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									ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                               HANDOUT FOR MATERIAL SCIENCE


                    ENGINEERING MATERIALS (CN104 double module)
                                             (1st Year C.Eng & E.Eng)


Learning objectives:

1.     To establish necessary base in material science for subsequent studying of concrete technology and
       metallurgy.
2.     To be able to use concrete properly as a construction material and assess its performance.
3.     To understand basic mechanism of physical and mechanical metallurgy and use ordinary metals
       adequately and efficiently.

Lectures

Part 1 : Material Science
Part 2 : Concrete Technology
Part 3 : Metallurgy

Laboratory class

Three sessions of concrete
Three sessions of metallurgy

Assessment

80% from final examination (which will be held after the second semester)
20% from course work (three sessions of concrete lab)

                                               INTRODUCTION

Learning objectives:
1. To be able to detail the elements of design.
2. To be able to list main constitutive models and major civil engineering materials.


The design process for all civil engineering structures can be reduced in the most general terms to the three
professional areas : a)
                     b)
                     c)

Looking at materials from engineer's sight, performance and properties of materials can be listed as
follows:
   1) performance : time-independent, time-dependent, temperature-dependent, moisture-dependent
                     properties, brittleness, ductility, durability etc. (isotropic, orthotropic and
                     anisotropic.)

     2) working conditions : high temperature and pressure (nuclear reactor), high humidity (under water),
                             high corrosion environment (chemical plant).

     3) loading conditions : uniaxial (tension, compression, bending, torsion), biaxial (pipe under internal
                             pressure) and triaxial (tip of crack, submarine in deep sea).

     4) modelling : elasticity, plasticity, creep(time-dependent), viscoplasticity, hyperelasticity (ruber-like
                    materials), hypoelasticity (concrete), deformation plasticity (crack growth),

Major Civil Engineering Materials : 1.

                                        2.

                                        3.

                                        4.


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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                         HANDOUT FOR MATERIAL SCIENCE

                                        5.

                                        6.

                                        7.

                                        8.

Course structure: also see Fig. 1

Section 1. Material Science
1.1 Atomic structure and bonding
1.2 Crystal structures and crystal geometry
1.3 Crystalline imperfections
1.4 Strength of materials.

Section 2. Concrete
2.1 Portland Cement
2.2 Cements of different types,
2.3 Properties of natural aggregates,
2.4 Design of concrete mixes,
2.5 Testing concrete,
2.6 Deformation of concrete,
2.7 Concreting,
2.8 Durability,
2.9 Specialised concrete.

Section 3. Metallurgy
3.1 Physical metallurgy,
3.2 Mechanical properties of metals,
3.3 Mechanical metallurgy,
3.4 Oxidation and corrosion,
3.5 Metals, their differences and uses.

                                    SECTION 1. MATERIAL SCIENCE

Session 1 learning objectives:
1. To understand four quantum numbers, their notations and ranges.
2. To be able to work out electron configuration, valence and electronegativity for given electron number
    of an element.
3. To understand both primary and secondary atomic bondings including their characteristics and
    drawing sketches.

1.1 Atomic structure and bonding

1.1a The structure atoms

Table 1.1 shows properties of subatomic particles.

1.1b Modern atomic theory and the periodic system

Four quantum numbers :

 1)


 2)

 3)


 4)


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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                              HANDOUT FOR MATERIAL SCIENCE

Table 1.2 shows permissible values for the quantum numbers of electrons.

1.1c Electron structure of multielectron atoms

a. Maximum number of electrons in each principal atom shell

Table 1.3 shows the maximum number of electrons in each principal atomic shell.


b. Electron configurations of the elements

Table 1.4 demonstrates electron configurations of the elements.


c. Valence

The valence of an atom is related to the ability of the atom to enter into chemical combination with other
elements and is often determined by the number of electrons in the outermost combined sp level.

d. Atomic stability

If an atom has a valence of zero, no electrons enter into chemical reactions and the element is inert.

