An Introduction to Engineering Systems

Systems engineering
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Presenter Information. Recommended for groups of 10 or more. Excellent materials to take home and review later at own pace. Comprehensive coverage of the subject. History and definitions Systems life cycle Benefits and relevance of systems engineering Systems engineering context Systems engineering process Analysis - synthesis - evaluation Systems engineering framework.

Subsystem requirements analysis Requirements allocation Interface identification and design Synthesis and evaluation Specifications and baselines Systems design reviews. Technical reviews and audits Systems test and evaluation Specifications and standards Technical risk management Configuration management Integration management Engineering management plan.

Project management Quality assurance Integrated logistic support ILS Operators Other engineering disciplines including software engineering. Since , he has held a number of positions in communications and systems engineering and in management and project management. His research and teaching interests are in communications and information systems, requirements engineering, systems engineering, project management, and technology management. He is the author or co-author of twelve books, three book chapters, and over technical papers and reports. Since , he has held a number of engineering, project management and academic positions in the fields of avionics, simulation, radar, communications and information systems.

He is the director of a Canberra-based engineering consultancy magpietech. He is the author or co-author of a number of books covering radar, radar electronic warfare, avionics systems and systems engineering. No dates? Or unable to attend dates shown? Your expression of interest will be pooled with others until the minimum has been met. The presenter will then announce a course delivery date and you will be notified via your submitted email.

Until you indicate to us via email, we will continue to update you on Professional Education courses. However, substantial loss minimisation is possible. Some suggested measures are: 9 reduction in size of fire areas; 9 installing supply facilities and safety apparatus in different fire areas; e. The quake lasted 40 seconds and was measured at Richter 7. Experts claim that the highway failure was initiated by shear failures of the piers. The total loss is estimated in billions of dollars Figure 1. Figure 1.

However, special design measures can be, and are, taken in earthquake-prone locations. In the parlance of insurance assessment, disasters caused by unpredictable natural events are referred to as acts of God. Until , the greatest loss in any one disaster was that due to hurricane Andrew, affecting the east coast of USA and the Bahamas. The rise in the scale and scope of natural disasters, linked to global warming and reflected in a report produced by Munidt Re, has alarmed underwriters so much that some are considering making parts of the world uninsurable.

In general, a very large proportion of industrial loss is incurred due to fire damage. Consequently, while mechanical failures may be embarrassing, occasionally very costly, and in some cases result in tragedy see for example Eddy Potter and Page, , on the scales of insured industrial loss, they are a rare event. This doesn't mean to imply that they happen rarely On the contrary, engineering components fail with [ Chapter l monotonous regularity.

However, in most cases, designers are aware of the limitations of engineering components and systems. Failures elicit planned responses, and damage and injury are minimised. In some critical designs, and in design against fatigue, mechanical failures are often used as a design tool to establish component life. Some very special engineering components are designed tofail in service. On critical pressure vessels a bursting-disc is used to limit the effects of overpressure to a specially designed part of the system. In some machinery, expensive parts are protected by specially designed, cheap, easily replaced, sacrificial components.

These types of mechanical fuses minimise damage and injury. What engineering designers fear most is unexpected and un-planned-for failures. Some are described in the next section. In general, if properly investigated and documented, they represent valuable case studies for future designs. The subject of this failure example was steam driven by a two stage turbine set. Following a major overhaul, carried out after 25, successful service hours, the MW t u r b o g e n e r a t o r set was being checked for overspeed protection.

The loss was estimated at SUS 40 million. The generator shaft was ruptured over half its length seen in Figure 1. One rotating part of the generating equipment, weighing about three tonne, was hurled into the machine hall and hit the girder of an overhead travelling crane. The insurer's experts speculated on the cause, but they could not find any substantive reason for the failure. No material defects were found and all shaft failures were attributed to exceeding the p e r m i s s i b l e b e n d i n g stresses. S u b s e q u e n t speculation by rotodynamics experts conjectured that a bearing or rotor instability might have caused this type of failure.

The subject of this failure was a 2m diameter, 10m high welded steel tank, with a 22mm outerlacket, operating at "C and 33 bars pressure 3. D u e to an e m e r g e n c y shutdown, the pressure in the preheater rose to The burst vessel and the devastated plant room are shown in Figure 1. Initial investigation of this failure focused on the vessel material.

There was some concern that other similar vessels in this plant could suffer similar failures from what appeared Figure I. Ultimately, however, the failure was attributed to faulty welding of this particular vessel. The faulty weld was a longitudinal weld, with a manufacturing fault, that remained undetected duringX-ray inspection. During 11, hours of operation the faults in the weld developed further, requiring only the small pressure rise to initiate the failure.

In some pile drivers the hammer has a centrifugal out-of-balance mass driven to provide a vibrating load for hammering. The angle of the hammer guides needs to be adjustable to allow shifting of the centre of gravity of the unit. This adjustment is particularly important when operating on uneven ground, where a very small lean out of vertical of the h a m m e r guides can result in substantial overturning moments acting on the unit.

