How much pressure does driving a rivet exert on its hole? 2

[KirbyWan]

So I'm looking at repairing a small access door using a doubler, but the door is plastic and I was wondering if the pressure from driving or squeezing a rivet might cause the door to crack. The door material is .150 thick epoxy sheet molding compoud per BMS8-327 Type 1 which indicates a design material strength of 30 ksi. The material spec gives a strenght of 35 ksi (Boeing being more conservative of course). Is there a reference I can use to calculate the rivet force on the internal face of its hole from being installed?

I suppose this is not a problem with metal materials becuase they are ductile and expand as needed when the rivet overcomes their yield strength, but on a brittle plastic material one pound of force over F(tu) and *POP* ... toss it in the trash and get a new one. And yes I have thought about a fiberglass layup repair, but it's just not optimal for the location of this crack repair.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[SWComposites]

If the customer won't accept 'This access door does not carry structural loads and no analysis is required.', then tell them to buy a new part. Sheesh.

SW

 

[rb1957]

is there any warrantee from Boeing ?

 

[KirbyWan]

SWC: Yeah well customers are picky and we still have to try and please them. This job would be great if it wasn't for the customers.

RB: No warrantee that I know of, but that's not my department.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

actually SW brings up a good point ... would failure of this cover compromise the structural integrity of the plane ? (obviously not) would you ground the plane because of the damage ? maybe order a replacement and inspect every so often to see if the crack reinitiates from the other side of the slot (and to check that the panel is still attached to the plane).

 

[kontiki99]

Sometimes it's just best to go with a nut and screw. A lot of time plastic parts need to be free to float a bit.

 

[madvlad]

If at all possible...throw the plastic in the garbage, install the hinge and latch on a 0.15" sheet of 2024, pass by comparison to original equipment, done.

Does it really matter if it adds half a pound?

 

[utspg1980]

This is a bit late, but I just wanted to add something:

Using MMPDS (or any other generic tables for joint strength) would not give you an accurate value for this repair.

The reason is related to what you originally asked about actually. In testing samples, 2 pieces of the same material are used (assuming single shear). When they install the rivet, it physically expands the same amount in each test sample, so that the rivet retains an overall straight, smooth shaft.

When a rivet is installed in 2 materials with very different compressive strengths, it will expand in the softer material more. Thus, there will be a little ridge, or crease, in the rivet where the 2 materials meet. This discontinuity in the rivet will cause it to have substantially weaker joint strength, and the rivet will shear off at that ridge much sooner than it otherwise would.

As such, common repair practices are to use steel fasteners when dealing with highly dissimilar materials.

 

[KirbyWan]

Excellent points utspg1980. Generally when I am doing a structural repair I'm coming back with the same material, perhaps on gauge thicker and fasteners. My original idea for repairing this was wrong-headed. I eventually swithced it to a scarf joint wet layup repair and it came out nicely.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[kellnerp]

What type of fatigue is this? Is it driven by tensile stresses or shear stresses?

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