Structural geometrical transitions under dynamic loading of materials

It is known that the destruction of solids, including brittle and quasi-brittle ones, in the field of external forces is preceded by the appearance of a certain density of crystal structure defects. Collective movements of such defects in the cooperative interaction with the structure of the material at different scale structural levels (from nano- to macro-) define the process of destruction. In fact, the destruction is the final stage of plastic deformation of solids. Over the past two or three decades, it was found that the profile and surface of dynamically destructible materials are fractal objects. In studies which had been carried out aimed at the possible use of the fractal dimension as a characteristic that allows linking the various parameters of the processes of destruction and the dynamic properties of the materials. Laboratory samples of the three alloys used for pipeline ship fittings were exposed to impact tensile; and then structural studies were carried out of the destroyed specimens. The experiments were performed according to the Kolsky method using a split Hopkinson bar (SHB) at strain rates from 103 to 3·103 s-1. Properties were investigated of the 3M titanium, as well as of 08Kh18N10T stainless steel and BrAZhNMts bronze. We obtained the dynamic stress-strain curves, strength properties and the limit characteristics of plasticity. It was found that the samples destruction was preceded by the acts of microplastic deformation within the activation volume not exceeding the volume of the material grain. Under high-speed loading the damage of material is implemented consistently with the participation of ensembles of the crystal structure defects due to accumulation and changing of their spatial organization. As а parameter for searching the correlations between the strain rate, the type of fracture and the mechanism of structural arrangement it is offered to use the fractal dimension of the contour in the fracture surface of the dynamically loaded material specimen. Also, the possibility of using the fractal dimension for the ranking of material properties was demonstrated.