This interview is republished with permission from NASA Tech Briefs
Dr. Ajay Koshti, Lead Nondestructive Evaluation Engineer, invented NASA Flash Infrared Thermography Software. Koshti also worked as a Nondestructive Evaluation (NDE) Engineer on NASA Space Shuttle Orbiter for 23 years.
NASA Tech Briefs: Ajay, Your September Webinar with us will focus on infrared (IR) flash thermography software. To set the stage, what is infrared (IR) flash thermography software?
Dr. Ajay Koshti: It is a post-processing software, so let me describe the flash thermography process. It’s a non-destructive evaluation process used to detect delaminations and voids in composites, primarily nonmetallic composites. These materials are used on NASA flight hardware, so they are critical structure materials. Flash thermography is one of the nondestructive evaluation methods to inspect these materials for internal flaws.
In this process, you have an infrared camera, used to acquire the image data. Flash lamps momentarily heat up the part under inspection. As soon as the flash occurs, you acquire the data of surface temperature, using the infrared camera, and that data is analyzed to detect internal flaws, such as voids and delamination.
Usually we use a commercial software to acquire the data and provide some post-processing to analyze it. I developed a separate software to analyze the data. Primarily, [with the separate software], we are looking for quantitative evaluation of the data. When you have internal flaws, you want to have a quantifiable response that can be measured repeatedly; even if you change the camera systems, you should get the same response on the same defect.
That was the starting point: to understand and define the signal response from the flaws. That’s where it started out: with the concept of normalized contrast. The response is normalized between -1 and +1. It’s very repeatable.
Going beyond that, once the response is the characteristics of a flaw, you can start characterizing the flaws for the depth and diameter. That’s based on calibration. You get these responses from various flaws, diameters, and depths. Then, you teach the system to identify or interpolate flaw sizes based on these responses. It’s a calibration process. Using the contrast methodology, you can also reduce the noise in the response. You essentially have a reflection component in infrared imagery, which comes from reflection of heat that is coming from the background. If you are able to remove that, you improve your signal response.
One of the other iterations I have is drawing the boundary around the defect indication. That’s called the half-max. It looks for a location of 50% signal drop around the indication area, and then it draws a boundary. That gives you a most accurate way of sizing the defect. You can estimate depth of the defect. Upon calibration, it even gives you an idea about gapping within the defect.
NTB: How are these subsurface flaws being formed?
Dr. Koshti: Flaws are formed due to many reasons. Many of these structures are laminated, which means that in the assembly itself, you use sheets or layers, which are then stacked, put in autoclave, and bonded. They start out with layered construction. Many times, in manufacturing between layers, they may not bond properly. That’s delamination, or separation between layers.
These are also damage-prone. If there is an impact, or if something falls on the part, it will have a tendency to have delamination. Layers will separate. You can also inspect these materials for handling damage. Currently, for example, on the Space Shuttle program, reinforced carbon-carbon composite material is being used on the wing leading edge and also on the nose cap. It’s a layered material that is exposed to reentry temperatures in excess of 3000 °F.
These materials heat up and expand, and form microcracks. Through those microcracks, a small amount of oxidation will occur. This results in separation below the outside layer, which is silicon carbide. The inner ones are carbon-carbon, which are providing structural strength, while the upper layer is refractory, providing high temperature capability.
NTB: How is the flash thermography software being used currently?
Dr. Koshti: Initially we started using it on the Space Shuttle program. Though microcracks are not detrimental, excessive oxidation caused by these microcracks can become structurally significant issues. Monitoring of very small indications is important so that you can confirm the structural integrity of the part.
There was a need to quantify an indication. That’s where the normalized contrast type approach was used initially: to assess if there is a growth of these indications between flights. More than that, because these responses are repeatable, you can then analyze them, once calibrated, for what size of flaw you have, what depth it is, and what kind of gapping is occurring. Now with image processing, you can actually use it for inspection: to detect and locate flaws in composite hardware for NASA at JSC. When you have to do finer analysis, you can go ahead and use this software.
NTB: What are some applications with nondestructive evaluation (NDE) testing?
Dr. Koshti: The Boeing 787 is primarily a composite structure. Thermography is very applicable there. The typical applications for thermography are for composites, even outside aerospace. Outside aerospace, composites are used in many places as reinforcement. Graphite fibers, for example, are often used to reinforce columns. Many small boats are made out of composite fiberglass. Again, to detect impact damage, you could use it for those structures. The software can be used anywhere that there are composite structures, even bathtubs that are made out of fiberglass.
NTB: Compared to traditional methods of measuring, what makes the thermography software a better option?
Dr. Koshti: The previous software is more qualitative in nature. When you take a video, the data itself is in the video. What I’m doing: I convert the video data to contrast data. So I’ve taken the first step to enhance the data.
Ultimately, visual detection is done by looking at the differences from an indication with respect to its surroundings. Our contrast is in a different domain. It’s in a contrast, and it’s not in a raw domain. I do a certain processing on it. I go ahead and look for where the contrast is maximum, and create composite images showing that. So things that you would do to go through the data by playing it back and forth: It’s done by the software. It gives you those images right away, and so it saves you time. Image processing enhances the results.
The primary idea was also to be able to quantify these images. The images have a scale. The software gives you the normalized contrast values, so you can actually read what the actual value you’re getting from various indications. Those indications can also be analyzed for their size, depth, based on calibration.
This is actually the philosophy of ultrasonic inspection. You calibrate your instrument on known flaws, and then use that to interpret your data. I’m bringing the well-known ultrasonic testing metrology of calibration and evaluation of data into flash thermography. This is the first time anyone has done that. This allows NDE personnel to understand flash thermography as analogous to pulse-echo ultrasonic testing. They can use the ultrasonic testing knowledge and apply it in thermography and do the inspection at the same level of rigor and same quantitative manner, knowing exact signal responses and basing the reject level on the known level of signal responses. This brings thermography from just visual, qualitative evaluation to a quantitative, objective evaluation.
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