2026-07-07 · EN
Ultrasonic testing (UT) fundamentals: principle, equipment and application
Ultrasonic testing (UT) is a nondestructive examination method that uses high-frequency sound waves to find discontinuities inside a material and to measure remaining pipe or shell wall thickness. It applies across a wide range of work — weld seams, pressure vessel shells, forgings, pipelines — and it is one of the primary tools both for detecting subsurface discontinuities and for tracking corrosion.
The strength of the method is that it "looks" deep into the material from a single accessible surface and reports the position of a discontinuity as a number. That strength only pays off with the right equipment choice, careful calibration and a competent operator. The sections below cover the method at the level of physical principle, equipment, wave modes, calibration, scanning and evaluation.
1. How it works
UT rests on sound propagating through a medium and reflecting from interfaces. A piezoelectric crystal (the probe) is excited electrically, vibrates, and sends a short sound pulse into the material. When that pulse meets a different medium — a crack, a void, or the back wall — part of it reflects and the same probe detects the returning echo.
What the instrument actually measures is elapsed time. Sound velocity in a given material is practically constant, so the travel time of the echo converts directly into distance, that is, depth. This gives both the position of the discontinuity and the wall thickness of the component. The arrangement is called pulse-echo, and it is the basis of single-probe work. Where the transmitter and the receiver are separate units, the setup is called through-transmission or pitch-catch.
2. Sound physics: velocity, frequency and resolution
Three quantities drive every ultrasonic examination:
- Sound velocity: Every material has its own velocity, and it stays practically constant within that material (in steel, roughly 5,900 m/s longitudinal and 3,230 m/s shear). Physically it depends on the elastic modulus and the density. Thickness and depth calculations rest on this velocity — enter the wrong value and every reading shifts.
- Frequency: Typically 1–10 MHz. Higher frequency gives better resolution, meaning a better ability to separate small discontinuities, but penetration drops and scattering increases in coarse-grained material. As sound travels it weakens through absorption and scattering at grain boundaries (attenuation), and this loss grows with frequency.
- Wavelength: Sound velocity divided by frequency. As a rule of thumb, expect reliable detection only when the discontinuity is larger than half a wavelength.
The sound beam also has a complex near field immediately in front of the probe and a spreading far field beyond it. Evaluate discontinuities in the far field where possible; amplitude readings taken in the near field can mislead. The near field length grows with the square of the probe diameter and shrinks with wavelength (N ≈ D²/4λ), so probe diameter and frequency together set where the usable evaluation zone begins.
3. Core components and equipment
- Probe (transducer): The piezoelectric element that generates and receives the sound wave.
- Instrument: The unit that fires the pulse, processes the echo and displays it on screen; it carries the gain (dB), range and gate settings.
- Couplant: Gel, oil or water that displaces the air between probe and surface so the sound can enter the material. The energy reflected at an interface depends on the difference in acoustic impedance (density × sound velocity) between the two media; the impedance mismatch between air and steel is so large that without couplant almost all the sound reflects at the surface and no examination is possible.
- Calibration blocks: Blocks of known geometry used to set instrument range, sensitivity and probe angle.
- Reference blocks: Blocks carrying reference reflectors, used to set the reference level the indications are then compared against.
4. Probe types
Different probe designs suit different jobs:
| Probe type | Use |
|---|---|
| Single element (0°) | Thickness measurement, straight beam examination |
| Dual element (TR, transmit–receive) | Thin wall, near-surface discontinuities, hot surfaces |
| Angle beam | Weld examination (45°, 60°, 70°) |
| Delay line | Very thin parts, near-surface resolution |
| Immersion | Automated or shop examination, scanning in water |
5. Wave modes
Sound travels through a material in several modes. Choose the mode to suit the examination:
- Longitudinal wave: Particle motion is along the direction of propagation; used for thickness measurement and straight beam examination (roughly 5,900 m/s in steel).
- Shear (transverse) wave: Particle motion is perpendicular to propagation. It is slower — roughly 3,230 m/s in steel, about half the longitudinal velocity — and it is what angle beam probes use for weld examination. Depth and distance calculations in angle beam work rest on this velocity.
- Surface (Rayleigh) wave: Used to scan for surface and surface-breaking discontinuities, with a penetration depth of roughly one wavelength.
In an angle beam probe, a wedge refracts the longitudinal wave according to Snell's law. Raise the wedge angle above the first critical angle and the longitudinal wave disappears entirely, leaving only the shear wave in the material. Standard angle beam probes (45°, 60° and 70°) work on exactly this principle to produce pure shear waves, and that is the foundation of weld examination.
6. Calibration
Reliable results depend on correct calibration. Before the examination, set the instrument against known references for sound velocity, range and sensitivity. Calibration blocks set the instrument against a known geometry:
| Calibration block | Purpose |
|---|---|
| V1 (IIW) block | Angle, beam exit point and range (100 mm radius reference) |
| V2 (mini IIW) block | Quick angle and range setup in the field (25 mm and 50 mm radii; shear wave verification) |
| Step block | Linearity check for thickness measurement |
The following are not blocks but sizing and evaluation techniques:
- DAC (distance amplitude correction): A curve that corrects for the amplitude difference an equally sized discontinuity produces at different depths.
