Thermographic detection of defects in wood and wood-based materials
Thermography is a remote and nondestructive technique for the detection of defects usually not visible on the surface, e. g. cracks, delaminations, splitting, resin pockets, and black knots. Experiences from industrial measurements show that the technique is capable to inspect a surface or a structure element in one view and to solve several production problems. Passive thermography can be applied for the detection of defects if the objects under study undergo a heat treatment in the course of the production process. In this case, the cooling of the surface after the respective process step (e. g., after leaving the press) is observed with an infrared camera. Usually the surface temperature decreases homogenously within the inspected area. However, above or in the neighborhood of typical hidden defects such as air inclusions or delaminations, the heat conduction in the material is reduced, and therefore invisible defects appear as "cold spots" on the surface. Examples from measurements in industrial processes show the capability of this method to detect poor and good bonding of decorative paper on particle boards as well as density distributions in OSB and poor pressing in the gluelam production. If production heat can not be utilized, the heat has to be provided externally (active thermography), for instance by an infrared heat source which the samples pass when moving on a conveyor belt. While passing the heater the surface temperature increases by a few degrees centigrade, and the heat diffuses into the cold part of the object more or less homogenously. However, the heat conductivity is different (normally lower) near hidden defects so that the corresponding surface area remains warm for a longer time. By observing the surface under investigation with an infrared camera the faults can thus be identified as "hot spots" in the thermal image. In some cases, the excitation of the material with high power ultrasound is the best method to detect defects within the material. Especially in the case of black/dead knots and very small cracks the ultrasonic excitation will create heat along the defects because of friction. The heat produced by friction can be detected with the infrared camera, and the area can thus be identified as defective. Some examples from modern composite materials such as glass-fiber-reinforced plastic glued on particle boards, laminated floor coverings, and lightweight table plates made from plywood and/or composite materials will be presented in the talk, showing the detection of typical defects using this technique.