Mechanical testing of notched specimens

[davepenney]

Hi there,

I am currently testing a certain metal in a notched tensile test piece and standard dog-bone/straight test piece. The notched test piece average ultimate stress is HIGHER than the straight test piece ultimate stress. Any reasons for this? I thought the notch would act as a stress-raiser and produce lower results?

Thanks very much,

Dave

 

[metengr]

davepenney;
Well, assumming your tensile test procedure and specimen geometry were in accordance with ASTM E8 and this certain metal (which you have not identified) exhibits work hardening and lower notch sensitivity characteristics; these factors alone could result in a ratio of notched/smooth bar tensile test values greater than 1.0.

You really would need to provide more information for a definitive answer. Also, you might be interested in reading the excerpt below

http://books.google.com/books?id=5u...e&q=notched tensile testing of metals&f=false

 

[Wrenchbender]

Sounds like you are seeinge effects of triaxial constraint as explained in metengr’s reference (last paragraph in first column). The same reasons explain why soldered and brazed joints are very strong when thin enough; the surrounding material is providing lateral constraint, minimizing shear, minimizing dslcn motion, etc...

 

[davepenney]

Hi there,

Good info, thanks for that. It's MP35N, the mother-of-all steels as far as I can tell. It's got ridiculous properties, but is apparently a good 25% stronger as a notched specimen.

Thanks again,

Dave

 

[TVP]

MP35N is not steel, as it does not contain any Fe. It is properly termed a multiphase alloy, consisting of Ni-Co-Cr-Mo. This alloy has incredible strength and toughness, hence its use in the highest strength aerospace fasteners and other exotic applications. It will definitely have a notch sensitivity ratio > 1, meaning it is notch strengthened. The following reference has some additional information regarding composition, microstructure, properties, etc.

http://www.latrobesteel.com/assets/documents/datasheets/MP35N_extended.pdf

 

[davepenney]

Yes sorry about that, I knew it wasn't a steel actually, slip of the keyboard!

Because it has no Fe, is that why the tri-axial stress concentration strengthens it? Is the strengthening phenomonon of tri-axial stress only applicable to some non-Fe materials?

Thanks again,

Dave

 

[TVP]

Dave,

No, the phenomenon of notch strengthening (also called notch ductility) applies to many different metals, including Fe-, Ni-, Al-, and Ti-based alloys. It is dependent upon the microstructure, not the predominant atomic species. Perform a Google search of "notch ductility" or "notch strengthening" and you will find more references on the subject. Here is one short article that shows some relative rankings of various alloys:

http://www.asminternational.org/pdf/amp_articles/AMP200606_Steels.pdf