Understanding the Definition of Yield

Thursday, May 3rd 2012

Most people in the bolting industry have a misconception of yield as far as studs are concerned. From what we have heard in the field, one normally thinks that “100% yield” is where the stud breaks. This is false. 100% yield is a reference to when a stud passes the “plastic” region and enters the elastic region of its load bearing capabilities. It is also where the stud will not come back to its original length (by 0.2%). Therefore if the stud is elongated past number 3 (as seen below) Hooke’s Law does not apply.

Bolt Yield Graph

What does this mean to the assembler? This basically states that just knock the wrench harder is not always the best way to assemble a flange. If you take the stud past 100% yield, the amount of load will not be directly proportionate to the length, and you don’t know how much load you have on the stud. Even though you won’t see failure in the stud, your joint can easily leak from not only higher bolt scatter but also you risk having complete failure in the stud. Most pressure vessels are very dynamic joints, and with temperature & pressure changes you can easily break the stud, if you take it to yield.