Advanced Nondestructive Evaluation I

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Authors: Asa Prateepasen, Pakorn Kaewtrakulpong, Chalermkiat Jirarungsatean
Abstract: This paper presents a Non-Destructive Testing (NDT) technique, Acoustic Emission (AE) to classify pitting corrosion severity in austenitic stainless steel 304 (SS304). The corrosion severity is graded roughly into five levels based on the depth of corrosion. A number of timedomain AE parameters were extracted and used as features in our classification methods. In this work, we present practical classification techniques based on Bayesian Statistical Decision Theory, namely Maximum A Posteriori (MAP) and Maximum Likelihood (ML) classifiers. Mixture of Gaussian distributions is used as the class-conditional probability density function for the classifiers. The mixture model has several appealing attributes such as the ability to model any probability density function (pdf) with any precision and the efficiency of parameter-estimation algorithm. However, the model still suffers from model-order-selection and initialization problems which greatly limit its applications. In this work, we introduced a semi-parametric scheme for learning the mixture model which can solve the mentioned difficulties. The method was compared with conventional Feed-Forward Neural Network (FFNN) and Probabilistic Neural Network (PNN) to evaluate its performance. We found that our proposed methods gave much lower classificationerror rate and also far smaller variance of the classifiers.
Authors: Asa Prateepasen, Chalermkiat Jirarungsatean, Pongsak Tuengsook
Abstract: In petroleum industry, corrosion failures of steel structures are common. The severity of corrosion in oil distillery inorganic compounds is higher than in those of organic compounds. Inorganic compounds such as sulfur are the most influential corrosive activators inside oil or chemical storage tanks. They normally have the tanks inspected and repaired along their life time. In addition the concentration of sulfur compound increases due to the accumulation of the residuals inside the tank, and so does the corrosive rate. In this paper, Acoustic Emission (AE) has been chosen to study the characteristic of AE signals received from the uniform corrosion mechanism of mild steel (A36) in various concentrations of Sulfuric acid (H2SO4) solution. AE signals were captured using a wide band sensor (WD) and recorded by AE system model LOCAN 320. The relationship between AE signals and sulfur concentrations as well as pH were exhibited.
Authors: Yun Hee Lee, Jong Seo Park, Seung Hoon Nahm, Yong Hak Huh
Abstract: A nondestructive rod compression has been proposed as a new strength characterizing technique for in-service components. Using electrical discharge machining, milling machining, and stamping, it was possible to machine small rods, typically about 0.5 mm in both diameter and height on the target surface. Static compressions of the rods were carried out using a flat punch and their deformation behaviors were recorded as load-displacement curves. Referring to initial dimension of the rods, engineering stress-strain curves implying the yield strengths were calculated from the deformation curves. Surface yield strengths from the rod compressions were directly compared with reference compression results and their slight discrepancies were discussed from the influences of pre-deformation and damage of the rod and additive compliance of the base.
Authors: Jai Won Byeon, S.I. Kwun, Kae Myung Kang
Abstract: In this study, mechanical degradations in 2.25Cr-1Mo steel were evaluated by quantitative morphological analysis of carbides. Based on the morphology, carbides were classified as globular, fine acicular, rod, and grain boundary one. Mean size of carbides were determined as a function of morphology and thermal degradation time at 630°C. Area fraction of grain boundary carbides and fraction of grain boundary M6C carbides were observed to increase rapidly in the initial stage of degradation and then gradually afterwards. Both mean size of globular carbide and fraction of grain boundary M6C carbides were linearly correlated with strength. Potentials of carbide morphology analysis as a health monitoring technique were discussed, in term of correlation coefficient with strength.
Authors: Bong Kook Bae, Chang Sung Seok, Jae Mean Koo, Hyung Ick Kim, K.Linga Murty
Abstract: A burst rupture test has been mainly used for evaluating the burst properties of internal pressurized tubes. Although the burst creep test provides accurate results, its complicated and material-wasting process makes it difficult to perform this test. In the burst test, it is known that the hoop stress is a main reason of burst, so it can be expected that the burst rupture properties are strongly related with the hoop creep rupture properties. A ring test is occasionally used for obtaining the hoop directional properties of tube-shaped structures. In this study, the creep rupture properties of Zirlo tubing are investigated at temperature ranging from 365 to 570  using the ring specimens. An analysis of the fractography was performed and the estimation of service life with Larson-Miller parameter was conducted. Finally, In view of the Dorn equation of power-law, the creep mechanism was determined and discussed.
