Fracture of foam-polystyrene plates with stress concentrators

Experiments were performed to determine the fracture of foam-polystyrene plates of trademark PSB-25 involving some holes, cuts and also without them. The plates of dimensions 1 × 1 m and thickness 5 cm are used. The ultimate strength of material, Young´s modulus, Poisson´s ratio, pre-fracturing strain and critical coefficient of stress intensity were determined. All stress concentrators in plates are made as central holes having circular, elliptic and square shapes. A major axis of elliptic holes is located normally to the stretching directions and at angle 45° to this direction. To analyze experimental data, a numerical algorithm is used because the relation among the sizes of holes and plates does not correspond to the known problems on stress concentrations for infinite planes. Using the gradient fracture criterion as presented in M. A. Legan (1993) and M. A Legan (1994) as the base and the boundary element method (the fictitious stress method), the above computational strength algorithm is constructed based in A. S. Sheremet and M. A. Legan (1999). The key feature of the numerical algorithm is that in calculating it is necessary to determine not only components of stress state but also their derivatives with respect to space coordinates as well for them to be used in the gradient fracture criterion. The experimental data are compared with the calculation results. The application of classic strength criterion resulted in lower evaluations of critical stresses and loads. In comparison with the classical criterion, the use of the gradient fracture criterion leads to better agreement between estimates and experimental data.