Light remembers where a part is squeezed
A clear polymer is optically isotropic at rest — light passes through unchanged, and between crossed polarisers the field stays black. Load it, and the molecular lattice stretches unevenly. The material turns birefringent, splitting each ray in two along the principal stress axes and letting them travel at different speeds.
Recombined through the second, crossed polariser, the two rays interfere. Where the retardation between them lands on a whole wavelength, that colour extinguishes and the survivors paint a fringe. Nothing is added to the part; we are only reading a stress that was already there, standing invisible until the light was asked the right way.
- Isochromatic
- A fringe of equal principal-stress difference. Count the bands and you have measured the shear, without touching the part.
- Fringe order · N
- How many wavelengths of retardation the light accrued crossing the material. Higher order, harder loaded — the colour tells you which.
- Stress concentration
- Where the fringes crowd tight: a fillet, a thread root, a weld toe. The part fails here first, long before the bulk gives way.
The colour is the number
Under white light the fringe order is written in colour. Zero stress leaves black extinction; the first order passes through grey and straw yellow to a deep first-order red, then the vivid blues and greens of the second, fading to pale pinks and sea-greens as the count climbs into the high orders.
This strip is the exact sequence our polariscope produces, computed the same way the hook above is coloured. Drive the load and watch the throat of the hook march up it, band after band.
What we put under the light
Lifting hook
Cast epoxy · 2 t ratedCurved-beam bending throws the peak onto the inner throat; the fringes stack there first while the back of the curl stays nearly black.
Combination spanner
Acrylic · open jawThe open jaw is a tuning fork of stress. Every flat crowds its bands into the reentrant corner where the fork meets the handle.
Bracket weld toe
Stress-frozen · filletThe toe of a fillet weld is a fringe magnet. We photograph the fatigue crescent years before it becomes a crack.
Involute gear tooth
Polycarbonate · single meshContact load walks along the flank; the root fillet lights up brightest at the instant each tooth takes the drive.
Bring us the part
Cast or machine it in stress-frozen epoxy, or hand us the acrylic prototype. We load it on the bench, photograph the fringes at every increment, and return a stress map keyed to the fringe order — the quiet places and the loud ones, mapped before the part ever sees service.
Book the polariscopeStress-freezing · 3D photoelasticity · live loading to 60 kN
Bench 4, Kelvin Works — response within two working days