Tensile testing of metallic materials is specified according to European EN 10002 standard. 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.
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 (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). In the case of
proportional test pieces, where original gauge length is
other than 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
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 - 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.