Tokuo Teramoto

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.

Evaluation of Cutting Process on the Tensile and Fatigue Strength of CFRP Composites

Carbon fiber-reinforced plastics (CFRP) composite is most attractive materials to reduce the weight of transportations. To increase the production volume and the efficiency in the field of CFRP component, fast, highly precise and cost-efficient technologies are required. Although laser cutting meets these requirements, it is not used because of insufficient knowledge about the effect of thermal damage on the material behavior. In this study, the effect of several cutting processes on the static tensile strength and the fatigue strength was evaluated for CFRP consisting of thermoset resin matrix and carbon fibers. The CFRP was cut using two different-type of lasers; a CO2 gas laser and a single-mode fiber laser, an abrasive water-jet and a conventional mechanical tool. The mechanical cutting specimen produced a cut of high quality. The water-jet cutting specimen showed a moderate quality though was seen a trace of abrasive grain. While, the laser cutting specimens clearly showed a heat-affected zone (HAZ). The static tensile strength and the fatigue strength by laser cutting specimens clearly decreased in comparison with mechanical or water-jet cutting specimen. The laser cutting specimen exhibited a linear dependency of the tensile strength on the HAZ, indicating that the main effect resulted from thermal destruction of CFRP within the HAZ.

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.

Strength and Fracture Characteristics of SUS304/AL-Alloy Scarf Adhesive Joint with Various Adhesive Thicknesses

In this study, strength and fracture toughness of epoxy adhesively bonded scarf joints of dissimilar adherends, namely SUS304 stainless steel and YH75 aluminium alloy are examined on several scarf angles and various bond thicknesses under uniaxial tensile loading. Scarf angles, θ = 45°, 60° and 75° are employed. The bond thickness, t between dissimilar metals is controlled to be ranged between 0.1 mm to 1.2 mm. Finite element (FE) analysis is also executed to investigate the stress distributions in the scarf joints by ANSYS 11 code. From analytical solutions, stress singularity exists most pronouncedly at the steel/adhesive interface corner of joints having 45° to 75° scarf angle. This is not only in agreement with the FE analyses results but also confirmed by fracture surfaces observation wherein the fracture has always been initiated at this point. The strength of scarf joints increases as the bond thickness decreases. Interface corner toughness, Hc approach can be applied when predicting the failure stress of scarf joints. Besides, for scarf joints with an interfacial crack, the fracture toughness, Jc values are independent of bond thickness and less sensitive to adherends. Moreover, Jc increases as mode mixity increases.

Irradiation Test of Stainless Steel by High Power CO2 Laser

Electron beam, plasma arc and ion beam are often employed to simulate the high heat flux applied to the first wall or the divertor plate in a fusion reactor. In this study, an irradiation test with high heat flux was carried out under atmospheric condition by using high power CO2 laser. The test material is SUS316 and the temperature change and the melting amount were measured. A thermal analysis code to take melting and evaporation behavior into account was developed. The laser absorption coefficient can be raised up to 95% before melting if special paint is coated on specimen surface. After melting, this coefficient is estimated to be 60% by thermal analysis. However, it was revealed that a precise modification of this model was indispensable. Although the effect of irradiation environment or heat source on material damage was also examined, there is no significant difference one another. In conclusion, it is found that CO2 laser is quite suitable for use as a heat source to simulate a high heat flux.

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 propagation tests of first wall materials by cyclic irradiation with an electron beam

Abstract In this study, cyclic high heat flux beam irradiation tests were performed for clarifying the crack propagation mechanisms of type 316 austenitic stainless steel and molybdenum alloy TZM. In the experiments, a surface-cracked plate specimen was set in a vacuum chamber in a specimen jig which was connected to a load-cell. And a pulsed electron beam was used to simulate the high heat flux of a fusion reactor. The electron beam has a peak value of 12–20 MW/m2 and 0.5 s operation time in every 30 s. After specified cycles of irradiation, the specimen was fractured by mechanical fatigue load. In all 316SS and some TZM specimens, thermal fatigue crack propagation was observed. To evaluate such a fatigue crack growth behavior only in 316SS, non-linear fracture mechanics analyses were performed based on finite element analysis. And these results were correlated with the non-linear fracture mechanics parameter.

AFM/MFM hybrid nanocharacterization of martensitic transformation and degradation for Fe-Pd shape memory alloy

Martensitic transformation and degradation characteristics for Fe-Pd ferromagnetic shape memory alloy were investigated by the developed AFM (Atomic Force Microscope)/MFM (Magnetic Force Microscope) hybrid nano-characterization technique. In AFM martensitic transformation was detected by the changes of surface topography of martensite plates. In MFM martensitic transformation was detected by the changes of magnetic domain structures. This technique has an advantage that martensitic transformation characteristics such as martensitic transformation temperature and reverse transformation temperature can be measured at microscopic and nanoscopic small area. Degradation characteristics of martensitic transformation under cyclic loading were also detected by the changes of AFM and MFM images. In AFM images surface topography of martensite plates became flat and in MFM images the morphology of magnetic domain structures became unfocused under cyclic loading. Then it was found that the hybrid nano-characterization was very high sensitive technique to evaluate degradation for Fe-Pd ferromagnetic shape memory alloy.

Fracture Characteristics of Shear Adhesive Dissimilar Joint

In this study, the effect of bond thickness upon shear strength and fracture toughness of epoxy adhesively bonded joint with dissimilar adherents was addressed. The bond thickness, t between the adherents was controlled to be ranged between 0.1 mm and 1.2 mm. Finite element analyses were also executed by commercial ANSYS 11 code to investigate the stress distributions within the adhesive layer of adhesive joint. As a result, shear strength of adhesive joint reduces with increasing bond thickness. The strength of shear adhesive joint was also depended on elastic modulus of adherent. Moreover, the failure of dissimilar adherents bonded shear joint originated at a location with critical stress-y which was the interface corner of ALYH75/epoxy. In the case of shear adhesive joint with an interface crack, the fracture also occurred at the ALYH75/epoxy interface even in the steel-adhesive-aluminum (SEA) specimens. Fracture toughness, Jc of aluminum-adhesive-steel (AES) joints was similar to those of SES and demonstrates strong dependency upon bond thickness. Furthermore, the interface crack in SEA specimen has relatively large fracture resistance if compared to those in AES specimen. Finally, Kc fracture criterion was found to be appropriate for shear adhesive joints associated with adhesive fracture.

Stress Analysis and Failure Characterizations of V-shaped Epoxy Adhesively Bonded Joint

This paper deals with stress and failure characteristics of V-shaped epoxy adhesive joint. Effect of scarf angle upon failure morphology was investigated by tensile test and monitoring using high speed camera. V-shaped specimens were fabricated having bond thicknesses, t = 1.0 mm and various scarf angles (i.e. θ = 30 ̊, 45 ̊, 60 ̊, 75 ̊, and 90 ̊). From failure surface observation, failure morphology can be divided into 5 types consisting of interface failure and/or cohesive failure. Stress singularity plays a major role in failure morphology where higher singularity favors cohesive failure in the specimens tested.