Multiscale experimental investigation and numerical simulation of deformation and failure in polycrystalline alloys under shear loading

Recently, there has been a revived interest in ductile fracture of metallic alloys, especially under dominant shear loading condition. Conventional ductile fracture models such as the Gurson–Tvergaard–Needleman model has been developed based on the mechanics of void growth due to volumetric strain and subsequent void coalescence leading to fracture. It has now become evident that these models are not capable of capturing fracture in dominant shear deformation scenarios, which arise commonly in the form of shear localizations in polycrystalline metallic materials. Ad-hoc phenomenological modifications by means of an artificial augmentation of void growth in shear have been introduced to extend the applicability of these models to low stress triaxiality regimes. However, the mechanics and physics of deformation and failure in dominant shear loading at the microstructure of polycrystalline alloys are not well understood. In this article, we report in-situ multiscale examination of deformation processes and failure mechanisms in Al 6061-T6 – a polycrystalline alloy with a dispersion of second phase particles in the microstructure – under low stress triaxiality levels using modified Arcan specimens. Strains at the grain and subgrain levels are measured in a scanning electron microscope by in-situ tracking of the changes in grain size and morphology, and at the macroscale level using digital image correlation. Grain level strains in the range of 2–2.5 are sustained in the material without any indication of failure and shown to be significantly higher than estimates of strain measured using a specimen dimension as the gage length. A continuum material failure model based on grain level strain measurement was introduced and used in numerical simulations to assess the suitability of the proposed failure model. The results from the failure model were compared to those from commonly used Johnson–Cook model. It was noted that although the proposed failure model based on grain based deformation was able to reproduce the essential features observed in the experiments, the results from Johnson–Cook model predicted a premature failure in the material. It was concluded that calibration of failure models requires a suitable length scale and the grain size as an intrinsic property of the material can be used as an appropriate length scale to define strain in failure model calibrations

Cavitation in rubber: an elastic instability or a fracture phenomenon?

It is by now well established that loading conditions with sufficiently large triaxialities can induce the sudden -appearance of internal cavities within elastomeric (and other soft) solids. The occurrence of such a phenomenon, commonly referred to as cavitation, can be attributed to the growth of pre-existing defects into finite sizes. In the first part of this discussion, I will present a new theory within the context of nonlinear elasticity to study the phenomenon of cavitation in rubber that contrary to earlier approaches: (i) allows to consider general 3D loading conditions with arbitrary triaxiality; (ii) applies to general classes of nonlinear elastic solids; and (iii) incorporates direct information on the initial shape, spatial distribution, and mechanical properties of the underlying defects at which cavitation can initiate. The basic idea is to first cast cavitation in elastomeric solids as the homogenization problem of nonlinear elastic materials containing random distributions of zero-volume cavities, or defects. Then, by means of a novel iterated homogenization procedure, exact solutions are constructed for such a problem. These include solutions for the change in size of the underlying cavities as a function of the applied loading conditions, from which the onset of cavitation – corresponding to the event when the initially infinitesimal cavities suddenly grow into finite sizes – can be readily determined. In the second part of the discussion, I will confront the theory with a variety of cavitation experiments with the objective of establishing whether the phenomenon of cavitation is an elastic instability (and hence depends only on the elastic properties of the rubber), or, on the other hand, a fracture process (and hence depends on the fracture properties of the rubber). REFERENCES [1] Lefèvre, V., Ravi-Chandar, K., Lopez-Pamies, O. Cavitation in rubber: An elastic instability or a fracture phenomenon? International Journal of Fracture. 2014. Submitted. [2] Lopez-Pamies, O., Nakamura T., Idiart, M.I. Cavitation in elastomeric solids: I – A defect growth theory. Journal of the Mechanics and Physics of Solids. 2011, 59, 1464–1487. [3] Lopez-Pamies, O., Nakamura T., Idiart, M.I. Cavitation in elastomeric solids: II – Onset-of-cavitation surfaces for Neo-Hookean materials. Journal of the Mechanics and Physics of Solids. 2011, 59, 1488–1505

