What is ultrasonic testing, and how does a Level II use it?

How ultrasonic testing works in three steps

A probe holds a piezoelectric crystal. The instrument pulses it, the crystal sends a short burst of sound into the part, and the same crystal listens for the return. Sound travels through a given material at a known speed, so the time between pulse and echo converts to distance.

Here is a quick worked example, invented for this article. Say a probe sits on a steel plate and the far-wall echo arrives 10 µs after the pulse. At a longitudinal velocity of 5.9 mm/µs, the sound covers 5.9 × 10 = 59 mm there and back, so the plate is 59 / 2 = 29.5 mm thick. Divide by two every time, because the echo makes a round trip. Forgetting that step is one of the most common slips in UT math, and the UT formulas guide works through the rest of the arithmetic one formula at a time.

A flaw between the surfaces reflects some of the sound early. That produces an extra echo on the screen at a shorter distance than the back wall. Its position tells you depth, and its height gives you a first read on size.

The equipment in a UT setup

• Instrument. It pulses the probe and displays the returning signals, with controls for gain, range and velocity.
• Probe. A straight-beam probe sends sound straight in. An angle-beam probe uses a wedge to send a refracted shear wave into the part at a set angle.
• Couplant. A gel or liquid film between probe and part. Sound does not cross an air gap well, so no couplant means no signal.
• Reference blocks. Blocks of known thickness and known reflectors that you use to calibrate distance and set sensitivity before you scan.

Calibration is where a UT result becomes believable. The same part scanned with a poorly set range will give you confident but wrong depths, so exam questions about setup carry real weight.

What UT finds, and what it does not

UT is strong on volumetric flaws and on flaws that sit at an angle to the sound. It measures remaining wall thickness from one side, which is why it shows up in corrosion work, and it reaches inside welds and forgings where the eye cannot.

It has limits. Coarse-grained or rough material scatters sound. Flaws that sit nearly parallel to the beam can reflect almost nothing back to the probe. Tight geometry can create echoes that mimic flaws. A Level II learns to recognize those cases and to choose a different probe or angle, or flag the result for further review.

Straight beam and angle beam in one paragraph

Straight-beam work is the choice for thickness and for flaws parallel to the surface, such as laminations in plate. Angle-beam work is the choice for welds, where flaws often sit at an angle to the surface and you need the beam to meet them squarely. The geometry of angle-beam work is where the exam math lives: beam angle, skip distance, surface distance and depth. If you want to see that geometry on numbers of your own, the skip distance calculator is a quick check.

Why the math is part of the method

UT is the one method where arithmetic is part of every examination, not just the exam. Locating a flaw in a weld means turning a position on the screen and a beam angle into a depth and a surface distance. Setting sensitivity means working in decibels. Checking a probe means relating frequency, velocity and wavelength.

The good news is that the set of formulas is small and the same ones return again and again. If you can rearrange an equation and use a right triangle, you have the algebra and trigonometry the General exam needs. Practice them on a sketch of the beam path, and the Snell's law calculator lets you check your hand results.

Level I, Level II and Level III in UT

Level I technicians perform tests following written instructions, usually under direction. Level II technicians set up and calibrate, perform the examination, interpret and evaluate results against a code or specification, and write the report. Level III is the senior tier, with responsibility that reaches past a single job. Our requirements guide lists the training days and experience days ASNT publishes for UT Level II, and the route comparison shows how employer certification differs from ASNT's central program.

Your next step: the General exam topics

The ASNT UT General exam is where this knowledge gets tested, in six topics: ultrasonic techniques, base material product forms, weldments, bonded structures, discontinuity detection and general evaluation. The UT General guide explains each one. If you want a plan around them, the 30, 60 and 90 day UT study plan turns the topics into weeks. The math refresher covers the algebra and trigonometry that sit under the formulas.

The free 10-question check shows exactly which areas to study first, so you spend your first week on the topics that cost you points rather than the ones you already know. Take the free UT check and see where you stand.

FAQ

How does ultrasonic testing work?

A probe sends a short burst of high-frequency sound into the part and listens for echoes. The instrument measures the time each echo takes to return and converts it to distance using the speed of sound in that material. Echoes from the back wall give thickness, and echoes from flaws give their position.

What is the difference between UT and ultrasound imaging?

Both use high-frequency sound and echoes. Industrial UT inspects metals, composites and welds, usually showing a signal trace on a screen, and medical ultrasound images soft tissue. This site covers industrial UT for NDT technicians.

What can UT detect?

UT detects internal flaws such as cracks, lack of fusion, porosity and laminations, and it measures thickness from one side. How well it finds a given flaw depends on the flaw's size and orientation, the material and the probe you choose. Choosing the right technique for that flaw is a core Level II skill.

What is the minimum thickness UT can measure?

It depends on the instrument, the probe frequency and the material. Thin-section work calls for higher-frequency probes and careful setup, and the written procedure for your job defines what is acceptable. For exam purposes, focus on how thickness is calculated and why resolution changes with frequency.