Micromechanical modeling of crack propagation in TBC systems

2016 ◽  
Author(s):  
Nasim Nayebpashaee ◽  
M. Aboutalebi ◽  
Hossein Vafaeenezhad ◽  
S. M. Hadavi ◽  
S. H. Seyedein ◽  
...  
Keyword(s):  
Crack Propagation ◽  
Micromechanical Modeling ◽  
Tbc Systems

Micromechanical modeling of crack propagation in weak snow layer

2020 ◽  
Author(s):  
Gregoire Bobillier ◽  
Alec van Herwijnen ◽  
Bastian Bergfeld ◽  
Johan Gaume ◽  
Jürg Schweizer
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Crack Velocity ◽  
Stress Component ◽  
Crack Tip ◽  
Micromechanical Modeling ◽  
Snow Layer ◽  
Mechanical Field ◽  
Fracture Dynamics ◽  
Micromechanical Approach

<p>Improving the prediction of snow avalanches requires a detailed understanding of the fracture behavior of snow, which is intimately linked to the mechanical properties of the snow layers (strength, elasticity of the weak and slab layer). While the basic concepts of avalanche release are conceptually relatively well understood, understanding crack propagation and fracture propensity remains a great challenge. About 15 years ago, the propagation saw test (PST) was developed. The PST is a fracture mechanical field test that provides information on crack propagation propensity in weak snowpack layers. It has become a valuable research tool to investigate processes and mechanical parameters involved in crack propagation.</p><p>Here, we use the discrete element method (DEM) to numerically simulate PST and therefore analyze fracture dynamics based on micromechanical approach. Using cohesive and non-cohesive ballistic deposition, we numerically reproduce the basic required layers for dry-snow avalanche: a highly porous and brittle weak layer covered by a dense cohesive slab.</p><p>The results of these numerical PTSs reproduce the main dynamics of crack propagation observed in the field. We developed different indicators to define the crack tip and therefore derive the crack velocity. Our results show that crack propagation on flat terrain reaches a stationary velocity if the snow column in long enough. The length of the snow column to reach stationary crack velocity depends on snowpack parameters. On sloped terrain our results show a transition in the local failure mode, this transition can be visualized from the crack tip morphology and from the main stress component.</p><p>Overall, our results lay the foundation for a comprehensive study on the influence of the snowpack mechanical properties on these fundamental processes for avalanche release.</p>

Fracture Toughness Phenomena in an Unaged Beta Titanium Alloy

1975 ◽  
Vol 97 (4) ◽  
pp. 330-337 ◽  
Author(s):  
H. J. Rack
Keyword(s):  
Fracture Toughness ◽  
Titanium Alloy ◽  
Crack Propagation ◽  
Cleavage Fracture ◽  
Critical Stress ◽  
Micromechanical Modeling ◽  
Cleavage Crack ◽  
Beta Titanium Alloy ◽  
Characteristic Distance ◽  
Beta Titanium

The fracture toughness behavior of a commercial beta titanium alloy, Ti-3Al-8V-6Cr-4Zr-4Mo, has been examined between −196°C and 380°C, The observed tough-brittle transition results from increasing amounts of intergranular and cleavage fracture with decreasing temperature. Crack initiation is associated with particle/matrix interface parting, while cleavage crack propagation at low temperature requires the attainment of a critical stress over a characteristic distance. Micromechanical modeling of the principal fracture event associated with critical stress cleavage crack propagation does appear to accurately predict the magnitude of this distance, although it is limited to initiation controlled cleavage fracture events.

scholarly journals Micromechanical modeling of snow failure

2019 ◽  
Author(s):  
Grégoire Bobillier ◽  
Bastian Bergfeld ◽  
Achille Capelli ◽  
Jürg Dual ◽  
Johan Gaume ◽  
...  
Keyword(s):  
Discrete Element Method ◽  
Crack Propagation ◽  
Computational Cost ◽  
Discrete Element ◽  
Micromechanical Modeling ◽  
Tensile Fracture ◽  
Macroscopic Behavior ◽  
Weak Layer ◽  
Snow Model ◽  
Element Method

