Aluminum local delamination versus rivet bearing strength

[trainguy]

Guys,

Imagine a riveted connection without much clamp (such as when using Huck BOM fasteners)in an application where one of the members (7004 aluminum, thickness 5/8) is connected to another member (6061-T6 thickness 3/16). The governing load case is a proof load case resulting from a train-to train impact. Rivet is a cadmium plated steel fastener (BOM) installed 35 yrs ago.

Now, imagine the thicker member has undergone local delamination (around the fastener in an approx. 2 inch diameter circle), at approx. 1/8 inch of depth below the shear plane.

The delamination has been established through macroscopic testing, and the specimen looks quite sound otherwise.

1) Does this affect the rivet's bearing strength (calculated by D * t * Fbru)?
2) Is there a reference for using a reduced bearing strength considering this damage?
3) Any thoughts on how I go about stopping the spread of delamination?

tg

 

[Ron]

trainguy....can you post a photo?

 

[boo1]

Minimally the reduction in bearing capacity is linear to the reduction in area.
This is not uncommon in aircraft structures and older marine aluminum structures. Cause probably is from galvanic action, or a base metal flaw. My guess is the galvanic cell.

Try Proseal sealant the joint and fastener.

 

[trainguy]

boo1,

Is this reduced bearing capacity documented in any aircraft structural repair manual or standard? If so, I suppose I can add a few fasteners in nearby sound metal, or is that inviting the possibility for more widespread delamination?

What I find difficult to grasp is that the delamination is near the shear plane, not the far side, so any slight rotation of the fastener has it bearing mostly on the "loose" material.

tg

 

[boo1]

We used engineering judgment, estimating the sound metal remaining.

 

[trainguy]

Does this de-lamination act as a floating shim, potentially making the fastener work in bending?

tg