The effect of size on the quantitative estimation of defect depth in steel structures using lock-in thermography

Abstract

An investigation into the effect of size on the quantitative estimation of defect depth in a steel specimen has been undertaken using lock-in thermography. Phase contrast measurements over circular defects of varying diameter and depth are presented for a range of excitation frequencies. It was found that the diameter of a defect had an appreciable effect on the observed phase angle which consequently has significant implications with regard to estimating defect depth. Phase contrast measurements for a range of defects in a 10 mm steel specimen indicate that an excitation frequency of 0.02 Hz is the optimal frequency for defect detection. Results obtained with an excitation frequency of 0.02 Hz are used to discuss the limitations of determining the size and depth of defects. A finite element analysis was found to have good correlation with experimental data and thus demonstrates potential in providing improved estimates of defect depth.

Quantitative determination of subsurface defects in a reference specimen made of Plexiglas by means of lock-in and pulse phase infrared thermography

“…7) due to a matched combination of depth-diameter-modulation frequency, resulting in little or no phase response (blind frequency [28]). Pickering and Almond [14] and Wallbrink et al [30] already observed the same effect for flat-bottomed artificial defects back-drilled in carbon fibre composite and steel plates, respectively, and concluded that blind frequency occurrence is related to the defect diameter/thermal diffusion length ratio. In our study we found that it also depends on the depth of the defect and appears for p/l* ratios higher than 0.30.…”

Investigation of lock-in infrared thermography for evaluation of subsurface defects size and depth

“…The phase and amplitude information calculated by processing of recorded thermal images for each of the pixels are stored in the form of 2D matrices and subsequently converted to images known as phase image and amplitude image. [26][27][28][29] The amplitude image displays total temperature increase on the system during power cycling and phase image represents the time delay between powering a device and subsequent heating on the surface. Amplitude images are quantitatively analyzed to acquire the defect shape & size and phase image for defect's depth.…”

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

“…The online-mode requires special software that performs the necessary calculations during runtime (section 3.2 in [3]). Alternatively, special signal electronics can be used if a phase-locked temperature signal is available during runtime [4]. Data evaluation at the end of the measurement is usually realized offline by a Fast Fourier Transformation (FFT) or Discrete Fourier Transformation of multiples of an oscillation period (section 2.2 in [3].…”