Mechanical Engineering as a Profession 1、Introduction "Scientists study the world as it is, engineers create the world that never has been.” What is engineering ? Engineering is the art of applying scientific and mathematical principles, experience, judgment, and common sense to create things that benefit people. In other words, engineering is the process of producing a technical product or system to meet a specific need. . What engineers do ? Engineers: Turning Ideas into Reality Engineers do practical design and production work using the principles of science and mathematics to come up with economically viable solutions to technical problems. . In shorter words, they design and build needed tools, products, and systems. What is mechanical engineering ? Mechanical engineering is the most diverse and exciting of all the engineering disciplines. . It is specifically concerned with the design, development, installation, operation and maintenance of just about anything that has moveable parts. . If an object is man-made, mechanical engineering skills will have been involved at some stage during its development and manufacture. Role of Mechanical Engineers We Make Machines… and More ! Mechanical engineers are professionals devoted to employing the principles of motion, forces, and energy. Mechanical engineers research, develop, design, manufacture test tools, engines, machines, and other mechanical devices. For society this means that mechanical engineers are using their wide range of skills to think of ways to improve the way we live. . 2、Top 10 Benefits The top 10 rewards and opportunities that an engineering career offers. #1 Job Satisfaction It is important to find a career that provides you with enjoyment and satisfaction. . After all, you might spend 40 or so years working eight hours or more a day, five days a week, 50 weeks a year.. Do you want to dislike every minute of that time, or would you rather do something that you enjoy? . #2 Varied Opportunities An engineering degree offers a wide range of career possibilities. If you are imaginative and creative, design engineering may be for you. If you like laboratories and conducting experiments, you might consider test engineering. If you like to organize and expedite projects, look into being a development engineer. If you are persuasive and like working with people, consider a career in sales or field service engineering. The analytical skills and technological expertise you develop as an engineering student can also be put to use in many other fields. . For example, you could combine engineering and business skills in a career as a technical manager or a salesperson for a high-tech company. #3 Challenging Work If you like challenges, engineering could be for you. In the engineering work world, there is no shortage of challenging problems. . When you get into the engineering work world, virtually all problems will be open-ended. There will be no single answer, no answer in the back of the book, no professor to tell you that you are right or wrong. . You will be required to devise a solution and persuade others that your solution is the best one. #4 Intellectual Development An engineering education will “exercise” your brain, developing your ability to think logically and to solve problems. These are skills that will be valuable throughout your life — and not only when you are solving engineering problems. . For example, your problem-solving skills can help you undertake tasks such as planning a vacation, finding a job, organizing a fund-raiser, purchasing a house, or writing a book. #5 Social Impact Just about everything that engineers do benefits society. For example, engineers develop transportation systems that help people and products move about easily. . As an engineer, you can also work on beneficial projects, such as cleaning up the environment, finding new sources of energy and increasing the standard of living in underdeveloped countries. #6 Financial Security While financial security should not be your only reason for choosing a career in engineering, if you decide to become an engineer, you will be well paid. #7 Prestige Engineers play a primary role in sustaining our nation’s international competitiveness, maintaining our standard of living, ensuring a strong national security, and protecting public safety. As a member of such a respected profession, you will receive a high amount of prestige. #8 Professional Environment As an engineer, you will work in a professional environment in which you will be treated with respect and have a certain amount of freedom in choosing your work. You will be also be in a position to influence what happens at your company. In most cases, you will receive adequate work space and the tools you need to do your work, including the latest computer hardware and software. You will probably also receive the secretarial and technical support staff you need to get your work done. . You will learn from experienced engineers in your organization and will be offered seminars and short courses to increase your knowledge. #9 Technological and Scientific Discovery An engineering education can help you understand how things in the world work. Furthermore, an understanding of technology will also provide you with a better understanding of many issues facing our society. For example: Should we have stopped building nuclear reactors? What will we use for energy when oil runs out? #10 Creative Thinking Because we are in a time of rapid social and technological changes, the need for engineers to think creatively is greater now than ever before. . Only through creativity can we cope with and adapt to these changes. If you like to question, explore, invent, discover, and create, then engineering could be the ideal profession for you! 3、Top 10 achievements The top 10 Great Achievements of 20th Century Mechanical Engineering. 1.The Automobile .Henry Ford freed common people from the limitations of geography, creating social mobility on a scale previously unknown. 2.Apollo Moon Landing From early test rockets to sophisticated satellites, the human expansion into space is perhaps the most amazing engineering feat. 3.Power Generation From street lights to supercomputers, electric power makes our lives safer, healthier, and more convenient. 4.Agricultural Mechanization The machinery of farms - tractors, cultivators, combines, and hundreds of others - dramatically increased farm efficiency and productivity. 5.The airplane Modern air travel transports goods and people quickly around the globe, facilitating our personal, cultural, and commercial interaction. 6.Integrated circuit From vacuum tubes to transistors to integrated circuits, engineers have made electronics smaller, more powerful, and more efficient. 7.Air conditioning and refrigeration No longer dependent on the weather for work or play, humans truly made the environment adapt to their needs. 8.CAD/ CAM and CAE Technology Computers and imaging technologies are used to design and test a variety of products. 9.Bioengineering Artificial organs, replacement joints, and biomaterials are but a few of the engineered products that improve the quality of life for millions. 10.Codes and standards Standards for product design and performance promote product quality, safety, and interchangeability of components. 