NASA’s System for In-situ Defect Detection in Composites During Cure Webinar

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2023-07-10 NASA's System for In-situ Defect Detection in Composites During Cure Webinar - YouTube

As many of you here know composite materials offer many advantages to aerospace applications they have higher strength to weight ratio increased fatigue properties. However defects such as porosity occur during the manufacturing of composites and that includes during the cure cycle.

Actually what you're going to hear about today is the cure cycle step in the manufacturing process. We have process models that can predict these cure defects, an example being the cure defects process model that was developed during the advanced composites project. However, there what we found is there's no direct validation of porosity during cure.

Current validation methods include resin pressure sensors during cure as well as inspection or microscopy after cure and that's actually what you're seeing in the images in the bottom right. They are optical micrographs of three different locations within a laminate and these dark areas are void so this is a laminate with very very high porosity.

The optical micrograph in the center is a very well consolidated laminate with very low porosity and then the optical micrograph on the left is something in between. so you still see voids however they're not nearly as severe as the optical micrograph on the right.

The objective of our project was to develop a system to perform porosity detection, localization and quantification during cure in both an oven as well as an autoclave and then compare those inspection results to the process model. That was the original question that started this work.

However, what we found as we did this research is that the system is capable of doing a lot more. So you acquire a real-time knowledge of porosity content and location in your part during cure and you could then use that to, for example, control your processing parameters based on those measurements. As well as a few other things and we're actually going to talk about that later in the presentation about the impact on the system.

But then a question we always want to ask is why is this system hard to develop.

• So first and foremost is that you have a limited accessibility to the composite part while it's being cured. If you look at the figure on the right, you have an autoclave. The composite part is inside this vacuum bag. It's in contact with the tool plate. there is material such as breather cloth and release film that all make access to the composite difficult.
• The defects themselves are very small. porosity feature size can be on the order of microns.
• And the autoclave is a harsh environment so you have elevated temperature. that's required to cure the composite, as well as pressure. so anything you send into the autoclave actually has to go through bulkhead fittings to maintain the pressure of the autoclave and then quite simply commercial off-the-shelf inspection systems cannot operate in this in this environment.

A little history about the system. So there were two technologies that came prior to the one you're hearing about today. Both were focused on a guided wave based cure process monitoring. The first had static piezoelectric transducers that were mounted onto what we called a smart call plate. The second system was similar. It used piezoelectric actuation but the sensing mechanism was an optical fiber with fiber brag gratings or phase shifted fiber brag gratings.

