Fatigue limit
Fatigue limit
(OP)
One of the product we design is a rotor can for an Electro Hydraulique Power steering for the automotive industry. To prove that our design is going to whistand its life expectancy our client ask us to perform a sort of fatigue test on the overall rotor can assembly. As all the parts involve use ferrous material we are planing a 10^6 cycles test with a load representing the worst condition. I would now want to back up this assumption with more robust data. My first idea is to look for the S/N curve of the different material and proove that at 10^6 cycles the material we use have reached their fatigue limit.
Do you know where I could find this information?
Thanks for your help
Do you know where I could find this information?
Thanks for your help





RE: Fatigue limit
Do you know what steel alloy is used?
Do you know how the manufacturing process (stamping, forging, etc.) affects the nominal strength of the material?
Do you have a good understanding of the surface characteristics of this part (surface roughness, presence of micro or macro-cracks, geometry changes that locally modify the stress field, etc.)?
Do you have a good understanding of the loading environment in terms of forces (min & max), cycles, temperature, etc.?
Is it appropriate to take the maximum stress and apply it for 106 cycles as a means to validate this design? The effects of mean stress and variable amplitude loading would not be captured by this type of test.
Now, good places for fatigue data are the following:
ASM HANDBOOK Volume 19 Fatigue And Fracture
http://www.asminternational.org/Template.cfm?Section=BrowsebyTopic&template=Ecommerce/ProductDisplay.cfm&ProductID=10494
Atlas of Fatigue Curves
http://www.asminternational.org/Template.cfm?Section=BrowsebyTopic&template=Ecommerce/ProductDisplay.cfm&ProductID=10557
S/N Fatigue Life Predictions for Material Selection and Design
http://www.asminternational.org/Template.cfm?Section=BrowsebyTopic&template=Ecommerce/ProductDisplay.cfm&ProductID=10332
Fatigue Data Handbook
http://www.ttp.net/web/search/b_detail.cfm?id=0-87849-803-6
Handbook of Fatigue Crack Propagation in Metallic Structures
http://www.elsevier.com/inca/publications/store/5/2/5/2/6/0/index.htt
MIL-HDBK-5H
http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=3
http://www.engineering-e.com/datamart/1319_DataMart2002_Datasheet.pdf
http://www.autosteel.org/bsfd.php3
http://www.swri.edu/4org/d18/mateng/matint/NASGRO/Overview/NASMAT.htm
RE: Fatigue limit
Here is some answers to your questions:
We use 4 different materials:
-Stamping: Half hard mild steel cold rolled HV135/165 CS4 to BS 1449
-machined: Mild steel 230M07 (EN1A-UNLEADED) to BS 970
-deep draw: Hot rolled steel sheet/coil HR1 to BS 1449
-machined: 16Mn Cr5 casehardned
We don't know the impact of the manufacturing process on the material characteristic
Our understanding of the surface finish charcteristic is very limited.
Our loading condition are as follow: the torque apply to the shaft vary from nil to 4Nm depending the output required from the EHPS. It is unidirectional as the motor always rotates in the same direction. It is very difficult to capture the number of cycle as the torque vary all the time. So our idea for the test was to say, let test it by oscilating the torque from 0 the maximum torque (4Nm)for 10^6 cycles (where I understand most of the steel alloy reach their fatigue limit). We also did a destructive test where we measured the troque we need to apply to strip the shaft / rotor can assembly. Our results show that the torque to apply is around 40Nm.
I now would like to check that our test is going to capture the fatigue aspect of our application. I would also like to find some technical information to prove that 10^6 cycles is enough (ie: S/N curve of the material used).
Thanks for your help.
RE: Fatigue limit
This way you don't even need the fatigue curves for the metals-they're all steel, and they all have a limit-unlike alum., for example. Just make sure your loads/stresses are realistic.
ASM has many refs. on fatigue.
RE: Fatigue limit
RE: Fatigue limit
RE: Fatigue limit
I just looked this up on a CD-ROM I have from the ASM (Failure Analysis). I found only a few references to the 1 million cycle limit for "steel", and nothing which indicated any other number of cycles.
You may have to do a Google-type search on the subject and find hard refs, for what you require.
RE: Fatigue limit
The answer to your question about the endurance limit of steel is yes, most steels under controlled fatigue testing reach an endurance limit by 106 cycles. The value of stress (maximum stress, stress amplitude, etc.) is what varies at the endurance limit. The references that I gave in my previous post will support this.
RE: Fatigue limit
RE: Fatigue limit
Fatigue strengths/limits don't correlate well with YS, but they do with UTS. The 45% is wrt UTS.
RE: Fatigue limit
Another method for evaluating this may be to perform a stress governed test: (IIRC stairstep method?) Load your part such that it break in btw 1000 and 10,000 cycles. Mark the load and cycles to failure. Reduce the load by some factor. 1/8th 1/4 1/2 etc... record load vs lifetime. With a large number of repetitions this could be used as proof that your part has sufficient strength to survive usage.
Nick
I love materials science!