4、New directions in ME Micro/Nano Technologies Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. MEMS (Micro-electro-mechanical systems) requires the solution of many mechanical engineering problems on the micro scale. . Fluid control : Microvalve/pump, Microsensor ; Computer : Magnetic head, Printer head, Laser scanner, Micro-mechanical memory ; Robot : Microrobots, Micro-teleoperator, Mobile sensor. Figure shows Nickel micromotor and gear train formed using the LIGA process at the University of Wisconsin ,Such structures combine extreme precision with high aspect ratios, can be driven magnetically, and provide one example of MEMS. The rotor diameter here is 150 mm. Magnetic micromotors have been driven at rates exceeding 50,000 rpm. Cellular and Molecular Biomechanics In recent years cellular and molecular bio-mechanics have gained in importance, and problems of biomechanics at that scale have begun to emerge. These problems, again, necessitate expansion and further development in the basic continuum mechanics theories and models that have long been the mainstay of ME. Information Technology IT has influenced ME in many significant ways. Computational methods in mechanics are becoming increasingly important, e.g., finite element methods (FEM). The availability of distributed information through networks reinforces the emphasis on collaborative systems design and analysis rather than a more isolated focus on the various components. Energy and Environmental Issues The focus in environmental engineering has shifted from remediation (e.g., wastewater treatment) to design of environmentally “friendly” products manufactured in an environmentally conscious manner. . Key principles with which mechanical engineers must deal are design for the environment, lifecycle design, and sustainable development. . 5. Communicating as a ME In both academia and industry, mechanical engineers need to speak and write their ideas. 5.1 Types of communication Lab Reports Lab work is an important part of every engineer's training. Lab Reports are factual presentations of test or experiment results completed in a lab or simulation. Typically, Lab Reports discuss procedures as well as describe the details of a test or experiment. Poster Sessions As an engineer, you'll participate in Poster Sessions during conferences and seminars. A Poster Session allows you to display and discuss your work on a project or the results of your research. . It combines text and graphics to make a visually-pleasing presentation. As viewers walk by, your poster should quickly and efficiently communicate your research. Proposals Engineers write Proposals to present a topic to be researched or to suggest a plan of action. . In academia, engineers write proposals to receive funding for their research or even to initiate a project. As an engineer, you may determine that a problem exists, and therefore, propose solutions to an organization. In this case, you must first convince the agency that the problem exists before proposing your solutions. Technical Reports Technical reports present facts and conclusions about your designs and other projects. Typically, a technical report includes research about technical concepts as well as graphical depictions of designs and data. . A technical report follows a strict organization. This way, when other engineers read what you write, they can quickly locate the information that interests them the most. . 5.2 Communication Conventions Headings & Subheadings They are good organizational techniques, and they also help readers locate information. This way, a reader interested in the necessary materials could quickly find this information without reading the whole report. Lists Lists are effective ways to present information. Lists are especially useful when you have to convey steps, phases, years, procedures, or decisions. When creating a list, consider writing phrases, fragments or even questions and answers. By avoiding full sentences in a list, your information is concise and more likely to engage your readers. Passive Voice Usually, the passive voice should be used in writing. 1. I used the electric identifier to solve the problem. ( active voice ) . 2. The electric identifier was used to solve the problem.( passive voice ) . Terseness Lengthy sentences and long paragraphs are signs that your writing is not terse. The reason why terseness is necessary to good engineering writing is because it helps your readers understand information quicker. . Good writing is descriptive, but it also gets to the point as quickly as possible. The information you present should always be relevant to your topic, as well as to your audience. . 5.3 Advice from engineers Present Information Logically Many engineering professors note that much of the writing they read from students often doesn't have a "logical flow." By this, they mean that the writing doesn't present ideas in an order that makes sense. You should also make sure that your entire document or presentation presents information logically. For instance, don't include conclusions or results in either the procedure section or the introduction. Format Your Documents Whenever you produce a document, you should always consider how you've organized your thoughts and how you can make this known to the reader. You should use a consistent style (according to the style guidelines in your discipline). This includes margin sizes, line spacing, and even the title page you attach to the front of your document. . Know Your Purpose and Audience By determining who your audience is and what your purpose is, you can then gather specific information instead of including everything that you might find on a particular topic. . This way, you don't have to worry about presenting information that may bore or confuse a particular audience. 6. Summary - Mechanical engineers are problem solvers who are employed in a wide variety of areas. They work in a team environment, often with professionals from other disciplines. . - Mechanical engineers work in many industries, often developing everyday products. They are also employed in government, doing research , and in academia, teaching the next generation of engineers. Thus, mechanical engineers play an important role in society. Statics, Dynamics and Mechanical Engineering 1、Introduction Mechanics: Science which describes and predicts the conditions of rest or motion of bodies under the action of forces. The field of Classical Mechanics can be divided into three categories : . 1) Mechanics of Rigid Bodies 2) Mechanics of Deformable Bodies 3) Mechanics of Fluids Rigid-body mechanics ( General mechanics ) Statics deals with bodies that are in equilibrium with applied forces. [ Such bodies are either at rest or moving at a constant velocity. ] . Dynamics deals with the relation between forces and the motion of bodies. [ Bodies are accelerating. ] Notes ?Rigid-body mechanics is based on the Newton’s laws ofmotion. ? These laws were postulated for a particle, which has a mass, but no size or shape. . ? Newton’s laws may be extended to rigid bodies by considering the rigid body to be made up of a large numbers of particles whose relative positions from each other do not change. Newton’s Laws of Motion 1st law. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. 2nd law. If the resultant force acting on a particle is not zero, the particle will experience an acceleration proportional to the magnitude of the force and in the direction of this resultant force. 3rd law. The mutual forces of action and reaction between two particles are equal in magnitude, opposite in direction, and collinear. 2.1 Vectors ? Scalar : Any quantity possessing magnitude (size) only, such as mass, volume, temperature. ? Vector : Any quantity possessing both magnitude and direction, such as force, velocity, momentum. The calculation of a vector must be in a reference frame. A scalar is independent of reference frames. Given two vectors, the vectors will only be equal if both the magnitude and direction of both vectors are equal. In Cartesian coordinate system, an arbitrary vector can be written in terms of unit vectors as Addition of Two Vectors Subtraction of Two Vectors Inner Product of Two Vectors Vector Product of Two Vectors 2.2 Forces Force is a vector quantity, a force is completely described by:1.Magnitude2.Direction3.Point of Application External force : Forces caused by other bodies acting on the rigid body being studied. ( Ex.-- weight, pushing, pulling. ) Internal force : Those forces that keep the rigid body together. Force in 3D A force F in three-dimensional space can be resolved into components using the unit vectors : The vectors i, j, k are unit vectors along the x, y and z axes respectively. . 