Papers by Author: Marc Fivel

Abstract: Interlocked materials are new examples of “hybrid materials”, mixing materials and structures at a millimetric scale. They consist of periodic assemblies of elementary blocks with specific shapes, maintained in contact by compressive boundary conditions. These “pre-fragmented materials” can simultaneously fulfil antagonistic properties such as high strength together with good damage tolerance. We performed indentation tests on two different structures: (i) an assembly of osteomorphic ice blocks and (ii) an assembly of plaster made cubes. The tests being performed up to the failure, it is found that these structures dissipate much more mechanical energy than similar monolithic plates and preserve their integrity up to much larger deformation. A numerical modelling is then developed in order to reproduce this behaviour. Using finite elements, we simulated the friction contact between two elastic cubes or blocks, for a given lateral load and friction coefficient. The outputs are then introduced as local contact rules in a “Discrete Elements code” specially developed for this study. The discrete code is then used to model the elastic and damage behaviour of assemblies of cubes or osteomorphic blocks. The comparison with experimental results is satisfactory. Finally, the code is used to model larger assemblies of interlocked structures for which the force path is analysed.

Abstract: The combined effect of cyclic thermal shocks and static tensile loading is investigated, in a 304L stainless steel. During these experiments, the stress state in the cylindrical specimen walls is nearly equi-biaxial (σZZ ≈ σθθ). In dislocation dynamics (DD) simulations carried out with σZZ = σθθ, the predominant slip directions b are nearly aligned with the free surface normal vector n, regardless of their associated activation ratio (A.R.). This effect is related to the "surface connected volume" (SCV) of the predominant slip systems. Hence, surface grains with n = <110> possess "large SCV slip systems" and therefore, constitute preferential sites for micro-crack initiation in thermal fatigue. During the tests, a marked effect of the superimposed static tensile loading (or mean stress) is also noted. This effect is explained with the help of DD simulations performed with a positive mean stress: slip irreversibility in the individual persistent slip bands systematically augments with increasing mean stress.

Abstract: Nanoindentation is an interesting technique used to probe the local mechanical properties of a material. Although this test has been widely used and developed over the world during the past few years, it remains a lot of uncertainties regarding the interpretation of nanoindentation data. In this study, we propose to simulate the nanoindentation test of FCC single crystals like Cu or Ni using three numerical models. At the lowest scale, molecular dynamics simulations give details of the nucleation of the first dislocations induced by the indentation. At an intermediate scale, discrete dislocation dynamics simulations are performed to study the evolution of the dislocation microstructure during the loading. Finally, at the upper scale, 3D finite element modelling using crystal plasticity constitutive equations give a continuum description of the indentation induced plasticity. It is shown how the different models are interconnected together.

Abstract: Fatigue simulations are performed by using the new parallel discrete dislocation dynamics code. The effects of particles (shearable or non-shearable) on the fatigue properties, e.g. the cyclic mechanical response and the surface markings, are presented. The simulated results are found to represent the features observed in the experiments well. Fatigue of materials containing both shearable and non-shearable particles (bimodal case) is also simulated. The Orowan loops accumulated around the non-shearable particles promote a dispersion of the slips by a local cross slip, and the fatigue features of the bimodal case are in between those of the shearable and the non-shearable particle case.

Abstract: The early stages of the formation of dislocation microstructures in low strain fatigue are analysed,using three-dimensional discrete dislocation dynamics modelling (DDD). A detailed analysis of the simulated microstructures provide a detailed scheme for the persistent slip band formation, emphasizing the crucial role of cross-slip for both the initial strain spreading inside of the grain and for the subsequent strain localization in the form of slip bands. A new ad-hoc posttreatment tool evaluates the surface roughness as the cycles proceed. Slip markings and their evolutions are analysed, in relation to the dislocation microstructure. This dislocation-based study emphasizes the separate contribution of plastic slip in damage nucleation. A simple 1D dislocation based model for work-hardening in crystal plasticity is proposed. In this model, the forest dislocations are responsible for friction stress (isotropic work-hardening), while dislocation pile-ups and dislocation trapped in Persistent Slip Bands (PSB) produce the back stress (kinematic workhardening). The model is consistent with the stress-strain curves obtained in DDD. It is also consistent with the stress-strain curves experimentally obtained for larger imposed strain amplitudes.