CHAPTER 7
Characteristics of
Mechanical Tests
1
7.1 Static Tension
A simple tension test is accomplished by
gripping opposite ends of the piece of
material and pulling it apart.
The use of tension test is largely determined
by the type of service to which a material is
to be subjected.
Metals and plastics generally exhibit
relatively high tenacity for resisting tensile
loads.
2
Specimens for tensile tests
Tensile strength test procedures are used to determine the
properties of;
wire,
reinforcement bars
rod,
fibers
tubing,
fabrics
The cross section of the specimen is usually round, square or
rectangular.
The central portion of the length is usually (but not always) of
smaller cross section than the end portions in order to
cause failure at a section where the stresses are not
affected by the gripping device.
3
Specimens for tensile tests
The gage length is the marked on which elongation or extensometer
measurements are made.
The shape of the ends should be suitable to the material and fit
properly to the gripping device.
The ends of the round specimens may be plain, shouldered or
threaded.
4
Specimens for tensile test
The percent elongation (or strain) of a ductile metal
specimen of given diameter depends on the gage length
over which the measurements are made.
For small cylindrical specimens for ductile materials, ASTM
(E8) calls for a gage length of four times the diameter.
TS 138 EN 10002-1 “Metallic materials- Tensile testingPart 1: Method of test at ambient temperature” is the
standard test method to obtain the tensile strength of
reinforcement bars and other metallic materials of
construction.
ASTM standards and TS 138 use specified gage length and
thickness or diameter as a base.
5
The properties obtained by
tensile strength test
In the commercial tension test of metals the properties
usually determined are:
• yield strength (yield point of ductile materials),
• tensile strength,
• ductility (elongation and reduction in area),
• type of fracture.
For brittle materials, only the tensile strength and
character of fracture are commonly determined.
In more complete tests, determinations of stress-strain
relations, modulus of elasticity and other mechanical
properties are included.
6
7
Application of tensile strength test
Prior to applying the load to a specimen, diameter
or width and the thickness of the critical section
are measured.
If elongation measurements are to be made, the
gage lengths are marked very light so as not to
damage the metal and influence the break.
The speed of testing should not be greater than the
rate at which the load and other readings can be
made with the desired degree of accuracy.
8
Elongation & Reduction in area
After the specimen has failed, it is removed from the
testing machine, and if elongation values are needed,
the broken ends of a specimen are fitted together and
the distance between gage points is measured to the
nearest 0.2 mm. The diameter of the smallest section is
calipered for determining the reduction in area.
The elongation is the increase in gage length, expressed
as a ratio of the original gage length. The reduction in
area is the difference between the area of smallest
cross section (at the break) and the original crosssectional area, expressed as a ratio of the original
cross-sectional area.
9
Tensile fracture types
Ductile metals experience observable
plastic deformation prior to
fracture. Brittle metals experience
little or no plastic deformation prior
to fracture.
Some metals behave in a transitional
manner - partially ductile/brittle.
Ductile fracture has dimpled, cup-cone
fracture appearance. The dimples
can become elongated by a lateral
shearing force, or the crack may be
in the opening (tearing) mode.
Brittle fracture displays either cleavage
or intergranular fracture.
A description of the fracture type
should be given in every test report.
10
Comparison of Breaks.
Steel
neck down
break
Aluminum
45 degree
break
Cast iron
straight break
11
Effect of size of the specimen
on tensile strength test
The size and shape of a specimen affect the
mechanical properties of a metal to a different
degree.
If the metal is of uniform quality, the size of
geometrically similar specimens does not
appreciably affect the results of the tension test.
The diameter and the gage length of the piece must
be fixed for obtaining comparable elongations on
metal specimens.
12
7.2 Static Compression
Compressive strength is the most significant
strength for concrete since the concrete
members are primarily designed for
compressive loads.
Furthermore, some reliable correlations exist
between the compressive strength and
other strengths and properties of practical
significance.
