Wikipedia
Stress-whitening
Stress whitening is a white line appearing along the bend or curve when a material is stressed by bending or punching operations.
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 materials, and also in some brittle polymers like PS, PMMA and Polycarbonate. The white colour is because of the light scattering by the crazes.