Recent advances in modelling ductile rupture

International audienceA brief account of recent advances in modelling ductile rupture is given. The importance of the inhomogeneity in the distribution of cavity nucleation sites is firstly emphasized. Then some recent extensions of the Gurson model to account for non spherical void shape are presented. Finally recent progress in modelling cavity coalescence is highlighted

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.

Determination of the intrinsic behavior of polymers using digital image correlation combined with video-monitored testing

AbstractThree methods for the determination of the large-strain behavior of ductile polymers are compared in both tension and compression. Each method relies on some (non-contact) measurement of the strain and some approximations in the calculation of stress. The strain measurement techniques include digital image correlation (DIC) and two techniques of video-based extensometry: marker tracking and area variation monitoring. Since the specimens are inevitably subject to structural plastic instabilities (necking in tension, barreling in compression) the strain and stress states are no longer uniform in the gauge section after the peak load. Under such circumstances, it is demonstrated that the three experimental methods can lead to significant differences. It is inferred from the comparative analysis that the method based on vertical marker tracking is not reliable. Validated by DIC, video-based area variation is shown to be a simple alternative way to obtain an excellent estimate of the intrinsic true stress–strain behavior of the polymer

Effect of UV-aging on the mechanical and fracture behavior of low density polyethylene

This research is possible by the NPRP award (NPRP 7-1562-2-571) from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the re-sponsibility of the authors. Special thanks to the Qatar Petro-chemical Company (QAPCO) for providing the film materials of the study and for fruitful discussions with Dr. Mabrouk Ouederni.In polyethylene, a transient, oxidation-induced strengthening is often observed over a narrow range of UV radiation dose. In addition, plastic deformation may not be volume-preserving due to cavitation. Here, we employ a suite of analytical experiments and mechanical testing on pristine and oxidized low-density polyethylene films in order to investigate the transient strengthening behavior as well as the propensity for cavitation to fracture. Emphasis is laid on connecting macroscopically observed behavior with microscopic information involving the competition between multi-scale phenomena: chain scission and cross linking at a fine scale, chemi-crystallization, oxidation-induced cracking and mechanical damage at the meso and coarse scales. The results provide an insight into the role of cavitation in the oxidative embrittlement of semicrystalline polymers

Failure of metals I: Brittle and ductile fracture

This is the first of three overviews on failure of metals. Here, brittle and ductile failure under monotonic loadings are addressed within the context of the local approach to fracture. In this approach, focus is on linking microstructure, physical mechanisms and overall fracture properties. The part on brittle fracture focuses on cleavage and also covers intergranular fracture of ferritic steels. The analysis of cleavage concerns both BCC metals and HCP metals with emphasis laid on the former. After a recollection of the Beremin model, particular attention is given to multiple barrier extensions and the crossing of grain boundaries. The part on ductile fracture encompasses the two modes of failure by void coalescence or plastic instability. Although a universal theory of ductile fracture is still lacking, this part contains a comprehensive coverage of the topic balancing phenomenology and mechanisms on one hand and microstructure-based modeling and simulation on the other hand, with application examples provided

Photo-oxidation of semicrystalline polymers: Damage nucleation versus growth

International audienceThe relative roles of damage nucleation and growth in the oxidative embrittlement of polymers has been investigated in tension. Additive-free polyamide-6 was used, taking advantage of prior knowledge of damage mechanisms in the unaged condition. Accelerated aging was carried out on bulk and thin specimens, which were then mechanically tested to fracture. Videos of the tests enabled the measurement of specimen deformation and detection of surface microcracks and their evolution. In addition, in situ synchrotron tomography was used to observe damage nucleation and growth in the thin specimens. It was found that distributed cracking occurred well before the load maximum and that the density of microcracks increased with both strain and oxidation levels. Using a combination of video-based extensometry and digital image correlation, significant strain-induced dilatations were measured for both unaged and oxidized specimens. In addition, damage nucleation by chain scission was found to be heterogeneous, as the molar mass between entanglements, inferred from the true strain hardening rate, was unaffected by oxidation. Finally, a chemical-stress based model is invoked to interpret gradual ductile-to-brittle transitions in oxidative embrittlement

Anisotropic ductile fracture, part II. theory

International audienceA theory of anisotropic ductile fracture is outlined and applied to predict failure in a low alloy steel. The theory accounts for initial anisotropy and microstructure evolution (plastic anisotropy, porosity, void shape, orientation and spacing) and is supplemented by a recent micromechanical model of void-coalescence. A rate-dependent version of the theory is employed to solve boundary value problems. The application to the studied steel relies on material parameters inferred from quantitative metallography measurements. The quantitative prediction of damage accumulation and crack initiation in notched bars is achieved without any adjustable factor and is discussed under various stress states and loading orientations

Anisotropic ductile fracture, part I. experiments

International audienceDeformation and fracture are investigated at room temperature in steel through tension and compression tests. Round smooth bars and cylinders are used to characterize the deformation behavior while round notched bars are used to explore stress state and specimen orientation effects on fracture. The microstructure is characterized to infer initial average values of porosity, void aspect ratio and void spacing ratio, all three playing a key role in the fracture process. Interrupted tests are used to determine the spatial distribution of those variables after crack extension thus providing a basis for comparison at the micro-scale with theoretical predictions. All damage stages are found to be inherently anisotropic, including void nucleation and crack propagation. An important finding of this investigation is that, in quantitative modeling of ductile fracture, any void growth model should be supplemented by a physically motivated void coalescence model

Evolution of the 3D plastic anisotropy of HCP metals: Experiments and modeling

International audienceA two-surface, pressure-insensitive plasticity model is further developed to represent the mechanical response of hexagonal close packed metals. The model describes the 3D plastic anisotropy of a material, the tension–compression asymmetry, and the consistent evolution upon straining of both the net anisotropy and the asymmetry. The model may be viewed as a reduced order quasi-crystal plasticity model whereby the two activation surfaces represent glide- and twinning-dominated flow. The two-surface formulation enables to represent independent, yet coupled, hardening laws in terms of effective plastic strains accumulated on either generic deformation system. Application of the model to a discriminating data set assembled for a magnesium alloy thick plate illustrates the capabilities and versatility of the modeling approach

The stored energy of cold work: Predictions from discrete dislocation plasticity

The stored energy of cold work is calculated for planar single crystals under tensile loading with plastic deformation occurring through dislocation glide. Superposition is used to represent the solution of boundary value problems in terms of the singular fields for discrete dislocations and image fields that enforce boundary conditions. Constitutive rules are used which account for the effects of line tension and three-dimensional dislocation interactions including dynamic junction formation. The stored energy is calculated both under load and after load removal and methods are devised to estimate the local plastic dissipation and to separate out the contribution of long-range stresses to the energy stored. Calculations are carried out up to imposed strains of 0.05–0.1 and the effects of strain level, dislocation structure and crystal orientation on the evolution of the stored energy are investigated. Although the flow stress and work hardening rate depend mainly on the dislocation density, the stored energy of cold work depends on details of the dislocation structure that forms, with any long-range dislocation stress field playing a significant role. The calculations exhibit a connection between the stored energy of cold work and the Bauschinger effect. It is also found that local energy storage values can differ substantially from the average value.