Other atoms also prefer to behave as if their outer sp levels are either completely full, with eight electrons,
or completely empty.

e. Electronegativity

Electronegativity describes the tendency of an atom to gain an electron.


1.1d The Periodic Table
The IA to VIIA elements The rows in the periodic table correspond to quantum shells, or principal
quantum numbers.

The IIIB to VIIIB elements. In each of these rows, an inner energy level is progressively filled

The IB and IIB elements. These elements, which include copper, silver, and gold, have complete inner
shells and one or two valence electrons.

Fig. 1.1 shows the Periodic Table.

Table 1.5 shows the electronic configuration of the transition elements.




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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                          HANDOUT FOR MATERIAL SCIENCE

                                                Questions

Part 1 Material Science

1.1 List general civil engineering materials.
1.2 Describe three elements for engineering design including their subareas.
1.3 Show main constitutive models for modelling the performance of engineering materials and the
     corresponding structures.
1.4 Draw a typical sketch for an atomic structure.
1.5 Indicate masses that a proton, a neutron and an electron have, respectively.
1.6 List four quantum numbers, their notations, and their ranges, also use them to describe the motion of
     an electron.
1.7 Write out the electron arrangements in different shells, see electron numbers of elements are 20,35 and
     40.
1.8 Calculate the maximum number of electrons in the N shell of an atom. Determine the atomic number if
     all energy levels in the K,L, M, and N shells are filled.
1.9 Find out valence for Sodium, Zinc and Tin and draw their electron configurations.
1.10 Using the electronic structures, compare the electronegativities of calcium and bromine.
1.11 Determine how many electrons can be contained in the following shell or subshells of an atom:
     i)       O shell (n=5)
     ii)      2p subshell
     iii)     4f subshell.




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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                             HANDOUT FOR MATERIAL SCIENCE

1.1e Atomic bonds
Generally, there are five mechanisms by which atoms are bonded together. In three of the five mechanisms,
bonding is achieved when the atoms fill their outer s and p levels.

1. The ionic bond
When more than one type of atom is present in a material, one atom may donate its valence electrons to a
different atom, filling the outer energy shell of the second atom. Hence both atoms now have fill (or
empty) outer energy levels but both have acquired an electrical charge and behave as ions. The atom that
contributes the electrons is left with a net positive charge, while the atom that receives the electrons
acquires a net negative charge. The oppositely charged ions are then attracted to one another and produce
the ionic bond. As a example, attraction between sodium and chloride ions produces sodium chloride, or
table salt, NaCl. Fig. 1.2 shows such typical ionic bond.

2. The covalent bond
A second type of primary atomic bonding is covalent bonding. Covalently bonded materials share electrons
between two or more atoms. Fig. 1.3 shows such bonding.

Even though covalent bonds are very strong, materials bonded in this manner have poor ductility and poor
electric conductivity. When a silicon rod is bent, the bonds must break if the silicon atoms are going to
permanently change their relationship to one another. Fig. 1.4 demonstrates the broken bonds.

Therefore, covalent bonded materials are brittle rather than ductile and behave as electric insulators instead
of conductors (reason for this is that for an electron to move and carry a current, the covalent bond must be
broken subject to the conditions of high temperatures or voltages).


3. The metallic bond
The metallic elements, which have a low valence, give up their valence electrons to form a "sea" of
electrons surrounding the atoms. The valency electrons can move freely between the positive metallic ions.
Fig. 1.5 shows such bond.

The positive ions are arranged in a crystalline lattice, and the electrostatic attraction between the positive
ions and negative free electrons provides the cohesive strength of the metal.

Metallic bonds are non-directional; the electrons holding the atoms together are not fixed in same place.

The metallic bond also permits metals to be good electrical conductors. The reason for this is that under the
influence of an applied voltage, the valence electrons can move easily which causes a current to flow if the
circuit is completed. Other bonding mechanisms (such ionic bonding) require much higher voltages to free
the electrons form the bond.

