Photon Induced Positron Annihilation: Detecting structural flaws at the atomic level

Nov. 1, 2002

Detecting Structural Flaws at the Atomic level

By Joe Escobar

Imagine a technology that would enable you to detect material damage at an atomic level prior to the appearance of cracks. Can you picture yourself telling your director of maintenance something like "Yea boss, we are going to have to down the aircraft because our NDT test has detected an atomic anomaly in the spar that could lead to catastrophic failure." Something out of Star Trek? Not necessarily. Photon Induced Positron Annihilation (PIPA) is a new non-destructive test method that can detect weakened areas where flaws can develop. In this article, we will take a look at how this new technology works.

What is PIPA?

PIPA is an NDT testing system that is manufactured by Positron Systems, Inc. The technology behind it was developed at the U.S. Department of Energy’s Idaho National Engineering and Environmental Laboratory and licensed to Positron Systems for commercial use. Unlike other traditional methods of NDT that are able to detect existing flaws in aircraft structure, PIPA is able to detect fatigue, embrittlement, and other forms of structural damage in materials at the atomic level, before cracks even appear. Consider for a moment the four stages of fatigue:

Phase 1: Early fatigue damage (no visible cracks)

Phase 2: Crack initiation

Phase 3: Crack growth

Phase 4: Fracture

Conventional NDT methods can only detect the last two phases of the fatigue cycle. In many cases, mechanics are able to find the cracks after they have developed to Phase 3 in a timely manner to prevent failure. But in some cases, the problem goes undetected until failure of the part. Depending on where the defect is, this could be catastrophic. PIPA is able to offer a proactive NDT choice that allows users to prevent problems from occurring in the first place.

PIPA involves penetrating the material being tested with a photon (X-ray) beam. When these X-rays go into the material, they knock a neutron off of a small fraction of the atoms in the material. When these atoms have a neutron knocked off, they change into an isotope that will actually sit there and decay while producing positrons for a period of time. The positrons that are generated will move around in the material. In the case of a material with a defect, the positrons are trapped at nano-sized defects in the material. They will slow down and stop in that defect area. Ultimately, the positrons collide with low momentum electrons in the defect area and are annihilated, thereby releasing energy in the form of gamma rays. The gamma ray energy spectrum that is released in the case of a defect creates a distinct and readable signature of the size, quantity, and type of defects present in the material. The gamma ray spectrums provide the PIPA equipment with baseline data for analysis. After recording the damage characteristics, the data is analyzed to provide a determination of the location and concentration of surface and subsurface anomalies.

If the material being tested has no defects in it, the positrons react a little differently. They just move around the material in a fairly uniform proportion if they are unable find a defect. They eventually annihilate with an electron that has energy momentum associated with it. When that happens, instead of the very singular gamma ray energy distribution that happens with a defect, the energy distribution is distinctly different. So by distinguishing between the very singular energy distribution associated with a defect and the different energy distribution associated with non-defects, PIPA is able to calculate very accurately the level of the defects in the material.

Detecting material defects

PIPA is able to detect numerous defects in material. It can detect flaws that could lead to cracks caused by fatigue, thermal damage, and corrosion. Basically any defect that affects the molecular lattice structure of the material can be detected. Most of the materials used in aerospace applications can be tested using PIPA technology. It can even be used to detect fatigue damage in composite structures.

Material geometry does not significantly affect the test. PIPA is able to work with varying materials in different geometries. In addition, it is able to detect defects in underlying layers. So, a crack in a titanium spar can be detected under a layer of aluminum skin. Another thing is PIPA is not sensitive to surface paint or surface conditions. It can be used to inspect to various depths and through multiple layers. This makes it a very feasible inspection because the user doesn’t have to take the product apart in order to test it.

Early detection

As Doug Akers, technical director for Positron Systems, put it, "If the crack is big enough to be seen using traditional NDT methods like dye penetrant and X-ray, then you don’t need to come to us. Where we can benefit the user is being able to detect the material defects and allow the user to initiate corrective action much earlier or to monitor suspect areas and know when it will probably fail."

A good way to think of it is that traditional NDT systems are able to detect cracks once they form. This is in the last two stages of the fatigue cycle. PIPA is able to address the first two stages of the fatigue cycle of the part before cracks ever appear.

Unit availability

Right now, most of the testing is done at Positron System’s testing facility in Idaho. The company is developing a portable system that will be able to be moved around to facilities for leasing and/or services for a few months. In addition, it is also developing a smaller system that is based on a similar concept that will be portable and will be able to be used for low-level continuous monitoring.

On portability, Akers shared, "What we are seeing are customers that are using PIPA technology to help characterize materials and help understand what the fatigue life and remaining life is, and using this information to update their models and eventually update maintenance schedules. You can do a lot of that from our facility, but eventually a mobile system to inspect parts on wing is where we are going to be positioned."

PIPA is a new technology that could significantly change the way we inspect aircraft. So, telling your director of maintenance, "We are going to have to down the aircraft because our NDT test has detected an atomic anomaly in the spar that could lead to catastrophic failure" isn’t so farfetched after all. It’s not science fiction, it’s science fact. AMT

Additional ReSource

Positron Systems

6151 N. Discovery Way

Boise, ID 83713

(208) 672-1923