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Discontinuous Fiber-reinforced Composites above Critical Length

University of Alabama at Birmingham, School of Engineering, Department of Biomedical Engineering, Hoehn Building, Room 330, 1075 South 13th Street, Birmingham, AL 35294, USA;

View larger version (35K): [in a new window]   Figure 1. Fully articulated four-point flexural bend mechanical test results for (A) flexural strength, (B) flexural modulus, (C) strain at peak load, and (D) work-of-fracture toughness (each value is the group mean for 10 sample tests; error bars ± 1 standard deviation). X-rays (E, F) with two of the most accentuated cracks propagated from all fiber-reinforced samples highlight the common fracture pattern, with initial tensile failure extending back toward the midline and neutral axis. Fiber-reinforced samples were all still intact and supporting a force averaging 60% of the maximum when testing was interrupted, whereas all particulate-filled composites failed immediately at only 0.00002 strain past peak load.

View larger version (68K): [in a new window]   Figure 2. Izod impact toughness. (A) Results for full fracture at high strain rate are charted, demonstrating remarkable toughness improvements when fiber reinforcement is added to particulate-filled dental pastes (each value is the group mean for 5 sample tests; error bars ± 1 standard deviation). (B) SEM (scale 5 microns) characterizes particulate-filled commercial composite Z100 with minimal debonding. (C) SEM (scale 5 microns) depicts reinforced Z100 with transverse fractured quartz fibers providing a large irregular debonding surface area. Particulate-filled composite essential for fiber-reinforced molding and radiopacity still resides between individual fibrils and as shattered debris.

View larger version (97K): [in a new window]   Figure 3. Atomic force microscope imaging demonstrating the molding ability of a nine-micron-diameter quartz filament by optical laser two-dimensional representation with true statistical information acquired through the three-dimensional counterpart (x-y scales in microns). Rare surface imaging exposes a quartz filament damaged during mixing, placement, and mold separation. Fibrils extending upward provide most of the relief, appearing as white higher-modulus material outlined sporadically by black surrounding deeper defects compared with the large mass of dark greyish particulate-filled paste. Although damage creates an overall vertical Z-range distance of 621 nanometers, due to the ability of the pure quartz fiber to deform by compressing parallel against a surface and particulate paste to fill in space, the average surface roughness is still distinctively only 16 nanometers.