23 Fracture Toughness and Impact Damage Resistance of Nanoreinforced Carbon/Epoxy Composites 175 VARTM processing. The same procedure was used for the reference and nano-reinforced (hybrid) composites. After curing, composite plates were rough cut into specimens by means of diamond-abrasive cutting wheels, and wet-polished with SiC abrasive papers to final dimensions and smoothness. 23.3 Mode-II Interlaminar Fracture Toughness Initial tests were performed to quantify Mode-II fracture toughness of the composite. The standard three-point End Notched Flexure (ENF) test was chosen for its overall simplicity in testing and analysis (Fig. 23.3) [16, 17]. Specimens were prepared with nominal dimensions LD25mm, a0 D7.6mm, bD12mm, 2hD4 mm (8 plies). Tests were carried out on numerous specimens of each material type (reference and hybrid composite), giving numerous curves of the form shown in Fig. 23.4. From these curves, calculation of the Mode-II strain energy release rate was made by various methods. From the peak load at crack advance alone, the strain energy release rate GII is given by the relation GII D 9a2P2 16E1b2h3 1C0:2 E1 G13 h a 2! (23.1) where ais the crack length bis the specimen width 2his the specimen thickness Pis the applied load at the central roller E1 is the elastic modulus along the beam span G13 is the shear modulus in the plane of bending For reasonably slender specimens of a material with a higher bending modulus than shear modulus (a>>handE1>>G13) may be approximated as GII D 9a2P2 16E1b2h3 (23.2) Fig. 23.3 Diagram of end notched flexure (ENF) specimen Reference Hybrid 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displacement (mm) Load (kN) Fig. 23.4 Representative curves of load-deflection behavior for Mode-II ENF fracture specimens
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