How to create voids PURPOSELY to test for NDT
How to create voids PURPOSELY to test for NDT
(OP)
Hey, I'm doing some NDT research here at my job and my boss has asked me to create artificial voids within the carbon panels using prepregs. I've tried two methods thus far which both didn't work:
1)I've tried curing small sections of a prepreg sheet by sandwiching it between two aluminum sheets with cut-out sections, and heating it in the radiation light source (so that the cut-out section will be cured, but the other parts won't since the UV will reflect off the aluminum sheets). I successfully cured the small sections of the prepreg sheet, then I laid this sheet up in the middle of a regular prepreg layup and cured the entire thing (with vacuum) in the oven hoping that would create a void somehow: but no, every layers sort of cured together, I couldn't even see the pre-cured layer.
2)This time I took 2 pieces of a small cured prepreg parts and stacked them together. I then sealed all four edges with a sealant tape and inserted this "pocket" of cured prepreg in between my layup of 14 prepreg plies, then cured the entire thing. I was so sure that this would create a void inside the panel, if not at least a piece of tape in there for my NDT device to pick up, but no, everything sort of melted together again and there is absolutely no void, no tape, na-da.
Anyone know a effective way of creating an artificial voids?
My NDT device is an infrared camera, FYI.
Panel must be made of prepregs as well.
Thank you all.
1)I've tried curing small sections of a prepreg sheet by sandwiching it between two aluminum sheets with cut-out sections, and heating it in the radiation light source (so that the cut-out section will be cured, but the other parts won't since the UV will reflect off the aluminum sheets). I successfully cured the small sections of the prepreg sheet, then I laid this sheet up in the middle of a regular prepreg layup and cured the entire thing (with vacuum) in the oven hoping that would create a void somehow: but no, every layers sort of cured together, I couldn't even see the pre-cured layer.
2)This time I took 2 pieces of a small cured prepreg parts and stacked them together. I then sealed all four edges with a sealant tape and inserted this "pocket" of cured prepreg in between my layup of 14 prepreg plies, then cured the entire thing. I was so sure that this would create a void inside the panel, if not at least a piece of tape in there for my NDT device to pick up, but no, everything sort of melted together again and there is absolutely no void, no tape, na-da.
Anyone know a effective way of creating an artificial voids?
My NDT device is an infrared camera, FYI.
Panel must be made of prepregs as well.
Thank you all.





RE: How to create voids PURPOSELY to test for NDT
-Kirby
Kirby Wilkerson
Remember, first define the problem, then solve it.
RE: How to create voids PURPOSELY to test for NDT
We get up to about 250 F or so.
RE: How to create voids PURPOSELY to test for NDT
What's the materials's spec.?
RE: How to create voids PURPOSELY to test for NDT
What you are attempting has been tried by many people before you. It is not easy to do with any degree of control. There are many different forms of porosity as well as degrees. The best approach is to get samples of rejected parts. You could also try fiberglass prepreg so that you can actually see the voids you are trying to detect by some other method.
RE: How to create voids PURPOSELY to test for NDT
Going to try what Kirby suggested with the two layers of non-perforated FEP.
Thanks ya'll
RE: How to create voids PURPOSELY to test for NDT
B.E.
RE: How to create voids PURPOSELY to test for NDT
You can buy PTFE particles from DuPont or cheaper from Shamrock.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: How to create voids PURPOSELY to test for NDT
It looks like most of the suggestions (from people who know more about composites then I do) are to get porosity. My suggestion was to just get a delaminated area. The mold release might work as well. How are you testing for voids? sonic tap testing (i.e. tap hammer) or an ultrasonic detector? With the right equipment you should be able to tell which layer is disbonded.
-Kirby
Kirby Wilkerson
Remember, first define the problem, then solve it.
RE: How to create voids PURPOSELY to test for NDT
We are aiming for the delamination to be exact, not so much the porosity.