2.3 Moments The moment of force F about point O is defined as the vector product : where r is the position vector drawn from point O to the point of application of the force F. . The right-hand rule is used to indicate a positive moment. ( torque ) 2.4 Couples A couple is formed by 2 forces F and -F that have equal magnitudes, parallel lines of action and opposite direction. The moment of a couple is a vector M perpendicular to the plane of the couple and equal in magnitude to the product Fd. Notes @ A couple will not cause translation only rotation. @ The moment of a couple is independent of the point about which it is computed. @ Two couples having the same moment M are equivalent. They have the same effect on a given rigid body. The direction of a couple is given by the right-hand rule. Therefore, a positive couple generates rotation in a counterclockwise sense. 2.5 Equilibrium of a Rigid Body Conditions for rigid-body equilibrium : where: ??Forces are “external forces” ( body force, applied force, support reaction ) ??Moment may be taken about any center of rotation “o” 2.6 Free Body Diagrams ( FBD ) Three steps in drawing a free body diagram: 1. Isolate the body, remove all supports and connectors. 2. Identify all external forces acting on the body. 3. Make a sketch of the body, showing all forces acting on it. 2.7 Solving a Statics Problem STEPS: 1. Draw a free body diagram. 2. Choose a reference frame. Orient the axes. 3. Choose a convenient point to calculate moments around. 4.Apply the equilibrium equations and solve for the unknowns 2.8 Frictional Forces In problems involving the contact of two bodies, if the contact is not smooth, a reaction will occur along the line of contact. This reaction is a force of resistance called the friction. Frictional forces inhibit or prevent slipping. Provided that there is no slipping at the contact surface and that the body is not accelerating, experimental studies have shown that the frictional force is related to the normal contact force by the equation : F = μs N Where F is the static frictional force and N is the normal contact force. The constant μs is called the coefficient of static friction. If the body is accelerating, then the frictional force has a value less than the static value. This frictional force, F, is called the kinetic frictional force and is related to the normal force as F = μk N where μk is the coefficient of kinetic friction. Values of μk are as much as 25% smaller than values for μs . 3. Dynamics Dynamics = Kinematics + Kinetics 1). Kinematics, branch of dynamics concerned with describing the state of motion of bodies without regard to the causes of the motion. [ displacement, velocity, acceleration, and time ] . 2). Kinetics, branch of dynamics concerned with causes of motion and the action of forces. . [ work, power, energy, impulse, …] Direct dynamics:Calculation of kinematics from forces applied to bodies. Inverse dynamics:Calculation of forces and moments from kinematics of bodies and their inertial properties. Applications : Analysis of cams, gears, shafts, linkages, connecting rods, etc. 3.1 Kinematics Types of rigid-body motion : Translation (3 degrees of freedom) Rotation about a fixed axis (1 DOF) (angular velocity ω, angular acceleration α ) General plane motion(3 DOF)( the sum of a translation and a rotation ) Motion about a fixed point (3 DOF) General motion (6 DOF) Equations of motion for rigid bodies : Where m is the mass of the rigid body, a is the acceleration of the body’s center of mass, I is called the mass moment of inertia (in kg·m2), and α is the angular acceleration of the center of mass (in rad/s2). 3.3 Solving a Dynamics Problem Free body diagrams Equations of motion The acceleration and angular acceleration must be indicated on the diagram. 4、Summary Rigid-body mechanics, which includes statics and dynamics, is a branch of science that deals with forces and motion of bodies that do not deform under the applied loads. In a free-body diagram, the body under considera- tion is isolated from its surrounding, and loads acting on the body are shown. The direction and magnitudes of the loads must be properly indicated or the analysis will fail. Solid Mechanics and Mechanical Engineering Objectives After learning this chapter, you should be able to do the following : ? Differentiate between the different types of basic loading conditions. . Understand the basic approach of the Finite Element Method(FEM). 1. Introduction During the analysis of an engineering design, a mechanical engineer is often faced with predicting the deformation of a body. . In some cases, the inverse problem is solved. That is, the maximum amount of desired deformation is known and the load that will produce the deformation is desired. Solid Mechanics : Structural Mechanics、Mechanics of Materials、Elastic Mechanics、Plastic Mechanics 2. Stress and Strain . Normal Stress, often symbolized by the Greek letter sigma, is defined as the force perpendicular to the cross sectional area divided by the cross sectional area. (axial stress) . . Axial Strain, a non-dimensional parameter, is defined as the ratio of the deformation in length to the original length. Strain is often represented by the Greek symbol epsilon(?). Application 1——（Tension & Compression） Suppose the force is perpendicular to the longitudinal axis. The stress will be a Shear Stress, defined as force parallel to an area divided by the area..Just as an axial stress results in an axial strain, so does shear stress produce a Shear Strain (γ). Application 2——Shearing Force Let’s consider a shaft, to which an external torque is applied (such as in power transmission). The shaft is said to be in torsion. The effect of torsion is to create an angular displacement of one end of the shaft with respect to the other. For a shaft of circular cross section, the relationship between the shear stress and torque is where J is the polar moment of inertia. Application 3——Transmission Shaft Notes In general, more than one type of stress may be active in a solid body, due to combined loading conditions.(tension, compression, shear, torsion, etc.) When faced with an engineering problem, an engineer must recognize if more than state of stress exists.. Because stresses are vector quantities, care must be taken when adding the terms together. Application 4——Transmission system of machine tools Notes The simple loading cases considered in this chapter form the basics of the study of strength of materials. .. The Finite Element Method is often used to solve problems involving complicated geometries or loading conditions for structural analysis. 3. Poisson Effect When a tensile load is applied to a uniform beam, the increase in the length of the beam is accompanied by a decrease in the lateral dimension of the beam. . The decrease or the increase in the lateral dimension is due to a lateral strain, which is proportional to the strain along the axial direction. The ratio of the lateral strain to the axial strain is related to the Poisson ratio, named after the mathematician who calculated the ratio by molecular theory. The minus sign in Equation is needed in order to keep track of the sign in the strain. For example, because tension corresponds to a decrease in the lateral direction, the lateral strain is negative. 