Is there life after yield...
Is there life after yield...
(OP)
Hi there
I have a particular application where the compression of my part induces stresses WAY beyond the yield stress. The small amount of elasticity remaining in the part is fundamental to the design as it has to survive approx one million cycles of +/- 20 microns. Unfortunately on the current design cracks are appearing and we need to somehow improve it. The part has a section of only a few mm, is made from stainless-steel and the compression is across the section NOT along the length.
My question is can fatigue calculation software model this kind of fatigue problem with high plasticity, low strain cycling and a million cycles? I'm looking at packages like MSC.Fatigue as we already have an account with them.
Any help or advice on my problem is appreciated.
Thanks in advance
Zatt.
I have a particular application where the compression of my part induces stresses WAY beyond the yield stress. The small amount of elasticity remaining in the part is fundamental to the design as it has to survive approx one million cycles of +/- 20 microns. Unfortunately on the current design cracks are appearing and we need to somehow improve it. The part has a section of only a few mm, is made from stainless-steel and the compression is across the section NOT along the length.
My question is can fatigue calculation software model this kind of fatigue problem with high plasticity, low strain cycling and a million cycles? I'm looking at packages like MSC.Fatigue as we already have an account with them.
Any help or advice on my problem is appreciated.
Thanks in advance
Zatt.





RE: Is there life after yield...
The resultant stresses you get could be simply assessed against a simple S-N curve of the material but if you've got a load history and complex tri-axial stresses then it might be worth considering some fatigue software.
corus
RE: Is there life after yield...
If the initial deformation is not too big (say less than 1% strain) it should not significantly impair the residual fatigue strength of the item. If on the contrary the initial strain is large or there are local effects (e.g. discontinuities at supports or load location) that are difficult to model, then even the best software wouldn't be of much help.
prex
http://www.xcalcs.com
Online tools for structural design
RE: Is there life after yield...
I have a wire 2mm thick and I want to compress it across it's thickness by 0.5mm (i.e. 25%). I then want to completely remove the load (allowing it to recover approx 20 microns) then reapply the same compression. Repeat this a million times and there's my problem. Most of the time the wire survives intact (albeit deformed). But sometimes the wire cracks.
I'm been researching this all day and am coming to the conclusion that Wohler and Goodman just can't be used beyond yield as they both predict almost instant failure.
I think I'll dig out my old Uni notes tonight and give myself a headache.
Zatt
RE: Is there life after yield...
There are 3 different methods for Fatigue Analysis:
1) S-N Method (Stress Life)
2) E-N Method (Strain Life, or Crack Initiation)
3) Crack Propagation Method
I think that Stress Life method is not a good approach for you, because, in this method you basically ignore plasticity. Strain life method can handle better your case. It is the most common life prediction method used in industry. It is also called the local strain approach. Defining the material properties for this method is more difficult than Stress life method. In MIL handbook you can find some S-N curves.
Regards,
Ali
http://www.geocities.com/fea_tek/asd/
RE: Is there life after yield...