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In our earlier study, the authors revealed that the fatigue limit of ductile cast iron (DCI) specimens whose shapes are similar to the welded joint shapes is about three times larger than that of the welded joint specimens. However, since many defects are usually included in the DCI specimens, the fatigue limit of DCI joints decreases with increasing the maximum defect size. In this paper, therefore, the maximum defect size is estimated by using statistics of extremes. Then, the lowest fatigue limit corresponding to the maximum defect size is estimated from the 4 parameter model and compared with the lowest fatigue limit of the welded joint. As a result, it was confirmed that the lowest fatigue limit of the DCI specimens is about twice as large as the welded joint.

ASME Grade 122鋼は，蒸気温度が600℃の超々臨界圧火力発電用ボイラの主要構造材として使われている．この材料の溶接継手のクリープ寿命の低下は，高温長時間使用中に溶接熱影響部に生じるType-IV損傷に起因する．本研究では，電磁超音波共鳴法(EMAR)を用いてASME Gr. 122鋼の溶接継手部のクリープ損傷中の2つの非線形音響特性: 非線形超音波スペクトロスコピー(NRUS）法による共鳴周波数の移動と非線形3波相互作用法により発生した第3の波の振幅から評価し，その2つの非線形音響特性の相関関係とクリープ損傷中の微細組織の変化との関係を明らかした．EMAR法は, 非接触で超音波を送受信できる電磁超音波探触子(EMAT)を共鳴測定に適用した計測法である．使用したEMATは，板厚方向に偏向しながら横波を送受信する体積波横波EMATを用いた．大型平板溶接継手試験片を製作し，600℃，100MPaの条件で，所定の時間までクリープ試験を行い, 中断材を作製した．2つの非線形音響量は，クリープ開始から寿命の約50％まで減少し，その後微増し，寿命の80％過ぎから破断まで急増した．これは細粒HAZ部のクリープボイドの発生・成長に関係する転位の回復，再結晶，再構築による組織変化に起因する. それはSEMやTEM観察による結果から裏付けられた． EMAR法を用いた非線形超音波法は，細粒HAZでのType-IV損傷の捕らえることができる可能性を持っている．, We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints of ASME Grade 122, one of high Cr ferritic heat resisting steels. EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests of thick welded joints specimens were carried out at 873 K, and 100 MPa and interrupted at several time steps. The two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 50% of creep life. After that they rapidly increased from 80% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring and the initiation and growth of creep void, with support from the SEM and TEM observation.

Expansion joints used in road bridges are divided into several types and considered to be low in fatigue durability because they support wheel loads directly and repeatedly. There were some experiences to suffer fatigue cracks at the welded joints between the middle beams and support beams in the modular expansion joints. This fact suggest that fatigue assessment method is needed for the joints. In this study, the fatigue load and fatigue strength have been discussed in order to indicate the fatigue assessment method for the modular expansion joints. The fatigue strength has been determined though fatigue tests on model specimens and FE analyses and fatigue load has been set through stress measurement tests using actual joints and previous vehicle traffic investigation results.

P(論文), The author's studies during the last 5 years are outlined. Contents of the studies are random vibration, nonlinear vibration, reduction of residual stress of welded joints using vibration, drilling ofFRP using vibration, reduction of seismic response using a base isolation device, dynamic characteristics of structures with bolted joints and strength and dynamic characteristics of FRP structures. Those studies contain a simplified estimation method of integral of the mean square valueof the response, reliability of the system subjected to seismic excitations, continuous system with collision, continuous system with nonlinear support, continuous system with elasto-plastic damper, reduction of response using impact damper, reduction of residual stress of welded joints using low cycle vibration and ultrasonic vibration, application of vibration to drilling of FRP, development ofbase isolation device for reduction of seismic response, the damping ratio and the natural frequency of structures with bolted joints, compressive strength, the damping ratio and the natural frequency ofFRP structures.

The safety factor for joints was investigated, which is at the welded joint of the steel structure in consideration of Column-Overdesign-Factor (COF). Considering the rupture of the welded joint in the steel structure, it is difficult to obtain the structural reliability containing brittle failure by general reliability analysis. In order to perform the reliability analysis including the welded joint rupture, these failure modes of the structure were classified to the mutually exclusive events by COF and the welded joint performance. The welded joint strength was predicted by Bayes’ theorem, using the beam strength as a prior distribution and the experiment's results of welded joint strength. As a result, the ratio of failure probability with the welded joint rupture became about 3% of the total failure probability. When it compared to the layer collapse probability, the COF is higher, the ratio of failure probability including the weld joint rupture is increased.