A 30m high, tonne, piledriver with a vibrating hammer was being used for earthworks near the edge of a river, when the unit, driven on a crawler tractor, slowly toppled over into the river Figure 1. Investigation of this failure found that ajammed hydraulic valve prevented successful hydraulic control of the 30m high hammer guides. Once the unit commenced to lean over in the uneven soil of the river bank, the hydraulics could not correct the lean by appropriate shift of the centre of gravity and the unit toppled over into the river.

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This was the first of 2, Ec-2 class ships that became known as the Liberty ships. War time pressures of production schedules demanded that these ships should be produced in record time. One such Liberty ship was delivered fourteen days after laying the keel in the shipyard. These ships were of low alloy steel, fully welded, construction. In earlier, riveted hull ships, structural cracks would be halted by the presence of rivet holes in the material. Some of the Liberty ships were scheduled to operate in very cold climates.

The combination of low temperature embrittlement lowered fracture toughness ]n low t e m p e r a t u r e s and the unimpeded propagations of cracks through the structure resulted in some spectacular failures in these ships Figure 1. A photo elastic model of a similarfirtree connector under load is also shown in the figure. Photo-elastic models are made of a photopolymer, which can cause light to split into two c o m p o n e n t s w h e n u n d e r stress double refracting. When polarised light is passed through a photopolymer under stress, the stresses show up as interference patterns.

These patterns can display the stress in the material, with closely spaced fringe lines indicating large stress gradients. From Figure 1. Failures of this type can result in spectacular and costly damage. Several fragments cut through the turbine casing, damaging it beyond repair. One fragment impacted a ,kVA transformer causing oil to leak out and ignite. The major damage was caused by the ensuing fire.

These engineering components are exposed to very high time-dependent loads. Consequently, the most common mode of failure of propellers is fatigue. Figure I. I I Failed aircraft propeller Figure 1. I0 Failed turbine disc In Figure 1. Also, the failure commenced at one of the bolt holes on the propeller, indicating that this was a region of high stress caused by the bolt holes acting as stress concentrators.

As can be expected, propeller failure will result in spectacular and occasionally tragic consequences. As a conservative safety measure, regulations require commercial aircraft to be able to fly and land with half of their normal complement of engines still operating. However, depending on the specific nature of the failure, corrective action by aircrew is not always possible. One aircraft lost a propeller as it was about to land and eventually landed safely, incurring only mechanical damage.

The other aircraft lost its propeller in mid flight, and had to make an emergency landing. Several people including the pilot were killed and the aircraft was lost. In both cases the failures were traced to a small surface indentation incurred during manufacture, resulting in a local stress concentration in an already highly stressed region near the root of the [ Chapter l I 13 blade. Given the resulting high local stress, the resulting fatigue failures were predictable. These investigations often focus on 'who is to blame' and 'who will bear the cost' of the failure.

Our focus is considerably more constructive. We focus on technical issues associated with failures, in order to learn from them. We hope to discover the underlying technical issues that might influence a whole class of such failures, and how to prevent this type of failure from happening again. There is an instructive story about a professional golfer, who was playing with a well known celebrity partner in a pro-celebrity golf tournament.

The celebrity hit off at the first tee and landed his ball in the local car park, out of bounds. We have examined a series of ten failures. At the beginning of this chapter we categorised failures as technical, operational or unpredictable. In addition, we have identified a notion of failure intensity as a measure of economic, environmental and social impact caused by the failure. We should finally comment briefly on the problem of assessing likelihood of predictable failures.

In general, designers are aware of normal, or expected, or most likely, operating conditions of their engineering component or system, so that they are at least able to design for its normal operation. The difficulty arises when they have to plan for the abnormal. Ultimately, one could conceive of a situations so abnormal that it would be a reasonable bet that they would never arise. The implication is that design judgement is a little like gambling. Most engineers, however, are conservative, and wish to avoid the risk of gambling with financial or human loss.

The most important judgement to be made early in any design, is the worst credible accident that is likely to befall our component or system. In many industries, codes of practice will prescribe allowable working stresses. In structural design, many codes also assign w o r k i n g loads. In mechanical design of engineering components, the assessment of working loads during the worst credible accident requires considerable experience and engineering wisdom.

An amusing, albeit instructive, story is about a visit by the physicist Leo Szilard to the bacteriology laboratory of Salvador Luria, who was awarded the Nobel prize for his work on bacteriophages viruses that attack bacteria. Szilard Introducing modelling and synthesis [ 14 I - - himself was by then an established figure in physics, partly responsible for the Manhattan project that developed the first atom bomb.

