Brittle fracture initiation characteristics of manual arc and submerged arc welded joints of 60 kg/mm² high strength thick steel plates were investigated by testing wide plate tension specimen with angular distortion and with a surface notch. The test results were analized by fracture mechanics. The results are summarized as follows.:
(1) The lowest V-charpy impact toughness in the weld zone was obtained at the weld fusion line.
(2) At temperatures above 0°C, the fracture initiation stress obtained from the wide plate specimens was not so lower than the yield strength of the steel.
At lower temperatures, however, the fracture initiation stress was greatly lowered. The decrease was particularly remarkable below -10°C in transversely welded W(1) joints and below -5°C in T-type welded W(1)+W(2) joints and also repair welded joints with some residual stress.
(3) The fracture toughness was calculated quantitatively from the fracture initiation stress. The values calculated in the transversely welded W(1) joints, the T-type welded joints and the repair welded joints decrease in a single band as a function of temperature.
(4) The fracture toughness in the weld fusion line of the welded joints for 60 kg/mm² high strength thick steel is high value above 0°C, but it is remakably lowered in the temperature range of -5-20°C and shows larger dependency on temperature than that for, 80 kg/mm² high strength thick steel. And this steel does not show the embrittlement by stress relief annealing. Therefore, there is little difference in frecture toughness between as-welded spcimen, stress relieved specimen and repair welded specimen.
(5) The fracture toughness values of the manual arc welded metal and of the submerged arc weld fusion line are almost equal to that of the manual arc weld fusion line.

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In former reports the authors described on many mild steels and Mn-Si type high tensile steels, of which tensile strengths are below about 57kg/mm². In the present report, the authors describe on high tensile steels YF and YG which have the tensile strength of about 60 kg/mm² and contain small amounts of Ni, Cr, Ti and V besides Mn and Si, on the heat-treated Mn-Si type steel "2HA" which has been water-quenched and tempered and has tensile sterngth of about 60 kg/mm², and furthermore on a heattreated steel "DT" made on trial, which has the tensile strength of about 90kg/mm² and containing small amounts of various alloying elements and is analogous to the steel T-1 in U.S.A.
(1) On the non-notch bending tests of the steels as received, the steel of which tensile strength is higher, has generally the larger absorbed energy as in the report 5.
(2) On the specimens which have been subjected to thermal cycles of maximum heating temperature 1350°C, the bended angle by impact and slow bending becomes lower in the case of shorter cooling times for all steels used. The absorbed energy by impact bending test at 0°C tends to become higher with the decrease of cooling time for all steels used.
(3) The bad effects of carbon on the non-notch bending test are relaxed by addition of a small amount of suitable alloying elements.
(4) The reduction of area by tensile test began to lower in the case of steel specimen cooled with about C'f cooling time, but the elongation began to lower from the case of a longer cooling time and lowered slowly with the decrease of cooling time to about 8% at the neighbourhood of C'f.

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断層における岩石の変形機構は，深さ(温度・封圧)により異なる。浅部は低温であるため，地震を伴う

