Investigations into the short crack nucleation and propagation in metals under monotonic and cyclic loading

Abstract

Fracture and fatigue failure of metals is a multiscale phenomenon. The nucleation and propagation of short cracks, originating with the atomic level separation of metal atoms, is dependent on various material specific processes at different length scales. Fundamental to this behaviour is the emission of dislocations and their interaction at the crack front that controls the fracture toughness at the single crystal level. At the inter-grain level, microstructural features such as grain orientation, grain size, phase/grain boundaries, precipitates and inclusions control the propagation of short cracks. Added factors such as presence of an embrittling element like hydrogen within the microstructure can also affect the short crack nucleation and propagation behaviour. Understanding the short crack nucleation and propagation behaviour under monotonic and cyclic loading is the key requirement for designing metallic materials with better resistance to fracture, fatigue and hydrogen embrittlement. Motivated by above fact, the thesis presents a compendium of investigations to understand the fundamental process associated with fracture of metallic single crystals to short fatigue crack propagation in reactor pressure vessel steel (SA508 Garde 3 Class I low alloy steel) along with its hydrogen embrittlement behaviour. The first part of thesis deals with atomistic modelling of crack propagation in precracked nickel single crystals to investigate the factors affecting the fracture behaviour at this scale. The pioneer work of James R. Rice [1], relates fundamental material property such as unstable stacking fault energy per unit area (𝛾􀯨􀯦􀯙) with the dislocation emission leading to ductile failure in comparison to the surface energy per unit area (𝛾􀯦 ) that governs brittle failure by cleavage at the crack front. Under plane strain condition (representing bulk material behaviour), Rice predicted ductile behaviour in various metals compared to brittle failure, if the critical stress intensity factor linked with dislocation emission under Mode I loading (KIe that scales with 􀶥𝛾􀯨􀯦􀯙) is less than the critical stress intensity factor for cleavage (KIc that scales with 􀶥𝛾􀯦 ). Using molecular static simulations, crystal orientation dependent transition in ductile to brittle fracture is investigated. The stress triaxiality at the crack front changes with crystal orientation due to transformation of stiffness tensor Cijkl. It is found that high stress triaxiality supresses the dislocation emission at the crack front leading to cleavage failure even for the cases when KIe< KIc, which is in contradiction to the Rice theory. This is explained based on increase in the γusf value with stress triaxiality which is not accounted for in the Rice theory. Next, transition in fracture toughness of metals with thickness is explained using atomistic simulations of crack propagation in pre-cracked nickel single crystals under Mode-I loading. Metallic materials show a typical transition in fracture toughness value, Kc (MPa√m), with decreasing thickness of the pre-cracked specimens. From thick to very thin specimens, Kc value peaks out before hitting much lower values compared to their constant plane strain fracture toughness value, 𝐾􀯖 􀯉􀬿􀰌 , obtained using standard fracture mechanics testing of thick material specimens. To understand this behaviour, 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, stressstate 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 provided 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 of thick and thin specimens are also studied and the mechanisms for dislocation multiplication in thin specimens compared to thick specimens are highlighted. In the second part of thesis, experimental investigations into short fatigue crack nucleation and propagation in reactor pressure vessel (RPV) steel, SA508 Grade 3 Class I low alloy steel which is used in Indian nuclear power plants, are performed. A considerable proportion of component fatigue life is spent during nucleation and propagation of short fatigue cracks. In general, cracks having length between 10 𝜇m to 1mm are considered short cracks. Most of the defects present in the material are in the range of short cracks, therefore it is important to study the propagation of short cracks with respect to its correlation with the microstructural features such as grain orientation and size, grain/phase boundaries, precipitates and inclusions etc. At first, a short fatigue crack measurement framework is established using travelling digital microscope fitted on the servo-hydraulic fatigue testing machine. Next, short fatigue crack propagated from the small edge notch ranging from 40 to 450 𝜇m in the single edge notch tension (SENT) specimens is studied to establish the correlation among the microstructural features and the propagation behaviour. Unlike long fatigue cracks, the propagation rate of short fatigue cracks cannot be predicted by LEFM based Paris law due to interaction of short cracks with microstructural features as explained for the subject RPV steel in this work. RPV steels are also reported to fail due to hydrogen embrittlement (HE) caused by absorption of even up to 2-6 ppm of hydrogen during operations under high temperature water environment. Due to the presence of hydrogen, diminished role of prior austenite grain boundaries in short fatigue crack growth in hydrogen charged RPV steel is reported that leads to increase in the crack growth rate which otherwise was found to form a strong barrier to short crack propagation in the un-charged steel specimens. Apart from short crack propagation, insitu investigations into short crack nucleation under scanning electron microscope (SEM) are also carried out during monotonic and fatigue loading using specially designed shallow notched specimens. These studies are performed both for un-charged and hydrogen charged specimens. Detailed microscopic analysis provided mechanistic understanding of HE as observed in the subject RPV steel.