ENGS 171 Wegst L08 11 May 2012 ff - Thayer School of ...d30345d/courses/engs171/Wegst... · CES...

© MFA 2011 © UGKW 2012

Ulrike G.K. WegstThayer School of Engineering

Dartmouth College, Hanover, NH [email protected]

Cummings 106

Lecture 8

Chapter 9

Tradeoffs in Product Purchase and Product Use: Cars

www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012

Eco-Informed Materials Selection


Tradeoff

The tradeoff between carbon footprint and car ownership


Tradeoff between Carbon Footprint and Car Ownership


Tradeoff between Carbon Footprint and Car Ownership

Constraints: 4 doors, gas fuel, >150 hp


The Tradeoff Between Embodied Energy and Use Energy

• When a steel component is replaced by one of a light alloy both the use energy and the embodied energy change.

• There is a net energy saving only if the sum of the two is negative.

• The diagonal lines are contours of constant (negative) sum.


The Tradeoff between Embodied Energy and Use Energy


The Tradeoff between Material Cost and Use Cost

Tradeoffs in Product Purchase and Product Use: Refrigerators


A Tradeoff Plot for Refrigerators

Static versus Mobile Use: Crash Barriers

www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012

Static barrierMobile barrier

Bending strengthper unit mass Criterion: Bending strength

per unit embodied energy

Function: Absorb impact, transmit load to energy-absorbing units or supports

Index:

m

3/2y

H

3/2y

Material dominates

M Mf Tr Use

Dominantphase of life:

Ener

gy

M Mf Tr Use

Use dominates

Minimize embodied

energy

Minimize mass

Materials for Crash Barriers


Static Barrier: The Material Index as Bar Chart

+10%-Precision

Selected materials: Cast irons, steels

m

3/2y

H


Mobile barrier: The Material Index as Bar Chart

+10%-Precision

Selected materials: CFRP, Mg alloys, Ti alloys, Al alloys

3/2y


CES EduPack Level 3 for More Detail

2100 metals,polymers & composites

Using Level 3 DB allows specific grades to be identified


Beam

Absorb impact, transmit load to energy-absorbing units or supportsFunction

Minimize mass and material costObjectives

Mass m per unit bending strength

Cost C per unit bending strength

Criteria

2 / 3y

m

2 / 3ymC

C Cm = Material [$/kg]

ρ = Density [kg/m3]= Yield strength [MPa]= exchange constant,

[$/kg]

y

Index: minimize

mCmCZ

2 / 3y

Penalty function

Tradeoff: Materials for Mobile Crash Barriers


Use the “Advanced” facility to make the penalty function

List of properties Density Price Tensile strength etc

^+ - */ ( )

(Density / (Yield strength^0.66))*(Price + 10)

Selection Using the Penalty Function in CES

The value of theexchange constant

mCZ

2 / 3y


α = $1/kg

Penalty Function with α = $1/kg

Best choice: steels

The penalty function Z is plotted as a bar chart:


Use the “Advanced” facility to make the penalty function

List of properties Density Price Tensile strength etc

+ - */ ^ ( )

(Density/(Tensile strength^0.66))*(Price + 10)

The value of theexchange constant


α = $10/kg

Best choice: light alloys




α = $100/kg

Best choice: CFRP



Other Exchange Constants

Carbon tax is an exchange constant

Set α = 0 Result: no abatement. Cost of CO2 is “externalised”

α = ∞ Result: total ban on CO2 release

Currently α = $0.03/kg. Is it enough?

Currently α = $0.12/kg. Is it enough?

Penalty function mαZ C

$/kg of CO2 to atmosphereCost of abatement, $

kg of CO2 to atmosphere

Landfill tax is an exchange constant

Penalty function mαZ C$/kg of landfill Cost of alternative to landfill, $

kg of material to landfill

Your Projects: What is each Product’s and

Component’s Function?

Strategies for Reducing Environmental Impact

Cooling and Heating


Coffee Maker

Minimize energy use

Component to be redesigned

Breanna DayNatasha HerringMelissa MurphyDouglas LambertWaheed Zarif


Component Functions


Use Phase: Heating and Cooling Systems

Use Heating and CoolingAim Design for minimum thermal loss. Actions Select material with the lowest value of the appropriate index—it may

be necessary to minimize both.Relevant material indices. • When the temperature difference across the wall is constant over

long periods of time, choose the material with the lowest thermal conductivity (or largest R value, where R ∝ 1/).

