size, as well as the type, shape and dimensions of precipitates. Due to its excellent strength and corrosion resistance at very high temperatures, in addition to its exceptional mechanical properties at cryogenic temperatures, this alloy is used extensively in the fabrication of rocket engine parts, primarily turbo pumps. In an attempt to reduce the manufacturing costs of using Alloy 718 in rocket engine components, the application of additive manufacturing (AM) has recently gained attention. As such, numerous studies have been undertaken in the past few years with a view to understanding the mechanical and fatigue properties of AMed Alloy 718. For example, Yadollahi et al. (2018) conducted fatigue tests on two types of AMed Alloy 718, both with and without defects. They reported that specimen fatigue strength was greatly affected by the existence of defects and also by the surface finishing conditions of specimens. Yamashita et al. (2018) also performed fatigue tests on AMed Alloy 718, successfully evaluating fatigue strength by applying the square root area ( √ area ) parameter model proposed by Murakami and Endo (1986). Nonetheless, the applicability of the model to this alloy has not been sufficiently addressed, especially insofar as the effects of factors such as microstructural morphology and testing environment are concerned. In cases where the fatigue strength of a material is strongly dependent on its microstructural features (as for Alloy 718), it is crucial to elucidate the impact of such factors on the crack-growth threshold. Moreover, for applications in components used in a hydrogen environment, the influence of hydrogen on the fatigue properties of the material should also be investigated. However, very little research has been focused on the effect of hydrogen on the fatigue limit of this alloy, although a few studies have examined the fatigue limits and crack-growth thresholds in other materials such as austenitic stainless steels (Matsuoka et al. (2016)), carbon steels (Ogawa et al. (2018)) and low-carbon steels (Ogawa et al. (2017)). For the previous reasons, this study aimed to conduct a comprehensive evaluation of the fatigue limit of defect-bearing Alloy 718, as well as an assessment of the impact of hydrogen on small-crack behavior in the alloy.