PSI - Issue 19

Motoki Nakane et al. / Procedia Structural Integrity 19 (2019) 284–293 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The fatigue design of nuclear components in Japan follows the JSME codes for Nuclear Power Generation Facilities, Rules on Design and Construction for Nuclear Power Plants (JSME S NC1-2016), and the design fatigue curves prescribed in this codes were originated from previous ASME B&PV Code Sec. Ⅲ . On the other hand, NUREG/CR 6909 (2007) published by US NRC and Argonne National laboratory reports new best-fit fatigue curves and new design fatigue curves in air at ambient temperature based on statistical analysis including latest fatigue data. New design factors such as effect of data scatter, surface finish, material variability and etc. are also investigated in this report. Proposed design fatigue curves and design factors by NUREG/CR-6909 (2007) are prescribed in the design fatigue curves in ASME B&PV Code Sec. Ⅲ (2007 Ed.-2009 Ad.). In order to develop new design fatigue curves and fatigue evaluation methodology for nuclear component materials, Design Fatigue Curve (DFC) Phase 1 and Phase 2 (DFC1/DFC2) subcommittees were launched in the Atomic Energy Committee of The Japan Welding Engineering Society (Asada et al., 2018).The DFC1/DFC2 subcommittees established domestic and international fatigue database of small scale specimens and developed new best-fit curves as a function of ultimate tensile strength of the materials. In addition, Smith-Watson-Topper approach is adopted as a mean stress correction method. One of the essential design factors for DFC is surface finish effect, and lathe machining finish effect on fatigue strength of materials in air have been widely studied (Siebel et al., 1957 and JSME Data book). However, disk grinding is also often used to repair the scratches of the surface of the nuclear components and also used to weld toe finish to mitigate undesirable stress concentration, and Le Duff et al. (2009) had studied grinding finish effect on low cycle fatigue behavior of austenitic stainless steel in a pressurized water reactor environment. This study investigates the effect of disk grinding surface finish on fatigue strength of the nuclear component materials in air at ambient temperature for relatively high cycle regime. Fully reversed tension-compression strain controlled fatigue tests of disk grinding finished austenitic stainless steel SUS316L plate were conducted at RT in air. Parallel, perpendicular and random disk grinding directions against to the cyclic loading direction, are considered in this study, and fatigue test results are compared to S-N curves obtained by emery polishing and lathe machining finished specimens. Metallographic observations and some measurements such as roughness, hardness and residual stress of the surface were carried out to discuss the mechanism of fatigue strength reduction due to the disk grinding. In addition, this study discusses the effect of stress concentration and configuration of the micro grooves introduced on the surface of the material by disk grinding are discussed.

Nomenclature DFC

Design Fatigue Curve Finite Element Analysis Japanese Industrial Standards fatigue life (cycles to failure) maximum height roughness

FEA

JIS K sf

Surface finish effect factor based on fatigue test results

N f R z

elongation

  ta

total strain amplitude

proof strength

 0.2

ultimate tensile strength at ambient temperature

 u