Ultrasonic testing

What ultrasonic testing measures

Ultrasonic testing (UT) uses the timing and amplitude of sound echoes to locate reflectors or measure thickness. Match the technique to the required coverage. ASNT, ultrasonic testing principles

In the normal-incidence pulse-echo example, corrected round-trip time is 4.00 µs. At 5.90 mm/µs the thickness is 11.80 mm. At 5.60 mm/µs it is 11.20 mm. The example assumes a homogeneous material and a valid backwall echo. The display cannot correct an incorrect material velocity.

A numeric thickness reading is not a comprehensive flaw examination. Conversely, a recorded reflector does not independently establish a crack's type, dimensions or acceptability. Define the material, component geometry, likely discontinuity locations/orientations, accessible surfaces, required coverage and governing acceptance rule before choosing equipment.

These arrangements have different access and interpretation requirements. Conventional contact UT generally needs a coupling medium, while specialized air-coupled and electromagnetic acoustic techniques use different arrangements. ASNT, modes and techniques

Time of flight and a bounded calculation

For a simple normal-incidence pulse-echo measurement:

thickness d = V × t / 2,

where V is material sound velocity and t is the corrected round-trip transit time. Division by two accounts for outward and return travel. Actual gauges also account for fixed electronic and probe delays through zero calibration. Evident, theory of ultrasonic thickness gauging

Original arithmetic example: assume a homogeneous material, a valid normal-incidence backwall echo, corrected time 4.00 µs, and an assumed calibrated velocity 5.90 mm/µs. Then:

d = 5.90 × 4.00 / 2 = 11.80 mm.

If the actual velocity were 5.60 mm/µs, the corresponding thickness would instead be 11.20 mm. These assumed velocities are not specifications for a named alloy. The example shows why an accurate timer and a finely resolved display cannot compensate for an incorrect material velocity.

For angle-beam inspection, the time-derived distance is a sound path, not automatically vertical depth. Wedge delay, refraction, actual beam angle, index position and any skip path must be accounted for in the selected geometry. Evident's tutorial treats angle-beam setup and reflector location separately from straight-beam thickness measurement. Evident, ultrasonic flaw detection tutorial

Probe selection, material and surface

Probe frequency, size and bandwidth involve tradeoffs. Evident explains that higher frequencies improve response to small reflectors through shorter wavelength, while lower frequencies usually penetrate farther because attenuation increases with frequency. Broadband probes improve near-surface resolution but can sacrifice penetration; probe diameter affects coverage and access. Select and demonstrate a suitable combination rather than assigning a universal minimum detectable flaw to a nominal frequency. Evident tutorial, section 4.1

Grain scattering, absorption, geometry and discontinuity orientation can limit performance. High gain does not necessarily recover useful information from material noise. A reflector that returns little energy toward the probe can be difficult to detect even when physically large. Address the anticipated flaw orientations through the scan plan and suitable beam directions; a clean-looking screen alone does not prove full coverage.

Conventional contact testing requires sound transmission across the probe/part interface. Evident explains that even a thin air gap creates a severe acoustic-impedance mismatch. Choose couplant for the surface, temperature, required wave mode and material compatibility. Rough surfaces may need a different couplant/probe arrangement, and residue removal can matter for corrosion-sensitive parts. A high-temperature couplant rating does not authorize the same temperature for the probe. Evident, ultrasonic couplants

Temperature also affects setup. Evident's dual-element setup guidance calls for redoing zero calibration at working temperature when the part is significantly hotter than the calibration condition. Use the instrument/probe instructions and validated procedure for temperature compensation and contact duty. Evident, dual-element setup

Three different calibration activities

Evident distinguishes:

1. Velocity/zero calibration: converts measured time to distance while accounting for material velocity and fixed delays; two known sound paths support a two-point setup.
2. Reference calibration: establishes the test response relative to suitable reference reflectors or blocks, including amplitude sensitivity.
3. Instrument certification: verifies specified instrument measurement performance under documented conditions.

A current instrument certificate does not replace material/probe setup or reference sensitivity. Check the complete configured system against suitable standards at the required intervals and after relevant changes. Keep the instrument, probe, wedge, cable and setup linked in the record. Evident, calibration concepts

Reference-reflector response is a comparison basis, not automatic proof that a real flaw has the reflector's dimensions. Surface transfer, attenuation, orientation and beam interaction affect amplitude. Apply the specified evaluation method instead of treating screen height as a direct ruler for every discontinuity.

Coatings and corrosion measurement

Paint can introduce thickness error if its transit time is interpreted using the metal's velocity. Evident describes echo-to-echo measurement, which times successive metal backwall echoes, and the separate THRU-COAT approach. The latter requires compatible specialized probes; the cited application identifies D7906-SM or D7908. Confirm instrument/probe support and usable conditions rather than assuming every gauge can ignore a coating. Evident, measuring metal thickness through paint

For corrosion mapping, choose scan spacing and validation appropriate to localized loss. A spot value describes its measurement location; it does not prove the minimum remaining wall elsewhere. Retain coordinates, signal-quality information and the basis for selecting the reported backwall echo, especially when readings change abruptly or disappear.

Advanced techniques and remaining blind regions

Phased-array UT uses timed excitation of multiple elements to steer and focus beams. TOFD uses separate transmitting and receiving probes and diffracted signals to support sizing. TWI explains that conventional echo-amplitude sizing is orientation-dependent and that TOFD can have difficulty with small near-scanning-surface flaws because of the lateral-wave response. Plan complementary coverage when the required inspection includes those regions. An image or multiple beam angles does not remove all orientation, material or access limits. TWI, ultrasonic examination Part 3

Report observations and acceptance separately

Document component/material identity, inspected volume and inaccessible regions, procedure and acceptance-standard revisions, probe/wedge configuration, calibration blocks and checks, velocity, range, sensitivity/gates, scan plan and coordinates. Preserve required raw data and representative signals together with located indications and the applied evaluation. Record deviations and coupling or attenuation problems rather than treating uninspectable areas as satisfactory.

The drawing, code or inspection agreement establishes acceptance criteria. UT provides evidence under the defined technique; it does not independently establish fitness for service, authorize repairs or replace other required examinations.

Related reference

Inspection and quality documents