How can holographic interferometry detect flaws between the layers of a tire?

Holographic interferometry is a powerful tool for the nondestructive testing of tires, specifically for detecting internal flaws such as debonds between tire plies.1

The process relies on the principles of holographic interferometry, which can detect minute changes in an object's surface by comparing wave fronts. To inspect a tire, a hologram is typically recorded while the tire is in an unstressed state. The tire is then subjected to a stress—such as a slight change in pressure or temperature—which causes the surface to deform.2

If there is a flaw or debond between the internal layers, that specific area of the tire will exhibit a different deformation pattern compared to the surrounding, well-bonded material. When a second hologram of the stressed tire is superimposed or compared with the original, the resulting interference fringes reveal these localized surface irregularities. These fringes act as a map of the deformation, allowing technicians to identify the exact location of internal structural weaknesses that would otherwise be invisible to the naked eye.21