A geometric approach to modeling microstructurally small fatigue crack formation: II. Physically based modeling of microstructure-dependent slip localization and actuation of the crack nucleation mechanism in AA 7075-T651

Abstract

The objective of this paper is to develop further a framework for computationally modeling microstructurally small fatigue crack growth in AA 7075-T651 (Bozek et al 2008 Modelling Simul. Mater. Sci. 16 065007). The focus is on the nucleation event, when a crack extends from within a secondphase particle into a surrounding grain, since this has been observed to be an initiating mechanism for fatigue crack growth in this alloy. It is hypothesized that nucleation can be predicted by computing a non-local nucleation metric near the crack front. The hypothesis is tested by employing a combination of experimentation and finite element modeling in which various slip-based and energy-based nucleation metrics are tested for validity, where each metric is derived from a continuum crystal plasticity formulation. To investigate each metric, a non-local procedure is developed for the calculation of nucleation metrics in the neighborhood of a crack front. Initially, an idealized baseline model consisting of a single grain containing a semi-ellipsoidal surface particle is studied to investigate the dependence of each nucleation metric on lattice orientation, number of load cycles and non-local regularization method. This is followed by a comparison of experimental observations and computational results for microstructural models constructed by replicating the observed microstructural geometry near second-phase particles in fatigue specimens. It

Fatigue crack nucleation: Mechanistic modelling across the length scales

“…Fatigue indicator parameters (FIPs) were employed in [51] in fatigue studies on a peak-aged aluminium alloy in which fatigue crack nucleation is premised to be preceded by the development of persistent slip local to a non-shearable cracked particle inclusion. Fatigue cracks are thought to develop more rapidly in this material than in those for which PSBs do develop, and typically, in the experimental study, the regions of slip localisation were found to be quite diffuse.…”

Modeling Near-Crack-Tip Plasticity at Nano- to Micro- Scales

“…Similar to the studies of nickel-based alloys, detailed three-dimensional finite element models of aluminum 7075-T651 have been generated to predict the initiation of cracking at the microstructural scale (Hochhalter, 2010). To better understand the slip accumulation during cyclic loading that precedes nucleation events, these finite element models were generated using observed microstructural data that included the configuration and location of constituent particles and grain microstructural details.…”

An Overview of Innovative Strategies for Fracture Mechanics at NASA Langley Research Center

“…By incorporating models for slip accumulation, a relationship between plastic exhaustion and crack growth can be computed 17 .…”

“…The discrepancy between simulations and experiments has attracted considerable attention among researchers because it prevents the reliable and accurate modeling of fracture in particular and puts doubt on the reliability of the atomistic simulations in general 17,31 . Most likely, the source of this discrepancy is related to the very different length (nanometers vs. millimeters) and time (nanoseconds vs. seconds) scales at which simulations and experiments are usually performed.…”