Won Yi

Assessment for Structural Stiffness and Fatigue Life on Self-Piercing Rivet of Car Bodies

The highest structural stiffness of an SPR joint was exhibited in the specimen constituted with 5J32 and ASPH, and the lowest stiffness in that constituted with the single 5J32 combination. Even though the structural stiffness of an SPR joint specimen is roughly the same with another type of joint, the fatigue life is different according to sheet material and its thickness. The results of numerical analysis are nearly identical to those of experiments. Thus, it can be that FEM modeling in accordance with FEMFAT is effective in assessing the stiffness and fatigue life of SPR joints, and can be an alternative to experimental analysis.

Probabilistic Analysis of the Creep Crack Growth Rate of Type 316LN Stainless Steel by the Monte Carlo Simulation

This paper presents a probabilistic analysis for evaluating the creep crack growth rate (CCGR) of type 316LN stainless steel. The CCGR data was obtained from the creep crack growth tests, which are conducted under various applied loads at 600°C. The crack growth rate was characterized as a function of the C* fracture parameter. In order to logically obtain the B and q values in the CCGR equation of a=B(C*)q, three methods of the least square fitting method (LSFM), a mean value method (MVM), and a probabilistic distribution method (PDM) were adopted. Also, using the Monte Carlo simulation, a number of random variables was generated, and the CCGR lines were predicted probabilistically. The three methods did not show a large difference in the CCGR lines, but the PDM was most useful because the CCGR line can be evaluated with a probabilistic reliability. Both the B and q coefficients followed a lognormal distribution, even though the B ones were a little scattered for the points of the data. In the case of a standard deviation of 1 σ for the probability variables, P (B,q), the results of the MCS and the PDM for the distribution ranges of the CCGR lines were compared and discussed.

The Defect Detection and Non-Destructive Evaluation in Weld Zone of Austenitic Stainless Steel 304 Using Neural Network-Ultrasonic Wave

In recent years, the importance of non-destructive evaluation has rapidly increased due to the collapse of large structures and the shooting up of safety accidents. The ultrasonic method. which is often used as a major non-destructive testing (NDT) technique in many engineering fields, is playing a significant role as a volumetric test regarded highly for evaluating a material’s integrity. This paper is recommended for publication the detecting any defects of the weld zone in austenitic stainless steel type 304 using ultrasonic waves, employing neural network on the basis of the detected defects and evaluating them. In detecting defects, we drew a distance amplitude curve on a standard scan sensitivity and a preliminary scan sensitivity shown in the correlation between the ultrasonic probe, the instrument and the materials on a quantitative standard, and quantitative evaluation is used to draw a distance amplitude curve. A total of 93. 3% of the defect types was distinguished by testing 30 defects after organizing a neural network system based on the defects on the ultrasonic evaluation and learning the neural network system. Thus the proposed ultrasonic wave-neural network in this work is useful for defects detection and Ultrasonic Non-Destructive Evaluation (UNDE) of the weld zone of austenitic stainless steel 304.

Creep-Life Prediction and Its Error Analysis by the Time Temperature Parameters and the Minimum Commitment Method

To predict long-term creep life from short-term creep life data, various parametric methods such as Larson-Mille. (L-M), Orr-Sherby-Dorn (O-S-D), Manson-Haferd (M-H) parameters, and a Minimum Commitment Method (MCM) were suggested. A number of the creep data were collected through literature surveys and experimental data produced in KAERI. The polynomial equations for type 316LN SS were obtained by the time-temperature parameters (TTP) and the MCM. Standard error (SE) and standard error of mean (SEM) values were obtained and compared with the each method for various temperatures. The TTP methods showed good creep-life prediction, but the MCM was much superior to the TTP ones at and . It was found that the MCM were lower in the SE values when compared to the TTP methods.

Nondestructive Evaluation of Adhesively Bonded Joints Using Ultrasonic Technique

Quantitative nondestructive evaluation of defects is particularly important to ensure the reliability and safety of structures. The pulse-echo method is one of the most widely used ultrasonic techniques for evaluation of structural integrity. In this study, ultrasonic signals were used to evaluate parameters related to attenuation and amplitude variation in adhesively bonded joints. Ultrasonic characteristics were measured in double cantilever beam (DCB) specimens under stress with mode I fracture. The FFT was used to determine bond-layer parameters such as effective thickness and frequency spectrum. A control experiment confirmed that the variation of frequency spectrum in the adhesive joints depends on the transition of stress variation. Furthermore, the adhesively bonded joint strength for DCB specimens was evaluated by the parameters of ultrasonic waves and the J-integral. The results reveal that the frequency analysis of ultrasonic waveform can be utilized as a tool for a nondestructive evaluation of adhesively bonded joints. Introduction The nondestructive evaluation method is commonly used to diagnose the safety of structures, since the object of inspection incurs no damage. The ultrasonic technique is widely favored because of its easy-to-handle convenience and high compatibility for field application. Recently, much research has been carried out utilizing ultrasonic techniques to diagnose the reliability of adhesively bonded joints. The current state-of-the-art method for defect detection in the adhesively bonded joints is pulse-echo transmission technology [1]. In the study of diffusion-bonded, defected mild steel plates, Kato et al. [2] obtained a correlation of the bonding strength and bonding condition by using ultrasonic wave reflected from adhesive surface. In their other study, the effects of material thickness on frequency characteristics of ultrasonic waveform were also reported [3]. Recent efforts have been directed toward strength evaluation of adhesively bonded joints by means of ultrasonic waveform analysis reflected on bonded interfaces. Yamada [4] calculated the J-integral as a fracture parameter for the bonded double cantilever type specimen. Song [5] analyzed frequency waveforms of the composite materials upon the transmission characteristics of the elastic wave. Kim et al. [6] considered the couplant effects on pulse-echo ultrasonic testing. It was found that the first back-wall echo and the apparent attenuation coefficient increased with acoustic impedance of the couplant. Using ultrasonic attenuation coefficients and velocity variation, the ultrasonic wave detection on the single lap bonded joint and the strength evaluation of adhesively bonded joints were performed by our research group [1]. DCB specimens were prepared using Al6061-T6, a common structural material used on automobiles and aircraft, and Cemedine 1500 as an adhesive. Quasi-static load was applied to the specimens. To analyze the correlation among fracture mechanics parameters the ultrasonic signals upon the load were acquired by means of the waveform analysis of the DCB specimen with mode I deformation. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1833-1838 doi:10.4028/www.scientific.net/KEM.270-273.1833

