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Islamic Azad University, Mahshahr Branch Department of ...
Transcript of Islamic Azad University, Mahshahr Branch Department of ...
In the name of God
Islamic Azad University, Mahshahr Branch
Department of Mechanical Engineering
Technical English for Mechanical
Engineering
Prepared by
Roozbeh Alipour
(BSc, MSc, PhD. Mechanical Engineering)
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Chapter 1 Metals
Introduction — Nature and Properties of Pure Metals
Metals achieve engineering importance because of their abundance, variety, and unique
properties as conferred by metallic bonding. Twenty-four of the 26 most abundant
elements in the Earth’s crust are metals, with only two nonmetallic elements, oxygen
and silicon. The two most abundant metallic elements, iron (5.0%) and aluminum
(8.1%), are also the most commonly used structural metals. Iron is the most-used metal,
in part because it can frequently be extracted from its enriched ores with considerably
less energy spending than aluminum, and also because of the very wide range of
mechanical properties its alloys can provide. The next 15 elements in frequency include
most common engineering metals and alloys: calcium (3.6%), magnesium (2.1%),
titanium (0.63%), manganese (0.10), chromium (0.037%), zirconium (0.026%), nickel
(0.020%), vanadium (0.017%), copper (0.010%), uranium (0.008%), tungsten (0.005%),
zinc (0.004%), lead (0.002%), cobalt (0.001%), and beryllium (0.001%). The cost of
metals is strongly affected by strategic abundance as well as secondary factors such as
extraction/processing cost and application. Carbon steels and cast irons, iron alloys with
carbon, are usually most cost-effective for ordinary mechanical applications. These
alloys increase in cost with alloying additions.
A variety of metal properties are unique among materials and of importance
technologically. These properties are conferred by metallic bonding. This bonding is
different from other types of solids in that the electrons are free to acquire energy, and
the metallic ions are relatively mobile, and quite interchangeable with regard to their
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positions in the crystal lattice. A crystal lattice is the three-dimensional repeating
arrangement of atoms in a solid.
Metals are good conductors of heat and electricity because thermal and electrical
energy can be transferred by the free electrons. These two properties tend to parallel
each other. For example, the pure noble metals (e.g., copper, silver, gold and platinum)
are among the best electrical and thermal conductors. As a broad generalization,
metallic elements with an odd number of valence electrons tend to be better
conductors than those with an even number. Thermal conductivity and electrical
resistivity have a reciprocal relationship. As metals are alloyed with other elements,
their electrical and thermal conductivity usually decreases significantly from that of the
pure, perfect, unalloyed metal. Electrical and thermal conductivities tend to decrease
proportionately to each other with increasing temperature for a specific metal. These
conductivities may be altered if heating introduces metallurgical change in the crystal
lattice.
Strength and Deformation, Fracture Toughness
Figure 1 shows a typical stress–strain diagram for a metal. The first portion is a linear,
spring-type behavior, termed elastic, and attributable to stretching of atomic bonds. The
slope of the curve is the “stiffness”. The relative stiffness is low for metals as contrasted
with ceramics because atomic bonding is less strong. Similarly, high-melting-point
metals tend to be stiffer than those with weaker atomic bonds and lower melting
behavior. The stiffness behavior is frequently given quantitatively for uniaxial loading by
the simplified expressions of Hooke’s law:
/ and / x x y z xE E (1)
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Where σx is the stress (force per unit area, psi or Pa) in the x direction of applied
unidirectional tensile load, εx is the strain (length per unit length or percent) in the same
direction, εy and εz are the contracting strains in the lateral directions, E is Young’s
modulus (the modulus of elasticity), and υ is Poisson’s ratio. As the elastic modulus
(stiffness) increases with atomic bond strength, the coefficient of linear expansion tends
to decrease.
Fig. 1: Typical engineering stress–strain curve for a metal.
At a critical stress the metal begins to deform permanently, as seen as a break in the
straight-line behavior in the stress-strain diagram of Figure 1. The stress for this onset is
termed the yield stress or elastic limit. For engineering purposes it is usually taken at
0.2% plastic strain in order to provide a predictable, identifiable value. In the case of
steel a small yield drop allows for clear identification of the yield stress. The onset of
yield is a structure-sensitive property. It can vary over many orders of magnitude and
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depends on such factors as grain size and structure, phases present, degree of cold
work, and secondary phases in grains or on grain boundaries as affected by the thermal
and mechanical treatment of the alloy. The extension to failure, the ductility, and
maximum in the stress–strain curve, the “ultimate stress” or “tensile strength” are also
structure- sensitive properties. The strength and specific strength (strength-to-weight
ratio) generally decrease with temperature.
