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

Abstract

International audienceIn the last decades, the use of structural adhesion has increased. It is indeed able to replace traditional bonded techniques such as rivets or bolts, and reduces the global weight of the structure, which is of great interest in the aeronautic industry. In addition, it allows the assembly of mixed or composite materials with better stress repartition. 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 ultrasound is known to detect easily decohesion or voids in a structure, its use for bond strength inspection is less straightforward. Constraining hypothesis have to be made to retrieve the bond strength [1]. However, in some cases, the effect of such bond degradation on ultrasound is the same as the effect of geometrical fluctuations of the structure (eg thickness of various layers)[2]. Another approach relies on the nonlinear signature of a bond defect inspected by high amplitude ultrasound [3], generated with a chaotic cavity transducer [4]. In the present paper, this method is applied to the inspection of various metallic bonded plates (titanium or aluminum) with several bond defects. The defects were introduced by degradation of the surface preparation process (introduction of PTFE spray, or finger prints). 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