Here is more on photoelasticity, the principle by which stress analysis with crossed polarisers works. It is particularly cool (and I didn’t know) that many computer flat panel screens are excellent sources of uniformly illuminated polarised light. Put on a pair of polarised sunglasses, bring up a blank white window on your computer, tilt your head (if necessary) until the screen goes the darkest, then look through various objects of clear plastic and see the stress contours and localisation. Apply some bending force to the object and watch the stress fringes dance.