The fracture toughness (KIc) was evaluated by fracturing
chevron notched beams (CNB) in flexure. The test bars were
45 mm × 3 mm × 4 mm (length by width by thickness, respectively)
and were notched with a 0.1 mm-thick diamond saw; the chevronnotch
tip depth and average side length were about 0.12 and 0.80 of
the bar thickness, respectively. The direction of testing is sketched in
Fig. 1b. The specimens were fractured using a fully-articulated steel
four-point fixture with a lower span of 40 mm and an upper span
of 20 mm using a screw-driven load frame (Instron, 6025). Three
specimens were loaded with a crosshead speed of 0.05 mm/min. The
“slice model” equation of Munz et al. [14] was used to calculate KIc.
Five additional composite specimens were also tested according to
ASTM C1421 using a similar, fully articulated four-point fixture at
a crosshead rate of 0.03 mm/min (Instron mod. 5881). The work of
fracture (WOF) was also calculated for each fracture toughness specimen.
The total work was determined by measuring the area under the
load deflection curve. The WOF was calculated by dividing the total
work by the newsurface area produced by fracture (twice the cross sectional
area of the chevron portion of the toughness specimens). The
room temperature flexural strength (σRT) was measured according to
the European standard EN 843-1 by fracturing five chamfered bars
that were cut along each of the major axes of each composite billet.
The bars had dimensions of 25 mm × 2.5 mm × 2 mm (length by
width by thickness, respectively).