13
Concrete Compressive Strength TestCylinder samples
14
Concrete Compressive Strength Test-Cube
samples
15
Standard specimens for concrete
compressive strengh test
For concrete, the standard specimens used in USA
are cylinders twice the diameter in height which
changes with the maximum aggregate size Dmax:
Dmax< 50 mm
150x300 mm
Dmax < 65 mm
200x400 mm
Dmax < 150 mm (mass concrete) 450x900mm
In Europe (Turkey) 150mm cubes or 150x300mm
cylinders are chosen as the standard specimens.
16
Standard Specimens
1. Cylinders: d: 150, h: 300
2. Cubes: 150x150
17
Standard specimens for mortar
and wood
For compressive strength testing of mortars ASTM
specifies cubes of 50 mm dimensions, however, EN
specifies the use of 40x40x160 mm for bending test and
the use of 40 mm modified cubes for compressive
strength tests.
Specimens for compression tests of small pieces of wood
parallel to the grain are 50x50x200 mm rectangular
prisms.
Compression tests perpendicular to the grain are made on
nominal 50x50x150mm specimens. The load is applied
through a metal bearing plate 50 mm width placed
across the upper surface.
18
Compression test of wood
perpendicular to the grain
19
Bearing Plates
Bearing plates should be flat and parallel surfaces
which should also be strong and hard relative to the
test specimen.
Usually one end of the specimen should bear on a
spherically seated block.
The purpose of the block is to overcome the effect of a
small lack of parallelism between the head of the
machine and the end face of the specimen.
It is desirable that the spherically seated bearing block
be at the upper end of the specimen. The specimen
should be centered with respect to the center of the
spherical surface.
20
Spherical bearing blocks for
compression tests
Spherical
seat
Concrete
specimen
21
Test Procedure
In commercial tests, the only property determined is
the compressive strength. The dimensions of the
test specimens should be determined with
appropriate precision. Metals: to the nearest
0.02 mm, concrete and wood: to the nearest
0.2 mm.
The rate of loading of the compressive strength test
should be within 1.4-3.5 kgf/cm2/sec.
Above the practical range, increasing the test speed
increases the ultimate strength, while decreasing
the test speed reduces it due to creep effect.
22
Types of failure of brittle
materials under compression
Brittle materials commonly
rupture either along a
diagonal plane, or with a
cone- (cylindrical
specimens) or a pyramidal(square specimen) shaped
fracture, sometimes called
an hourglass fracture.
Shear
cone or
hourglass
(mortar or
stone
cubes)
23
Mohr’s Theory of Rupture
A material that resists deformation and failure by
internal friction as well as by cohesion and that
behaves in accordance with the Mohr theory of
rupture, the angle of rupture  is not 45o (the
plane of maximum stress) but is a function of the
internal friction angle ; the angle  which the
plane of failure makes with the axis of loading is
equal to 45 - /2.
The internal friction angle for cast iron, sandstone,
brick and concrete is approximately 20° for
specimens long enough for normal failure
surfaces to develop.
24
Relation between angle of
rupture& angle of friction
25
Deviation from the theory
The behavior of materials such as iron, concrete or ceramics
do not conform exactly to Mohr’s theory due to two main
reasons:
• Their nonhomogeneous composition causes irregularities in
the stress pattern.
• The angle of rupture may deviate from the theoretical value
due to the stress conditions induced in the end portions of
compression specimens by restraint to lateral expansion
caused by friction of the bearing plates on the end surfaces.
The second effect is more pronounced for the short specimens
26
If the specimen is so short that a normal failure
plane cannot develop within its length, then the
strength is appreciably increased, and other
types of failure, such as crushing may occur.
With brittle materials in short specimens, when
there is a combination of high compressive
strength and unrestrained lateral expansion at
the ends, the pieces often fail by separation into
columnar fragments, known as splitting failure or
columnar fracture.
27
Wood under compression
Wood under compressive loading exhibits a behavior
peculiar unto itself. It is an isotropic material
composed of cells formed by organic growth that
align themselves to form a series of tubes and
columns in the direction of the grain.
