Key Engineering Materials Vols. 321-323

Abstract: In this research, we proposed nondestructive evaluation method of a flaw in an optical glass lens by resonant ultrasound spectroscopy (RUS). An optical glass lens is used for improving signal coupling between fibers, emitters and detectors. The shape of optical glass lens is an aspherical lens with 3mm in length and 6.35 mm in diameter. For nondestructive evaluation of an aspherical lens, we made of the measuring system by RUS. For flaw evaluation by the RUS to verify the data obtained from the experiment, we carried out simulation by explicit finite elements method, and compared the experimental results with simulation results. Also, we compared the resonant frequencies of specimens with some defect with those of acceptable specimens. The histogram drawn from these data and the available resonance mode surveyed were used to classify the acceptable specimens in the plant.

Abstract: A laser-based ultrasonic inspection system is a non-contact scanning device with a high spatial resolution and a wide spectrum bandwidth. In this paper, we developed a laser-based ultrasonic inspection system by fabricating a pulse laser and a Confocal Fabry-Perot Interferometer (CFPI) with a dynamic stabilizer. The dynamic stabilizer generates ultrasound by adaptively obtaining the time of the maximum gain of the CFPI whose gain is varied with the time. Then, a computer adaptively corrects the measurement error by using the gain of the CFPI at the measuring time. In this paper, we describe the hardware configurations and the control algorithms to build the stable laser-based ultrasonic inspection system. We have investigated an ultrasonic signal in the time and frequency domain to detect micro cracks through experiments. We experimentally confirmed that the proposed error correction method is effective for improving the performance of the system and the configured ultrasonic inspection system is stable

Abstract: In recent years, a thermal barrier coating technology becomes more important because it is required to develop materials which avoid high-temperature oxidation and corrosion. In order to ensure reliability of coating, it is necessary to understand microfracture criterion in terms of stress distribution and fracture morphology. The objective of this study is to clarify the damage generation process by comparing Acoustic Emission
AE) generation behavior and thermal stress simulation result based on heat history measurement. Al2O3 as the top coat (TC) and NiCrAlY as the bond coat (BC) were sprayed to SUS304 disc or plate as a base material. AE in cooling process were detected by laser heterodyne interferometer after spraying finished. Two-dimensional axisymmetric mesh for samples was made and transient heat conductivity was analyzed for various boundary conditions. Then, using the obtained heat history, transient heat stress was analyzed and the distribution of stress elements was calculated. The fracture process of ceramic coating was evaluated from these experiments and analyses.

Abstract: The magnetic camera using magnetic lens is proposed to satisfy the demands such as obtaining the distribution of the magnetic flux leakage (DMFL) on the high lift-off, and improving sensitivity. However, DMFL is strongly affected by the magnetization direction. Therefore, the DMFL has low intensity when the crack is not perpendicular to the magnetizing direction. Also, quantitative nondestructive evaluation would be difficult. This paper proposes an improved magnetization method to evaluate a crack quantitatively, regardless of the crack direction. The secondary magnetic source, which is perpendicular to the primary magnetic source, is introduced in the scan type magnetic camera. The intensity of two values of ∂B/∂x (1st differential to the magnetization direction) of the DMFL can be used to evaluate the crack volume. The experimental and the numerical analysis results are considered to verify this phenomenon.

Abstract: Magnetic-optical film (hereafter, MO film) is a bismuth-doped iron garnet film grown on a gadolinium gallium garnet substrate by liquid phase epitaxial method. Because an MO film has physical characteristics such as Faraday rotation and magnetic anisotropy, the distribution of its external magnetic field can be visualized. The MOI (Magneto-Optic/Eddy Current Imager) was developed to detect far-side cracks and corrosions in aluminum alloy materials on an aircraft. The detection probability of 2.5mm-length cracks is 90/95% when the MO film and the induced area current are used. However, the quantitative evaluation algorithm for quantifying crack shapes and sizes has yet to be proposed. This paper investigates a quantitative evaluation algorithm for magnetic optical eddy current imaging by using the finite element analysis methods (FEM) and experimental results. The MOI including a primary induction coil was simulated by FEM software. The position, depth, shape, and the induction current frequency, lift-off were simulated and examined. A simplified quantitative evaluation algorithm for MOI is proposed by using the analysis results.

Abstract: We observed the surface resistance of metal thin films by a nondestructive characterization method using a near-field scanning microwave microprobe (NSMM). The NSMM system was coupled to a dielectric resonator with a distance regulation system. To demonstrate the ability of local microwave characterization, the surface resistance dependence of the metallic thin films has been mapped nondestructively.

Abstract: Galvannealed steels are produced by reheating of Zinc-coated steels, in which the iron and the zinc are interdiffused to form Zn-Fe intermatallic compound. In recent years, application of galvannealed steel has increased due to its excellent corrosion resistance, good weldability and paintability. Galvannealed production requires strict control of processing conditions to obtain an optimal alloying degree in the coatings. The analysis of the alloying degree is very important since they are closely related to the corrosion and mechanical properties. This study is measurement of alloying degree of the galvannealed steel by using the XRD. Partial least squares (PLS) model is a powerful multivariate tool that has been successfully applied to the quantitative analysis of data in XRD. The alloying degree was determined by using PLS regression to a concentration accuracy of 0.2%. The XRD analysis and proposed PLS model can be successfully used to determine the alloying degree of industrially produced galvannealed steel.

Abstract: As widely-used methods to measure metallic resistivities, Four-terminal method, van der Pauw method, Four-Point Probe(FPP) method and eddy current method were compared and analysed to find the method with lowest uncertainty and accurate method for applying nondestructively to industry. STS 316, non-magnetic metal is used as sample for the four methods. Conductivity ratios by the four-terminal method and van der Pauw method had the lowest uncertainty of 0.25 % and the ratios by the FPP method had measurement uncertainty of 0.45 % when probe spacing was more than two times of sample thickness and dc 10 A is applied. Also, the ratio values by the eddy current method had 1 % or more difference from the four-terminal or van der Pauw method and measurement uncertainty was obtained as 0.93 %. From the results, former two methods had the lowest uncertainty but are destructive methods so that those are difficult to apply to the field of industry. The FPP method is an accurate and nondestructive measurement method so that it can be used in the field. The eddy current method is one of nondestructive method but it has the highest uncertainty and low accuracy.

Abstract: Besides well-known destructive methods for material degradation, the electrical resistivity method has been used as one of nondestructive evaluation methods because of easy measurement. To use the method, however, careful geometrical corrections and thickness measurements are required. The high current probe assembly and accurate thickness measurement device were developed to improve overall measurement uncertainty. If dual configuration Four-Point Probe method with the developed devices is used, overall measurement uncertainty of electrical resistivity could be reduced to 0.44 % at 95 % confidence level. Proper selection of the probe spacing matched with sample thickness is very important to determine accurate electrical resistivity. When sample thickness is less than probe spacing, it is expected that dual configuration Four-Point Probe technique can be used in nondestructive evaluation of plant equipment materials.

Abstract: We attempted to estimate the residual stress which evolved during the shot peening of Al 7075 alloy using leaky surface acoustic wave (LSAW). Shot peening was conducted to produce a variation in the compressive residual stress with the depth from the surface at a shot velocity of 30m/s. The LSAW velocity was measured using a scanning acoustic microscopy (SAM). The Vickers hardness profile obtained inwards from the surface showed significant work hardening of the near surface layer with a thickness of about 0.3mm. The variation in the LSAW velocity through the shot peened surface layer was in good agreement with the distribution of the residual stress measured by X-ray diffraction.