Stress Analyser Using Polarizers

The Usual method to identify stress in a material is to use the moduli like Young's Modulus, Bulk Modulus which are mathematical functions and require input parameters which are to be measured manually or obtained from a source.

Knowing stress points in materials is helpful for handling, or just study, or explore stress in different materials.

This Strain Analyser is based on a experimental method called photo-elasticity, which in turn is an outcome of a material property called birefringence (Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light, materials exhibiting it are categorised as birefringent). This method is purely experimental and gives a general idea of stress in a material.

This test requires light to pass through the sample under analysis and is therefore restricted to materials which allow light to pass through like glass(ware), plastics [even coloured ones provided that they allow light to pass through them, same goes for glass(ware)]. Even you can check stress points of transparent minerals like Calcite or translucent ones like Potash Alum.

• Our Stress analyser is a simple one with a strong light source, a polariser, an analyser, and a sample holder all contained in a frame.

Now Lets move to DIY(ing) section!

Let's check these items first

Materials

1. Polarising sheets (x2).^
2. Cardboard.
3. A Collimated white LED light Source OR A strong Light Source(Strong White LED)

^ You can buy them, or can salvage them from a broken LCD display (I got them from a broken digital calendar.)

Tools

Well it actually depends upon you what you want to use, still giving a list of tools that I used. Might come in handy.

• Scissors
• Tape (the one used to stick things) /* I used packaging tape */
• Stapler

Get them together and move to the next step

I preferred cardboard (Packaging Cardboard) here but you can make it more rigid, if you may use wood.

Lets look at the design first (The pictures above describe the design)

The whole structure is a box with dimensions (10 x 10 x 20) centimetres (L x B x H).

Now lets construct

First we make three walls and base

Cut out the four walls, a top and a bottom and a sample holder which will also hold the polariser sheet.

• The dimensions of walls are 20cm x 10cm (/!\ - Don't forget to include extra length's because they'll be needed to attach the whole setup together.).
• The dimensions of Top and Bottom are 10cm x 10cm (/!\ - Don't forget to include extra length's because they'll be needed to attach the whole setup together.)
• Cut out Spaces in sample holder and top to install polarisers (The dimensions of cuts depend upon the dimensions of the polarising sheets you have.)

Don't forget to install the polarisers crossed (! - It isn't necessary but gives a good background to samples being examined)

• Now attach one of the polariser sheets to the sample space and the top as is shown in the picture.
• Attach two walls facing each other with base and then install sample holder at an appropriate height.
• Now attach the other two walls.(I stapled them together)
• Now attach the top ( you can keep it fixed, I preferred it open to give me more control over sample space)
• Cut out a section of cardboard from any of the four installed walls to give you access to sample space.

/!\ - I made a cut in the lighting section as well because I installed a removable light.

If you want to make it self sufficient you may choose to give it an independent light source, I used a torch and kept it removable.

The light source sits on the bottom and shines light through the analyser(the first polariser)

Ahem!... Now that we made it, let's check some samples.

I ordered the prints on cardboard to make them look a bit better