124 C. Barile and C. Casavola Table 17.1 Pico sensor main characteristics Characteristics of the pico sensor Peak sensitivity 54 dB Operating frequency range 200–750 kHz Resonant frequency 250KHz Temperature range 65 to177 ıC Dimension 5 4mm Fig. 17.4 Final recorded image of CCD camera of one representative delaminated specimen where a is the crack length, B is the specimen width and U represents the total elastic strain energy of the specimen. Energy release rate for Mode I DCB test is evaluated according to standardized formula referring to the beam theory as follows: GBT I D 3Pı 2Ba (17.2) where P is the maximum load and • is the deflection in correspondence of the load. In Fig. 17.4 is reported a representative image of delaminated specimen recorded by CCD camera. The image reported also the time information used to identify the maximum load for fracture toughness evaluation. 17.3 Results and Discussion In this work, the DCB specimens were loaded according to the procedures represented in ASTM D5528 so as to investigate the delamination in carbon/epoxy composites under mode I loading. Figure 17.5 shows a representative load-displacement graph overlapped on crack-growth displacement diagram for one of the 9 specimens. As can be seen from Fig. 17.5 by increasing load up to 98 N, strain energy is accumulated in the specimen. When load reaches to 98 N, stored strain energy in the specimen (GI) reaches to the critical value (GIC) and delamination initiates. Beyond the maximum load, the nonlinearity is considerable. Initial growth of the delamination seems not to be affected by fiber bridging phenomenon that generally occurred behind the crack tip in glass fiber reinforced plastics (GFRP) [4]. It is interesting to note in the crack growth trend that after the maximum load there is a heavily increasing slope of the curve during the loading, confirming that delamination occurs.
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