As a result of this structure, its elastic limit is relatively
low without any definite yield point. These
properties vary with the orientation of the load with
respect to the direction of grain.
28
Wood under compression
For loads normal to the grain, the load that causes
lateral collapse of the tubes or fibers is the
significant load.
For loads parallel to the grain, the elastic strength as
well as the strength at rupture is of importance.
Ductile and plastic materials bulge laterally and take
on a barrel shape as they are compressed,
provided that the specimen does not bend or
buckle.
29
Types of failure of wood under
compression paralled to grain
30
Effect of Specimen Shape&Size
For uniform stressing of the compression
specimen, a circular cross section is preferred,
square or rectangular shapes are often used.
The selection of the ratio of height to diameter of a
compression specimen is of importance.
The effect of frictional restraint at the ends of the
specimen (end-effect) becomes unimportant by
increasing h/d ratio.
h/d: 2 is commonly employed.
31
Relative compressive str. %
Effect of height of concrete on
strength
200
180
160
140
120
100
80
0
1
2
3
4
h/d ratio
32
Effect of Specimen Shape&Size
The actual size of the specimen depends on the type
of the material, the type of the measurements to
be made and the testing apparatus available.
The ends to which the load is applied should be flat
and perpendicular to the axis of the specimen or
made so by using caps or adjustable bearing
devices.
It is desirable for the capping material to have a
modulus of elasticity and strength at least equal to
that of the material of the specimen. The cap
should be as thin as is practicable (3-5 mm).
33
Effect of size on concrete
cylinders on strength
34
7.3 Shear Tests
A shearing stress acts parallel to a plane,
as distinguished from tensile and
compressive stresses, which act normal
to a plane.
Loadings causing shear conditions:
• Direct shear
• Torsion shear
35
Direct Shear
If the resultants of parallel but opposed
forces act through the centroids of sections
that are spaced very small distances apart,
the shearing stresses over the sections
would be uniform and a state of pure direct
shear would exist.
This condition may be approached but is
never realized practically.
36
Direct shear loadings
Rivet in double shear
Wood block in single shear
37
Direct Shear Test
For the direct shear of metals, a bar is usually sheared in
some device that clamps a portion of the specimen while
the remaining portion is subjected to load by means of
suitable dies.
In the direct shear test, the testing device should hold the
specimen firmly and preserve good alignment, the load
should be applied at right angles to the axis of the piece.
The only critical value that can be observed in the direct
shear test is the maximum load P. If A is the area
subjected to the force, then the average shearing
strength is taken as P/A. The shape and the fractured
surface of the specimen should be reported.
38
Methods of testing metals in
direct shear
39
Torsion
The applied forces are parallel and opposite but
do not lie in a plane of longitudinal axis of the
body, thus a couple is set up that produces a
twist about a longitudinal axis.
Torsional shearing stresses on circular cross
sections vary from zero at the axis of twist to a
maximum at the extreme fibers.
40
Torsional loading
41
Torsion Test
The purpose of torsion tests is usually
parallel to those for tension tests.
Often used for testing brittle materials and
can be tested in full-sized parts, i.e.
axles and twist drills which are subjected
to torsional loading in service.
42
Apparatus for torsion tests
Although some UTMs have torsional capacity,
special testing machines for torsion testing are
available. Torsion-testing equipment consists of:
1) A twisting head, with a chuck for gripping the
specimen and for applying the twisting moment to
the specimen.
2) A weight head, which grips the other end of the
specimen and measures the twisting moment of
torque.
43
Torsion test macines
44
Torsion test specimens
A circular cross section specimen is
normally used since in the elastic range,
shear stress varies linearly from a value
zero at the centre of the bar to a
maximum value at the surface.
Thin-walled tubular specimens are
frequently used.
45
Test procedure
A twisting moment is applied to the
specimen and the torque is measured.
The angular displacement (or degree of
rotation) of a point near one end of the
test section of the specimen with respect
to a point on the same longitudinal
element at the opposite end is
determined by using a troptometer
(twistmeter).
