Signal Processing for air coupled acoustic characterizationof thin (adhesive) multi layers

Wöckel, Sebastian; Institut fur Automation und Kommunikation eV; Germany

Wöckel, S.; ifak – Institute for automation and communication; Germany

ID: ECNDT-0049-2018
Download: PDF
Session: Additive Manufacturing – ultrasound
Room: H1
Date: 2018-06-12
Time: 10:00 - 10:20

The non-destructive evaluation of adhesive layers is one of the most crucial information in modern manufacturing technology. The physical state of the thin adhesive layer, including its thickness, material distribution, density and elasticity, determines the stability and durability of the assembly. In aerospace and automotive industry the inspection with ultrasound is a common technique. Depending on the application a direct contact, immersion or air-coupled techniques are applied. Although the first provides the highest signal-to-noise ratio and resolution, it is limited to point-selective measurements. The second water coupled technique provides an automatic scanning, keeping the SNR and resolution, whereat the construction component needs to be immersed in a water bed. The inspection with air coupled ultrasound provides the highest degree of freedom – concerning the technical realization. Its main drawbacks are the low defect sensitivity caused by the high reflection loss at the air-solid interface and the low resolution due to the wavelength accordingly frequency ranges (< 1 MHz). Further, varying process conditions (e.g. temperature, inhomogeneity of the media) hinder their extensive and reliable usage. This contribution introduces a signal oriented method to overcome the mentioned drawbacks of air-coupled inspections. The idea bases on the fast estimation of the sound propagation in the multi layered media including an automated derivation of media properties (mainly density, and elasticity) by using a genetic algorithm. The modelling and the estimation technique are exemplified on simulation data of multiple stacked solid and elastic (adhesive) layers, which are “thin” (< 1 mm) in relation to the acoustic wavelength (> 5 mm).
The main outcome, for air coupled non-destructive evaluations, is an algorithmic method which delivers supplemental information by a complete acoustic characterization of the medium. This enables an increase of the reliability in the non-invasive remote monitoring and the access to new applications of material characterization.