4. Van der Waals bonding
Different from previous three primary bonds, Van der Waals bonds join molecules or groups of atoms by
weak electrostatic attractions. Therefore such bonds are so weak that their effect is negligible when any
primary bonds are present. Fig 1.6 illustrates such bonding.

5. Hydrogen bond
Hydrogen bond is another secondary bond which can be made between atomic groups which have no
electrons to spare. It bears a close resemblance to the Van der Waals bond. When hydrogen links
covalently with, for example, oxygen to form water, the electron donated by the hydrogen atom spends the
greater part of its time between the two atoms, and bond requires a definite dipole with hydrogen which
becomes a positively charged ion. Since the hydrogen nucleus is not separated by any other electron shells,
it can attract to itself other negative ends of dipoles, and the result is hydrogen bond.


1.2 Crystal structures and crystal geometry

Session 2 learning objectives:
1. To understand the definition of crystalline materials and space lattice, including unit cell,
2. To be able to indicate how many basic types of unit cells and their names,
3. To be able to draw three most important crystallographic planes of cubic crystal structures,
4. To understand procedures for determining the Miller indices,
5. To understand Allotropy.

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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                                HANDOUT FOR MATERIAL SCIENCE

From engineering importance the physical structure of solid materials mainly depends upon the
arrangements of the atoms, ions, or molecules which make up the solid and the bonding forces between
them. If the atoms or ions of a solid are arranged in a pattern that copies itself in three dimensions, they
form a solid which is said to have a crystal structure and is referred to as a crystalline material. Metals,
alloys, and some ceramic solid are crystalline materials.

Atomic arrangement plays an important role in determining the micro structure and behaviour of a solid
material.


1.2a The space lattice and unit cells
Atomic arrangements in crystalline solids can be described by referring the atoms to the points of
intersection of a network of lines in three dimensions. Such a network is known as a space lattice; it can be
described as an infinite three-dimensional array of points. Fig. 1.7 shows space lattice of ideal crystalline
solid.


1.2b Elementary classification of crystal structures
By specifying specific values for axial lengths and interaxial angles, unit cells of different types can be
constructed. Specialist for crystallograph have discovered and verified that only seven different types of
unit cells are necessary to create all point lattices. Table 1.5 displays such crystal systems.

Many of the seven crystal systems have variations of the basic unit cell. French crystallographer A. J.
Bravais verified that 14 standard unit cells could describe all possible lattice networks. Fig. 1.8 shows those
14 unit cells.


 There are four basic types of unit cells, i.e.

 1)

 2)

 3)

 4)


1.2c Directions in cubic unit cells
Usually it is important to refer to specific directions in crystal lattices. This is specifically useful for metals
and alloys that have properties which vary with crystallographic orientations.

To clearly indicate a direction on a cubic unit cell, we draw a direction vector form an original point which
is usually a corner of the cubic cell until it reaches to the cube surface. Fig 1.9 shows some directions in
cubic unit cells.

The position co-ordinates of the unit cell where the direction vector emerges from the cubic surface after
being converted to integers are the direction indices. The direction indices are enclosed by square brackets
with no separating commas.


1.2d. Miller indices of plane for cubic and hexagonal unit cells
Sometimes it is necessary to refer to specific lattice planes of atoms within a crystal structure, or it may be
of interest to know the crystallographic orientation of a plane or group of planes in a crystal lattice. To
identify crystal planes in cubic crystal structures. the Miller notation system is utilised. The Miller indices
of a crystal plane are defined as the reciprocals of the fractional intercepts (with fractions cleared) which
the plane makes with the crystallographic x, y, and z axes of the three nonparallel edges of the cubic unit
cell. The cubic edges of the unit cell represent unit lengths, and the intercepts of the lattice planes are
measured in terms of these unit lengths.

