Fatigue life 3

[paramathma]

Fatigue life

I need to find the life of a bolt (no of cycles)

I have a bolt, which is subjected to fluctuating Tensile load of 0 to 2000 pounds.
The minimum cross section of the bolt i.e. at the root diameter is .095”.

I then calculated the max stress =max stress/Area, and found the max stress.
The stress ratio is 0.
Can we find the bolt life using the SN curve and the max stress?

I have a simple question
Normally our stress group only provides us the max stress.
They apply the load directly and finds the max stress.
In this the fluctuating load does not comes in to picture.
Then how can we use the stress obtained from the FEA to find the life (component) using the S-N curve?

Please advice

 

[tmacm]

What preload is applied to the bolt? How is the load applied? Describe the joint.

If the preload is over 2000 pounds, then the bolt will not see any fluctuating load.

 

[paramathma]

the bolt is just hand tighted.
the bolt application is different to what we usually encounter.
the bolt is used compress an elastomer.

 

[GregLocock]

"The stress ratio is 0. "

How do you figure that out?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

what material is the bolt ? ... MMPDS-01 should have a s/n curve for you. pick the highest Kt, 3 maybe 5 ... look up Petersen if you want something more definite.

but i wouldn't bother ... 2000 lbs on a 3/32" bolt is a stress of 282ksi ... the answer is "zero fatigue life"

 

[rb1957]

to answer your second question, get your stress office to understand fatigue (and not just at the end of the day).

your final question is (i'm sorry) too basic ... if you know the stress and you have the appropriate s/n curve, then it is pretty straight-forward. careful with using FE stresses ... there are factors (like preload) that affet the problem (though maybe not in this case, since you have no preload). the s/n curve depends on the Kt, there are some simple Kt calulators available online, i'd invest in a copy of Petersen.

 

[CoryPad]

VDI 2230 Systematic Calculation of High Duty Bolted Joints has the following equation for a bolt subjected to alternating stresses:

[σ][sub]ASV[/sub] = 0,85 (150/d + 45)

where

[σ][sub]ASV[/sub] is the fatigue stress (infinite life) of the bolt
d is the bolt diameter

There are several assumptions built into this, but it is a start.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drawoh]

paramathma,

The minimum cross section of the bolt i.e. at the root diameter is .095".

Are you telling us the minor diameter of your bolt, or the stress area. If it is the minor diameter, you have somehow managed to find a #5 bolt, and I doubt it will hold 2000lb.

I searched my bolt tables for something with a stress area of .095in[sup]2[/sup], and I did not find anything. A 3/8-24UNF is .0878in[sup]2[/sup], and a 7/16-14UNC is .1063in[sup]2[/sup]. Is your bolt metric by any chance? I get .0899in[sup]2[/sup] for an M10X1.5 thread.

JHG

 

[dinjin]

It sounds like you only have a static condition.
Where does the fluctuating load come from?

 

[hydtools]

The bolt will fail in the first application of the 2000 lb load. I agree with rb1957. 282,000 psi exceeds the material strength of the bolt. Unless you are using some super strength material.

http://www.k-tbolt.com/bolt_chart.html

Ted

 

[plasgears]

To help your cause use high strength bolts and use self aligning washers under the head and the nut. This will negate bending stresses due to lack of parallel faces.

 

[dinjin]

I would be more concerned about the stress on the
elastomer than that the bolt.

 

[hydtools]

This may be of general help relating part stress to part life:

http://books.google.com/books?id=70...=X&oi=book_result&resnum=4&ct=result#PPA87,M1

Ted

 

[dinjin]

I think one of the keys is that you must have enough clamp force on the bolt so that the alternating stresses stay
within the range of clamped force and that the joint not
be allowed to open up.

 

[paramathma]

Thank you.

My apologies for the delay in replying

The material we use have yield strength on 340ksi and Uts of 350 ksi.

 

[arunmrao]

Are you sure about your strength values given. Please check.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[rb1957]

ok, the bolt can statically react the load (i guess it's something like inconel).

i'd use washers to distributed the the plastic, maybe shear stress = pi*d*t ?

as other posters have already noted, rather than finger tight i think you should apply so preload (something like your applied load) to reduce the stress cycle in the bolt) ... a preload of 1/2 the applied load reduces the stress cycle to 1/2.

then i'd fatgiue test the design to determine the fatigue life.

 

[MechEng2005]

From Shigley's I have that the endurance strength calculation begins with 0.504 * (UTS), with a maximum of 107000 psi. I got this value from a spreadsheet I created, and not directly from the text. Therefore, there may be assumptions or conditions that invalidate this. However, as a starting point, your stress just based on (Max Load/Area) assuming 0.095" diameter is over 200000 as stated. May want to look into this...

Maybe later I will try running it through the spreadsheet completely for you, but don't have time now...

-- MechEng2005

 

[MechEng2005]

The aforementioned calcuation is for determining the endurance limit, which is infinite cycles (theoretically). I suppose the OP did ask about number of cycles, so maybe this wasn't what the OP was looking for. However, I still think this indicates that the number of cycles will not be very large, since your maximum stress is more than 2x the value given as a maximum for infinite life.

-- MechEng2005

 

[dinjin]

I have a bolt, which is subjected to fluctuating Tensile load of 0 to 2000 pounds.
The minimum cross section of the bolt i.e. at the root diameter is .095".

I have not seen the question answered.
Is the cross sectional area .095 inches squared?
What size bolt are you using?