13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- its transparency gives the chance to see a crack extension and to control it by photo recording methods. Crack propagation was registered by a streak camera which allowed to receive the time sweep of crack trajectory with the resolution 40-400 ns. 2. Experimental Technique Our experiments were performed on the samples of injection molding PMMA with the parameters: density ρ = 1180 kg/m3, longitudinal sound velocity CL = 2750±25 m/s, transverse sound velocity CTR = 1400±25 m/s, modulus of elasticity E = 5.9 GPa. The quasi-static loading was carried out on a tensile testing machine. The dumbbell-like plane samples with the dimensions of the working part of 93×35×5 mm were used. To initiate the crack, a notch was made by a razor blade in the middle of the working area. The notch depth was 0.4 - 1.3 mm. The dynamic loading was realized by means of the setup for electrical explosion of conductors. The capacitor capacity was С = 1.0 µF; the charge voltage, U ≤ 25 kV; the stored energy, Е ≤ 312 J. The samples were in the form of square plates (200×200 and 98×128 mm) with a side notch. The sample thickness was 5, 8 and 10 mm. The notch was 50 mm in length and 0.5 mm in width. The size of the greater samples was selected so that to remove the influence of reflected waves on the process of crack propagation during the registration time. The localized pressure pulse was generated by electrical explosion of a wire. The exploding wire (∅ 0.2 mm) was placed between the edges of the notch perpendicular to the plane of the plate at the distance of 24-31 mm from the notch base. A lavsan film was used to create the acoustic contact between the wire and notch edges. The same sample was used several times. The registration of crack front extension was carried out by means of a streak camera (K008) using the method of slit-type scanning of an image. For this purpose, a beam of light was directed at the expected place of crack propagation at an angle to the plane of the sample. The camera recorded the space-time scan of the beam of light reflected from the surface of the growing crack. In the quasi-static tests, the synchronization of the camera was made on the change of intensity of a laser beam passing through the notch base. In the case of dynamic loading, the synchronization was carried out on the signal from a current sensor. The detailed diagrams of the experiments are presented in [5]. The registration of caustic near with a crack tip was carried out on the basis of schemes of receiving shadow optical images described in [6]. The change of caustic was registered by scanning the real image with help the streak camera. However, the diameter of caustic was registered along crack extension rather than across (as generally accepted). Note that it may affect quantitative estimates because of the large singularity at the back of caustic. The calculation of the stress intensity factor was made according to the following formula [6]
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