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

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 10mm steel specimen indicate that an excitation frequency of 0.02Hz is the optimal frequency for defect detection. Results obtained with an excitation frequency of 0.02Hz 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.