Luria was initially embarrassed at showing the famous physicist through his laboratory. Szilard" he asked, "what should l assume, how much should l explain? In what follows we apply Szilard's mildly arrogant measure to readers of this text. The origin of S I units, based on the decimal system, dates to the aftermath of the French revolution, when in the French Academy of Sciences adopted the metre as the unit of length. This measure was based on v times the quadrant of a great circle of the Earth meridian passing through the poles and through Paris. It took six years to survey the arc from Barcelona to Dunkirk, resulting in a value of The units ofgram, 10 - ' times the mass of a m 3 of water at its maximum density 4"C , and litre, the volume of m 3 of water, are both derived from the metre.

In a National Bureau of Weights and Measures was established at Savres, near Paris, where representatives of 40 countries meet every six years to update the international units of measurement. For the metre, this was the distance between two marks on a specified bar of platinum, and for the kilogram, a carefully constructed and certified mass of platinum.

This is the quantity of visible light emitted per unit time per unit solid angle or steradian recall that there are 4Tt steradians in a sphere. The lumen is related to the visual sensation caused by light and is dependent on the wavelength used. For example, light at a wavelength of nanometres 10 -9 metre , to which the eye is most sensitive, has a luminous intensity of lumens per watt of radiant power.

While we make exclusive use of SI units t h r o u g h o u t this text, some e n g i n e e r i n g organizations still make use of the older Imperial System of units. It is expected that within the foreseeable future engineers will need to be familiar with both sets of units. A table of conversion factors is provided in the Appendix. I Chapter l i 15 1. Engineering science eschews guessing or estimating by the very nature of its analytical content.

Engineering design, however, relies on estimation for synthesis. We cannot sensibly evaluate the outcomes of all possible choices in synthesising a solution to an engineering design problem. Hence, we often rely on estimation as a heuristic rule of thumb , for r e d u c i n g the task of evaluating outcomes to only a few, informed, solution choices.

Naturally, there is a risk involved in making guesses, or estimates. But then engineering design synthesis is a risky business, not for the fainthearted. There are many industries relying on estimation for their business, and accepting risk as an integral part of that business. Perhaps the most well recognised of these risky businesses is insurance. If we wish to be insured against some loss, we estimate the value of the loss, and contract with an insurance underwriter to carry the loss should it actually occur.

The insurer estimates the nature and extent of the risk involved in the loss event, and based on this estimation, calculates an appropriate premium, or annual payment, to cover the payout when, and if, the loss event is incurred. In essence, the whole process is a gamble, where the insured bets with the insurer that the loss will occur, and continues to lay an annual wager to this effect.

Since, generally, the wager is only a small proportion of the eventual loss value, both parties, the insurer and insured, benefit from the process. In the business of insuring against industrial loss, a key parameter of insurance risk is experience with the technology involved. Insurers go to inordinate length in estimating risk with new technologies. The industrial insurance of space technology involved an almost unacceptable risk at the b e g i n n i n g of the s.

N o t only was the technology new, with no data to rely on for loss estimation, but also the losses that might be incurred could be very large. One would be tempted to question the value of offering industrial insurance in such a risky technological environment. However, the benefits to be gained are also substantial. At the end of there were some , mostly geo-stationary, satellites in orbit around the Earth. As experience with the t e c h n o l o g y grows, the i n s u r a n c e estimation becomes less risky. Also shown in Figure 1.

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Clearly, while the insurers made substantial early losses, the industry appears to be breaking even cumulatively. In future years the space insurance industry can look forward to strong gains from the experience built up in these early startup years. As already noted, engineering judgement is a key determinant of design success.

The underlying skill in engineering judgement is the capacity to marshal Figure I, 12 European space lab in cargo bay of the space shuttle and space insurance loss data: After Schaden Spiegel, 'Space Flight and Insurance' , Introducing modelling and synthesis I 16f all the available data and to make informed, intelligent estimates about loads and failure scenarios. Typically, we can find almost limitless examples where quite realistic population estimates may be drawn from personal experience and observation.

An example will demonstrate the process of estimation. Suppose we were interested in designing hospital equipment to support birthing in hospital maternity wards. As a matter of course, we would need to estimate the birthrate to asses the size of our market. Clearly, we were able to make a close estimate of birthrate from the demographic data of Figure 1.

Engineering estimation proceeds in a way not very different from the above example. O f course, for success in estimation and engineering judgement, we need to start with a list of what we expect you to know. These are listed in Table 1. Where calculations are needed, these should be done mentally. Total time estimate for answering all these questions successfully is 10 minutes.

While we focus here on factual knowledge, it is useful to recall the a d m o n i s h m e n t of Henri Poincarrd : "Science is built up o f facts, as a house is built of stones; but an accumulation of facts is no more science than a heap of stones is a house. This means that every female will, on average, give birth to 2 children in a lifetime of 75 years. Bureau of Statistics data shows that in there were 60, live births to a Victorian population of 4.