• When, instead, the temperature difference across the wall fluctuates, choose the material with the lowest value of (Cp)1/2.

• Use as large a 'recycled content' in the material as possible. • Use as little material as possible while retaining enough redundancy

for safety. Conflicts • Watch out for conflict with the Materials phase: the material with the

lowest direct eco-impact may not be the lightest or the cheapest. • Use trade-off methods to resolve the conflict.


Heating and Cooling Temperature Profiles

Examples:

Coffee kept warm in ‘pot’ with hotplate

Repeated use of coffee maker: heating and cooling of ‘pot’


Material Indices: Thermal Design


Selection Chart for Minimum Thermal Loss

Property Chart: Thermal Conductivity, , versus Thermal Diffusivity, a = /Cp

Why?Because it not only allows to evaluate the materials based no their Material Indices, here and (Cp)1/2, but also to determine how their material properties, here thermal conductivity and volumetric specific heat Cp, which determine the thermal diffusivity, a = /Cp, affect the indices.

ExampleCork, a renewable material, has one of the lowest thermal conductivities, , and thermal diffusivities, a, of all materials shown. This makes it an excellent candidate for applications, when the temperature difference across the material wall is constant (e.g. when coffee is supposed to be kept hot in the pot). By this criterion it outperforms most other polymer foams. However, cork cannot match their performance, when the function is to reduce thermal losses when the temperature across the wall fluctuates or oscillates (e.g. frequent sequential coffee making). This is, because cork has a higher volumetric specific heat than other polymer foam.

The designer (you) has to tradeoff and decide, which function dominates.Try it out: plot the indices against each other other, tradeoff and discuss your choice.


Selection Chart for Minimum Thermal Loss

When the temp. difference across the wall is constant over long periods of time, minimize

When the temp. difference across the wall fluctuates, minimize (Cp)1/2

Prepare a Tradeoff Plot to

determine optimal

combination.


Renewable Alternative: Cork

Are there others?Biodegradable polymer foams?

The CES database will give you a first indication.

Strategies for Reducing Environmental Impact

Lighting


Use Phase: Static Mode

Use Aim Design for energy use. Actions • Replace energy consuming component with a more energy

efficient option (e.g. incandescent lamp with LED).• Select material with the lowest value of the appropriate index.

Conflicts • The new energy consuming component may not minimize embodied energy or cost.

• The material choice that minimizes mass may not minimize embodied energy or cost.

• Use trade-off methods to resolve the conflict.


Desk Lamp

Minimize embodied

energy

Minimize energy use


Component Functions


Can You Prepare a Similar Plot for Incandescent Lamps and LEDs?

Strategies for Reducing Environmental Impact Stiffness, Strength, Damage Tolerance

First Step: Determine Function of Product and Components


Material Indices: Stiffness-limited Design I(tables in CES EduPack 2012 Help)


Material Indices: Stiffness-limited Design II

(tables in CES EduPack 2012 Help)


Material Indices: Strength-limited Design I


Material Indices: Strength-limited Design II


Material Indices: Damage-tolerant Design

Second Step: Evaluate and Reduce Environmental Impact


Materials Phase

Materials Aim Minimize embodied energy CO2 footprint or water use per unit

of function. Actions • Select material with lowest embodied energy and CO2

footprint per unit of function. • Use as large a 'recycled content' in the material as possible. • Use as little material as possible while retaining enough

redundancy for safety. Conflicts • Watch out for conflict with the Use phase: the material with

the lowest direct eco-impact may not be the lightest or the cheapest.

• Use trade-off methods to resolve the conflict.


Minimize Environmental Impact: Material Examples


Selection Charts to Minimize Material Impact

Bubble Charts for selecting materials that minimize impact of Material phase:

• Modulus (y-axis) vs. Embodied energy × Density (x-axis) • Yield strength (y-axis) vs. Embodied energy × Density (x-axis) • Modulus (y-axis) vs. CO2 footprint × Density (x-axis) • Yield strength (y-axis) vs. CO2 footprint × Density (x-axis)

(See Lecture

Alternatively use the graph stage 'Advanced' function to create bar charts of the material index.


Underwater Disposable Cameras

Minimize embodied

energy

Ryan BirjooEmil CashinLauren HendricksMichael JohnsenPaul Russell


Component Functions

Ext. Winding Knob

BodyFilm CanisterFilm Wrist Strap Gasket

Hard Case

Winder & Shutter Actuator Assy.

