The influence of optical properties of paints and coatings on the efficiency of infrared nondestructive testing applied to aluminum aircraft structures

IR NDT (Infrared Nondestructive Testing) is a popular method for detecting defects in composite, ceramic, and metallic structures. The effectiveness of IR NDT depends on various thermal and optical properties of the material being tested. The thermal properties, including thermal conductivity, thermal diffusivity, specific heat and density are important and have been discussed extensively in many treatises on IR NDT. However the optical properties of the surface are equally important and while the thermal properties cannot be changed, sometimes the optical properties can be. Bare metal surfaces have high reflectivities and low emissivities, and as a result, they are generally not good candidates for IR NDT. Painted, coated, anodized, and oxidized metal surfaces can, in some cases, be successfully tested with IR NDT, but the effectiveness depends on the optical properties of the surface. It is well known by IR NDT practitioners that the easy solution to the testing of reflective materials is to paint the surface black. However, this is not always practical and it may not be permitted by the “owner” of the part. This paper demonstrates a process of analyzing the interaction of spectral curves that are relevant to the IR NDT process. This process can be used to evaluate the effectiveness of an IR NDT process for use on real parts with specific coatings and can help select a coating that may improve the effectiveness. This paper shows examples of optical properties for some typical paints and coatings that may be used on aluminum aircraft structures. It shows the spectrum of a generic incandescent radiant heat source and how the energy from this source is absorbed by several of these paints. Further, it shows the interaction between an IR camera detector response curve and the other curves. And finally, it shows how these three can be combined to produce an “IR NDT” efficiency rating for several examples.