Determination of thermal wave reflection coefficient to better estimate defect depth using pulsed thermography

Adisorn Sirikham,Yifan Zhao,Jörn Mehnen

doi:10.1016/j.infrared.2017.08.012

Adisorn Sirikham, Yifan Zhao + Show 1 more

Open Access

https://doi.org/10.1016/j.infrared.2017.08.012

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Abstract

Thermography is a promising method for detecting subsurface defects, but accurate measurement of defect depth is still a big challenge because thermographic signals are typically corrupted by imaging noise and affected by 3D heat conduction. Existing methods based on numerical models are susceptible to signal noise and methods based on analytical models require rigorous assumptions that usually cannot be satisfied in practical applications. This paper presents a new method to improve the measurement accuracy of subsurface defect depth through determining the thermal wave reflection coefficient directly from observed data that is usually assumed to be pre-known. This target is achieved through introducing a new heat transfer model that includes multiple physical parameters to better describe the observed thermal behaviour in pulsed thermographic inspection. Numerical simulations are used to evaluate the performance of the proposed method against four selected state-of-the-art methods. Results show that the accuracy of depth measurement has been improved up to 10% when noise level is high and thermal wave reflection coefficients is low. The feasibility of the proposed method in real data is also validated through a case study on characterising flat-bottom holes in carbon fibre reinforced polymer (CFRP) laminates which has a wide application in various sectors of industry.

Highlights

Nowadays, non-destructive testing (NDT) is playing a more and more important role to inspect defects/damages of various industrial components
peak contrast time method (PCT) measures the peak time of the temperature contrast between the considered point and the reference point, and peak slope time method (PST) detects the peak time of the first derivative of temperature contrast
The aim of these numerical simulations is to compare the performance of the proposed New Least-Squares Fitting method (NLSF) method with other existing methods against different values of R and noise levels

Summary

Introduction

Non-destructive testing (NDT) is playing a more and more important role to inspect defects/damages of various industrial components. PCT measures the peak time of the temperature contrast between the considered point and the reference point, and PST detects the peak time of the first derivative of temperature contrast The LSF method uses a curve fitting technique to fit the temperature decay curve based on a theoretical heat transfer model to determine the defect depth directly This method is less susceptible to noise but it presumes that the thermal wave reflection coefficient (R) is 1, which is not true for most real situations [39].

Principle of active thermography

Numerical simulations

Experimental results

Under a noise condition