Surprising simplicity in the modeling of dynamic granular intrusion

Granular intrusions, such as dynamic impact or wheel locomotion, are complex multiphase phenomena where the grains exhibit solid-like and fluid-like characteristics together with an ejected gas-like phase. Despite decades of modeling efforts, a unified description of the physics in such intrusions is as yet unknown. Here we show that a continuum model based on the simple notions of frictional flow and tension-free separation describes complex granular intrusions near free surfaces. This model captures dynamics in a variety of experiments including wheel locomotion, plate intrusions, and running legged robots. The model reveals that three effects (a static contribution and two dynamic ones) primarily give rise to intrusion forces in such scenarios. Identification of these effects enables the development of a further reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of granular locomotion of arbitrarily shaped intruders. The continuum-motivated strategy we propose for identifying physical mechanisms and corresponding reduced-order relations has potential use for a variety of other materials.Comment: 41 pages including supplementary document, 10 figures, and 8 vide

2nd International Workshop on Physics-Based Modelling of Material Properties and Experimental Observations with special focus on Fracture and Damage Mechanics: Book of Abstracts

This report covers the book of abstracts of the 2nd International Workshop on Physics Based Modelling of Material Properties and Experimental Observations, with special focus on Fracture and Damage Mechanics. The workshop is organized in the context of European Commission’s Enlargement and Integration Action, by the Joint Research Centre in collaboration with the TOBB University of Economics and Technology (TOBB ETU) on 15th-17th May 2013 in Antalya, Turkey. The abstracts of the keynote lectures and all the technical presentations are included in the book. This workshop will give an overview of different physics-based models for fracture and degradation of metallic materials and how they can be used for improved understanding and more reliable predictions. Models of interest include cohesive zones to simulate fracture processes, ductile-brittle transition for ferritic steels, ductile fracture mechanisms such as void growth or localized shear, fatigue crack initiation and short crack growth, environmental assisted cracking. Experimental studies that support such models and case studies that illustrate their use are also within the scope. The workshop is also an opportunity for scientists and engineers from EU Member States and target countries to discuss research activities that could be a basis for future collaborations.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

Fast algorithms for material specific process chain design and analysis in metal forming - final report DFG Priority Programme SPP 1204

The book summarises the results of the DFG-funded coordinated priority programme \"Fast Algorithms for Material Specific Process Chain Design and Analysis in Metal Forming\". In the first part it includes articles which provide a general introduction and overview on the field of process modeling in metal forming. The second part collates the reports from all projects included in the priority programme

On the evolution of elastic properties during laboratory stick-slip experiments spanning the transition from slow slip to dynamic rupture

The physical mechanisms governing slow earthquakes remain unknown, as does the relationship between slow and regular earthquakes. To investigate the mechanism(s) of slow earthquakes and related quasi-dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration. We reproduced the full spectrum of slip behavior, from slow to fast stick slip, by altering the elastic stiffness of the loading apparatus (k) to match the critical rheologic stiffness of fault gouge (kc). Our experiments show an evolution from stable sliding, when k>kc, to quasi-dynamic transients when k ~ kc, to dynamic instabilities when k<kc. To evaluate the microphysical processes of fault weakening we monitored variations of elastic properties. We find systematic changes in P wave velocity (Vp) for laboratory seismic cycles. During the coseismic stress drop, seismic velocity drops abruptly, consistent with observations on natural faults. In the preparatory phase preceding failure, we find that accelerated fault creep causes a Vp reduction for the complete spectrum of slip behaviors. Our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults, depending on frictional properties. In agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by Vp changes during the seismic cycle. The observed reduction in Vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high-resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

CRYSTAL PLASTICITY FINITE ELEMENT MODELING OF MAGNESIUM ALLOYS AND EXPERIMENTAL CHARACTERIZATION OF A TRIP HIGH ENTROPY ALLOY

This work presents two crystal plasticity finite element studies on magnesium alloys and an experimental characterization of a high entropy alloy. The first of two crystal plasticity studies presents a high strain rate deformation characterization via a split Hopkinson bar Taylor impact of a WE43 magnesium alloy. This study showed that crystal plasticity finite element modeling (CPFE) was able to model WE43 texture evolution, twin volume fraction along the length of the cylinder, and anisotropy with four different material orientations at high strain rates when compared to experimental data. The second study investigated the Taylor-type model homogenization response of the virtual polycrystal and how to best spread the crystal orientations over the finite element (FE) mesh for accurate modeling of Mg alloys specifically AZ31. It was found that 6 embedded crystals per integration point proved most optimal when compared to a full-field explicit grain mesh model. The third study investigated phase transformation hardness values and strain hardening characteristics for a four-phase high entropy alloy by nanoindentation. The material exhibited great strength based on phase transformation during plastic deformation upon compression

