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Peak frequency analysis via wavelet transform for impact damage mechanisms in woven composites
우성충,김태원 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.6
Impact damages have been a major concern in the design of laminated composite structures since the damages mainly occur within the materials in a very short time. In this article, impact induced damage mechanisms of woven composites were investigated by means of novel SHPB-AE coupled tests. A step-consisting new methodology was developed; first, damage mechanisms of woven composites were determined by peak frequency analysis via wavelet transform (WT); second, the extraction of a range of acoustic emission (AE) parameters was achieved from AE energy, duration, and amplitude on the basis of peak frequency analysis; finally, the quantification of damage mechanisms was accomplished through a normalized cumulative AE count. In order to confirm the features of damage, the microscopic characterization of damage mechanisms was performed for the materials. As a result, the determination of impact damage mechanisms in tested composite materials was possible by peak frequency analysis via WT along with waveform and intensity examinations. In addition, the mechanisms could be identified by confirming the progress-based distribution and band formation of peak frequencies. AE parameter ranges were also extracted in terms of amplitude, duration, and energy according to the damage mechanisms. The damage process of the woven composites under impact loading began in the matrix and immediately caused fiber breakage to occur. Subsequently, multiple damage mechanisms were shown, such as fiber-matrix debonding, fiber pull-out, severe deformation and rupture in Kevlar fiber tips. The proposed methodology for the novel SHPB-AE coupled test of woven composites therefore showed an effective way to figure out in situ information on the damage under impact loading.
우성충(Sung-Choong Woo),구남서(Nam Seo Goo) 대한기계학회 2008 大韓機械學會論文集A Vol.32 No.10
Damage mechanisms in bending piezoelectric actuators under electric fatigue loading are addressed in this work with the aid of an acoustic emission (AE) technique. Electric cyclic fatigue tests have been performed up to 10? cycles on the fabricated bending piezoelectric actuators. An applied electric loading range is from -6 ㎸/㎝ to +6 ㎸/㎝, which is below the coercive field strength of the PZT ceramic. To confirm the fatigue damage onset and its pathway, the source location and distributions of the AE behavior in terms of count rate and amplitude are analyzed over the fatigue range. It is concluded that electric cyclic loading leads to fatigue damages such as transgranular damages and intergranular cracking in the surface of the PZT ceramic layer, and intergranular cracking even develops into the PZT inner layer, thereby degrading the displacement performance. However, this fatigue damage and cracking do not cause the final failure of the bending piezoelectric actuator loaded up to 10? cycles. Investigations of the AE behavior and the linear AE source location reveal that the onset time of the fatigue damage varies considerably depending on the existence of a glass-epoxy protecting layer.
우성충(Sung-Choong Woo),최낙삼(Nak-Sam Choi) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5
Fracture characteristics of monolithic aluminum, fiber reinforced plastics and glass fiber/aluminum hybrid laminates under tensile loads have been evaluated using plain coupon and single-edge-notched specimens. Elastic modulus and ultimate tensile strength of GFMLs showed different characteristic behaviors according to the fiber orientation and composition ratio. Fracture toughness of A-GFML-UD which were determined as parameters of K<SUB>IC</SUB> and G<SUB>IC</SUB> based on critical load were similar to those of GFRP-UD and much higher than monolithic Al. Therefore, A-GFML-UD presented superior fracture toughness and prominent damage tolerance in comparison to its constituent Al.