Common Terms in Materials Testing
The terms below are commonly referenced in the world of materials testing. Many of these definitions are taken from ASTM E6 Standard Terminology Relating to Methods of Mechanical Testing.
Ductility: The ability of a material to deform plastically before fracture.
Elastic limit: The greatest stress that a material is capable of sustaining without any permanent deformation remaining upon complete release of stress.
Elongation at break or total elongation: The amount of plastic or permanent deformation determined after fracture by realigning and fitting together the broken ends of the specimen.
Elongation at fracture or maximum elongation: The elongation at the time of fracture, including both plastic and elastic deformation of the tensile specimen. Usually determined by keeping an extensometer on the specimen through break.
Engineering strain: A dimensionless value that is the change in length per unit length of the original linear dimension along the loading axis of the specimen. This is frequently expressed in inches per inch or percent.
Engineering stress: The normal stress, expressed in units of applied force per unit of original cross-sectional area.
Gage length: The original length of that portion of the specimen over which strain or change in length is determined.
Hardness: The resistance of a material to deformation, particularly permanent deformation, indentation or scratching.
Hooke’s Law (Hookean behavior): Within certain force limits, the stress in a material is proportional to the strain that produced it.
Mechanical properties: Those properties of a material that are associated with the elastic and plastic reaction when force is applied.
Offset yield strength: The engineering stress at which, by convention, it is considered that plastic elongation of the specimen has commenced. Offset yield strength is determined by constructing a line parallel to the linear elastic region of the stress-strain curve and shifting it in the strain direction an amount specified as a percentage of gage length.
Plastic strain ratio: The ratio of the true strain in the width direction to the true strain in the thickness direction of a sheet material loaded beyond yield.
Proportional limit: The greatest stress that a material is capable of sustaining without deviation from a linear relationship of stress to strain.
Reduction of area: The difference between the original cross-sectional area of a tension-test specimen and the area of its smallest cross-section after fracture.The reduction of area is expressed as a percentage of the original cross-sectional area.
Strain hardening exponent, n: A measure of the increase in hardness and strength caused by plastic deformation of a tensile specimen. A power curve is used to approximate the shape of the stress-strain curve between yield and ultimate stress. “n” is the calculated exponent of the power curve.
Ultimate tensile strength or tensile strength: The maximum tensile stress that a material is capable of sustaining. Tensile strength is the maximum load during a tension test carried to rupture divided by the original crosssectional area of the specimen.
Yield by extension under load (EUL): The engineeringn stress at which, by convention, it is considered that plastic elongation of the specimen has commenced. EUL is determined by constructing a line perpendicular to the strain axis of a stress-strain curve at a strain value that is specified as a percentage of gage length.
Yield strength: The engineering stress at which, by convention, it is considered that plastic elongation of the material has commenced.
Young’s Modulus of Elasticity: The ratio of stress to corresponding strain below the proportional limit.
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