Characterization of adhesion within structural materials by means of guided elastic waves

Assembly methods in industry have improved considerably during last decades and mechanical fastening is increasingly abandoned in favor of structural bonding and/or welding methods. The need for improved nondestructive testing and evaluation (NDT&E) methods becomes therefore important particularly for thermoplastic composites welding. Indeed, the latter welding process generates subtle defects (Kissing Bonds) that can hardly be detected, hence the need to develop specific well-calibrated techniques. In addition to bonding testing which is based on the existing contrast between high and weak bonded areas, it remains necessary to determine the adhesion quality, particularly in the case of multi-layered materials. In this contribution a nondestructive quantification method of the adhesion strength corresponding to welded assemblies is presented. The work is based on the use of an ultrasonic contactless experimental device based on air-coupled ultrasound propagating within welded composites. The work takes advantage of the existing interaction between the generated guided waves and the composite structure along the propagating distance in order to locate defects and evaluate the adhesion of the tested thermoplastic welded assemblies. This rely on the basis of an inverse method based on the dispersion curves of Lamb waves, which revealed to be sensitive to the deterioration in adhesion quality.