Experimental Study of Crack Branching Velocity in Polymers

One of the little-studied problems of modern mechanics and the physics of destruction is crack branching, which is observed in materials of varying nature. In order to study this phenomenon, the criteria and mechanisms for crack branching are analyzed. Studies of crack branching in polymers and steel, as well as fractographic studies of the fracture surface, are considered. It is found that the crack at branching in brittle plastics reaches the limiting propagation velocity V * = 500–800 m/s. Tensile tests of flat samples made of polymethyl methacrylate (PMMA) at temperatures of +20 and –60°C are carried out with measurement of crack velocity by the method of rupture of conductive strips. A measuring device based on a TERCON precision converter of signals of resistance thermometers and thermocouples connected to a computer is created. Crack velocity is measured for (i) rectilinear crack propagation in the mirror, matte, and feather zones of PMMA fracture surface, (ii) single branching of the crack, and (iii) multiple crack branching with parallel movement of the front of several cracks. A hypothesis for the physical mechanism of crack branching is proposed.