46
The shear strain γ is given by
Θ: is the angle of twist or degree of rotation, radian
L: is the test length of the specimen
r: radius of the specimen
In the elastic range, the extreme fiber stress т is related to the torque T by the
torsion formula for circular shafts:
 
Tr
J
т: shear stress, Pa
T: torque (or torsional moment), Nm
r: radius of the bar, m
J: polar moment of inertia, m4
47
Types of failure in torsion
Solid barductile
material
Solid barbrittle
material
48
Types of failure in torsion
49
7.4 Static Bending
If forces act on a piece of material in such
a way that they tend to induce
compressive stresses over one part of a
cross section of the piece and tensile
stresses over the remaining part, the
piece is said to be in bending.
Bending action in beams is often referred
to as flexure.
50
The variations in total transverse shear
and in bending moment along a beam
are commonly represented by shear and
moment diagrams.
It should be noted that symmetrical two
point loading given a condition of pure
bending (constant moment) over the
central portion of the span.
51
Shear and moment diagrams
52
Scope of Bending Tests
Most structures and machines have members
whose primary function are to resist loads that
cause bending; e.g., beams, slabs and
columns under eccentric loads.
The design of such members is based on tensile,
compressive and shearing properties
accounted for in various bending formulas. The
bending test may serve as a direct means of
evaluating behavior under bending loads.
53
Scope of Bending Tests
Bending test is often as a control test for
brittle materials such as cast iron and
concrete.
For wire and sheet metals relative flexibility
may be measured by a simple bend test.
54
Specimens in Bending Test
A beam to be tested in flexural failure must be so
proportioned that it does not fail by lateral buckling
or in shear before the ultimate flexural strength is
reached.
In order to avoid shear failure, the span must not be
too short with respect to its depth.
L = 6d, L=12d where L: length, d: depth
Against lateral buckling: L<15b where b: width
Standardized specimens are used for routine testing
of common materials such as concrete, iron, brick
and wood.
55
Test Apparatus
Many flexure tests are conducted in UTMs with the
supports placed on the platen and with the
loading block fastened under the movable head.
Some hand-operated machines are also
employed.
Fiber strains are measured by deformeters or
strainometers supported to the bending fixture or
the beam itself.
Test speeds should be planned so that the readings
can be taken accurately.
56
Third-point loading
57
Center loading
58
Rupture
The failure of beams of brittle materials such as iron and plain
concrete always occurs by sudden rupture.
Failure finally occurs in the tensile fibers, the ratio of tensile to
compressive strength is ~25% for cast iron and ~10% for
concrete. Ultimate tensile strength of such materials measured
by bending test is referred to as flexural tensile strength.
For beams of brittle material, the nominal fiber stress at rupture
as computed by the flexure formula (the modulus of rupture in
bending) is usually greater than the true tensile strength of the
material. The ratio of modulus of rupture to the true tensile
strength is ~1.8 for cast iron and ~1.5 to 2 for concrete.
59
Effect of Loading type in
bending test
The relative magnitudes of the modulus of
rupture for three common types of loading are
as follows:
1. In a simple span, the largest value of the
modulus of rupture is obtained from center
loading.
2. Third point loading on a simple span gives
results somewhat less than center-point
loading (~10-15%).
60
7.4 Hardness
Hardness refers to various properties of
solid matter that gives it high resistance
to various kinds of shape change when
force is applied.
There is no single measure of hardness,
however there are different definitions of
hardness.
61
Definitions of Hardness
1. Resistance to permanent indentation under
static and dynamic loads – indentation hardness
2. Energy absorption under impact loads- rebound
hardness
3. Resistance to scratching- scratch hardness
4. Resistance to abrasion-wear hardness
5. Resistance to cutting or drilling- machinability
62
Results of a hardness test may
be utilized as follows:
1. It should be noted that a hardness number cannot be utilized
directly in design and analysis, for example, rebound hammer
(Schmidt hammer) test results do not indicate the compressive
strength of concrete.
