Energy dissipation via acoustic emission in ductile crack initiation

The final publication is available at Springer via http://dx.doi.org/10.1007/s10704-016-0096-8.This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture

Crack growth in Fe-Si (2 wt%) single crystals on macroscopic and atomistic level

This paper is dedicated to experimental and atomistic study of the influence of so called T-stress (acting along the crack plane) on fracture processes in bcc iron. We analyze experimental results from fracture tests performed at room temperature on bcc iron-silicon single crystals with a long edge crack (1¯ 1 0)[1 1 0] (crack plane/crack front). The specimens were loaded in tension mode I under different border conditions inducing different sign of the T-stress. The brittle-ductile behavior at the crack front was monitored on-line via optical microscopy together with external force and prolongation of the specimens. Topology of the specimens has been investigated before and after the fracture tests via the white light interferometer. The microscopic processes produced by the crack itself were studied at 300 K via 3D molecular dynamic (MD) simulations in bcc iron under equivalent boundary conditions and the T-stress was examined by means of stress calculations on the atomistic level. The experimental and atomistic results show that the sign of the T-stress affects the fracture behavior. MD simulations reveal that positive T-stress makes the emission of blunting dislocations 〈1 1 1〉{1 1 2} from the crack front more difficult. As a consequence, higher external loading is needed for crack blunting in the experimental specimens with T > 0 in comparison with the specimen having T < 0