Photoelastik

Photoelasticity, or photoelastik in Turkish, is an experimental method used to visualize and quantify internal stresses in transparent elastic materials. The technique exploits birefringence that develops in a stressed transparent material: light traveling through the specimen experiences a difference in refractive indices along two principal stress directions, producing phase retardation and interference fringes when viewed between polarizers.

The optical principle involves the stress–optic law: Δn = C (σ1 − σ2), where Δn is the difference in refractive indices, σ1 and σ2 are the principal stresses, and C is the material’s stress-optic coefficient. The accumulated retardation δ = (2π/λ) Δn t leads to isochromatic fringes of constant σ1 − σ2, with fringe order N proportional to (t/λ) C (σ1 − σ2). Isochromatic lines reveal magnitudes of principal-stress difference, while isoclinic lines (if observed) indicate the orientation of principal stress directions. A two-polarizer setup, called a polariscope, is used: a light source, a polarizer, the specimen, and an analyzer. Materials can be transparent polymers, epoxies, glass, or clear resins; loading can be static or dynamic.

Applications include evaluation of stress concentration around notches or holes, verification of finite-element models, and educational demonstrations. Modern approaches extend to digital photoelasticity, where full-field fringe data are captured with cameras and analyzed computationally to derive quantitative stress maps.