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Elongation Testing

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Elongation is a measure of the ductility of a material as determined by a tension test. It is the increase in the gauge-length of a test specimen after fracture divided by its original gauge-length. Higher elongation means higher ductility.

• Elongation is important in components which absorb energy by forming plastically such as crash barriers and car bumpers.
• High elongation to failure is important for "plastic hinges" such as the hinges on video cassette boxes.
• Elongation is important in manufacturing as it measures how much bending and shaping a material can withstand without breaking.

Elongation is inversely proportional to hardness, tensile strength, and modulus. The greater a material’s hardness, tensile strength, and modulus, the less it will elongate under stress. It takes more force to stretch a hard material having high tensile strength and high modulus than to stretch a soft material with low tensile strength and low modulus. Because elongation is equal to the failure strain it has no units, but is often given in % strain.

Usually strength refers to the ability of a structure to resist loads without failure. Failure may occur by rupture because of excessive stress or may take place because of excessive deformation. Tensile properties include the resistance of materials to pulling or stretching forces. The amount of force required to break a material and the amount it extends before breaking are important properties. For most materials, the initial resistance to force, or modulus and the point of permanent deformation, are obtained from plots of force against elongation. Analysis of force elongation or stress-strain curves can convey much about the material being tested, and it can help in predicting its behavior.

As an example, if an O-ring is overstretched, elongation becomes an important installation factor, especially as gland and seal dimensions decrease. What might be a small percentage increase in a larger seal can be a large increase in a smaller seal.

TEST STANDARDS:

• ASTM E8 / E8M - 11 Standard Test Methods for Tension Testing of Metallic Materials
• ASTM D76 / D76M - 11 Standard Specification for Tensile Testing Machines for Textiles
• ASTM D5278 - 09 Standard Test Method for Elongation of Narrow Elastic Fabrics (Static-Load Testing)