In shearography, the tyre is subjected to two pressure conditions with the aid of a vacuum chamber and the inner surface of the tyre is illuminated with laser light. The pressure difference between the two conditions causes the surface to bend (“dent”) as a result of the enclosed air. The optical superposition of the images of both pressure states leads to interference lines at the “dents”, which allow indications of separations between the rubber layers. In this way, separations in the tire structure can be detected at a very early stage.
The prerequisite, however, is that the air inclusion is completely enclosed by material, which is often no longer the case with destroyed tires.
Shearography is used in tire production to evaluate carcasses for retreading. However, I often use this method for damage analysis because it is relatively simple and inexpensive to use.