2. The quality level of materials or products may be checked or
controlled by hardness tests. They may be applied to determine
the uniformity of samples of a material or the uniformity of some
treatment such as surface hardening, alloying, heat-treatment etc.
3. By establishing a correlation between hardness and some other
desired property, e.g., tensile strength, simple hardness tests may
serve to control the uniformity of the tensile strength and to
indicate rapidly whether more complete tests are warranted.
63
Indentation Hardness Tests
Hardness measurement by resistance to indentation
is the basis for a variety of instruments. The
indenter, either a ball or a plain or truncated cone
or pyramid, is usually made of hard steel or
diamond.
Either the load that produces a given depth of
indentation or the indentation produced under a
given load can be measured.
The loads may be applied either static or dynamic
ways. In dynamic tests, the force is developed by a
drop or a spring load and the height of rebound of
the indenter is taken as a measure of hardness.
64
Hardness Tests
Most commonly used hardness tests for
metals are Brinell and Rockwell tests.
Increased use of very hard steels or hardened
steel surfaces has brought into use of
several other tests, e.g., Shore scleroscope,
Vickers, Monotron, Rockwell superficial and
Herbert machines.
65
Static Hardness Tests/a.Brinell Test
Exerting a static load on an indenter
deforms the specimen.
10mm hardened steel ball is used
Load exerted by a mass of ;
• 3000 kg for hard metals,
• 1500 kg for metals of intermediate
hardness,
• 500 kg for soft metals.
66
Brinell Test
BHN 
F

2
D(D  D 2  Di2 )
67
b. Rockwell Test
Similar to Brinell test, the hardness number is a
function of the degree of indentation of the
test piece by action of an indenter under a
given static load.
This test differs from Brinell test in that the
indenters and load are smaller (60, 100, or
150 kg) and the resulting indentation is
smaller and shallower.
68
Rockwell Test
69
c. Vickers Test
An indentation is made and the hardness
number is determined from the ratio P/A of
the load exerted by a mass P (kg) to the
contact surface area A (mm2) of the
indentation.
The indenter is a square based diamond
pyramid, the mass varies between 1-120 kg.
70
Vickers Test
71
d. Other Static Hardness Tests
Monotron hardness test measures the
pressure in kg/mm2 necessary to give a
fixed indentation depth of 45 μm.
Microhardness testers such as Knoop
indenter is used to determine the hardness
of a material over a very small area.
A static ball indentation test is standardized
for use with wood, but its usage is not
common.
72
Dynamic Hardness Tests/
a.Shore scleroscope
The hardness measured by this instrument is
often referred to as rebound hardness.
Scleroscope hardness is expressed by a
number given by the height of rebound of a
small pointed hammer after falling within a
glass tube from a height of 254 mm against
the surface of the specimen.
73
Shore Scleroscope
74
b. Schmidt Hammer
A Schmidt hammer is a device to measure the
surface hardness of concrete or rock. The hammer
measures the rebound of a spring loaded mass
impacting against the surface of the sample. When
conducting the test the hammer should be held at
right angles to the surface which in turn should be
flat and smooth.
The rebound reading will be affected by the
orientation of the hammer, when used in a vertical
position gravity will increase the rebound distance
of the mass and vice versa for a test conducted on
a floor slab.
75
Types of Schmidt Hammer
Classified according to
their impact energy:

Type L-0.735 Nm

Type N-2.207 Nm

Type M-29.43 Nm
76
Schmidt Hammer
Schmidt hammer should be calibrated using a
calibration test anvil supplied by the manufacturer
for that purpose.
The average of 15 readings should be obtained.
Using this method of testing is classed as indirect as it
does not give a direct measurement of the strength
of the material.
It simply gives an indication based on surface
properties, it is only suitable for making comparisons
between samples.
77
Wear-Hardness Tests
Abrasion or wear tests have found their principal use in
connection with paving materials, a number of tests
are standardized.
Abrasion resistance of concrete may be determined by
sand blast method (TS 3262).
For mineral aggregates or brick, the resistance to
degradation or abrasion resistance may be
determined by use of Los Angeles machine (TS EN
1097-2).
