Building Materials 10 - Testing Methods
14. TESTING OF STEEL
14.1 Marking of Steel
The most common type of marking in Czech republic is by five-digits number (eventually six-digits), for
example 10 335,or 11 373, where
first two digits
material group sign
10 ….. building steel,
11 ….. machine steel
second two digits for steels from group 10
33 … 1/10 of yield strength in MPa
other steels
37 …
some important property, for example
5 …..
8 …..
complementary sixth digit state of steel
for ex. 1 …..
1/10 of tensile strength in MPa
fifth digit
good weldability
cold worked steel
normalised steel
14.2 Basic Properties of Reinforcing Steel
3
Volume weight
7850 kg/m
Modulus of elasticity
210 000 MPa (N/mm )
Thermal expansion coefficient
11.10 – 13.10
Thermal conductivity
75 W/mK (pure iron). With increasing carbon
2
-6
-6
1/K
content decreases to 50 W/mK
Tensile strength
250 – 2000 MPa
in relation to carbon content
0,1-0,15 % of C……….. 370-450 MPa
0,58 % of C ………….. 700 MPa
in relation to temperature
at temperature > 300°C decreases
at temp. > 500°C decreases to 50 % of
original value
The basic properties of common types of Czech reinforcing steels are given in Tab.:32 and Tab.:33
Tab.:32 Main properties of reinforcing steel
Mark of steel
10 216 E
11 373 EZ
Standard requirement [ MPa]
minimum yield
strength
tensile strenght
206
max. 539
∅ > 16
226
∅ ≤ 16
235
minimum
ductility
[%]
min. 363
24
26
27
10 245 K
245
min. 363
18
10 335 J
325
min. 471
18
10 338 T
325
min. 390
12
10 425 V
410
min. 569
14
10 505 R
490
min. 550
12
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Chapter 14 -Testing of Steel
Tab.:33 Shape of some types of reinforcing steel
Mark of steel
Nominal
diameter
10216 E
5,5 – 12
10 373 EZ
> 16
≤ 16
Shape and surface
10 245 K
6 – 50
10 335 J
10-32
6,5
10 338 T
8
6
10 425 V
8 and 10
10-32
10 505 R
6 – 36
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Building Materials 10 - Testing Methods
14.3 Tensile Test
Tensile test is one of the most common tests for steel. The test is described by standard EN 10 002.
The test involves straining a test piece by tensile force, generally to fracture, for the purpose of
determining tensile strength, yield strength, event. ductility and reduction of area.
14.3.1 Definitions
gauge length (L) - length of cylindrical or prismatic portion of the test piece on which elongation is
measured at any moment during the test [m]
original gauge length (L0) - gauge length before application of force [m]
final gauge length (Lu) - gauge length after rupture of the test piece [m]
elongation - increase in the original gauge length at the end of the test
ductility – percentage elongation after fracture (A) - permanent elongation of the gauge length
after fracture , expressed as the percentage of the original length:
A=
Lu - L0
L0
[%]
extension – increase of the original length at a given moment of the test
percentage reduction of area (Z) - maximum change of cross sectional area, which was
occurred during the test, expressed as a percentage of the original cross-sectional area .
Z=
where S0 is
Su
S 0 - Su
S0
[%]
2
original cross-sectional area before testing [m ]
2
minimum cross-sectional area after fracture [m ]
maximum force (Fm) - the greatest force which the test piece withstand during the test [N ]
stress (σ
σ) - force at any moment during the test divided by the original cross-sectional area (S0) of
the test piece :
σ=
F
S0
tensile strength (Rm) - stress, corresponding to the maximum force Fm :
Rm =
Fm
S0
[MPa]
yield strength (Ry) – when metallic material exhibits a yield phenomenon, a point is reached
during the test at which plastic deformation occurs without any increase in the force :
Ry =
where Fy is
[MPa]
Fy
S0
[MPa]
force at the point of yield [N]
proof strength (Rp) – stress at which extension is equal to a specified percentage of the gauge
length. the symbol used is followed by a suffix giving the prescribed percentage,for example Rp, 0,2
page 74
Chapter 14 -Testing of Steel
Fig.:36 Stress –strain diagram
a) steel with yield point
b) steel with proof strength Rp,0,2
σ [N/mm2]
σ [N/mm2]
( F [N] )
( F [N] )
tensile strength
Rm
Rp,0,2
proof strength
fracture
Ry
yield strenght
elastic limit
proportionality limit
ε [-]
ε [-]
( ∆l [mm] )
0,2 %
( ∆l [mm] )
14.3.2 Test Pieces
The shape and dimensions of the test pieces depend on the shape and dimensions of the metallic
products the mechanical properties of which are to be determined.
The test piece is usually obtained by machining a sample from the product. However product of
constant cross-section may be subjected to test without being machined. The cross section of the test
pieces may be circular, square, rectangular, annular or, in special cases, of some other shape.
