Damage mechanical investigations at ductile fracture of free-cutting steels by means of microscopy - void growth and fracture surface topography

Abstract
At ductile fracture of metallic materials the growth of micro-voids generated by particles in the metallic matrix as for example manganese sulfides can be observed as the dominating damage mechanism. In order to measure and model this damage usually damage parameters such as for example the void radius are used. The final size of grown voids at crack initiation can be estimated by measuring the dimple sizes on the fracture surfaces. Beside these values topometric parameters were applied to characterise the fracture behaviour of the material by its fracture surface. The final void sizes at craak initiation measured on the fracture surfaces partly show a decreasing tendency with higher stress triaxiality, that means a competition between stress state and void growth can be observed. The measured void sizes are generally comparable to values from model calculations. Using the fractal dimension it is possible to distinguish fracture surface topographies resulting in materials showing different fracture toughness values.