yeild strength of bolts (measured and in the spec)
yeild strength of bolts (measured and in the spec)
(OP)
dear friends:
need your help. we ran some tensile tests (fasterner held between two grips on an instron) and pulled it to failure. we reviewed the resultant data (stress calculated as instanaeous load/orggonal reduced thread area) and strain (cross head displacement divided by displcament of the cross head). we then calcuated the 0.2% yield strength. this number (yield?) is approxomately 2X greater than the 35-40 ksi published in the fastener specification. however the ultimate load is only 10% higher than the published ultimate.
Design folks want to know what gives: we may have preloaded the bolts to beyond yield if the spec data is to be believed.
Hint: the threads on the bolts during the test were not lubrictaed. Is it our test method, or is it something more fundamental in nature. we have some theories but we need your help. What am I missing
need your help. we ran some tensile tests (fasterner held between two grips on an instron) and pulled it to failure. we reviewed the resultant data (stress calculated as instanaeous load/orggonal reduced thread area) and strain (cross head displacement divided by displcament of the cross head). we then calcuated the 0.2% yield strength. this number (yield?) is approxomately 2X greater than the 35-40 ksi published in the fastener specification. however the ultimate load is only 10% higher than the published ultimate.
Design folks want to know what gives: we may have preloaded the bolts to beyond yield if the spec data is to be believed.
Hint: the threads on the bolts during the test were not lubrictaed. Is it our test method, or is it something more fundamental in nature. we have some theories but we need your help. What am I missing





RE: yeild strength of bolts (measured and in the spec)
Note that just preloading beyond yield isn't all bad; just don't re-use them.
RE: yeild strength of bolts (measured and in the spec)
If you are comparing direct axial tensile yield to what clamp load you expect to see at yield during installation, dont forget the torsion stress. You are in a combined stress state during tightening of a bolt. That will account for roughly 15% of the yield stress.
(If you need more, please elaborate on your question)
RE: yeild strength of bolts (measured and in the spec)
Are you speaking of proof load of the
bolt?
It is normal to see three values ie
ultimate tensile strength
ultimate yield strength
proof load.
It is common to allow a 2 times safety
factor in bolt loading which might explain
the 2 times value that you are getting.
RE: yeild strength of bolts (measured and in the spec)
From what you explained, it appears you have not pre loaded. But did you follow ASTM A 370 method of testing. Also have a close look at the material spec and the super scripts along the yield column of the material spec and the foot notes.
Now to testing: In case of fasteners testing, it always prudent to use extensomenter to get the 0.2% yied or 0.5% yield (Some spec calls for 0.2% and some other considers 0.5%). As regards breaking load and the breaking strength it is simple. Break Load divided by the initial area.
Now to 0.2 % yield. To get the yield by drop of beam method is not correct. To get it from stress strain graph (With out using extensomenter) does give some error, but the error will not be very great, the effect will be marginal and in case of failures of the fasteners (To meet the spec) you can hair split and argue but not in your case.
If the fastener is not tempered properly or if not tempered at all. Consider Double tempering test (Normally done on some grades and spec and mant times in case of Nuts!!).
Here you take the hardness of the fastener (Which has doubtful heat treatment background) and subject it to tempering, then take hardness again, if you find the variation (How much is tolerable? check the spec) between the first reading and the next reading then you have a fastener which was not tempered (Or was cold worked after tempering). thsi will work for low aloy and CS fastners.
For further probe you may even conside Micro (Check for Untempered martensite), assuming you are dealing with low alloy fastener.
Good Luck
B.Moorthy
moorthykar@yahoo.com
RE: yeild strength of bolts (measured and in the spec)