4. Hooke’s Law Hooke's Law says that the stretch of a spring is directly proportional to the applied force. Engineers say "Stress is proportional to strain". This law is formulated in terms of the stress and strain and may be written as : where E is a material constant known as Young’s modulus. Example 1 Suppose that a 4-inch-diameter round bar is extended with a 50.000-lb axial load. The bar has an initial length of 5 feet and extends 0.006 inches. What is the Young’s modulus for the material from which the bar is made? . Solution We can obtain the Young’s modulus by using Hooke’s law, and Equation (1) and Equation (2). The stress in the bar is The strain is Thus, the Young’s modulus for this material is 5. Stress Concentration When an elastic body with a local geometrical irregularity is stressed, there usually is a localized variation in the stress state in the immediate neighborhood of the irregularity. The maximum stress levels at the irregularity may be several times larger than the nominal stress levels in the bulk of the body. This increase in stress caused by the irregularity in geometry is called a stress concentration. Stress Concentration Factor Where the stress concentration can not be avoided by a change in design, it is important to base the design on the local value of the stress rather than on an average value. The usual procedure in design is to obtain the local value of the stress by use of a stress concentration factor. σmax , maximum stress in the presence of a geometric irregularity or discontinuity, σnom, nominal stress which would exist at the point if the irregularity were not there. Typical Kt Curve 6. Fatigue Loads or deformations which will not cause fracture in a single application can result in fracture when applied repeatedly. Fracture may occur after a few cycles, or after millions of cycles. . This process of fracture under repeated loading is called fatigue. Fatigue is one of the three common causes of mechanical failure, the others being wear and corrosion. Consider a situation in which the stress at a point in a body varies with time. Experiments show that the alternating stress σa is the most important factor in determining the number of cycles of load a material can withstand before fracture, while the mean stress level σm is less important. Fatigue Curve Notes It is customary to designate the stress which can be withstood for some specified number of cycles as the fatigue strength of the material. . Fatigue cracks are most likely to form and grow from locations where holes or sharp corners cause stress concentrations. . In designing parts to withstand repeated stresses, it is important to avoid stress concentrations. Keyways, oil holes, and screw threads are potential sources of trouble and require special care in design in order to prevent fatigue failures. 7. Finite Element Method (FEM) The FEM is a numerical analysis technique for obtaining approximate solutions to engineering and design problems. Assume a problem with an infinite number of unknowns. The finite element discretization procedures reduce the problem to one of a finite number of unknowns: dividing the solution region into discrete elements；expressing the unknown field variable in terms of assumed approximating functions within each element. Notes The discrete elements can be used to represent exceedingly complex geometric shapes, since these elements can be put together in various ways. . The approximating functions ( or interpolation functions) are defined in terms of the values of the field variables at specified points called nodes. Nodes usually lie on the element boundaries where adjacent elements are considered to be connected. Notes Clearly, the nature of the solution and the degree of approximation depend not only on the size and number of the element used, but also on the interpolation functions selected. . .In essence, a complex problem reduces to considering a series of greatly simplified problems. FEM Steps To summarize in general terms how the finite element method works, the following steps are listed. 1、 Discretize the continuum. The first step is to divide the continuum or solution region into elements. . 2、Select interpolation functions. The next step is to assign nodes to each element and then choose the type of interpolation function to represent the variation of the field variable over the element. 3. Find the element properties. Once the finite element model has been established, the matrix equations can be determined, thus expressing the properties of the individual elements. . 4. Assemble the element properties to obtain the system equations. Combine the matrix equations expressing the behavior of the elements and form the matrix equations expressing the behavior of the entire solution region or system. 5. Solve the system equations. The assembly process of the preceding step gives a set of simultaneous equations that must be solved to obtain the unknown nodal value of the field variable. . 6. Make additional computation if desired. Sometimes it is desirable to use the solution of the system equations to calculate other important parameters. Example 1 (ANSYS) Example 2（Crash Analysis for a Car） 8. Summary ? A solid material under applied load will deform. This deformation may be under tension, compression, shear, and torsion, depending on how the loads are applied. It is also possible that one or more of these loading cases may exist simultaneously. . ? In the realm of engineering design, it is important to know the behavior of a structure under applied loads. Unsafe designs are characterized by extensive deform-ations. In such cases, the engineer must redesign the component or structure. . ? The theory of solid mechanics is used to predict the amount of deformation under applied loads and hence whether or not a design is safe. . Fluid Mechanics and Mechanical Engineering Sections ?Fluid Properties?Fluid Statics?Fluid Dynamics?Aerodynamics Fundamentals?Summary 1. Fluid Properties A fluid is a substance that deforms continuously under an applied shear stress. . ? Liquids ? water ? Gases ? air The difference between the two is that liquids occupy a definite volume, independent of the volume in which they are contained, whereas gases expand to fill the entire volume of the container in which they are placed. . Viscosity Viscosity is a fluid property that relates the magnitude of fluid shear stresses to the fluid strain rate. . For a Newtonian fluid , τ is the shear stress, μ is a constant called the viscosity (in N·s/m2). Newtonian fluids For a large class of fluids, the coefficient of viscosity(μ) is independent of the velocity gradient. Such fluids are called Newtonian fluids. . Most fluids familiar to us, such as water, air, and oil, behave as Newtonian fluids. However, certain other fluids, such as blood, toothpaste, and paint, are non-Newtonian. . Fluid mechanics is concerned with Newtonian fluids. . Notes: The viscosity is a property of the fluid , largely a function of temperature for most Newtonian fluids. The fluids that we commonly deal with — water and air — possess relatively low viscosities. Consequently, over most of the flow field, the fluid can be treated as nonviscous. . The magnitude of the coefficient depends on the cohesive force between molecules and the momentum interchange between colliding molecules. Temperature Effect The cohesive force is dominant for a liquid, so that as the temperature of a liquid is raised and the cohesive force between molecules decreases, the viscosity also decreases. The momentum interchange is dominant for a gas, and as the temperature of the gas is raised, providing for greater momentum interchange, the viscosity of the gas increases. . Surface Tension Surface tension of a liquid is due to the forces of attraction between like molecules, called cohesion, and those between unlike molecules, called adhesion.. Molecules on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension. 2. Fluid Statics ? Fluid statics is the branch of fluid mechanics which deals with situations in which there is no relative motion between fluid elements. The fluid can be either at rest or in uniform motion. . In a static fluid, there is no motion of one layer of fluid relative to an adjacent layer, so there are no viscous shear forces. Thus, the only forces we shall consider in a study of fluid statics are pressure forces and gravity. Application : manometer The liquid in the tube will reach an equilibrium position where its weight will be balanced by the difference between the tank pressure and the local atmospheric pressure exerted on the liquid at D. Thus, Automobile Hydraulic Lift Hydraulic Drum Brake Archimedes’ principle The principle of buoyancy: . A submerged body is subject to an upward force FB equal to the weight of the fluid displaced. 