tUpdate README - slidergrid - grid of elastic sliders on a frictional surface
 (HTM) git clone git://src.adamsgaard.dk/slidergrid
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 (DIR) commit ce975080d4afb2c66265f61f2cf93ff2937007bf
 (DIR) parent 01f9042427f6b9e5933c803f55b9fdafe5b4a33c
 (HTM) Author: Anders Damsgaard <anders@adamsgaard.dk>
       Date:   Sat,  7 Sep 2019 08:25:37 +0200
       
       Update README
       
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 (DIR) diff --git a/README.md b/README.md
       t@@ -7,3 +7,26 @@ Licensed under the GNU Public License v.3, see `LICENSE` for details.
        
        Author: Anders Damsgaard, [adamsgaard@ucsd.edu](mailto:adamsgaard@ucsd.edu), 
        [http://cs.au.dk/~adc](http://cs.au.dk)
       +
       +
       +## Notes
       +- [ ] Implement beam equation in slidergrid
       +- [x] change in displacement is wrong
       +- [x] velocity set in test.c is not transfered to main.c
       +- [x] forces set in slider.c are not transfered to main.c. I can set mass but not force
       +- [x] system stable only with elasticity
       +- [ ] viscosity should be serial to elasticity (Maxwell material). From Walker 2013: We set Young's modulus and Poisson's ratio for ice to 4.8 GPa and 0.4, respectively, following Gudmundsson (2011), who showed that these values make the behavior of a Maxwell material essentially identical to that of the more complicated Burgers material used by Reeh et al. (2003). We find that a viscosity of ν=10^15 Pa*s allows for noticeable departure from purely elastic behavior while remaining high enough to prevent underdamping, consistent with observations. Model sensitivity to these and other parameters is discussed in the next section.  DEM implementation: Abbas et al 2007
       +- [ ] Add driving stress related to surface slope
       +- [ ] Add basal friction
       +
       +### Interaction outline
       +```
       +slider_neighbor_interaction (main.c)
       +  bond_deformation (slider.c)    // resolve strains
       +    bond_parallel_deformation      // resolve tension/compression
       +    bond_normal_deformation        // resolve shear/bending/twisting
       +  slider_interaction (slider.c)  // resolve forces and torques from strains
       +    bond_parallel_kelvin_voight    // resolve forces from tension/compression
       +    bond_normal_kelvin_voight      // resolve forces and torques from shear/bending/twisting
       +```
       +