Tensile Testing Of Metallic Materials

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Introduction to Tensile Testing Standards

Tensile testing of metallic materials is specified according to European EN 10002 standard. This standard 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

This article describes the terms, definitions, and designations for tensile tests conducted at ambient temperature. The test involves straining a test piece in tension, generally to fracture, for the purpose of determining fundamental mechanical properties of the material.

Key Terminology 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 (Lo) - gauge length before application of force
  • Final gauge length (Lu) - gauge length after rupture of the test piece

Parallel length (Lc) - parallel portion of the reduced section of the test piece

Elongation - increase in the original gauge length (Lo) at any moment during the test

Percentage elongation - elongation expressed as a percentage of the original gauge length (Lo)

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 (Lo)

Percentage elongation after fracture (A) - permanent elongation of the gauge length after fracture (Lu - Lo), expressed as a percentage of the original gauge length (Lo). For proportional test pieces, where the original gauge length differs from 5.65√So, the symbol A should be supplemented by an index indicating the coefficient of proportionality used (A11,3 for Lo=11.3√So) or by an index indicating the original gauge length (A80 mm for Lo=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 (Lo)

Extensometer gauge length (Le) - 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 (Le) 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 (Le)

Percentage yield point extension (Ae) - 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 (So - Su) expressed as a percentage of the original cross-sectional area (So)

Maximum force (Fm) - the greatest force which the test piece withstands during the test once the yield point has been passed. For materials without a 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 (So) of the test piece

Tensile strength (Rm) - stress corresponding to the maximum force (Fm)

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 (ReH) - value of stress at the moment when the first decrease in force is observed
• Lower yield strength (ReL) - lowest value of stress during plastic yielding, ignoring any initial transient effects

Proof strength, non-proportional extension (Rp) - stress at which a non-proportional extension is equal to a specified percentage of the extensometer gauge length (Le). The symbol used is followed by the suffix giving the prescribed percentage, such as Rp0,2

Proof strength, total extension (Rt) - stress at which total extension (elastic extension plus plastic extension) is equal to a specified percentage of the extensometer gauge length (Le). The symbol used is followed by the suffix giving the prescribed percentage, such as Rt0,5

Permanent set strength (Rr) - stress at which, after removal of force, a specified permanent elongation or extension expressed respectively as a percentage of the original gauge length (Lo) or extensometer gauge length (Le) has not been exceeded

Fracture - phenomenon which is deemed to occur when total separation of the test piece occurs or force decreases to become nominally zero

Test Piece Specifications and Preparation

The shape and dimensions of test pieces depend on the shape and dimensions of the metallic product from which they are taken (Figure 1). 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, 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

Testing Procedure and Measurement

The test piece shall be held by suitable means such as wedges, screwed grips, parallel jaw faces, shouldered holders, etc. Every effort should be made to ensure that pieces are held in such a way that the tension is applied as axially as possible to minimize bending. This is particularly important when 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 axes lie in a straight line. Elongation after fracture shall be determined to the nearest 0.25 mm with a measuring device with sufficient resolution, and the value of percentage elongation after fracture shall be rounded to the nearest 0.5%.

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

Strength Parameters and Result Interpretation

The designations and related curves for yield, proof, and tensile strength are provided in Table 3 and illustrated in 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

Test Reporting Requirements

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
• 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 testing.