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Ductility Measurement: Reduction of Area

The performance characteristics of a material are determined by its mechanical properties, which in turn are determined by subjecting a test sample to a tensile test. There are a number of mechanical properties which include, amongst others, the yield strength, ultimate tensile strength and ductility (defined by percentage reduction of area or percentage elongation) of a material.

Designing and manufacturing high-quality products requires an in-depth knowledge of a material’s mechanical properties and how they affect the performance of a product within its operating environment. Since engineers have access to a wide range of materials and manufacturing processes, selecting the most appropriate material whilst minimising manufacturing and product costs is a major challenge. Knowledge of a material’s mechanical properties assists engineers in selecting the most appropriate material and help them to understand how such properties affect the workability of a material.

An important characteristic to consider is ductility. Ductility is determined through either percentage reduction of area or percentage elongation. This article will focus on reduction of area.

What is reduction of area?

Reduction of area is a comparison between the original cross-sectional area of a sample and the minimum cross-sectional area of the same sample after complete fracture failure. It is used as an indicator to show to what extent a material will deform when subjected to a tensile load. Reduction of area is normally displayed as a percentage.

How is it determined?

Reduction of area is determined by applying a tensile load to a test sample. The test sample is normally circular in cross section, but other profiles may also be used. The test sample consists of a single piece of material (no chemical or mechanical joints) and has enlarged ends which allow it to be clamped into a tensile testing machine.

The tensile testing machine applies a tensile force along the longitudinal axis of the test sample, gradually pulling its ends further apart. The sample elongates along the longitudinal axis causing it to thin, or ‘neck’. Necking continues until failure occurs and the sample fractures, breaking into two separate pieces. A tensile test can be performed on any type of material.

How is it measured?

The two fractured test samples are repositioned so that they touch each other at the fracture surfaces. For cylindrical samples, the reduction of area is measured at the minimum neck diameter. For non-cylindrical samples, the reduction of area is measured at the smallest circumference of the neck area.

Once the reduced surface area is calculated, it is displayed as a ratio of the test piece’s original cross-sectional surface using the following equation:

The variables are as follows:

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% RA – percentage reduction of area

Ai – original cross-sectional area (in m2) of the test sample

Af – minimum cross-sectional area (in m2) of the test sample after failure

What does reduction of area represent?

Reduction of area indicates ductility – which represents a material’s ability to withstand plastic deformation before experiencing fracture failure. A more ductile material will experience a greater reduction in area, and a less ductile material a smaller reduction in area.

For which materials is it relevant?

Reduction of area is relevant for all materials from metals to polymers.

Factors affecting reduction of area

Temperature has a major effect on a material’s actual reduction of area compared with its standard reduction of area. Increasing temperature will increase reduction of area, whilst reducing temperature will reduce reduction of area and may even cause a material to transition from ductile to brittle.

Cold working of a material also reduces the reduction of area. Cold working occurs in a temperature and time region where plastic deformation occurs, but strand hardening is not relieved, resulting in a more brittle product.

The intentional or unintentional inclusion of impurities has a marked effect on reduction of area and may also even change a material from ductile to brittle.

Conclusion

Engineers can use any of a material’s mechanical properties to determine which is best for a particular application. When selecting materials, instances may occur where the strength and hardness of two materials may be similar, and it is difficult to select either one. Usually, the additional information on a material’s ductility will aid in determining which material is more appropriate for a specific application.

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