Stress intensity factor due to surface crack propagation was analyzed by using the Influence function method and inherent strain analysis of residual stress fields caused by welding. Initial residual stress in a plate-welded butt-joint was calculated by using inherent strain analysis, and redistributed residual stress and stress intensity factor due to crack propagation were analyzed as changes of structural shape. Stress intensity factor was also calculated by using the influence function database. The stress intensity factor in the residual stress fields due to crack propagation obtained by using inherent strain analysis completely agreed with that obtained by using the Influence function method. The results of the crack propagation analysis were compared with the experimental results of a fatigue test. And it was validated that both the inherent strain analysis and the influence function method were efficient for analyzing stress intensity factor in residual stress fields caused by welding., 溶接残留応力のある応力拡大係数は，影響関数法と固有ひずみ解析法で求めるができる．き裂進展解析の結果と疲労試験の結果と比較することにより影響関数法と固有ひずみ解析法の妥当性を評価する．

H-2ロケットは1994年2月に1号機、8月に2号機そして1995年3月には3号機が打ち上げられ、宇宙輸送手段としての機能は確立されている。このロケットの第1段主エンジンのLE-7には数多くのインコネル-718材溶接継手が使用され製作が行われている。LE-7はエンジン始動時から停止時にわたり厳しい熱応力にさらされ、開発過程で経験した不具合の多くはこの溶接継手部で発生した。このため、不具合の改善およびLE-7の信頼性を向上するための重要課題として、溶接継手に関する特性の向上が挙げられ、また、エンジン設計の見直しが検討された。この様な背景の下、これまでに1228Kと1338Kの温度で溶体化処理されたGTA(ガスタングステンアーク溶接)溶接継手の引張特性を調べた。今回は、組織検査、硬さ検査を通して、1228Kと1338Kの温度範囲内での溶体化処理条件(温度と時間)の影響を調べた。その結果、母材の結晶粒の粗大化は、溶体化処理温度、時間の狭い範囲で急速に起こり、それは時間よりも温度に敏感であること、および溶接部に析出したNi3Nbの溶解は、母材の結晶粒の成長と同時に進行することが明らかになった。また、代表的な溶体化処理条件を選択し、その条件で溶体化処理を施した溶接継手の引張特性を調べた。その結果と前回までに報告した結果とを比較検討した結果、Ni3Nbの連続性を減少させることにより、引張強度の低下を招くことなく溶接継手の延性の向上が可能であることが明らかになった。, The H-2 rockets have been successfully launched three times since its first success in February 1994, and Japan has established its own means of space transportation. The main engine of the H-2 rocket, LE-7, uses many welded Inconel-718 joints. The hot section materials of the LE-7 engine has suffered from severe thermal shock during the start and cutoff sequence, and these welded joints were the cause of many of the accidents during development. Hence, the welded joints were redesigned to increase the reliability of the LE-7 engine. Against this background, the tensile properties of the GTA (Gas Tungsten Arc welding) welded joint with solution treated at 1,228 K and 1,338 K were studied in a previous report. This report shows the effects of solution treatment condition (time and temperature) ranging from 1,228 K to 1,338 K on the microstructure, hardness and tensile properties. The solution treatment condition study shows that the grain growth of the parent metal occurs rapidly in a very narrow window of time and temperature and that these phenomena are more sensitive to temperature than to time. This study also shows that the dissolution of the Ni3Nb phase and the grain growth of the parent metal occur simultaneously. The tensile property test for different solution treatment conditions shows that the appropriate condition increases the ductility without loss of tensile strength by partially dissolving the continuous Ni3Nb phase to a discrete one., 資料番号: AA0000444000, レポート番号: NAL TR-1285

