Inverse model for defect characterisation of externally glued CFRP on reinforced concrete structures: comparative study of square pulsed and pulsed thermography

Abstract

The objective of the study summarised, hereafter, is to compare square pulsed and pulsed thermography for defect detection and characterisation of carbon fibre-reinforced polymer (CFRP) plates used as structural reinforcement in Civil Engineering applications. For this purpose, two specimens built with cement concrete support were manufactured in the laboratory. They were reinforced with CFRP plates bonded to their surface and different artificial defects were inserted during gluing. Two types of thermal excitations (pulse and square pulse optical heating) have been studied and applied to these specimens. In parallel, numerical simulations were carried out as well. Defect detection was rapidly performed using singular value decomposition both on experimental and simulated thermal image sequences. A 1D multi-layer thermal quadrupole model coupled to an estimation procedure was studied. Validity domain of the 1D hypothesis is studied. Then this model is used to characterise the defects and the glue layer thickness. In addition, a sensitivity study of the proposed model is also presented. Furthermore, 3D numerical simulations were conducted to study the reliability of the estimation procedure vs. the two types of thermal excitations. The estimation procedure on numerical data provides results closer to experimental observations when applied to the square pulse method and it is less disturbed by white noise, even though a preliminary estimation residue analysis indicated that the heat pulse method should be, in theory, more precise. Finally, the estimation procedure studied herein was applied to experimental data acquired on laboratory specimens. Results obtained are analysed and perspectives for the presented method are discussed.