T.O. 33B-1-15-765.4.6 MethodsAssociatedWithInstrumentsDedicatedtoBondInspection.Refer back to the bottom half of Table 5-6 for a summary of these methods, which are described in detail below.5.4.6.1 ResonanceMethod.An appropriate ultrasonic contact probe is placed on a test sample with couplant and is driven at its resonancefrequency by an oscillator in the instrument. The detector in the instrument measures the phase and amplitudecomponents of the electrical impedance of the probe, which are affected by changes in the acoustic impedance of thetest part. The acoustic impedance of a part is altered by a lack of bond or a delamination. Bonded laminates act like athin plate, which vibrates and generates a standing wave. Changes in the effective thickness caused by the unbonds ordelaminations will significantly affect the phase and amplitude of the acoustic wave in the part. With the resonancemethod the instrument indicates the probe's impedance with a "flying" spot on an ultrasonic impedance plane display.Amplitude changes in impedance are indicated by the radial distance of the spot from the center of the display (nullreference point), and changes in the phase are indicated by the rotation of the spot around the center null point. Figure5-75A is an example of an ultrasonic impedance plane display which shows the spot positions corresponding todifferent depths of unbonds (delaminations) in the bonded laminate in Figure 5-75B. The laminate is an example of atypical reference standard that would be used for calibration. The positions can be gated, so that a disbond produces analarm, or the display can be monitored to determine between which layers a disbond occurs. The resonance modeworks very well for detecting disbonds at metal/metal, metal/composite and composite/composite interfaces, for findingdelaminations within composite materials, and for detecting skin-to-core disbonds in honeycomb sandwich structures.Figure 5-75. Resonance Method.5.4.6.2 Pitch/CatchImpulseMethod.A pulse of a single ultrasonic frequency is transmitted into the part by one while a second transducer, mounted about3/4 inch away in the same probe assembly, receives the returned signal. Contact with the part is made through 1/4-inchdiameter nylon wear tips on spring-loaded metal rods attached to the respective transducers. The ultrasound travelsthrough the material between the two probe tips. The received signals are displayed in various ways depending on theinstrument: a) amplitude and phase components are displayed on separate meters, b) the resultant signal activates light-emitting-diode (LED) display or c) the phase and amplitude components are combined to position a "flying" spot on animpedance plane display. The first type of display is the old style used on the Sondicator, The box in the middle of thedisplay in Figure 5-76 is the gate that sets off an alarm if the spot lands inside, indicating a disbond.
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