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I'm looking for a material that changes optical properties in response to changes in stress or density. The idea is to make a solid motor coupling to measure torque using photo emitter and detector mounted on the stationary housing. Perhaps something like an acrylic rod made with a pigment suspended during casting so that the pigment particles move closer as torque is applied to the shaft increasing it's opacity.

Are there any materials that would demonstrate this property at the microscopic level and does the effect I'm looking for have a technical name?

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  • $\begingroup$ You might also get good answers on the Engineering SE site. $\endgroup$
    – Steeven
    Jun 1, 2019 at 13:44

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Photoelasticity is the phenomenon that will most likely be useful to you. There are a lot of resources and literature online, such as this: Manual on Experimental Stress Analysis, and this: Stress Analysis of a Rotating Body by Means of Photostress Method and Using Solidworks Programme.

The principle is that certain transparent materials alter the polarization of transmitted light according to the amount and direction of stress in the material.

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You can probably use the fact that the breakdown voltage in gas depends on the gas pressure (https://en.wikipedia.org/wiki/Electric_discharge_in_gases). So pressure decrease in gas subjected to electric field can trigger gas discharge and make the gas opaque.

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