Fatigue limit evaluation on Ni-based superalloy 718 based on shear-mode threshold

2022 Fiscal Year Final Research Report

• Project/Area Number: 21K20407
• Research Category: Grant-in-Aid for Research Activity Start-up
• Allocation Type: Multi-year Fund
• Review Section: 0301:Mechanics of materials, production engineering, design engineering, fluid engineering, thermal engineering, mechanical dynamics, robotics, aerospace engineering, marine and maritime engineering, and related fields
• Research Institution: Fukuoka University (2022); Kyushu University (2021)
• Principal Investigator: Tanaka Yuya 福岡大学, 工学部, 助教 (50904588)
• Project Period (FY): 2021-08-30 – 2023-03-31
• Keywords: Alloy 718 / 破壊力学 / せん断型疲労き裂 / 開口型疲労き裂 / き裂進展駆動力 / き裂進展下限界値 / 疲労限度

Outline of Final Research Achievements

In this study, to establish the comprehensive evaluating method of the fatigue limit of Ni-based superalloy 718 (Alloy 718) via fracture mechanics, (i) a ΔKI-decreasing, (ii) torsional fatigue tests and (iii) tension-compression fatigue tests were carried out. All the tests were performed at the stress ratio, R, of －1. In addition, the elastic stress fields near the vicinity of the crack tip in some loading conditions were analyzed. The present results and the authors’ previous findings indicated that the competition of stress intensity factor ranges associated with the opening- and shear-mode crack growths and the rivalry between the respective threshold conditions dominate the crack arresting mode. Based on the above experimental findings and analyses, a novel strategy for evaluating the crack-arresting mode and fatigue limit of Alloy 718 was proposed.

Free Research Field

材料力学，破壊力学

Academic Significance and Societal Importance of the Research Achievements

本研究では，疲労き裂の開口型およびせん断型進展に要する下限界の駆動力（き裂進展下限界値）の大小関係とき裂先端近傍の応力状態に着目し，Ni基超合金Alloy 718の疲労限度を定量評価する手法を提案した．この知見は，主要構成材料としてNi基超合金が採用されている再利用ロケットブースターの安全保障に大きく貢献するものである．また，き裂進展下限界値の競合関係とき裂先端近傍の応力状態が複合した条件により，材料の疲労限度が支配されていることを定量的に示した研究は前例がなく，本研究は学術的にも意義深いものである．