PHYSICS-BASED MODELING FRAMEWORK INCORPORATING MICROSTRUCTURAL EVOLUTION FOR PREDICTING DEFORMATION AND RECRYSTALLIZATION OF METALLIC MATERIALS

Large number of metallic parts is produced from the raw materials by a set of mechanical shaping and heating operations. Considering the need for metallic components in the modern industry, the field of thermo-mechanical processing of metallic materials is of immense importance. Even modest improvements of the existing thermo-mechanical processes could potentially result in great savings of time, recourses and energy. Finite element modeling of the forming and heating operations introduced in the past decades has allowed for the optimization of the thermo-mechanical processes and has thus resulted in significant advancements. However, due to the limitations of the presently used constitutive models, certain aspects of the process and effects of the process parameters on the component properties cannot be simulated accurately. Work presented in this dissertation is a contribution to the development of a physics-based crystal plasticity model capable of accurately simulating both the mechanical shaping and heating portions of the process and their effect on the microstructure of the component. The well-established visco-plastic self-consistent polycrystal plasticity (VPSC) model is advanced in several aspects in an effort to develop a coupled deformation-recrystallization model. First, different numerical implementation of the VPSC constitutive model into the finite element framework is developed. In addition, two methods for the accurate representation of the material rate sensitivity within the finite element framework are proposed. The proposed models are verified on Taylor impact tests of Zr and Ta cylinders. Next, an algorithm for statistical description of intragranular fluctuations of crystallographic orientation is developed. The effects of the fluctuations of crystallographic orientation within the grains on the fluctuations of stress and rotation rates are considered as well. The developed model is applied to compression and plane strain compression of fcc material and verified by direct comparison with experimental measurements and full-field predictions. Finally, a physics-based recrystallization model coupled with the developed VPSC model capable of predicting intragranular crystallographic orientation fluctuations is proposed. The coupled deformation-recrystallization model is applied to the recrystallization of fcc and bcc materials and reasonable agreement is observed. Combination of different models proposed in this dissertation allows for the simulation of both shaping and heating portions of the thermo-mechanical process

Interaction of weak shock waves with cylindrical and spherical gas inhomogeneities

The interaction of a plane weak shock wave with a single discrete gaseous inhomogeneity is studied as a model of the mechanisms by which finite-amplitude waves in random media generate turbulence and intensify mixing. The experiments are treated as an example of the shock-induced Rayleigh-Taylor instability. or Richtmyer-Meshkov instability, with large initial distortions of the gas interfaces. The inhomogeneities are made by filling large soap bubbles and cylindrical refraction cells (5 cm diameter) whose walls are thin plastic membranes with gases both lighter and heavier than the ambient air in a square (8.9 cm side shock-tube text section. The wavefront geometry and the deformation of the gas volume are visualized by shadowgraph photography. Wave configurations predicted by geometrical acoustics, including the effects of refraction, reflection and diffraction, are compared to the observations. Departures from the predictions of acoustic theory are discussed in terms of gasdynamic nonlinearity. The pressure field on the axis of symmetry downstream of the inhomogeneity is measured by piezoelectric pressure transducers. In the case of a cylindrical or spherical volume filled with heavy low-sound-speed gas the wave which passes through the interior focuses just behind the cylinder. On the other hand, the wave which passes through the light high-sound-speed volume strongly diverges. Visualization of the wavefronts reflected from and diffracted around the inhomogeneities exhibit many features known in optical and acoustic scattering. Rayleigh-Taylor instability induced by shock acceleration deforms the initially circular cross-section of the volume. In the case of the high-sound-speed sphere, a strong vortex ring forms and separates from the main volume of gas. Measurements of the wave and gas-interface velocities are compared to values calculated for one-dimensional interactions and for a simple model of shock-induced Rayleigh-Taylor instability. The circulation and Reynolds number of the vortical structures are calculated from the measured velocities by modeling a piston vortex generator. The results of the flow visualization are also compared with contemporary numerical simulations

The acousto-ultrasonic approach

The nature and underlying rationale of the acousto-ultrasonic approach is reviewed, needed advanced signal analysis and evaluation methods suggested, and application potentials discussed. Acousto-ultrasonics is an NDE technique combining aspects of acoustic emission methodology with ultrasonic simulation of stress waves. This approach uses analysis of simulated stress waves for detecting and mapping variations of mechanical properties. Unlike most NDE, acousto-ultrasonics is less concerned with flaw detection than with the assessment of the collective effects of various flaws and material anomalies. Acousto-ultrasonics has been applied chiefly to laminated and filament-wound fiber reinforced composites. It has been used to assess the significant strength and toughness reducing effects that can be wrought by combinations of essentially minor flaws and diffuse flaw populations. Acousto-ultrasonics assesses integrated defect states and the resultant variations in properties such as tensile, shear, and flexural strengths and fracture resistance. Matrix cure state, porosity, fiber orientation, fiber volume fraction, fiber-matrix bonding, and interlaminar bond quality are underlying factors