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In reply to Discussion of fracture paper #39 - Dynamic Fracture on a Molecular Level

Dear Per,

First and foremost, thank you for selecting our paper to appear in the blog.

A reply to your questions: 

1. Dislocation emission near a crack front in silicon crystal usually occurs first at about 900K. There is no dislocation emission at room temperature since the activation energy for dislocation emission at room temperature is much higher than the cleavage energy. The covalent bonds are responsible for the high activation energy for dislocation emission. This is regardless of the suggested kinks along the crack front. The kinks are always there, even when the theory of kinking is quite new. At room temperature, silicon crystal may be considered as an ideally brittle material.
2. To answer this question (is the kink propagating under mixed mode closer to mode I or possibly closer to mode II?) we emphasize that kink motion, both kink advance and kink formation are planar entities in a 3D coordinate system. They propagate along the cleavage system (plane of propagation and direction of the crack front velocity, e.g., (110)[110] or (111)[112]); kink advances advancing parallel to the crack front, kink formations are generated normal to the crack front (by thermally activated processes). The boundary value problem is fully symmetric to the x-axis, the cleavage system is in the middle of the specimen and parallel to the specimen edges, where the deformation vector is normal to the edges. While the fracture surface is smooth at the scale of several nanometers (confocal optical microscope), scanning tunneling microscope (STM) scans show atomistic scale misalignment steps due to unavoidable misalignment between the maximum KI plane and the cleavage plane of the material. And yet no significant shear stresses exist to activate Mode II deformation.

   Dov Sherman

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In reply to Avoid 4th order tensors if you can

The google drive link in the parent comment is broken. Please use this one:

https://drive.google.com/file/d/0BzO5SDANDZyjQUVScDRTRnRleVk/view?usp=sh...

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In reply to International vacancies for post-doctoral researcher positions and other positions

Postdoc position Nazarbayev University

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In reply to Process parameter sensitivity of the energy absorbing properties of additively manufactured metallic cellular materials

Dear iMechanicians,

I hope this paper is of interest to the additive manufacturing, architected materials, and dynamic impact (high strain rate) communities. 

Quasi-static and dynamic mechanical testing on stainless steel 316L cellular geometries is combined with material characterisation techniques to understand the interplay between additive manufacturing process parameters, geometry and mechanical performance (energy absorption, strength). Four architected materials were studied, finding that the geometry had a greater influence on the investigated mechanical properties, relative to the Laser Powder Bed Fusion (LPBF) process parameter sets.

M. Simoes et al. Process parameter sensitivity of the energy absorbing properties of additively manufactured metallic cellular materials. Materials and Design, 224, 111398 (2022)

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