4Definitions

IS 1608:2005, Metallic Materials — Tensile Testing at Ambient Temperature

Last Updated 29 Oct 2026

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For the purpose of this International Standard, the following definitions apply.

4.1

gauge length (L): Length of the cylindrical or prismatic portion of the test piece on which elongation shall be measured. In particular, a distinction is made between:

4.1.1

original gauge length (Lo): Gauge length before application of force.

4.1.2

final gauge length ( $L_{\mathrm{u}}$ ): Gauge length after rupture of the test piece (see 11.1).

4.2

parallel length ( $L_{\mathrm{c}}$ ): Parallel portion of the reduced section of the test piece.

NOTE — The concept of parallel length is replaced by the concept of distance between grips for non-machined test pieces.

4.3

elongation: Increase in the original gauge length (Lo) at any moment during the test.

4.4

percentage elongation: Elongation expressed as a percentage of the original gauge length (Lo).

4.4.1

percentage permanent elongation: Increase in the original gauge length of a test piece after removal of a specified stress (see 4.9), expressed as a percentage of the original gauge length (Lo).

4.4.2

percentage elongation after fracture (A): Permanent elongation of the gauge length after fracture $\left(L_{\mathrm{u}}-L_{\mathrm{o}}\right)$ , expressed as a percentage of the original gauge length $\left(L_{\mathrm{o}}\right)$ .

In the case of proportional test pieces, only if the original gauge length is other than $5,65 \sqrt{S_{o}}{ }^{2)}$ where $S_{o}$ is the original cross-sectional area of the parallel length, the symbol A shall be supplemented by an index indicating the coefficient of proportionality used, for example:

$A_{11,3}=$ percentage elongation of a gauge length $\left(L_{o}\right)$ of $11,3 \sqrt{S_{o}}$ .

In the case of non-proportional test pieces, the symbol A shall be supplemented by an index indicating the original gauge length used, expressed in millimetres, for example:

$A_{80 \mathrm{~mm}}=$ percentage elongation of a gauge length $\left(L_{o}\right)$ of 80 mm.

4.4.3

percentage total elongation at fracture ( $A_{\mathrm{t}}$ ): Total elongation (elastic elongation plus plastic elongation) of the gauge length at the moment of fracture expressed as a percentage of the original gauge length ( $L_{\mathrm{o}}$ ).

$5,65 \sqrt{S_{\mathrm{o}}}=5 \sqrt{\frac{4 S_{\mathrm{o}}}{\pi}}$

4.4.4

percentage elongation at maximum force: Increase in the gauge length of the test piece at maximum force, expressed as a percentage of the original gauge length ( $L_{o}$ ). A distinction is made between the percentage total elongation at maximum force ( $A_{\mathrm{gt}}$ ) and the percentage non-proportional elongation at maximum force ( $A_{\mathrm{g}}$ ) (see figure 1).

4.5

extensometer gauge length ( $L_{\mathrm{e}}$ ): Length of the parallel portion of the test piece used for the measurement of extension by means of an extensometer.

It is recommended that for measurement of yield and proof strength parameter $L_{\mathrm{e}} \geq L_{\mathrm{o}} / 2$ .

It is further recommended that for measurement of parameters "at" or "after" maximum force, $L_{\mathrm{e}}$ be approximately equal to $L_{\mathrm{o}}$ .

4.6

extension: Increase in the extensometer gauge length ( $L_{\mathrm{e}}$ ) at a given moment of the test.

4.6.1

percentage permanent extension: Increase in the extensometer gauge length, after removal of a specified stress from the test piece, expressed as a percentage of the extensometer gauge length (Le).

4.6.2

percentage yield point extension (Ae): In discontinuous yielding materials, the extension between the start of yielding and the start of uniform work hardening. It is expressed as a percentage of the extensometer gauge length (Le).

4.7

percentage reduction of area (Z): Maximum change in cross-sectional area $\left(S_{o}-S_{u}\right)$ , which has occurred during the test expressed as a percentage of the original cross-sectional area $\left(S_{o}\right)$ .

4.8

maximum force (Fm): The greatest force which the test piece withstands during the test once the yield point has been passed.

For materials, without yield point, it is the maximum value during the test.

4.9

stress: At any moment during the test, force divided by the original cross-sectional area (S₀) of the test piece.

4.9.1

tensile strength ( $R_{\mathrm{m}}$ ): Stress corresponding to the maximum force $\left(F_{\mathrm{m}}\right)$ .

4.9.2

yield strength: When the metallic material exhibits a yield phenomenon, a point is reached during the test at which plastic deformation occurs without any increase in the force. A distinction is made between:

4.9.2.1

upper yield strength (R_eH): Value of stress at the moment when the first decrease in force is observed (see figure 2).

4.9.2.2

lower yield strength (R_eL): Lowest value of stress during plastic yielding, ignoring any initial transient effects (see figure 2).

4.9.3

proof strength, non-proportional extension ( $R_{\mathrm{p}}$ ): Stress at which a non-proportional extension is equal to a specified percentage of the extensometer gauge length $\left(L_{\mathrm{e}}\right)$ (see figure 3). The symbol used is followed by a suffix giving the prescribed percentage, for example: $R_{\mathrm{p} 0,2}$ .

4.9.4

proof strength, total extension ( $R_{\mathrm{t}}$ ): Stress at which total extension (elastic extension plus plastic extension) is equal to a specified percentage of the extensometer gauge length ( $L_{\mathrm{e}}$ ) (see figure 4). The symbol used is followed by a suffix giving the prescribed percentage for example: $R_{\mathrm{t} 0,5}$ .

4.9.5

permanent set strength ( $R_{\mathrm{r}}$ ): Stress at which, after removal of force, a specified permanent elongation or extension expressed respectively as a percentage of the original gauge length $\left(L_{\mathrm{o}}\right)$ or extensometer gauge length $\left(L_{\mathrm{e}}\right)$ has not been exceeded (see figure 5).

The symbol used is followed by a suffix giving the specified percentage of the original gauge length ( $L_{\mathrm{o}}$ ) or of the extensometer gauge length $\left(L_{\mathrm{e}}\right)$ , for example: $R_{\mathrm{r} 0,2}$ .