Ultrasonic Weld Testing (UT): How Sound Waves Find Defects

Q: "What defects can ultrasonic testing find in welds?"

"UT detects internal defects including cracks, lack of fusion, incomplete penetration, slag inclusions, and porosity. It's particularly good at finding planar defects (cracks and lack of fusion) because these reflect sound waves strongly. Volumetric defects like scattered porosity are harder to detect with UT but show up well on radiographic testing."

Q: "What's the advantage of UT over radiographic testing?"

"UT is safer (no radiation, no evacuation zone), faster (real-time results vs. film processing), and better at detecting planar defects like cracks and lack of fusion. UT can also size defects more accurately and determine their depth in the weld. RT's main advantage is producing a permanent image that anyone can review. UT results depend on the operator's skill and judgment."

Q: "Do I need a special certification for ultrasonic testing?"

"Yes. UT technicians must be certified to ASNT SNT-TC-1A Level II or III (in the US) or equivalent international standards. Level II certification requires classroom training, practical hours, and passing written and practical exams. UT is the most operator-dependent NDT method, so qualification and experience directly affect reliability."

Published August 15, 2025 · Updated March 17, 2026 · 8 min read

Ultrasonic testing (UT) sends high-frequency sound waves through a weld and reads the reflections to find internal defects. When the sound wave hits a crack, lack of fusion, slag inclusion, or void, part of the energy bounces back to the transducer. The timing and amplitude of that echo tell the technician where the defect is, how big it is, and what it likely is.

UT is replacing radiographic testing on many jobs because it’s faster, safer (no radiation), and better at finding the planar defects (cracks, lack of fusion) that cause structural failures. AWS D1.1 Section 6, Part F covers UT requirements for structural steel weld inspection.

How Ultrasonic Testing Works

A transducer (probe) generates sound waves at frequencies between 1-10 MHz, far above human hearing. These waves travel through the weld metal at a known velocity (about 0.230 inches per microsecond in steel). When the wave hits a boundary between different materials (metal-to-air at a defect, for example), it reflects back.

The transducer also acts as a receiver. It picks up the reflected echo and displays it on a screen. The position of the echo on the horizontal axis shows the distance (depth) to the defect. The height of the echo on the vertical axis indicates the relative size of the reflector.

The A-Scan Display

The A-scan is the basic UT display. It shows:

Horizontal axis: Time (which translates to distance/depth in the material)
Vertical axis: Signal amplitude (echo height, which correlates to defect size)

Initial pulse: The first spike on the left represents the transducer sending the sound wave.

Back wall echo: A spike on the right represents the sound wave reflecting off the far surface of the material. If the material is 1 inch thick and the sound velocity is known, this echo appears at the 1-inch mark.

Defect echoes: Any spike between the initial pulse and the back wall echo indicates a reflector (potential defect) inside the material.

UT Probe Types

Straight Beam (0-Degree)

Sends sound straight down into the material. Used for thickness measurement and detecting laminations (delaminations in plate). Not effective for weld inspection because the weld cap reinforcement blocks direct contact.

Angle Beam

The standard probe for weld inspection. A wedge on the transducer introduces the sound at an angle (typically 45, 60, or 70 degrees). The angled beam travels through the base metal, enters the weld from the side, and reflects off any defects in the weld volume.

Beam Angle	Refracted Angle in Steel	Primary Use
45 degrees	45°	Thick sections, first inspection pass, deepest penetration
60 degrees	60°	Most common for weld inspection, good balance of coverage
70 degrees	70°	Thin sections, root area inspection, shallow defects

AWS D1.1 requires inspection from both sides of the weld with at least two angles to achieve full weld volume coverage.

Phased Array UT (PAUT)

Phased array probes contain multiple transducer elements that can be fired in sequence with controlled delays. This allows:

Electronic beam steering (multiple angles from one probe)
Sector scans (sweeping through a range of angles)
Linear scans (faster coverage of long welds)
S-scans (sector displays that show a cross-section image of the weld)

PAUT is faster and produces more detailed images than conventional UT. It’s becoming the standard for pipeline and critical structural work.

Calibration

UT is only as good as the calibration. Before every inspection session, the technician calibrates the instrument using reference blocks with known reflectors.

IIW Block (International Institute of Welding)

A standardized reference block with machined notches, side-drilled holes, and known dimensions. Used to:

Verify beam angle
Set distance calibration (screen range)
Check probe index (exit point)
Verify sensitivity with known reflectors

AWS D1.1 Reference Block

AWS D1.1 requires a specific calibration procedure using a reference block with a 1/16 inch diameter side-drilled hole. The sensitivity is set so the hole produces a reference amplitude on the screen. Indications in the weld that exceed this reference level (after distance-amplitude correction) require evaluation.

Distance-Amplitude Correction (DAC)

Sound waves lose energy as they travel (attenuation). A defect deep in the weld produces a smaller echo than the same-size defect near the surface. DAC curves compensate for this, setting a consistent rejection threshold at all depths.

UT Inspection Procedure for Welds

Step 1: Surface Preparation

The scanning surface (base metal adjacent to the weld) must be clean, smooth, and free of loose scale, paint thicker than about 10 mils, and heavy rust. Grind if needed. The weld cap doesn’t need grinding for angle beam inspection unless the cap is so rough it blocks scanning access.

Step 2: Apply Couplant

A thin layer of couplant (gel, glycerin, or oil) goes between the probe and the scanning surface. The couplant eliminates the air gap that would block sound transmission. Apply enough for consistent contact but not so much that it runs everywhere.

