Passing a UT weld test comes down to three things: a clean calibration, a methodical scanning technique, and confident signal interpretation. Most candidates who fail don’t miss defects because they lack knowledge. They fail because they rush calibration, move the probe too fast, or misclassify an indication they actually found. Here’s how to get each step right.
Understand What the Test Actually Evaluates
A UT weld test typically requires you to scan one or more welded samples, locate any discontinuities, classify them by type and severity, and report whether each indication is acceptable or rejectable under the applicable code. Under AWS D1.1, the dominant structural welding code in the U.S., indications are rated on a decibel (dB) scale and sorted into classes (A through D) using the code’s acceptance tables. Class A indications are rejectable regardless of their length, while Class D indications are acceptable regardless of length. The tricky part: the same dB reading can fall into different classes depending on which acceptance table applies to your joint. Table 6.2 and Table 6.3 use different thresholds, so a +10 dB indication might be Class D (acceptable) under one table and Class A (rejectable) under another.
Before you touch a probe, know which code and which acceptance table your test uses. Read the written procedure you’re given, confirm the weld joint type and thickness, and identify the correct table before scanning. Getting the evaluation criteria wrong will sink your results even if your scanning technique is flawless.
Nail Your Calibration
Calibration is where most practical mistakes begin. You’ll calibrate your instrument on a reference block with known reflectors (usually side-drilled holes or notches) to set your sensitivity and distance range. Every indication you find during the scan is measured against this baseline, so errors here cascade through the entire test.
Set up your Distance Amplitude Correction (DAC) curve or Time Corrected Gain (TCG) carefully. A DAC curve plots the expected echo amplitude from the reference reflector at various distances, creating a curved line on your screen. Signals closer to the probe appear taller than identical reflectors farther away, and the DAC curve accounts for that drop-off. TCG does the same job differently: it automatically adjusts the instrument’s gain over time so that the same-size reflector produces the same screen height no matter how deep it is. If your test instrument supports TCG, it simplifies evaluation because you’re comparing every indication against a flat horizontal reference line rather than a curve. Either method works, but make sure you know which one your procedure calls for and that you’ve verified it on all calibration reflectors before you start scanning.
Double-check your calibration at the end of the test as well. If your instrument drifted during scanning, your evaluator will likely require you to rescan, and running out of time is a common way to fail.
Choose the Right Probe Angles
Almost all angle beam weld testing uses 45, 60, or 70 degree probes. The general rule is straightforward: use higher angles (60 or 70 degrees) for thinner material, roughly under 25 mm (1 inch), and lower angles (45 degrees) for thicker material, generally over 50 mm (2 inches). For material in between, you’ll often use two or even three angles to ensure full coverage of the weld volume.
The probe angle needs to be high enough that a first-leg signal can reach the weld root without the probe physically bumping into the weld crown or running off the edge of the plate. Before scanning, do a quick sketch of the sound path geometry. Calculate where the beam hits the back wall and where it enters the weld zone on the first and second leg. This tells you your minimum and maximum standoff distances from the weld centerline and ensures you’re actually insonifying the areas you need to inspect.
Be aware that leg selection affects your results significantly. A reflection from the same flaw can produce very different dB readings depending on whether you catch it on the first leg (one bounce) or the second leg (two bounces). In one documented example using AWS D1.1, a side-drilled hole produced an indication rating of +1 dB on the first leg, which was rejectable, but +6 dB on the second leg from the opposite face, which fell into the acceptable range. This isn’t a trick. It’s a real consequence of beam spread and attenuation over longer sound paths. During your test, scan from both sides of the weld and with multiple angles when the procedure allows, and report findings from the leg and angle that give you the strongest, most reliable indication.
Scanning Technique That Catches Everything
Move the probe slowly and overlap each pass. A common guideline is to advance the probe no more than half the crystal width per pass. This feels tediously slow, but it’s the difference between finding a tight crack and skating right over it. During the test, you’re being evaluated on completeness as much as accuracy.