Shutter Base

Connector

O-ringsFilm Spool

Printed Wrapper

Viewfinder Cover

Film Sprocket

Viewfinder Lens

Shutter Assy.


Office Chair

Minimize embodied

energy

Sharang Biswas. Max Langford, Lucas Sanford-Long, Scott Sottosanti, Paul Rosenfield


Component Functions


Hand Mixer

Minimize embodied

energy

14.55

9.76

0.51

Plastics

Various Metals

Others

0.00 5.00 10.00 15.00 20.00MJ

Material break down

Kofi Amoako-Gyan, Kellie Katako, Olutosin Osibodu. Dimitris Vantzis


Component Functions


Desk Telephone

Anthony Giralo, Natasha Mohan, Vedant Rathi, Patrick Vander Neut, Joseph Zabinski

Minimize embodied

energy

Optimize Function per unit embodied energy


Component Functions

handset & cord

casing

circuit board

transmitter/receiver

packaging


Bicycle

Minimize embodied

energy

0.0%2.0%4.0%6.0%8.0%

10.0%12.0%14.0%16.0%18.0%20.0%

Frame Wheels AirTube

Fork HandleBar

Crank RearWheelFrame

FrontWheelFrame

% o

f Tot

al E

nerg

y

Materials: Components with Highest Embodied Energy

Kevin DahmsRobert MossJose Ordonez CoronelAlan SalasJessica Zarker


Component Functions


Remember Other Important Considerations

Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

Be aware of the Multifunctionality of Components

• The mass of specific components may need to be preserved (e.g. wheel of the bicycle, to give it stability)

• Transparency may be a constraint (e.g. the material protecting the camera lens)• Thermal properties may be critical (e.g. the lamp shade―with potential for a

‘chain reaction’)

‘Chain Reaction’ Example:

• A high thermal conductivity of the lamp shade is important as long as it has to dissipate the heat of an incandescent light.

• When the light is ‘cold’ such as that of an LED, this is no longer a design requirement, allowing a much broader range of materials to be used.

• The new lamp shade material may have a considerably lower density (e.g. paper versus steel).

• As a result, the lamp stem and base can be lighter, too, as they will need to support and balance a much lower mass than before (note the elegant ‘trim’ design of LED lighting designs in comparison to traditional ones).


Transparency

Transparency is ranked on a four-point scale, from water-clear to opaque.

Remember the Benefit of Shape


Shape factors: Structural Elements Subject to Bending



Example: Aircraft Wing Spars


CFRP ( = 10) Outperforms Al ( = 20) and Wood ( = 1)



Bicycle Forks


Square beam section compared: left, with a tube of the same area (but 2.5 stiffer); right, with a tube with the

same stiffness (but 4 lighter).


Material Indices with Shape


Remember the Importance of Tradeoffs


Systematic Tradeoff Strategies

Simultaneously minimize bothmass (performance metric P1) and cost (performance metric P2), while meeting other constraints.

A solution is defined as a viable choice of materials that meets all constraints but does not necessarily optimally satisfy either of the objectives.

Each bubble describes a solution.

Non-dominated solutions lie on the tradeoff surface.

Example

The non-dominated set of solutions is called the Pareto set.


The Penalty Function Superimposed on the Tradeoff Plot

The contours of Z have a slope of 1/.

The contour tangent to the tradeoff surface identifies the optimum solution.

Values for exchange constants, , can be real or perceived:

Here, utility [$] of saving 1kg of weight.


Exchange Constants

Often a single material (or subset of materials) is optimal over a wide range of values of the exchange constant.

Then, approximate values for exchange constants are sufficient to reach precise conclusions about the choice of materials.

Example: Bicycle


The Tradeoff between Bicycle Mass and Cost

The tangent to the trade-off surface gives an estimate of the exchange constant.

To a consumer seeking a cheap bike for shopping, the value of weight savings is low: $20/kg.

To an enthusiast who wants performance, it can be high: $2000/kg.



Preparation for Next Class with Prof. Wegst

Textbook and CES Software

• Read Chapters 10 in Ashby Textbook

Project/Case Study

• Analyze the function of the different components in your respective product:

• Tie in tension, Beam or Plate in bending, etc.?

• Are stiffness, strength, toughness, thermal conductivity, etc. of concern?

• What are the objectives?

• What are the constraints?

• Tradeoffs

ENGS 171 Wegst L08 11 May 2012 ff - Thayer School of ...d30345d/courses/engs171/Wegst... · CES...

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