Author: RAS Thermal Co., Ltd. · Updated 2026-10-07
TL;DR Manual variation: inter-operator sizing spread of 15-25 % on the same defect. Automated repeatability: same indication sized within plus or minus 0.5 mm on repeat scans. Positioning: encoder indexing at plus or minus 0.1 mm, same coordinates every pass. Outcome: a two-campaign comparison that separates growth from operator change.
A crack found at one campaign is measured again the next, and the question is whether it is growing or holding. A crack only becomes a decision when two scans of the same crack can be compared. Manual UT sizing varies between operators by 15-25 % on the same defect, which is wider than the year-on-year growth of many cracks, so a manual comparison cannot tell growth from a change of hand. An automated scan removes the operator from the sizing, and the same indication repeats within plus or minus 0.5 mm. That repeatability is what turns a crack measurement into a trend line.
Common root causes
| Root cause | What you see | Where it bites |
|---|---|---|
| Fatigue under cyclic loading | a crack that lengthens along a known path as cycles accumulate | cap edges, bond lines and root transitions |
| Bond-line disbond feeding the crack | growth tracking a separated interface | web to shell and trailing edge joints |
| Impact-initiated crack | a crack starting from a load concentration and extending | shell laminate after impact |
| Manufacturing flaw under load | a defect that stays quiet until it starts to extend in service | cure and infusion anomalies |
What the numbers allow
The whole point is comparability. Manual sizing carries 15-25 % operator variation, and repeat scans on an automated crawler size the same indication within plus or minus 0.5 mm, which is a far tighter band than the growth a monitored crack shows between campaigns. With encoder indexing at plus or minus 0.1 mm, each measurement lands at the same coordinates, so the trend is real growth and not a change of position.
- Manual variation: inter-operator sizing spread of 15-25 % on the same defect
- Automated repeatability: same indication sized within plus or minus 0.5 mm on repeat scans
- Positioning: encoder indexing at plus or minus 0.1 mm, same coordinates every pass
- Outcome: a two-campaign comparison that separates growth from operator change
How to confirm it
Work down this list in order. The first check that changes the result has found the cause.
- Fix the scan geometry. Reproduce the same scan path and probe angle as the previous campaign, because a different path measures a different crack even if the length looks similar.
- Size at the same coordinates. Use the encoder indexing to land on the previous coordinates at plus or minus 0.1 mm, so the two measurements describe the same point on the crack.
- Compare against the 0.5 mm band. Read the change against the plus or minus 0.5 mm repeatability the automated scan holds, and treat anything inside that band as stable rather than as growth.
- Cross-check the neighbours. Compare the crack against the manual baseline of 15-25 %, so any claim of growth is bounded by what the method can actually resolve.
- Trend across campaigns. Store each measurement against the blade serial and plot the length across campaigns, because one pair of readings shows a difference and three show a rate.
Prevention
Standardise the scan on a crawler so every campaign is measured the same way, and store each result against the blade serial. Then a crack becomes a monitored value with a rate, and the repair decision rests on a trend rather than on two readings that may differ only because a different hand held the probe.
Values quoted here come from the same verified data the product pages carry, so this page and its sources agree line for line.
Frequently Asked Questions
Why does crack sizing vary between manual operators?
Because manual UT sizing carries 15-25 % variation between technicians on the same defect, which is often wider than the year-on-year growth of the crack itself. That is why an automated scan is used for monitoring rather than a repeat manual measurement.
How repeatable is an automated crack measurement?
The same indication is sized within plus or minus 0.5 mm on repeat scans, and the encoder indexes each pass to plus or minus 0.1 mm. That band is tighter than the growth most monitored cracks show between campaigns.
How many scans before a crack shows a trend?
Two scans show a difference, and three show a rate. Storing each result against the blade serial is what converts the readings into a line, and the line is what the repair decision uses.
What if the crack is inside the repeatability band?
Then it reads as stable for that campaign. Because the automated scan holds plus or minus 0.5 mm, a change inside that band is not distinguishable from measurement noise, so the crack is monitored rather than reclassified.
Bottom line: A crack is only a decision when two scans can be compared, and manual sizing varies by 15-25 % where an automated scan repeats within plus or minus 0.5 mm. Fix the scan geometry, land on the same coordinates, and read the change against the band the method actually holds.
Standards and sources
- IEC 61400-5:2020, Wind energy generation systems, Part 5: Wind turbine blades
- DNV-ST-0126, Support structures for wind turbines
- DNV-ST-0376, Rotor blades for wind turbines
- ISO 9712:2021, Non-destructive testing, qualification and certification of NDT personnel
- ASTM E2580, Standard practice for ultrasonic testing of flat panels of polymer matrix composites
- ASTM E426, Standard practice for electromagnetic examination of tubular products
- ASME BPVC Section V, Nondestructive Examination
- ASNT SNT-TC-1A, Personnel qualification and certification in nondestructive testing