Namas Chandra Introduction to Mechanical engineering Chapter 3-1 EML 3004C CHAPTER 3 Engineers and...

Namas ChandraIntroduction to Mechanical engineering Chapter 3-1

EML 3004C

CHAPTER 3

Engineers and the Real World?


EML 3004C

Objectives

Examine Society’s View of EngineeringLearn about the Role of Failure in Engineering DesignDiscuss Classic Design Failures as Case StudiesLearn How to Accept and Utilize Failure in Your Own Design

Sections

3.1 Society’s view of Engineering

3.2 How Engineers Learn From Mistakes

3.3 The Role of Failure in Engineering Design: Case Studies

3.4 Preparing for Failures in Your Own Design


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Engineers are not perceived as glamorous but as talented professionals.

Engineers do the impossible as a matter of routine They create science fiction stories into realities Recall Captain Kirk with a flip-box talking to anyone in the

world in 1960 >>>> Today’s cell phone Rewind to 1902

• Would you have believed that man would have landed on the moon• Explore another planet Mars

As Engineers, we need to hold ourselves to high standards in expanding the quality of life and maintain Moral and Ethical Standard Clear many popular scientific/engineering misconceptions.

3.1 Society’s View of Engineering?


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Perception of the public may distort truth. Do you know?

Cell phone communications still need land lines• Hurricanes can affect both equally likely

NMR (Nuclear Magnetic Resonance) was renamed to MRI (Magnetic Resonance Imaging) since public fear Nukes• NMR (or MRI) has nothing in common with Nuclear power

MP3 music has a better quality than one in regular CD • One is the format and the other storage medium.

Too many cell phone calls can bring down the entire network. • Cell phones reach the nearest tower and has no influence on the

overall network.

3.1 Myth and Perception?


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Only he who does nothing makes no mistake (French proverb)

Failure is delay, but not defeat. It is a temporary detour but not a dead-end street. (William Ward)

The fellow who never makes mistakes takes order from the one who does (Herbert Prochnow)

To make no mistake is not in the power of man; but from their errors and mistakes the wise and good learn wisdom for the future (Plutarch)

3.2 How Engineers Learn from Mistakes?


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Consider a simple hypothetical design competition in EML 3004C.

The goal is to build a vehicle with an articulated arm that would pick up an opposing vehicle and throw it off the track.

Two students are assigned per group and they are supposed to complete the design, construction, operation and ready for the competition in a six week period.

3.2 Hypothetical Design Competition


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Mike and Heather form a team. They design a vehicle on paper, draw it Pro-E CAD,

test it out using animation software. They draw all the 58 parts needed for the vehicle Go to the machine shop, machine all the parts

accurately and assemble them perfectly In one week, they do a mock up test on another

vehicle. During the test the vehicle lifts the opposing one and

throws it off the track.

3.2 Scenario 1


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Mike and Heather have been working now for more than a month. After many trials and tribulations, they design an arm capable of

lifting objects. They first use two mouse-trap springs but found power to be

inadequate. Needed to replace with battery powers. They buy electric motors with fine controls. Holes in the arm

needed to be re-drilled. Too many wrong holes. Five weeks past, no working vehicle, project over budget. When the arm is mounted on the vehicle, it does not fit-needs

rework. When it all fits, during testing the opponent vehicle is lifted but

both the vehicles fall of the track. Abandon the arm concept and redesign with a ram approach.

3.2 Scenario 2


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“If anything can go wrong, it will”- Murphy’s Law All great discoveries are made by mistake An expert is one who knows more and more about less and less

until he knows absolutely everything about nothing Experienced engineers know that design never works the first time

and are never discouraged by initial failures. Failures are inevitable part of the design process. It is better for a defect to surface during the design and testing,

rather than in the field. The best way to avoid field failure is to test, re-test and do some

more re-testing. Computers based virtual testing complement the actual testing but

does not replace it.

3.2 Welcome to reality!


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Despite great attempts by engineers to weed out design mistakes, they eventually show up and result in catastrophes.

Mistakes are made by engineers who do their best but fail due to lack of prior experience or/and lack of engineering judgment.