78
Scratch Hardness Tests
For a qualitative
classification of materials
over a wide range,
perhaps the most
applicable type of test is
the scratch test:
A scale is set up in terms
of several materials,
each of which will just
scratch the material of
next lower hardness
number.
Mohs’ Scale
Hardness No
Reference Material
1
Talc
2
Gypsum
3
Calcite
4
Fluorite
5
Apatite
6
Feldspar (orthoclase)
7
Quartz
8
Topaz
9
Sapphire or Corundum
10
Diamond
79
7.5 Impact
An important type of dynamic loading is that in which the
load is applied suddenly, as from the impact of a moving
mass.
The energy of a blow may be absorbed in a number of ways,
through:
- elastic deformation,
- plastic deformation,
- histeresis effects in the parts,
- frictional action between parts and,
- effects of inertia of moving parts.
80
7.5 Impact
In the design of many types of structures that must take
impact loading, the aim is to provide for the absorption of
as much energy as possible through elastic action.
In such structures, the resilience of the material is a
significant property.
In most tests to determine the energy-absorption
characteristics of materials under impact loads, the object
is to utilize the energy to cause rupture (toughness) of the
test piece.
81
Impact
Toughness depends fundamentally on strength and
ductility and appears to be independent of the type of
loading.
However, the rate at which the energy is absorbed may
markedly affect the behavior of a material, different
(more or less) measures of toughness may be obtained
from impact loadings than from static loadings.
82
Impact resistance of materials
The form of a piece may have a marked effect on its
capacity to resist impact loads.
In order to induce fracture to take place under a single
blow, test specimens of a ductile material are notched.
The use of a notch causes high localized stress
concentrations, causes most of the energy of rupture to
be absorbed in a localized region of the piece and tends
to induce a brittle type of fracture.
The tendency of a ductile material to act like a brittle
material when broken in the form of a notched specimen
is referred to as notch sensitivity.
83
Types of Impact Tests
In an impact test, the load may be applied in
flexure, tension, compression or torsion.
The impact blow may be delivered through
the use of;
• a dropping weight,
• a swinging pendulum, (Izod and Charpy
tests)
• a rotating flywheel.
84
Pendulum Devices
The principle features of a single-blow pendulum
impact machine are:
1. a moving mass whose kinetic energy is great
enough to cause rupture of the test specimen
placed in its part,
2. an anvil and a support on which the specimen
is placed to receive the blow,
3. a means for measuring the residual energy of
the moving mass after the specimen has been
broken.
85
Pendulum Principle
In pendulum tests, calculation of the
energy required for fracture is of
primary importance.
Energy loss due to friction or air
resistance of the pendulum is
generally less than 1% in quality
machines.
86
Pendulum Principle
Initial energy :
Ei=m g a = W a
Energy after rupture:
Er=m g b = W b
Energy absorbed by impact:
Eabs=m g (a - b) = W (a - b)
a = R (1 – cos α)
b = R (1 - cos β)
87
A universal pendulum impact tester
(Charpy, Izod or tension impact
tests)
88
Charpy & Izod Tests
These tests are made on small notched specimens
broken in flexure.
In the Charpy test, the specimen is supported as a
single beam and in the Izod test, it is supported
as a cantilever.
These tests do not simulate shock loading in
service, they simply give the relative resistance
of a particular notched metal specimen to
fracture under a particular type of blow.
89
Charpy Test -Specimens
The standard metal specimen for the Charpy test
is a square notched prism of 10x10x55 mm.
The specimen is arranged as a simple beam
with a span of 40 mm, the notch being on the
tension side.
90
Charpy Testing Machine
91
Izod Test
For the Izod test 10x10x75 mm notched prisms are
clamped to act as vertical cantilevers
92
Drop-weight machines
In drop weight machines the principal
features are the moving mass of known
kinetic energy.
In contrast to pendulum type tests, the
drop-weight tests do not necessarily
break a specimen.
There may be an automatic rebound brake
to limit the impact to a single blow.