The system you're hearing about today incorporated a mobile piezoelectric transducer so the transducer actually moves around during the cure cycle and what that allows for is defect detection localization and quantification in real time in-situ during cure in addition to defect detection which is what you're going to hear a lot about today but the system also provides very high spatial resolution cure monitoring um the experiments that we were doing had a one millimeter by one millimeter per pixel resolution um it could actually be even um higher resolution than that the system is deployable to existing production lines um so it is very very scalable from rnd all the way to uh to industry and a key feature is that no change is required to your current part design so the inspection system can work without having to go back in and redesign your part so our approach we had a few key design parameters that we were working with so our raster scanner had a maximum desired operating temperature of 100 f or 38 c so we knew right out of the gate that we knew that we were going to have to do cooling of those of those components we wanted to make sure that the system did not affect the curing of the composite as well as making sure that the system could answer the original question which was porosity detection localization and quantification during cure some additional challenges or design parameters was that the autoclave is pressurized so anything that we were sending into the autoclave had to go through through bulkhead fittings we were spatially constrained in the autoclave as well as we wanted to maintain an adequate scan area in the current embodiment of the system we can do 7 inches by 18 inches as well as the ability to do cure monitoring with that extremely high spatial resolution uh the image that you are seeing in the bottom right is a um view of our system and how it was operating so our scanner is enclosed into a unsealed cooling container the transducer that's actually doing the inspection is out in the autoclave and the motion is controlled from the raster scanner that's inside the inside the container and when the temperature inside the container gets to a predetermined temperature it sends a signal to the solenoid valve which opens at which point a liquid nitrogen tank which is always at a higher pressure than the autoclave would then send ln2 through the hose by the time it actually gets to the container it's gaseous and so you wind up with cold nitrogen gas inside the container and hot nitrogen gas inside the autoclave we began with testing in an oven for vacuum bag only or out of autoclave composite materials and then transitioned the system to an autoclave that is what you are seeing in the upper right is a picture of our system in an autoclave the scanner is housed inside the cooling enclosure the ln2 that we talked a little bit about on the previous slide is delivered through this hose from a tank external to the autoclave the motion of the transducer is controlled using a bellow system on the side of the enclosure that allows for side to side and front to back motion and the ultrasonic transducer is high temperature so it is allow is able to operate at the cure temperature of the composite the question you may be asking is where is the composite material in all of this it is actually vacuum bagged to the underside of the tool plate there are other orientations possible it can be flipped over and so the composite being on top but in our particular system it was uh we have the composite underneath and that's what you're seeing here in the bottom right is kind of a side view cutaway of how the the system operates for those that might not be familiar with how ultrasound works is you have an ultrasonic transducer that emits a wave that then travels through the tool plate into the composite and that same transducer measures those those reflections at which point the transducer moves side to side and front to back and those reflections ultimately reveal the internal structure of the of the composite panel so some early successes from oven testing we began with intentionally embedded hollow glass microspheres that were emulating porosity beginning with the image on the left this is a top view where you see three distinct areas of porosity and then the image directly beside it is a cross-section view so this is the tool plate composite interface this is the composite vacuum bag interface and then all of the reflections between are coming from within the composite and so you see this first area of porosity between plies six and seven uh this second area porosity between um 12 and 13 and then this third area between plies 18 and 19. after doing these intentionally embedded experiments we moved on to processing induced porosity where we drove varying levels of porosity within the composite using uneven pressure this is again a top view of the laminate and the color scheme here is such that the red color indicates low porosity and the blue color indicates high porosity the key result in this uh from these testing was that we demonstrated defect detection and localization both in plane as well as through thickness uh during the cure cycle so all of the images that you see above were measured at the highest cure temperature for the for the composites after finishing testing in the oven we transition to autoclave and we'll talk about these figures i'm in clockwise fashion the first figure you see here is our composite panel a simple photograph prior to cure it is a tapered laminate such that we have 32 plies in the center of the panel it is eight plies thick at the edge of the panel and then the thickness is changed through ply drops from the center to the center to the edge the panel is symmetric so we actually are only scanning one half of the of the panel and a small slice directly in the in the center so all the image that uses images that you will see on previous or on future slides will be from from this scan area the image on the top right is of our part thermocouple so we have thermocouples measuring part temperature um as well as autoclave temperature temperature of the motors temperature inside the inside the enclosure so we were very well instrumented as far as far as temperature goes the cure cycle here as you can see is a we have a 30 minute b stage hold with a two hour full cure at 177 c the photo in the bottom right is a cutaway or cross-section view of the system in a little bit more detail so we have our ultrasonic transducer below there we have a a flat tool plate at which point we then have our tapered laminate again showing where we are thicker in the center and thinner near the near the edge and then a flat call plate so the call plate initially is um is initially as flat at the beginning of the cure cycle but due to pressure that is applied during cure the call plate actually bends and causes uneven pressure across the across the composite panel so we have high pressure out near the edge of the call plate a low pressure regime at the end of the ply drops a transition zone where the pressure is again high and then in the center of the panel the pressure is reduced as well the ultrasonic transducer as mentioned before sends sends the waves through the tool plate into the composite and then measures those reflections and then it moves side to side and front to back throughout the throughout the cure cycle some results so we were taking scans about every five minutes throughout the entire six hour cure cycle our scan area we already saw where that was at on the previous slide but it was an area of 406 by 13 millimeters and we had a resolution of one millimeter by one millimeter per pixel um the results um that you're seeing on the right are specifically the amplitude of the reflections from the composite call plate interface that's actually not as important as as long as you understand