Dear Carlos Romero,

Both Percentage elongation (%EL) and Reduction of area (RA) are the conventional and relevant measures of ductility obtained from tensile test. Both %EL and RA are obtained by keeping the fractured specimen together and taking measurements.

If Load vs elongation (or Engg stress-strain curve) is represented, then measurement of elongation can be directly compared among samples by plotting in a single graph; even though plastic strain is also included.

However, %EL is gauge length dependent, while this difficulty is not associated with RA.

Anas

Hello Carlos,

Ductility measures both percentage elongation (%EL) and Reduction of area (RA) that a material can withstand without breaking. Ductility is important to both designers and manufacturers. The designer of a component prefers a material that displays at least some ductility, so that, if the applied stress is too high, the component deforms before it breaks.

The distance between the gage marks on the test specimen can be measured before and after the test. The percent elongation describes the extent to which the specimen stretches before fracture

A second approach is to measure the percent change in cross-sectional area at the point of fracture before and after the test.

Infact both of methods is important as far as ductility is concerned but based upon our calculation and requirement any of one is choosen.

Best Wishes

Both %EL and RA are important ductility parameters. As far as I know, for titanium alloys, the %El is more relevant to material selection as a measure of ductility.

No doubt both % elongation and RA are equally important measures of ductility .To decide which is more important depend on material behaviour under static test conditions.You may do tensile tests at various strain rates and study stress/strain relationship by area under the curve as measure of ductility.

Carlos:

As most responses to your question have indicated, both measures of ductility are equally applicable and useful. However, Dr. Anas is absolutely correct in that the %elongation is a function of the gauge length of the specimen - due to the fact that the uniform elongation scales with the gauge length whereas the post maximum load, non-uniform (necking) elongation does not. The %reduction in area does not suffer from this problem and thus, in my mind, is the preferred measured of ductility.

ROR

For the most case, the purpose of tensile test is to get the curve of force and displacement. Traditional parameters used to discribe the ductility are both elongation and RA. However, ductility can be considered as a "state". It means ductile material can show brittle behaviou under some conditions, for example, lower temperature. Which ductility parameter is more important will depend on the application. For example, if using in fatigue application, RA or similar parameter will be more important.

That depends on what you're going to do with the numbers. In aerospace, there is a lot of focus on %elongation as an important material property to measure and consider when choosing a material for a specific application. It's a nice number to know, but I've rarely encountered a scenario where it was the limiting factor in the design. Early in my career, we were much more interested in %RA because the equations our customers approved for predicting fatigue life (derived from Coffin-Manson theory) depended on %RA, not %EL.

I very much agree with the answers of Anas, ROR and Hall.

From my point of view (in my opinion):

The %EL is specific to the tensile testing standard, particularly the ratio of the diameter of a round bar tensile specimen to its parallel length (and to the specified test conditions, rate etc.). Since, the main purpose of a tensile testing standard is to check that a new batch of the metallic material conforms to a minimum requirement for yield/proof strength, tensile strength and ductility. Then the %EL is perfect for that purpose, especially since the specimens dimensions are specified in the standard.

With regards to mechanical design most design codes for metallic materials implicitly assume that only ductile metallic materials are being used (I might be being controversial here). Therefore, the design codes reference material standards and contain a list of approved materials (all of which are ductile, i.e. conform to a minimum value of %EL). Some of the deficiencies of these design codes arise when they are incorrectly used for brittle materials. Here the materials engineer can often be to blame! They look at the requirements of a design code and say to themselves that they could, by alloying and heat treatment, create a new high strength metallic material which they perceive to be "better". However, when the materials engineer inadvertently cross the line and creates a brittle, high strength material then problems arise. The problem being that all practical engineering structures contain stress concentrations and/or welds and it is important for a metallic material to be ductile in order to be resilient at those stress concentrations and/or welds. In practice this means that some parts of the structure will deform plastically and it is important that the metallic material is ductile enough to accommodate that plastic deformation without initiating a crack (crack growth and fracture are important too but that is for another question).

The use of the %RA is somewhat less clear, it has disadvantages and advantages. I am personally firmly in the camp that believes that %RA is very useful. However, I recognise its problems. One of the problems with %EL is that it is an average strain over the whole parallel length of which only a small proportion has actually failed. Therefore, %EL can be considered to be a pessimistic estimate of the “strain at failure” of a round bar (with specified dimensions). Whereas, the %RA is actually measured very close to failure. However, %RA is not actually a measure of strain! After all %RA is calculated from the square of the diameter. In addition, %RA measurements have considerable uncertainty; measurements of diameter at the failure location of a round bar are not as easy and accurate as the diameter measurements before test. Furthermore, the failure is often not round and the state of stress within the necked specimen is not uniaxial!

I could go on about the problems with %RA. However, I want to close with a statement that %RA is the only measure made in a tensile test which actually gets close to the true failure point of the specimen and for that reason it can be useful for the physical understanding of failure mechanisms in metallic materials.

Final summary: %EL is useful for confirming a metallic materials conformity with a standard and %RA for research into physical understanding of failure.

If you remain interested in ductility then look up Uniform Elongation and see what you think that measure might be used for.

Both % Elongation and % Reduction in Area are considered important, but often % Elongation have been used mostly during Tensile testing. In my research using Gleeble high temperature simulation, % Reduction in area are used to measure ductility as it relate to Nil Ductility temperature and other parameters.

The are of TRUE-Stress vs. TRUE-Strain diagram measured in the necking, in the fracture cross-section. This value is some kind "fracture energy" The % elongation has gauge length dependence.

Both the properties have different implications in practical use and materials characterization. However, it is generally understood that the measure of reduction in CA gives more understanding of materials behaviour especially w.r.t. flow characteristic.