Step 3: Scan the Weld

Move the probe along the base metal beside the weld, sweeping it toward and away from the joint while moving along the weld length. The beam enters the weld from the side and reflects off any internal defects.

Scan pattern:

First pass: scan from one side at 70 degrees to examine the root area
Second pass: scan from the same side at 45 or 60 degrees for the fill and cap area
Third and fourth passes: repeat from the opposite side to ensure full volume coverage

Step 4: Evaluate Indications

When an echo appears on the screen, the technician must:

Locate the reflector: Calculate its position in the weld (depth and lateral position)
Measure the amplitude: Compare to the DAC curve or reference level
Characterize the defect: Sharp, clean echo = planar defect (crack, lack of fusion). Scattered, low-amplitude echoes = volumetric defect (porosity, slag)
Size the defect: Determine length by scanning along the weld until the echo drops below threshold

AWS D1.1 UT Acceptance Criteria

AWS D1.1 uses a classification system based on defect severity:

Class	Description	Acceptance
Class A	Large reflectors: echo amplitude exceeds reference + 6 dB	Rejected (any length)
Class B	Medium reflectors: echo amplitude between reference and reference + 6 dB	Rejected if longer than specified length
Class C	Small reflectors: echo amplitude between reference - 6 dB and reference	Rejected if longer than 2 inches (non-tubular)
Class D	Minor reflectors: echo amplitude below reference - 6 dB	Accepted (below evaluation threshold)

The specific length limits for Class B and C depend on weld thickness and whether the structure is statically or cyclically loaded. AWS D1.1 Table 6.3 provides the details.

UT vs. RT: Choosing the Right Method

Factor	Ultrasonic Testing (UT)	Radiographic Testing (RT)
Planar defect detection (cracks, LOF)	Excellent	Poor to moderate (orientation dependent)
Volumetric defect detection (porosity, slag)	Moderate	Excellent
Permanent record	Limited (conventional); yes (PAUT with encoded scans)	Yes (film or digital image)
Safety	No radiation, no exclusion zone	Radiation, exclusion zone required
Speed	Real-time results	Hours to develop film
Cost per weld	Lower	Higher (film, processing, source rental)
Operator dependence	High (skill-intensive)	Moderate (film interpretation is more standardized)
Thickness range	5/16" minimum (conventional) to unlimited	Any thickness with appropriate source

UT Technician Requirements

UT is the most operator-dependent NDT method. The technician must understand wave physics, material acoustics, weld joint geometry, and defect characterization. A poorly trained UT technician can miss critical defects or call non-existent ones.

ASNT SNT-TC-1A defines three levels of UT qualification:

Level I: Can perform UT under direct supervision, following a written procedure. Cannot evaluate or report results independently.
Level II: Can set up equipment, calibrate, perform examinations, and evaluate results against acceptance criteria. This is the working level for most weld inspection UT.
Level III: Can develop procedures, train Level I and II technicians, and interpret codes and standards. Usually a supervisory or technical role.

Most project specifications require Level II minimum for UT weld inspection. Level III certification requires additional education and experience beyond Level II.

The industry trend is toward UT (especially PAUT) replacing RT. The safety advantages alone (no radiation, no clearing the area) save time and money on active job sites.

Common Mistakes

Poor calibration. If the calibration is off, every indication is evaluated against the wrong reference. Calibrate at the start of each inspection session and re-verify periodically.

Insufficient couplant. Dry spots between the probe and surface create echo loss. This can mask defects. Apply couplant consistently.

Scanning from one side only. Some defects (especially lack of fusion on the far bevel face) are only detectable from the opposite side. AWS D1.1 requires scanning from both sides of the weld.

Misinterpreting geometry echoes. The weld root, cap reinforcement edge, and backing bar all produce echoes that can be mistaken for defects. Know the joint geometry and expect these echoes at known locations.

Skipping the 70-degree probe. The 70-degree angle is critical for root area inspection on thinner materials. Relying only on 45 or 60 degrees can miss root defects.

Not accounting for weld access. Some joints have obstructions (stiffeners, flanges, adjacent welds) that prevent scanning from both sides. If you can only scan from one side, note the limitation in the report. Some codes require additional angles or techniques to compensate for single-side access.

For surface and near-surface inspection methods, see the visual weld inspection guide, dye penetrant testing, and magnetic particle testing guides. Return to weld inspection or the welding techniques pillar for the full topic list.

Frequently Asked Questions

What defects can ultrasonic testing find in welds?

UT detects internal defects including cracks, lack of fusion, incomplete penetration, slag inclusions, and porosity. It's particularly good at finding planar defects (cracks and lack of fusion) because these reflect sound waves strongly. Volumetric defects like scattered porosity are harder to detect with UT but show up well on radiographic testing.

What's the advantage of UT over radiographic testing?

UT is safer (no radiation, no evacuation zone), faster (real-time results vs. film processing), and better at detecting planar defects like cracks and lack of fusion. UT can also size defects more accurately and determine their depth in the weld. RT's main advantage is producing a permanent image that anyone can review. UT results depend on the operator's skill and judgment.

Do I need a special certification for ultrasonic testing?

Yes. UT technicians must be certified to ASNT SNT-TC-1A Level II or III (in the US) or equivalent international standards. Level II certification requires classroom training, practical hours, and passing written and practical exams. UT is the most operator-dependent NDT method, so qualification and experience directly affect reliability.