Stress-whitening is where a white line appears along a bend or curve when a material is stressed by bending or punching operations. Polymer Stress Whitening

The appearance of white line indicates that there is an onset of failure of the corresponding material. This phenomenon is known as "stress-whitening". This is more common in

amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...

materials, and also in some brittle

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

s like PS, PMMA and

Polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...

. The white colour is because of the light scattering by the crazes and changes in the

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

of the area under stress.

References

* {{cite book , last1=International , first1=ASM , last2=Lampman , first2=S. , title=Characterization and Failure Analysis of Plastics , publisher=ASM International , year=2003 , isbn=978-1-61503-073-6 , url=https://books.google.com/books?id=RJWiilJLdxYC&pg=PA405 , access-date=2021-05-01 Machining