that this is operating on the premise that porosity increases attenuation of the wave and such that a lower amplitude or a black color here indicates increased porosity and the key result is that we observed high porosity at the expected locations i mean those were near the end of the ply drops in the middle of the panel if you specifically look at a scan 20 it's in large tier scan 20 was taken 130 minutes into the cure cycle when the composite was at 122 degrees celsius so the resin was still in the liquid state and it's during the second ramp of the cure cycle moving starting out at the call plate edge we have low porosity as indicated by the increased amplitude at the end of the ply drops we have high porosity indicated by the low measured amplitude in this transition regime we have low porosity again as indicated by higher amplitudes and then in the center of the panel we have moderate to high porosity indicated by the decreased amplitude putting this all together we started with a simulation of our porosity from the physics-based process model that's what you're seeing here are cutaways or cross-section views of the of the predicted porosity we can then compare that to our in-situ inspections so we take are taking those inspections throughout the entire cure cycle the images you're seeing here a cross section view or a b uh b scan um at every every stage if we pull out uh the specific scan that corresponds to the same time um as the single image of the of the process model and put those side to side i'm going to explain exactly what you're seeing here and then we'll talk about how they how they compare so first off this this reflection here is from the front wall or the tool plate composite interface um this reflection here is the back wall or the composite call plate interface now if we line these two up if we start at the edge of the panel or near the call plate edge you see low porosity predicted from the process model you compare that to our inspections and we're seeing high amplitude indicating low porosity at this same location near the end of the ply drops there is this high porosity region we again are seeing that same region in our in situ inspections from the decreased amplitude in this transition regime there is a low porosity again we're seeing this in our inspections and then at the center of the panel there's moderate two to three percent porosity and again we're picking that up with our in-situ inspection which is very good that we're even able to detect even low amounts of low amounts of porosity so in addition to these cross-section or through thickness views you've already seen some of the top views are referred to as a a c scan anytime you develop a new system you need to have some type of validation so we had two ways to do that so the first is post cure immersion uh ut very well understood um from a completely different system and if you look inside this scan area you can see that we're seeing um almost more identical trends in the data post cure using another system compared to uh the measurements that we were taking during the cure cycle this particular image is when the resin is in the in the liquid state um and then next we had optical micrographs for direct observation of the porosity postsecure um if you look at this optical micrograph this is from the bottom of the or the end of the ply drops and we're seeing very very high porosity as indicated by these voids in the in the optical micrographs then at the beginning of the ply drop region we see a very well consolidated low porosity laminate just as we uh have seen in our inspections and then in the center of the panel we're seeing moderate levels of porosity that is also detected in our in our inspection we'll point out that these specific optical micrographs are from a previous panel with identical conditions so identical layup geometry call plate thickness and cure cycle um optical micrographs from this specific panel will be taken once we return to on-site work um at langley after um covet 19 passes uh in summary uh we developed a system that performs defect detection localization and quantification deering cure and it can operate in an oven or an autoclave it features high spatial resolution cure monitoring of resin state as well as material properties and that's in addition to the defect detection capability it's scalable from rnd all the way to existing production lines and a key feature is that there would be no change required to current part design or processing and only very limited changes to the processing equipment so a few bulkhead fittings in the side of your autoclave to pass everything in and this this system can can work with your existing composite structures the impact is you wind up with a real time knowledge of porosity or any other defect we've talked mostly about porosity today it's location and quantity deering cure you could use this to validate process models like we've done or you could validate your processing parameters during certification um you could also use it to control your processing parameters during the cure cycle based on these real-time measurements so the case study for that would be let's say that two percent porosity is an acceptable level in your part um you're measuring three percent porosity at the end of the b stage hold you could decide to either extend the b stage hold or increase the pressure of your autoclave to alleviate or reduce the pressure or i'm sorry reduce the porosity in your part down to that acceptable level and prevent having to scrap that part or repair that part ultimately all these a result in a more efficient process development a shortened certification time a reduction in all spec parts and combined for increased production throughput so some commercialization uh applications include aircraft launch vehicles satellites automotive wind turbine blades really anytime that you have structural composites that are cured in an oven or autoclave the system would be applicable and the users are also as broad so it could be oems tier one or two suppliers um an inspection equipment manufacturer may want to license it and then resell it and then it could be other oga's universities or research labs so really a broad um broad base of people that may want to use this technology so a quick um demonstration of what this looks like um while the experiment is going on so it's a four monitor system uh on screen one we have a live video of the scanner operating inside the enclosure the reason the video is fairly fuzzy is this is you're looking through a very thick sheet of glass in the side of the or the front of the autoclave and what you're seeing out here is the transducer moving um and performing uh performing the inspection on screen two uh we have our ultrasound measurements a b and uh c scans uh in real time uh so you're not requiring post processing um afterwards everything is processed in real time uh screen three are your temperature monitoring and we look at uh the scanning motors the enclosure autoclave temperatures composite panel um measurements throughout the for multiple locations of your of your process and then on screen four contains all of your autoclave controls this is how you control the autoclave temperature uh the pressure the vacuum um that's how you actually set the cure cycle is through the um is all on screen four uh the takeaway is that this provides visual real-time feedback on all aspects of your all aspects of your processing this was definitely not a one person project so i want to uh specifically thank frank hoy shawn eric jeff jamie bryan tom bryce ken abiel joey and patrick for their help on all aspects of this project this would not have been possible with uh without your help as well as the technology transfer office for um for hosting uh this webinar thank you sean kim and chris and at this point we will stop and take questions.