Effect of Autofrettage on Fatigue Crack Growth Rate for High Pressure Reactors For high pressure reactor vessels such as for polyethylene production,

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1 Effect of Autofrettage on Fatigue Crack Growth Rate for High Pressure Reactors For high pressure reactor vessels such as for polyethylene production, autof rettage is used extensively to have the fatigue life elongated. In this study, to evaluate the material degradation based on the autof rettaged strain, the repeated internal pressure fatigue tests were conducted using the actual reactors, where the effect of autof rettage on the fatigue crack growth rate was investigated. (1) Fatigue crack growth rate of the autof rettaged cylinder is smaller than that of the non-autof rettaged one, especially near the bore surface. (2) Autof rettaged stain near the bore surface does not affect the material degradation, which means that the delay of fatigue crack growth rate is mainly caused by the highly compressive residual stress at the bore surface. (3) The stress intensity factor expression of an autof rettaged thick cylinder was developed successfully which is based on the use of the effective stress intensity factor, Kef, f, considering the highly compressive residual stress.

2 Fig. 1 Configuration of specimens used for repeated internal pressure tests. Table 1 Chemical compositions of the material used for repeated internal pressure tests. Table 2 Mechanical properties of the material used for repeated internal pressure tests.

3 Fig. 2 Residual stress distribution measured by Sachs' method for the autofrettaged cylinders. Table 3 Chemical compositions of the material with prestrain used for fatigue crack growth rate. Table 4 Mechanical properties of the as-received material before prestrain used for fatigue crack growth rate.

38 Fig. 3 超高圧容器の疲労き裂進展速度におよぼす自緊の影響 Configuration of the for prestraining するように配慮し,Fig.3のしたここで,予 3%/min.で 5T-CT試 tensile specimen used of 0.5 T - CT specimens.

4 38 Fig. 3 超高圧容器の疲労き裂進展速度におよぼす自緊の影響 Configuration of the for prestraining するように配慮し,Fig.3のしたここで,予 3%/min.で 5T-CT試 tensile specimen used of 0.5 T - CT specimens. ごとく試験片を採取ひずみ加工時のひずみ速度はある疲労き裂進展速度試験には0. Fig. 4 験片を用い,き裂長さと試験片幅の比 a/ω が0.35か Relationship between pressure and fatigue Change of crack repeated internal life of all test cylinders. ら0.75の範囲で,コンプライアンス法を用いてき裂進展速度の測定を行なった 3.試 3.1内験結果および考察圧疲労強度とき裂進展挙動 Fig.4は実体および小形試験片によって得られた内圧と疲労寿命の関係を示したものである図中には自緊処理を施してない処女円筒試験片の結果も示してあるが,自緊円筒の疲労寿命は明らかに処女円筒に比べて大きく,その傾向は内圧が小さくなるにつれて著しくなっている.このような自緊処理による疲労寿命の向上は,通常の切欠を付さない平滑円筒での結果4)5)よりも顕著である Fig.5は実体試験片に観察されたビーチマークをもとに得られたき裂深さ αとき裂長さ2Cの関係である処女円筒のアスペクト比a/2Cは外表面(き裂深さで40mm)付近まで0.45と一定の Fig. 5 cylinders during configuration of large test the test. 値を示し,疲労き裂は一様な形状を保って進展しし数の関係を示したものであるき裂深さが小さていることがわかる一方,自緊円筒のアスペクい疲労き裂の進展の初期段階においては,自緊円ト比は,深さ約15mmま筒のき裂の進展は処女円筒に比べて著しく遅く応力のたたにa/2C=0.5とで内表面の高い圧縮残留半円の形で進展しているしかし,き裂深さが15mm以いに処女円筒のき裂形状a/2C=0.45に上ではしだ近づく傾向を示している Fig.6は 38 自緊円筒と処女円筒のき裂深さと繰返なっている Fig.7は疲労き裂進展速度におよぼす自緊の影響を調べるたたに,Fig.6のき裂深さと繰返し数の関係から,疲労き裂進展速度da/ dlvを求た,き裂深さaとの関係で示したものである自緊円筒の内表面近傍の疲労き裂進展速度圧力技術第26巻第1号

5 Fig. 6 Relationship between crack depth and number of cycles for large test cylinder. Fig. 7 Relationship between fatigue crack growth rate da/dn and crack depth for large test cylinders.

6 Fig. 8 Comparison between calculated stress intensity factor solution and experimental results for thick-wall cylinder(wall ratio r2/r1=2.3).

8 Fig. 13 Fatigue crack growth rate obtained using the CT specimens with prestrain of 0, 4.0 and 6.0 percent.

9 (1)T. E. Davidson et al., Case Studies in Fract. Mech., AMMRC. (2)H.Ford et al., 72nd National Meeting of AICE (1972).

Fig. 4. Configuration of fatigue test specimen. Table I. Mechanical property of test materials. Table II. Full scale fatigue test conditions and test

Fig. 4. Configuration of fatigue test specimen. Table I. Mechanical property of test materials. Table II. Full scale fatigue test conditions and test (J. Soc. Mat. Sci., Japan), Vol. 52, No. 11, pp. 1351-1356, Nov. 2003 Fatigue Life Prediction of Coiled Tubing by Takanori KATO*, Miyuki YAMAMOTO*, Isao SAWAGUCHI** and Tetsuo YONEZAWA*** Coiled tubings,