PSI - Issue 20

A.A. Alexeev et al. / Procedia Structural Integrity 20 (2019) 254–258 A.A. Alexeev et al / Structural Integrity Procedia 00 (2019) 000 – 000

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A side cut was made on samples from one end in the middle part with a depth of 3 mm and a width of 1.5 mm with a milling tool, then copper contact pads were glued to the surface in the upper and lower parts, then recording strips of conductive glue on a silver base were applied. A power supply was connected to the upper common copper site, a measuring circuit connected to the lower contact pads was made up of a battery of resistance from 10 Ohm to 60 Ohm, in series increasing in 10 Ohm steps. When the crack is broken by conductive lines, a uniform step voltage drop is obtained on the measuring resistance connected in series to the battery. A chromel-alumel thermocouple is fixed in the middle part of the plate on the reverse side to record the temperature of the plate. The processing of the data with the construction of graphs made in the software environment Mathcad. 3. Results and discussion Two series of tensile tests of flat samples from polymethylmethacrylate (PMMA) were carried out on an Instron tensile testing machine at temperatures of +20 C and – 60 C with measuring the crack velocity. At +20 C, the crack propagated in a straight line in a plane perpendicular to the direction of maximum tensile stresses; fracture stresses were р =10 MPa. Measurements of the crack velocity showed the following: the mirror and matte zones of the fracture surface correspond to low values of the crack velocity 144 – 373 m/s, the feather zone corresponds to the velocities of 418 – 454 m/s, the crack branching occurred at a velocity of about 500 m/s (Fig. 2). At – 60 C, fracture occurs at higher fracture stresses р =20 – 30 MPa, the crack propagated from the notch in a straight line at a distance of 12 – 15 mm to the branch point at a speed of about 750 m/s, after which the fracture occurred through multiple branching and parallel movement of the front of several cracks at speeds of 750 – 920 m/s, the trajectories of movement and branching of cracks are shown in fig. 3. The scatter of measured velocities at the stage of multiple branching is explained by the fact that when moving the front of multiple cracks moving at different angles to the horizon (Fig. 3), the path of each crack is different, and it is impossible to establish which crack first crossed the reference speed measurement line.

Fig. 2. The rate of crack velocity at various stages of propagation, branching cracks.