Abstract

Defect-Selective Imaging by Nonlinear Air-Coupled Ultrasound

R.Stoessel, H.Pfaff, N.Krohn, G.Busse (University of Stuttgart, Institute for Polymer Testing and Polymer Science (IKP), Non-Destructive Testing (ZFP), Stuttgart, Germany)

e-mail: stoessel@ikp.uni-stuttgart.de

Improved non-destructive testing techniques are required for new kinds of materials. One of the new inspection methods is non-linear air-coupled ultrasound. Defects in solids cause a non-linear transfer function. Therefore, the injection of a sinusoidal elastic wave results in higher harmonics being generated that can be used to indicate a local defect. By using these harmonics, defect-selective imaging is performed in which only the defect itself appears while intact features are suppressed. In this study, this technique has been applied to non-destructive imaging in which a transducer attached to the sample (or embedded into it, e.g. in shape-adaptive structures) injects ultrasound into the sample to be inspected, while a focused narrowband air-coupled ultrasound receiver scans across the sample to monitor the local vibration. If the transducer is tuned to an integer fraction of the frequency to which the receiver responds, one has a method in which only overtones are used for imaging. In this paper, we have applied this technique to monitor the integrity of shape adaptive structures (e.g. delaminations) and to detect defects (e.g. impact damages) in carbon fibre reinforced polymers (CFRP).

Section : 3