Enhanced method for flaws depth estimation in CFRP slabs from FDTC thermal contrast sequences

Andrés David Restrepo Girón

doi:10.15446/ing.investig.v35n3.50552

Abstract

After the detection of internal defects in materials, the characterization of these plays a decisive role in order to establish the severity of these flaws. Finite difference thermal contrast (FDTC) is a new technique proposed recently for contrast enhancement in sequences of thermal images in order to allow the detection of internal flaws in composite slabs with greater probability of success. Besides FDTC, a criterion was also conceived for the estimation of the depth of the detected defects, which brings good results for shallow and strong contrast defects, but poor estimations for deeper and weaker defects. Considering this problem, a revision of the original criterion is carried out in this paper to define a new and robust criterion for estimating the depth of defects, applied after FDTC en-hancement and flaws detection. Results of the execution of the revised algorithm on a synthetized thermal sequence from an artificial CFRP slab (using ThermoCalc6L software) show a better performance of the estimation task, reducing the average relative error by more than half.

Highlights

Nowadays, composite materials like Carbon Fiber Reinforced Plastic (CFRP) play a crucial role for aeronautical and automotive industry due mainly to the better strength/ weight relation with respect to common metallic materials
In an experiment of Pulsed Thermography (PT), a flash is used to heat the material under inspection and a sequence of infrared (IR) images or thermograms are recorded by a thermal camera; later, cooling profiles at every pixel are extracted from these images to be processed and make possible the detection and characterization of internal defects or flaws (Benítez, Loaiza & Caicedo, 2011)
To assess the performance of the modified Finite Difference Thermal Contrast (FDTC) depth estimation, without the influence of noise, emissivity or any optical distortion, a sequence of IR images obtained from a simulated squared CFRP slab, 2 mm thick, with 9 air-filled defects (Figure 3) was used

Summary

Introduction

Composite materials like Carbon Fiber Reinforced Plastic (CFRP) play a crucial role for aeronautical and automotive industry due mainly to the better strength/ weight relation with respect to common metallic materials. Finite Difference Thermal Contrast (FDTC) method was proposed to work with a discretized and reduced 3D heat propagation model to give a relative thermal estimation error, leading to a normalization procedure that brings stronger contrast profiles and, greater probability of detecting deeper defects without requiring an a priori selection of a sound area or a reference image (Restrepo & Loaiza, 2014a).

Results

Conclusion