Contact, Immersion and Special Pulse-Echo Techniques

Learning outcomes

By the end of this chapter, the learner should be able to compare contact, immersion, through-transmission and pitch-catch arrangements; design immersion water paths and scanning controls; explain multiple-backwall, tandem and delta applications; recognise the boundaries of air-coupled, EMAT and laser methods; and select specialised techniques by inspection purpose.

8.1 Contact and immersion systems

Contact testing places the probe or wedge against the component through a thin couplant layer. It is portable and adaptable but sensitive to surface condition, pressure and coupling consistency.

Immersion testing separates the probe from the component with a controlled liquid path. It supports repeatable coupling, focused probes, high-speed mechanised scanning and complex beam angles by manipulating probe orientation. The system must control water temperature, path length, alignment, bubbles, surface condition and motion accuracy.

The water path should separate the entry response and required component echoes in time and avoid multiple-water-path echoes overlapping the inspection region. Calculation must use the liquid and component velocities, and the proposed path must be verified on the actual setup.

8.2 Transmission arrangements

Pulse-echo uses the same probe or co-located aperture to transmit and receive. Through-transmission places transmitter and receiver on opposite sides and detects reduced transmitted energy. It can be effective for laminations, disbonds and highly attenuating products, but it normally does not provide direct depth information and needs two-sided access.

Pitch-catch uses separated transmitter and receiver positions, often on the same surface. Tandem weld techniques arrange probes so energy reflected from a near-vertical fusion face reaches the receiver. Delta arrangements use mode conversion or scattered energy to improve response from certain vertical reflectors and can support depth assessment when geometry is controlled.

8.3 Multiple-backwall technique

Successive backwall echoes can reveal thickness, attenuation, bond quality or local loss. Their amplitude decay depends on material attenuation, interface reflection, beam spread and coupling. A missing multiple may result from roughness, curvature, poor alignment or a discontinuity. The technique requires a controlled baseline rather than a simplistic echo-count rule.

8.4 Non-contact and specialised generation

Air-coupled ultrasound avoids liquid couplant but faces a severe air-solid impedance mismatch, so specialised low-frequency transducers, matching and signal processing are used. Electromagnetic acoustic transducers generate ultrasound in conductive materials through electromagnetic mechanisms; they can operate without liquid couplant but depend on material conductivity, magnetic properties, lift-off and mode configuration.

Laser ultrasonics uses optical energy to generate and detect elastic waves remotely. It may suit hot, moving or inaccessible products, but system complexity, surface response, optical safety and cost are significant. These methods are not drop-in replacements for contact probes and require specific competence and validation.

8.5 Technique selection example

A flat composite panel requires rapid disbond mapping. Immersion pulse-echo or through-transmission may provide stable area coverage. If only one side is accessible, through-transmission is excluded. If water exposure is prohibited, air-coupled or a validated dry-contact array may be considered. The final choice follows product constraints and demonstrated performance, not the method with the most dramatic marketing animation.

Failure modes and decision control

Weak practice commonly includes choosing an immersion path without checking echo overlap; ignoring water-temperature velocity change; using through-transmission when depth is a mandatory output; applying an echo-count criterion without a baseline; assuming non-contact means calibration-free; and using specialised methods without competent technical support.