Rivet Shear Strength Question from A&P Course
Rivet Shear Strength Question from A&P Course
(OP)
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
www.mechanicsupport.com
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
www.mechanicsupport.com





RE: Rivet Shear Strength Question from A&P Course
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.
RE: Rivet Shear Strength Question from A&P Course
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.
RE: Rivet Shear Strength Question from A&P Course
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, ...)
RE: Rivet Shear Strength Question from A&P Course
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.
RE: Rivet Shear Strength Question from A&P Course
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.
RE: Rivet Shear Strength Question from A&P Course
RE: Rivet Shear Strength Question from A&P Course
Thankyou for these nuggets of information. I am going to email the author this url. As has been elegantly stated above "vast over-simplification?"
RE: Rivet Shear Strength Question from A&P Course
RE: Rivet Shear Strength Question from A&P Course
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.
RE: Rivet Shear Strength Question from A&P Course