The steps to determine the Miller indices for a cubic crystal plane are as follows:

   a.   Choose a plane that does not pass through the origin at (0,0,0),

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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                               HANDOUT FOR MATERIAL SCIENCE

   b.   Determine the intercepts of the plane in terms of the crystallographic x, y, and z axes for unit cube.
        Please note that these intercepts may be fractions,
   c.   Form the reciprocals of these intercepts,
   d.   Clear fractions and determine the smallest set of whole numbers which are in the same ratio as the
        intercepts. These whole numbers are the Miller indices of the crystallographic plane and are
        enclosed in parentheses without using commas. The notation (h k l) is used to indicate Miller
        indices in a general sense, where h, k, and l are the Miller indices of a cubic crystal plane for the x,
        y, and z axes, repetitively.

Fig 1.10 shows three of the most important crystallographic planes of cubic crystal structures.

Fig 1.11 shows cubic crystal plane (6 3 2) which is fractional intercepts.

The above system of indexing applies to all crystal systems, but it has been found convenient to modify it
somewhat by the addition of a fourth axis in the case of the hexagonal system. For crystals of this habit, the
axes are arranged as in the following overhead.

Fig 1.12 shows the crystal plane for hexagonal system.


1.2e. Allotropic transformations
Materials that can have more than one crystal structure are called allotropic or polymorphic. Some metals,
such as iron and titanium, have more than one crystal structure. At low temperatures iron has the body-
centred cubic structure but at high temperature iron transforms to a face-centred cubic structure. Allotropy
provides the basis for the heat treatment of steel and titanium.




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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                         HANDOUT FOR MATERIAL SCIENCE

                                               Questions

1.12 Describe five types of atomic bondings and draw their sketches.
1.13 Why are covalent bonding materials brittle? Why do metallic bonded materials have good ductility and
     conductivity?
1.14 Draw a sketch showing the atomic arrangement and bonding you would expect to find in carbon
     dioxide ( CO2 ). (It is possible that atoms can be joined by more than one covalent bond.)
1.15 What is the formula of the ionic compound produced when magnesium and sulphur are brought
     together?
1.16 How stronger is Hydrogen bond than other Van der Waals bond? How weaker is it than any other
     primary bonds?
1.17 Draw a sketch for space lattice and indicate unit cell.
1.18 How many are there Bravais conventional unit cells grouped according to crystal system? How many
     are there basic types of unit cells and what are they called?
1.19 Draw three of the most important crystallographic planes of cubic crystal structures.
1.20 Write out procedures to determine Miller indices of a plane in space.
1.21 Define the term Allotropy and give a typical example.




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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                              HANDOUT FOR MATERIAL SCIENCE

1.3 Crystalline imperfections

Session 3 learning objectives:
1. To understand three crystal lattice imperfections and be able to draw their sketches,
2. To be able to draw typical stress-strain curve for a mild steel under tension including all
    characteristics,
3. To understand engineering stress and strain, true stress and strain,
4. To understand Young’s modulus, Poisson’s ratio, yield stress, tensile strength, breaking strength,
    creep, and fatigue failure.


All materials contain various types of imperfections and defects which have a profound effect on their
physical and mechanical properties.

Crystal lattice imperfections are classified according to their geometry and shape. The three main divisions
are:

(1)


(2)


(3)



1.3a Point defects
The simplest point defect is the vacancy. Fig. 1.13 shows such defect.

A vacancy is produced when an atom is missing from a normal lattice point. Vacancies are introduced into
the crystal structure during solidification, at high temperature, or as a consequence of radiation damage.

Sometimes an atom in a crystal can occupy one interstitial site between surrounding atoms in normal atom
sites. Fig. 1.14 shows such point defect (interstitialcy). This type of point defect is called a interstitialcy.

A Frenkel defect is a vacancy-interstitial pair formed when an ion jumps from a normal lattice point to an
interstitial site, leaving behind a vacancy. Fig. 1.15 shows Frenkel defect.