I General knowledge quiz What is the speed of light in vacuum in m. What is the density of aluminium in kg. What is the speed of sound in air in m. What is the area of a sphere, as a function of its radius? What is the volume of a sphere, as a function of its radius? What is the yield stress of mild steel? What is the current human population of the earth to one sig.

One of the oldest clich6 hackneyed, overused phrase in engineering is the back of the envelope calculation. N o doubt, this phrase predates computers and electronic calculators. It was often used by practising engineers as a pseudonym for estimation involving afew facts and numbers.

In essence, the above estimation of birthrates is an example of a back of the envelope calculation. We don't need to calculate any of the results in this quiz, or, at least, if there is to be calculation, it should be of the back of the envelope type. In this second quiz, you should try to answer all the questions in 60 seconds. How high is it? Approximate answers: a 30kg; b 3m; c 0. Correspondingly more care is needed in estimating answers than in the previous two quizzes.

This has b e c o m e particularly important since the widespread use of electronic calculators. It is so easy to slip a digit or two when entering data in a complicated set of calculations. Order of magnitude, and dimensional consistency, checks must be routinely used for all calculations. Moreover, the number of significant figures available from electronic calculators bears no relation to the real precision of our results.

Consequently, we never present more significant figures in our results than we can credibly justify, within the levels of approximations made along the way. Ultimately, these procedures build confidence and credibility in evaluations for both engineer and client. Our guiding principle must be that we engineers are in the credibility business.

T h e case c o n c e r n e d the design and construction of a relatively simple and inexpensive dolphin enclosure. A crucial calculation, using an equation known as Merion's equation, was made at conceptual design stage. The calculation contained a simple and easily identifiable arithmetical error. However, it was never checked. The judge held them both to be equally liable for the failure. The case illustrates the importance of risk management systems where all designs are reviewed and checked. There are two billion children persons under 18 in the world.

At an average of 3.

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Planar 2D Rigid Body Kinematics II -In this section students will continue to learn about planar 2D rigid body kinematics, relative velocity equation, rotation about a fixed axis, instantaneous center of zero velocity, and relative acceleration equations. This is an introductory course in the concept of time value of money and financial management for engineering managers. It focuses on defining customer needs and required functionality early in the development cycle, documenting requirements, then proceeding with design synthesis and system validation. Long Range Planning, 7 6 , 2— Toggle navigation.

Allowing for time zones and the rotation of the Earth, assuming he travels east to west, Santa has 31 hours of Christmas to work with. This represents Assuming the 86 million households are evenly distributed on the Earth's surface a gross approximation for the sake of this estimate , the average distance between households is 1. Assuming each child gets no more than say a simple construction kit -- 1 kg , neglecting r e i n d e e r and Santa usually seen as gravitationally challenged , the total payload of the sleigh is 86 thousand tonnes, roughly the weight of the ship Queen Elizabeth, one of the largest passenger liners ever built.

One could continue to speculate on the energy required to stop and start this mass 86 million times, even as it is reducing at a rate of 1 kg per stop, and the energy generated by air resistance at the phenomenal speed it is travelling. However, even if the sleigh and its payload were to travel in the stratosphere, it would most likely burn up on re-entry. We can safely conclude that either Santa's existence is, at best, questionable, or that he does it by mqic. In engineering design, we regard this process as the very essence of engineering thinking.

Hence our term of structural distillation, the process which distils the essence of a complex engineering structure into its recognisably tractable elements.

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While most of our engineering education tends to focus on analysis, structural distillation requires substantially different skills. We focus on the way a structure is to be used, and the type of loading it will be subjected to, during its operational life. These operational loads determine the underlying nature of the structure. After all, it was these loads that the structure or component was designed to Introducing modelling and synthesis I 22!

A typical example is shown in Figure 1. The supporting legs of the director's chair are decomposed into a pair of beams. We note that legs are really beam-columns, due to the large axial load component, particularly in the upper half of the scissor.

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Beam-columns are quite tricky to model see also section 3. In this chair there are two pairs of legs, front and rear, interconnected by a central hinge element and two side 'beams' to which the cloth seat is attached. A more complex structure is offered by the example s h o w n in Figure 1. Here we have decomposed the trolley into two structural elements, a solid blob for the loaded basket, and a simple portal frame, for the base and support arms.

Needless to say, there are parts of the whole structure we have not considered in this simple decomposition. The objective is to illustrate the process. Moreover, we recognise that this simple distillation process is only the first step in the design synthesis. One could go on to model the complete structure in a finite element analysis modeller to achieve mass optimisation for example. T h a t can certainly happen, once the final embodiment of the structure is determined, but it may not be warranted for the sake of potential cost savings.

Structural distillation precedes the analytical evaluation process by providing early evaluation of the structure before final embodiment is decided upon. Hence, the process we are proposing here is essentially one of synthesis. While the examples we offer are those of existing embodiments, the intent is to provide design tools for the simple and credible evaluation of engineering structures as they are being created.