Of course, it is easy to pick the flaw in hind sight- Monday morning quarterback!!

Spotting a flaw becomes difficult as the complexity of the system increases.

A good engineer becomes a great engineer when he/she can spot flaws before the production or operation.

3.2 Case Studies of Engineering Disasters!


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3.2 Succeed by Averting Failures! The study of failure is an important part of the

engineering design process Studying failures which occur during product testing helps to discover

hidden flaws which were not foreseen during the initial design and prototype process

Subsequent redesign and adjustment will help to prevent similar failures from occurring once the product has been released to the public

Classic engineering failures Well documented cases where inadequate design or testing resulted in

catastrophic failure

Caused both loss of life and/or major loss of property

The study of these failures helps us to avoid the mistakes of the past


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3.2 Hubble Space Telescope!

Hubble Space Telescope

Powered by large solar panels which resembled wings

Alternately heated and cooled as they moved in and out of earth’s shadow

Resulting expansion and contraction cycles caused the solar panels to “flap”

Problem caused by not taking operating environment into account during the design phase


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3.2 What went wrong with Hubble?

Perkin-Elmer fabricated mirrors

A pair of calibrated mirrors to direct light on hubble surface

Diameter of the mirror is 2.4 m and to be maintained to 0.001 wavelength in micrometers

During assembly there were warning signs-Range was falling short; engineers added 1.3 mm spacers to correct the problem.

During mock-up test they found spherical aberration but ignored the concern as trivial.


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3.2 Tacoma Narrows Bridge!

Tacoma Narrows Bridge

Built across Puget Sound in Tacoma, WA, in 1940

Longest suspension bridge of its day

Engineers copied designs used for shorter bridges, and simply lengthened them, without testing for structural integrity

Bridge collapsed after less than one year, due to high winds which the structure could not withstand


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3.2 Tacoma Narrows Bridge!On November 7, 1940, at approximately 11:00 AM, the first Tacoma Narrows suspension bridge collapsed due to wind-induced vibrations. Situated on the Tacoma Narrows in Puget Sound, near the city of Tacoma, Washington, the bridge had only been open for traffic a few months

Source: http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/index.html


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3.2 Hartford Civic Center!

Hartford Civic Center Built in mid-1970’s

The roof used a novel “space frame” construction which did not require support columns

Computer models were used to evaluate the structure and to verify that the roof could withstand all plausible applied loads. The validity of these computer models were never tested using actual structures or prototypes

Roof collapsed under a heavy snow load in 1978


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3.2 Space Shuttle Challenger!

Space Shuttle Challenger Exploded in 1986 about two minutes after it

was launched

One of the O-rings used to seal the solid rocket booster failed after it became brittle due to low (below freezing) temperatures on the day of the launch

This low temperature failure was unforeseen because the O-ring was never tested at sub-freezing temperatures


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3.2 Hyatt Skywalk Collapse!

Kansas City Hyatt Skywalk Collapse Hanging, two-layer, open-air pedestrian

walkway designed to span a lobby atrium

The original design required a set of support shafts which could not easily be fabricated


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3.2 Hyatt Skywalk Collapse!

Kansas City Hyatt Skywalk Collapse Construction engineers modified the

original shaft design. While this modification allowed the walkways to be built, it reduced the maximum load that could be supported

The reduced capacity of the walkways did not come to light until their collapse during a party, which resulted in over 100 fatalities


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3.2 Three Mile Island Power Plant!

Three-Mile Island Power Plant Second-worst nuclear accident (Chernobyl was the worst)

Flaw in reactor design: an indicator light showed that power had been delivered to a pressure relief valve actuator coil, but could not verify that the valve had actually opened. This is an example of a poor man-machine interface


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3.2 My personal experience!

Atomic Power Project Construction Design and site engineer

Large stainless steel pressure vessel designed, constructed, fabricated and shipped to site

During routine hydrostatic testing …..


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CHAPTER 3…concludes

Engineers and the Real World?

Namas Chandra Introduction to Mechanical engineering Chapter 3-1 EML 3004C CHAPTER 3 Engineers and...

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