Note the following:

The standard tensile test procedure leads to an notable instability.

(1) Engineering stress is the ratio of the tensile stress and the initial cross-sectional area of a specimen

(2) The product of cross-section area to the true stress is the tensile force

(3) As the cross-section area reduces, the true stress increases even the engineering stress remains constant, leading the further decrease of the cross-section

From this viewpoint, the reduction of area is more important, as it leads to the instability.

Actually, the most important is the question concerns the behavior of true stress in the necked cross-section.

Namely, if the product of true stress to the cross-sectional area increases with true deformation, the material behaves stable. Otherwise, the material is unstable and possesses unfavorable structural properties.

Another important question is the material behavior in two-dimensional stress state. Unfortunately it is impossible to acquire directly the material model in an one-axial tensile test.

Both elongation and reduction in area are important in measuring ductility of a material. But elongation is more relevant to express the concept.

of ductility.

with best wishes

For experimental determination see the attached slides

If you are working with true stress and true strain, you require % RA, otherwise just for reporting a value or for any engineering purpose % El is enough

True strain for a cylindrical specimen can be calculated in the following expression

TSTRAIN=2ln(d/d0)

d- instantaneous diameter

d0 - initial diameter

From this expression the unit directly follows.

Neither is a good measure of ductility.

Elongation is certainly influenced by strain rate, specimen geometry, and machine compliance (the tensile strain rate sensitivity changes as the instability grows).

Strain rate and machine compliance are also important in RA, which includes a strong influence of complex stress.

By the time fracture occurs, the state of the material (mechanical and microstructural) is quite different from the starting conditions that you are trying to characterize.

Neither is a good measure of ductility.

Elongation is certainly influenced by strain rate, specimen geometry, and machine compliance. The tensile strain rate sensitivity also changes as the instability grows.

Strain rate and machine compliance are also important in RA, which includes a strong influence of complex stress.

By the time fracture occurs, the mechanical and microstructural state of the material is quite different from the starting conditions that you are trying to characterize.

Neither is a good measure of ductility.

Elongation is certainly influenced by strain rate, specimen geometry, and machine compliance. The tensile strain rate sensitivity also changes as the instability grows.

Strain rate and machine compliance are also important in reduction in area, which includes a strong influence of complex stress.

By the time fracture occurs, the mechanical and microstructural state of the material is quite different from the starting conditions that you are trying to characterize.

Neither is a good measure of ductility.

Elongation is certainly influenced by strain rate, specimen geometry, and machine compliance. The tensile strain rate sensitivity also changes as the instability grows. Strain rate and machine compliance are also important in reduction in area, which includes a strong influence of complex stresses.

By the time fracture occurs, the mechanical and microstructural state of the material is quite different from the starting conditions that you are trying to characterize.

Neither is a good measure of ductility.

By the time fracture occurs, the mechanical and structural state of the material is quite different from the starting conditions that you are trying to characterize.

Neither is a good measure of ductility. Elongation is certainly influenced by strain rate, specimen geometry, and machine compliance. The tensile strain rate sensitivity also changes as the instability grows. Strain rate and machine compliance are also important in reduction in area, which includes a strong influence of complex stresses.

By the time fracture occurs, the mechanical and structural state of the material is quite different from the starting conditions that you are trying to characterize.

I should erase all but one of the above. But the system kept telling me it could not accept an operator--hence the multiple attempts. Maybe this is the best way to get a point across?

Both elongation and reduction of area are important, but normally reduction in area is provided as an additional information. Ductility is defined as the ability of a material to deform plastically before fracturing, so in my opinion reduction in area is closer to this definition. Although both the properties are used to define ductility but their major contribution comes from different zones of the plastic deformation portion of a tension test. %elongation is outcome of uniform elongation of whole gauge length and is contributed by whole plastic zone after yield point. While reduction of area is more a measure of the deformation required to produce fracture and its major contribution results from necking.

Both elongation and reduction in area are important depending on the area on which you are focused. It is worth to mention the case of high hardness hardened steels, specimen surface finish is very important to get good values of both characteristics. In my view point ASTM E8 is very subjective on its recommendations for this characteristic.

You might be facing this problem because of possible error in measurement of area. Which happens, when reduction in area is not significant or ductility is low.

Both % EL and % RA are important parameter from the point of ductility. % RA is more relevant but again the feasibility of measurement come into the picture in case of complex geometry specimens. The uniform % EL would be the relevant ductility parameter.

Both the Elongation % and reduction of Area% is the same for measuring the ductility. However, the %E is more easy to do which is the difference between the final length and the original length divided by the original length %.

I agree with Hedia's answer, that Both the Elongation % and reduction of Area% is the same for measuring the ductility.

E - depends on gauge length, R independent (in principle), but the best ductility parameter@ is the AREA under the true stress-true strain diagram measured in the necking part of the cylindrical specimen.

El. and RA both are the measures of ductile behaviour of metallic material. But their implications in use is different. For general structural applications where consideration of Engineering strain is primarily required the El. is a suitable parameter. But in the cases of considering plastic deformation behaviour as essential criterion RA always serves most appropriately. You may consult mechanical metallurgy of metals and alloys.

Ductile parameter, which you'll choose for your investigation depends on material (i.e. standards for material and for testing) and purposed use of product made of the material.

For example, for hot rolled reinforcing steel bars use elongation (A5, A10) and for brittle reinforcing materials (strands and bars for prestressed constructions, cold deformed wire and bars) use uniform elongation of Agt as a measure of the ability to deform with concrete (especially uniform elongation).

But for wire rod intended for cold drawing (for cold reduction of area under tensile force) the main characteristic of ductility and deformability is ROA.