yeild strength of bolts (measured and in the spec) 1

[PGODAVARTI]

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

 

[Metalguy]

If I'm reading this right, it appears that you have stronger fasteners than you expected, so how/why were they "preloaded beyong yield" in your assembly?

Note that just preloading beyond yield isn't all bad; just don't re-use them.

 

[nashjp]

As Metalguy stated, most bolt strength specs only provide minimum strength numbers, a hardness range, and maybe a loose %content list. It is common to recieve fasteners at the high end of the hardness range, meaning they will have significantly better yield performance than the ones at the minimum.

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)

 

[diamondjim]

Where did the ultimate load come from.
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.

 

[bmoorthy]

A fastener manufacturer generally does not manufacture fastener or choose/stock raw material to siut only one specific grade. For example if you look at A 193 B 7, A fastener manufacturer would use the same rod/Raw material for multiple grades. So that the product range is widened. So a given fastener may be in the high end or in the low end of the range. So long as the values meets the specification then you may rest in peace, unless you have specific requirement limiting the Yield to a maximum limit.

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

 

[vonlueke]

PGODAVARTI: (1) Was your bolt preloaded due to an installation torque? (2) What do you mean by "reduced" in your phrase "original reduced thread area"? I think a bolt specification refers to the initial tensile stress area. Did you use tensile stress area in your stress calculation? (3) What do you mean by "cross head displacement divided by displacement of the cross head"? Please explain. And what do you mean by "cross head"?