On the double transition in the failure mode of polycarbonate

In the present work, the transition in the mode of failure from brittle to ductile, observed in certain polymeric materials, is explored both experimentally and numerically, focusing on polycarbonate, a polymer of wide industrial use. The limit between both behaviours depends on several intrinsic factors, such as temperature and deformation rate, and extrinsic factors such as notch radius and specimen thickness. The parameters that have been explored in this work are the thickness of the specimen and the offset of the initial notch from symmetry. We explore this transition through experiments on polycarbonate and numerical simulations using a global damage model. In order to accomplish this, a VUMAT user subroutine has been developed in the finite element commercial code ABAQUS/Explicit, which takes into account both failure criteria (brittle and ductile), independently. Thus, it has been possible to reproduce the transition in the failure mode of polycarbonate specimens subjected to three point bending dynamic fracture tests. These numerical results have allowed to observe that a double transition may occur, depending on the thickness and strain rate.The authors thank the Comisión Interministerial de Ciencia y Tecnología of the Spanish Government for partial support of this work through the Research Project DPI2011-23191, and the University Carlos III of Madrid for the financial support provided through the "Aid for the mobility of the own research program". Prof. K. Ravi-Chandar acknowledges the support of Universidad Carlos III of Madrid with a Cátedra de Excelencia funded by Banco Santander.Publicad

Experiments and numerical simulations of initiation and growth of cracks under mixed mode I + III loading

Non UBCUnreviewedAuthor affiliation: Department of Aerospace Engineering and Engineering Mechanics - University of Texas at AustinFacult

Experimental Investigation of Deformation and Failure in Ductile Alloys under Shear Loading

Conventional ductile fracture models such as the Gurson-Tvergaard-Needleman model has been developed based on the mechanics of void growth due to volumetric strain and subsequent void coalescence leading to fracture. It has now become evident that these models are not capable of capturing fracture in dominant shear deformation scenarios, which arise commonly in the form of shear localizations in polycrystalline metallic materials. In this paper, we report in-situ multiscale examination of deformation and failure mechanisms in Al 6061-T6 under low stress triaxiality levels using modified Arcan specimens. Strains at the grain and subgrain levels were measured in a scanning electron microscope by in-situ tracking of the changes in grain size and morphology, and at the macroscale level with the means of digital image correlation. Strains in the range of 2-2.5 were measured at the grain level without any indication of damage. High strain heterogeneity at the grain was evidenced and quantified, and its relation with grain size and morphology was evaluated. The interaction of matrix grains and particles was monitored and correlated with force data to examine the effect of particle fracture on load carrying capacity and final failure of the material under shear loading

Tensile and ductile fracture properties of as-printed 316L stainless steel thin walls obtained by directed energy deposition

International audienceMechanical properties of as-printed 316L stainless steel thin-walled structures obtained by directed energy deposition are investigated. In-situ tensile and fracture tests are performed on small samples obtained from a additively manufactured square section tube and extracted with three different orientations with respect to the part build direction. Despite a strongly oriented microstructure resulting from the process, as-printed specimens exhibit a reduced anisotropy in comparison with thick or polished samples commonly reported in the literature. Moreover, it is shown using a simple model that the reduced dentified anisotropy can be explained by considering the material thickness variation pattern only, resulting from the layer stacking process. Fracture tests are analyzed using an adapted digital image correlation procedure that evaluates the specimen fracture toughness from experimentally computed J-integrals. Using time reversal, strain fields in regions close to the crack path are identified. Stress fields are then computed from the constitutive behavior identified in tensile tests. A regularization procedure is proposed to enforce the stress equilibrium. Finally, the J-integral is computed using various integration contours in order to validate its path-independance. On this basis, a nearly isotropic fracture toughness is identified. Additional scanning electron microscope observations show that fracture surface features are independent from specimen orientation. This apparent isotropy is explained by the isotropic distribution of lack-of-fusion defects driving crack initiation and propagation