Methods to make photoelastic samples
Most of the transparent material around us have photoelastic properties. However for most of them we would need to load them so strongly to observe a photoelastic signal that we cannot really use it as photoelastic materials. Another reason not to use any random transparent material to make a photoelastic sample is that most of the time no special attention have been paid while forming the material so that it is full of residual stresses and not really usable. As an example here is a pair of glasses where one glass is observed in between crossed polarisers:
For all this reason, making a photoelastic sample is not a simple task. So, before making a photoelastic sample, it is necessary to choose the material you will use as a function of:
- the load you want to applied,
- the shape of the sample you want to get,
- how many samples you need,
- what you expect to measure
- how you expect to image your sample and
- the money you have.
This different criteria have to be carefully considered before choosing how you will make your photoelastic sample among the different methods presented below. This section will guide you to make samples from simply using a piece of gel, to casting polyurethane or cutting photoelastic pads. So first, you have to choose your material so that its photoelastic ability is enough to observe something significant in the range of stresses you want to apply. Second you have to make your sample without trapping any residual stress in it. Let's see it more carefully
Cutting samples out of a sheet
The first historical method consists in cutting the sample out of a sheet of photoelastic material. This can be simply a plate of plexiglass or a rubber pad that you cut with a milling machine, a cookie cutter or waterjet. You pick the material whose deformability is compatible with the load you want to apply and you cut it using different methods listed in this tutorial. This is convenient to get 2D samples. However, most of the time cutting methods induce residual stresses so machining the pads can be tricky. Also, it can be quite expensive depending on the material you use. All the details are available here.
The sample on the left (cross) is cut using a milling machine, while one on the left (cylinder) is cut with a rotating cookie cutter.
Molding urethane samples
A second method consist in directly molding your sample in the shape you want. In this case you need first to make a mold of the sample shape you want. Then, you pick urethane with the Young Modulus corresponding with the load you want to apply and finally you can cast the liquid urethane and get your sample. The complete method and tips are given in this tutorial. This is the most versatile way to make sample since you can make the shape you want as long as you can make a mold, and you can easily tune the stiffness of the sample varying the cross-linker quantity. Also it is a very cheap way to make very clean photoelastic samples since raw urethane is not expensive at all.
Squeezed piece of urethane between crossed polarisers.
Molding gel samples
Not only plastic polymers are good photoelastic materials convenient to mold. Some gels can also be used to make photoelastic samples:
- Gelatine for example have been used in different labs by lim et al., Kilcast et al. and Workamp and Dijksman. This material is very clear, easy to make and to mold and nicely photoelastic but not very stable on the long run even if it can be cross-linked for a better stability as explained here.
Squeezed piece of gelatine between crossed polarisers.
- Agar-agar gel can be used as a photoelastic material too, just like Tamlinson and Taylor did. Just like gelatine, it is easy to make and to mold and quite nicely photoelastic. However it is not perfectly transparent and the material is quite brittle when loaded dynamically and plastic-like when loaded very slowly.
Squeezed agar gel of urethane between crossed polarisers.
- Many other biologic or synthesized gels like konjac...
The different methods associated with these different materials are explained in this tutorial.
Printing polymer samples
Even if the method is just at its beginning it is also possible to 3D print photoelastic samples. Stratasys proposes for example to print VeroClear a transparent polymer that has a quite good photoelastic response (read this article for more example). Just like polyurethane molding, this method is very versatile in terms of shape. However if not properly used it let massive residual stresses and do not have a very large range of material properties. More details are available in this section