13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Using both experimental and numerical methods the time to fracture was determined to be 17.5 µs. This yields an apparent dynamic fracture toughness for PMMA of 2.75 MPa√m from the above analysis, Fig. 4. A number of other tests were carried out at varying loading rates. Fig. 5 shows the stress intensity evolution for a test performed at a velocity of 9.5 m/s. The variation in amplitude of the oscillations between the two tests is as a result of the increased rate of loading. Figure 5. Stress intensity evolution For PMMA at 9.5m/s Fracture time for the above test was 14.5 µs, which corresponds to an apparent toughness of 2.19 MPa√m. These results agree well with other authors, [5] and show fracture toughness for PMMA increases at dynamic loading rates. The use of Eq. (5) was shown to greatly underestimate the temporal SIF. For this equation to yield accurate results it is important that the strain measurements be made within the singular zone. The strain gauges used in these tests were 1.5mm in length. Relative to the samples being tested these dimensions are quite large, and while care was taken in their placement, the strain measurements were recorded outside the singular zone therefore leading to inaccurate results. As a result of this the use of strain gauges will be restricted to determining the fracture time for later tests. It should be noted that the use of smaller strain gauges might avoid this problem. A third method was also tested for comparison. This test procedure uses an impact response curve (IRC) for determination of dynamic fracture toughness, [16,17,18]. This method makes use of the relationship between the dynamic fracture toughness and time to fracture. The IRC is determined once for a specific test condition (impact velocity, specimen material and geometry) using FVM simulation with a contact procedure and subsequently only requires the time to fracture found through experimentation in order to determine the dynamic fracture toughness for successive tests of similar setup. Fig. 6(a) and (b) show the IRC’s determined for an OPB test setup at a particle velocity of 6 m/s using a Young’s modulus of 6 GPa and 3 GPa respectively, corresponding to the dynamic and static moduli used by Ivanković and Williams, [19]. In order to determine the IRC for each case the sample was prevented from fracturing. The strain was determined directly at the crack tip in the model within the singular zone, and Eq. (5) was used to calculate the temporal SIF’s for these cases. Using the time to fracture determined experimentally from an equivalent test setup the apparent
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