Detection of defects in adhesive bonds under static and dynamic loading using ultrasonic testing methods

Defects such as voids, cracks, delaminations, and so-called kissing bonds may influence the mechanical load capacity of adhesive joints. The purpose of this paper is two-fold. On the one hand it deals with the ultrasonic detectability of defects in bonds of epoxy (EP) and polyurethane adhesives (PUR) which had typical thicknesses of 0.2 and 4 mm, respectively. The experiments were carried out on bonded tubes and shear test samples with artificially introduced defects of up to 2 mm diameter in the case for PUR. The diameter of tubes, their wall thickness and their length were 60×5×120 mm3. On the other hand the behaviour of the bonds with the defects and their influence on the load capacity was investigated under static and dynamic load in a tensile-torsion test machine using standardized testing procedures. Ultrasonic A-, B- and C-scans were made using compressional and shear waves, both under perpendicular and oblique incidence, and by employing surface waves. All artificial defects including the kissing bonds were detected in addition to natural defects present. Furthermore ultrasonic measurements were carried out during mechanical loading in the test machine in order to study the behaviour of the defects. The measurements showed that changes in the sound velocity and ultrasonic transmission through the bonds become noticeable during loading, e.g. softening and hardening, as well as defect formation and growth of delaminations. During loading considerable generation of higher harmonics could be observed. In particular kissing bonds turned out to be a local source of the higher harmonics. The ultrasonic data were compared to the results of the destructive tests. The experiments showed that small adhesive and cohesive defects have little influence on the breakage strength in the quasi-static loading test but reduce especially the lifetime of the bonds under dynamic loading.