Evaluation of metallic bonded plates with nonlinear ultrasound and comparison with destructive testing

Abstract

In the last decades, the use of structural adhesives has increased. Indeed, they are able to replace advantageously traditional assembly techniques such as riveting or bolting, and generally tend to reduce the global weight of the structure and/or improve the strength of the structure thanks to more homogeneous stress distribution. In addition, it allows the assembly of composite materials or materials having different nature, which is of great interest in the aeronautic industry. However, to be used for structural joining and critical application, reliable non-destructive testing techniques are compulsory for evident safety reasons, be it after fabrication or during the whole life of the structure. While linear ultrasounds have demonstrated their capability to detect easily decohesion or voids in a structure, their use for bond strength inspection is less straightforward. Another approach relies on the analysis of nonlinear signature of a bond defect inspected by high amplitude ultrasound. In the present paper, a Chaotic Cavity Transducer is used to inspect various metallic bonded plates (titanium or aluminum) with several bond defects. The defects were introduced by depositing localized surface pollution (introduction of PTFE spray, release agent, or fingerprints) before the adhesive is deposited on top of the surface. Combined with the pulse inversion technique, high amplitude plane waves were sent in the structure, leading to harmonic components observed in the defect region. Mechanical destructive tests were performed and display a good agreement with nondestructive tests.