Creep Curve Modeling of Hastelloy-X Alloy by Using the Nonlinear Regression Method in the Kachanov-Rabotnov Creep Model

To design HTGR components for up to 1000oC, their creep curves are necessary during a design process. In this study, the full creep curves were modeled by the nonlinear least square fitting method using the Kachanov-Rabotnov (K-R) creep model. A series of creep data was obtained experimentally under various stress levels for Hastelloy-X at 950oC, and the data was used to model the creep curves. The K-R model gave a poor description of modeling creep curves, but the modified K-R one, which has another variable, K in the K-R model, was in better agreement than the K-R one. It was found that the λ parameter in the K-R model was constant regardless of the stress variations. The λ value was about 3.9 for the K-R model and about 5.8 for the modified one.

Creep-Life Prediction of Type 316LN Stainless Steel by Minimum Commitment Method

This paper presents the results of the Minimum Commitment Method (MCM) applied to predict the creep rupture life of type 316LN SS. Constant A, and the function of P(T) and G(σ) being used in the MCM equation were determined. To determine a proper value of the constant A, a focal point method and a trial and error one were adopted, respectively. It was found to be A=-0.02~-0.05 for type 316LN SS. Each prediction curve with the A values were presented up to 106 hours and compared to the experimental data at each temperature. Using the short-term creep rupture data for under 2,000 hours, a long-time rupture reaching up to 106 hours was predicted by the MCM.

Cutting Characteristics and Deformed Layer of Type 316LN Stainless Steel

The cutting characteristics and the deformed layer of nitrogen(N)-added type 316LN stainless steel were comparatively investigated to type 316L stainless steel. The cutting force, the surface roughness(Ra) and the tool wear in face milling works were measured with cutting conditions, and the deformed layers were obtained from micro-hardness testing method. The cutting resistance of type 316LN was similar to type 316L in spite of its high strength. The surface roughness of type 316LN was superior to type 316L for all the cutting conditions. In particular, in the high cutting speed above 345m/min, the surface roughness of the two stainless steels was closely same. The deformed layer thickness of the two stainless steels was generated in the 150-300 ranges, and its value of type 316LN was lower than that of type 316L. This is due to the high strength properties by nitrogen effect. It was found that type 316LN was higher in the tool wear than that type 316L, and flank wear was dominant to crater wear. In face milling works of type 316LN steel, tool wear is regarded as a important problem.

Chaoticity Evaluation of Ultrasonic Signals in Welding Defects by 6dB Drop Method

This study proposes the analysis and evaluation method of time series ultrasonic signal using the chaotic feature extraction for ultrasonic pattern recognition. Features extracted from time series data using the chaotic time series signal analysis quantitatively welding defects. For this purpose analysis objective in this study is fractal dimension and Lyapunov exponent. Trajectory changes in the strange attractor indicated that even same type of defects carried substantial difference in chaoticity resulting from distance shills such as 0.5 and 1.0 skip distance. Such differences in chaoticity enables the evaluation of unique features of defects in the weld zone. In experiment fractal(correlation) dimension and Lyapunov exponent extracted from 6dB ultrasonic defect signals of weld zone showed chaoticity. In quantitative chaotic feature extraction, feature values(mean values) of 4.2690 and 0.0907 in the case of porosity and 4.2432 and 0.0888 in the case of incomplete penetration were proposed on the basis of fractal dimension and Lyapunov exponent. Proposed chaotic feature extraction in this study enhances ultrasonic pattern recognition results from defect signals of weld zone such as vertical hole.

Analysis of stress and stress intensity factor in bonded dissimilar materials by boundary element method

Currently it is increasing to use th bonded dissimilar materials in the various field of advanced engineering such as the highly rigid and lighter vehicle, plastic molding LSI package and metal/ceramic bonded joint. In spite of such a wide application of the bonded dissimilar materials, the evaluation method of the bonding strength has not been established yet. Therefore in this paper we analyze the interface crack problem by introducing fracture mechanics parameters as the basic research about estimating of the strength of adhesive joints. The variation of stress intensity factor according to the elastic modulus of adherend and thickness of bonded layer are investigated. Numerical results are based on the results of boundary element analysis of four different type butt joints subjected to uniaxial tension loading.