Upload
ngotruc
View
229
Download
1
Embed Size (px)
Citation preview
© 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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Tradeoff
The tradeoff between carbon footprint and car ownership
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Tradeoff between Carbon Footprint and Car Ownership
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Tradeoff between Carbon Footprint and Car Ownership
Constraints: 4 doors, gas fuel, >150 hp
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
The Tradeoff between Embodied Energy and Use Energy
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
The Tradeoff between Material Cost and Use Cost
Tradeoffs in Product Purchase and Product Use: Refrigerators
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Static Barrier: The Material Index as Bar Chart
+10%-Precision
Selected materials: Cast irons, steels
m
3/2y
H
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Mobile barrier: The Material Index as Bar Chart
+10%-Precision
Selected materials: CFRP, Mg alloys, Ti alloys, Al alloys
3/2y
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
CES EduPack Level 3 for More Detail
2100 metals,polymers & composites
Using Level 3 DB allows specific grades to be identified
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012
α = $1/kg
Penalty Function with α = $1/kg
Best choice: steels
The penalty function Z is plotted as a bar chart:
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012
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
Penalty Function with α = $10/kg
α = $10/kg
Best choice: light alloys
The penalty function Z is plotted as a bar chart:
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012
Penalty Function with α = $100/kg
α = $100/kg
Best choice: CFRP
The penalty function Z is plotted as a bar chart:
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Coffee Maker
Minimize energy use
Component to be redesigned
Breanna DayNatasha HerringMelissa MurphyDouglas LambertWaheed Zarif
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Heating and Cooling Temperature Profiles
Examples:
Coffee kept warm in ‘pot’ with hotplate
Repeated use of coffee maker: heating and cooling of ‘pot’
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Thermal Design
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Renewable Alternative: Cork
Are there others?Biodegradable polymer foams?
The CES database will give you a first indication.
Strategies for Reducing Environmental Impact
Lighting
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Desk Lamp
Minimize embodied
energy
Minimize energy use
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Stiffness-limited Design I(tables in CES EduPack 2012 Help)
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Stiffness-limited Design II
(tables in CES EduPack 2012 Help)
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Strength-limited Design I
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Strength-limited Design II
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices: Damage-tolerant Design
Second Step: Evaluate and Reduce Environmental Impact
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Minimize Environmental Impact: Material Examples
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Underwater Disposable Cameras
Minimize embodied
energy
Ryan BirjooEmil CashinLauren HendricksMichael JohnsenPaul Russell
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Office Chair
Minimize embodied
energy
Sharang Biswas. Max Langford, Lucas Sanford-Long, Scott Sottosanti, Paul Rosenfield
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Desk Telephone
Anthony Giralo, Natasha Mohan, Vedant Rathi, Patrick Vander Neut, Joseph Zabinski
Minimize embodied
energy
Optimize Function per unit embodied energy
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
handset & cord
casing
circuit board
transmitter/receiver
packaging
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Component Functions
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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).
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Transparency
Transparency is ranked on a four-point scale, from water-clear to opaque.
Remember the Benefit of Shape
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Shape factors: Structural Elements Subject to Bending
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Example: Aircraft Wing Spars
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
CFRP ( = 10) Outperforms Al ( = 20) and Wood ( = 1)
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Bicycle Forks
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby
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).
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
Material Indices with Shape
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby
Remember the Importance of Tradeoffs
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby
www.grantadesign.com/education/resourcesM. F. Ashby, 2011 © UGKW 2012© UGKW 2012
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