Thermoplastic polymers may exhibit a change in color under tension. This phenomenon is known as stress whitening and may occur in both semi-crystalline and amorphous thermoplastics.
What is really happening here?
When an axial load is applied to a thermoplastic, the polymer chains reorganize during stretch. The polymer chains will straighten, slip and shear within the plastic’s microenvironment. In some cases, occlusions or holes are formed by the movement of fillers and the polymer chains. Together these occlusions form microvoids. When microvoids cluster to a size greater than or equal to the wavelength of light (380-750nm), the transmitted light is scattered and the object appears white. Thus, the microvoids change the refractive index of the plastic.
It is important to understand stress whitening when measuring strain after yield with a video extensometer. When testing with a video extensometer, contrasting dots are used to track specimen strain (i.e. white dots on a black specimen). In a stress whitening plastic, a dark specimen marked with white dots will change color after yield. In this scenario, the video may have difficulty tracking the dots and a more complicated marking solution would be needed. An alternative solution for measuring strain after yield in stress whitening plastics is to use a high travel contacting extensometer, such as the AutoX750.