Photoelasticimetry : Seeing Mechanical Stress With Optics

Photoelasticimetry is a way to visualize strains in materials. In this Instructable, we will see how you can do some samples to experimentally determine the stress distribution in some materials under mechanical load !

Step 1: Some Explanations About How This Works

A "birefringent" material is a material in which the refractive index (i.e. the speed of light) depends on the polarization and propagation direction of light.

When you apply some mechanical stress, the birefringence of the material changes locally depending on the strain, and at some points it acts like a "wave plate" that change the polarization state of the light.

A "polarizer" is an optical component that only let some types of polarization to pass through it. If you superpose two "linear" type polarizers oriented in the perpendicular directions, the light will be blocked but if you add a "wave plate" conveniently oriented between them, the light will pass through and you will see light.

Combining these two effects allows to see in real-time different colors that pass or not (as the modification of polarization also depends here on the wavelength of light)

To better understand how a wave plate allows to change the polarization of light, you can read the following article :

https://en.wikipedia.org/wiki/Waveplate

The photoelasticimetry article also goes further than my lightweight explanation :

https://en.wikipedia.org/wiki/Photoelasticity

Step 2: Build Some Mechanical Frames to Apply Stress to Plastic Parts

Here are some frames and samples I imagined to visualise frames

Step 3: Realize the Frames

Thanks to a french I.U.T. Fablab in the city of Cachan (at the south of Paris), the InnovLab ( http://innovlab-iut-cachan.blogspot.com/ ), I had the chance to have access to a waterjet cutter in order to realize the frames !

http://innovlab-iut-cachan.blogspot.com/2018/10/po...

If you want to do the sames, you could water cut them or maybe use others types of C.N.C. machining. Here, I used a 12mm thick aluminum material.

Then, you can drill some holes and tap them to add some screws that will help you to press the samples. You can also do a mechanical deformable structure that will locally press your sample.

Step 4: Realize the Samples

You can also cut some samples (some bars, or an Eiffel-like tower) in plastic (I successfully used some 7mm thick PolyCarbonate sheet, glass also works but breaks more easily)

Step 5: Enjoy Your Experiments

Put your sample in the frame and view it between an L.C.D. screen (that emit polarized light) and a polarizer (I got mine there : https://www.ebay.com/itm/123232829944 )

Then apply you strain and watch the colors change.

Enjoy !

Comments

[smooth_jamie]

Interesting instructable. So where do you place the polarising film? Either side of the frame with the polycarbonate part in the middle? What orientation should the film be in?

[LaPuge]

Thanks !

The screen is behind the polycarbonate part (it includes by itself a polariser), while the polarising film is in the front of it.
You can also do a sandwich with the polycarbonate between two polarizing films and look at a soft light source (for example, a piece of paper illuminated with a desktop lamp)

The orientation of the films and sample will determine which polarisation you can let pass / shutdown. Before cutting your final piece, you can "cross" the axis of the linear polarisers by turning the closest one of you (they should become "dark", and block the light), and put the raw polycarbonate sheet between them.
Depending on the orientation of the sheet, you will see either no light (in this case it means that the axis of the polarisers are aligned with the axis of birefringence of your sample that act like a waveplate), either if you turn it at 45°, you will see a colored light (see https://en.wikipedia.org/wiki/Interference_colour_... for more details about the colors)