What is Stress and Strain Curve?

Relevant specifications

BS EN 10002 Methods of tensile testing of metallic materials.
BS EN 876 Destructive tests on welds in metallic materials – longitudinal tensile test.
BS EN 895 Destructive tests on welds in metallic materials – transverse tensile test.
BS EN ISO 7500-1 Tension/compression testing machines. verification and calibration of the force measuring system.
ASTM A370 Mechanical testing of steel products.
ASTM E8 Tension testing of metallic materials.
ASTM B557 Tension testing wrought and cast aluminum and magnesium alloy products

In engineering and materials science, a stress–strain curve for a material which showing the relationship between stress and strain of a material section subjected to tension. It is obtained by gradually applying load to a test coupon and measuring the deformation, from which the stress and strain can be determined. These curves reveal many of the properties of a material, such as the Young’s modulus, the yield strength and the ultimate tensile strength.

Stress is a geometry-independent measure of the load on a material. Thus, independently of the actual cross-sectional area, a comparable statement on the load intensity is obtained when specimens with different cross-sections are stressed in the tensile test.

Strain is the measured elongation of the specimen in the tensile test is also still a geometry-dependent quantity. For example a steel specimen may be considerably greater elongated than a wooden specimen despite the same force, provided that the steel specimen is considerably longer than the wooden sample (see blue curve). However, this does not mean that steel can be stretched more than wood.

For this reason, the elongation of the sample must be related to an identical initial length. It is therefore advisable to specify the elongation of the sample as a percentage (%), i.e. in relation to the initial length. This means nothing else than to determine the quotient of elongation Δ-L and initial gage length L-o

This quantity is also called strain ϵ and is a geometry-independent measure of specimen elongation. Thus, a comparable statement about the intensity of the lengthening of a specimen is obtained independently of the initial length. For example, a strain of 4 % of a steel specimen means in principle a lower ductility than an elongation of 10 % of a wooden specimen, regardless of the respective initial lengths.

The yield strength σy indicates the stress limit below which the material undergoes a purely elastic deformation and always reaches its initial length again after removal of the force (elastic limit), Yield strength is one of the most important material parameters in structural engineering.

A more precise distinction can be made between the upper yield strength σyuσyu, which marks the beginning of plastic deformation, and the lower yield strength σylσyl, to which the stress subsequently decreases minimally during the flow of the material (more on this see section yield point elongation).

Typical terms and definition for further understanding the Tension test:

yield strengthstress below which the material is subjected to purely elastic stresshighly stressed materials should have highest possible yield strength
Young’s modulusmeasure of the stiffness of a material (proportionality factor between stress and strain within the elastic region)materials for components that may only deform slightly (elastically) must have a high modulus of elasticity
ultimate tensile strengthmaximum loadable stress from which the material necks and finaly fracturesmaterials for components with high safety relevance must have high tensile strengths
yield-tensile ratioratio of yield strength to tensile strength. Measure of the risk of fracture if the yield strength is exceededmaterials for safety-relevant components should have the lowest possible yield-tensile ratio
offset yield strengthstress at which the material experiences a certain permanent strainhighly stressed materials should have the highest possible offset yield strength (analogous to the yield strength)
unifrom strainmeasure of the formability of a material without neckingmaterials for forming technology should have the highest possible uniform strain.
fracture strainmeasure of the ductility of a material. In combination with a high yield strength, this means a high energy absorption capacity.materials for components that have to absorb a lot of energy in the event of failure should have a high fracture strain.
reduction in areameasure of the brittle fracture resistance of a materialin general, a high reduction in area value of materials is desirable

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