For the dynamic toughness tests, it is essential to ascertain the critical time at crack initiation. The strain measured from the strain gauge would drop dramatically when the crack was about to propagate because the strains accumulated around the crack-tip would be released once the crack moved away. Figure 6 shows the strain signal-time curve from which the stress intensity factor-time curve can be determined. The quasi-static loading rates were 0.04 MPa·√m/s for the crosshead rate of 1 mm/min and 5 MPa·√m/s for 100 mm/min, respectively. The SHTB tests gave a much higher loading rate at ~3× 104 MPa·√m/s. The results of all tested materials are shown in Figure 7. Figure 7 Effect of loading rate and nano-rubber weight fraction on fracture toughness 4.2 Nano-rubber fraction and loading rate effects Figure 7 shows clearly the improving toughening effect with increasing nano-rubber fraction. This trend is true for all three loading rates implying nano-rubber particles can improve the toughness of epoxy even at a high loading rate. However, compared to the lower loading rates, 5 and 0.4 MPa√m/s, the toughness increased by the rubber content under high loading rate, 3×104 MPa√m/s, is less significant. For example, under quasi-static loading, 0.04 MPa√m/s, the toughness of R6 is more than 3.0 times the toughness of epoxy. But under high loading rate, 3×104 MPa√m/s, the toughness of R6 is only ~1.8 times the toughness of epoxy. It is possible that rubber particle cavitation is reduced due to the increased cavitation strength of the nanorubber under high loading rate. Future study will be conducted to clarify the rubber particle toughening mechanisms under different loading rates. The results in Figure 7 show the crack growth responding to different loading rates. For neat epoxy, the toughness KICm is ~1.8 times larger at the dynamic loading rate of 3×10 4 MPa√m/s, compared to the quasi-static loading rates of 5 and 0.4 MPa√m/s, due to thermal blunting of the crack-tip
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