The ultrasonic inspection method uses a transducer to generate a short pulse of ultrasound (typically between ~2MHz and ~10Mhz) into a component or structure, then analyses the reflected sound pulses. Conventionally, the ultrasonic transducer is scanned directly on the surface of the test part, by an operator.

Immersion ultrasonics is a variation on the method whereby the test part is submerged in a water filled tank. The transducer is mounted in a multi-axis manipulator, a few cm distant from the component. Ultrasound then travels through water before entering the component.

The NDT Validation Centre uses two immersion systems. In both systems the ultrasonic probe is manipulated by a motion controller. Immersion testing offers the following advantages compared with the contact method: -

• Water coupling eliminates the variability associated with contact coupling.
• High frequencies, up to 100Mhz can be used. Higher frequencies give reduced ultrasonic beam width and much higher resolution. Flaws as small as 0.2mm are readily resolved.
• Focused beams can be generated either by using a phased array or with concave contoured single element transducers.
• Inspection of complex geometries.

The system shown in Fig.1 has very high frequency capability (100MHz), enabling extremely fine focusing. The pulser/receiver operates under the control of Utex Winspect software. This collects the entire waveform and enables the data to be presented in a variety of views (plan, cross-section, etc.). The probe manipulator has 4 degrees of freedom.

Figure 2 shows the output for an aircraft wing component made from a carbon fibre reinforced plastic tested at the onset of impact damage (A-scan on the right and C-scan on the left).

Figure 3 shows the Ultrasonic Sciences immersion system. This has a number of alternative manipulation devices.

• A 750mm diameter turntable enabling discs to be inspected.
• A billet table allowing the rotation of tubular rolls.
• A 5-axis manipulator for manoeuvring the probe head.

It is being used here to inspect a cylindrical metal billet to detect embedded inclusions. The close-up shows the transducer mounted in the 5-axis manipulator. The tank can accommodate components that are up to 2.5m x 1.5m.

Immersion testing offers many potential benefits over its 'dry' alternative:

• Inspection over uneven surfaces
• Ultra-high resolution C-scan mapping
• Fast large area mapping area
• Inspection of complex geometry

The system is equipped with a dedicated ultrasonic data analysis system, together with integral scanner controllers.