Transient thermal conductivity measurements: comparison of destructive and nondestructive techniques

Transient thermal conductivity methods have benefited from the rapid nature of the testing, but at the same time these methods are destructive because a sample must be prepared. Laser flash diffusivity is an example of a well-established transient method of measuring thermal conductivity, which requires samples to be prepared to a diameter of 12.4 mm. The newly introduced modified hot wire (MHW) and transient plane source (TPS) techniques are transient as well, but are nondestructive in nature. This is possible because both of these new techniques are reflectance methods that operate by applying and measuring heat at the same surface. Essentially any flat surface can be measured, with no maximum size constraints. These transient techniques have been used to evaluate materials of differing size, structure, and homogeneity. While results compare well for homogeneous materials from foams to ceramic, composite materials differ considerably. This is because the two methods penetrate the samples to different degrees, and the results are a reflection of the testing depth. As test times are varied in the reflectance techniques, information was gathered that has been related to the thermal conductivity as a function of position normal to the sample. Techniques are discussed and results on homogeneous materials from two different transient techniques are compared.