How to calculate force, frequency and cycle needed in fatigue testing machine

[student85]

Hi,

 

I'm now facing problem on how to calculate the force, frequency and cycle needed for fatigue testing of a test sample. The force is refer to compression and tensile stress. Please kindly advice an solution regarding the above problem. thanks

 

 

 

Edited December 7, 2012 by student85

[Enthalpy]

To decide on your apparatus, you need knowledge about the material to be tested. It's called a Wöhler curve and relates the number of cycles with the admissible stress.

 

Deforming a solid, even elastically, take much energy (and force) which is impractical to produce, store and give back many times, so machines for such trials bend the probe and rotate it so the compressed and extended sides alternate. Other processes use to reduce the number of cycles.

 

http://en.wikipedia.org/wiki/Fatigue_(material)#The_S-N_curve

 

Books consider the frequency isn't important. Useful to conduct tests more quickly.

Edited December 7, 2012 by Enthalpy

[student85]

Hi Enthalpy, thanks for your information. By the way, the material that i'm going to test is a strainless steel rod.

 

So Enthalpy,do you mean that before i can do the calculation for force needed and number of cycle, i need to come out with the S-N curve? How do i come out with the curve? Either i go online to a find a similar one or i go collect information like the maximum stress of strainless steel and start doing experiment to produce a S - N curve myself? Sorry Enthalpy. just to inform you that actually i'm a electrical student. I do not have any background regarding material science.So thats why i need to ask a bit more detail.

 

By the way, i found a website that indicates a kind of calculation for the force for the fatigue test, without using the S - N curve.

 

http://blog.mechguru.com/machine-design/fatigue-stress-design-calculation-example/

 

May i ask is it accurate and am i able to use it?

[Enthalpy]

The Wöhler curve depends essentially on each alloy and its condition, so a few formulas like on the cited website don't make sense. Especially an austenitic stainless steel behaves very differently from a classical tempered steel.

 

Fatigue resistence also depends very much on surface hardness and smoothness of the part, so any attempt to relate it with the material's bulk properties would be futile. It depends also on the thermal history of the part, for instance welding.

 

This is a very variable quantity, where quick estimates are unusable. That's why people measure it.

 

To organize a test, you must know:

- How many cycles are needed in the projected use, if you design a part;

- Or how many cycles you're willing to test;

- And have an idea of the corresponding stress.

- Alternately, you can know the stress and measure the corresponding number of cycles, but to project the test, you must know approximately when the sample will break or how long it needs to resist.

 

Depending on the desired number of cycles, you can use one apparatus or need an other one, so yes, you must check the material in advance, by comparison in case yours isn't well documented. One first split among stainless steel: austenitic / ferritic / martensitic, and less commonly duplex / martensitic with precipitation hardening / austenitic with precipitation hardening.