3. Fluid Dynamics Laminar flow — fluid moves along smooth paths — viscosity damps any tendency to swirl or mix Turbulent flow —fluid moves in very irregular paths —efficient mixing —velocity at a point fluctuates 3.1 Control Volume Approach Solving problems involving fluids in motion, ? to pick a fixed region in the fluid and watch the fluid as it enters and leaves the region. Conservation laws such as conservation of mass, momentum and energy are applied. We don’t need to know the flow details within the control volume ! Example 1 Suppose we are designing a water-piping system for a building with two apartments and wish to supply water to two faucets, which are connected by a tee configuration, as shown in the figure. We want the velocity of the water to be same when it leaves both faucets. What should be the velocity of the water source? Analysis ?Assume that the mass of the fluid per unit time entering the control volume is equal to the amount leaving. And assume that the water is steady, not trapped in control volume. . ? The density of the water is the same throughout (incompressible fluid) , the diameter of the pipe is the same everywhere. Conservation of mass gives the solution. . 3.2 Fluid Force we assume steady one-dimensional flow, then we may write that the force in the kth direction, Fk , as : where is the mass flowrate and (V2k－V1k) is the difference in the velocity of the fluid from station 1 to station 2 in the kth direction. Example 2 Water with a velocity of 10 m/s strikes a turbine used for power generation and is rotated 60? from the horizontal by the blade, as shown in the figure. The cross section of the inlet water is 0.003 m2. What is the force on a turbine blade shown in the figure? Solution We may assume incompressible flow, so that ρ1=ρ2 , also A1=A2. The water is initially horizontal means that v1y=0. Applying the idea of conservation of mass indicates that : Notice that because A1 = A2, then V1 = V2 . Applying equation to the x direction yields, Substituting the appropriate numbers into this expression gives: The arrow indicates the direction of the x component of the force. Likewise, in the y direction, we have Application : Propeller A propeller consists of several rotating blades attached to a hub that is connected to a shaft. The application of a torque to the propeller shaft causes the ambient fluid ahead of the propeller to move past the blades, imparting to the fluid an exit velocity that is greater than the approach velocity. . Application : Windmill The purpose of a windmill is to extract power from the wind. Due to an increasing shortage of conventional energy sources, the windmill is currently being examined as a potential source of at least some of our electrical power. With the wind itself quite variable, means must be found for wind energy storage to handle electrical demands during periods of low wind velocity. Aerodynamics Fundamentals Aerodynamics is the branch of dynamics that treats the motion of air (and other gaseous fluids) and the resulting forces acting on solids moving relative to such fluid. . Basic Forces for Aircraft Flight Weight — The force due to gravity. Lift — To overcome gravity, you need to create an upward force. That's what an airplane's wings are for. Thrust — To create lift, you need forward motion. That's what an airplane's engines are for — they produce a force called thrust. . Drag — To keep moving, you need to overcome the resistance of the air — a force called drag. Lift Force ?With a certain angle of attack, the upper surface of the airfoil has more curvature than the lower, the air must speed up in order to flow over the surface . . For a moving fluid, an increase in velocity cor-responds to a decrease in pressure, the higher pre-ssure on lower surface essentially pushes the wing up. This force is called the lift. Stall ? Increasing the angle of attack can increase the lift, but it also increases drag so that you have to provide more thrust with the aircraft engines. . At too high an angle of attack, the air will not be able to remain attached to the upper surface of the airfoil, turbulent flow increases the drag dramatically and will stall the aircraft. Sonic Boom Sonic boom is an impulsive noise similar to thunder. It is caused by an object moving faster than sound. An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship's bow. . When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves, dropping sonic boom along its flight path. The sonic booms can be sometimes quite loud. For a commercial supersonic transport plane (SST), it can be as loud as 136 decibels. . Concorde Problems : Excessively powerful engine /Sonic boom . Civil Aviation Materials and Mechanical Engineering Sections 1. Introduction、2. Atomic Bonds、3. Material Properties、4. Selection of Materials Objectives After learning this chapter, you should be able to do the following : Understand how material properties are used to qualify materials for engineering design. . . Understand how traditional and composite materials are used in engineering design. 1.Introduction Material science encompasses the study of the structure and properties of any material, as well as using this body of knowledge to create new types of materials, and to tailor the properties of a material for specific uses. ? Materials are the foundation and fabric of manufactured products. ? Mechanical design is dependent on and limited by materials. ? New materials and processes enable other new technologies to be commercially successful. .Properties of materials are usually the deciding factor in choosing which materials should be used for a particular application. . Material properties depend on the material microstructure, which in turn results from its composition and processing. 2. Atomic Bonds Metallic Bonds In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. . Since the electrons are free to move, they lead to good thermal and electrical conductivity. Ionic Bonds Atoms like to have a filled outer shell of electrons. Sometimes, by transferring electrons from one atom to another, electron shells are filled. The donor atom will take a positive charge, and the acceptor will have a negative charge. The charged atoms or ions will be attracted to each other, and form bonds. Covalent Bonds Some atoms like to share electrons to complete their outer shells. Each pair of shared atoms is called a covalent bond. Hydrogen Bonds Hydrogen bonds are common in covalently bonded molecules which contain hydrogen, such as water (H2O). Notes There are two types of bonds: 1.Primary bonds ? ( Metallic Bonds, Covalent Bonds, Ionic Bonds ) 2.Secondary bonds (Hydrogen Bonds, etc.) Primary bonds are the strongest bonds which hold atoms together. Secondary bonds are much weaker than primary bonds. They often provide a "weak link" for deformation or fracture. 3. Material Properties The principal properties of materials which are of importance to the engineer in selecting materials can be broadly divided into : Mechanical Properties ( concerned mainly with strength ) Physical Properties ( such as melting temperature,density, etc ) 3.1 Tensile Test The mechanical properties used in engineering are determined by performing a tensile test. . Typical test machines may test the specimen in different ways including tension and compression. Stress-Strain Curve In a static tensile test, the stress-strain curve is produced. Characteristics of the curve include a linear region and a region of rapid elongation known as the plastic region. The point at which the linear region ends is called the proportional limit. The slope of the curve in the linear region is called the Young’s modulus. The proportional limit defines the point where a small increase in the stress yields a large deformation. This phenomenon is called yielding. Most engineering designs tend to avoid the plastic region. If the goal of a design is to design within the linear region, then all stresses in the structure or component must be below the yield stress. A material that can undergo large plastic deformation before fracture is called a ductile material. A material that exhibits little or no plastic deformation at failure is called a brittle material. 