Temperature Diffusivity Measurement and Nondestructive Testing Requiring No Extensive Sample Preparation and Using Stepwise Point Heating and IR Thermography

Abstract

This chapter describes a modification to the laser flash method that allows determining temperature diffusivity and nondestructive testing of materials and constructions without cutting samples of predefined geometry. Stepwise local heating of the studied object surface at a small spot around 0.1 mm radius with simultaneous high temporary-spatial resolution infrared (IR) filming of the transient temperature distribution evolution with a thermal camera provides a wide range of possibilities for material characterization and sample testing. In case of isotropic and macroscopic homogeneous materials, the resulting transient temperature distribution is radially symmetric that renders possible to improve temperature measurement accuracy by averaging many pixels of the IR images located at the same distance from the heating spot center. The temperature diffusivity measurement can be conducted either on thin plates or on massive samples. The developed emissivity independent in plain IR thermographic method and mathematical algorithms enable thermal diffusivity measurement for both cases with accuracy around a few per cent for a wide range of materials starting from refractory ceramics to well-conducting metals. To detect defects, the differential algorithm was used. Subtracting averaged radial symmetric temperature distribution from the original one for each frame makes local inhomogeneities in the sample under study clearly discernible. When applied to crack detection in plates, the technique demonstrates good sensitivity to part-through cracks located both at the visible and invisible sides of the studied object.

Relationship between Thermal Diffusivity and Mechanical Properties of Wood

“…The thermal diffusivity tensor components a ij = λ ij /ρC p -where λ ij represents the thermal conductivity tensor components, ρ is the material density, C p is the specific thermal capacitance-were measured using our original non-destructive thermal imaging technique described in details in [11][12][13]. The idea at the basis is to perform a local stepped heating at small spots on the sample surface by a laser beam while continuously monitoring the surface temperature distribution with a thermal camera.…”

“…Several instances of thermal images at three perpendicular faces with some processing are shown at Figure 3. Every recorded frame was then processed according to the original models and algorithms described in [11][12][13]43,44]. In particular, each frame in a time interval such as 1-3 s after heating onset, with a pixel-by-pixel subtracted base frame obtained before heating, was used to determine the main axes of several tens of elliptical isotherms.…”

“…In particular, thermal conductivity λ and diffusivity a [2][3][4] of wood and other anisotropic composite materials depend on density, porosity, humidity [5][6][7], and specifics of interconnections between microstructural units of the material as hardness, Young's modulus, and strength do [8][9][10]. Once revealed, such relations could allow switching from laborious and material extensively destructive mechanical tests to non-destructive measurements of TPP using express methods, as described in [11][12][13]. Typically, they require around 1 min per measurement and do not demand for cutting samples of any specific geometry.…”

Multiscale Mechanical Performance of Wood: From Nano- to Macro-Scale across Structure Hierarchy and Size Effects

“…The thermal diffusivity tensor components a ij = λ ij / ρC p —where λ ij represents the thermal conductivity tensor components, ρ is the material density, and C p is the specific thermal capacitance—were measured using an original non-destructive thermal imaging technique described in detail in [ 167 , 168 , 169 ]. The method is based upon local stepped heating at small spots on the sample surface by a focused laser beam while continuously monitoring the surface temperature distribution with a thermal camera.…”

“…Heat propagation in such a setup is close to spherical symmetry in isotropic materials, while in orthotropic materials, the isothermal surfaces are close to three-axis ellipsoids with the axes fully determined by the main components of a ij tensor and the time elapsed since heating onset, provided that the distance to the heating center is at least several times higher than the heating beam radius. Therefore, the a ij values were determined by processing dynamic thermal images obtained on lateral, radial, and transverse crosscuts of wood samples, as described in [ 167 , 168 , 169 ].…”