Masakatsu Saito

Fatigue strength for stainless steel irradiated by high power laser beam

It is important to investigate the effect of irradiation on the fatigue damage of high heat flux components which directly face a fusion plasma. The materials tested are 316SS and 304SS. The former is the reference first wall plate for the fusion experimental reactor. The specimen was irradiated with a high power laser beam and then a cyclic load was applied to the specimen up to fracture. The laser beam can provide only a heat flux because it is effective as a microwave only. The melt layer surface looks rough and seems to be oxidized. Although there exists no difference in fatigue strength between the irradiated 316SS and 304SS, the fatigue strength for the irradiated specimen is greatly decreased according to the quantity of irradiation. The microscopic observation was made on the section including the melt layer and on the fracture surface. The near melt surface is characterized by a blow hole, solidification cracking, and a columnar structure. This means that the melt layer was quickly cooled by heat loss under atmospheric conditions. In conclusion, the microscopic defects can cause the fatigue strength to be much degraded. Other factors such as surface roughness could be attributed to the reduction of strength.

Crack propagation in first wall by thermal fatigue and creep

The operation of the international thermonuclear experimental reactor (ITER) is assumed to be a pulsed mode, of which the duration time varies up to 1000 s depending on the experimental condition. If the structural material 316 stainless steel (SS316) of the first wall has a defect, its crack propagation behavior will depend on the duration time. This study deals with the crack propagation behavior on SS316 under high thermal load cycles. The high heat flux tests were carried out on the 5-mm thick specimen under four test conditions in order to investigate the creep behavior on the thermal fatigue. Two-dimensional (2-D) thermal-structure analyses including creep effect were performed to simulate the thermal and residual stress distribution around the crack tip. It was concluded that the creep effect increased the crack propagation length induced by thermal fatigue although the compressive stress occurred around the crack tip during high temperature.

Strength of copper alloys in high temperature environment

Abstract The first wall of ITER is expected to be hot isostatic pressing (HIP) bonded structure of copper-alloy/SS316. Firstly, fracture toughness and crack propagation tests were performed on DS-Cu and DS-Cu/SS316 HIP joints at ambient temperature and 573 K T. Yamada, M. Uno, M. Saito, Fall Meeting of the Atomic Energy Society of Japan, vol. I, 1998, p. 187 (in Japanese). J IC values of DS-Cu and DS-Cu/SS316 decreased significantly at 573 K. In crack propagation test, DS-Cu lost its ductility at 573 K. Secondly, we performed fracture toughness tests on CuCrZr and CuCrZr/CuCrZr, CuCrZr/SS316 HIP joints at ambient and 573 K. CuCrZr base metal had higher J IC values than DS-Cu. Concerning CuCrZr/CuCrZr and CuCrZr/SS316 HIP joint, its J IC value decreased to less than that of CuCrZr base metal.

Fracture strengths of HIPed DS-Cu/SS joints for ITER shielding blanket/first wall

Abstract Fracture toughness and crack propagation tests were performed to investigate the effect of HIP temperature and fracture behavior of HIPed DS-Cu/SS joints. Test specimens of DS-Cu/SS HIPed joints were manufactured by bonding flat plates of DS-Cu and SS under HIP temperatures of 980°C, 1030°C and 1050°C. J Q of the joint at HIP temperature of 1050°C was larger than the other two joints. For the crack propagation test, two types of test specimens were prepared. One had a notch along the HIPed interface and the other in DS-Cu and normal to the interface. The crack in the former specimen propagated along the interface. On the other hand, the crack in the latter specimen propagated in the DS-Cu perpendicular to the loading direction, stopped at the interface, and then exfoliated along the HIPed interface. In the fracture tests, the crack was observed propagating in DS-Cu side at approximately 5–10 μm away from the interface.

Crack propagation in first wall of fusion reactor by cyclic thermal stress

Abstract To clarify the crack propagation behavior of first wall materials under high thermal load cycles, cyclic high heat flux loading tests were carried out on SUS316. A specimen with a thickness of 5 mm was used, and the tests were performed under three types of external load conditions in order to investigate the effect of stationary tensile stress, simulating gravitational load on the first wall. To get rid of the effect of creep, the duration time of the beam was limited to 0.5 s in every 30 s high heat flux loading cycle. Two-dimensional thermal and elasto-plastic stress analyses and non-linear fracture mechanics analysis were carried out, and the fatigue crack propagation lengths were evaluated using Ĵ-integral amplitude ΔĴeff. Through this study, it was concluded that crack propagation lengths were decreased by applying stationary moderate tensile load, and that the evaluated crack lengths roughly agreed with experimental ones.