14.3.3 Determination of Original Cross-Section Area
The original cross-section area S0 shall be calculated from measurements of the dimensions of the test
piece.
for products of circular cross-section and smooth surface S0 may be calculated from formula:
S0 =
π.d 2
4
[mm ]
2
where d is the arithmetic mean of two measurements carried out in two perpendicular direction
for products of ribbed surface S0 may be determined from the mass of a known length L and its
3
density (7850 kg/m ) according the formula :
ρv =
m
m
=
V
S0 × L
and from it :
S0 =
m
ρv × L
[m ]
2
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Building Materials 10 - Testing Methods
14.3.4 Determination of Original Gauge Length
Elongation is not equal through the whole length of the test piece. At the point of fracture is biggest and
decreases with the distance from this point. This is the reason, why the percentage elongation after
fracture is determined on special length – original gauge length. Test pieces could be proportional and
non-proportional.
proportional test pieces have the original gauge length in relation with the original cross-section
area according the formula:
L 0 = k . S0
where k is equal to 5,65 ( eventually 11,3 )
S0
original cross-sectional area
In the case of test pieces with the circular cross-section this formula gives:
Lo = 5 d ( and for k = 11,3 Lo = 10 d ), where d is diameter of the test piece
non-proportional test pieces may be used if specified by the product standard.
Test pieces of circular cross-section shall preferably have the dimensions given in Tab.:34
Tab.:34 Dimensions of circular cross-section pieces
k
Diameter d
mm
Original crosssection area S0
Original gauge
length
Total length Lt
L0 = k S0
mm
5,65
2
mm
20 ± 0,150
314,2
100 ± 1,0
10 ± 0,075
78,5
50 ± 0,5
5 ± 0,04
19,6
25 ± 0,25
mm
Depends on the method of
fixing the test piece in the
machine grips.
In principle: Lt > Lc + 2d
14.3.5 Determination of Final Gauge Length
Standard EN 1002 – 1 says that measurement of final gauge length is valid only if the distance
between the fracture and the nearest gauge mark is not less than one third of the original gauge
length. In order to avoid having to reject test pieces in which fracture may occur outside the limits, the
method based on sub-division of L0 into N equal parts may be used:
Before the test sub-divide the original gauge length into N equal parts. Recommended value of N is 10
and the size of one part is than L0/10. Make the complementary scale (scale division is equal to one
part) along the total test piece.
After fracture the two broken pieces of the test piece are carefully fitted back together so that their axes
lie in a straight line. Special precaution shall be taken to ensure proper contact between the broken
parts.
From the point of fracture measure five parts on each side (together 10 parts) and it is final gauge
length Lu. If there is not enough parts (less than five) at one side, than final gauge length is determined
in this way (see Fig.:37):
on the shorter piece measure the distance from the fracture to the last mark. This distance is La
on the longer piece measure the distance from fracture to the mark, corresponding to five parts -Lb
on the longer piece find the parts, symmetrically (from fracture) corresponding to the parts, which
miss on the shorter part. This distance is Lc
final gauge length is than equal to :
Lu = L a + L b + L c
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Chapter 14 -Testing of Steel
Fig.:37 Determination of the final gauge length
Lc
Lb
La
Lc
Lu
14.3.6
Testing Procedure
before testing measure diameter of the test piece, determine cross-sectional area S0 and original
gauge length L0. Complementary scale (according 14.3.5) shall be marked along the whole test
piece. The marks could not result in premature fracture.
grip the test piece in the jaws of the test machine. Ensure that test pieces are held in such a way
that the force is applied as axially as possible.
prepare writing device for making of stress-strain diagram
apply load by prescribed rate of stressing. Within the elastic range the rate of stressing shall be
within the limits given in Tab.:35. Within the plastic range the straining rate shall not exceed
0,0025/s for determination of yield strength and 0,008/s for determination of tensile strength
after fracture put down the maximum force Fm, measure the final gauge length Lu (according
14.3.5) and minimum diameter after fracture. From stress-strain diagram find the force at the point
of yield Fy (by the rule of three).
determine tensile strength Rm, yield strength Ry, percentage elongation after fracture A, minimum
cross-sectional area and percentage reduction of the area Z according chap. 14.3.1.
Tab.:35 Rate of stressing
Rate of stressing
Modulus of elasticity of the
material
2
N/mm . s
-1
min.
max.
< 150 000
2
10
≥ 150 000
6
30
N/mm
2
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Building Materials 10 - Testing Methods
14.3.7 Evaluation of Results
Determined values of tensile strength yield strength and ductility (i.e. percentage elongation after
fracture) should be compared with requirements given in the appropriate standards. Basic
requirements for common types of steel are given in Tab.:32. Value of the percentage reduction of the
area is only informative and is not given in standards.
Note: Integral part of the laboratory report is stress-strain diagram, obtained from testing machine,
completed by axes, description and scale.
Vocabulary
strain
deformace
cold worked steel
ocel tvářená zastudena
ductility
tažnost
elongation
prodloužení
extension
protažení
final gauge length
konečná měřená délka
gauge length
měřená (odměrná)délka
grip
pevně uchytit
jaw
čelist
original gauge length
počáteční měřená délka
percentage elongation after fracture
tažnost
percentage reduction of the area
stažnost (kontrakce)
proof strength
smluvní mez kluzu
reinforcing steel
betonářská ocel
ribbed
žebrovaný, s vroubkovaným povrchem
stress
napětí
stress-strain diagram
pracovní diagram
weldability
svařitelnost
yield
kluz, průtažnost
yield strength
mez kluzu
page 78