14.2 Ultrasonic Testing (UT)

14.2 Ultrasonic Testing (UT)

Quick Answer: Ultrasonic testing uses high-frequency sound pulses (typically MHz range) introduced through a couplant. In pulse-echo mode, echoes from the back wall and from discontinuities give location (time of flight) and relative amplitude. Angle-beam shear waves are workhorses for weld planar flaws; normal-beam longitudinal waves support thickness and lamination checks. IWI-S reviews reports and technique adequacy; Level 2 UT performance belongs to ISO 9712-certified NDT personnel. TOFD and PAUT are advanced methods you should recognise at Standard level, with deeper management at Comprehensive.

Radiography images attenuation through the volume. Ultrasonics interrogates the volume with mechanical waves. Together they form the core of WI2.7 volumetric NDT knowledge for welding inspectors.

Pulse-Echo Basics

A piezoelectric probe (transducer) converts electrical pulses to sound and vice versa. In pulse-echo:

1. A short pulse enters the material
2. Part of the energy reflects from boundaries and discontinuities
3. The instrument displays A-scan (amplitude vs time) and, in more advanced systems, B-/C-/S-scans
4. Time of flight × velocity → depth / path length
5. Amplitude relates to reflector size, orientation, and roughness — but only when calibrated and interpreted with training

Key inspector concepts:

• Velocity depends on material and wave mode (longitudinal vs shear)
• Attenuation increases with path length, grain size, and frequency
• A large amplitude is not automatically a rejectable crack — sizing and characterisation follow procedure, reference blocks, and acceptance criteria

Longitudinal and Shear Waves

Longitudinal (compression) waves — particle motion parallel to propagation. Higher velocity. Used for:

• Thickness measurement
• Lamination / mid-wall checks with normal-beam probes
• Some weld root or special techniques as procedure allows

Shear (transverse) waves — particle motion perpendicular to propagation. Generated in the test piece commonly by angle probes (plastic wedge + mode conversion). Used for:

• Weld inspection of bevel fusion faces, cracks, and many planar reflectors
• Typical refracted angles in steel such as 45°, 60°, 70° (procedure-specific)

Skip-path thinking: the beam bounces between surfaces, so a reflector may be seen on a half-skip or full-skip path. Inspectors reading reports should understand leg and skip language so they can ask coherent questions — without claiming they performed the scan.

Couplant and Surface Conditions

Air gaps kill coupling. Couplant (gel, oil, paste, water for immersion/automated systems) fills the probe–surface interface. Inspector-relevant points:

• Surface must be sufficiently smooth and clean (scale, heavy spatter, and loose coating impair testing)
• Temperature extremes change couplant behaviour and velocity assumptions
• After testing, couplant residue may need cleaning for painting or hygiene-critical service
• Dry coupling or specialised wheels exist for some automated systems but still require procedure control

If the surface preparation specified in the UT procedure was not achieved, the test may be invalid even if an operator “got some echoes.”

Calibration Awareness

Calibration is not optional bureaucracy. Typical elements (standards and procedures vary — IIW/V1/V2 blocks, DAC/TCG curves, transfer corrections):

• Range / time base so depth readings are correct
• Angle verification and probe index point
• Sensitivity setting using side-drilled holes, notches, or other reflectors defining a DAC (distance–amplitude correction) or equivalent
• Transfer correction when the calibration block surface/material differs from the job
• Periodic checks during the shift when procedure requires

IWI-S review questions on a UT package:

• Procedure ID and revision applicable to thickness, joint type, and material
• Equipment identity and calibration status
• Reference block identity and sensitivity method (DAC, DGS, etc.)
• Scanning pattern (both sides, both directions as required), coverage, and scanning sensitivity
• Operator certification level and validity
• Indication table: location (along weld / depth / leg), amplitude vs DAC, length, characterisation if given
• Clear accept/reject against the contract acceptance standard, not informal “looks small”

Strengths Relative to RT

UT strengths

• Often superior for planar defects oriented as reflectors to the beam (lack of fusion, many cracks)
• Provides depth and position information useful for repair excavation
• No ionising radiation; often faster access for thick sections when automated
• Thickness monitoring and corrosion mapping variants in service inspection

UT limitations

• Requires competent operators and good access for probe manipulation
• Coarse grain materials (some stainless, castings) and austenitic welds can be difficult — specialised techniques
• Geometry: complex nodes, restricted access, rough as-welded caps may need grinding for scanning
• Permanent image record is not automatic on basic A-scan (though modern systems store data; TOFD/PAUT produce rich records)
• Small volumetric porosity fields that RT shows clearly may be harder to characterise on conventional UT

Complementary use: many specifications require RT or UT (or both) by joint class. Method selection is not a personality preference — it follows code, risk, geometry, and expected discontinuity type (see 14.3).

Advanced Techniques — TOFD and PAUT (IWI-C Depth Note)

At IWI-S you must recognise advanced volumetric UT families:

• TOFD (Time-of-Flight Diffraction) — uses diffracted signals from defect tips; strong for sizing and detection of many cracks with good imaging of through-wall extent when set up correctly
• PAUT (Phased Array UT) — multi-element probes steer/focus beams electronically; sectorial scans and encoded data improve coverage and recording

IAB-041 places advanced NDT and non-conventional applications more clearly in IWI-C management competence. Standard-level inspectors still see TOFD/PAUT on modern projects: verify that procedure, personnel certification scope, and reporting match the ITP. Do not invent a PAUT interpretation if you are not certified for it.

Inspector Review vs Performing Level 2 UT

Exam trap: “IWI-S certification automatically includes UT Level 2 scanning rights.” False. Welding inspector diplomas and NDT personnel certification are separate. Dual competence requires both routes of training and assessment.

Practical Scenario

A pressure piping ITP calls for 100% UT on critical butt welds. The report shows DAC evaluation, both-side scanning, and three indications above the recording level. As IWI-S you:

1. Confirm operator certification and procedure applicability
2. Confirm calibration/block references and coverage statements
3. Check that indication coordinates map to the physical weld ID
4. Apply the acceptance criteria in the contract (e.g. length/amplitude rules)
5. Open NCR/repair process if rejectable; require retest after repair
6. You do not re-scan “by ear” with a borrowed probe unless you hold the NDT qualification and are authorised

Link Forward

Section 14.3 unifies surface vs volumetric selection, IQI concepts in context, other tests (pressure, dimensional), and ISO 9712 levels 1/2/3 so you can defend method choices and personnel requirements on the oral and written exams.