Лако-красочные материалы — производство

7.6.4.1 Impulse-Induced Resonance

This technique was developed to test assemblies made from sheet metals and nonmetallic backing elements.

Pulses that are created by broadband ultrasonic transducers and long enough to produce stationary waves are transferred to the metal sheet via an appropriate medium. The metal sheet has a thickness resonance spectrum of frequencies within the band of the pulse. The signals detected are not corrected. The first echo observed is the reflected pulse from the surface of the specimen, followed by a second signal produced by the stationary wave in the metal. This signal is partly captured and transferred to a spectroscope that displays the frequencies of the fundamental and harmonic vibrations of the thickness resonance of the half-wave. When a nonmetallic plate is bonded to the metal as a backing element, the resonances are damped to an extent that corresponds to the degree of bonding. Consequently, a high resonance amplitude corresponds to ‘no bonding’, while that of a partly bonded assembly will be average, and a perfect bonding will have a low amplitude.

During the early 1950s Fokker first introduced a bondtester based upon a well — known impulse-induced resonance technique, and indeed the ultrasonic resonance — impedance-based Fokker Bondtester is still used today, primarily for manufacture or maintenance purposes in the aircraft industry. The procedure involves a piezo­electric transducer being excited with a high frequency and then coupled with the sample such that changes in resonance frequency and impedance can be detected, depending on the characteristics of the sample and the physical properties of the adhesive joint. This allows conclusions to be drawn with regards to the quality ofthe joint.

The sensor consists ofa disk-shaped, spring-loaded transducer coupled with to the upper end of the sample by a viscous substance, for example a fatty substance or other highly viscous medium. The transducer must be supported so as to allow axial and radial vibration and to forward the elastic waves at given resonance frequencies. A commercially available Fokker bondtester is shown in Figure 7.18.

The Fokker bondtester is especially suited to the testing of metal-to-metal joints, and sheet-to-honeycomb epoxy resin-bonded metal assemblies to detect delamina­tions in the outer sheets.

Unfortunately, the method is very time-consuming and so is particularly well suited to small sample quantities. Another disadvantage is the extremely high spread of the measured values, so that only large-sized defects such as large-area delamina­tions are detected.