Understanding, observation, modelling and simulation of ductile damage mechanisms – COMINSIDE

The objective of the present proposal is to study the intimate interaction between strain and damage mechanisms at the micrometer scale during ductile failure in the bulk for different levels of stress triaxiality, and to understand the underlying physical mechanisms. These experimental results will be investigated further via microstructural and macroscopic finite element simulations trying to account for the observed micro-mechanisms including particle fracture/debonding or void nucleation and subsequent void growth and coalescence. To achieve this goal, three modern techniques will be seamlessly coupled, namely, laminography to image in situ tested large flat samples made of ductile materials, volume correlation to measure 3D displacement (and strain) fields in the bulk, and 2D/3D simulations using the experimental information on multiple length-scales.

In situ experiments assessing damage events in 3D for various large flat specimen geometries and associated levels of stress triaxiality ratios ranging from pure shear to highly triaxial stress states are made possible for the first time by synchrotron laminography. Laminography is particularly adapted to obtain reconstructed volumes of regions of interest in objects that are laterally extended (i.e., in two directions) and thin in the third direction, i.e. sheet-like objects. Strain localization and necking phenomena during ductile tearing can be studied in unprecedented manner due to the possibility of using large specimens. Displacement (and strain) fields will be measured in the bulk of the observed material using the natural 3D contrast of the microstructure via digital volume correlation.

In the subsequent simulations an attempt will be made to reproduce the initial microstructure and the evolution of coarse particles and voids during damage nucleation, growth and final failure. Numerical simulations will use level-set functions to define interfaces between particles-voids-matrix and an advanced anisotropic mesh adaptation technique will enable the evolution of 3D microstructures to be accurately described under various loading conditions. For the first time, modeling of voids nucleation and coalescence will be made possible in 3D under large plastic strain. In addition, boundary conditions for the simulations will be varied from idealized configurations to actual fields measured via digital volume correlation. The measured strain fields will be compared to the model predictions for the different used modeling approaches.

Two different materials will be used, namely, nodular cast iron that provides large particles whose evolution can be followed in detail in situ via laminography at the achievable micrometer resolution and that also provides strong 3D image contrast for digital volume correlation. In addition, a recently developed aluminum alloy for aerospace applications with smaller particles and low particle volume fraction will be assessed to investigate the transferability of the results.

The combination of a new promising observation technique (laminography), of advanced digital volume correlation, and of accurate 2D/3D modeling of particle failure/debonding and void growth/coalescence makes this project particularly suitable to get a better understanding of ductile damage mechanisms at the microscale for various loading conditions, and their interactions with plasticity.