Induced Positron Analysis

Oct. 25, 2007
Detecting structural flaws at the atomic level.

Five years ago, AMT published an article on photon induced positron annihilation (PIPA). In the article, we discussed a new technology that allows an inspector to detect damage to material at an atomic level before any visible damage is apparent. In this article, we will discuss some advances made by Positron Systems in this inspection technology that allows inspectors greater flexibility to inspect airframe and engine components.

No longer PIPA
The technology formerly known as PIPA is now known as induced positron analysis (IPA) There are two types of IPA tests; IPA - volumetric (IPA-V), which is the old PIPA test, and a newer test developed called IPA - surface (IPA-S).

IPA technology is based on a nuclear physics concept called positron annihilation. It was developed by the U.S. Department of Energy at the Idaho National Laboratory, and Positron Systems now owns the worldwide license to the patents on the technology.

The process starts by inducing positrons into a component. The energetic positrons slow down until they “annihilate” with electrons, emitting low-energy gamma rays. During this process, the positrons seek defects in the material such as dislocations in the lattice sites. When the positrons become trapped in a lattice structure defect, they annihilate with free electrons, which have a very low momentum level.

The test is highly sensitive at low levels of damage induced either during fabrication, or from fatigue, embrittlement, high-temperature creep, corrosion, or other failure mechanisms. IPA offers the ability to detect these defects before visual indications of the defect are present.

As mentioned earlier, there are two types of IPA test methods, IPA-V and IPA-S. The IPA-V test has a large advantage. It is able to measure defects up to 7 inches (184 mm) deep. But there’s a catch – the test requires a linear accelerator. The test is not portable – it has to be performed at Positron System’s Idaho facility. As you could imagine, the TSA, airport security, and other security agencies might have a small problem with a linear accelerator located within the secure area of an airport.

The linear accelerator generates electron beams that pass through a tungsten target to generate high energy photons. The photons then enter the material being tested and interact with its atoms, dislodging neutrons from the atoms. This results in an unstable neutron-deficient isotope. As the isotope decays into a more stable element, a positron is ejected. Then the generic process of reading the gamma response takes place.

Positron System’s new IPA-S test now offers a solution for end-users. The test can detect surface damage between 2 and 3 mm deep, depending on the probe and material being tested. There is no linear accelerator involved, so all the safety and security issues around that are not an issue. The test can be performed using a mobile work bench. This test technique is being developed for widespread commercial use.

In order to test using IPA-S, a test probe is positioned in a jig in close proximity to the part being tested. In order to determine the proper placement of the probe, and whether or not a test can be performed successfully, sample tests are performed on the part at various stages. Martin Hedley, president of Positron Systems, shares, “When a customer approaches us about using IPA-S, we first prove the feasibility and business case for the test. We take a material that is brand new, that is partially used, that is halfway used, that is used up, and one that is broken. Upon development of a business case, we take lots of samples so that we have a statistically valid curve. And we show what exactly the response from the test would be as the part goes through its service life.”

Once the test is proven, then the customer can perform tests. The test probe needs to be secured in the jig at the position the test was proven by Positron Systems. Each test takes about five minutes. The test probe is commonly about ¼ inch diameter, and it tests an area of about ½ inch. Even though the test area seems small, Hedley shares that it is effective. “The test is effective because the engineers know where these parts are going to break, if they do break. So we don’t have to scan an entire engine blade. That would take a long time with our test. That’s not really the purpose of it. It’s down by the root, or it’s in one of the air cooling holes where you get a fracture. Or you may get creep at the edge of the blade. There is always going to be a failure mechanism in place. So we design the test to look for defects in those places.”

Hedley shares the benefits of the test. “There are three basic benefits. The first one, which is an overriding benefit, is that no other test can tell how the material is degrading over time until you get to the crack. We are way earlier than that. So the benefits that come out of that overriding benefit are two. The first thing is if something is cracking prematurely or is going to crack prematurely, we can see it in most cases. On the other hand, if you get a part that has been life-expired based on the number of hours it has flown, for example we could test it and if it looks like a part that has only flown 30 percent, then it may be reasonable to say that part can be used for one more maintenance cycle before it is discarded. And when you take a look at the type of parts that we are testing – turbine blades, disks, pressure bulkheads, and things like that, the costs of replacing those things is pretty high. And if I could get one more maintenance cycle out of it before I have to throw it away, I’m going to save some significant money – and know that my part still has integrity.”

Testing capabilities
Hedley tells AMT there are numerous applications for using IPA-S. “The best two materials currently are steel and titanium. Also, nickel alloys. We can also see some defects in aluminum. We are improving our probes right now so we get a better response from aluminum, but it’s not wonderful yet. We’re only days away from developing a test that works on composite materials. There are a number of other exotic metals we could test. Basically, we have to prove it each time as to whether or not the test works. Because obviously, looking at the atomic structure, the signal that we get back from the test is going to be dependent on not only what the material is, but also the shape of it. So even if you have two engine blades that are made of exactly the same material, but they are different shapes, then it’s going to be a different response. So we actually have to prove the test out on each part a customer wants to test.”

Like any other non-destructive inspection method, induced positron analysis has its limitations. But it does offer potential users one thing that has not really been possible before – the ability to track how the material is degrading over its life. The ability to detect flaws at the atomic level is a technology we can now add to our arsenal of non-destructive inspection techniques. 

About the Author

Joe Escobar