A R Boccaccini et al. / Composites Science and Technology 65(2005)325-333 331 Fig. 9. Macrograph showing the high level of deformation achieved during the 4-point flexural strength test by a sample that had been impacted at an energy of 10.35 J. The sample did not break into two fragments, demonstrating a true composite"pseudo-plastic behaviour high level of deformation achieved dur iring 4-point flex- ral strength test in a sample that had been impacted at an energy of 10.35 J. The sample did not break into two fragments, demonstrating a true composite, ""pseu do-plastic"behaviour. a plot of relative Youngs modulus as a function of impact energy is presented in Fig. 10(a). The elastic modulus of the as-received material ther lot (71.6 GPa), but comparable to that of similar oxide/ oxide composites as reported in the literature [10, 15]. 30 Decrease in elastic modulus after ballistic impact is ob- served up to the point of penetration of the projectile$20 (4.2 J impact energy) where structural damage imum Samples impacted with higher energy projectiles show an increase in Youngs modulus, indicating less structural damage. This accordance with the Impact Energy (J) existing understanding that structural damage under ballistic impact increases to a point where impact en- ergy is just sufficient to cause penetration as observed 2 Iso in polymer and glass-ceramic matrix composit 140 The continuous decrease of Youngs modulus with increasing impact energy of projectile(below 4.2 J) which is related to the cumulative development of 80 microstructural damage in the sample, may be ana lysed by considering a model linking elastic constant and microcracking density. Assuming that micro- 20 cracking in the matrix is the dominant damage mech anism, the approach proposed by Budiansky and 20 OConnell [32] for the elastic modulus of a cracked Impact Energy (J) body could be appropriate, which introduces a dam age parameter based on area and perimeter of uni Fig. 10. (a) Elastic modulus and(b) fracture load of ballistic impacted mullite fibre reinforced-mullite matrix composites as a function of formly distributed cracks. However the damage impact energy The values shown are averages of five measurements ntroduced in the present composites under increasing and the relative error was in all cases <10%high level of deformation achieved during 4-point flexu￾ral strength test in a sample that had been impacted at an energy of 10.35 J. The sample did not break into two fragments, demonstrating a true composite, ‘‘pseu￾do-plastic’’ behaviour. A plot of relative Young
s modulus as a function of impact energy is presented in Fig. 10(a). The elastic modulus of the as-received material is rather low (71.6 GPa), but comparable to that of similar oxide/ oxide composites as reported in the literature [10,15]. Decrease in elastic modulus after ballistic impact is ob￾served up to the point of penetration of the projectile (4.2 J impact energy) where structural damage is max￾imum. Samples impacted with higher energy projectiles show an increase in Young
s modulus, indicating less structural damage. This is in accordance with the existing understanding that structural damage under ballistic impact increases to a point where impact en￾ergy is just sufficient to cause penetration as observed also in polymer and glass–ceramic matrix composites [30,31]. The continuous decrease of Young
s modulus with increasing impact energy of projectile (below 4.2 J), which is related to the cumulative development of microstructural damage in the sample, may be ana￾lysed by considering a model linking elastic constants and microcracking density. Assuming that micro￾cracking in the matrix is the dominant damage mech￾anism, the approach proposed by Budiansky and O
Connell [32] for the elastic modulus of a cracked body could be appropriate, which introduces a dam￾age parameter based on area and perimeter of uni￾formly distributed cracks. However the damage introduced in the present composites under increasing Fig. 9. Macrograph showing the high level of deformation achieved during the 4-point flexural strength test by a sample that had been impacted at an energy of 10.35 J. The sample did not break into two fragments, demonstrating a true composite ‘‘pseudo-plastic’’ behaviour. 0 10 20 30 40 50 60 70 80 0 5 10 15 20 25 30 35 Impact Energy (J) Young's Modulus (GPa) (a) 0 20 40 60 80 100 120 140 160 180 0 10 20 30 4 Impact Energy (J) Failure Commencement Load (N) (b) 0 Fig. 10. (a) Elastic modulus and (b) fracture load of ballistic impacted mullite fibre reinforced–mullite matrix composites as a function of impact energy. The values shown are averages of five measurements and the relative error was in all cases <10%. A.R. Boccaccini et al. / Composites Science and Technology 65 (2005) 325–333 331