Fatigue life
Fatigue life
(OP)
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
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





RE: Fatigue life
If the preload is over 2000 pounds, then the bolt will not see any fluctuating load.
RE: Fatigue life
the bolt application is different to what we usually encounter.
the bolt is used compress an elastomer.
RE: Fatigue life
How do you figure that out?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Fatigue life
but i wouldn't bother ... 2000 lbs on a 3/32" bolt is a stress of 282ksi ... the answer is "zero fatigue life"
RE: Fatigue life
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.
RE: Fatigue life
σASV = 0,85 (150/d + 45)
where
σASV 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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Fatigue life
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 .095in2, and I did not find anything. A 3/8-24UNF is .0878in2, and a 7/16-14UNC is .1063in2. Is your bolt metric by any chance? I get .0899in2 for an M10X1.5 thread.
JHG
RE: Fatigue life
Where does the fluctuating load come from?
RE: Fatigue life
http://www.k-tbolt.com/bolt_chart.html
Ted
RE: Fatigue life
RE: Fatigue life
elastomer than that the bolt.
RE: Fatigue life
http://boo
Ted
RE: Fatigue life
within the range of clamped force and that the joint not
be allowed to open up.
RE: Fatigue life
Thank you.
My apologies for the delay in replying
The material we use have yield strength on 340ksi and Uts of 350 ksi.
RE: Fatigue life
Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)
RE: Fatigue life
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.
RE: Fatigue life
Maybe later I will try running it through the spreadsheet completely for you, but don't have time now...
-- MechEng2005
RE: Fatigue life
-- MechEng2005
RE: Fatigue life
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?
RE: Fatigue life
The min root dia of the thread is .095"
The bolt is custom made.
the bolt material has a yield strength of 340kSi (18 ni maraging steel)
RE: Fatigue life
you haven't given us a lot to go on ... how often is the load applied ? how many load app's are you looking for ? is the bolt going to be lifed (and regularly replaced in service)?
why can't you torque the bolt (and create a preload) ? maybe the plastic would just squish out of the way ?
be worried about the strength of the plastic.
are you going to test the installation ?
RE: Fatigue life
So it is a #5 screw!
You should be working out the tensile stress area of your screw. You can get this from all sorts of handbooks. The Handbook of Bolts and Bolted Joints, Bickford and Nassar, show how to calculate this. The values are different for UNJ and MJ threads, as opposed to regular threads. Machining the thread, as opposed to rolling it, might be an issue too.
This is a weird size. You might have problems finding standard mating fasteners, as well as fabrication and inspection tools. Why so tiny?
JHG
RE: Fatigue life
RE: Fatigue life
http://j
This one has some references to other investigators. I noticed that there is reference to Shigley.
htt
RE: Fatigue life
A simple P/A analysis is not adequate unless you are applying a very conservative FoS. There is the stress concentration (Kt)at the thread root and under-head fillet you must add, you must also analyze using the combined tensile and torsional loads due to wrenching at installation. And don't forget to include some bending due to misalignment of the seating faces.
As for fatigue, the surface finish of the parts will have an effect too. Finally, don't forget to account for any thermal mismatch strains due to dissimilar materials.
RE: Fatigue life
Now I know the difficulty in calculating the fatigue life of bolt.
Thanks