Is the difference between tensile strength and yield strength a good measure of ductility?

[jzhaan55]

Since a 100% brittle material will have no difference between tensile strength and yield strength, would taking the difference between these two values for a ductile metal be a good indication of its ductility? Is there some sort of formula I can use for this?

 

[metengr]

jzhaan55;
You are referring to the yield/tensile strength ratio and, yes as the ratio decreases, the material has increased capacity for strain (aka ductility).

 

[mrfailure]

You asked this question in a different form in another thread. I would be leery of using this as an indicator. It is true that a totally brittle material will fail before the defined offset for yield strength is reached, resulting in equal yield and tensile strength, but that is a boundary condition that does not reflect true material behavior. The slope of individual stress-strain curves vary considerably from the yield offset to maximum tensile load. HSLA steels, for example, can have a very small ratio but still have good ductility. As mentioned before, an illusion of low ductility is created by a low ratio since calculation of tensile stress is based on original cross-sectional area and does not account for actual ductile behavior of the material.

Aaron Tanzer

http://www.lehightesting.com

 

[Metalhead97]

jzhaan55,

Many tensile testing software have the ability to report plastic elongation. I would go with that rather rather than the (UTS-YS). Many people use (UTS-YS) when a extensometer isn't being used, but it still isn't a good indicator.

MH

http://www.linkedin.com/pub/luke-autry/1b/510/566

 

[FennLane]

None of the answers really address the issues that could be tied up in this question.

To keep it simple if we only consider 'Engineering' Stress Strain' Data then:

For the Brittle Elastic Solid you describe the ratio of tensile strength to yield strength will be 1.00.

If the material were, however, Elastic-Ideally Plastic ie non-work hardening the ratio would still be 1.00. (Plasicine would have this behaviour)

Clearly the two materials are very different and if you compare the True Stress vs True Strain Curves the behaviour would be very different but it would be very difficult to produce this data.

Clearly most metals are Elasto-Plastic Work Hardening Solids and there is a huge amount of data needed to fully define and understand their behaviour across a range of engineering applications.

Ductility is sometimes considered a guide to a material's impact strength, sometimes its ability to be used for pressing or other process involving sheet metal deformation and generally requires information with regard to the level of strain that is associated with this deformation. Again bend test data can be used to give some subjective guide to performance.

If you are concerned about Fracture Tougness you will need Charpy Impact Data or better still KIc values. If you are worried about low temperatures the FATT data will be good to have.

If you are concerend about pressings you will need either Forming Limit Data of Work Hardening Exponents etc, etc.

You need to consider the application and seek the relevant data to be confident.

I think that if if you try to define a single non-dimensional parameter you will potentially make disastrous selections.