- TCG (time-corrected gain): Applies the same correction electronically through gain.
- DGS/AVG: Reports an equivalent reflector size — a flat-bottom hole — against a reference reflector.
Wherever possible, the calibration block should have the same material and sound velocity as the part under examination; otherwise depth and sizing readings come out wrong. Verify the calibration periodically during the shift, because temperature changes and probe wear both cause drift.
7. Scanning techniques and displays
Conventional UT presents the result as an A-scan: a plot with distance or time on the horizontal axis and echo amplitude on the vertical axis. Advanced systems add further displays:
- A-scan: Amplitude-versus-time plot from a single position.
- B-scan: Cross-sectional view (depth versus position).
- C-scan: Plan view, a map of the scanned area.
Scanning arrangements include pulse-echo with a single probe, pitch-catch with separate transmitter and receiver, and the tandem arrangement for thick section welds. In weld examination, move the probe systematically along the seam and perpendicular to it in a raster pattern.
8. Pipe and shell wall thickness measurement
One of the most common uses of UT is tracking wall loss from corrosion. Take readings at fixed, repeatable points — condition monitoring locations (CMLs). A sound thickness program does three things:
- It repeats the reading year after year at the same point, with the same probe and the same couplant.
- It calculates long-term and short-term corrosion rates.
- It derives remaining life and the next examination interval from those rates.
Recording the measurement locations is worth far more than any single report, because the real decision comes out of the trend over time.
9. Weld examination
Weld seams are usually scanned with an angle beam probe using shear waves, for two reasons. First, the imperfections that matter most — planar ones such as lack of fusion (LOF, incomplete fusion in ASME/AWS wording), lack of penetration (LOP, incomplete joint penetration) and cracks — tend to lie vertically or close to it. An angled shear wave strikes such a plane perpendicular and returns a strong echo. Second, the weld cap blocks straight beam 0° access from above. For planar imperfections such as LOF and cracks, UT outperforms radiography (RT); for volumetric ones such as porosity and slag, the two methods complement each other.
Before the examination, check the base material for laminations, then scan the seam at several angles to establish the position and size of any discontinuity. The heat-affected zone (HAZ) is one of the main targets of the examination, particularly for toe cracks.
10. Evaluation and acceptance
Evaluate every echo against a reference level before you accept or reject it:
- 6 dB drop technique: The points where echo amplitude falls by half mark the edges of the discontinuity; used for sizing.
- DAC/TCG: Corrects the amplitude difference an equally sized discontinuity produces at different depths, which makes the decision objective.
An indication only becomes a defect once it exceeds those limits; until then it stays an indication. ASME Section V itself sets no acceptance limits. They live in the construction or in-service code the examination is performed to — for example ASME Section VIII for pressure vessels, or ASME B31.1 for power piping.
11. Common mistakes
- Poor surface preparation: Rust, loose paint or roughness ruin couplant performance.
- Wrong sound velocity entry: Shifts every depth and thickness reading.
- Calibration never verified: Temperature and probe wear cause drift.
- Evaluating in the near field: Amplitude readings mislead.
- Non-repeatable measurement points: Invalidates the corrosion trend.
12. Personnel and competence
UT results depend heavily on operator interpretation, which makes personnel competence an inseparable part of the method. Certification typically follows ISO 9712 (Level I/II/III) or an employer-based scheme written to ASNT SNT-TC-1A:
- Level I: Sets up the equipment and takes readings; does not evaluate independently.
- Level II: Performs the examination to the procedure, evaluates accept/reject and reports.
- Level III: Writes procedures, selects methods, trains and examines personnel.
On critical equipment, use personnel certified at the appropriate level and experienced in that product group. The same indication reads as "noise" to an inexperienced eye and as "a growing crack" to an experienced one.
13. Related standards
- ASME BPVC Section V, Article 4 / Article 5: Ultrasonic examination of welds and of materials (Article 4 for welds, Article 5 for materials and product forms).
- ISO 16810: Ultrasonic testing — general principles.
- ISO 17640: Ultrasonic testing techniques for welded joints.
- ISO 11666: Acceptance levels for ultrasonic testing of welds.
- In-service examination (thickness and corrosion): API 510 for pressure vessels, API 570 for process piping, API 653 for storage tanks.
From the field
The step people skip most often is surface preparation and coupling. On a rough surface with air trapped under the probe, even the most expensive instrument will walk straight past a real crack. A reliable ultrasonic examination starts before the instrument does: a clean surface, the correct sound velocity, and a calibration you have verified yourself. For corrosion monitoring, everything rests on the repeatability of the measurement points. The value of this method sits in the examination plan and the operator, not in the equipment.
Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.
At DoaWise we carry out non-destructive testing services — ultrasonic testing (UT/PAUT) above all — to international standards, producing recordable, auditable results. Through weld examination, corrosion mapping and remaining life studies, we support operators in making safe, well-planned maintenance decisions.