tensile stress in bolt
tensile stress in bolt
(OP)
I am using Austenitic SS 304 annealed bolts (3/8"-16) and hypothetically say i am torquing it to 290 in-lb. using the following relationships i determine the tensile stress developed in the bolt...
F = T/[KxD] = (290)/(0.2x0.375) = 3875 lb
P = F/A = (3875)/(0.0775) = 50,000 psi
where
F is force in pounds
T is torque in inch pounds
D is nom diameter of bolt
P is stress
A is tensile stress area
I get a stress of 50,000 psi and I am a little confused on how to use this value. the yield strength for this bolt is 30ksi while the tensile strength is 75ksi. For such an application would i use the yield strength or tensile strength?
F = T/[KxD] = (290)/(0.2x0.375) = 3875 lb
P = F/A = (3875)/(0.0775) = 50,000 psi
where
F is force in pounds
T is torque in inch pounds
D is nom diameter of bolt
P is stress
A is tensile stress area
I get a stress of 50,000 psi and I am a little confused on how to use this value. the yield strength for this bolt is 30ksi while the tensile strength is 75ksi. For such an application would i use the yield strength or tensile strength?





RE: tensile stress in bolt
I would advise you use the yield stress and not the tensile stress.
Depending on application, bolts are torqued to either 70% or 90% of the proof stress or yield stress.
However all joints should be analyised based on external loads and the strength of materials being clamped together.
Look at this site;-
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desertfox
RE: tensile stress in bolt
You should also know that the correlation between torque and bolt stress is very inexact. This has been discussed many times, a site search will no doubt provide you further reading.
Regards,
Mike
RE: tensile stress in bolt
Stephen Seymour, PE
Seymour Engineering & Consulting Group
www.seymourecg.com
RE: tensile stress in bolt
Tobalcane
"If you avoid failure, you also avoid success."
RE: tensile stress in bolt
normally limit preload to 70% of yield, then compare this preload to the maximum limit in-service load (preload should be higher, to prevent the joint gapping).
then check the ultimate condition, with the ultimate load only (as the joint wil have gapped and all the load is in the bolt)
RE: tensile stress in bolt
RE: tensile stress in bolt
In simple terms, all joint load will be lost well before the bolt breaks.
Regards,
Mike
RE: tensile stress in bolt
like SnT says ... exceed yield strength and the bolt will yield, exceed ultimate strength and you have two pieces of bolt.
do you think you want to yield the bolt from preload ?
RE: tensile stress in bolt
Now you're scaring me. I'm not trying to be crass, but if you don't know or understand Failure Theories (did not you take this back in college?), I suggest hiring a Mechanical Engineer.
Tobalcane
"If you avoid failure, you also avoid success."
RE: tensile stress in bolt
Some say 50% or 55% of Minimum Tensile Ultimate while others use 75% of Minimum Tensile Yield and so on.
Hence the ASTM/SAE Grade designation lists Ultimate, Yield and Proof strengths.
Other factors like using a washer or not, the friction of the bolt versus the clamped parts, the strength of the clamped material, etc also play a role. If you are designing to a spec or code most of these decisions have already been assumed for you and the math is just a rote exercise or chart lookup.
RE: tensile stress in bolt
RE: tensile stress in bolt
My meaning is that some code books may list the proper amount of pre-load in terms of ultimate strength of the bolt rather than in terms of yield strength. One may say "use 35% of ultimate" and another may say "75% of yield". In effect they are saying the same basic thing since .35*Fu is about the same as .75*Fy with common steel fasteners. Machinery's Handbook explains this better on their section on preloading bolts.
Maybe the OP didn't understand the definition of Ultimate vs Yield but I didn't read it that way. I thought his question was more of "why doesn't everyone use the same point of reference?".
RE: tensile stress in bolt
RE: tensile stress in bolt
For a non gasketed joint , leakage is not the criteria. Although exceeding the yield stress will loosen the joint , it is good to know when it will come apart(joint failure), if you compare the bolt stress with ultimate tensile strength.
I would however use only yield stress criterion for a bolted joint design