2018 Volume 59 Issue 8 Pages 1280-1287
In a highly CaCO3 filled compression molded short glass fiber polyester GFRP-BMC composite with high solidification texture angle of 71 ± ∼7 deg with respect to specimen length, Charpy impact strength, auc at both low −79°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 −62°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 auc with asymptote maxing out at about auc = 16 to 18 kJ m−2 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 CaCO3 powder filler appears to play a role of increasing the auc 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 CaCO3 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.
