Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.Comment: 35 figures, which are small (and blurry) due to the space limitations; submitted (with original figures) to Physical Review B. Final versio

A New Model for Void Coalescence by Internal Necking

A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.

Numerical Simulations of Void Linkage in Model Materials using a Nonlocal Ductile Damage Approximation

Experiments on the growth and linkage of 10 μm diameter holes laser drilled in high precision patterns into Al-plates were modelled with finite elements. The simulations used geometries identical to those of the experiments and incorporated ductile damage by element removal under the control of a ductile damage indicator based on the micromechanical studies of Rice and Tracey. A regularization of the problem was achieved through an integral-type nonlocal model based on the smoothing of the rate of a damage indicator D over a characteristic length L. The simulation does not predict the experimentally observed damage acceleration either in the case where no damage is included or when only a local damage model is used. However, the full three-dimensional simulations based on the nonlocal damage methodology do predict both the failure path and the failure strain at void linkage for almost all configurations studied. For the cases considered the critical parameter controlling the local deformations at void linkage was found to be the ratio between hole diameter and hole spacing

Study of a 2024 aluminium rod produced by Rotary Forging

An investigation of the rotary forging process of a 2024 aluminium rod is summarised. Some dispersion in mechanical properties and chemical composition of the base material is permitted. Samples of two material batches were selected: one just stays near the upper limit of tolerance and the other has mean properties. Tensile and compression tests confirm the different mechanical behaviours and allow the identification of constitutive laws parameters. Optical metallography after T3 and T10 thermal treatments and differential thermal analysis provide the grain size and precipitation characteristics of each material batch, which explain their different mechanical behaviours. The industrial rod studied is usually forged in two operations: a first forging process, then a T10 thermal treatment followed by a second forging step. Industrial practise shows that manufacturing the rod with one forging step fails. FEM simulations of the process coupled with a fracture criterion confirm the advantage of a two-step process compared to a single forging step

SOI membrane-based pressure sensor in stress sensitive differential amplifier configuration

This paper introduces a pressure sensing structure configured as a stress sensitive differential amplifier (SSDA), built on a Silicon-on-Insulator (SOI) membrane. Theoretical calculation show the significant increase in sensitivity which is expected from the pressure sensors in SSDA configuration compared to the traditional Wheatstone bridge circuit. Preliminary experimental measurements, performed on individual transistors placed on the membrane, exhibit state-the-art sensitivity values (1.45mV/mbar). © 2012 IEEE

On localization and void coalescence as a precursor to ductile fracture

Two modes of plastic flow localization commonly occur in the ductile fracture of structural metals undergoing damage and failure by the mechanism involving void nucleation, growth and coalescence. The first mode consists of a macroscopic localization, usually linked to the softening effect of void nucleation and growth, in either a normal band or a shear band where the thickness of the band is comparable to void spacing. The second mode is coalescence with plastic strain localizing to the ligaments between voids by an internal necking process. The ductility of a material is tied to the strain at macroscopic localization, as this marks the limit of uniform straining at the macroscopic scale. The question addressed is whether macroscopic localization occurs prior to void coalescence or whether the two occur simultaneously. The relation between these two modes of localization is studied quantitatively in this paper using a three-dimensional elastic-plastic computational model representing a doubly periodic array of voids within a band confined between two semi-infinite outer blocks of the same material but without voids. At sufficiently high stress triaxiality, a clear separation exists between the two modes of localization. At lower stress triaxialities, the model predicts that the onset of macroscopic localization and coalescence occur simultaneously. [GRAPHICS] .The support of the Belgian Science Policy through the IAP 7/21 project is gratefully acknowledged

Evolving texture-informed anisotropic yield criterion for sheet forming

In this work we compare the predictions of the phenomenological anisotropic plane-stress plasticity model BBC2008, calibrated either classically by means of mechanical tests, or by crystal plasticity virtual experiments, to those of a HMS type model with continuous calibration of the same phenomenological model BBC2008. An industrial-grade aluminum alloy AA-6016 is chosen for the test case. Experimental part of the study includes tensile tests and deep drawing of cylindrical cups, preceded by measurements of crystallographic texture. It was found that the material exhibits a noticeable through-thickness gradient in terms of both the texture and plastic anisotropy. The classical calibration of the 16 parameters of BBC2008 was done from tensile experiments (yield stresses and Lankford coefficients) in directions every 15° from the rolling direction and the biaxial yield stress and anisotropy coefficient. The initial texture for the HMS-BBC2008 model was determined from as received samples. The ALAMEL model was used to estimate polycrystal plasticity from texture information. Finally, the earing number and height profile was measured for experimentally drawn cups and compared to the results of macroscopic simulations, based on both the classically calibrated BBC2008 model and the continuously calibrated HMS-BBC2008 model.status: publishe