3.2 Young’s Modulus Young's modulus measures the resistance of a material to elastic (recoverable) deformation under load. . A stiff material has a high Young's modulus and changes its shape only slightly under elastic loads. A flexible material has a low Young's modulus and changes its shape considerably. . A stiff material requires high loads to elastically deform it - not to be confused with a strong material, which requires high loads to permanently deform (or break) it. Measurement (Tensile Test) 3.3 Strength The strength of a material is its resistance to failure by permanent deformation (usually by yielding). . A strong material requires high loads to permanently deform (or break) it - not to be confused with a stiff material, which requires high loads to elastically deform it. Measurement (Tensile Test) 3.4 Toughness （韧性） Toughness is the resistance of a material to being broken in two, by a crack running across it - this is called "fracture" and absorbs energy. . A tough material requires a lot of energy to break it, usually because the fracture process causes a lot of plastic deformation. . A brittle material may be strong but once a crack has started the material fractures easily because little energy is absorbed (e.g. glass). Measurement (Compact Tension Test) 3.5 Elongation （延展性）Measurement (Tensile Test) 3.6 Density Density is a measure of how heavy an object is for a given size, i.e. the mass of material per unit volume. . The weight of a product is a very common factor in design. In transport applications, lightweight design is very important - for example, to reduce the environmental impact of cars, or to increase the payload of aircraft. 3.7 Max. Service Temperature The strength of a material tends to fall quickly when a certain temperature is reached. This temperature limits the maximum operating temperature for which the material is useful. 3.8 Resistivity Resistivity is a measure of the resistance to electrical conduction for a given size of material. Resistivity is affected by temperature - for most materials the resistivity increases with temperature. An exception is semiconductors (e.g. silicon) in which the resistivity decreases with temperature. Measurement: The resistivity can be calculated quite easily be measuring the resistance of a piece of wire of constant cross-section and known length. 4. Selection of Materials The main criteria to influence the selection of materials for any particular engineering product can be summarized as the following: Property requirements、Processing requirements、Economic requirements Ultimately our final choice will involve a compromise. There is rarely, if ever, an ideal solution. 4.1 Mild Steel Steels are the most important engineering materials, and cover a wide range of alloys based on iron and carbon. . Mild steel contains 0.1-0.2 %C. They are cheap, strong steels used for construction, transport and packaging. . All steels have a high density and a high Young's modulus. The strength of mild steel is improved by cold working. It is inherently very tough. . Mild steel rusts easily, and must be protected by painting, galvanizing or other coatings. Design strengths: ? High strength-to-weight ratio ? High stiffness-to-weight ratio ? Good strength with high toughness ? Very cheap ? Easy to shape ? Easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.2 Alloy Steel Alloy steels are mostly fairly cheap, covering a range of carbon contents (0.1-1.0%). The high carbon content steels respond well to heat treatment to give very high strength and good toughness for gears, drive shafts, pressure vessels, tools. . Alloy steels containing other elements as well as carbon are classified into low alloy and high alloy, depending on the amount of additional alloying elements. Heat-treated high alloy steels give very high strengths, but are more expensive. . Alloy carbon steels rust easily, and must be protected by painting or other coatings. Design strengths: ? High strength with good toughness ? High stiffness ? Mostly very cheap ? Quite easy to shape ? Easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.3 Stainless Steel Stainless steels are more expensive steels containing typically 25% of Chromium and Nickel, which gives excellent corrosion resistance and also high strength and toughness (used for chemical plant and surgical instruments). Design strengths: ? High strength with good toughness ? High stiffness ? Mostly very cheap ? Quite easy to shape ? Quite easy to weld ? Easy to recycle Design weaknesses: ? High density ? Poor electrical and thermal conductivity 4.4 Aluminum Alloy Aluminum is a lightweight, reasonably cheap metal widely used for packaging and transport. It has only been widely available and used for the last 60 years. . Raw aluminum has low strength and high ductility (ideal for foil). Strength is increased by alloying and heat treatment. Some alloys are cast, others are used for wrought products. . Aluminum is quite reactive, but protects itself very effectively with a thin oxide layer to resist corrosion. Design strengths: ? High strength-to-weight ratio ? High stiffness-to-weight ratio ? High electrical and thermal conductivity ? Easy to shape ? Easy to recycle Design weaknesses: ? Difficult to arc weld 4.5 Titanium Alloys Titanium alloys are quite low density, stiff, strong alloys and are expensive. They are used most in sports products (e.g. golf clubs and bicycles) and in aircraft (e.g. engine fan blades). . Pure titanium has moderate strength, but the standard titanium alloy contains 6% aluminum and 4% vanadium, which gives the high strength needed in a jet engine. . Titanium is a reactive metal when hot, but has good corrosion resistance at room temperature. Design strengths: ? High strength, even at high temperatures ? High stiffness ? Corrosion resistant, even resistant to salt water Design weaknesses: ? High cost ? Chemically very reactive when hot ? Quite difficult to shape - usually cast 4.6 Silicon Silicon is doped with very low levels of other elements to give it the particular "semiconducting" electrical properties needed for transistors and microchips. . To supply the huge demand for computer chips, processes have developed so that it can be produced as very large high purity crystals. . Silicon is the base material used for the manufacture of computer chips, and is therefore one of the most important materials. Design strengths: ? Semiconducting properties Typical products: ? Transistors ? Computer chips 4.7 Diamond Diamond is covalently bonded pure carbon, and has the highest Young's modulus and hardness of all materials. . Diamond is naturally occurring but can also be manufactured. It is increasingly used for its very high hardness in cutting tools. Design strengths: ? Excellent corrosion resistance ? Low density ? High electrical resistance. ? High hardness Design weaknesses: ? Low tensile strength ? Low toughness ? Difficult to shape 4.8 Composite Composites are formed from two or more types of materials. Examples include polymer/ceramics and metal/ceramics composites. . Composites are used because overall properties of the composites are superior to those of the individual components. . For example: polymer/ceramics composites have a greater modulus than the polymer component, but are not as brittle as ceramics. There are two types of composites: Fiber Reinforced Composites Particle Reinforced Composites 4.9 Example 1 (Automobile) The new Lincoln LS represents a current example of the use of light weight materials on a high volume production vehicle Aluminum, plastics and magnesium are selected to achieve weight reduction. (Totally more than 20% of vehicle weight) The Ford P2000, is a good example of what the mix might be for vehicles by the end of the next two decades The P2000 meets the goal of 50% weight reduction in the body and chassis. To do this light weight materials are used in every application where feasible. 