Which Inspection Method Works Best?

Pros and cons of X-ray and CT techniques

Published: Tuesday, March 21, 2023 - 12:03

Many times per week we’re asked, “Is X-ray or CT the correct inspection method for my project?” The answer, of course, isn’t always straightforward. This article will highlight some strengths and weaknesses of each method.

The subject for inspection is a carbon fiber board with inclusions throughout. The piece is very large at 26 in. x 26 in. Using X-ray to inspect it is fine, but with computed tomography (CT) it could prove difficult.

Fact-finding with focus scans

Because this project was intended for R&D and technique development, we were able to cut a section out of the sheet. This probably should have been done by professionals; we ended up running through five blades cutting through the armor-grade material.

The purpose of the small section is to acquire the best possible data to be used as a gauge for the more complex development. If we can understand what the material looks like at higher resolution, it will help inform our higher-energy, lower-resolution technique development.

The focus scan highlights carbon fiber layers, layer orientation, and resin gaps.

General inspection and sorting

There are many ways to optimize scans. This focus scan can also be used as a gauge.

By using a representative quality indicator (RQI), which is a part with known defects or acceptable level of defects, we can gauge the balance of speed or quality while maintaining the ability to detect defects. For example, if we can decrease resolution to image the large sheet and still detect defects as soon as we do with the focus scan, we may be able to batch-scan a larger area at once, significantly reducing cost. Or, if the highest possible resolution is required, we can then optimize exposure or the quantity of projections.

X-ray and CT side by side

Here are three benefits of an X-ray-based project:
1. Speed: It’s much faster to acquire 2D shots than process CT data sets.
2. Higher resolution: For large parts, it’s easier to optimize for resolution from a single orientation than to consider a full 360-degree scanning envelope.
3. Fewer artifacts: Less X-ray attenuation and beam hardening.

And here are three benefits of a CT-based project:
1. CT gives an additional perspective into the part. An X-ray might detect a default in one planar view, but CT tells you exactly where the defect is located. (Depth of inclusions is seen in the final image of this article.)
2. CT works well when you have complex measurements like GD&T and CAD comparisons.
3. Some defects can only be seen in a 3D data set. If a crack is perpendicular to the 2D view, it will be missed. In multi-wall assemblies, individual features can’t be determined in 2D.

High energy and large detector

With a 5-ft working diameter, we rely on our 450 kv vault to evaluate very large, heavy, or complex products. This carbon fiber sheet isn’t heavy, but it is large and thin. The opposing 0.5-in. thickness and 26-in. path length is well suited for a limited-angle scan to reduce beam hardening. However, our goal wasn’t the highest resolution but the largest single volume. We’re able to detect all of the inclusions, apply different filters for enhanced analysis (i.e., only show high-density features), and measure the exact XYZ location or depth from surface.

Another purpose of cutting the sample was to mimic two different-width boards. You’ll see above the unequal contrast between the wider portion of the scan vs. the thinner. The shorter path length reduces CT artifacts and leads to cleaner, more easily interpretable data.

To learn more about radiographic testing and CT technologies, visit this page.

First published Jan. 30, 2023, by Haven Metrology.

About The Author

Keith Irwin

Keith Irwin is the metrology manager at Haven Metrology, a Cadrex company. He has worked as a metrologist and manager within the company, and has expertise in projects requiring inspection such as dimenstional analysis and non-destructive testing methods.