現在、H-2ロケット第1段エンジンとして使用されるLE-7エンジンの開発が進められている。このLE-7エンジンの製造および組立の際には数多くのInconel-718材溶接継手が用いられている。しかし、開発の当初から発生している不具合は、この溶接継手部に起こっており、そのため、溶接継手特性の向上が重要課題となっている。著者らは、以前、1228Kで溶体化処理された溶接継手の引張特性を調べた。今回は1338Kで溶体化処理を行なった溶接継手の引張試験片について調べ、1228K溶体化処理の結果と比較し、検討を行なった。その結果、溶体化処理温度を1228Kから1338Kに上げることにより、破断位置は溶接部から母材部に変化し、静的荷重下において安定的な破壊形態が得られることが明らかになった。, The LE-7 engine, which will be used as the first stage main engine for the H-2 rocket, has been developed in Japan; however, problems have arisen in connection with the various welded joints. In the previous report, the results of an investigation of the tensile properties of Inconel-718 welded joints, solution annealed at 1228 K, were reported. In this report, those properties, solution annealed at 1338 K, were investigated and compared with the previous results. It was observed that the site of the break in the specimen shifted from weld metal to parent metal as the solution annealing temperature was increased from 1228 K to 1338 K., 資料番号: AA0004210000, レポート番号: NAL TR-1195

現在、日本では、宇宙開発事業団(NASDA)を中心として、H-2ロケット第1段エンジンとして使用されるLE-7の開発が進められている。このLE-7の製造および組立の際には数多くの溶接継手が用いられている。開発の当初よりこれらの溶接継手部において様々な不具合が生じており、溶接継手強度に関し改めて強度特性を把握する必要が生じている。そこで、LE-7を構成する主要材料であるInconel-718材溶接継手の引張試験片を製作し、GHeおよびGH2雰囲気中での引張試験を実施し、常温および高温での溶接継手の強度および水素脆性感受性を調べた。また、その結果を基に、強度とミクロ組織との関係について検討した。その結果、溶接継手は、母材と比較し著しく延性が劣ること、またそれは溶接部にみられるNi3Nbの析出形態に強く依存していることが明らかとなった。, The LE-7 engine which will be used as the first stage main engine for the H-2 rocket has been under development in Japan. This engine has many welded joints that have developed a variety of problems. Therefore, tensile properties and microstructures of Inconel-718 test specimens with welded joints were investigated at various temperatures in high pressure hydrogen and helium environments to study the fundamental characteristics of the welded joints. The results indicated that the ductility of the welded joints was less than that of the parent metal and that the deterioration of ductility was due to the dendrite structure of the Ni3Nb phase., 資料番号: AA0004203000, レポート番号: NAL TR-1188

In this paper, firstly the elasto-plastic finite element analyses of a center cracked plate are performed based on deformation and incremental plasticity theory to investigate the difference between solutions obtained through both theory and some factors determining the path independence of J-integral, and loading condition go an element in the vicinity of crack tip are examined. Secondly, the analysis of a longitudinal bead-welded joint with a center crack is conducted based on incremental theory to discuss the path independence of J-integral and loading condition of elements on phase boundary.

Basic research on the joint performance in the through-thickness direction of welded joints is seldom found, although those practical applications to the assessment of lamellar tearing susceptibility have been rather extensively studied. The present report therefore aims at examining tensile and fatigue properties in the through-thickness direction of the structural steel, SS41, with 40mm in thickness as the fundamental data for the study of the joint performance in that direction, and investigating both properties in that direction of welded joints.

In welded joints with a crack on the phase boundary such as weld bond, the crack is exposed to tension and shear, which leads to mixed mode cracking. Consequently, the crack extension force both perpendicular and parallel to the plane of the crack acts on the cracked body. In this paper, finite element analyses of the J-integral were performed on welded joints with a crack on and near the weld bond. The effects of the crack situation and the strength ratio of weld metal to base metal on the J-values were confirmed through analytical results. Furthermore, it was proved that the J-integral controlled the stress-strain field in the vicinity of the crack tip of welded joints with a crack on bond.

第1章 緒論||第2章 溶接継手を球状黒鉛鋳鉄継手で一体化することによる疲労強度の強化||第3章 最大欠陥を想定した球状黒鉛鋳鉄継手の疲労限度と溶接継手との比較||第4章 球状黒鉛鋳鉄継手の疲労強度におよぼす引張強度の影響||第5章 総括, 九州工業大学博士学位論文 学位記番号：工博甲第486号 学位授与年月日：令和2年3月25日, 令和元年度