But who knows, with application of enough mold release, it could cause a delam along with the porosity.
And Kirby, our focused testing method currently is using the Infrared camera. There would be light source behind the specimen and we would look at the camera from the front.
Hopefully it is accurate and sophisticate enough to detect a small delaminations.
RE: How to create voids PURPOSELY to test for NDT
LOL
RE: How to create voids PURPOSELY to test for NDT
You're kool, not a tool. Sorry for the error, I'm just a fool.
-Kirby
Kirby Wilkerson
Remember, first define the problem, then solve it.
RE: How to create voids PURPOSELY to test for NDT
The mold release deal should work well.
It might be hard to get measurable voids in a small sample, easy to evacuate. You may need to flex it a little to get your intranal structure to "stop touching". Maybe a thin sheet of Teflon plastic, again, it would be able to be well defined.
What about sliding a needle between and intentionally injecting an air pocket in the sandwich before cure...like making bismarks?
My thoughts anyway. Good luck!
RE: How to create voids PURPOSELY to test for NDT
Steven Fahey, CET
RE: How to create voids PURPOSELY to test for NDT
RE: How to create voids PURPOSELY to test for NDT
Did you use just a single layer? I would think even teflon would have some bonding with the resin on both sides which is why I was thinking two layers would be needed. I do understand that for thermal through transmission that may not make a difference, but for sonic or ultrasonic I think it might.
-Kirby
Kirby Wilkerson
Remember, first define the problem, then solve it.
RE: How to create voids PURPOSELY to test for NDT
The teflon was plainly obvious in the fiberglass part, as you'd expect. It was easily detectable by coin-tapping, too. We didn't bother getting out an ultrasonic scope because the plane requires very little ultrasonic anyway. The load test procedure was so elaborate that I was too busy to spend any more time thinking about the flaw inclusion than that.
Steven Fahey, CET
RE: How to create voids PURPOSELY to test for NDT
We successfully produced voids in our test specimens by stamping out 1/8, 1/4 and 1/2 inch circles (defects) of Grafoil (flexible graphite) and placed them at various depths within the laminate. We used both prepreg fabric, unidirectional tape and a combination of both for our tests, cured using our normal production run. It worked like a champ.
Mike
RE: How to create voids PURPOSELY to test for NDT
I tried both Teflon tape and Release tape (wrapped a cured prepreg sheet).
The two panels came out, I couldn't see the defect using my IR camera, so we cut the panel to check out the cross-section.
So void/delam.
Frustration is amounting.
Post from Radagast - regarding Grafoil, would be my next attempt, but going to step aside with this for a bit.
RE: How to create voids PURPOSELY to test for NDT
By putting in a piece of steel or plastic would do the same trick IF it's just picking up a foreign object, no?
RE: How to create voids PURPOSELY to test for NDT
Definitely created a dis-bond, just didn't notice until I cut out the piece.
So in a summary, a pre-cured prepreg sheet wrapped in a Teflon tape or blue Flash tape worked.
Too bad that our IR camera doesn't seem to pick up the disbond however...
Anybody worked with Infrared NDT?
RE: How to create voids PURPOSELY to test for NDT
Again, I am out of my expertise, but when it comes to NDI, the methods are not perfect. If there is not a physical separation between the parts, I don't believe you can easily detect the delamination.
Now, here is the problem, just because there is a delamination, does not mean there is a separation. It is very possible to have "kissing" surfaces actually making contact with each other, though the have no ability to resist shear deformation.
Perhaps that will help to understand the situation.
Brian
www.espcomposites.com
RE: How to create voids PURPOSELY to test for NDT
Have you tried ultrasound instead of IR. I think you may have a better chance of picking up the back face reflection of a dis bond that is close. Especially if you scan at a tangent.
B.E.
RE: How to create voids PURPOSELY to test for NDT
The defects were seen as voids/delaminations during scan. We have another set of test panels for FOD etc.
Mike