The tensile strength Rm (also tearing strength) is a material characteristic value for the evaluation of strength behavior. The tensile strength is the maximum mechanical tensile stress with which a specimen can be loaded. If the tensile strength is exceeded, the material fails: the absorption of forces decreases until the material specimen ultimately tears. The material however undergoes plastic deformation (residual) before reaching the actual tensile strength value.
The tensile strength Rm is determined with a tensile test (e.g. in accordance with the ISO 6892 series of standards (for metallic materials), or the ISO 527 series of standards (for plastics and composites)).
It is calculated from the maximum achieved tensile force Fm and the specimen cross-section surface at the start of the test:
Tensile strength Rm = maximum tensile force Fm / specimen cross-section surface S0
The tensile strength is specified in MPa (megapascal) or N/mm².
In the stress-strain diagram (also stress-strain curve), the tensile stress of the specimen is plotted over its relative change in length in the tensile test.
This curve can be used to determine the different characteristic values for the material to be tested; for example, the elastic behavior or the tensile strength. In the stress-strain diagram, the tensile strength is the maximum stress value reached in the tensile test after renewed increase of the tensile stress.
For metallic materials with a pronounced yield point the maximum tensile force is defined as the highest reached force after the upper yield strength. The maximum tensile force after exceeding the yield strength can also lie below the yield point for weakly work-hardened materials, therefore the tensile strength in this case is lower than the value for the upper yield strength.
The stress strain curve image to the right shows a curve with a high level of work-hardening (1) and with a very low level of work-hardening (2) after the yield point.
For plastics with yield point and subsequent stress, on the other hand, the tensile strength corresponds to the stress at the yield point.
For the evaluation of strength properties, upper and lower yield points, as well as breaking strength or tear strength are determined in addition to the tensile strength.
Yield point is generally used to describe the stress at the transition from elastic to plastic deformation. It is the generic term for elastic limit, upper and lower yield strength (tensile test), compressive yield strength (compression test), flexural yield strength (flexure test) or torsional yield strength (torsion test).
Offset yield points are stresses that already include a certain residual or total elongation. They are used with metallic materials to mark the continuous transition from the elastic to the plastic range.
The term yield point is commonly used in rheology and describes the stress value from which the material starts to flow (especially with plastics). The flow is characterized by the fact that plastic, that is irreversible, deformation of the material occurs when the yield point is exceeded.
In many materials, after the maximum force Fm has been reached, the force and thereby the nominal tensile stress decrease with increasing elongation, until the specimen breaks or tears. The breaking force related to the initial cross sectional area is also called breaking strength or tear strength. It is an important parameter especially for plastics. In the case of brittle metallic materials, elastomers and tough plastics without yield point, the tear strength generally corresponds to the tensile strength.
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