93
Drop-weight machines
94
Drop-weight machines
The tup is raised by an electric motor to a specified or
desired release position.
Machines with drop heights up to about 2.5 m are
available, but ~0.6 m is more usual.
Thus velocities may be as high as 7 m/s but are typically
around 3 m/s. The tup masses usually range from ~10
to 200 kg. The energy achieved may be as high as
18kJ.
The kinetic energy of the tup equals the potential energy
before release, there are minor energy losses due to
friction and air resistance.
95
7.6 Fatigue
Fatigue is the progressive and localized
structural damage that occurs when a
material is subjected to cyclic loading.
The maximum stress values are less than
the ultimate tensile stress limit, and may
be below the yield stress limit of the
material.
96
The stress at which a metal fails by fatigue after a
certain number of cycles is termed the fatigue
strength.
For most materials, there is a limiting stress
below which a load may be repeatedly applied
and indefinitely large number of times without
causing failure, this limiting stress is called
fatigue limit or endurance limit.
For most constructional materials, the fatigue limit
in completely reversed bending varies between
about 0.2 and 0.6 of the static strength.
97
S-N Curve
From a test series, a record may be
produced that relates the number
of cycles N to which the
specimen had been subjected
before failure occurred to the
fatigue strength S (S-N curve or
Wöhler curve).
The red points in the chart represent
the cyclic stress for each test and
the number of cycles at which
the specimen broke. The blue
points represent the stress levels
and number of cycles applied to
specimens which did not fail.
This diagram clearly
demonstrates the statistical
nature of metal fatigue failure
98
S-N Curve
If the applied stress level is below the
endurance limit of the material, the
structure is said to have an infinite life.
This is characteristic of steel and titanium,
typical S-N curve corresponding to this
type of material is shown Curve A.
Many non-ferrous metals and alloys, such
as aluminum, magnesium, and copper
alloys, do not exhibit well-defined
endurance limits. In such cases
a fatigue strength Sf for a given
number of cycles must be
specified. An effective endurance limit
for these materials is sometimes
defined as the stress that causes
failure at 1x108 or 5x108 loading
cycles.
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Types of Fatigue Loadings
Zero-to-max-to zero: a part which is carrying no load is then
subjected to a load, and later, the load is removed, so the part
goes back to the no-load condition.
Varying load superimposed on a constant load: The
suspension wires in a railroad bridge are an example of this
type. The wires have a constant static tensile load from the
weight of the bridge, and an additional tensile load when a train
is on the bridge.
Fully-reversing load. One cycle of this type of fatigue loading
occurs when a tensile stress of some value is applied to an
unloaded part and then released, then a compressive stress of
the same value is applied and released. A rotating shaft with a
bending load applied to it is a good example of fully reversing
load.
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Fully reversing load
Varying load superimposed
on a constant load
Zero-max-zero if σmin=0
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Fatigue Tests
One of the simplest and widely used type of
fatigue test is completely fully reversed flexural
loading on rotating beam specimens, the
maximum stress being computed with the
simple flexure formula.
A constant amplitude test for metals is described
by ASTM E 466.
A fatigue test is generally unsuitable for an
inspection or a quality control test, owing to the
time and effort required for collecting data.
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Fatigue test specimens
These laboratory samples
are optimized for fatigue
life.
They are machined with
shape characteristics
which maximize the
fatigue life of a metal,
and are highly polished
to provide the surface
characteristics which
enable the best fatigue
life.
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Fatigue Testing Machines-1
Machines for making fatigue tests under cycles of
repeated or reversed stress may be classified
according to the type of stress produced:
1. Machines for cycles of axial stress (tension,
compression)
2. Machines for cycles of flexural stress
3. Machines for cycles of torsional shearing stress
4. Machines for axial, flexural, or torsional shearing
stresses or combinations of them
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Fatigue Testing Machines-2
All repeated stress testing machines must be provided with a
means for applying load to a specimen and with a means for
measuring the load. Also there must be a counter for recording
the number of cycles applied and some device that
automatically disengages the counter when the specimen
breaks.
