Takashi Wakui

Identification of the constitutive equation by the indentation technique using plural indenters with different apex angles

This paper describes a novel technique for determining the constitutive equation of elastic–plastic materials by the indentation technique using plural indenters with different apex angles. Finite element method (FEM) analyses were carried out to evaluate the effects of yield stress, work hardening coefficient, work hardening exponent, and the apex angle of indenter on the load–depth curve obtained from the indentation test. As a result, the characterized curves describing the relationship among the yield stress, work hardening coefficient, and the work hardening exponent were established. Identification of the constants of a constitutive equation was made on the basis of the relationship between the characterized curves and the hardness given by the load–depth curve. This technique was validated through experiments on Inconel 600 and aluminum alloy. The determined constitutive equation was applied to the FEM analyses to simulate the deformation including necking behavior under uniaxial tension. The analytical results are in good agreement with experimental results.

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (II) : Bubbling Effect to Reduce Pressure Wave

Liquid mercury was suggested to be used as target material for high-power pulsed spallation neutron sources. In order to realize the high-power target, however, the pressure wave is a critical issue, which is caused by the thermal shock in mercury and causes cavitation at the moment when highly intense proton beams bombard mercury. R&D on pressure wave mitigation technologies is carried out for Japan Spallation Neutron Source (JSNS; 1MW/25 Hz). Microbubble injection into the mercury is one of prospective technologies to mitigate the pressure wave. The microbubble effect was experimentally investigated from the viewpoint of pitting damage due to the cavitation in the mercury loop with an electro-magnetic impact testing machine (MIMTM) and numerically examined from the viewpoint of bubble dynamics. In the present study, we confirmed that the microbubble injection is very effective to reduce pitting damage and the amplitude of negative pressure, which causes explosive growth of cavitation bubble.

Failure Probability Estimation of Multi-walled Vessels for Mercury Target

A liquid mercury target for MW-class pulse spallation neutron sources is being developed. Failure probability analyses on the mercury target that will be installed at the material and life science facility in the Japan Proton Accelerator Research Complex (J-PARC) were carried out taking account of the stress condition caused by various types of loading and material degradation due to neutron irradiation and pitting damages. The mercury target consists of multi-walled vessels; a mercury vessel and a safety hull, to prevent mercury leakage to the outside, i.e., the mercury vessel that is in contact with mercury is covered by the safety hull. The failure probability of the safety hull was estimated to be lower than 10−6 for the expected designed lifetime. On the other hand, the failure probability of the mercury vessel directly subjected to thermally shocked pressure waves in mercury increased with the operation time and the protonbeam power, and was estimated to be ca. 99.9% for the designed lifetime of the MW-class target.

Dynamic effect on strength in SiCf/SiCm composite

Abstract Loading rate dependence of mechanical properties of SiC fibre-reinforced SiC composites (SiC f /SiC m ) has been experimentally investigated as to the fibre volume fraction and coating materials for SiC fibre. The composites consisting of monolithic SiC and SiC fibre (Hi-Nicalon) coated with Boron-Nitride (BN) or Carbon (C) with fibre volume fractions of 20, 30 and 40% were fabricated by polymer infiltration–pyrolysis (PIP) process. The stress–strain response and strength were measured in tension over a wide range of strain rate,10 −4 ∼200xa0s −1 . It was shown that the higher volume fraction, the larger tensile strength regardless of the kind of coating and strain rate. The interface friction stress evaluated by the fibre pullout length that is measured through microscopic observations of fractured specimens is larger in dynamic loading than in static loading. The BN-coated fibre gave the composite superior tensile strength to the C-coated fibre. This trend results from the variety of the interface friction stress associated with the coating thickness.

Mechanical-property evaluation of thin corroded surface layers of ceramic materials by the microindentation technique

Abstract An instrumented depth-sensitive microindentation technique was applied to evaluate mechanical properties of thin corroded layers of some ceramics which were immersed in boiling, highly condensed sulfuric acid up to 1000 h. Indentation load–depth curves were analyzed using a finite element model to systematically investigate the effect on the ceramics of film or corroded layer. The mechanical properties of both the oxide films and the corroded layers were evaluated through the combination between analytical and experimental results on the indentation curves. It was found that the relationship between thickness T of oxide films or corroded layers and the depth d determined by the slope change on the indentation load/depth–depth curve is described as T ≈10 d . The bending strengths of Si 3 N 4 and Al 2 O 3 after immersion were evaluated using the d associated with the corroded layer thickness and the K IC obtained before immersion. It was confirmed that the strength degradation of Si 3 N 4 and Al 2 O 3 due to the boiling sulfuric acid corrosion can be estimated using K IC and d -value.

