Competing mechanisms of particle fracture, decohesion and slip-driven fatigue crack nucleation in a PM nickel superalloy

Abstract

Fatigue cracks may initiate around non-metallic inclusions via particle fracture, particle decohesion and slipdriven nucleation. Cohesive zone techniques within microstructurally faithful crystal plasticity modelling validated by micromechanical experiments (HR-DIC and HR-EBSD) are employed to investigate these nucleation phenomena. Particle fracture and decohesion lead to stress redistribution which influences subsequent energy storage driving slip-driven fatigue crack nucleation. Particle fracture and decohesion strengths were determined and using a stored energy criterion, the number of cycles to initiation of the fatigue microcrack was predicted. A threshold applied stress below which decohesion and fracture do not occur was obtained, thus modestly increasing fatigue life.

Role of Non-metallic Inclusions and Twins on the Variability in Fatigue Life in Alloy 718 Nickel Base Superalloy

“…Therefore, an in-depth understanding of the role of NMIs and their neighboring microstructure will provide critical insights on the causes of variation in fatigue life. Plasticity associated with inclusions embedded in metallic matrix (nickel or steel as a continuum medium) was investigated using finite element analyses (FEA) [18][19][20][21] and more recently FEA coupled with high resolution-digital image correlation (HR-DIC) techniques [22]. These studies demonstrated accumulation of plastic deformation at the "NMI/metallic matrix" interface under cyclic loading in the presence or not of initial damage at the NMI/matrix interface.…”

A mechanistic and stochastic approach to fatigue crack nucleation in coarse grain RR1000 using local stored energy

“…An uncoupled relationship between normal and shear traction is used since we expect the normal traction to be the primary driver of decohesion as demonstrated by Zhang et al 34 and Bergsmo and Dunne. 35 In these studies, decohesion between the metal-oxide interface and oxide particle fracture is observed experimentally and shown to correlate with high interfacial normal stresses and maximum principal stress normal to the fractured plane, respectively. Also, the significant stress redistribution that occurs once the oxide agglomeratematrix interface decoheres is shown to significantly change the energy distribution local to the oxide particle.…”

“…Material data for the metal-oxide interface was obtained from Bergsmo and Dunne. 35 This work argued that the process of decohesion is stress controlled and that failure occurs in a brittle manner releasing all its traction after reaching a peak value. Thus, the choice of a linear traction-separation relation is justified as the shape of the curve would be of secondary importance only.…”

“…There are many different types of possible inclusion that can form, including different compositions and morphologies. There are three ways in which an inclusion can result in initial crack nucleation, Bergsmo and Dunne, 35 (a) fracture of the particle itself, (b) decohesion of the particle-matrix interface and (c) facet nucleation in the matrix surrounding the particle. In any of these cases, the initial crack in the particle or matrix may or may not lead to growth of the facet into a fatigue crack; for instance, if the particle cracks, it may not lead to the development of further crack growth within the matrix.…”

Analysis of the Root Causes of Damage to the Edges of Tank Manholes on the Main Deck of Handy-Size Bulk Carriers

“…Fatigue decohesion occurs when the maximum normal stress around a concentrator is equal to the metal cohesive strength. This area was studied by Gołaski, Zakrzewski, Lardner, Bergsmo [ 22 ] and others.…”