The designer needs to be sufficiently confident to perform structural distillations of new embodiments as they occur. That, after all, is the real essence of engineering design. The components shown in Figure 1. The distinguishing feature of these components is their capacity to carry loads which result in more complicated stress fields than those in the simple axially loaded components of Figure 1. They are the designer's friends. It is important to realise that these generic components are defined and indeed recognised during the process of structural distillation by both their shape and the directions and types of load they sustain.

The designer must be able to see these c o m p o n e n t s - often disguised- within larger and more complex structures. In each case we identify the component by the type of loading it is expected to withstand, its generic name, as well as other names that are in common usage. Clearly, these types of components are exposed to uniaxial loading only. Later on, in Chapter 2, we deal with the relationships between loading and stress conditions, and hove these influence c o m p o n e n t failure.

However, in structural distillation we are only concerned with the nature of loading the component is expected to carry. Their design is, in general, the subject of statutory codes of practice, since the stress fields generated by their loads are complex, and their failures can be devastating refer Figure 1. However, for the purposes of this elementary consideration we regard the vessel as a smooth, closed, thin walled, cylindrical container, capable of carrying internal pressure.

In our discussion of loading we need to establish when a component is subjected to: 9 time varying loading fatigue - Figure 1. In general, these types of elements are beyond the cope of an introductory text. Some examples are shown in Figure 1.

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We consider a beam of u n i f o r m rectangular cross section under the influence of a bending moment M. We accept the notion that the deflection of this beam is sufficiently small to allow the following simplifying assumptions to hold: 9 the radius of curvature R is large compared to the span of the beam; 9 plane sections remain plane during the deflection process. The resulting relationship between beam section geometry, and radius of curvature is M Figure I. The radius of curvature R is taken to be that of the only undeflected neutral layer in the beam. The line in which any cross section of the beam intersects this neutral layer is called the neutral axis.

Although some earlier conventions have measured positive distances in the beam in the direction of increasing radius of curvature, here our convention follows that adopted by more recent texts on applied mechanics see for example Young, Also conventionallg tensile stresses are regarded positive and, correspondingly, compressive stresses are regarded as negative. Using these conventions, we can write down the extension of the small element AB at distance y from the neutral axis. Refer to Chapter 5 for a brief discussion on sign conventions. Beams with a positive bending moment as indicated in Figure 1.

Both point loading and distributed loading are shown. Notably, there is a duality between simply supported beams and cantilevers, and this duality is drawn out in the point loaded case. Clearly, with the loading shown, the cantilever beam behaves precisely like one half of the simply supported beam. With distributed loading, the duality still exists, but it is a little more subtle than for the point load case. While the maximum moments are equal, Figure I. The steps involved are: 9 identify applied forces, moments i.

Shear-force and bending-moment diagrams are plotted, to indicate the nature of loads experienced by the component. The force F is that - - I 27 applied to some item held in the jaws and q is the distributed load applied by the human hand to the handle of the device. Clearly, the operation of each device is limited by the maximum load, q, a human is capable of applying.

The loading is essentially the same as for a simple lever, and the arms of the pliers behave like simple beams. The arms of the multigrips behave like a frame commonly referred to as a bellcrank. The structural behaviour of the bell crank is considered as two simple cantilevers at right angles to each other, joined at their built-in ends. Freebody diagrams are drawn for each of the two parts of the arm, using local coordinates, as shown in Figure 1.

It is a ubiquitous device, since apart from its use in paper clipping it is often employed in other roles. For example, a string of paper clips, joined end-to-end, can be formed into a ring to create the office stationery equivalent of a daisy-chain. Some desktop computer m a n u f a c t u r e r s r e c o m m e n d the use of a straightened paper clip for dislodging jammed floppy disks.

However, for our example we consider only the most commonly intended use of the clip, namely that of acting as a compliant connector for sheets of paper. The two cutting blades need to be analysed separately, since they have slightly different shapes. However, for the purposes of this simple example we have only plotted bending-moment and shear-force diagrams for blade 'A'. The diagrams for blade 'B' will be similar, but of different signs.

In operation the two loops of wire are deformed out of their plane and the forces generated by this elastic deformation hold the sheets of paper together. Perhaps the main objective of the structural distillation process we are advocating is that it focuses on a key aspect of design, even when applied to such humble artefacts as the paper clip. Someone had to think about the parameters of successful performance for this device. This involves the use of insight and simple physics, to comprehend the loads which are being imposed on a device. Often, the engineer will proceed by sketching a simplified diagram of the device, which reduces it to a set of simpler components, such as beams, cantilevers, shafts and columns.

