Material constant for Neuber's method

[gurmeet2003]

Gentlemen and Ladies,

I am working with a Ductile Iron material (65-45-12): uts-65000 psi, yield pt-45000 psi, elongation-12%. I have a part made from this material, which has a bore and a hole prependicular to the bore. Hole dia is about 1/4 inch and bore diamter is 5 inch. I have carried out a finite element analysis of the part and the maximum stresses occur inside the hole and not at the edge which breaks out in to the bore. This is in line with Peterson's handbook.

Based on the FEA stress concentration inside the hole is 3. I am trying to use Neuber's method for analysis of notches. But I do not have the material constant A required for Nueber's formula. However I found a graph in Ductile Iron Handbook, which plots the S-N curves for a notched and un-notched sample (bar with a v-groove notch). Based on this Kf value is 1.67. The root radius is 0.01 inch. I ran an axi-symmetrical analysis to find out the Kt value for the notch. Using a very fine mesh and ensuring convergence I found Kt = 11.

I calculated the sqrt(A) for Neuber's method and come up with a value of 1.4 inch for the material. The value does not seem to be reasonable. Please give me your comments on above.

Gurmeet

 

[gwolf2]

If you have an FE model you don't need any of the peterson stuff. If your model is a true representation of the geometry and there are plenty of elements then that is the stress which you use with Neuber.

I use Neuber to correct a large linear stress beyond the elastic limit to a nonlinear stress. This gives me my elastic-plastic stress and strain which I then use in subsequent fatigue calcs.

That's it really.

 

[gurmeet2003]

gwolf2,

Thanks for your comments. I think that stresses predicted for my model are good based on loads, bcs etc. For sharp notches some people do not use the peak stress computed by FEA for calculation of cyclic failure factor, but a some what lower stress based on notch sensitivity of the material. Notch sensitivity is not purely a material property but also depends on the notch radius. For sharp notches fatigue notch factor Kf drops off.

I understand where you use Neuber's formula to estimate plastic stresses based on elastic stresses. However I was trying to use the Neuber's formula to estimate the fatigue notch factor Kf. The Nueber's formula in question is as follows:

Kf= Kt(1/(1+sqrt(A/r))

Where A is a material constant, r is the notch radius and Kt is the stress concentration factor (which I obtain from the FEA results).

In the past I have made the assumption Kt=Kf, which is conservative.

Gurmeet

 

[gwolf2]

OK, understood, out of my area then. Hope for another post.