Browsing by Subject "Ductile failure"

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A combined experimental and modeling study of low velocity perforation of thin aluminum plates
(2015-12) Simpson, Gary Forest Jr.; Ravi-Chandar, K.; Landis, Chad
This work conducts a combined experimental and modeling study of low velocity projectile perforation of thin AA5083-H116 aluminum plates. Experiments were performed in order to characterize the candidate material and calibrate simple and easy to implement empirical models for both the material response and ductile failure behavior. Quasi-static tensile tests were performed in order to investigate the Portevin-Le Chatelier effect common to 5xxx series aluminum as well as to calibrate a Ramberg-Osgood representation for the material stress-strain curve. The material response at strain rates up to and exceeding 104 s-1 was investigated by means of an electromagnetically driven ring expansion test, characterizing the potential strain rate sensitivity of the material. Additionally, the failure behavior and potential damage accumulation of the material were evaluated using an interrupted, multiple loading path strain-to-failure test, validating a Johnson-Cook failure model for use in numerical simulation. Low velocity ballistic impact and perforation experiments, investigating several specific mechanisms of deformation and failure, were conducted and modeled by implementing the developed material and failure model in 3D finite element simulations.
Localization and failure of Aluminum 6061-T6 under biaxial loading
(2019-06-19) Scales, Martin Filipp; Kyriakides, S.; Kovar, Desiderio; Liechti, Kenneth; Mear, Mark; Ravi-Chandar, Krishnaswamy
The adoption of new materials for light-weighting purposes in the automotive industry has been hindered by these materials’ limited ductility and more-complicated constitutive models. Establishing the onset of failure through experiment is challenging, and numerical predictions depend strongly on the adopted material model. With this in mind, a series of experiments was developed with the goal of providing directly-measurable strains and stresses at failure. Custom-designed Al 6061 T6 tubular specimens with a thin-walled test section are loaded in radial stress paths in the nominal axial-shear stress space. Stereo digital image correlation is used to monitor the specimen surface throughout the experiment. The stress and deformation within the test section are uniform until a load maximum is reached, beyond which deformation localizes into a circumferential band with width the order of the wall thickness. The series of experiments shows that the strain at failure monotonically increases as the triaxiality decreases, a result that is contrary to previously-reported results for this alloy. The strains at failure are also significantly larger than previously-reported values, with equivalent strains around 1.5 at low triaxialities. This experimental methodology provides a robust means of directly establishing failure strains that can be employed as failure criteria in numerical simulations. In support of a separate effort to numerically reproduce the responses and localization in these tension-torsion experiments, a series of combined tension and internal pressure experiments on the same tube stock was conducted. In these experiments, the tubes are loaded in radial paths in the nominal axial-hoop stress space. The data obtained proved sufficient for calibrating the non-quadratic, 18-parameter, anisotropic constitutive model of Barlat and coworkers. With the calibrated constitutive model, a large-deformation material stress-strain curve was inversely extracted from the post-necking response in a uniaxial tension test. The pressure-tension experiments were then studied numerically through a finite element (FE) model that incorporated the calibrated constitutive model and hardening response. The analysis shows that properly-calibrated plasticity with an accurate stress-strain curve and suitable FE mesh is capable of reproducing the measured responses as well as the localized deformations that developed prior to burst.
On the ductile failure of thin-walled aluminum alloy tubes under combined shear and tension
(2012-12) Haltom, Scott Sumner; Kyriakides, S.; Ravi-Chandar, Krishnaswamy
The aim of this thesis is to establish the extent to which materials can be deformed under shear-dominant loadings. Custom Al-6061-T6 tubular specimens are loaded under radial and corner paths of tension and shear to failure. During the experiments, the deformation is monitored in a test section designed to have nearly uniform stress and deformation at large strains while providing minimum constraint to the development of localization that precedes failure. The recorded shear stress-rotation and axial stress-displacement responses exhibit maxima beyond which deformation localizes in a narrow band that is of the order of the 1 mm wall thickness of the test section. For the mainly shear dominated stress paths followed, deformation remained nearly planar allowing for the establishment of both the true stresses and the local deformation strictly from measurements. Results from thirteen radial path experiments as well as from four corner path experiments show the strain at failure to monotonically increase as the mean stress decreases, a result that is in contrast with previously reported results for Al alloys. Also, the measured failure strains are significantly larger than previously reported values. Analysis of corner stress paths investigates the path dependence of localization and failure. Results show little path dependence on the failure strains, but some path dependence on stress maxima and failure stresses. Furthermore, statistical grain-level strain estimates from five of the stress paths revealed a significant variation in strain across the macroscopically observed localization zone. In the neighborhood of the crack tip strains with 25-100% higher levels than the macroscopic values were recorded. This indicates that localization also occurs at a smaller scale than hitherto understood. The difference between the macro strain at failure and the average grain level values increased as the axial/shear stress ratio increased.
