ultimate strain of SS?

[ozzkoz]

I've been looking for the ultimate strain for some different materials and having trouble. Particularly 13-8 H1000, MIL-HDBK-5 has a stress strain curve but it only ges up to about 1.2% strain. They also have a "full range" but it shows the same Ramburg-Osgood curve and then at 1.2% strain the curve starts coming down. This seems a little low for a material which is considered ductile. Is this value right or is there another source I can check for a better stress strain curve. As an aside mil-hdbk says the fracture strain is 10%, which I guess I believe a little more than the ultimate strain.

Thanks

 

[cloa]

For the strain-curves- buy it from here

http://tss.asminternational.org/por...toid=e5ee5a68ba56e110VgnVCM100000701e010aRCRD

 

[TVP]

ozzkoz,

The fracture strain is the same as the ultimate strain, which is required to be 10% for PH13-8Mo according to AMS 5629 (Table 2.6.6.0(b) on page 2-158 of MIL-HDBK-5J). The full range curves shown in Figure 2.6.6.1.6(c) show the same thing: the H1000 specimen fractures at an ultimate strain of ~ 0.10. Keep in mind that these are engineering stress-strain curves, and that the shape of the true stress-strain curve will be different.

 

[metengr]

You can also use other technical resources for comparing engineering stress/strain information on 13-8PH as shown in the attachment below regarding tensile elongation values;

http://www.alleghenyludlum.com/ludlum/Documents/13-8(062606).pdf

Tensile elongation (total strain to failure) in 2" gage is 14% typical for an H1100 condition.

 

[ozzkoz]

The elongation is the total stretch at fracture, I'm actaully looking for the ultimate strain (strain at ultiamte strength, or the strain at onset of necking).

I'm doing an elastic / perfectly plastic analysis and I don't think I can assume I carry the full plastic load all the way out to fracture since I'd have necking after the ultimate strain is reached.

 

[metengr]

Ok, I now understand what you are asking. At the onset of necking, the true strain = strain hardening exponent. If you know the strain hardening coefficient, you can use this value for the strain at necking.