How Small a Defect Can UT Find

Customers often ask LTI how small a defect ultrasonic testing (UT) can find. This question is very difficult to answer without actually attempting a test. A better question to ask is this: How big a defect will be a problem? Material specifications and the engineering knowledge which go with a particular part are the first place to look. LTI can suggest some possibilities in some cases but can’t go as far as making engineering recommendations.

As an aid to our customers, LTI can give some information on what factors make a UT test easier or more difficult. This can help you decide how ultrasonic testing could best be applied to your product. First, it will help to get an overview of how UT works.

The Ultrasonic Testing Process

Ultrasonic testing is performed by sending sound pulses into a material, typically thousands of times per second. These sound pulses have a frequency much higher than what people can hear, thus the term “ultrasound”. Higher frequencies can detect smaller defects. A device called a transducer receives an electrical pulse from the UT test system. It converts this pulse into sound. The sound travels into and through the test material and reflects off of surfaces. These surfaces can be the outer surfaces of the part, boundaries associated with the metallurgical structure of the material, or actual defects. In general, any location where there is a big difference between material properties will reflect sound.

This reflected sound is detected by the transducer and converted back into an electrical pulse. The UT test system displays this pulse on a screen. We call this an indication. We can get some information about what the sound reflected from based on the size, shape, and location of the indication on the screen.

In general, we’ll compare this indication to an indication made from an artificial defect we placed in similar material. This artificial defect is usually a notch or a flat bottom hole. Notches are usually made with electrical discharge machining and simulate cracks. Holes are usually drilled and simulate inclusions, bursts, or other internal defects. The flat bottom helps generate a repeatable response. We call a piece of material with such notches or holes a reference standard.

The Impact of Material Characteristics

Numerous material characteristics can affect what size defects ultrasonic testing can find. Among these are:

• The alloy of the material
• How the material was processed and heat treated
• The surface finish of the material
• The geometry of the material

Alloys
Carbon steel and aluminum alloys do not have very complicated metallurgical structures, and therefore sound travels through these materials fairly easily. By contrast, 6Al-4V titanium has two different crystal structures, which interact with each other, and thus there are more things to affect how sound travels through such an alloy. In general, the more complicated the alloy, the more difficult it is to UT.

Processing and Heat Treatment
Most of the material LTI performs ultrasonic testing on is wrought. The forming processes used can have a significant effect on how difficult UT can be.

One example is forged material. When properly forged, a forging will have a fairly uniform grain structure throughout. This makes the UT test easier. When improperly forged, the forging will have a fine grain near the outer surface, and very coarse grains near the centerline. The coarse grain structure is more difficult to penetrate, and the transition region between the two structures is even more difficult.

Castings tend to be more difficult to test than forgings. The cast grain structure tends to be nonuniform, which causes the same difficulties as improper forging.

With tubular products, seams, die scratches, and wall thickness variations can appear to be defects even when they are otherwise acceptable.

Certain heat treatments, especially on more complicated alloys, can also make ultrasonic testing more difficult. One important consideration is that UT results are generally invalidated if the material is heat treated after the UT test because the heat treatment itself can generate defects which weren’t present during UT.

Surface Finish
The surface finish has a significant effect on how much sound can get into the material. A rough surface will diffuse the sound all over; a smooth surface allows the sound to pass straight into the material. Most specifications require a surface roughness of 250 Ra or smoother, which is comparable to medium sandpaper. The reference standard also must not be smoother than the test material.

Geometry
Geometry is probably the most significant factor affect the feasibility of UT. Thick sections require more sound amplification, which makes for a noisier test. Extremely thin sections, besides being fragile, require specialized transducers to properly distinguish between the material surfaces and true defects. Tubular products with very heavy walls require special techniques for directing the sound waves to inspect the full volume of the material. Finally, material which has machined features can be difficult or impossible to UT – the UT system can’t tell the difference between a hole which should be there and a defect which shouldn’t. In general, the more complicated the geometry, the more difficult UT will be.

In general, flat bottom holes smaller than about 1/16 inch can be difficult to find. Notches less than about 0.004 inches can be difficult to find. All these factors affect how successful ultrasonic testing will be on a particular material. LTI’s UT department is always happy to discuss your testing requirements.