The ductility usually decreases as the strength (yield or ultimate) increases for a
particular metal. Reduction in the grain size of the metal will usually increase yield stress
while decreasing ductility (Figure 2). Either yield or ultimate strength is used for
engineering design with an appropriate safety factor, although the former may be more
objective because it measures the onset of permanent deformation. Ductility after yield
provides safety, in that, rather than abrupt, catastrophic failure, the metal deforms.
Fig. 2: The effect of grain size on yield stress and elongation to failure (ductility) for brass.
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A different, independent measure is needed for impact loads — “toughness.” This is
often treated in design, materials selection, and flaw evaluation by extending Griffith’s
theory of critical flaw size in a brittle material:
1/2
1 /f ck c (2)
where σf is the failure stress, Klc is a structure-sensitive materials property, the “fracture
toughness” or “stress intensity factor” for a normal load, γ is a constant depending on
orientation, and c is the depth of a long, narrow surface flaw or crack (or half that of an
internal flaw). This is a separate design issue from that of strength. It is of particular
importance when a metal shows limited ductility and catastrophic failure must be
avoided. In some applications the growth of cracks, c is monitored to prevent
catastrophic failure. Alternatively, sufficient energy absorption as characteristic of a
metal is determined when it is fractured in a Charpy or Izod impact test.
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Chapter 2 Fatigue
Fatigue is the repeated loading and unloading of metal due to direct load variation or
eccentricity in a rotating shaft, or differential thermal expansion of a structure. Even
substantially below the yield point (elastic limit) of a metal or alloy this repeated loading
can lead to failure, usually measured in terms of the number of cycles (repeated load
applications) to failure. Some studies have suggested that well over 80% of all
mechanical failures of metal are attributable to fatigue.
High-stress, low-cycle fatigue usually occurs at stresses above the yield point and
lifetimes are tens or hundreds of cycles (to about a thousand cycles). Failure occurs as a
result of the accumulation of plastic deformation, that is, the area (energy) under the
stress–strain curve. A simple lifetime predictive equation can be used to predict lifetime:
2
/ 2u pfN (3)
where N is the number of cycles to failure, εu is total strain from the stress–strain curve,
and εpf is the plastic strain amplitude in each fatigue cycle. More commonly, metals are
used well below their yield point and fail after many cycles of repeated loading in low-
stress, high-cycle fatigue. Early in the fatigue process surface flaws or in some cases
severe internal flaws begin to propagate. The fatigue crack propagation in areas of high
stress has a small distance with each tensile loading. The propagation on each cycle
frequently leaves identifiable marking on the failure surface termed fatigue striations
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which mark the progress of the subcritical crack. When the crack becomes so large than
the fracture toughness criterion, the catastrophic overload failure occurs.
Figure 4 shows typical metal S–N curves (stress vs. number of cycles to failure) for a
high- strength aluminum and for a titanium alloy.
Fig. 3: S–N (fatigue) curves for high-strength titanium (upper curve) and aluminum alloys.
Note that the convention is to make stress the vertical axis and to plot the number of
cycles to failure on a logarithmic scale. For high-stress, low-cycle fatigue (<103 cycles)
the curve is flat and linear, consistent with the model of Equation 3. For high cycle
fatigue the lifetime is a rapidly varying function of stress until very low stresses (long
lifetimes occur).
A number of mathematical relationships have been proposed to predict fatigue life, but
none works with complete success and all require experimental data. Perhaps the most
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successful of the so-called "fatigue “laws” are the “cumulative damage” laws. The
simplest is Miner’s law:
/ 1i ii n N (4)
where ni is the number of cycles applied and Ni is the number of cycles for failure at a
particular stress level, σi. The conceptual basis is that the number of fatigue cycles at a
stress level may be correlated to fatigue crack propagation.
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Chapter 3 Vibrations
Vibrations in machines and structures should be analyzed and controlled if they have
undesirable effects such as noise, unpleasant motions, or fatigue damage with
potentially catastrophic consequences. Conversely, vibrations are sometimes employed
to useful purposes, such as for compacting materials.
The simplest vibrating system has motion of one degree of freedom (DOF) described by
the coordinate x in Figure 4. (An analogous approach is used for torsional vibrations,
with similar results.)
Fig. 4: Model of a simple vibrating system.
Assuming that the spring has no mass and that there is no damping in the system, the
equation of motion for free vibration (motion under internal forces only; F = 0) is
20 or 0mx kx x x (5)
where ω is the natural frequency in rad/sec that can be expressed as
10
/k m (6)
The displacement x as a function of time t is
1 2sin cosx C t C t (7)
where C1 and C2 are constants depending on the initial conditions of the motion.
