Scanning Force Microscopy as a Tool for Fracture Studies

Abstract

Dynamic simulations of the fracture toughness as a function of the orientation and tem- perature were carried out and compared with experimental results obtained by in-situ loading pre-cracked NiAl single crystals inside a scanning force microscope (SFM). In order to compare the simulations with the experiments the problem of the short crack with dislocations was solved for general loading and arbitrary slip line directions. The stress and strain field obtained could be directly connected to FEM calculations which allowed the examination of the stability of micro cracks at notches. The effect of different fracture conditions for biaxial loading were studied in detail.The dynamic simulation yielded predictions of K1C, slip line length and dislocation distri- butions as a function of loading rate, temperature and orientation. These predictions were tested by in-situ loading NiAl single crystals inside a SFM at various temperatures. The local COD, slip line length and apparent dislocation distribution at the surface were measured as a function of the applied load and the temperature. The experiments clearly demonstrated that dislocations emit from the crack tip before unstable crack jumps occur. The local COD could be directly related to the number of dislocations emitted from the crack tip. With increasing temperature the number of dislocations and the local COD increased before unstable crack jumps or final fracture occurred.