Scientific report 1992-1994

Chapter 9:
Non-destructive testing and quality control

Contents

NDT-specialists are continuously challenged to develop new techniques for non-destructive testing and to offer new methods for data handling, in order to increase the reliability of structural materials, such as composites and coated materials.

The acousto-optic technique, has experimentally been elaborated in the Department MTM. The theoretical modelling was supervised by prof. O. Leroy of the Interdisciplinary Research Centre of the K.U.Leuven Campus Kortrijk. By this technique the modulation of ultrasound, reflected by a layered material, is optically measured in amplitude and phase, using two low power He-Ne laser beams crossing the ultrasonic beams (incident and reflected) of low frequency. The technique is based on the diffraction of light by ultrasound. The diffraction area can be separated into two regions, the near field and the far field. The variations in light intensity in the near field are proportional to the density variations in ultrasound.

The following characteristics of electrolytically deposited copper on stainless steel were investigated: thickness and thickness variations within the layer, bonding quality and the presence of defects (porosity, artificial). Only when the ultrasound is incident in the Rayleigh angle of the substrate material, the phase changes obtain important information on the quality of the layered structure. The experimental results confirm the theoretical predictions [IJ94 WEV2], [PR94 DEV], [PR94 WEV2].

Improvements of the technique are now focused on and include measurements in the near field with only one laser and measurements in the far field based on the visualisation of ultrasound.

High frequency ultrasound from a 50 MHz focused PVDF foil transducer is used on thin laminated composites and electronic packaging devices in order to increase the axial and lateral resolution to find defects using the ultrasonic C-scan technique. Typical defects are porosities, disbonds and delaminations [PR93 WEV], [PR93 WEV2]. A computer program was developed to process the ultrasonic C-scan reflections to end up with quasi-three-dimensional plots of the damage or defects. With the exact knowledge of the extent and depth of the defects, further modelling of their influence on properties and performance is possible.

For equal layer thicknesses within a laminated composite the high-frequency US-signal in the time-domain is difficult to interpret, because the echoes of defected areas cause multiple reflections at different interfaces, and also due to the ringing of the transducer. Research on carbon fibre reinforced plastic composite plates with artificial defects at certain depths, indicated the Cepstrum analysis is helpful to interpret the signals. The Cepstrum analysis is a mathematical technique which can be used to remove the "ghost" echoes from US-signals in the time domain. Therefore the peaks resulting from defects are much sharper, which leads to an enhanced depth resolution [PR94 DES].