Alternatively,
sin( )x A t (8)
where C1 = Acosφ, C2 = Asinφ, and φ is the phase angle. Another constant, A, is the
complete cycle of the motion occurs in time τ, the period of simple harmonic motion,
22 (sec/ cyc)
m
k
(9)
The frequency in units of cycles per second (cps) or hertz (Hz) is f = 1/τ.
The simplest case of forced vibration is modeled in Figure 4, with the force F included.
Using typical simplifying assumptions as above, the equation of motion for a harmonic
force of forcing frequency Ω,
0 sinmx kx F t (10)
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Chapter 4 Thermodynamics and Energy
Thermodynamics can be defined as the science of energy. Although everybody has a
feeling of what energy is, it is difficult to give a precise definition for it. Energy can be
viewed as the ability to cause changes. The name thermodynamics stems from the
Greek words thermo (heat) and dynamics (power), which is most descriptive of the early
efforts to convert heat into power.
Today the same name is broadly interpreted to include all aspects of energy and energy
transformations, including power generation, refrigeration, and relationships among the
properties of matter.
One of the most fundamental laws of nature is the conservation of energy principle. It
simply states that during an interaction, energy can change from one form to another
but the total amount of energy remains constant. That is, energy cannot be created or
destroyed. A rock falling off a cliff, for example, picks up speed as a result of its potential
energy being converted to kinetic energy.
Forms of Energy
Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential,
electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy (E)
of a system. The microscopic forms of energy are those related to the molecular
structure of a system and the degree of the molecular activity, and they are
independent of outside reference frames.
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The sum of all the microscopic forms of energy is called the internal energy of a system
and is denoted by U. The portion of the internal energy of a system associated with the
kinetic energies of the molecules is called the sensible energy and the other portion
associated with the phase of a system is called the latent energy.
Properties of a system
Any characteristic of a system is called a property. Properties are considered to be
either intensive or extensive. Intensive properties are those that are independent of the
mass of a system, such as temperature, pressure, and density. Extensive properties are
those whose values depend on the size-or extent-of the system. Mass, volume and total
energy are some examples of extensive properties.
An easy way to determine whether a property is intensive or extensive is to divide the
system into two equal parts with an imaginary partition. Each part will have the same
value of intensive properties as the original system, but half the value of the extensive
properties.
Thermodynamics deals with equilibrium states. The word equilibrium implies a state of
balance. In an equilibrium state there are no unbalanced potentials (or driving forces)
within the system. A system in equilibrium experiences no changes when it is isolated
from its surroundings.
There are many types of equilibrium, and a system is not in thermodynamic equilibrium
unless the conditions of all the relevant types of equilibrium are satisfied. For example, a
system is in thermal equilibrium if the temperature is the same throughout the entire
system. That is, the system involves no temperature differential, which is the driving
force for heat flow.
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Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium
if there is no change in pressure at any point of the system with time. However, the
pressure may vary within the system with elevation as a result of gravitational effects.
Open system and Closed system
A system is defined as a quantity of matter or a region in space chosen for study. The
mass or region outside the system is called the surroundings. The real or imaginary
surface that separates the system from its surroundings is called the boundary. The
boundary of a system can be fixed or movable. Note that the boundary is the contact
surface shared by both the system and the surroundings. Mathematically speaking, the
boundary has zero thickness, and thus it can neither contain any mass nor occupy any
volume in space.
Systems may be considered to be closed or open, depending on whether a fixed mass or
a fixed volume in space is chosen for study.
A closed system (also known as a control mass) consists of a fixed amount of mass, and
no mass can cross its boundary. That is, no mass can enter or leave a closed system, but
energy in the form of heat or work, can cross the boundary; and the volume of a closed
system does not have to be fixed. If, as a special case, even energy is not allowed to
cross the boundary, that system is called an isolated system.
An open system, or a control volume, as it is often called, is a properly selected region in
space. It usually encloses a device that involves mass flow such as a compressor,
turbine, or nozzle. Flow through these devices is best studied by selecting the region
within the device as the control volume. Both mass and energy can cross the boundary
of a control.
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Chapter 5 Internal combustion engine
An internal combustion engine (ICE) is a heat engine where the combustion of a fuel
occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of
the working fluid flow circuit. In an internal combustion engine the expansion of the
high-temperature and high-pressure gases produced by combustion apply direct force
to some component of the engine. The force is applied typically to pistons, turbine
blades, or a nozzle. This force moves the component over a distance, transforming
chemical energy into useful mechanical energy. The first commercially successful
internal combustion engine was created by Étienne Lenoir around 1859 and the first
modern internal combustion engine was created in 1864 by Siegfried Marcus.
