Papers by Author: Jae Jung Hwang

Abstract: The time-temperature dependence of the compressive behavior of polypropylene (PP) foam was investigated to make predictions about what sort of behavior for wide ranges of temperature and strain rate. Compressive stress relaxation tests were conducted at 213 K and 373 K. Compression tests were also conducted. The strain rate was 2×10-3 1/s at 213 K and 373 K. The compressive stress-strain curves were roughly linear and dependent on temperature until the maximum stress was reached. The maximum stress occurred at 5% strain regardless of temperature. The plateau stresses decreased as temperature increased. By plotting compressive behavior of the PP foam at the master curve of the stress relaxation modulus, its temperature dependence could be explained by the thermo-viscoelastic properties. Therefore, the behavior of PP foam at different strain rates could be approximately predicted from the stress relaxation modulus with the timetemperature equivalence principle.

Abstract: We experimentally investigated compressive behaviors of a paper and a paper coated by polyethylene resin. The initial thickness of sheets of paper measured under a compressive stress of 4.5kPa was widely distributed in comparison with the basis weight, although the results based on the ISO 534 standard (under compressive stress of 100kPa) had relatively little dispersion. For compressive deformation, the thickness immediately decreased under low compressive stress, because the gap between fibers in the paper collapsed. After that, the variation of the thickness under higher compression was small due to the compression of fibers. We found that although the resin-coat layers did not have an insignificant effect on compressive compliance above 250kPa, the compliance of the coated paper was larger than that of uncoated paper.

Abstract: In order to assess material properties and part homogeneity in carbon matrix composite (CMC) brake disks we have performed nondestructive evaluation, which are originally developed for aerospace applications. In this paper we have adopted several ultrasonic techniques to evaluate carbon matrix composites for the material properties that are attributable to the manufacturing process. In a carbon matrix composite manufactured by chemical vapor infiltration (CVI) method, the spatial variation of ultrasonic velocity was measured and found to be consistent with the densification behavior in CVI process in order to increase the density of the CMC composites. Ultrasonic velocity and attenuation depend on a density variation of materials. Low frequency through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Optical micrograph had been examined on the surface of the CMCs using a destructive way. Also a motorized system was adopted to measure ultrasonic velocity on the point of the CMC materials under the same coupling conditions. Manual results were compared with those obtained by the motorized system with using dry-coupling ultrasonics and through transmission method in immersion.