Systematic errors in the evaluation of uncorrected data from thermographic lock-in measurements

Abstract

Lock-in thermography (LT) is based on the correct evaluation of phase differences between the temperature oscillations at different surface regions of the object under test during periodic heating. Since the usual heating procedures contain a DC component, the actual heating pattern achieved is not harmonic. This causes systematic deviations when phase differences are determined by means of harmonic analysis, e.g. with FFT analysis. The resulting errors depend clearly on the ratio between DC and AC amplitude, which is demonstrated at simulated and experimentally recorded temperature transients. Further experimental LT data obtained by different oscillating energy inputs showed a variety of possible shapes of transients with different DC components..

Thermal wave interferometry measurements and microstructural analysis of twin wire arc spray cylinder coatings for passenger car engines

“…The surface temperature of the cylinder coating is measured 5 s for each excitation frequency. However, well known systematic errors during lock in thermography result from lock in measurements during early non linear heating of the sample [23]. To avoid non linear sample heating during the actual measurements, the cylinder coating is pre heated in advance of each measurement for additional 5 s at each ex citation frequency.…”

Thermal wave interferometry measurements correlated to microstructural properties and bonding behavior of thermally sprayed coatings

“…However, to even further minimize the influence of lateral heat flow, temperature values are only evaluated within a spot diameter of 6.7 mm, since lateral heat diffusion dominantly occurs in the outer region of the excited area. Following recommendations by Krankenhagen et al [38], the sample is preheated for 5 s in advance of every applied frequency to avoid non-linear sample heating during early excitation. Preliminary experiments have shown that this time scale is sufficient to eliminate non-linear heating effects for all applied modulation frequencies.…”