Masaru Sekihara

Fatigue Life Estimation Method Considering Inelastic Behavior of Ni-Based Directionally Solidified Superalloy With Multiple Holes

Low cycle fatigue tests at high temperature were conducted on test specimens with small holes made of a Ni-based directionally solidified superalloy, which are intended as the cooling structures formed in the components in the fossil fuel power plant. The tests included those cases with and without a strain holding process, i.e., fatigue creep interaction (FCI) tests and low cycle fatigue (LCF) tests, respectively. The number of LCF crack initiation cycles of the one- and seven-hole specimens decreased compared to that of the smooth one. The number of FCI crack initiation cycles of a compressive hold case for the seven-hole specimen decreased compared to that of the LCF test, while that of a tensile hold case decreased further. The test results were evaluated based on the inelastic behavior around the center hole of the specimens, where the most serious inelastic strain occurred, using finite element analysis that takes into account the inelastic anisotropy of material properties. The number of crack initiation cycles of the LCF and the compressive FCI tests correlated with the maximum tensile stress around the hole, while that of all the tests correlated with the frequency-modified strain energy. We propose a method for evaluating cyclic inelastic behavior around a hole using cyclic Neuber’s rule for anisotropic materials to simply evaluate the failure life of actual components.© 2012 ASME

304 The Influence of Long Term Operation on Delamination Behavior of Thermal Barrier Coatings under Thermo-mechanical Fatigue.

1 緒 言 燃料価格の上昇および環境問題などから,発電効率の 向上が強く求められている.ガスタービンの高効率化に は高温化が効果的であるものの,クリープ強度に対する 材料の制限から,遠心力が作用するタービン動翼の信頼 性を維持するべく,冷却の強化を図る必要がある. 近年,冷却強化の手法の一つとして,熱伝導率の低い セラミック皮膜を動翼表面に施工することで,内部冷却 翼の温度を低減する熱遮蔽コーティング (TBC, Thermal Barrier Coatings) の適用が主流となりつつある. TBCはすでに運用中の翼に施工することも可能であ り,耐食耐酸化性向上も期待できることから,その適用 範囲は今後さらに拡大することが予想される.しかしな がら,TBCの損傷挙動を定量的に評価するには,まだ不 透明な点も多い.1) 従来 TBCに関する研究は,皮膜の性質に関する内容 とはく離強度に関する内容に大別されてきた.膜質につ いて Fujiiらは,高温曝露に伴う焼結によりセラミック層 の気孔率が減少するとともに熱伝導率が増加する挙動を LMP (Larson-Miller Parameter) を用いて整理する手法を 提案している.2)TomimatsuらはEB-PVD (Electron Beam Enhanced Physical Vapor Deposition) により製膜した TBCを対象として,レーザー光を用いた皮膜内の残留応 力測定手法を提案するとともに,高温曝露に伴い残留応 力の値が増加することを報告している.3)また脇らはセラ ミック層の残留応力を応力開放法により求めるとともに, 繰り返し負荷下での皮膜の変形特性を示している.4), 5) 一方強度については,西野らが TBCの施工により等 温下での低サイクル疲労寿命が低温で短寿命,高温で長 寿命となることを示し,これはボンド層の延性の温度依 存性に起因すると説明している.6)しかしながらガスター ビンの高温部品には,Fig. 1に示すように起動・停止に 熱機械疲労下における熱遮蔽コーティングのはく離挙動 吉 竹 茂 飯 尾 哲 史 船 尾 淳 喜 関 原 傑 有 川 秀 行 市 川 国 弘 Delamination Behavior of Thermal Barrier Coatings under Thermo-Mechanical Fatigue

Influence of Creep and Fatigue Pre-damage on Creep-Crack- and Fatigue-Crack-Growth Rates of CrMoV Rotor Steel

The effect of mechanical degradation in a long-term-operated component on the creep- and fatigue-crack growth rates was studied. Creep-crack-growth tests and fatigue-crack growth tests were performed on creep-pre-strained and cyclic-strained CrMoV rotor-steel specimens.It was found that under constant load, creep-crack growth rates, da/dt, of the creep-pre-damaged specimens were higher than those of virgin specimens. It was also found that under the same stress intensity factor K, da/dt in the case of about 10%-crept and creep-void-induced specimens was increased 5 times, while in the case of 0.8%- and 2.8%-crept specimens, it only increased a little. However, all the data fall in a narrow scatter band in the relationship between creep J integral C* and da/dt.Another result showed that fatigue-crack growth rates, da/dN, of creep- and fatigue-pre-damaged specimens were higher than those of virgin specimens. Under the same stress intensity factor range ΔK, da/dN in an about 10%-crept specimen was increased 20 times. And da/dN in fatigue-pre-damaged specimens were accelerated 10 times. However, the results of the strain-controlled crack growth test fall in a narrow scatter band in the relationship between J integral range ΔJ and da/dN.These results suggest that the creep remaining lives and fatigue remaining lives of the mechanically damaged CrMoV rotor steel can be estimated by using C* & ΔJ of the damaged material and the da/dt and da/dN values of the virgin material.

The Influence of Metallurgical and Mechanical Pre-Damage on Creep-Crack and Fatigue-Crack Growth Rates of CrMoV Steel

The effects of mechanical degradation on the creep- and fatigue-crack growth rates in power plants operated long-term were studied. Creep-crack growth tests and fatigue-crack growth tests were performed using creep-pre-strained and repetitive-strained CrMoV rotor-steel specimens. It was found that the creep-crack growth rates, da/dt, of the creep-pre-damaged specimens were larger than those of virgin specimens under constant load. It was also found that under the same stress intensity factor K, da/dt in the case of about 10%-crept and creep void induced specimens was increased five times, while in the case of 0.8%- and 2.8%-crept specimens, it only increased a little. However, all the data fell in a narrow scatter band in the relationship between C* and da/dt. The effect of long-term operating degradation appeared on the creep- and fatigue-damage under the Slow-Fast waveform. Also the crack density was larger in the damaged specimens compared with the virgin specimens. Other results showed that the fatigue-crack growth rates, da/dN, of creep- and fatigue-pre-damaged specimens were larger than those of virgin specimens. Under the same stress intensity factor range ΔK, da/dN in a specimen with approximately 10%-creep was increased 20 times. Moreover da/dN in fatigue-pre-damaged specimens was accelerated 10 times. However, the results of the strain-controlled crack growth test fell in a narrow scatter band in the relationship between J integral range ΔJ and da/dN. These results suggest that the creep remaining lives and fatigue remaining lives of mechanically damaged CrMoV steel can be estimated using the C* & ΔJ values considering the creep rate and the deformation rate of the pre-damaged materials and the da/dt and da/dN values of the virgin material.Copyright