On the transition of fracture toughness in metallic materials with thickness: An atomistic viewpoint

Abstract

From thick to very thin specimens, fracture toughness of metallic materials peaks out before hitting much lower value compared to the constant plane strain fracture toughness value obtained using standard fracture mechanics testing of thick specimens. Understanding this behavior is essential for improving reliability of smallscale metallic devices currently being used for various critical applications. To understand this behavior, it is essential to study the process of dislocations emission and interaction at the crack front and its variation with specimen thickness. To this end, atomistic fracture simulations of pre-cracked single crystal FCC metal (Nickel) specimens representing both thick and thin specimens are performed. At first, stress-state dependent single crystal yield function based on the generalized Schmid-law is associated with the dislocation emission process at the crack front for both thin and thick specimens. Fracture simulations are then performed on single crystal specimens representing different thickness cases. Due to low stress triaxiality prevailing throughout the thickness of thin specimens, dislocation interaction with each other inside the specimen and then with the specimen surface leading to wedge-shaped groove formation on opposite surfaces at crack front is found to be the responsible mechanism of crack propagation in thin specimens. This mechanism provides enhanced fracture toughness to thin specimen compared to thick specimen in which crack propagation is based on high stress triaxiality at the core of the crack front making formation of microvoids and their coalescence as a dominant mechanism of crack propagation. The dislocation configurations generated at the crack front for thick and thin specimens are also studied and the mechanisms for dislocation multiplication in thin specimens compared to thick specimens are highlighted.