Dye penetrants for inpsection of composites

[blakmax]

Does anyone have information on the effectiveness/risks of using dye penetrants for inspecting composites for damage such as impact damage?

My thoughts are that firstly if it is BVID there may not be sufficient surface damage (compared to sub-surface delaminations) to make the process viable, and conversely if there is sufficient surface damage, then there is a risk of the penetrant migrating into the damage, thus rendering repair efforts futile because of contamination of surfaces which would probably be bonded during repair.

 

[berkshire]

blakmax
I have used vegetable dye and detergent to penetrate damaged composites. In particular for the detection of crazing in Gel Coat going into the sub surface, also for detection of cracks in carbon structure.
The technique has been to remove all of the visible dye by grinding to trace out the damage. Then flush off the surfaces with water to remove any traces of detergent left by the grinding operation, followed by denatured alcohol then acetone. Dry , then regrind with clean discs.
My feeling has been ,if you can still see the dye then that material needs to come out and be replaced.
Some people use Spotcheck dye penetrant, but I am not sure you can ever get all of that stuff out and it is an oil.

I have a question about your comment about the penetrant migrating into the damage, why would you leave a damaged section in, unless you were doing a scab patch over the top?
B.E.

 

[blakmax]

Thanks BE.

My personal approach would be to always remove the damage, but there are people out there who would want to perform an injection repair for delaminations. Many aircraft repair manuals do exactly that.

Another concern I have is that the size of delamination damage is always significantly greater than the size of the impact site. There is a risk that not all damage would be addressed without removing material. I like your comment about removing damage until there is no evidence of penetrant visible.

Regards

blakmax

 

[berkshire]

blakmax,
I would not do a dye penetrant inspection for an inject-able delamination. Unless you intend to grind out, in which case you do not need to inject.
Injecting a peeling failure on a solid layup involves the risk that you will not always get the resin to the start of the fracture, leaving a weak spot in the structure.
A core to skin failure is a good candidate for injection provided the core is not honeycomb in which case you would need a syntactic resin to avoid weight buildup.The other heartburn I have had with honeycomb is a golf ball sized impact requiring a 2 square foot repair. Try injecting that. Foam cores are much more damage tolerant.
B.E.

 

[blakmax]

Thanks again BE, but I would never do an injection repair for a disbond, any disbond. I have seen some evidence that injection repairs for impact damage in composites may (or may not) provide limited benefit. I balance that data against some early 1980's test data which showed injection repairs to impact damage in 24 ply composites provided a compression fatigue life which was one third of the life for an unrepaired impact damage. One manufacturer (same one we discussed elsewhere with the wing root bonding design) acctually admits that the best an injection repair can do is to slow down delamination progression, not prevent it.

The problem with injection for disbonds is that to get an effective adhesive bond requires that the surface is chemically active, and the inner surface of the adhesive is fully cross-linked and is not chemically active. I have many examples of cases where injected adhesive did nothing other than replicated the surface of the adhesive and disbonded core, and "hide" the defect so NDI could not find it.

Thanks for the comment on removing the damage. This reinforces my assessment of the inspection process.

Regards

blakmax

 

[Demon3]

If you chose a dye that is soluble in the resin you inject to do the repair then it should be OK and not reduce adhesion.

If the dye is insoluble it will sit at the damaged surface creating a weak boundary layer and prevent adhesion.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[ESPcomposites]

Some other thoughts:

If the scenario is only a few layers of delamination, what about using field fasteners to button up the delamination? This should prevent damage propagation and detrimental sublaminate buckling. Injecting with a resin can also used in conjunction, but as mentioned, you can't rely on it alone for strength.

In tension, you still have significant residual strength after delamination. I am not a big fan of removing structure if a minimally invasive repair can be done. The problem is that once you remove significant amount of original material, a failed repair is a much bigger problem than what you started out with.

If the delamination occurs at many layers through the thickness, then this may require a different approach. You could cut it out and repair, but you can also bolt a doubler over it.

Brian

http://www.espcomposites.com