Abstract. Dry-snow slab avalanches start with the formation of a local failure in a highly porous weak layer underlying a cohesive snow slab. If followed by rapid crack propagation within the weak layer and finally a tensile fracture through the slab appears, a slab avalanche releases. While the basic concepts of avalanche release are relatively well understood, performing fracture experiments in the lab or in the field can be difficult due to the fragile nature of weak snow layers. Numerical simulations are a valuable tool for the study of micromechanical processes that lead to failure in snow. We used a three-dimensional discrete element method (3D-DEM) to simulate and analyze failure processes in snow. Cohesive and cohesionless ballistic deposition allowed us to reproduce porous weak layers and dense cohesive snow slabs, respectively. To analyze the micromechanical behavior at the scale of the snowpack (~ 1 m), the particle size was chosen as a compromise between a low computational cost and a detailed representation of important micromechanical processes. The 3D-DEM snow model allowed reproducing the macroscopic behavior observed during compression and mixed-modes loading of dry snow slab and weak snow layer. To be able to reproduce the range of snow behavior (elastic modulus, strength), relations between DEM particle/contact parameters and macroscopic behavior were established. Numerical load-controlled failure experiments were performed on small samples and compared to results from load-controlled laboratory tests. Overall, our results show that the discrete element method allows to realistically simulate snow failure processes. Furthermore, the presented snow model seems appropriate for comprehensively studying how the mechanical properties of slab and weak layer influence crack propagation preceding avalanche release.

scholarly journals Micromechanical modeling of snow failure

2020 ◽  
Vol 14 (1) ◽  
pp. 39-49 ◽  
Author(s):  
Grégoire Bobillier ◽  
Bastian Bergfeld ◽  
Achille Capelli ◽  
Jürg Dual ◽  
Johan Gaume ◽  
...  
Keyword(s):  
Discrete Element Method ◽  
Crack Propagation ◽  
Discrete Element ◽  
Micromechanical Modeling ◽  
Tensile Fracture ◽  
Snow Layer ◽  
Macroscopic Behavior ◽  
Weak Layer ◽  
Snow Model ◽  
Element Method

Abstract. Dry-snow slab avalanches start with the formation of a local failure in a highly porous weak layer underlying a cohesive snow slab. If followed by rapid crack propagation within the weak layer and finally a tensile fracture through the slab, a slab avalanche releases. While the basic concepts of avalanche release are relatively well understood, performing fracture experiments in the laboratory or in the field can be difficult due to the fragile nature of weak snow layers. Numerical simulations are a valuable tool for the study of micromechanical processes that lead to failure in snow. We used a three-dimensional discrete element method (3-D DEM) to simulate and analyze failure processes in snow. Cohesive and cohesionless ballistic deposition allowed us to reproduce porous weak layers and dense cohesive snow slabs, respectively. To analyze the micromechanical behavior at the scale of the snowpack (∼1 m), the particle size was chosen as a compromise between low computational costs and detailed representation of important micromechanical processes. The 3-D-DEM snow model allowed reproduction of the macroscopic behavior observed during compression and mixed-mode loading of dry-snow slab and the weak snow layer. To be able to reproduce the range of snow behavior (elastic modulus, strength), relations between DEM particle and contact parameters and macroscopic behavior were established. Numerical load-controlled failure experiments were performed on small samples and compared to results from load-controlled laboratory tests. Overall, our results show that the discrete element method allows us to realistically simulate snow failure processes. Furthermore, the presented snow model seems appropriate for comprehensively studying how the mechanical properties of the slab and weak layer influence crack propagation preceding avalanche release.

Effect of Thermal Cycling on Lifetime and Failure Mechanism of EB-PVD TBC with NiCoCrAlYZr Bondcoats

2013 ◽  
Vol 652-654 ◽  
pp. 1826-1829
Author(s):  
Peng Song ◽  
Jian Sheng Lu
Keyword(s):  
High Temperature ◽  
Crack Propagation ◽  
Thermal Cycling ◽  
Failure Mechanism ◽  
High Frequency ◽  
Cyclic Oxidation ◽  
Rapid Propagation ◽  
Small Cracks ◽  
Cracks Propagation ◽  
Tbc Systems

EB-PVD TBC with conventional MCrAlY bondcoats was widely used within the high temperature environments. The effect of oxidation frequency on the lifetime of TBC with NiCoCrAlYZr and the failure mechanism was investigated in this paper. The TBC systems with Zr-doped MCrAlY bondocats presented a longer lifetime after discontinuous oxidation than cyclic oxidation. Formation of thick TGO in the TBC-system with Zr-containing bondcoat did not result in an immediate failure. The crack propagation in the case of the Zr-doped NiCoCrAlY bondcoat at the TGO/bondcoat interface was hindered due to the inhomogeneous TGO morphology. The inner oxidation and pores hindered the above small cracks propagation and then result a longer lifetime. However, the lifetime of TBC-system with NiCoCrAlY+Zr bondcoat is significantly shorter in the cyclic than in the discontinuous test due to the rapid propagation of cracks under high frequency thermal cycling conditions.

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