The effect of anisotropy on the localization and failure of aluminum alloys under biaxial loads
(2019-08) Chen, Kelin; Kyriakides, S.; Corona, Edmundo; Liechti, Kenneth; Ravi-Chandar, Krishnaswamy; Landis, Chad
Development of a robust finite element model capable of simulating ductile failure of thin-walled Al-alloy structures under complex loading conditions requires: (a) A suitably calibrated constitutive model of the material that includes the prevailing plastic anisotropy, and (b) an appropriately extracted material hardening response to large enough strains. This work addresses these two issues through the analysis of the response up to failure of thin sheets in hydraulic bulge tests, and the large deformation response of tubes under combined shear and tension. Hydraulic bulge tests have been used to extract the material hardening response of sheet metals to large strains. The extraction requires proper knowledge of the plastic anisotropy in the sheets. To this end, the non-quadratic anisotropic yield function of Barlat et al. [2005] was calibrated using a series of uniaxial and biaxial tests and two data from the bulge test. The calibration uses an iterative scheme for evaluating the stress state at the apex. The calibrated yield function is then used to extract the material hardening. The veracity of the scheme is demonstrated by using the calibrated yield function and hardening response in a 3-D finite element model to successfully simulate the bulge test up to failure. The second part of the project simulates the response of Al-alloy tubes under combined tension and torsion. Experiments conducted in parallel with this study have shown that following initial plastic deformation, strain localizes in a narrow zone and grows significantly before rupture. Here, the non-quadratic anisotropy yield function is calibrated using the set of tension-torsion experiments conducted, supplemented by a set of pressure-tension experiments. The constitutive model is then used to extract the material hardening response from a simple shear test accounting for the rotation of the material frame. The two constitutive components are then implemented in a 3-D finite element model to simulate a set of tension-torsion experiments. It is demonstrated that the constitutive model and hardening material response can reproduce the experimental structural responses, the onset of localization and its evolution to strains that correspond to the measured failure strains. This is achieved without the artificial introduction of material softening.
Towards the predictive modeling of ductile failure
(2015-12) Gross, Andrew Jeffrey; Ravi-Chandar, K.; Kovar, Desiderio; Landis, Chad; Liechti, Kenneth; Kyriakides, Stelios
The ability to predict ductile failure is considered by an experimental examination of the failure process, validation exercises to assess predictive ability, and development of a coupled experimental-numerical strategy to enhance model development. In situ loading of a polycrystalline metal inside a scanning electron microscope is performed on Al 6061-T6 that reveals matrix-dominated response for both deformation and failure. Highly localized deformation fields are found to exist within each grain as slip accumulates preferentially on a small fraction of crystallographic planes. No evidence of damage or material softening is found, implying that a strain-to-failure model is adequate for modeling fracture in this and similar material. This modeling insight is validated through blind predictive simulations performed in response to the 2012 and 2014 Sandia Fracture Challenges. Constitutive and failure models are calibrated and then embedded in highly refined finite element simulations to perform blind predictions of the failure behavior of the challenge geometries. Comparison of prediction to experiment shows that a well-calibrated model that captures the essential elastic-plastic constitutive behavior is necessary to capture confidently the response for structures with complex stress states, and is a prerequisite for a precise prediction of material failure. The validation exercises exposed the need to calibrate sophisticated plasticity models without a large experimental effort. To answer this need, a coupled experimental and numerical method is developed for characterizing the elastic-plastic constitutive properties of ductile materials using local deformation field information to enrich calibration data. The method is applied to a tensile test specimen and the material’s constitutive model, whose parameters are unknown a priori, is determined through an optimization process that compares these experimental measurements with iterative finite element simulations. The final parameters produce a simulation that tracks the local experimental displacement field to within a couple percent of error. Simultaneously, the percent error in the simulation for the load carried by the specimen throughout the test is less than one percent. The enriched calibration data is found to be sufficient to constrain model parameters describing anisotropy that could not be constrained by the global data alone.