Papers by Author: Kwang Hwan Oh

Abstract: Universal test method to evaluate sheet metal formability was developed using finite element method based on axiomatic design. The newly developed formability test intended to generate the various modes of deformation and to control the onset of failure independently under each mode of deformation. The functional requirements (FRs) and the design parameters (DPs) of the test system were defined on axiomatic design approach and decomposed until the design reaches final stage. The independence axiom was applied throughout the design process to maintain the hierarchical independence of the formability test system. The flow diagram representing the system architecture was introduced after decomposition to give a help to establish the systematic design procedures and to determine the design parameters. Numerical simulation was carried out to determine the specific value of DPs which satisfies the FRs. Numerical results showed that modes of deformation varies accompanying various strain paths and good controllability of sheet forming is obtained for different kinds of materials. Experimental work was finally conducted to validate the proposed design. Stamping results represented that the outcome of the deformed geometry and strains are in good agreement with the numerical results.

Abstract: Acoustic emission (AE) technique was applied as a non-destructive test method to detect and evaluate the localized damage at high temperature environment. The creep-fatigue crack growth tests were carried out with the acquisition of AE signal at 1000°F. Under trapezoidal waveform loadings, AE results showed different features according to each damage mode. During the creep period, low and steady emissions were shown, while emissions were burst and high counts rate was recorded during the fatigue loading. Based on these characteristics, damage contribution was expressed in terms of acoustic emission parameter as a part-time monitoring method. Comparisons of damage contribution with respect to lifetime showed the transient behavior from cycle-dependent to time-dependent process. In case of full-time monitoring, bilinear behavior between AE counts and life was represented. From both monitoring results, it was confirmed that creep and fatigue damage can be characterized by means of emission features and AE is possible way to evaluate the localized damage at elevated temperature.

Abstract: Graphite nanofibers (GNFs) reinforced aluminum matrix composites have been fabricated successfully by powder metallurgy methods. The GNF-Al mixtures were prepared through ultrasonication and ball milling. The GNF-Al composites were consolidated by hot isostatic pressing (HIP) and then a high density of compacts could be obtained. The microstructure and the distribution of nanofibers in matrix material were investigated by microscopy observations. A uniform distribution of nanofibers in aluminum matrix was obtained. The mechanical properties were measured by microhardness and compression tests. The optimal contents of nanofibers were determined in view of the mechanical properties. The results of thermal tests indicate that the addition of nanofibers were enhanced the thermal conductivity but, the dimensional stability such as thermal expansion was not improved significantly.

Abstract: A series of laboratory investigations concerned about fatigue assessment with acoustic emission method was presented. Fatigue aspects including cumulative fatigue damage, fatigue crack growth and creep-fatigue interaction were considered. As a basic approach, residual strength and acoustic emission characteristics of fatigue damaged materials were considered from the nominal stress-life (S-N) viewpoint. Acquired signal indicated that counts emission quantity can be a good measure of cumulated fatigue damage. In the fatigue crack growth approach, interrelationship between acoustic emission parameter and stress intensity factor was examined with different stress level and crack length. Experimental results were somewhat scattered since sensitive characteristics of acoustic emission method. However, their empirical relation indicated that counts rate correlated with fracture mechanics parameter. Finally, acoustic emission application was extended to the creep-fatigue interaction at elevated temperature. Emission response under each damage mode was compared and characterized. Based on these characteristics, creep-fatigue interaction was evaluated by use of acoustic emission parameter. Overall investigations concluded acoustic emission is very effective tool of fatigue assessment.

Abstract: This study investigated acoustic emission behavior during fatigue crack growth test under constant and variable amplitude loading in 304 stainless steel. To describe the acoustic emission behavior, counts rate(dη/dn) was related with stress intensity factor range (SIFR, ΔK) in log-log plot. As a result of test, the relationship was represented a curve, which forms rise and fall behavior in counts rate as the SIFR increases. AE response to a single overload was sudden drop and slow recovery in counts rate, which was similar to crack growth retardation behavior. Under block loading, counts rate of each loading block was same as that of constant amplitude loading. Overall experimental results indicated that stress intensity factor controls the counts rate (dη/dn) as well as crack growth rate (da/dn) regardless of load range or crack length.