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# Objectives

• This section will have 2 parts
• The first part aims to introduce the basic procedure of running dislocation dynamics using the code MDDP and it's supporting routine, FCCdata.
• Obtain a plot of the dislocation density evolution as a function of time.
• Obtain a plot of the stress vs time and verify the actual strain rate.
• Obtain a stress-strain curve
• For 2013 HW2 Only - Using the same setup, modify the DDinput file so that the dislocation is of pure screw character. Repeat the same steps 1, 2, and 3 and compare the results
• The second part of this section deals with the upscale bridging of dislocation mobility from the nanoscale to the microscale. The validity of dislocation dynamics hinges on the quality of the mobility law used.

# Part 1 Frank Read Source Operation

The FRSO is an elementary problem consisting of a dislocation multiplication of a dislocation segment pinned at both ends.

• Simulation box is cube with dimensions 12000 x 12000x 12000 (units: magnitude of the Burger's vector).
• Crystal is oriented so that the normal to the (111) plane is along the z-axis, the crystallographic direction [-211] is along the x-axis, and the [0-11] direction is along the y-axis.
• Initial dislocation structure consists of a dislocation line of length 2000b extending along the x-axis with its line sense in the negative x direction.
• The Burger's vector points in the negative y-direction
• Rigid wall boundary conditions are used on all sides. Dislocations cannot pass through the walls, rather they will stack up against them.
• Under the effect of the shear stress, the dislocation bows out, forms a loop and continuous operating in this mode generating increased number of loops. This process becomes harder as more dislocations pile up against the walls and induce back stress on the source, ultimately shutting it down.

## Preparation

### Step 1

• Obtain the 2 required input files and save them in your working directory.

### Step 2

• For 2013 HW2 navigate to
`/cavs/general/ICME_2013/`
• For 2015 HW2 navigate to
`/scratch/ICME_2015/HW2/MDDP/`
• Get a copy of the MDDP08 executable and save it to your working directory.

### Step 3

• In a terminal window, navigate to the new directory and open permissions for all the files with the command:
`chmod 770 *`

## Running the Calculations

• In a terminal window, navigate to the directory containing the newly saved files.
• Enter
`MDDP08`
in the command line
• Hit 'Y' to respond to the prompt that occurs.

## Output

The output files from MDDP are configured to be ready used with TecPlot. The information describing the dislocation structure is dumped into a chain of files starting with “tech” followed by a series number of the file (tech002, tech004, etc). Each file has the dislocation structure data for 500 time steps. The first 500 steps are written to tech002, the next 500 steps are written to tech004, and so on. This is done in order to keep the files’ size reasonable.

## Post Processing

To visualize the dislocation structure, the visualization package TecPlot360 will be used (available at CAVS) with instructions found here.