Ultrasonic finite-sized beam transmission through an immersed plate: Towards the local mechanical evaluation of adhesive bonds
Résumé
Industries such as aeronautics or aerospace tend to replace classical assembly processes (riveting, screwing or welding) by adhesively-bonded joints in order to reduce the mass and to obtain uniform concentrations of stresses in assemblies. However, the use of adhesive bonds is limited because of the lack of non-destructing methods to guarantee their quality. Echogenic defects located at interfaces between substrates and adhesive, e.g. disbonds, can be revealed <i>via</i>via ultrasonic measurements, but weakness in the interfacial adhesion while contact is ensured is difficult to detect. A recent work showed that ultrasonically measured elastic moduli of the whole adhesive bond (adhesive layer and interfaces) exhibit abnormal anisotropy when interfaces are not of good quality [1]. The used characterization method was based on the resolution of an inverse problem that minimizes the difference between measured and calculated ultrasonic transmission coefficients, for immersed assemblies under a plane-wave assumption, thus interrogating large areas of the bonds. The current work extends that method to finite or focused ultrasonic beams in order to locally characterize mechanical properties. For the moment, normal incidence measurements and predictions under collimated-bounded-beam conditions show unusual minima in the ultrasonic transfer functions, which are linked to the material shear modulus. It means that for an isotropic material, both Young and shear moduli could be measured in normal incidence, using finite-sized beams. Work in progress tends to assess more than 2 elastic moduli in order to detect abnormal anisotropy in case of weak interfaces, like previously under the plane-wave assumption. The interest of finite-sized beams will be the mapping of material properties, first for validation cases (single material plates) and then for adhesive bonds between substrates. [1] E. Syriabe, M. Rénier, A. Meziane, J. Galy, M. Castaings, Ultrasonic 79, p.34-51, 2017.