Scan with a slight oscillation (side-to-side rocking of about 10 to 15 degrees) as you move the probe along the weld. This “wobble” helps catch reflectors that aren’t perfectly oriented to the beam. Combine this with a slow forward travel and you’ll maximize the chance of hitting every discontinuity at the best possible angle.
Cover the full weld volume systematically. Start your probe at the maximum standoff distance for the second-leg sound path and walk it inward toward the weld until you’ve covered the first-leg path to the weld root. Then do the same from the other side. If the weld crown hasn’t been ground flush, you may only be able to scan from one side, which means you’re relying more heavily on second-leg coverage. Adjust your scanning plan accordingly and note any areas you couldn’t fully inspect.
How to Read the Signals
Signal interpretation is the skill that separates candidates who pass from those who don’t. Each defect type produces a characteristic pattern on the A-scan display, and learning to recognize these patterns quickly is essential.
Porosity
Porosity is the easiest defect to identify. Gas pores are small, round, and scattered, so they produce multiple small echoes rather than one clean peak. When you see a cluster of low-amplitude signals that rise and fall as you move the probe, that’s porosity. No other common weld defect looks quite like it.
Center Cracks and Lack of Fusion
Cracks running through the center of a weld tend to produce two strong peaks on the A-scan. These double echoes are too strong to be diffraction signals, and they appear relatively close together. Lack of fusion at the sidewall can produce a similar double-peak pattern, but the two peaks are typically more widely separated in time. If you see two prominent peaks and can rule out porosity, you’re likely looking at a planar defect: a crack or lack of fusion.
Sidewall Cracks
Unlike center cracks, sidewall cracks generally do not produce double echoes. You’ll see a single strong reflection. The challenge here is that sidewall cracks and slag inclusions can look almost identical on the screen. Distinguishing between them is one of the hardest calls in UT weld inspection. If you’re stuck, note the signal’s response to probe movement: slag inclusions tend to produce a broader, more diffuse signal as you scan across them, while a crack will give a sharper, more directional peak that drops off quickly as you move off-axis.
Root Area Defects
Defects at the weld root each have distinct signatures. Lack of penetration produces a clean, simple pulse shape. Over-penetration is characterized by a long tail of small ringings after the main echo, caused by the irregular geometry of excess weld metal protruding through the root. Root cracks generate a strong reflection from the corner where the crack meets the bottom surface, and in some cases you’ll see a small “pre-echo” appearing just before the main pulse. That pre-echo, even if it’s faint, is a strong indicator of a root crack.
Evaluating and Recording Indications
Once you find an indication, maximize it by adjusting the probe position until you get the highest possible echo amplitude. Record that peak amplitude relative to your DAC or TCG reference level. This gives you the indication rating in dB. Then determine the indication’s length by moving the probe along the weld axis and noting the points where the signal drops to the threshold level (typically 6 dB below the reference, depending on your code). The combination of indication rating and indication length determines the classification.
Plot each indication on your report form with its location (distance along the weld and depth in the joint), its dB rating, its length, and your accept/reject call based on the applicable table. Be precise with your measurements. Evaluators check your math, your plotting accuracy, and your classification logic. A common failure mode is finding the right defect but recording the wrong depth because of a sound-path calculation error.
Mistakes That Cost People the Test
The most frequent errors are procedural, not technical. Forgetting to verify calibration at the end of the test. Using the wrong acceptance table for the joint configuration. Scanning too fast and missing a discontinuity entirely. Calculating the wrong depth because you forgot to account for which leg the signal traveled. Confusing slag inclusions with sidewall cracks (a genuinely difficult distinction, but one that evaluators still expect you to attempt with reasoning).
Time management also matters. Most practical tests give you a fixed window to calibrate, scan, evaluate, and report. Candidates who spend too long on calibration or get stuck trying to classify one ambiguous indication can run out of time before completing the scan. Practice the full sequence, calibration through reporting, on sample welds until you can do it within the time limit with room to spare. Familiarity with the physical steps, the calculations, and the paperwork is what turns a stressful test into a manageable one.