Chu Sakae

Three-Dimensional Fracture Mechanics Analysis of Pit ormation Mechanism Under Lubricated Rolling-Sliding Contact Loading

The mechanism of so-called arrow headed pit formation on contact surfaces under lubricated rolling-sliding cyclic contact loading was analyzed by fracture mechanics. The stress intensity factors of Modes I, II and III for an arrow-headed crack were calculated by three-dimensional numerical analysis. The analytical results support the hypothesis that an arrow headed crack in pitting phenomenon first propagates in shear mode, and that crack growth by tensile mode follows due to a hydraulic effect on crack faces by the oil that has penetrated into the crack. The analysis predicts that the values of contact pressure and frictional force dominantly determine the final surf ace apex angle of an arrow headed crack. The shape of an arrow headed crack observed under service loading is considered to be useful to estimate the operating conditions in the service loading.

Experimental and Fracture Mechanics Study of the Pit Formation Mechanism Under Repeated Lubricated Rolling-Sliding Contact: Effects of Reversal of Rotation and Change of the Driving Roller

Five rolling contact fatigue tests, Tests (1)-(5) have been conducted. In Tests (1)-(3), when a fatigue crack was initiated on the surface of a follower, the test was halted. Then, in Test (1) the rotating direction was reversed. In Test (2) the follower and driver were interchanged, and in Test (3) the test was continued unchanged. In Test (3) the original crack grew to a pit. In Tests (1) and (2) the original crack immediately stopped propagating. In Tests (4) and (5), mating with a harder roller, a softer roller was used as the follower in Test (4) and as the driver in Test (5). A typical pit occurred in Test (4). In Test (5), surface damage substantially different from a typical pit was generated. Based on these experimental results, a 3-D crack analysis including the effect of frictional force on the contact surface and oil hydraulic pressure on crack surfaces, was conducted to elucidate the mechanisms of pit formation and surface damage in contact fatigue.

Effect of Stress Relief Groove Shape on Fretting Fatigue Strength

The objective of this study is to evaluate the effect of stress relief groove on fretting fatigue strength. Fretting fatigue tests and finite element analyses were done. The shape of groove was controlled by groove radius R and tangential angle θ. The depth of groove was specified by R and θ. Fretting fatigue strength was increased with an increase of θ and then it turned into a decrease. The decrease was caused by the transition of failure mode from fretting fatigue at the contact part to plain fatigue at the groove root. The transition was caused by an increase of stress concentration at the groove root with an increase of the groove depth. Therefore, the maximum improvement of fatigue strength was achieved by the largest θ limited by fatigue strength of the groove root. In the analysis, the groove generates high compressive stress field at the contact edge, where small cracks never propagate. Therefore, assumptions to relieve the contact pressure concentration at the contact edge were taken into the analysis model. The values of stress intensity factor ranges for small cracks introduced near the contact edge were almost the same between grooved and non-groove specimens.

Interpretation of Material Hardness, Stress Ratio, and Crack Size Effects on the ΔKth of Small Cracks Based on Crack Closure Measurement

Fatigue tests were performed on materials containing small cracks to investigate the effects of material hardness, mean stress, and crack size on the threshold stress intensity factor of small cracks. The crack closure measurement on a very small crack was done. Although most of those effects could be explained by the characteristic behavior of crack closure in small cracks, it was also shown that (ΔKeff)th was also affected by crack size. The combination of (ΔKeff)th and the crack closure behaviors caused the peculiar characteristics of ΔKth in small cracks. At an extremely high R region, an unusual decrease in ΔKth was found to occur. The large reduction occurred under the conjunction of three factors: extremely high stress ratio higher than 0.8, shallow crack less than a few tenths of a millimeter, and hard material whose HV is higher than 300. The near-threshold fatigue crack propagation rate could be uniquely evaluated even in such a short crack regime using the effective stress intensity factor with the following modification {ΔKeff − (ΔKeff)th,a}. This expression could be applicable in the short crack regime. ΔKeff still plays a role as the governing parameter for fatigue crack propagation of short crack as short as 0.04 mm.

Effect of hydrogen gas environment on fretting fatigue strength

1 緒 言 本研究の目的は,水素利用機器用材料のフレッティン グ疲労に及ぼす水素ガス環境の影響を明らかにすること である.水素エネルギーの利用はエネルギー問題,環境 問題対策の一つとして期待されている.水素社会の実現 には,水素利用機器の安全性確保が非常に重要であり, 現在この観点から活発な研究がなされている.水素ガス 中では疲労き裂進展速度が大気中に較べて増加する 1)こ と,高圧水素ガス環境下で使用される材料への水素吸蔵 を模擬した水素チャージ材の疲労試験では疲労強度が顕 著に低下する 2)ことなどが報告されており,水素雰囲気中 の疲労特性には未だ研究の余地がある.水素ガス環境中 のフレッティング疲労については 1956年にはすでに文献 3) にみられるが,影響の機構解明と水素利用機器に対する 材料選定などの観点からさらなる研究が不可欠である. 機械・構造物の疲労破壊は部品同士の接合部,接触部 で生じることが多い.そのような場所ではフレッティング をともなうことも多く,機器の強度設計においてフレッ ティング疲労は充分に検討されるべき問題である.水素 利用機器には用途に応じて様々な材料が用いられるため, 各種金属材料について研究が必要とされている.本報で は,このような研究の一環として行った,それぞれ水素 ガス供給配管系と高圧水素タンクなどに使用が想定され るステンレス鋼とアルミニウム合金に関する結果を報告 する. 2 実 験 方 法 2・1 供試材 実験にはオーステナイト系ステンレス鋼 SUS 304溶体 化処理材と耐食性アルミニウム合金 A6061-T6処理材を 用いた.TableIに供試材の化学成分,TableIIに機械 的性質を示す. 2・2 フレッティング疲労試験 フレッティング疲労試験装置を Fig. 1に示す.装置は 2個の接触片,疲労試験片,接触面圧を付与する棒状ば ねからなる.これらを組立てて疲労試験片に振幅一定の 繰返し曲げモーメントを与えると,疲労試験片と接触片 フレッティング疲労に及ぼす水素ガス環境の影響 久保田 祐 信 納 山 尚 樹 笛 田 宗 広 栄 中 近 藤 良 之 笛 田 宗 広 Effect of Hydrogen Gas Environment on Fretting Fatigue