How to compare percentage elongation of steel 1

[Alan John]

Hi,

I'm trying to compare the mechanical properties of aisi 8620 steel against a British Standard. All of the data sheets list a percentage elongation using a gauge length of 2 inches (typically 16%). However the British Standard gives a minimum requirement of 19% with a gauge length of 5.65√s0. What is the effect of the different gauge length? Can i directly compare the two?

Thanks

 

[swall]

First of all, it appears that you are trying to compare a data sheet stating typical properties with a spec that has minimum properties. You can't do that. Second, I don't understand the units you have provided for the gauge length in the British Standard.

 

[Alan John]

The data sheets are giving actual (not typical) elongation values and the British Standard gauge length is defined as: Gauge length = 5.65 √So (i.e. 5.65 x square root of the original cross-sectional area).

 

[metengr]

The stated values are not directly comparable between gage length values and values obtained by formula based on sample geometry.

 

[MagBen]

Gauge length of 2 inches is normally 4D, while 5.65√s0 is 5D. In this case, the specimen is machined to .5'', 4D=2'', 5D=2.25''. Normally 4D will give you a bit higher elongation.

 

[rob768]

5.65√So = 5D, which makes it a standard DP5 round test bar. A longer gauge length might give you a bit longer elongation.

 

[MagBen]

Generally speaking, shorter gauge length leads to a higher percentage of elongation, reason being the portion near breaking always deformed /elongated more, while the measured elongation is the average of the whole gauge length.

 

[rob768]

Ben you are right of course. my mistake.

 

[ulyssess]

A conversion of elongation values on various gauge length (proportional or non-proportional) to other gauge lengths is given in ISO 2566. Part 1 applies to carbon and low-alloy steels with tensile strength from 300 to 700 MPa.
To compare values obtained on non-proportional to proportional gauge lengths the cross section of the test specimen has to be considered.

 

[Alan John]

Thanks everybody. Your replies have been very helpful.