What Is Ultrasonic Testing (UT)?

Quick Answer

Ultrasonic testing (UT) uses high-frequency sound waves (0.5-25 MHz) to detect internal defects in materials and welds. A transducer sends a pulse into the test piece; reflections from internal flaws or back walls return to the transducer and display as signals on a screen. UT measures both flaw location and remaining wall thickness, making it necessary for piping fabrication, pipeline inspection, and in-service integrity assessment.

How UT Works

A piezoelectric transducer converts electrical energy into ultrasonic pulses. These travel through the material at a velocity dependent on the medium (5,920 m/s in carbon steel). When the pulse encounters a discontinuity—crack, inclusion, porosity, or the opposite wall—part of the energy reflects back. The instrument displays the reflected signal, and the technician evaluates the amplitude and time-of-flight to determine flaw size, depth, and type.

UT Techniques

Technique	Probe Type	Application
Straight beam (0-degree)	Normal probe	Wall thickness, laminations, inclusions in plate/pipe
Angle beam	Angle probe (45, 60, 70 degrees)	Weld inspection (detects lack of fusion, cracks, porosity)
Immersion UT	Focused probe in water bath	Mill inspection of seamless/welded pipe, tube stock
Phased array UT (PAUT)	Multi-element probe	Advanced weld inspection with sector/linear scans
TOFD	Pair of angled probes	Crack sizing, weld root inspection
Guided wave	Ring of transducers	Long-range pipeline screening (corrosion under insulation)

UT vs RT Comparison

Parameter	UT	RT
Defect type	Excellent for planar (cracks, LOF)	Excellent for volumetric (porosity, inclusions)
Depth measurement	Yes (accurate flaw depth/sizing)	No (2D projection only)
Wall thickness	Measures directly	Not applicable
Radiation hazard	None	Requires exclusion zones, licensed operators
Surface access	One side sufficient	Two-side access preferred
Permanent record	Encoded scans (PAUT/TOFD) or manual logging	Film or digital image
Thick sections	Effective to >300 mm	Limited by source energy
Governing standard	ASME V Article 4/5	ASME V Article 2

Key Standards

Standard	Scope
ASME Section V, Article 4	UT examination procedures for welds
ASME Section V, Article 5	UT examination of materials (straight beam)
ASTM A388	UT of steel forgings
ASTM A435/A578	UT of steel plates (straight beam)
ASTM E213	UT of metal pipe and tubing
API 1104	Pipeline weld UT (alternative to RT)
ASME Code Case 2235	Allows UT in lieu of RT for pressure vessel welds

Acceptance Criteria

ASME B31.3 and Section VIII reference acceptance criteria from ASME V and the applicable construction code. API 1104 Section 9 provides UT acceptance limits for pipeline girth welds based on flaw height, length, and interaction rules.

For manual UT, evaluation is based on amplitude (signal height) compared to a reference reflector (side-drilled hole, notch, or flat-bottom hole). PAUT and TOFD provide encoded data with measurable flaw dimensions, enabling engineering critical assessment (ECA) per BS 7910 or API 579-1/ASME FFS-1.

Pro Tip

ASME Code Case 2235 permits UT as an alternative to RT for pressure vessel and piping welds, provided the UT procedure meets enhanced requirements (angle beam + straight beam, calibration per Article 4 Mandatory Appendix). This eliminates radiation hazards and speeds up shop and field inspection cycles significantly.

UT results are recorded in the weld data package alongside mill test certificates and hydrostatic test records.

Read the full guide to non-destructive testing