Characterizing the Thermal Diffusivity of Single, Micrometer-Sized Fibers via High-Resolution Lock-In Thermography

Abstract

Many advanced materials consist of fibers. They are used as nonwovens, fabrics, or in composite materials. Characterization of individual fibers allows us to predict resulting material properties. We present a measurement setup and analysis software to characterize individual, micrometer-sized fibers fast and reliably. The setup is based on the lockin thermography principle. Thermal diffusivity values of seven reference samples agree very well with previously reported values. We use our setup to investigate critical measurement parameters like excitation frequency, excitation power, pixel size, and fiber orientation. Our results show that fibers with subpixel diameters can be measured even if they are not aligned. However, special care has to be taken to choose an adequate excitation power. Measurements at high intensities can underestimate thermal diffusivity even though the raw data looks reasonable. By automatically measuring at different excitation powers, our setup solves this issue.

A non-destructive thermographic procedure for the estimation of mechanical properties in steel through thermal diffusivity measurements

“…In fact, it has been demonstrated that as hardness increases, there is a corresponding reduction in thermal diffusivity [18,[33][34][35][36][37]. Among several methods present in literature for the non-destructive measurement of thermal diffusivity by thermography, using different sources, configurations and heating modes [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]45,46], the pulsed laser spot method in reflection mode has been used [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]45]. This method is based on the solution of the heat equation in the case of Gaussian distribution of energy, in adiabatic conditions, in a finite body after a Dirac pulse heating which is reported below:…”

“…Significant advantages can be obtained using non-destructive techniques (NDT), which can provide a check without influencing the component. Among all the NDT, active thermography has been proven to be effective for defects detections in composites and metals [7][8][9][10][11], process monitoring [12][13][14][15], thermophysical properties measurements [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] overcoming the limits of the traditional controls, because of a full field and no contact evaluation; thus 100% check is possible with a test duration compatible with high volume productions. Several past research works presented an anti-correlation between the hardness and thermal diffusivity [18,[33][34][35][36][37] and a thermographic procedure to evaluate the effectiveness of heat treatment in boron steel has been previously proposed [38,39] based on the use of spot pulsed laser method [19,21,24,[40][41][42].…”