Authors: Seung Wan Woo, Jae Do Kwon, Young Hwan Choi
Abstract: In a primary reactor cooling system, a dissimilar weld zone exists between cast stainless steel (CF8M) in a pipe and low-alloy steel (SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330, while no effect is observed in SA508 cl.3. The aged specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at 430, respectively. The specimens for elastic-plastic fracture toughness tests are prepared two types, which notch is created in the center of deposited zone and the heat affected zone of CF8M. From the experiments, the plastic-elastic fracture toughness values (JIC) with the increase of aging time decrease as the notch is created in the HAZ of CF8M, while that is different slightly as the notch is created in the deposited zone. Also, JIC values in the deposited zones are smaller than the HAZ of CF8M at all aged specimens.
Authors: Noh Yu Kim, Cheol Kyou Lee
Abstract: Scanning acoustic microscopy (SAM) has been applied successfully to measure leaky surface wave(LSAW) velocity from V(z) curve for material characterization. Besides LSAW, SAM produces a variety of wave modes in the material propagating with different phase speeds and features, one of which is the mode-converted ultrasound. In this paper, a novel method for simultaneous measurement of longitudinal and transverse acoustic wave velocities using the mode-converted ultrasound is developed and studied at SAM. Each ultrasound arriving at the acoustic lens with different speed and phase (or time-of-flight (TOF)) is measured by the defocused transducer to calculate longitudinal and transverse wave velocities. Simple mathematical ray analysis for the mode conversion is conducted to determine the relationship of acoustic velocities with TOF of the waves in plate. Experimental results for metals and glass show that the longitudinal and transverse wave velocities measured by the proposed method are as accurate as maximum 3% in error.
Authors: Noh Yu Kim, Hwan Seon Nah, Sang Soon Lee
Abstract: In order to evaluate the degradation of the epoxy coating in nuclear power plants, acoustic wave velocities of epoxy films are measured using defocused scanning acoustic microscopy system(SAM). Unlike metals, the surface of the epoxy coating on the concrete liner is so thin and wavy that the conventional ultrasonic techniques for acoustic velocity of epoxy coating are hard to apply. Acoustic velocities of bulk waves are determined from V(z,t) curves of mode-converted waves generated in the film by SAM. Epoxy films are fabricated and degraded under various accelerated aging conditions, and both of longitudinal and shear wave velocities of the epoxy film are measured. Approximately 10% of reduction in acoustic wave velocity is observed from experimental results when the aging is developed fully in epoxy films. It is also found that longitudinal wave is more sensitive to deterioration of epoxy coating than transverse wave.
Authors: Hee Dong Kim
Abstract: To assess the degradation of stator winding insulation in high voltage motor, a total of forty-eight coils were manufactured. These coils are classified into five groups and there are twelve strands in each coil. Four coil groups are artificial defects such as shorted turn of strand, internal separation between conductor and insulation, large void within insulation and removal of semi-conductive tape. The other coil group is normal. Before ac breakdown test was performed, nondestructive test that included ac current, dissipation factor and partial discharge (PD) was done with five coil groups. This paper carried out by comparing the results between nondestructive test and ac breakdown test. When the values of ac current increase rate, tip-up dissipation factor and PD magnitude are large, the insulation is occurred the deterioration. AC breakdown voltage is also low in degraded insulation.
Authors: Rong Guang Wang, Mitsuo Kido, Yoshiko Shinhara
Abstract: An electronic speckle pattern interferometry (ESPI) method was applied to nondestructive detection of cracks and delaminations of ceramics coating (Al2O3/NiCr) on SUS304 stainless steel. A drop of surface strain on the coating was found when cracks in the coating or delaminations from the coating/substrate interface occurred.

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