Description

Tensile test is a method for determining the behavior of materials under uniaxial tension loading. Load –elongation data are used to determine elastic limit, modulus of elasticity, proportional limit, reduction in area, tensile strength, yield strength. Tensile tests at elevated temperatures provide creep data. Procedures for tests are detailed in ASTM E8.
Other methods are : ASTM D 638, ASTM D 2289, ASTM D 882, ASTM D 2343, ASTM D 897 and ASTM D 412.

Ductility : extent to which a material can sustain plastic deformation without rupture. Elongation and reduction in area are common indices of ductility in a tensile test.

Elastic limit : greatest stress that can be added to a material without causing permanent deformation. For metals and other materials that have a significant linear portion on their stress/strain curve, elastic limit is approximately equal to proportional limit. For materials that do not exhibit a significant proportional limit, elastic limit is an arbitrary approximation.

Elasticity : ability of a material to return to its original shape when load causing deformation is removed.

Elongation : measure of ductility of a material determined in a tension test. It is the increase in gauge length (measures after fracture) divided by original gauge length. Higher elongation indicates higher ductility.

Flow stress : stress required to cause plastic deformation.

Fracture stress : true stress generated in a material at fracture.

Modulus of elasticity : rate of change of strain as a function of stress. The slope of the straight line portion of a stress strain curve. Tangent modulus of elasticity is the slope of the stress strain curve at ant point. Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It is also called stress-strain ratio. Tangent and secant modulus of elasticity are equal up to proportional limit.
Depending on the type of loading represented by the stress-atrain curve, modulus of elasticity may be reported as : compressive modulus of elasticity; flextural modulus of elasticity; shear modulus of elasticity; tensile modulus of elasticity; torsional modulus of elasticity.

Modulus of elasticity may be determined by dynamic testing, where it can be derived from complex modulus. Modulus alone generally refers to tensile modulus of elasticity. Shear modulus is almost equal to torsional modulus and both are called modulus of rigidity. Moduli of elasticity in tension and compression are approximately equal and are known as Young’s Modulus.

Necking : localized reduction of cross-sectional area of a specimen under tensile load. It is disregarded in calculating engineering stress but is taken into account in determining true stress.

Offset yield strength : arbitrary approximation of elastic limit. It is the stress that corresponds to the point of intersection of a stress-strain curve and a line parallel to the linear portion of the curve. Offset refers to the distance between the origin of the stress –strain curve and the point of intersection of the parallel line and the stress axis. Offset is expressed in terms of strain.

Plastic deformation : deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material. It is also called plastic strain and plastic flow.

Plasticity : tendency of a material to remain deformed, after reduction of the deforming stress, to value equal to or less than its yield strength.

Proportional limit : highest stress at which stress is directly proportional to strain. Its is the highest stress at which curve in a stress-strain diagram is a straight line. It is equal to elastic limit for many materials.

True strain : instantaneous percentage of change in length of spaecimen in mechanical test. It is eual to the natural logarithm of the ration of length at any instant to original length.

True stress : applied load divided by actual area of the cross section through which load operated. It takes into account the change in cross section that occurs with changing load.