Fatigue Life and Fatigue Strength 1

[Schuyler]

Manufacturing processes such as chrome plating can have an adverse effect on both fatigue life and fatigue strength. Let's say you make the following statements:

1. The process may reduce the fatigue life of a component by 50%.

2. The process may reduce the fatigue strength of a component by 50%.

It's my understanding that a 50% reduction in fatigue strength is much worse than a 50 % reduction in fatigue life (i.e., a 50% reduction in fatigue strentgh can result in much more than a 50% reduction in fatigue life).

Can someone explain

 

[TVP]

The results of fatigue tests are often plotted in the following manner:

y axis/ordinate = maximum stress, alternating stress, strain amplitude, etc.

x-axis/abscissa = log number of cycles to failure

The following two links show examples of how fatigue curves are plotted:

http://www.sintef.no/units/matek/2460/fatigue/images/fat2.gif

http://www.emrc.com/webpages/en_1.htm

As you can see, small changes in stress have a large effect on the number of cycles to failure. A change in the maximum stress of 50% will change the number of cycles to failure by a lot more than 50%.

 

[machineryguy]

Hi.

I will add to the great info that TVP supplied above.

The question you pose is a good one. The answer and reason for peoples confusion (including myself at times) is just nomenclature.

In general terms:
When someone refers to a material having a FATIGUE LIFE they are implying a given loading (forces, stresses, strains, etc). So if someone tells you that they have a material that has a fatigue life of say 250,000 cycles, they need to relate that to some loading, geometry, etc to make it significant. (If the usage of the material in some specific part in some application is implied, then one can probably get by with just throwing out a cycles to failure number).

Conversely, when you refer to something have a FATIGUE STRENGTH you are implying a given number of cycles (and geometry, load spectrum, etc). So if someone tells you a material has a fatigue strength of say 50 ksi, then that number must be related to some number of cycles (otherwise it is meaningless unless the context is implied).

In even more general terms:

Something has a fatigue strength of XXX psi given you want YYY cycles. That exact same something has a fatigue strength of XXX2 psi given you want YYY2 cycles. etc, etc.

Something has a fatigue life of

http://WWW cycles

given you apply ZZZ loading to it. That exact same something has a fatigue life of WWW2 cycles given you apply ZZZ2 loading to it, etc, etc.


ASTM E1823 definitions:
The fatigue life, Nf, is the number of cycles of stress or strain of a specified character that a given specimen sustains before failure of a specified nature occurs.
Fatigue strength, SNF, is a hypothetical value of stress at fialure for exactly Nf cycles, as determined from an S-N diagram. The fatigue limit, Sf, is the limiting value of the median fatigue strength as Nf becomes very large.

(I do not have E1823 so I pulled the ASTM defintions from Metal Fatigue in Engineering, 2nd Edition, Stephens, Fatemi, Stephens, and Fuchs)

Hope this helps.

 

[MASSEY]

Along the vein of what machineryguy is discussing, is the concept of Basic Dynamic load Ratings (BDRs) for bearings.

Bearing manufacturers supply a given BDR for a given bearing. This data is helpful in determining which bearing can carry a heavier load for the same amount of time as the next bearing.

However, the BDRs are misleading if you are trying to see how much load you can apply to your bearing because:

BDRs are the loads a bearing will carry for 500 hours rotating at 33 1/3 rpm.

Obviously 500 hours is ridulous for the life of a bearing. That is less than 3 weeks. So the need arises to use the formulae to determine a desired life for your bearing. The loads change drasticly.

This is just one of those things that you have to accept and deal with. In defense of the manufacturers: at least they all use the same formula and expected life to determine the published BDRs.

 

[Schuyler]

We have some components that are selectively carburized. Sometimes there is case leakage in undesignated areas due to insufficient copper plating. This creates a localized hardened (60 HRC)region (up to .250" x .250&quotadjacent to a softer (35 - 40 HRC) region. I have heard that this can cause a metallurgical notch (in effect, a stress concentration). It would appear to me that as long as the part is stressed only in the elastic region of the material that this would not be an issue (i.e., modulus is the same regardless of hardness). I can see a point where the notch sensitivity of the material would be increased (due to a smaller plastic zone), therefore the fatigue life may be effected. Also, I can see where a nonmetallic inclusion may act as stress concentrator because it may not distribute/carry load. Can someone please comment?

 

[CoryPad]

I agree with your comments - smaller plastic zone, inability to accomodate stresses due to inclusions, etc. This is a definite fatigue crack initiation site.

 

[mantill]

Fatigue occurs even if the material is only stressed in the elastic region. This is because fatigue damage occurs around microscopic stress raisers in the material, on a similar scale to the microstructure. Non metallic inclusions can be one source, or just dislocation pile up at a free surface or grain boundary can be the source of fatigue crack initiation.

The localised carburizing will have two effects. It will first alter the microstructure of the material, as evidenced by the increase in hardness. The fatigue life/strength data you have is for a specific material and, if the effect of carburising on the microstructure is significant, it will be invalid for the hardened material. An alteration in the notch sensitivity is one aspect of this.

The second significant effect is that the locally harder material will itself act as a stress raiser. Hence, the stresses around the locally harder region maybe larger than you think. Further, stresses around any defect in the material will also be concentrated and when the two effects are combined, at any defect in or around the hardened region, the stress concentration effect will be magnified with possibility of rapid fatigue crack intiation.

A futher side effect of the metallurgical notch will be that unless the locally hardened region is of simple uniform shape and under simple load, a complex multi-axial stress will form. This will also make you fatigue life data inapplicable as this almost certainly only applies to Mode I (Tensile) type loading.