Several types of fatigue testing machines are broader in
application. They have one stationary head or fixed platen and
one vibratory platen. The vibratory platen exerts a controlled
motion or force on the specimen: if exerted axially, tensile and
compressive stresses will be developed. By use of fixtures,
torsional or flexural stresses can be developed.
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106
Testing Procedure
To determine the fatigue limit of a metal, it is necessary to
prepare a supply of identical specimens that are
representative of the material.
The first specimen is tested at a relatively high stress so
that failure will occur after a small number of
applications of stress. Succeeding specimens are then
tested, each one at lower stress. The number of
repetitions required to produce failure increases as the
stress decreases.
Specimens stressed below the fatigue limit will not
rupture.
S-N diagram, as described earlier, is then prepared.
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7.7 Creep
The phenomenon of gradual increase in
strain with time under a given sustained
stress is called creep. The physical
process that brings about failure is a slow
but progressively increasing strain.
Material deformation occurs as a result of
long term exposure to levels of stress that
are below the yield strength or ultimate
strength of the material.
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Effect of Creep on Materials
While most materials are subject to creep,
structural differences among metals, plastics,
rubber, concrete and other materials cause
considerable dissimilarities in the creep
mechanisms.
It is not surprising that the effects of temperature,
as well as stress, vary widely.
In steel, temperatures of several hundred degrees
Celcius may make creep a problem, whereas
some plastics, concretes and lead may undergo
creep at normal atmospheric temperatures.
109
In a creep test a constant load is applied to a tensile
specimen maintained at a constant
temperature. Strain is then measured over a period
of time.
Primary creep, Stage I, is a period of
decreasing creep rate. During this
period, deformation takes place
and the resistance to creep
increases until stage II.
Secondary creep, Stage II, is a period
of roughly constant creep
rate. Stage II is referred to as
steady state creep.
Tertiary creep, Stage III, occurs when
there is a reduction in cross
sectional area due to necking or
effective reduction in area due to
internal void formation.
The slope of the curve is the strain rate
of the test during stage II or the
creep rate of the material.
110
If a specimen undergoing creep is unloaded, some of
the strain is recovered, but an appreciable plastic
strain has become permanent, its amount
depending on the material and test conditions.
Change in length
111
Stress Relaxation
The phenomenon of gradual decrease in
stress with time under a given sustained
strain is called stress relaxation.
The specimen is first subjected to a fixed
strain at an initial stress and the load
required to maintain the strain is
observed progressively by time.
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Creep Test for Metals-1
Creep tests at high temperatures appear to be the only
satisfactory guide to the performance of metals for high
temperature service.
Creep tests are inherently long-time tests, but the test
periods may nevertheless be short in comparison with
periods of high temperature service in actual structures,
so that extrapolation of creep-test data must be made
with judgment.
Four variables are involved in a creep test for metals: stress,
strain, time and temperature. The test may be conducted
on individual specimens at each of several loads and
several temperatures.
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Creep Test for Metals-2
The determination of creep characteristics of metals
at high temperatures requires the use of three
pieces of major equipment:
(1) an electric furnace with suitable temperaturecontrol,
(2) an extensometer,
(3) a loading device.
Temperature measurements are made with
thermocouples.
114
Creep Test for Metals-3
115
Creep Tests for Concrete-1
Two similar test methods for determining
the creep of concrete are;
 ASTM C512- Standard Test Method for
Creep of Concrete in Compression.
 TS 3454 Test Method for Determining
the Creep of Concrete in Compression.
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Creep Tests for Concrete-2
Creep of cylindrical concrete specimens
having Dmax< 50mm is determined under
constant compressive load.
Creep test specimens were loaded until
40% of their compressive strength
(compressive strength is determined on
identical specimens from the same batch).
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Creep Tests for Concrete-3
Strain values are recorded just before and
after loading, 2 hours and 6 hours later,
respectively. Following records were
made everyday up to one week, every
week up to one month and every month
up to one year.
At the end of the test period, creep-log t
diagram is plotted.
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