| Summary: | 碩士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === In this research, two approaches are adopted to study high damping and high stiffness (HDHS) composites. One utilizes the conventional method in the viscoelastic composite theory to combine metallic materials with polymer, and the other adopts the negative-stiffness concept through phase-transforming particulate inclusions such as VO2 and BaTiO3. For the conventional method, the polymers, such as hot melt adhesive (HMA) and polyamide, are embedded into stainless steel, as a core, to form composite materials. Although this may not yield ideal HDHS composites, this approach ensures the outer surface of the composite as stiff and high strength as the metal matrix. Its overall damping is significantly improved, as oppose to its metal counterpart. It is found that, with small amount of polymer inclusion, the steel-polymer composites exhibit large increases in loss tangent, in expense of reducing overall modulus through the identification of resonant peaks in the low frequency regime. Through resonant ultrasound spectroscopy (RUS) experiments, we found that the torsional resonant frequency in relation to the hole size in the steel cubes. For the stainless steel cube with a volume of V=25mm x 25mm x 25mm cube, the constant torsion resonant frequency was around 57 kHz. Loss tangent increases as the volume of polymer increase, and the tan delta of stainless steel with polyamide is larger than that of steel with the HMA inclusion. For the steel-polymer composite with a hole size of 24 mm, its tan delta with polyamide inclusion is 3.952x10^{-2} larger than the hollow stainless steel cube, and is 2.1517x10^{-2} larger than that of steel with HMA inclusion. As for the negative-stiffness composites, the phase-transforming particles are placed in the polymers. Experimental investigations with the resonant ultrasound spectroscopy and dynamic shear rheometer are conducted. Phase-transforming particles in the polymer matrix, in some cases, show increase of damping, but no effects on modulus. It is hypothesized that the stiffness of polymer matrix may not be large enough to dance with the particulate inclusions in the vicinity of phase transformation. However, it is found that polyamide matrix, albeit weak in stiffness, still showed anomalous signals in loss tangent and dynamic modulus around the transformation temperature of the inclusions. By DSR, the polyamide+VO2 (5%) shows anomalous increase in tan delta by about 0.0264, as to all other samples. The amount of inclusions and matrix stiffness must be balanced to observe the anomalies. In addition to mechanical enhancements in the ferroelastic polymer composites, it has been reported in the literature that flexible electronic devices that contain ferroelastic inclusions in polymer matrix may exhibit unusual electrical properties.
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