Rivet Shear Strength Question from A&P Course 2

[johnSchwaner]

Hello,
While studying sheet metal repairs in the Aviation Maintenance Airframe Study Series I found the following statement that I question?

"The rivet size and material must be chosen so that the shear strength of the rivet is slightly less than the bearing strength of the sheets of material being joined. This allows the joint to fail by the rivets shearing rather than the metal shearing."

Is this true? Is this the strength criteria that engineering approves recommends (endorses)? So stronger isn't always better?

Thankyou
John Schwaner

http://www.mechanicsupport.com

 

[berkshire]

johnSchwaner (Aerospace)
Most of the rivet spacing formulas promoted by the FAA in AC 43 13 ,are for a rivet allowable shear stress of 40 to 50 % of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160% of the sheet allowable tensile stress.
B.E.

 

[40818]

It would have to depend upon the criteria that is set for the item.
However, if you have the rivet shear strength less than the joint bearing allowable, then the rivets will fail first, and if one rivet fails, then the others can follow in a domino effect. The basic theory behind a bolt group analysis, is that all the bearing faces are in a semi-yielded state where they are all working in union. This works good for rivets as they "fill" the holes, but for bolts that arnt interference fit, some bolts dont take up the slack. Now if in this case you have the bolt with a lower shear allowable than the bearing joint allowable, the bolt will fail before the rest of the fasteners can carry load. So having a bearing allowable greater then the shear allowable is the preferred option.
So the Aviation Maintenance Airframe Study Series is unfortunately wrong.
There are times when it is desireable that the rivet or bolt will fail before the sheet, though not for general purpose installations.

 

[rb1957]

the joint is going to fail one way or another. this advice is for shear critical joints, which i think is unusual ... where i am they prefer bearing crtical ones.

furthe, assuming that the maintenance study is primarily concerned with "maintenance" rather than "design", i'd've thought the key in maintenance would be that the replacement rivet should be equivalent to the original one ... size, material, head, solid/blind, ... and the installation should mimic the OEMs (wet install, ...)

 

[johnSchwaner]

Thankyou for your informative replies. I think this is interesting information for mechanics doing repair work. Sometimes you just can't duplicate the original construction. Especially on 50 year old aircraft.

It gets me thinking about the whole joint rather than just the rivet that needs replacing. Am I correct to assume, if you are replacing a rivet, or a few rivets in a joint, that they should be equilivant? Anything stronger or weaker will create uneven loading among all the rivets. Such as a weaker rivet deforming and causing the remaining rivets to carry its share of the load. Or, a stronger rivet taking more of the load than the rest and failing.

 

[MintJulep]

Can you say vast over-simplification?

Presumably the original rivet selection was made by the designer taking into account the nature of the joint, and some known or assumed load.

Selecting a replacement by a rule-of-thumb doesn't seem like the best idea.

 

[rb1957]

i think they should be as near as possible to the original type. Sure you may not get exactly the same, repairing a 50 year old plane (you gotta love those DC3s ...) but i wouldn't worry too much about using a tronger rivet ... sure it may be stiffer than the surrounding 50 year old ones (either by spec or by practical in situ values) which would mean that it'd attact more load than the original ones, which would "only" mean it would yield slightly sooner than the original, and off-load to the adjacent ones. i don't think that you would have singificantly changed the joint strength (upwards or downwards).

 

[johnSchwaner]

rule-of-thumb appeals to the instant gratification element. I also hear quite often "they're all the same" Whenever I hear this I get cautious.

Thankyou for these nuggets of information. I am going to email the author this url. As has been elegantly stated above "vast over-simplification?"

 

[FEMdude]

I'd in most cases prefer bearing failure of the sheet over shear failure of the rivet. Rivet failure releases an amount of elastic energy that can at worst, when compared with increased load for the remaining fasteners, cause catastrophic failure (especially in composite material).

 

[berkshire]

From a repair point of view, shear failure of the rivets is easier to handle than failure of the skin. The skin is left relatively undamaged.
When the sheet has failed you are often required to replace complete sheets or do extensive patching.

With composite panels you very rarely see fastener failure, the edge pulls off the panel as the hole breaks out ( bearing critical failure).
These are observations from the field, and bear in mind, I work on part 23 aircraft not part 25.
B.E.

 

[thruthefence]

The way it was explained to me, ( back in the Pliesticene ) was that inspection programs 'SHOULD' show rivits failing before the structure gives way. ie "smoking" rivits, cocked heads, etc.