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Cutting samples out of photoelastic material
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Historically the first method to get photoelastic sample was to cut them out of a photoelastic sheet using any blade. This works fine for 2D samples but necessitate skills to cut the sheet without inducing residual stresses. Also it is quite expensive especially if you consider using [Vishay](http://www.vishaypg.com/micro-measurements/) material. In the following section we go more into detail about the material choice and the different ways to cut it.
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Historically the first method to get photoelastic samples [1] was to cut them out of a photoelastic sheet using any blade. This works fine for 2D samples but necessitate skills to cut the sheet without inducing residual stresses. Also it is quite expensive especially if you consider using [Vishay](http://www.vishaypg.com/micro-measurements/) material. In the following section we go more into detail about the material choice and the different ways to cut it.
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Material sheets
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Most of the time, photoelastic sheets or pads are made of transparent urethane rubber. But for strong loading you can also consider using plexiglass or polycarbonate. This latters can be bought from any local supplier. Just make sure that they have not been bent or knocked so that there are no residual stresses. If it is the case, you can still get ride of them by annealing them in an oven at ~90 degrees during ~2 hours (parameter to adapt as a function of the material you use and the amount of residual stresses you have). The same process can be used if residual stresses are induced by the cutting process.
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Most of the time, photoelastic sheets or pads are made of transparent urethane rubber. But for strong loading you can also consider using plexiglass or polycarbonate. This latter can be bought from any local supplier. Just make sure that they have not been bent or knocked so that there are no residual stresses. If it is the case, you can still get ride of them by annealing them in an oven at ~90 degrees during ~2 hours (parameter to adapt as a function of the material you use and the amount of residual stresses you have). The same process can be used if residual stresses are induced by the cutting process.
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Rubber-like sheets or pads are more dedicated for lower stresses. They are the most used. You can buy them from [Vishay](http://www.vishaypg.com/micro-measurements/photo-stress-plus/) which is the most well known company for photoelastic pads. But companies like [Precision Urethane](http://www.precisionurethane.com/) also have a large diveristy of [sheets](http://www.precisionurethane.com/urethane-sheets.html) and [pads](http://www.precisionurethane.com/polyurethane-pads.html) of different thickness hardness and color. They are not initially dedicated to photoelasticity but works perfectly well. For the most common use, pads of thickness 1/4'' and of hardness 60A are usually good. Also, if you buy harder material (>70A), because the material can be knocked or folded during the shipping, residual stress can be induced. So be carefully to specify to the company that plates must not be folded and must be properly packed.
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Rubber-like sheets or pads are more dedicated for lower stresses. They are the most used. You can buy them from [Vishay](http://www.vishaypg.com/micro-measurements/photo-stress-plus/) which is the most well known company for photoelastic pads. But companies like [Precision Urethane](http://www.precisionurethane.com/) also have a large diveristy of [sheets](http://www.precisionurethane.com/urethane-sheets.html) and [pads](http://www.precisionurethane.com/polyurethane-pads.html) of different thickness hardness and color. They are not initially dedicated to photoelasticity but works perfectly well. For the most common use, pads of thickness 1/4'' and of hardness 60A are usually good [1]. Also, if you buy harder material (>70A), because the material can be knocked or folded during the shipping, residual stress can be induced. So be carefully to specify to the company that plates must not be folded and must be properly packed.
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Cookie cutter method
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... | ... | @@ -37,18 +37,20 @@ For more details about the milling machine method you can refer to the [Duke Phy |
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Water jet cutting method
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The photoelastic pads bought from [Precision Urethane](http://www.precisionurethane.com/)or [Vishay](http://www.vishaypg.com/micro-measurements/) can be hydro-cut into designed size and shape particles. Usually for 60A (Precision Urethane) 1/4 inch thickness or PSM-4(Vishay) 1/4 in thickness pads, the waterjet is set as 60,000psi(~410MPa) with 80 mesh abrasive. [ADR Hydro-Cut, Inc.](http://www.adrhydrocut.com) is where we usually cut our particles.
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We used waterjet to cut gear particles to study the friction effect on shear jamming.
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A last method to cut photoelastic rubber-like pads is to use water-jet cutting. This method were first used by [3] and have been developped further in Behringer's lab at Duke University [4]. Just like the previous one, hydro-cutting is a very versatile method which permits to design any sample shape. It permits to get very straight and smooth edges with few residual stresses. Usually for a [Precision Urethane pad](http://www.precisionurethane.com/polyurethane-pads.html) of hardness 60A and thickness 1/4'', the water-jet is set as 60,000 psi (~410 MPa) with 80 mesh abrasive. [ADR Hydro-Cut, Inc.](http://www.adrhydrocut.com) is where Duke University photoelastic samples are usually cut. Below is a picture of geared cylindrical samples obtained with this method. The narrow cutting thickness (~0.1 mm) permits to make very complex edges.
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![Capture_du_2019-01-07_16-06-00](uploads/9371789aa0cdd74aa4094a059d6de3ea/Capture_du_2019-01-07_16-06-00.png)
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References for further reading
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(1) [D. Howell and R. P. Behringer, Fluctuations and dynamics for a two-dimensional sheared granular material. In Powders and Grains 97, edited by R. Behringer and J. Jenkins (Taylor & Francis, 1997), pp. 337–340.](https://www.crcpress.com/Powder-and-Grains-97/Behringer-Jenkins/p/book/9789054108849)
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(1) [D. Howell and R. P. Behringer, Fluctuations and dynamics for a two-dimensional sheared granular material, Powders and Grains, Taylor & Francis, (1997).](https://www.crcpress.com/Powder-and-Grains-97/Behringer-Jenkins/p/book/9789054108849)
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(2)[Karen E. Daniels, Jonathan E. Kollmer, and James G. Puckett, Photoelastic force measurements in granular materials, Review of Scientific Instruments 88, 051808 (2017)](https://aip.scitation.org/doi/pdf/10.1063/1.4983049?class=pdf)
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(2)[Karen E. Daniels, Jonathan E. Kollmer, and James G. Puckett, Photoelastic force measurements in granular materials. Review of Scientific Instruments 88, 051808 (2017)](https://aip.scitation.org/doi/pdf/10.1063/1.4983049?class=pdf)
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(3) [D. M. Wendell, K. Luginbuhl, J. Guerrero and A. E. Hosoi, Experimental investigation of plant root growth through granular substrates, Experimental Mechanics 52, (2012).](https://link.springer.com/article/10.1007/s11340-011-9569-x)
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(4) [H. Zheng, D. Wang, J. Barés and R. P. Behringer, Jamming by compressing a system of granular crosses, EPJ Web of Conferences 140, p. 06014 (2017)](https://www.epj-conferences.org/articles/epjconf/abs/2017/09/epjconf162393/epjconf162393.html)
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