Tensile Testing Of Metallic Materials

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Tensile testing of metallic materials is specified according to European EN 10002 standard. This standards consists of five parts:

• EN 10002-1 - Method of testing at ambient temperature
• EN 10002-2 - Verification of the force measuring system of the tensile testing machine
• EN 10002-3- Calibration of force proving instruments used for the verification of uniaxial testing machines
• EN 10002-4 - Verification of extensometers used in uniaxial testing
• EN 10002-5 - Method of testing at elevated temperatures

In this article the terms, definitions and designation for tensile test made at ambient temperature is described. The test involves straining a test piece in tension, generally to fracture, for the purpose of determining mechanical properties.

Terms and definitions

For the purpose of this European Standard, the following terms and definitions apply:

• gauge length (L) - length of the cylindrical or prismatic portion of the test piece on which elongation is measured. In particular, a distinction is made between:
• original gauge length (L_o) - gauge length before application of force
• final gauge length (L_u) - gauge length after rupture of the test piece
• parallel length (L_c) - parallel portion of the reduced section of the test piece
• elongation - increase in the original gauge length (L_o) at any moment during the test
• percentage elongation - elongation expressed as a percentage of the original gauge length (L_o)
• percentage permanent elongation - increase in the original gauge length of a test piece after removal of a specified stress, expressed as a percentage of the original gauge length (L_o)
• percentage elongation after fracture (A) - permanent elongation of the gauge length after fracture (L_u - L_o), expressed as a percentage of the original gauge length (L_o). In the case of proportional test pieces, where original gauge length is other than 5.65√S_o, the symbol A should be supplemented by an index indicating the coefficient of proportionality used (A_11,3 for L_o=11.3√ So) or by an index indicating the original gauge length (A_{80 mm} for L_o=80 mm)
• 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)
• extensometer gauge length (L_e) - length of the parallel portion of the test piece used for the measurement of extension by means of an extensometer
• extension - increase in the extensometer gauge length (L_e) at a given moment of the test
• percentage permanent extension - increase in the extensometer gauge length, after removal from the test piece of a specified stress, expressed as a percentage of the extensometer gauge length (L_e)
• percentage yield point extension (A_e) - in discontinuous yielding materials, the extension between the start of yielding and the start of uniform work hardening
• percentage reduction of area (Z) - maximum change in cross-sectional area which has occurred during the test (S_o - S_u) expressed as a percentage of the original cross-sectional area (S_o)
• maximum force (F_m) - 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
• stress - force at any moment during the test divided by the original cross-sectional area (S_o) of the test piece
• tensile strength (R_m) - stress corresponding to the maximum force (F_m)
• yield strength - when the metallic material exhibits a yield phenomenon, stress corresponding to the point reached during the test at which plastic deformation occurs without any increase in the force. A distinction is made between:
• upper yield strength (R_eH) - value of stress at the moment when the first decrease in force is observed
• lower yield strength (R_eL) - lowest value of stress during plastic yielding, ignoring any initial transient effects
• proof strength, non-proportional extension (R_p) - stress at which a non-proportional extension is equal to a specified percentage of the extensometer gauge length (L_e). The symbol used is followed by the suffix giving the prescribed percentage, such as R_p0,2
• proof strength, total extension (R_t) - stress at which total extension (elastic extension plus plastic extension) is equal to a specified percentage of the extensometer gauge length (L_e). The symbol used is followed by the suffix giving the prescribed percentage, such as R_t0,5
• permanent set strength (R_r) - stress at which, after removal of force, a specified permanent elongation or extension expressed respectively as a percentage of the original gauge length (L_o) or extensometer gauge length (L_e) has not been exceeded
• fracture - phenomena which is deemed to occur when total separation of the test piece occurs or force decreases to become nominally zero

Symbols and designations

Symbols and corresponding designations of the test piece are given in table 1.

The shape and dimensions of the test pieces depend on the shape and dimensions of the metallic product from which the test pieces are taken (Figure1). Their cross-section may be circular, square, rectangular, annular or, in special cases, of some other shape. The test piece is usually obtained by machining a sample from the product or a pressed blank or casting. However, products of constant cross-section and as cast test pieces may be tested without being machined.

Table 1. Symbols and designations of the test piece.

Reference (Figure1)	Symbol	Unit	Designation
1.	a	mm	Thickness of a flat test piece or wall thickness of a tube
2.	b	mm	Width of the parallel length of a flat test piece or average width of the longitudinal strip taken from a tube or width of flat wire
3.	d	mm	Diameter of the parallel length of a circular test piece, or diameter of round wire or internal diameter of a tube
4.	D	mm	External diameter of a tube
5.	Lo	mm	Original gauge length
-	L`o	mm	Initial gauge length for determination of Ag
6.	Lc	mm	Parallel length
-	Le	mm	Extensometer gauge length
7.	Lt	mm	Total length of test piece
8.	Lu	mm	Final gauge length after fracture
-	L`u	mm	Final gauge length after fracture for determination of Ag
9.	So	mm2	Original cross-sectional area of the parallel length
10.	Su	mm2	Minimum cross-sectional area after fracture
-	k	-	Coefficient of proportionality
11.	Z	%	Percentage reduction of area: (So - Su) / So x 100
12.	-	-	Gripped ends

Figure 1. Typical standard test piece

The test piece shall be held by suitable means such as wedges, screwed grips, parallel jaw faces, shouldered holders, etc. Every endeavour should be made to ensure that pieces are held in such a way that the tension is applied as axially as possible in order to minimize bending. This is very important for testing brittle materials or when determining proof or yield strength.

For determination of percentage elongation, the two broken test pieces are carefully fitted back together so that their axis lie in a straight line. Elongation after fracture shall be determined to the nearest 0.25 mm with a measuring device with a sufficient resolution and the value of percentage elongation after fracture shall be rounded to the nearest 0.5% (Table 2). On the Figure 2 schematic definitions of elongation are given.

Table 2. Different types of elongation

Reference (Figure 2)	Symbol	Unit	Elongation
13.	-	mm	Elongation after fracture: Lu - Lo
14.	A	%	Percentage elongation after fracture: (Lu - Lo) / Lo x 100
15.	Ae	%	Percentage yield point extension
-	Lm	mm	Extension at maximum force
16.	Ag	%	Percentage non-proportional elongation at maximum force (Fm)
17.	Agt	%	Percentage total elongation at maximum force (Fm)
18.	At	%	Percentage total elongation at fracture
19.	-	%	Specified percentage non-proportional extension
20.	-	%	Percentage total extension
21.	-	%	Specified percentage permanent set extension or elongation

Figure 2. Definitions of elongation

The designations and related curves for yield, proof and tensile strength are given in the Table 3 and on the Figure 3.

Table 3. Symbols and designations for different types of strength

Reference (Figure 3)	Symbol	Unit	Force and strength
22.	Fm	N	Maximum force
-	-	-	Yield strength -Proof strength -Tensile strength
23.	ReH	MPa	Upper yield strength
24.	ReL	MPa	Lower yield strength
25.	Rm	MPa	Tensile strength
26.	Rp	MPa	Proof strength, non-proportional extension
27.	Rr	MPa	Permanent set strength
28.	Rt	MPa	Proof strength, total extension
-	E	MPa	Modulus of elasticity

Figure 3. Definitions of upper and lower yield strengths for different types of curves

The test report shall contain reference to the standard, identification of the test piece, specified material, type of the test piece, location and direction of sampling test pieces and test results. In the absence of sufficient data on all types of metallic materials it is not possible, at present, to fix values of uncertainty for the different properties measured by tensile test.