The aim is then, to specify the major external loads forces and moments acting on these components, and the internal forces and moments to which they give rise. A relatively simple and useful way to view the devices in this assignment is to imagine they are made of some compliant material, like rubber, and imagine h o w they would deform u n d e r the normally applied loads. In essence, we are encouraging the use of a thought-experiment to aid thinking in these simple examples. You may choose any set of fifteen of these for structural distillation.

For your chosen set of devices, distil the essence of the loading situation as described in the preamble above. Specifically, you should: 9 identify the major loads which will concern the designer in determining the structural integrity of the device; 9 d e c o m p o s e the device into those basic structural components which could be used in a first-order analysis for design don't do the analysis.

Prepare structural distillations for any set of fifteen out of 30 devices shown in Figure 1. In each case sketch the shear-force and bendingmoment diagrams for the parts of the structure critical to the successful function of the device. Also identify the loading situation that could be considered the worst credible accident for the device. The case examples, shown in Section 1. One half A4 page per device should be ample, and each device should take no longer than 10 minutes to complete to the standard indicated in case examples 1.

Who shaves the barber? We assume that human hair grows, on average, at the rate of about 30 m m in 40 days a guess that may need e x p e r i m e n t a l evidence for its s u p p o r t. Additionally, we assume that most people feel a need to be trimmed up once they have sustained this growth.

The volume of a grain of wheat is estimated as a cylinder of 1ram diameter and 2ram long. Hence, we can calculate the total volume of the sparrow as 29 Introducing modelling and synthesis j 30 f - - Figure 1. A farm is to be connected to the electricity grid, requiring the installation of a set of low tension transmission poles and a single cable over a distance of 2. Your responsibility is to design the poles and cable so that they possess adequate structural strength at the lowest possible installed cost.

The poles will be made from steel and have a hollow cylindrical shape. The electrical cable will be copper with an allowable stress of 45 MPa, and designed to withstand yielding due to axial tension under selfweight. Electrical transmission requirements dictate that the cable must be no less than 10mm in diameter it can be regarded as being effectively a single solid wire.

For safety reasons, the cable must always be kept at least 6. Joule could not have detected this with ordinary thermometers. Initially you should concentrate on producing a design which fulfils j u s t the structural integrity requirements; b In as systematic a fashion as possible, improve your design by reducing its cost; c Prepare a report setting out clearly the reasoning leading to your results. In your report, use a flow chart to show your sequence of calculations and the major steps in your design. Discuss which of the variables you have treated as design variables.

List all the assumptions used in your design calculations and critically review them. Some important background formulae and definitions are given below. Notes 1. The system will require one pole to be installed at the household end, one at the grid tie-in point, and several in between. At the termination points, and at any intermediate poles where the horizontal direction of the cable may change, it may be assumed that sufficient guy wires are provided to ensure that no net sideways force is imposed on the poles due to the tension in the electrical cable.

Although the poles experience downwards axial force due to self weight and the weight of the electrical cable, it is assumed that this effect will be of minor significance compared to the effect of lateral bending due to wind forces. It should be ignored. The design of this simple system is an introductory exercise to illustrate features c o m m o n to all design problems. T h e important ideas of allowable stress, design uariable and optimization are introduced. The cable's curved shape is called a catenary, after the Latin word catena, meaning chain.

Some useful nomenclature is summarised in Figure 1. This expression is exact. This nonuniqueness is a natural feature of all design problems. We fully expect that the creative problem solver will find some more elegant solution. However, the one presented here is one that certainly seems to work. Two interesting features of the problem, which are not obvious at first glance, are: it is not possible to obtain an explicit relationship between the sag and the span of the cable i. This is because the shape of the cable c a t e n a r y turns out to be independent of d.

One good way to start the solution is to work out an expression for the total cost of the system. This will quickly identify the major design variables. Two important symbols which have not yet been provided are: the maximum allowable stress in the copper cable, Sa,cu ; the maximum allowable stress in the steel pole, Sa. J Ckd. Page I0 of 14 Ckd.

O00 OOO g4. John Ure, Engineer, Design against failure might seem a thoroughly pessimistic objective when compared to the more optimistic objective of studying successful designs. The title of this chapter intends to signify that, in general, designers are a pessimistic breed of engineers. In an unpredictable world a pessimist, designing against failure, is less often disappointed than an optimist designing for success. In the perfect world of the scientist's model of reality, stresses and loads, material properties and environmental variables all behave in some ideal, perfectly predictable way.

In that model world manufacturing errors don't exist. After all, this is the way a model world is expected to be. In engineering design we also deal with idealised models, but we know that they are merely simplified to make them tractable for analysis. The results of our analyses need to be adjusted for the uncertain nature of the real world, where we can never be certain of loads or environmental variables, and where manufacturing errors abound. O f course, we are not alone in considering failure as a critical starting point in a discourse on engineering design.