4.10 Example 2 (Aircraft) Structural materials mass distribution on the Boeing 747 and 777 : Aluminum alloys constitute by far the biggest proportion of structural mass of most modern aircraft, with steels, titanium alloys and structural composites all accounting for approximately 10%. A new series of aluminum alloys have recently been developed which contain the element lithium. These alloys are lighter and stiffer than existing alloys, and are now finding use on the latest aircraft designs. Notes Titanium has a density approximately twice that of aluminum, but when alloyed with other elements, can exhibit very high mechanical properties. The reason titanium alloys are not used more extensively on airframes is due to cost. Titanium alloys cost up to 10 times more than aluminum alloys Structural composite materials are finding increasing use on modern aircraft (especially military aircraft) because of their very attractive low density and high mechanical properties. Thermal Sciences and Mechanical Engineering Sections 1. Introduction、2. The Concept of Temperature 、3. Heat Transfer 4. Thermodynamics 、5. Thermal Deformation、6. Heat Treatment 7. Energy and Environment、8. Summary Objectives After learning this chapter, you should be able to do the following : Understand the concept of temperature and convert a temperature from one scale to another. . Discuss the different modes of heat transfer. . Understand that the design or analysis of a machine operating in a thermal cycle is governed by the laws of thermodynamics. 1、Introduction Thermal science is an area of scientific thought encompassing heat transfer and thermodynamics. . ( Heat transfer deals with the transfer of thermal energy. ( Thermodynamics deals with converting heat to work and understanding the role of energy and other properties of matter in this conversion process. Thermal Machinery Thermal Machinery wherein the working fluid or substance undergoes a thermal cycle includes: Power system ( Heat engine ) Refrigeration system Some systems produce work, such as internal combustion engines, fossil-fuel power plants; . (power system) The thermal efficiency ηth of a power system : @Thermal efficiency is a measure of how efficiently a heat engine converts the heat that it received to work.. Other systems require work input to produce other effects , such as refrigerators, air-conditioning systems. (refrigeration system) Coefficient of performance (COP) for a refrigeration system : An objective of a mechanical engineer working in designing thermal machinery is to improve the efficiency or COP of these devices. 2、 The Concept of Temperature Temperature is a measurement of the energy of molecules due to their rapid movement. (the degree of hotness or coldness of a substance.) . Thermometer measures the temperature by the expansion and contraction of a liquid (usually mercury) in a glass tube. The equality of temperature is the only requirement for thermal equilibrium. 2.1 The 0th Law of Thermodynamics If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. Notes It may seem silly, but it cannot be concluded from the other laws of thermodynamics, and it serves as a basis for the validity of temperature measurement. . . Its value as a fundamental physical principle was recognized more than half a century after the formulation of the 1st and the 2nd laws. It is so named because it logically precedes the 1st and the 2nd laws. 2.2 Temperature Scales The Fahrenheit scale places the freezing point of water at 32 degrees and the boiling point of water at 212 degrees. The Celsius scale places the freezing point at zero degrees and the boiling point at 100 degrees. The relationship between the Celsius scale and the Fahrenheit scale : The Kelvin scale is an absolute scale, with zero defined as when the motion of all molecules ceases. This point occurs at . The Kelvin scale is independent of the properties of any substance. The magnitude of each division of 1K and 1℃ are identical, i.e., the temperature interval on both scales is the same. 2.3 Specific Heat The specific heat is defined as the amount of heat required to raise the temperature of a unit mass (or unit quantity, such as mole) of a substance by one degree Celsius. . The relationship between heat and temperature change is usually expressed in the form shown below where c is the specific heat. . Notes The relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature. . The specific heats of most solids at room temperature and above are nearly constant. At lower temperatures the specific heats drop as quantum processes become significant. . 2.4 Calorie calorie, abbr. cal, is the unit of heat energy in the metric system. The calorie, is the quantity of heat required to raise the temperature of 1 gram of pure water 1°C. Heat is commonly expressed in the calorie, an older metric unit. . Calorie is usually used in describing the energy content of food. . Scientists express heat in terms of the joule, a unit used for all forms of energy. 3. Heat Transfer Heat transfer will occur whenever there is a temperature difference. And heat transfer is from the high-temperature medium to a lower-temperature one. . Heat is transferred by three mechanisms: conduction, convection, and radiation. Heat transfer is quantified in terms of the heat Q (in Joules. J), heat rate (in Watts, W), or heat per unit area (W/m2), called the heat flux. Conduction Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Conduction can take place in solids, liquids, or gases. Metals and metallic alloys are good conductors of heat whereas air, wood, plastic, glass are poor conductors. |Fourier’s law of heat conduction : k is the thermal conductivity of the material, ▽T is the temperature gradient. It indicates that the heat flux of conduction in a direction is proportional to the temperature gradient in that direction. Example 1 On a cold day, the temperature outside of a house reaches -30℉, whereas the temperature inside the house is 70 ℉. What is the heat transferred through a wall of the house with thickness of 4.5 inches if the wall is made of concrete with a thermal conductivity of 1 W/(M·K) ? Solution First, the temperature values are converted to Celsius. We find that –30 ℉ = – 34.4 ℃ and 70 ℉ = 21 ℃. Also, 4.5 inches = 0.1143 m. Then, by substitution into Equation 7-4, we get : Convection Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heat transfer. In the absence of fluid motion, heat transfer between a solid surface and adjacent fluid is by pure conduction. . Forced convection : if the fluid is forced to flow in a tube or over a surface by external means such as a fan, pump, or the wind. . Natural convection : if the fluid motion is caused by buoyancy forces induced by density differences due to the variation of temperature in the fluid. . (free convection) |Newton’s law of heat convection : h is called the convection coefficient [W/(m2·K) ]; yThe convection coefficient h is not a property of the fluid. Its value depends on all the variables that influence convection. Radiation Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. The transfer of energy by radiation does not require the presence of an intervening medium. . |The Stefan-Boltzman law gives the maximum amount of energy that may be transmitted : σ= 5.67×10-8 W/(m2·K4) is the Stefan-Boltzman constant Ts is the temperature of the radiating surface. yA surface that radiates energy according to the Stefan-Boltzmann law is called an ideal radiator, or blackbody. A real surface emits radiation at a lower value is called a graybody. . The expression for a real radiator : ε is called the emissivity of the surface, which has a value between zero and one. 4. Thermodynamics In the most general sense thermodynamics is the study of energy—its transformations and its relationship to the properties of matter. . In its engineering applications thermodynamics has two major objectives. . One is to describe the properties of matter when it exists in what is called an equilibrium state, a condition in which its properties show no tendency to change. . The other objective is to describe processes in which the properties of matter undergo changes and to relate these changes to the energy transfers in the form of heat and work which accompany them. 4.1 The 1st Law of Thermodynamics The principle of energy conservation for a system with a thermodynamic state: 4.2 The 2nd Law of Thermodynamics There are two common statements for the 2nd law of thermodynamics. . The Kelvin-Planck statement : It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. . No heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment. The Clausius statement : It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body. In other words, the spontaneous flow of heat from hot to cold bodies is reversible only with the expenditure of mechanical or other nonthermal energy. If we want to transfer heat from a cooler temperature to a warmer temperature, then work input is required. This is how a refrigerator works.. Notes Both the two statements of the 2nd law are negative statements, and a negative statement cannot be proved. . The two statements are equivalent in their consequences, and either statement can be used as the expression of the 2nd law. . Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa. . Any device that violates either 1st law or 2nd law of thermodynamics is called a perpetual-motion machine. . Example 3 A car engine produces 136 hp on the output shaft with a thermal efficiency of 30%. The fuel it burns gives 35 000 kJ/kg as energy release. Find the total rate of energy rejected to the ambient and the rate of fuel consumption in kg/s. Solution From the definition of a heat engine efficiency and the conversion of hp we have: The energy equation for the overall engine gives: From the energy release in the burning we have: Notes An actual engine rejects energy to the ambient through the radiator cooled by atmospheric air, as heat transfer from the exhaust system and the exhaust flow of hot gases. Entropy （熵） The second law is expressed mathematically in terms of the concept of entropy. When a body absorbs an amount of heat Q from a reservoir at temperature T, the body gains and the reservoir loses an amount of entropy S = Q / T . If an amount of heat Q flows from a hot to a cold body, the total entropy increases; because S=Q/T is larger for smaller values of T, the cold body gains more entropy than the hot body loses. . The statement that heat never flows from a cold to a hot body can be generalized by saying that in no spontaneous process does the total entropy decrease. 4.3 Reversible & Irreversible Process A process is reversible if the system and its surroundings can be returned to their initial states. . The system and its surroundings cannot be restored to their initial states if the process is irreversible. . Notes All real processes are irreversible but many of our calculations ignore that fact because the assumption of reversibility makes the calculations easier. . We go ahead assuming most processes are reversible and then correct our calculations to get approximate answers. The corrections are based on experience. . Our goal as engineers is to minimize the degree of irreversibility. 4.4 The 3rd Law of Thermodynamics A postulate related to but independent of the 2nd law is that it is impossible to cool a body to absolute zero by any finite process. Although one can approach absolute zero as closely as one desires, one cannot actually reach this limit. . The 3rd law of thermodynamics, formulated by Walter Nernst and also known as the Nernst heat theorem, states : . The limiting value of the entropy of a system can be taken as zero as the absolute value of temperature approaches zero. 5. Thermal Deformation Thermal deformation means that as the thermal energy (and temperature) of a material increases, so does the vibration of its atoms/molecules ; . and this increased vibration results in a stretching of the molecular bonds - which causes the material to expand. Thermal expansion Thermal contraction 5.1 Linear expansion For a long rod the main thermal deformation occurs along the length of the rod, where α is the linear coefficient of expansion for the material, and is the fractional change in length per degree change in temperature. . The term 'L' represents the initial length of the rod.. Over small temperature ranges, the thermal expansion is described by the coefficient of linear expansion. 5.2 Thermal Stress If the structure or members of the structure are constrained such that the thermal expansion can not occur, then a significant thermal stress may arise which can effect the structure substantially. . There are many cases where structures and materials are near or at their allowable stresses. In that case, if a thermal stress develops, the total stress may well exceed the allowable stress and cause the structure to fail. This is the reason bridges are built with expansion joints which allow the structure to expand and contract freely and thus avoid thermal stresses. . Additionally, this is why concrete sidewalks are built with spaces separating adjacent slabs, allowing expansions to avoid thermal stresses. 6. Heat Treatment Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. There are five basic heat treating processes: hardening (quenching), tempering, annealing, normalizing, and case hardening. . Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. Basic Steps Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific temperatures. . Once a metal part has been heated to the desired temperature, it must remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking. . The third step is cooling. Metals can be made to conform to specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth. . Heat Treatment of Ferrous Metals Hardening ( Quenching ) 淬火 A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine. . The hardening process increases the hardness and strength of metal, but also increases its brittleness. Tempering 回火 Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness. . Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool in still air. . Temperatures used for tempering are normally much lower than the hardening temperatures. Annealing 退火 Metal is annealed by heating it to a prescribed temperature, holding it at that temperature for the required time, and then cooling it slowly back to room temperature. Annealing is used to relieve internal stresses, soften them, make them more ductile, and refine their grain structures Normalizing 正火 Normalizing is achieved by heating the metal to a specified temperature (which is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air. Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding. . Steel is much tougher in the normalized condition than in any other condition. Parts that will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized. Case Hardening 表面强化 During the case-hardening process, a low-carbon steel is heated to a specific temperature in the presence of a material which decomposesand deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the inside of the piece soft but very tough. Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common case-hardening processes are carburizing and nitriding. Carburizing is a process of diffusing Carbon into the surface of steel. (渗碳) . Nitriding is a process of diffusing Nitrogen into the surface of steel. (渗氮) 7. Energy and Environment Pollutants emitted during the combustion of fossil fuels are responsible for smog, acid rain, and global warming and climate change (the greenhouse effects). . The environmental pollution has reached such high levels that it became a serious threat to vegetation, wild life, and human health. Air pollution has been the cause of numerous health problems including asthma and cancer. Acid Rain The sulfur oxides and nitric oxides react with water vapor and other chemicals high in the atmosphere in the presence of sunlight to form sulfuric and nitric acids. . The acids formed usually dissolve in the suspended water droplets in clouds or fog. These acid-laden droplets, are washed from the air on to the soil by rain. 8. Summary Mechanical engineers involved in the design and analysis of machinery must be aware of the change in the thermodynamic state during the cycle of the machinery. . The total energy must be conserved, as stated by the first law of thermodynamics. Real processes always obey the first law of thermodynamics. As the second law of thermodynamics states, not all the available heat can be converted to work, since some heat is always lost to the surroundings. . Natural heat transfer is from a body at a high temperature to a body at a low temperature. The potential of heat to go naturally from high to low temperatures can be utilized to produce work through a heat engine.