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How to Calculate Ductility

One of the core factors to consider when choosing a metal for a specific project is its ductility. Determining the ductility of metal is more crucial when the metal in question is going to be used in high tension applications.

What is Ductility?

Before you can calculate ductility, it’s vital to understand what it is and how it can affect the performance of any given structure.

Ductility is defined as the amount of tensile stress a material can take before enduring deformation. This damage is often in the form of decrease of the cross section area but without fracturing.

Why Calculating Ductility is Important

Calculating ductility is an important process for all metals for the following reasons:

  • The ductility of the metal advises on the forming and machining methods that can be used on the test metal.

  • It determines the applications that the materials are best used for.

  • Determining the maximum load of the material whose ductility you’re calculating.

How to Calculate Ductility

There are two measures required when calculating ductility:

Elongation

The increase in the gage length of the material, being subjected to tensile forces, divided by the original gage length. The elongation is often expressed as a percentage of the original gage length.

Use the following formula to calculate elongation:

Elongation (%) = 100 x ∆L/Lo 

Lo – the original gage length

∆L – the change in length after subjecting the material to tensile stress and causing a fracture.

The reduction of the cross sectional area

The second measure is the reduction of the cross-sectional area. This is also expressed as a percentage and is calculated using this formula:

Reduction of the C.A area (%) = 100 x (Ao – Af)/Ao 

Ao is the original cross-sectional area.

Af is the cross-sectional area after the fracture occurs. It should be the narrowest part of the material.

Both of these figures are expressed as percentages and indicate the ductility of the material being tested.

While performing the tensile stress test, it’s important to note that temperature is a major factor in the ductility of the tested metal.

Parameters that Affect Ductility

When conducting the tensile stress test, some parameters could affect the results of the test. It’s not only vital to know what these parameters are but also to keep them uniform to ensure the most accurate results.

Gage length

Gage length is a critical parameter to observe, when conducting a tensile stress test. But as the gage length increases, the value of elongation becomes less dependent on the gage length.

Specimen dimensions

The cross-sectional area of a specimen has a direct effect on the elongation measures. It’s important to keep the specimen dimensions uniform during the test to obtain accurate results.

Test speed or strain rate

A higher or faster strain will have an adverse effect on ductility hence decreasing the elongation value. Brittle materials are more sensitive to strain rate and the elongation values decrease, as strain rate increases across the board.