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Ductility Testing

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Ductility involves determining the extent by which a material can withstand plastic deformation without rupture.

A bend test for ductility provides a simple way to evaluate the quality of materials by their ability to resist cracking or other surface irregularities during one continuous bend. With a ductile fracture, there is a considerable amount of plastic deformation prior to failure.

Ductility is extremely important in metalworking, as materials that crack or break under stress cannot be manipulated using processes for metal forming, such as hammering, rolling, and drawing. Malleable materials can be formed using stamping or pressing, whereas brittle metals and plastics must be molded.
High degrees of ductility occur due to metallic bonds, which are found predominantly in metals and leads to the common opinion that metals are ductile in general. In metallic bonds valence shell electrons are delocalized and shared between many atoms. The delocalized electrons allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.

A metal can be both ductile and malleable, or it can be only one. For example, a metal might have both, meaning it holds up well under pressure and can be stretched into thin wires. Another metal, however, might have the ability to hold up well under pressure, but begins to break apart when stretched. This second metal would be malleable but not very ductile.

Manual Bending Machine

When a material specimen is stressed, it deforms elastically (i.e., recoverably) at first and then deformation becomes permanent. A cylinder of steel, for example, may assume an hourglass shape in response to stress. With sufficiently high stress, fracture occurs. Increases in temperature will increase ductility whereas decreases in temperature will reduce ductility.

A bend test helps provide a visual indication of the ductility of the material. The guided bend test method requires the specimen to be loaded at its center point with a mandrel or plunger while being supported at the ends. The specimen is bent to a predetermined angle or until the specimen fractures. The convex side of the specimen is visually inspected for cracks or defects, and failure is determined by the size of the cracks and imperfections allowed by the material specification.

TEST STANDARDS:
ASTM A720 / A720M - 02(2011) Standard Test Method for Ductility of Non-oriented Electrical Steel
ASTM D113 - 07 Standard Test Method for Ductility of Bituminous Materials
ASTM E290 – Test Method for Bend Testing of Material for Ductility