13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- dynamic fracture toughness is found to be 3.44 MPa√m in the case where the material modulus is taken to be 6 GPa, and 2.84 MPa√m when the modulus is 3 GPa. The model with the higher, dynamic, Young’s modulus is shown to overestimate the dynamic fracture toughness when compared to the load-point method presented above. Conversely, when the static Young’s modulus of the material (3 GPa) is employed the model predicts an apparent dynamic fracture toughness similar to that of the load-point method, 2.84 MPa√m versus 2.75 MPa√m. This suggests that the static Young’s modulus may be a more appropriate measure for this test setup. This point is further reinforced by noting that the quarter period of oscillation (14 µs) of the IRC developed using the dynamic Young’s modulus, Fig. 6(a), is less then the experimentally determined fracture time of 17.5 µs, Fig. 4. Figure 6(a). Impact Response curve Young’s Modulus: 6 GPa, (b) Impact Response curve Young’s modulus: 3 GPa 4.2 Advanced Ceramic Tests A number of tests were carried out on ceramic samples. Due to the inherent difficulty in introducing pre-cracks into hard brittle materials all samples were tested with a blunt notch of 150µm in radius. As a result of this the dynamic fracture toughness values are apparent values. Static fracture toughness values were taken from previous testing [12]. Fig. 7 shows a typical strain trace superimposed on a FVM simulation test with a contact procedure implemented at the incident bar/specimen interface [20]. Very good agreement is observed between experimental and numerical tests. The samples were not instrumented during this round of tests and so only the SIF evolution from the one-dimensional analysis will be calculated. Fig. 8 shows the incident and reflected strain for two tests on samples A and B. The two grades of advanced ceramic vary in both inclusion grain size and second phase material and are denoted A and B. It can be seen from the trace that the degree to which the incident wave is reflected is very similar in both cases. This is to be expected, as at increased rates of loading the values of fracture toughness for both grades of material have been previously shown to be similar [21]. Fig. 9 shows the SIF for both grades of advanced ceramic. The time to fracture for these events was not recorded as mentioned above so a value of fracture toughness at these rates cannot be obtained. It is of interest for future work to instrument the samples using minute strain gauges close to the crack tip as per the PMMA testing. The wave speed velocity in these samples is in the region of 11,000 m/s, resulting in an extremely rapid fracture event. With this in mind future work will also focus on developing the numerical model further to ensure the most accurate determination of fracture time possible. Quarter Period Oscillation Fracture Time = 17.5µs K1db = 2.84 MPa√m
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