Nonlinear vibro-acoustic intermodulation to detect weak adhesive bonds

This paper presents the results of the application of nonlinear vibro-ultrasonic frequency mixing to detect bondline damage and weak bonds in adhesive joints. The samples investigated were circular aluminum disks bonded to one another. Some samples contained controlled bondline damage introduced by applying a consistent amount of Frekote mold release agent to the surfaces during manufacture. Samples were analysed using nonlinear vibro-acoustic methods. They were placed between a set of 100kHz NDT transducers, which generate a steady-state high frequency probing wave. A low frequency (pumping) excitation was generated by affixing a small loudspeaker to the upper transducer. A custom built 24-bit, 2.5 MSPS measurement system along with a LabVIEW interface was used to record and calculate sideband amplitudes. Measurements were taken at various excitation amplitudes. Some of the disk samples were also mechanically tested in a single lap shear configuration to determine their adhesive bond strength. Two batches of disk samples were manufactured and tested. The first batch showed promising results with regards to correlating nonlinear mixing behavior and mechanical strength. The results for the second batch were inconclusive. No clear, quantitative correlation could be established between the ultrasonic non-linearity coefficient and adhesive bond strength. Thus, the paper concludes with recommendations for further research to develop a reliable testing methodology for weak adhesive bonds.