×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Rivet Shear Strength Question from A&P Course
2

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

RE: Rivet Shear Strength Question from A&P Course

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.

RE: Rivet Shear Strength Question from A&P Course

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.

RE: Rivet Shear Strength Question from A&P Course

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, ...)

RE: Rivet Shear Strength Question from A&P Course

(OP)
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.



  

RE: Rivet Shear Strength Question from A&P Course

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.

RE: Rivet Shear Strength Question from A&P Course

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).

RE: Rivet Shear Strength Question from A&P Course

(OP)
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?"

RE: Rivet Shear Strength Question from A&P Course

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).

RE: Rivet Shear Strength Question from A&P Course

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.
 

RE: Rivet Shear Strength Question from A&P Course

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources