How can infrared testing reveal faulty adhesive bonds in plywood-like sandwich materials?

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In the context of materials testing, infrared techniques are used to evaluate the integrity of adhesive bonds in sandwich construction materials like plywood by monitoring heat dissipation.1

When heat is applied to the surface of the material, the core acts as a heat sink, causing the surface temperature to drop evenly where the bond is continuous and secure. Conversely, if an adhesive bond is faulty, missing, or inadequate, the core cannot effectively draw heat away from that specific area. Consequently, these defective regions remain at a higher temperature than the surrounding well-bonded areas.1

By using infrared photography to capture the surface temperature distribution, inspectors can visually identify the precise location and shape of the bond failure. Alternatively, this process can be performed using thermal coatings that change color when they reach specific temperatures, providing a clear visual indicator of the bond's quality.1