Professor Henry Petroski Petroski, in his introduction to the nature of design notes: "The concept offailure is central to the design process, and it is by thinking in terms of obviating failure that successful designs are achieved. It has long been practically a truism among practicing engineers and designers that we learn much more from failures than from successes. Indeed, the history of engineering is full of examples of dramatic failures that were once considered confident extrapolations of successful designs; it was thefailures that ultimately revealed the latent flaws in design logic that were initially masked by largefactors of safety and a design conservatism that became relaxed with time.

The history 21 of mechanics of materials dates to the construction of the earliest structures that required practical prediction of material behaviour under load. It is now generally agreed that the Djoser pyramid in Upper Egypt, known as the Great Stepped Pyramid, is the earliest important stone structure requiring engineering estimation of material performance. This extraordinary structure, built during the third Dynasty in Egypt BCE has a base measuring m by m and rises to a height of 60 m. The fourteen most impressive columns in this temple rise 24 meters in height to massive capitals and lintels still standing today.

Clearly, the Egyptian constructors of these large and complex structures needed a considerable store of practical predictive wisdom for assessing the behaviour of their materials under load, however no written records of these predictive skills have been handed down to us. The beginnings of the science of structural mechanics dates to George Bauer a German scholar and scientist, who adopted the Latin name Georgius Agricola.

A physician by 2. The sources for these brief historical notes are the Encyclopaedia Britannica, 15th Ed. His major work, De re Metallica, dealing with mining and smelting, was published in This work was translated into English in by the mining engineer Herbert Hoover later US president. Together with his contributions to almost all other aspects of technology, Leonardo da Vinci also recorded some early experiments on the mechanics of materials.

Among several other experiments on structures, Leonardo investigated the strengths of columns and the tensile strength of wire. However, the first recorded study of the safe dimensions of structural elements appear in Galileo's book intorno d clue nuove scienze Introduction to Two New Sciences , published in In this work Galileo describes studies on columns and beams Timoshenko, In the Ecole Polytechnique was established in Paris, mostly due to the inspired leadership of the French mathematician, Gaspard Monge, who is widely regarded as the father of analytical geometry.

In what follows, we first briefly examine those properties of elastic materials that substantially influence our material choices for the design of engineering components. We then proceed to study the way in which the mechanical properties of our materials of choice allow us to predict component behaviour under complex, practical, loading conditions.

Our focus in this chapter is on technical issues that relate to structural integrity, and the design of engineering components to resist some form of structural failure. We begin by considering elementary failure mechanisms of some materials of construction used in engineering. This work was substantially edited and republished in by Saint-Venant; 9 A.

Barr8 de Saint-Venant, , French engineer, who derived the torsional or shear modulus of elasticity; 9 Alberto Castigliano, Italian mathematician and engineer, , developer of the notion of strain energy; based on the principle of virtual work; Figure 2. I Most important materials of construction, to I. Source: Bureau of statistics Although there has been a consistent, almost monotonic, increase in the use of polymers in engineering components since the turn of this century, we will concentrate on elastic materials, and principally metals, only. I Chapter 2 Figure 2.

Aluminium production, by far the largest c o m p o n e n t of all nonferrous metal production, has remained moderately constant over the last 30 years. Based on data, the world steel production was million tonnes and aluminium production was 12 million tonnes. In the corresponding production was million tonnes for steel and 16 million tonnes for aluminium.

The most dramatic change in materials of prodiuction has been the increase in polymers and resins. Figures for USA and European production of plastics in are 28 million and 25 million tonnes respectively; thereby significantly exceeding world productions of nonferrous metals. Clearly, even if proportionately the use of ferrous metals is declining, in absolute terms they still represent the bulk of all engineering and structural materials used in the world today.

Hence our concentration on their behaviour is based on the evidence that most engineering components are made of steel, or some other ferrous material. Design against failure is a complex, multifaceted process, in which we need to examine each facet separately for its influence on the eventual successful avoidance of failure in our design. Figure 2. Thefailuref0cus identifies the type or mode of failure the designer will focus on, or specifically design against, given the material characteristics and the operating conditions of the component.

The marked cells in the matrix indicate where there is a strong interaction between specific design variables and the mode of failure the designer is primarily attempting to avoid. In general, engineering components can fail by suffering one or more of the following effects: 9 the component breaks. This type of failure is generally signified by some active part of our c o m p o n e n t breaking u n d e r load.

The technical terms for this type of failure are rupture, or fracture; 9 the c o m p o n e n t sustains plastic, n o n recoverable, deformation. This type of failure is most commonly localised and often leads to eventual rupture of the component. The c o m m o n technical term for this type of plastic deformation is yielding; 9 the component experiences a time dependent failure mechanism known asfatigue. This type of mechanism is still the subject of intense research, since it is not yet fully understood.

Many engineering components are exposed to time dependent, dynamic, loading. Under these conditions the material of construction becomes tired and will fail at a significantly lower stress intensity than it could sustain in a static manner; 9 the component experiences a form of elastic instability with large lateral deflections under compressive loading.

