ULTRASONIC AND THERMOGRAPHIC NON-DESTRUCTIVE TECHNIQUES FOR BONDING EVALUATION IN EXTERNAL FRP STRUCTURAL STRENGTHENINGS

摘要

In this work two different Non-Destructive Techniques (NDT) have been investigated in order to point out their effectiveness in experimental bonding defects detection for external FRP structural strengthenings, and in order to arrange standard operative procedures for in-situ applications. An amplitude-based ultrasonic technique has been developed and has been experimentally demonstrated to be effective in detecting both bonding defects and stress induced detachments of CFRP and GERP applied on different structural materials like concrete, brick masonry and polyurethane. A theoretical model based on acoustic impedance mismatch between FRP layer and bonding discontinuities has been developed as well in order to predict defects detectability. Then, thermography has been examined in order to find an alternative to the ultrasonic technique, which is time consuming and requires well-experienced operators. A theoretical model has been developed again, describing the thermal behaviour of the FRP layer, heated (or cooled) through the free surface, in presence of a bonding defect. Experimental results, fully in accordance with the model, have evidenced that thermography effectiveness is heavily influenced by thermal conductivity of the FRP material, and for this reason it is very critical for CFRP materials. At the end, a combined thermographic-ultrasonic testing procedure has been theorised through a comparison between the two single techniques that have been experimentally applied to the same specimen.