Determination of mechanical properties of impacted human morsellized cancellous allografts for revision joint arthroplasty.

This paper deals with the characterization of mechanical properties of impacted morsellized cancellous allograft (IMCA) produced by dynamic compaction of allograft femoral heads ground by commercially available bone mills, i.e. rotating rasp and reciprocating type bone mills. Various ranges and profiles of particle size in the graft aggregates were obtained using these bone mills, and the effect of number of compaction as well as the distribution of particle sizes on the mechanical properties of IMCA under quasistatic compression and shear loading conditions was discussed. The morsellized cancellous allograft prepared by the reciprocating type bone mill showed a broad distribution of particle sizes, and gave IMCA superior mechanical properties to the graft with a more uniform size distribution, or prepared by the rotating rasp type bone mills. The increase of number of compaction also improved the mechanical properties of IMCA in compression.

Study on the evaluation of erosion damage by using laser ultrasonic integrated with a wavelet analysis technique

Spallation targets are the key components of accelerator driven systems (ADSs) that are being developed in the world. Erosion damages on the target vessels are anticipated. To prevent accidents occurrence due to erosion of spallation target vessel, the damage evaluation technique is desirable. The excited vibration of LBE target vessel will be monitored remotely to establish the technique. In this study, the basic researches were carried out through experiments and numerical simulations to investigate the interaction between ultrasonic waves and damage to understand the correlation between structural vibration and damage degree. Specimens with distributed erosion damage was irradiated by laser shots, and the vibration was detected by a laser vibrometer subsequently. A technique, Wavelet Differential Analysis (WDA), was developed to quantitatively and clearly indicate the differences caused by damage in the vibration signals. The results illustrated that the developed technique is sensitive to erosion damage with small size and is capable of quantitatively evaluating erosion damage. It is expected that the developed techniques can be applied to monitor the real spallation targets in the future.

Pitting Damage and Residual Stress Induced by Cavitation Erosion on Mercury Target Vessel

Cavitation damage that might be imposed on the mercury target vessel of a pulsed spallation neutron source was evaluated from the standpoints of residual stress and plastic deformation. The residual stress distribution and plastic region of a type-316L austenitic plate, called 316SS, and that with 20% cold rolling, called 316CW, were measured using an X-ray diffraction technique. As a result of the peak width distribution in each specimen with the cavitation damage, the plastic region in the 316SS was deeper than that in the 316CW. In addition, the internal compressive residual stress of the 316SS was higher than that of the 316CW. The distributions of plastic strain and residual stress affect the crack propagation from the bottom of the pits. Taking into account the energy balance in each specimen subjected to the cavitation damage based on the distributions of plastic strain and residual stress, the difference in the fatigue limit degradation between 316SS and 316CW was explained.

Fracture Behaviour of Plasma Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs) of plasma sprayed yttria stabilised zirconia (YSZ) are increasingly utilised for heat exposed components of advanced gas turbines1,2. An important reason for the application of zirconia coatings is the low thermal conductivity of this ceramic material which is further diminished in a TBC by the high concentration of spraying induced microstructural defects, e.g. crack-shaped defects between and within the spraying splats. Thus with TBCs on gas cooled turbine components stiff temperature gradients can be realised as an important prerequisite for an increased thermal efficiency of the energy conversion process.

Effects of Grain Size on Ultrasonic Attenuation in Type 316L Stainless Steel

A lead bismuth eutectic (LBE) spallation target will be installed in the Target Test Facility (TEF-T) in the Japan Proton Accelerator Research Complex (J-PARC). The spallation target vessel filled with LBE is made of type 316L stainless steel. However, various damages, such as erosion/corrosion damage and liquid metal embrittlement caused by contact with flowing LBE at high temperature, and irradiation hardening caused by protons and neutrons, may be inflicted on the target vessel, which will deteriorate the steel and might break the vessel. To monitor the target vessel for prevention of an accident, an ultrasonic technique has been proposed to establish off-line evaluation for estimating vessel material status during the target maintenance period. Basic R&D must be carried out to clarify the dependency of ultrasonic wave propagation behavior on material microstructures and obtain fundamental knowledge. As a first step, ultrasonic waves scattered by the grains of type 316L stainless steel are investigated using new experimental and numerical approaches in the present study. The results show that the grain size can be evaluated exactly and quantitatively by calculating the attenuation coefficient of the ultrasonic waves scattered by the grains. The results also show that the scattering regimes of ultrasonic waves depend heavily on the ratio of wavelength to average grain size, and are dominated by grains of extraordinarily large size along the wave propagation path.