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3.5.3 Lateral force (friction) mode

If the AFM probe is put in contact with the surface and dragged sideways across it, the cantilever will be sensitive to variations in the lateral force between the probe tip and the sample. Figure 8 illustrates the principle.

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3.5.2 Non-contact (tapping) mode

If the probe is taken further away from the sample, into the region beyond the maximum on the force-separation curve of Figure 7, it can be operated in non-contact mode. Typically, the separation required is of the order of 10 nm. The attractive force is much smaller than the repulsive force of the contact mode
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2.2 The challenge for innovation

For a Pirani sensor, the basic task is to provide a reliable measurement of pressure in a vacuum system as it varies from atmospheric pressure down to a value at least as low as 1 Pa. This statement can be further qualified by saying that unless its performance or cost is a fantastic improvement on the existing type, the micromachined sensor must be compatible with existing interface electronics, such that only minor modifications to its design are needed. This implies an electrically resisti
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Learning outcomes

After studying this course, you should be able to:

  • understand how to relate physical dimensions and materials properties to static and dynamic behaviour

  • demonstrate an awareness of how small features are cut out in solid materials, and how small features are built up in solid materials

  • describe the piezoelectric effect and its use for producing small-scale movement in mechanical devices

  • state the relative significance of different for
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References

Collins, S., Ghey, J. and Mills, G. (1989) The Professional Engineer in Society, London: Jessica Kingsley Publishers.
Foster, J., with Corby, L. (illustrator) (1996) How to Get Ideas, San Francisco: Berrett-Koehler Publishers, Inc.
Haaland, J., Wingert, J. and Olson, B. A. (1963) 'Force required to activate switches, maximum finger pushing force, and coefficient
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7.1 The engineer and society

Section 2 outlined some of the needs for engineering. Society relies on engineers to create solutions to the problems involved in meeting those needs.

This is a good time to pause and point out that inevitably, in return for all this fun and power, engineers have a responsibility to society. The people who employ our services, directly or indirectly, have to have an assurance that we are working within certain social, safety and ethical boundaries. Particularly given the increasing tren
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6 A problem with sensors

The problem we will look at in this section concerns the analysis of the design of a component used in cars that are fitted with airbags. The airbag has to be inflated rapidly when an electronic circuit in the system decides that a serious collision is taking place. The crucial component in the electronics is the accelerometer, which therefore has to be extremely reliable. Motor manufacturers have turned to a technology called MEMS (micro-electromechanical systems) for these accelerometers, b
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4.5 Model the best solution

In moving from the 'possible solutions' to the 'best solution' box, Figure 12, we have to assume that a certain amount of evaluation has been done in the previous loop. The solution is still on paper, and probably not much more than a sketch, but something is badly wrong if the best solution to come forward has n
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4.4 Evaluate solutions

If the obvious solution has been identified and everyone is in agreement, then a formal evaluation of solutions is unnecessary, and we would move on to modelling the design. However, if there is dissent then some stricter method of elimination is required, and this is usually achieved through a process of rank-ordering. There is little to be lost and potentially much to be gained by returning to the customer at this point for opinion, clarification or guidance.


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4.1 Advancing knowledge

Over the centuries, engineers have faced and solved a huge number of problems of one sort or another. Each time a problem is solved, knowledge is advanced, something usually gets written down, and so today we have a wealth of experience to draw on. Equally, problem-solving techniques have also been developed and evolved through use and refinement, which is rather handy. Not only do we have some idea of existing solutions to similar problems, but we also have an indication of how to go
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Conclusion

This free course provided an introduction to studying Engineering. It took you through a series of exercises designed to develop your approach to study and learning at a distance, and helped to improve your confidence as an independent learner.


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6.3 Tacoma Narrows suspension bridge failure

Such over-design could not be sustained for long and bridge designers gradually pared back their margins of safety. There is elegance and economy in having the lightest structure compatible with function. But history has a habit of repeating itself.

In 1940 a new suspension bridge with a central span of 2800 feet was built over the Tacoma Narrows in the United States. It was soon noticed the bridge deck was prone to oscillate in certain winds. The vertical amplitude of the oscillations
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Myths persist

Many myths still surround the Tay Bridge disaster, the most pervasive being it was brought down by wind action alone. Rothery's report (see Paper 3) should dispel that particular myth, in addition to the numerous examples shown in this unit of the way the structure had deteriorated by the time of the storm in late 1879.

Click 'View document' below to open Paper 3 (35 pages, 39 MB).

5.15 Further investigation is possible

There are still many mysteries that surround the Tay Bridge disaster, largely because so little was recorded at the time of construction. For instance, questions remain about the details of reject rates for the castings, and modifications made to the first designs of the piers and their component parts.

Although enlargement of the BoT set of pictures has helped clarify the various failure modes described by Henry Law and others at the enquiry, it has also revealed yet more mysteries. Wh
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5.13 Conclusion of the BoT enquiry

The BoT enquiry issued two reports at the end of the enquiry, one authored by the chair, Mr Rothery, the other by the two other assessors. The Rothery report is Paper 3, linked below. They agreed about most of the issues in contention, as follows (Paper 3, page 47 of report).

  1. There is no evidence to show that there has been any movement or settlement in the foundations of the pier
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5.12 Pole and Stewart report

Apparently prepared using the same methodology as Law, Pole and Stewart produced a report that calculated the loads at various points in the bridge under live locomotive loads and wind loading at various pressures. Stewart was employed by Bouch to perform the original design calculations for the bridge, while Pole was brought in as an independent expert. He had extensive experience of use of different materials in bridges, and indeed, had written a standard text book for engineers on the subj
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5.11 Further evidence on stability

Given the importance of establishing the nature of the stability of the bridge, further witnesses were called at a later stage in the enquiry to shed some light on the problem. If Mr Noble had observed chattering of the joints in the tie bars, had similar phenomena been observed earlier?

The key witnesses were the engineers in charge of erecting and finalising the structure before it was opened in May 1878, Major-general Hutchinson, the BoT inspector who approved the structure for publi
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5.10 Bridge stability

Any fracture of the diagonal wind brace tie bars could allow substantial lateral movement at the top of the piers. If these tie bars had already been injured by the previous train to cross the bridge, it would have only taken a little extra effort to complete the process as the mail train arrived over each pier supporting the high girders. Once the wind braces had failed completely, and the struts fractured at their connections each pier would behave as two separate supporting structures.


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5.9 Mechanical tests by David Kirkaldy

In order to determine which of the several parts of the joint were weakest, and gain some idea of the scatter in strength, David Kirkaldy was employed by Henry Law to test various samples he had collected from the bases of the fallen piers. David Kirkaldy had a good reputation for accurate and rigorous mechanical testing of materials using a large tensometer he had designed and built in London (see Input 9, linked below).

Click 'View document' below to open Input 9

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5.8 Design problems

Table 7 summarises the many design problems of the piers uncovered by Mr Law and his team. We have already seen the numerous fractured lugs in the remains of the bridge, shown in Author(s): The Open University

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