Abstract: Graphite materials are used for structural components in the core of high temperature gas-cooled reactors (HTGRs) because of their excellent thermo/mechanical properties. When the core temperature is raised at an accident, the thermal stress of the components is induced, and it enhances the fracture probability of them. In general, the thermal conductivity of graphite is decreased by neutron irradiation due to irradiation-induced defects preventing heat conduction by
phonon. It is hence expected that decreased thermal conductivity is recovered to some extent by thermal annealing at the accident. Therefore, the consideration of the thermal annealing effect is placed as much important subject in the fracture/strength evaluation of the graphite components at the accident. In the present study, the thermal stress and the fracture probability of graphite components
influenced by the thermal annealing were investigated by a finite element method (FEM) analysis. It was shown that the annealing effect decreases the thermal stress and a certain level of the fracture probability.
Abstract: Operating experience of steam generators has shown that cracks of various morphologies frequently occur in the steam generator tubes. These cracked tubes can stay in service if it is proved that the tubes have sufficient safety margin to preclude the risk of burst and leak. Therefore, integrity assessment using exact limit load solutions is very important for safe operation of the steam generators. This paper provides global and local limit load solutions for surface cracks in the steam
generator tubes. Such solutions are developed based on three-dimensional (3-D) finite element analyses assuming elastic-perfectly plastic material behavior. For the crack location, both axial and circumferential surface cracks, and for each case, both external and internal cracks are considered. The resulting global and local limit load solutions are given in polynomial forms, and thus can be simply used in practical integrity assessment of the steam generator tubes, because the comparison between experimental data and FE solutions shows good agreement.
Abstract: The effect of a AlN gradient interlayer on the surface mechanical properties was
investigated by nano-indentation for the TiN film on N+-implanted Al substrate. The AlN interlayer, 80nm thick measured by Auger Electron Spectrometer (AES), was formed by high energy N+-implantation prior to magnetron sputtering the TiN film in our custom designed multifunctional ion implanter. The nanohardness of N+-implanted aluminum was 450HV at extremely small depth, but quickly decreased to a constant value (65HV). The hardness and Young’s modulus of the TiN film on two different substrates, one with and the other without N+-implantation, kept almost constant up to a small depth of 200nm, and then decreased to the values of the Al substrate with increasing indentation depth, but with a lower decreasing rate for the N+-implanted system. It was found from the load-displacement curves that the interfaces cracked when the indentation load was 38mN for the N+-implanted system, while 18mN for the unimplanted system. Therefore the N+- implantation
improved the surface mechanical properties significantly.
Abstract: Pure titanium substrate specimen was sputter-coated with SiC or CoCrNi-oxide thin film and the wear test was carried out by using a “ball-on-disk” type testing machine. The semi-circular parallel cracks appeared on the film surface with angles of 45 degree to the sliding direction of the SiC ball and the delamination of film quickly occurred after the cracking. With the aid of finite element methods, stress distribution before and after cracking in the film was obtained. The maximum tensile
stress existing in the film at the back-contact edge of ball is the reason for the initiation of semi-circular parallel cracks. After the film is cracked, the tensile stress normal to interface as well as the shear stress along interface appear at crack tip. The combination of these two stresses is the main reason for the delamination after the film is cracked.
Abstract: An ultra-precision instrument with concomitant micromanipulation techniques is designed and set up to measure the damage strength of a single biomimetic microcapsule. It can provide the capability of simultaneously measuring the applied force and resultant displacement of a single microcapsule, with maximum force range of 5mN, resolution of 0.1µN and ultimate traveling distance up to 12mm, resolution of 1nm, respectively. By armed high magnification side-view system, it can offer extra and withal valuable information for the supervened analyzing. The bursting force of urea-formaldehyde biomimetic microcapsules of diameter 65µm in glucose solution was measured by this technique. The microcapsule was burst when the deformation reached a value of 56.2% of its diameter and the corresponding resonant force is about 1700µN. The technique provides an effective means to characterize elastic properties of micro biomimetic capsules and compare
mechanical strength of microcapsules made of different ingredients.
Abstract: A new formulation for an adhesive force between a substrate and an indenter is presented. The boundary condition taking into account surface stresses is used for the present analysis. The surface stress is originated from surface energy. A paraboloidal indenter is pressed to the substrate, and then adhesion occurs between both surfaces. Surface energy and surface stress will vary at the adhesion surface, and then the surfaces deform in a concave way. An attractive force occurs to keep the contact of two adhesion surfaces. In the present paper, an effect of surface stress on the adhesive force will be clarified.
Abstract: Diamond film was deposited on the pure titanium substrate by CH4-H2 gas mixture using MPCVD method. In order to carboxylate the surface of the diamond film, it was chemically treated in H2SO4:HNO3 (9:1, case 1) stirred at room temperature or in H2SO4:HNO3 (3:1, case 2) stirred at reflux. The oxidized diamond film was successively treated with 0.1M NaOH for 2hours and 0.1M HCl at 363K for 2hours, and then washed by distilled water. The surface of diamond film was observed by scanning electron microscopy (SEM). The diamond film was characterized using Raman spectroscopy and X-ray diffraction (XRD). Carboxylated diamond film was evaluated by Fourier transform infrared spectrometer (FT-IR), Mini secondary ion mass spectrometer (Mini SIMS) and X-ray photoelectron spectroscopy (XPS). In the FT-IR spectrum, the peak at 1640cm -1 was assigned with C=O stretching vibration of carboxylic acid. In the Mini SIMS profile, the peak intensities of mass number 16 (-O-) and 17 (-OH) were increased after the chemical treatment. The XPS results indicated COO- group and C=O group on the surface of diamond film.
Abstract: In order to evaluate the variation of fatigue data of turbine blade steel in low pressure (LP) steam, it is important to estimate probabilistic stress-life (P-S-N) curve to accurately define the probability distribution. In this study, a new procedure was introduced to determine the expression of P-S-N curves. For this purpose, 3-parameter Weibull distribution was found to be the most appropriate among assumed distributions when the probability distributions of the fatigue life were examined by a comparative analysis. Furthermore, the parameter of P-S-N curve was evaluated using various optimization techniques to maximize the correlation coefficient. As a result, the sequential linear program method is used for estimation of P-S-N curve.
Abstract: In this paper, a Z-parameter method is proposed to relate creep rupture data and
microstructure deterioration of three low carbon steels: Cr5Mo, 12Cr1MoV and 20 steels. A new analytic expression of the Larson-Miller parameter verse stress is supposed as: P =Z + C1 lgσ + C2 σ, where C1 and C2 are constants which determine the shape of the curves and related to materials, and the value of parameter Z represents the magnitude deviated from the master curve which corresponds with the deterioration in rupture properties. As the deterioration in rupture properties closely connects with the degradation of microstructure, namely the spheroidization of carbides in pearlite style steel, a linear relationships have been found between the value of Z-parameter and the level of spheroidization E which can be expressed as: Z = a1 + a2 E.