178 J.S. Fenner and I.M. Daniel Fig. 23.8 Ultrasonic C-scans of impact damage specimens, backwall echo (ply 16), impactor energy 46 J. Left: Reference composite, Right:Hybrid composite Reference Hybrid 10 15 20 25 0 2 4 6 8 10121416 Ply Number Mean Diameter (mm) Fig. 23.9 Plot of mean damage zone diameter from ultrasonic C-scan vs ply number (13.5 J impactor energy for both samples) an average produced values for mean damage zone diameter (½dmajor C½dminor Ddmean) at each ply. Plots of mean damage zone diameter (Fig. 23.9) consistently showed a smaller damage zone size at a given ply and impactor energy for the hybrid composite over the reference material. This behavior was observed regardless of the specific impactor energy applied to the pair of specimens. For comparison purposes, the ratio of mean damage zone diameters (dhybrid/dref) was computed for each comparable pair (same impact energy) of specimens at each ply. This allowed simultaneous comparison of test pairs at different impact energies (Fig. 23.10). In spite of the scatter at lower ply numbers, which occur nearer the site of impact, there is a clear overall tendency of the mean diameter ratio to remain less than unity at any given ply. This reaffirms the notion that the hybrid composite requires more energy to form a given amount of damage than the reference composite. From Fig. 23.10, the average diameter ratio across all plies and all energies is about 0.85, showing an overall 15 % decrease in mean damage zone diameter in hybrid composite over reference composite. As an approximate means of characterizing the Mode-II fracture toughness from the impact test data, the total absorbed fracture energy of a test may be assumed as Eimpact GIIAdamage (23.5) where Adamage is the mean damage zone area as observed ultrasonically This may be rewritten as GII 4 Eimpact d2 (23.6) where d is the mean damage zone diameter
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