The term internal combustion engine usually refers to an engine in which combustion is
intermittent, such as the more familiar four-stroke and two-stroke piston engines, along
with variants, such as the six-stroke piston engine and the Wankel rotary engine. A
second class of internal combustion engines use continuous combustion: gas turbines,
jet engines and most rocket engines, each of which are internal combustion engines on
the same principle as previously described. Firearms are also a form of internal
combustion engine.
Internal combustion engines are quite different from external combustion engines, such
as steam or Stirling engines, in which the energy is delivered to a working fluid not
consisting of, mixed with, or contaminated by combustion products. Working fluids can
be air, hot water, pressurized water or even liquid sodium, heated in a boiler. ICEs are
usually powered by energy-dense fuels such as gasoline or diesel, liquids derived from
fossil fuels. While there are many stationary applications, most ICEs are used in mobile
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applications and are the dominant power supply for vehicles such as cars, aircraft, and
boats.
Typically an ICE is fed with fossil fuels like natural gas or petroleum products such as
gasoline, diesel fuel or fuel oil. There's a growing usage of renewable fuels like biodiesel
for compression ignition engines and bio-ethanol or methanol for spark ignition engines.
Hydrogen is sometimes used, and can be made from either fossil fuels or renewable
energy. Figure 5 shows a schematic for a four-stroke gasoline engine.
Fig. 5: a schematic for a four-stroke gasoline engine.
4-stroke engines
The top dead center (TDC) of a piston is the position where it is nearest to the valves;
bottom dead center (BDC) is the opposite position where it is furthest from them. A
stroke is the movement of a piston from TDC to BDC or vice versa together with the
associated process. While an engine is in operation the crankshaft rotates continuously
at a nearly constant speed. In a 4-stroke ICE each piston experiences 2 strokes per
crankshaft revolution in the following order. Starting the description at TDC, these are:
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1. Intake, induction or suction: The intake valves are open as a result of the cam lobe
pressing down on the valve stem. The piston moves downward increasing the volume of
the combustion chamber and allowing air to enter in the case of a CI engine or an air
fuel mix in the case of SI engines that do not use direct injection. The air or air-fuel
mixture is called the charge in any case.
Fig. 6: Intake stroke
2. Compression: In this stroke, both valves are closed and the piston moves upward
reducing the combustion chamber volume which reaches its minimum when the piston
is at TDC. The piston performs work on the charge as it is being compressed; as a result
its pressure, temperature and density increase; an approximation to this behavior is
provided by the ideal gas law. Just before the piston reaches TDC, ignition begins. In the
case of a SI engine, the spark plug receives a high voltage pulse that generates the spark
which gives it its name and ignites the charge. In the case of a CI engine the fuel injector
quickly injects fuel into the combustion chamber as a spray; the fuel ignites due to the
high temperature.
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Fig. 7: compression stroke
3. Power or working stroke: The pressure of the combustion gases pushes the piston
downward, generating more work than it required to compress the charge.
Complementary to the compression stroke, the combustion gases expand and as a
result their temperature, pressure and density decreases. When the piston is near to
BDC the exhaust valve opens. The combustion gases expand irreversibly due to the
leftover pressure—in excess of back pressure, the gauge pressure on the exhaust port—
; this is called the blowdown 1.
Fig. 8: working stroke
1. The removal of solids or liquids from a container or pipe using pressure.
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4. Exhaust: The exhaust valve remains open while the piston moves upward expelling
the combustion gases. For naturally aspirated engines a small part of the combustion
gases may remain in the cylinder during normal operation because the piston does not
close the combustion chamber completely; these gases dissolve in the next charge. At
the end of this stroke, the exhaust valve closes, the intake valve opens, and the
sequence repeats in the next cycle. The intake valve may open before the exhaust valve
closes to allow better scavenging.
Fig. 9: Exhaust stroke
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Chapter 6 Greek alphabet
The Greek alphabet has been used to write the Greek language since the 8th century BC.
It was derived from the earlier Phoenician alphabet,[3] and was the first alphabetic
script to have distinct letters for vowels as well as consonants. It is the ancestor of the
Latin. Apart from its use in writing the Greek language, in both its ancient and its
modern forms, the Greek alphabet today also serves as a source of technical symbols
and labels in many domains of mathematics, science and other fields.
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Chapter 7 Mathematical Symbols
This is a list of symbols found within some branches of mathematics to express a
formula or to represent a constant.