Compression-molded short-fiber GFRP-BMC panels have random distribution of solidification texture angles from zero to 90 degrees in the center of the mother panels. Hence, there is significantly lower impact strength in the panel center than in the outside. However, experimental results showed homogeneous low voltage electron beam irradiation (HLEBI) applied to the center region apparently enhances the Charpy impact values (a uc ) 5 to 25%. Fracture mechanism was observed to convert at a uc > ³5.46.7 kJ·m ¹2 from clean to secondary microcrack proliferation and/or bends near the main crack, with increasing fracture surface area as a uc increased. SEM observation revealed 0.86 MGy HLEBI treated GFRP had much more polymer adhering to fibers than the untreated. This increased matrix adhesion can be explained by electron spin resonance (ESR) peaks indicating dangling bonds are generated creating repulsive forces between outer shell electrons in the polymer matrix, apparently exhibiting increased compressive stress on the fibers increasing adhesion force. Moreover, the lone pair electrons generated in the matrix may have bonded with the fibers more efficiently. For these reasons, increased fiber-matrix adhesion seen in the 0.86 MGy samples appears to assist for more internal cracking, increasing resilience to impact of the GFRP-BMC, raising the a uc .
In a highly CaCO 3 filled compression molded short glass fiber polyester GFRP-BMC composite with high solidification texture angle of 71 « ³7 deg with respect to specimen length, Charpy impact strength, a uc at both low ¹79°C (194 K) and high 70°C (343 K) temperatures were apparently increased 69 and 32%, respectively over that at RT (293 K). This result was highly unexpected. Test temperatures were beyond presently accepted extreme operating temperature range of commercial air flight of ¹62°C (211 K) to 53°C (326 K). As expected, optical observation of specimens showed number and size of surface cracks on tensile side increased, i.e. brittleness increased with decreasing temperature. In the 194 K samples, number of parallel cracks spanning most or all of specimen thickness increased exponentially with increasing a uc with asymptote maxing out at about a uc = 16 to 18 kJ m ¹2 absorbing increased fracture energy. SEM observation of fracture surfaces showed increased bare fiber exposed length from the 194 K sample indicating brittleness. There was smoother fracture surface in the 343 K sample indicating ductility compared to that of 293 K sample. The high 55 mass% of CaCO 3 powder filler appears to play a role of increasing the a uc of the composite at both low and high temperatures. Two strengthening mechanisms proposed are: 1) difference in coefficient of thermal expansion (CTE) between polymer matrix and CaCO 3 nanoparticles creating residual compressive stresses with temperature change during either cooling or heating; and 2) crossing thermal transitions in the polymers during cooling and heating to enhance these residual stresses.
The change in viscoelastic response of an unsaturated polyester during the course of thickening with magnesium oxide was evaluated, Rheological measurements were obtained to describe the effects of thickening as functions of time and magnesium oxide concentration. Previous researchers have made similar evaluations utilizing Brookfield viscosities. However, the test geometry asociated with a Brookfield T‐bar spindle is not well defined and data interpretation is consequently obscured. A Rheometrics Thermal Mechanical Spectrometer was used to obtain dynamic mechanical measurements ranging across three decades of frequency. Values of the elastic shear modulus, G′, the viscous shear modulus, G″, and the complex viscosity, η*, were calculated.
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