Papers by Author: Sung Choong Woo

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Authors: Sung Choong Woo, Dae Joon Kim, Nak Sam Choi
Abstract: Acoustic emission (AE) characteristics have been studied for single-edge-notched monolithic thin aluminum (Al) plates and glass fiber/Al hybrid laminates. Traveling microscope was used for observing the plastic deformation and damage zone around the initial notch tip. Frequency characteristics of AE signals processed by fast Fourier transform (FFT) from monolithic Al could be classified into two different types. Type I signal had a relatively low frequency band of 96~260kHz, while Type II signal had broad band frequencies of 192~408kHz. In case of glass fiber/Al hybrid laminates, AE signals with high amplitude (>80dB) and long duration (>2msec) were additionally confirmed on FFT frequency analysis, which corresponded to macro-crack propagation and/or delamination between aluminum layer and glass fiber layer. Also, distributions of the first and the second peaks in frequency spectrum were related with local fracture behaviors of the hybrid laminates. AE source location determined by signal arrival time showed the extent of fracture zones. On the basis of the above AE analysis, characteristic features of fracture processes of single-edge-notched glass fiber/aluminum laminates were elucidated according to different fiber orientations.
Authors: Sung Choong Woo, Nak Sam Choi
Abstract: Dominant frequency characteristics of acoustic emission (AE) from single-edge-notched (SEN) glass fiber aluminum laminates (GFAL) under tensile loads were analyzed in relation to fracture mechanisms. The first and second peak frequencies expressed the characteristic changes of fracture processes in SEN-GFAL specimens such as macro-crack propagation and/or delamination between aluminum and fiber layers. On the basis of the above frequency analysis and of the fracture observation with ultrasonic through-scan and various microscopes, characteristic models for crack propagation of SEN-GFAL was proposed according to various orientations of fiber layer.
Authors: Sung Choong Woo, Nam Seo Goo
Abstract: A performance evaluation of plate-type piezoelectric composite actuators (PCA) having different lay-up sequences was experimentally carried out at simply supported and fixed-free boundary conditions. The actuating displacement of the manufactured PCAs was measured using a non-contact laser displacement measurement system. It was shown that the actuating displacement with increasing applied electric field at a drive frequency of 1 Hz increased nonlinearly at the simply supported boundary condition whereas it almost linearly increased at the fixed-free boundary condition. In contrast, the actuating displacement of the PCAs depended on the applied electric fields in a drive frequency range from 1 Hz to 10 Hz. However, the displacement behavior of PCAs varied significantly at a higher range of drive frequency, i.e., beyond 15 Hz, due to the occurrence of resonance. On the basis of these experimental results, the bending characteristics of PCAs in relation to applied electric field, drive frequency, and boundary conditions were elucidated.
Authors: Sung Choong Woo, Nam Seo Goo
Abstract: Characteristics during the fracture process of a plate-type piezoelectric composite actuator (PCA) using acoustic emission (AE) monitoring were investigated under a bending load. The fracturing of a monolithic PZT ceramic shows typically brittle behavior; furthermore, the AE signal at the maximum load, which corresponds to the final fracture, has a high amplitude and long duration. Analysis of dominant frequency bands by a fast Fourier transform (FFT) in conjunction with AE parametric analysis expressed the characteristic changes of the fracture process in the PCA. For the PCA, a brittle fracture in a PZT ceramic layer induces the local delamination between the PZT ceramic and adjacent fiber composite layers. Based on the AE analysis and damage observations through optical microscopy, the features of AE associated with fracture process can be elucidated for the PCA.
Authors: Sung Choong Woo, Nam Seo Goo
Abstract: In this article, the flexural displacement of plate-type piezoelectric composite actuators (PCA) with various lay-up configurations has been evaluated considering the thermal deformations induced by a temperature drop using a three-dimensional finite element simulation. The results reveal that the flexural displacement of PCA subjected to electrical loadings is significantly affected by their lay-up configurations, the thickness of constituent PZT ceramic and applied boundary conditions, which is associated with the location of the neutral plane by moment equilibrium and the bending stiffness of PCA. For the displacement performance of PCA, an adequate choice of layup configuration together with the proper thickness of PZT ceramic is required.
Authors: Sung Choong Woo
Abstract: The objective of this study is to investigate the tensile behavior and fracture toughness of glass fiber reinforced aluminum hybrid laminates (GFAL) in association with the fracture process using plain coupon and single-edge-notched specimens. The tensile properties of GFAL, such as elastic modulus and ultimate tensile strength, were clearly dependent on the fiber orientations. In particular, the superiority of GFAL0 in KIC and GIC was much more pronounced than that of monolithic Al 1050. However, a transverse crack parallel to the fiber orientation reduced the toughness of GFAL considerably. Microscopic observations of the fracture zone in the vicinity of the crack tip exhibited various modes of micro-fracture in the respective layers as well as fiber fractures and interface delamination between fiber composite and Al layers. Such a damage evolution in GFAL depending on the fiber layer orientation had strongly influence upon the tensile behavior and the toughness of GFAL.
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