Fracture Properties and Residual Stresses in Small Dimensions: Fracture Mechanisms and Modeling
Sponsored by: TMS Structural Materials Division, TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Daniel Kiener, University of Leoben; Marco Sebastiani, Roma TRE university; Nagamani Jaya Balila, Max Planck Institut fuer Eisenforschung GmbH; William Gerberich, University of Minnesota; Siddhartha (Sid) Pathak, University of Nevada, Reno

Wednesday 8:30 AM
March 1, 2017
Room: 21
Location: San Diego Convention Ctr

Session Chair: Erik Bitzek, Friedrich-Alexander Universität Erlangen Nurnberg; Karsten Durst, Technical University Darmstadt

8:30 AM Introductory Comments

8:35 AM  Invited
Atomistic Simulations of Crack Nucleation and Propagation along Grain Boundaries : Erik Bitzek1; Johannes Möller1; 1Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
     Understanding the influence of the grain boundary structure and the relative orientation of the slip systems to the GB/crack plane on crack initiation and propagation is key to modeling intergranular fracture in polycrystalline materials. Here, atomistic simulations are ideally suited to investigate the details of grain boundary fracture. However, most of the simulations are currently performed using quasi-2D setups with straight crack fronts on perfect GBs.Here we present the results of large-scale 3D atomistic simulations using EAM potentials for various bcc metals. Crack nucleation by dislocation pile-ups were compared to crack nuclei formed by agglomerated vacancies. Crack propagation was studied for penny-shaped GB cracks, which showed distinct differences to the behavior of long, straight cracks. Further simulations revealed the influence of absorbed dislocation contents on intergranular fracture. The results are discussed in the context of fracture and fatigue of nanoscale objects as well as crack nuclei in bulk metals.

9:05 AM  
Directional Dependency of the Fracture Behavior of High Strength Pearlitic Steel Wires: Bernhard Völker1; Marlene Kapp2; Reinhard Pippan2; Anton Hohenwarter1; 1Montanuniversität Leoben; 2Erich Schmid Institute, Austrian Academy of Sciences
    High strength pearlitic steel wires are gaining recently more interest, since they are some of the strongest bulk materials produced with a strength of up to about 7 GPa. Two different wires with a drawing strain of 3.10 and 6.52 were investigated. Directional dependence of the fracture toughness was measured in drawing direction and perpendicular to it. To be able to test the fracture toughness of the samples in drawing direction micro-bending beams had to be fabricated utilizing a focused ion beam. For the investigation perpendicular to the drawing direction, tensile and bending tests with FIB notched wires were performed. The fracture toughness experiments for both directions were performed in-situ in the scanning electron microscope and ex-situ (perpendicular direction). It was revealed that in drawing direction the wires seem to be significantly less fracture tolerant than in the perpendicular direction. These findings are supported by the in-situ and ex-situ experiments.

9:25 AM  
Brittle-to-ductile Transition of Quasicrystals at Small Scales: Cracking, Serrated flows, Diffusion and Phase Transformation: Yu Zou1; Jeffrey Wheeler1; Alla Sologubenko1; Pawel Kuczera1; Walter Steurer1; Johann Michler2; Ralph Spolenak1; 1ETH Zurich; 2Empa Thun
    Since quasicrystals were first discovered in the early 1980s, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures; at low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behavior of icosahedral Al-Pd-Mn and decagonal Al-Ni-Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25-500 °C), strain rates, and sample sizes accompanying microstructural analysis. Here we show interesting phenomena relevant to small-length scales , including: cracking, serrated flows, diffusion and phase transformation. Deformation of small-scale quasicrystals reveals a unique mechanism to with crystalline and amorphous metals which is peculiar to quasicrsytals rather than regular crystals and amorphous solids.

9:45 AM  
Coupling Discrete Dislocation Plasticity and Cohesive Zone Models: Edmund Tarleton1; Angus Wilkinson1; 1Oxford University
    The competition between plasticity and fracture results in a brittle-ductile transition in several bcc metals. In order to simulate this, cohesive elements have been incorporated within a discrete dislocation plasticity code which requires a modified superposition scheme to be used. Cohesive elements allow a traction separation law to be implemented within continuum FEM and localised damage and crack propagation can then be simulated in the presence of dislocations. Details of how to correctly couple a cohesive zone model within discrete dislocation dynamics are presented. Finally simulations of micro cantilever bend tests on silicon and tungsten will be shown and the competition between plasticity and fracture is discussed.

10:05 AM Break

10:30 AM  Invited
Constitutive Modeling of Indentation Cracking in Fused Silica: Karsten Durst1; 1Technical University Darmstadt
    Fused silica shows three distinct regimes during nanoindentation, i.e. plastic deformation, inelastic densification and cracking. Cohesive zone FEM is used to study these regimes for different indenter geometries. In a three dimensional model, the median/radial cracking is considered by introducing cohesive element planes. In addition to comparing indentation cracking data with experimental data, the role of densification on indentation crack growth is critically examined using a pressure independent Von Mises and a pressure dependent Drucker-Prager-Cap constitutive model. The results show that the Drucker-Prager-Cap model delivers an accurate description of the elastic-plastic deformation conditions for all examined indenter geometries. Material densification leads to shorter crack lengths and thus larger fracture toughness values. Once the crack was initiated its propagation is comparable for blunt indenter geometries (Berkovich), while densification leads to a slower crack propagation for sharper indenter geometries.

11:00 AM  
Critical Stresses in Intermittent Plasticity: Peter Derlet1; Robert Maass2; 1Paul Scherrer Institut; 2University of Illinois at Urbana-Champaign
    The extreme value statistics of the first discrete irreversible plastic event is investigated for experimental nano-indentation and dislocation dynamics simulation data. It is found that the average of the critical stress and the Weibull fluctuations around it, is related to the deforming crystal volume via an exponentially truncated power-law. When the underlying master distribution of critical stresses is assumed to be a power-law, it becomes possible to extract the corresponding exponent, the density of discrete plastic events available to the crystal, and to understand the exponential truncation as a break-down of the asymptotic Weibull limit due to the micron length-scale of the deforming volume.

11:20 AM  Invited
Tensile Deformation Behaviour of Notched Nano-scale Metallic Glass Specimens: Narasimhan Ramarathinam1; Indrasen Singh1; 1Indian Institute of Science
    Nano-scale metallic glass (MG) specimens exhibit homogeneous plastic deformation and necking under tensile loading in contrast to bulk specimens which fail by unstable shear band (SB) propagation. Necking is found to be initiated by shallow notches on the specimen surface. Although molecular dynamics simulations indicate similar response, the mechanistic origin of the above quasi-brittle to ductile transition is not well understood. The aim of the present work is to gain insights on the above phenomenon from continuum finite element simulations using a non-local plasticity model. The results show that the plastic zone in front of the notch attains a saturation size rps when a dominant SB spreads across the specimen. The SB width as well as rps scale with an intrinsic material length that characterizes the interaction stress between flow defects. Failure mode transition from unstable SB propagation to necking is governed by ratio of rps to uncracked ligament length.