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

Abstract

In this study, the investigation on lock-in infrared thermography was done for the detection and estimation of artificial subsurface defects size and depth in stainless steel sample. The experimental and the finite element analysis were performed at several excitation frequencies to interrogate the sample ranging from 0.182 down to 0.021 Hz. A finite element model using 'ANSYS 14.0' was used to completely simulate the lock-in thermography. The four point method was used in post processing of every pixel of thermal images using the MATLAB programming language. A signal to noise ratio analysis was performed on both phase and amplitude images in each excitation frequency to determine the optimum frequency. The relationship of the phase value with respect to excitation frequency and defect depths was examined. Amplitude image was quantitatively analyzed using Vision Assistant, a special tool in LABVIEW program to acquire the defects size. The phase image was used to calculate the defects depth considering the thermal diffusivity of the material and the excitation frequency for which the defects become visible. A finite element analysis result was found to have good correlation with experimental result and thus demonstrated potentiality in quantification of subsurface defects.

Thermographic Inspection of Internal Defects in Steel Structures: Analysis of Signal Processing Techniques in Pulsed Thermography

“…In contrast, active thermography utilizes an external heating source to produce thermal contrast in the region of interest. Based on the exciting method, active thermography can be further classified as pulsed thermography (PT), lock-in thermography (LIT), vibrothermography (VT), and step heating thermography (SHT) [ 33 , 34 , 35 ]. PT and LIT are the two primary conventional practices.…”

Influence of defect shape in quantitative thermography

“…Lock-in and flash thermography have become increasingly popular in recent years as methods not only for detecting hidden defects, but also for quantifying them. Most studies have focused on determining some of the geometric dimensions of defects -most often the depth at which it is located [1,2]. Determining each of the geometric dimensions of the defects is difficult because it is influenced by many other parameters.…”

A Study on Crack Depth Measurement in Steel Structures Using Image-Based Intensity Differences

“…Nondestructive testing methods include radiographic testing (RT), ultrasonic testing (UT), magnetic practice testing (MT), penetrant testing (PT), eddy current testing (ET), leak testing (LT), and infrared thermography (IRT), and so on, and the demand is fast growing with the development of high-valueadded industries [4]. Among the various nondestructive techniques that can measure the depth of cracks, the noncontact testing typically includes eddy current testing [5][6][7] and infrared thermography [8][9][10], and it is the inspection method using the characteristics that the size and depth of the crack affect the formation of the temperature distribution of an object. However, the eddy current testing has a disadvantage in that a probe must be located very close to the surface of the object and is very sensitive to the surface roughness.…”