This type of failure is mostly experienced by slender columns under axial loads, and thin walled pressure vessels subject to external pressure e. The technical term for this type of failure is buckling. A component will buckle under a load significantly lower than the yield compressive load for the same component. Once buckled, the component can no longer sustain any significant load; 9 at elevated temperatures there is long-term, relatively slow, plastic deformation of the component under steady load.

The effect is attributed to the fact that key material properties such as yield strength and ultimate tensile strength are generally obtained from tests of the material at ambient temperatures. Loaded c o m p o n e n t s exposed to higher temperatures, such as gas turbine blades, or heat exchanger piping, suffer this type of slow deformation. In technical jargon this effect is known as creep; 9 the component experiences elastic or plastic deformation beyond some permitted bound.

The technical term for this type of failure is excessive-deflection; 9 one or more surfaces of the component suffer local failure due to high local rubbing or concentrated loads. This type of failure usually wears away some part of the surface locally and is technically termed wear. Also under the same heading is erosion, a type of surface damage caused by heavy particles impacting on a surface locally. A typical examples are found in jet exhausts of steam turbines, w h e r e the c o n d e n s i n g water particles erode the local surface, and in conveyor systems used for transporting abrasive materials.

Another effect of wear is a type of localised spalling of the surface r e s u l t i n g from surface fatigue. M o s t commonly, this type o f wear failure occurs in rolling element bearings; 9 the material suffers some form of degradation or chemical conversion, most commonly through oxidation.

The technical term for this type of failure is corrosion. It is often the result of unacceptable or unpredictable environmental exposure of materials to corroding agents. It can also be a result of a Galvanic current set up by a combination of electrode potential differences in adjacent materials in the presence of a suitable electrolyte, such as sea water or bodily fluids.

The process is often exacerbated by the presence of stresses at the grain boundaries of the metal. This type of corrosion is a significant problem for specific combinations of materials and environments. A typical example is experienced with certain grades of stainless steel in chloride surroundings such as the seaside. This effect is known as stress corrosion. Examples are: Teflon - for the product poly-tetra-fluoro-ethylene PTFE ; and mild steel - for a range of low [ Chapter2 53 carbon steels used in general engineering and designated by a typical 0.

Materials are chosen for a variety of reasons but generally three dominant criteria are used in the selection process: 9 first of all, the material must meet the specified duty for the product strength, elasticity or wear resistance for example ; 9 secondly, the material must be readily available. Material suppliers are notorious for listing exotic products which can only be produced if the buyer is prepared to wait many months or is prepared to order a large tonnage; 9 thirdly the material must meet production requirements. The product must be capable of being produced.

It may be exciting to design a product in a special ceramic, but the excitement soon evaporates when it is discovered that the only furnace capable of p r o d u c i n g the desired m a n u f a c t u r i n g conditions is in some remote location, or has inadequate capacity.

Ultimately, material selection is determined in consultation with the producer or distributor of the materials concerned. This will ensure that our material specification for the design is up-to-date. Iron is abundant in the form of iron ore haematite or iron-oxide , it is readily reduced from ore to metal, and when alloyed with carbon it provides a vast variety of properties. However the variety of steels commercially available for the production of commodities is vast and the range can be encompassed only through consultation with metals handbooks or manufacturers.

The range is further complicated by the changes in properties available through heat treatment. This text is intended to provide only an overview of the range of steels. Consequently, the brief survey that follows is broken up into the segments indicated on the tree of steels shown on Figure 2. Plain carbon steels range in carbon content from 0.

Mechanical Engineering Systems

To a very large extent the properties of the final product are affected by the way the steel, poured from the furnace, behaves during the cooling process. In the final stage of processing, the molten steel is surrounded by a great deal of free oxygen. This oxygen reacts with the carbon in the steel and forms carbon monoxide CO. As the steel cools, the C O bubbles rise to the surface, and the resulting metal structure is significantly less homogeneous than it would be without the bubbles. The resulting material is known as rimming steel, named after the CO bubbling process which is referred to as rimming.

Thin plate used in metal beverage can production is a typical application of rimming steel, where the thinning out process during rolling tends to reduce the effect of the rimming bubbles of CO and generally improve structural integrity. Where the steel maker wishes to produce steel without the CO bubbles, additives are used at the last stages of production. Quantities ofaluminium or silicon combine with free oxygen in preference to the formation of CO. The result is a very still or killed cooling process. In contrast to the rimming steel, fully killed steels tend to suffer cooling shrinkage.

In general the uneven end of the cast steel ingot is cut off and returned to the furnace. The lower volumetric efficiency of this production process results in a more expensive steel, but a steel vastly superior to rimming steel in structural properties. C u r r e n t s t e e l - m a k i n g processes rely on continuous casting and, in general, additives are used to produce a fully killed steel.