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.

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22

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Examples

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