Thermal fatigue crack propagation behaviour of F82H ferritic steel

Abstract This paper presents an issue obtained from thermal fatigue research, which attempts to examine the fusion reactor first wall by fracture mechanics. The research is organised with two different approaches: 1. Studies of the thermal fatigue crack propagation behaviour on notched 5-mm thick plate specimens of ferritic steel F82H (9Cr-1W), compared with 9Cr-1Mo ferritic steel and type 316 stainless steel; 2. Numerical simulations of the stress field caused by thermal loads including fracture mechanics. It is concluded that the stress intensity factor ΔKI is substantial for crack growth while cyclic thermal loading.

Stress analysis around DSCu/SS316 HIP bonded interface

Abstract HIP bonded structure of DSCu/SS316 is proposed for ITER plasma facing components. In 20th SOFT, we reported the fracture strength of HIP bonded interface at ambient temperature. But first wall is at high temperature in operation, we estimate in this study the fracture strength of HIP bonded interface at 473K as a first step of high temperature. The results of fracture strength test indicate that the fracture strength of HIP bonded interface at high temperature is lower than that at ambient temperature. The fracture surfaces of specimens ruptured at ambient temperature and at 473K were observed. Crack surfaces were nearly 10 μm apart from the bonded interface into DSCu side. Scanning electron microscope observation is considered to show the existence of the thin new layer in DSCu region in which the dispersed Al2O3 was diminished. Assuming the thin layer to be composed of Cu, fracture behavior of HIP bonded structure is simulated by three layers model DSCu/Cu/SS316 and the fracture strength is estimated by the stress intensity factor K. The analytical results conclude that crack propagates in DSCu/Cu interface more easily, which is good agreement with fracture surface in experiments.

Numerical analysis on fracture behavior of a single-edge-cracked plate subjected to electromagnetic force

Abstract A strong electromagnetic force due to a plasma disruption may damage the structural integrity of fusion reactor components. Since these components usually consist of thin plates, out-of-plane stress and deformation analyses are important. Besides, some components will inevitably contain cracks under the severe disruption conditions. In this study, first, an eddy current analysis on a single-edge-cracked plate was numerically conducted with the use of the magnetic vector potential method. As a result, the eddy current has a singularity at crack tip and its magnitude gradually increases with increasing crack length. Second, a static elastic bending analysis was conducted under three different mechanical boundary conditions. The neighborhood of the crack tip is deformed in a rather complicated manner, strongly depending on the type of boundary condition. The mode II and mode III stress intensities were significant when compared with mode I stress intensity. According to the definition of equivalent stress intensity, the possibility of crack initiation was discussed for three different boundary conditions.

Crack growth in first wall made of reduced activation ferritic steel by transient creep due to long pulse operation

The long pulse operation is assumed in ITER and future reactor. If the first wall has a defect, the crack may be propagated by cyclic thermal loads. In addition, flattop of more than 300 s during plasma burning is expected in ITER, so the crack propagation behavior will depend on the operation duration period. This study deals with the crack propagation behavior on F82H under high thermal load cycles. The high heat flux tests were performed under three types of duration periods to investigate creep fatigue behavior. To clarify the crack growth mechanism and the effects of transient creep, three-dimensional analyses were performed. It was concluded that the creep effect during the operation duration period enlarges stress intensity factor K in the cooling period and that consequently, the crack propagation length was increased.

Fracture Strength of CuCrZr in High Temperature Environment

ABSTRACT The fracture strength was estimated for copper-alloy CuCrZr, and their HIPed joints with SS316L and CuCrZr, which were the candidate materials consisting of the First Wall of ITER. Fracture toughness and fatigue crack propagation ratio of those materials were superior to those of copper-alloy DS-Cu and its HIPed joint with SS316. It was confirmed that the cracks near the CuCrZr/SS316L HIPed boundary was propagated in copper-alloy along the interface at a distance of about 10μm from the interface. The lost of ductility of all materials caused the decrement of fracture toughness and increment of crack propagation ratio in high temperature (573K).