Stress whitening occurs due to microstructural changes under mechanical deformation, especially in semi-crystalline polymers. When a polymer is bent beyond its elastic limit, localized plastic deformation begins. This leads to: • Formation of crazes: microvoids bridged by fibrils • Cavitation within amorphous regions • Alignment and stretching of polymer chains These features create interfaces with different refractive indices (polymer vs air voids). Light hitting these regions undergoes strong scattering (Mie scattering regime when feature sizes are comparable to visible wavelengths ~400–700 nm). Key factors influencing whitening: • Polymer type (amorphous vs semi-crystalline) • Strain rate and temperature • Degree of crystallinity • Presence of additives or plasticizers Importantly, stress whitening is often a precursor to fracture, since craze formation concentrates stress. In some engineered materials, this effect is actually used as a visual indicator of mechanical fat...