Honeycomb Sandwich inspection 3

[Worldtraveller]

I'm helping write/review the process specs for some of our repairs, particularly with regard to composite (solid laminate and honeycomb).

We have a very old spec that just says, "Inspect for moisture and corrosion." <--- That's not vague at all. It doesn't call out a process or process spec at all (this was probably written in the 70s, or late 60s). This is specifically for inspecting metallic face sheets and core, but could just as easily apply to non-metallic composites.

What method is most reliable for detecting otherwise non visible corrosion, or moisture in a metallic honeycomb structure? I think eddy current (HFEC) would work if corrosion is present on the interior face of the face sheet(s), but I don't know about finding moisture.

Trying to find some non-proprietary specs/processes now as well to use as a possible model/reference.

Thanks.

 

[Compositepro]

Infrared inspection works pretty well for water-logged honeycomb. The thermal mass of water-filled core is significantly higher than normal core, so when you apply some heat and look at surface temperature the water filled areas will be cooler. Delaminated core can be seen with a similar technique where heat is momentarily applied and then a laser interferometer visualizes the slight displacement of the skin.

 

[blakmax]

You should also be aware of other defects caused by water. The core may disbond from the adhesive fillets and this can cause a severe loss of flatwise tension strength. See Link
This type of defect is difficult to detect because there is sufficient contact to pass UT signals even though the bond is defective. I am aware of at least 20 rudder failures on military aircraft because of this defect.

The second type of defect is also discussed in that paper. This is where the core node bonds disbond at the nodes. This results in a loss of shear strength for the core. It also is hard to detect externally. I have observed a slight separation of the core foils using X-ray but large area scanning using X-ray would be very expensive. Be aware of these defects and if you do find water in sandwich structure then look adjacent to the water for these type of defects.

And please- if you find water do not simply drill vent holes and dry the core. The water got in there through some defect. Simply drying it will get rid of the water but it will soon be replaced once the panel goes back into service.

Regards

Blakmax

 

[berkshire]

In addition to Max's comments beware of moisture intrusion in northern climates or aircraft that fly with wet honeycomb. when the water freezes it tends to dis-bond the honeycomb, or stretch the core before it ruptures.
B.E.

You are judged not by what you know, but by what you can do.

 

[SWComposites]

WT - have you looked at the documents written by the SAE CACRC committee?

 

[Worldtraveller]

SWComposites, I have some references, and since I work for an company that is a .edu, I have access to a lot of research and some industry studies. I've now got 4-5 papers that I'm going to start going through. Any additional references are welcome though. I've got a fair bit of industry experience with repairs, and second hand experience with inspection, but I want to learn a little more before I start re-writing the specifications and processes.

Cheers.

 

[SWComposites]

https://www.sae.org/works/documentHome.do?comtID=TEAAMSCACRC

 

[WKTaylor]

WorldTraveler... for grins...

A few additional references that are extremely useful for composite laminate and honeycomb inspections are as follows.

AFWAL-TR-86-4033 Failure Analysis of Composite Structure Materials

AFWAL-TR-86-4137 Compendium of Post-Failure Analysis Techniques for Composite Materials

AFWAL-TR-87-4117 Nondestructive Evaluation of Large Scale Composite Components

WL-TR-91-4032 [V-I] Composite Failure Analysis Handbook – Program Overview
WL-TR-91-4032 [V-II-Part 1] Composite Failure Analysis Handbook - Procedures and Techniques
WL-TR-91-4032 [V-II-Part 2] Composite Failure Analysis Handbook - Atlas of Fractographs
WL-TR-91-4032 [V-II-Part 3] Composite Failure Analysis Handbook – Case Histories
WL-TR-93-4004 Composite Failure Analysis Handbook – Update 1

DOT/FAA/AR-08-51 Structural Testing and Analysis of Honeycomb Sandwich Composite Fuselage Panels

DOT/FAA/AR-08-54 Guidelines for the Development of a Critical Composite Maintenance and Repair Issues Awareness Course

MIL-HDBK-337 [old, partially current] Adhesive Bonded Aerospace Structure Repair

MIL-HDBK-731 Nondestructive Testing Methods of Composite Materials - Thermography

MIL-HDBK-732 Nondestructive Testing Methods of Composite Materials Acoustic Emission

MIL-HDBK-733 Nondestructive Testing Methods of Composite Materials - Radiography

MIL-HDBK-787 Nondestructive Testing Methods of Composite Materials-Ultrasonic

MIL-HDBK-793 Nondestructive Testing Techniques for Structural Composites

AND, IF available...

NAVAIR 01-1A-21 Organizational and Intermediate Maintenance - General Composite Repair

USAF T.O. 1-1-690 General Advanced Composite Repair Processes Manual

USAF T.O. 33B-1-1 Nondestructive Inspection - Methods, Basic Theory

USAF T.O. 33B-1-2 Nondestructive Inspection - General Procedures and Process Controls


Regards, Wil Taylor

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