PSI - Issue 30

A.A. Alexeev et al. / Procedia Structural Integrity 30 (2020) 1–5 A.A. Alexeev et al. / Structural Integrity Procedia 00 (2020) 000–000

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To determine the crack velocity at various stages of crack propagation and branching by method of current conducting strips breaking, an installation was created (Fig. 2), based on a high-speed board for PCI bus LA n10M8-100 and a precision converter “Tercon” signals of resistance thermometers and thermocouples, connected to the computer. The converter “Tercon” was used to measure the temperature of the pipe surface and the environment. A system of conductive strips, resistors was used to measure the velocity of the crack.

Fig. 2. Scheme of the measurement installation.

On a previously prepared, covered with a layer of dielectric paint, surface of the vessel, strips of aluminum foil were applied closing at the copper contact pads (Fig. 1, 3). The contact pads were glued to metal strips welded on both sides of the vessel, which were made so that during the experiment as a result of the expansion of the surface of the pipe, the contact pads would not peel off. The strips are spaced at an equal interval of 50 mm; the first strip is glued close to the edge of the artificial defect. The surface with glued strips and contact pads was additionally waterproofed with several layers of dielectric paint (Fig. 1, 3). Voltage was applied to the system of conductive strips, resistors from the power supply and recorded by the PCI LA-n10M8-100 board at a typical resistance of 10 Ohms (Fig. 2). In the process of movement and rupture by a crack of conductive strips, the voltage at the measuring resistance dropped and the recorded signal was stepwise. The board is configured to capture and save signal changes to a predetermined value. Thus, at the beginning of the crack motion and a corresponding change in the signal, automatic registration and storage of the obtained step signal in the computer memory occurred. Dividing the distance between the conductive strips by the time interval of the corresponding signal stage, we obtained the average crack propagation rate in this section. 3. Results and discussion In tests of vessels, crack propagation in all cases was initiated from an notch (Fig. 3). The duration of experiments to fracture ranged from 8 to 10 hours, depending on the temperature of the outside air and the volume of liquid in the cylindrical shell. The ambient temperature during the tests ranged from -15° C to –20° C. During fracture, the crack moved straight from the notch in both directions, as the velocity increased, the crack began to curved the trajectory, then the crack branched (Fig. 3). The crack branching was accompanied by the formation of microbranches at an angle of 8-10 degrees to the main crack, both before and after branching of the main crack, the angles of branching of the crack are 40-60 degrees, which is significantly larger than the angles of microbranches.