The Schottky defect is a pair of vacancies in an ionically bonded material. When two oppositely charged
ions are missing from an ionic crystal, a cation-anion divacancy induce Schottky imperfection. Fig. 1.16
shows Schottky imperfection.

In concrete and timber, point defects are not important since small irregularities in a structure already
highly irregular will have little effect. The mechanical effects included by such imperfections are negligible
by comparison with larger imperfections such as cracks and pores. However, in metals the point defects aid
crystal transformations and chemical reactions by accelerating diffusion rates.

1.3b Line defects or dislocations
Dislocations are line imperfections in an otherwise perfect lattice. Two types of dislocations are identified,
i.e.
     a)
     b)

The screw dislocation can be illustrated by cutting partway through a perfect crystal, then skewing the
crystal one atom spacing. Fig. 1.17 shows screw dislocation.

An edge dislocation can be demonstrated by slicing partway through a perfect crystal, spreading the crystal
apart, and partially filling the cut with an extra plane of atoms. Fig. 1.18 shows edge dislocation.

there is also the mixed dislocation which is a combination of edge and screw dislocations. Most
dislocations in crystals are of the mixed type, having edge and screw components. Fig. 1.19 shows the
mixed type dislocation.

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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                           HANDOUT FOR MATERIAL SCIENCE

1.3c Surface defects
Surface defects are the grain boundaries that separate a material into regions, each region having the same
crystal structure but different orientations.

The microstructure of metals and many other solid materials consist of many grains. A grain is a portion of
the material within which the atom arrangement is identical. However, the orientation of the atom
arrangement, or crystal structure, is various for each adjoining grain. Fig. 1.20 shows three grains
schematically.

A grain boundary is the surface that separates the individual grains and is narrow zone in which the atoms
are not properly spread. The atoms are too close at some location in the grain boundary, causing a region of
compression, while in other areas the atoms are too far apart, causing a region of tension.

The properties of a metal can be controlled by grain size strengthening. By reducing the grain size, the
number of grains can be increased and thence the amount of grain boundary be increased. Any dislocation
moves only a short distance before encountering a grain boundary and the strengthen of the metal is
increased. Fig. 1.21 shows the effects of grain size on the yield strength of a steel at room temperature.


1.4 Strength of materials
Usually, we use terminology strength to measure how strong a material is. There are a few parameters
which control such measurements, i.e. Young's modulus E, Poisson's ratio  , yield strength  y , ultimate
tensile strength    t .   Of course, here we mean the static strength of a material. To obtain above the
parameters, a series of uniaxial tensile tests up to failure should be carried out. Furthermore, a repeatable
stress-strain relationship can be achieved.

1.4a Stress-strain curve
As we mentioned above, through the uniaxial tensile tests we can obtain the stress-strain curve.

Fig. 1.22 shows a typical stress-strain curve based on engineering stress and strain and sample dimensions.

Before giving precise explanation for all parameters such as Young's modulus, Poisson's ratio, yield stress,
etc., the definitions for strain and stress should be drawn. There are two stress and strain definitions,
respectively, i.e. engineering stress and strain, true stress and strain. Engineering stress is based on the
original cross section area of test sample. Therefore,


          eng 


Whereas, true stress is defined as


          true 


Engineering strain is defined as follows


          eng 


Finally, true strain or logarithmic strain is defined as



                                     (l  l )
                    li
                       dl      l
          true          ln i  ln o         ln(1   eng )
                    lo
                       l      lo        lo



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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                               HANDOUT FOR MATERIAL SCIENCE


where   l o is the original length of the sample and l i is the instantaneous extended gauge length during the
test. It is very convenient for measuring large deformations in rubber like materials or polymers to use true
strain.

Obviously, if the stress-strain curve is based on true stress and strain, it will be shaped differently. Fig. 1.23
shows such curve. Explain elastic strain and plastic strain.

The reasons to use true stress instead engineering stress (or nominal stress) is that it is convenient to study
strain hardening for ductile materials and develop plastic constitutive modelling.