変形が卓越する。変形領域では、せん断強度は Byerlee の法則に従い、深くなるほど封圧の増加により大きくなる。一方で，深部では温度が上昇することにより、地震を伴わない塑性変形が卓越する。塑性変形領域ではせん断強度は流動則に従い、温度が高いほど小さくなる。この変形機構の入れ替わりが-塑性遷移である。-塑性遷移が起こる深さ領域(-塑性遷移領域)は領域の下限部にあたり、せん断強度が高くなるため、しばしば巨大地震の震源となる。本研究では-塑性遷移領域の条件下でせん断実験を行い、回収試料の微細構造から変形によるせん断歪量と塑性変形によるせん断歪の割合の変化を見積もることを試みる。　せん断実験は Griggs 型固体圧式高温高圧三軸変形試験機(Griggs 型試験機)を用いて行い、試料には地殻内の断層や沈み込み帯プレート境界を想定し、大陸地殻および海洋堆積物の主要鉱物である石英の多結晶体を用いた。実験条件は封圧 1000 MPa、せん断歪速度 2.5*10-4 /s の一定条件にして、温度条件を石英の-塑性遷移領域を含むとされる 400-1000 °Cの範囲 とした。実験中には力学データを測定し、回収した試料は薄片に加工し、画像解析プログラムを用いて解析した。画像解析では、粒子の長軸方向の角度、試料全体の歪量、R₁面の角度を測定し、これらからNoda(2021)のモデルを用いて全体の歪に占める塑性変形による歪量の割合を算出した。 　実験の結果、力学データでは400-700 ℃で摩擦則に従う強度を，800 -1000 ℃で流動則に従う塑性強度を示し，-塑性遷移領域は700-800℃であるように見受けられた。また、微細構造からの計算の結果、Noda(2021)のモデルで求められた塑性変形による歪量の割合は，高温領域では力学データと同程度の結果が得られた。しかし、低温領域では、微細構造からの計算では400℃でも塑性変形の割合が60％程度という力学データと矛盾した結果が得られた。このような結果になった原因としてNoda(2021)のモデルが長軸方向とアスペクト比の初期のばらつきを考慮してないこと、y面での滑りの寄与が大きいことがあげられる。本発表ではそれらについて吟味していく。

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Effects of residual stress and repair welding on the brittle fracture initiation in weld fusion line of the manual arc welded joints for 80kg/mm² high strength steel were investigated using the wide plate tension test specimen with an angular distortion and a surface notch. And the results were analyzed by application of the fracture mechanics. The main results are summarized as follow:
(1) The maximum residual stress of T-shaped welded joints is about 50kg/mm² in the direction of weld line. That of repair weld zone is about 35kg/mm² in the direction of right angle with weld line and is about 50kg/mm² in the direction of weld line.
(2) The brittle fracture strength of the T-shaped welded wide plate specimen with a surface notch, is lower than that of the transversely welded W(1) specimen. The test results with the specimen in which residual stress is superimposed can be adjusted quantitatively by taking stress intensity factor K_R of Eq. (4) into consideration. The fracture toughness obtained by the above procedure agrees with that of a transversely welded W(1) specimen without residual stress.
(3) The fracture toughness of the repair welded wide plate specimen almost equals to the minimum values of stress relieved welded joints when the notch tip is heated to the temperature range, 300-550°C, in repair welding. Therefore, the fracture stress of the repair welded joints is extremely lowered if defects remain in the fusion line. But it is as high as the fracture stress of the transversely welded W(1) joints as welded, when the maximum temperature of notch tip is below 250°C in repair welding.
(4) It is important that the maximum heat temperature of fusion line is below 250°C in repair welding. Removal ofhydrogen by heating is effective, but its temperature should be below 250°C.
(5) We calculated relation between fracture stress and crack length of T-shaped welded joint and repair weld zone for welded structure.

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In order to evaluate safety of the structures, establishment of the method of fracture strength assessment with high accuracy is needed. In England, two widely applied methods for fracture assessment have been developed, namely, BSI PD6493 and CEGB R6 approach. The BSI procedure is now under revision, so that it can take into account the benefit of CEGB R6 approach and the recent advancement of fracture mechanics. The CEGB R6 approach has lately attracted considerable attention. The notable feature of it is that the approach interpolates between two criteria, that is, LEFM failure and plastic collapse.
On the other hand, the fracture assessment method based on the CTOD design curve approach has been developed in Japan, that is, WES standard 2805. This standard adopted an elastic-plastic fracture parameter CTOD as a judgement criterion, because brittle fracture may occur from defects at structural stress concentration where plastic size generally seems to be greater than defect size.
This paper describes the main points of the above three fracture assessment methods. Although these three methods alike are based on the elastic-plastic fracture mechanics, it seems that the methods of BSI and R6 are quite different from WES approach in appearance. The purpose of this paper, therefore, is to clarify the relation of these methods. Recent numerical and experimental studies on fracture strength assessment are also presented.

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