Environmental Degredation

[ICbrokethings]

Not knowing an awful lot about composite engineering, would a visual inspection pick up environmental damage of a composite aircraft frame? i.e. damage caused by UV, humidity etc, or would a more invasive technique such as ultrasound required? I'm thinking of 6 to 12 year inspections, not a couple of weeks!

Thanks in advance

 

[SWComposites]

In general no, visual inspection will not find all potential damage. It can find surface evident damage, such as scratches, cracks, impact damage that leaves a surface dent, UV degradation, etc. UV degradation typically only occurs if paint is missing and the composite resin is exposed; the paint is missing and UV damage occurs, but the part is then painted, then the damage will no longer be visible. Visual inspection will not find damage internal to the structure, such as delaminations, disbonds, fiber damage, etc.

 

[gwolf2]

One of the big dangers with composite is that a very large delamination area can hide behind an apparently small impact mark. You will only be able to find out with a nondestructive inspection method like ultrasound or some of the newer thermal techniques.

 

[Compositepro]

Resins degraded by UV or moisture become brittle and weak. I know of no way to detect this except by destructive testing or by detecting damage that has already occurred.

With carbon fiber the degradation is usually only on the surface and that can be visible without there being any problem below the surface. Fiberglass will often degrade though its thickness if not protected.

 

[RPstress]

Composite is often painted or at least primed. Inpsection is for dents, not generally for non-mechanical degradation.

If there is bare composite exposed to direct sunlight you might see surface crazing of the resin. Even if it happens this is generally not too deleterious to overall laminate integrity. (Someone may have a horror-story to correct me, here.)

 

[PanelGuy]

I agree with what is here. It is the downside of any material is that it is easy to miss the defects inside. As to evironmental degradation effects on aircraft? I would agree the only non-destructive method would be an ultrasound.

I did a repair on a sailboat rudder (4 ft long). The onwer saw a small crack on the edge and wanted it fixed. Turned out that as I ground back the edge a little that the laminate was split in plane, I took a little bit more and found that the Balsa core was missing, more grinding found where it rotted away...I rebuilt the lower 30% out of foam and fresh glass. In short, years of leaking and freezing had significantly compromised the laminate due to a wear point in a mounting bolt. The crack was just the last seasons freeze manifesting the problem.

 

[berkshire]

Visual inspections will only pick up those defects that are evident on the surface as SW says. Even then failures such as a compression shake may be dismissed as a surface scratch by persons without enough experience in viewing such failures.
On the other hand Gel-coat cracks caused by wing flexing may be interpreted as major damage, when in fact they are only superficial.This whole area is highly subjective and depends entirely on the experience of the person doing the inspecting.
Ultrasound, thermal, and dye penetrant methods take some of the uncertainty out of inspections of composites but not all. For a major inspection like a 10 year C of A, I would use all you can get.
Some of the first type certified all glass fibre and resin aircraft such as the Windecker Eagle are now pushing 40 years of age.
The first generation of glass fibre sailplanes are now over 44 years of age. Check the service difficulty reports for these aircraft to get an idea of how these structures are faring. Kevlar and carbon fiber laminates of course are newer, but a data base is being established of how they perform. I realise that you are working on military aircraft with tighter parameters, but he same ground rules apply.
B.E.