From the stress-strain curve, we know that in the initial straight part the relationship between stress and
strain is linear. Thus we have

               
          E
               
E is called Young's modulus, which is parameter to measure linear elastic deformations. The modulus of
elasticity is related to the bonding strength between the atoms in metals or alloys.

The yield strength   y   is a very important value for use in engineering structural design since it is the
strength at which a material shows significant plastic deformation.

The ultimate tensile strength   t   is the maximum strength reached in the engineering stress-strain curve.
For any ductile material, its   t   could be much higher than its yield strength since there is very large
deformation before the sample is actually failed. Of course, for low ductile materials, their ultimate tensile
strength are close to their yield strength since there are only little deformations before they are broken. The
ultimate tensile strength is the important parameter in the studying of strain hardening of material and the
developing of plastic constitutive modelling for ductile materials.

1.4b Creep
The phenomenon that the structural deformations are increasing with time under a constant load is known
as creep. There are many examples for creep in engineering, i.e. buildings, bridges, dams, etc. under their
own weights, water mains under their service pressure (10 bar). Creep can be generalised as three stages,
i.e. primary creep, secondary creep and tertiary creep. Fig. 1.24 shows a typical creep path.

Creep deformations are actually viscoelstic or viscoplastic ones, i.e. such deformations are mainly
permanent and time-dependent. To model creep performance of a material, a series of creep tests should be
carried out which could last reasonably long term (usually a few months). Therefore any necessary material
parameters which govern the creep performance will be obtained. Such parameters then can be put in the
developed constitutive model which can simulate the creep performance analytically or numerically.

1.4c Fatigue
The structural response under cyclic loading is termed as fatigue. The engine shafts for aeroplanes, car,
ships, etc., ships under sea wave force, bridges under traffic loading and so on are subject to fatigue
loading. Fatigue failure usually causes disasters since there is no any warning prior to the failure.
Historically, some plane crash, bridge falling down, ships breaking into two halves were induced by fatigue
failure. Thus fatigue strength is a very important item to assess civil engineering materials. Fatigue failure
is usually divided into the procedures as follows:

 a)

 b)

 c)


Therefore the crack initiation is the prerequisite for fatigue failure. However, the reasons for crack
initiation are mainly the imperfections of crystal structures of materials.



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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                           HANDOUT FOR MATERIAL SCIENCE

To prevent from fatigue failure you have to stop crack growth. The way to stop crack growth is to make
crack tip blunt. Generally speaking, the more ductile a material, the easier the crack blunt. In other words,
brittle materials, such as glass, some high strength but low toughness alloys, are easily suffered with crack
growth. Thus to avoid fast crack growth more ductile materials should be selected.




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ENGINEERING MATERIALS (1ST YEAR C.ENG & E.ENG)                              HANDOUT FOR MATERIAL SCIENCE

                                                   Questions

1.22 What are three crystal lattice imperfections? Which dimension imperfection does dislocation have?
1.23 Draw sketches for vacancy point defect, screw dislocation, edge dislocation.
1.24 Draw sketches demonstrating Frenkel and Schottky point defects.
1.25 Draw a sketch of grain boundary and explain how such a defect affects the strength of a material.
1.26 Draw a sketch for a typical stress-strain curve for an alloy and indicate yield strength, tensile strength
    and breaking strength. Using above sketch show elastic and plastic strains.
1.27 Explain why it is necessary to use a stress-strain relationship instead of a load-displacement
    relationship to describe the performance of a material.
1.28 What are Young's modulus and Poisson's ratio? If there are two materials, one is hard and one is soft,
    which material has higher Young's modulus.
1.29 Give the definitions for engineering stress and strain, true stress and strain, and their formula.
1.30 Define Poisson’s ratio and give a typical value for steel, concrete and a polymer.
1.31 Draw a sketch for creep curve and indicate three stages.
1.32 Give the three stages in the formation of the fatigue failure mechanism.
1.33 Why is fatigue failure critical?




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