Georgia Tech School of Aerospace Engineering Aerospace Systems Engineering A Modern Approach Dr....

Georgia Tech

School of Aerospace Engineering

Aerospace Systems EngineeringA Modern Approach

Dr. Daniel P. Schrage

Professor and Director,

Center of Excellence in Rotorcraft Technology(CERT)

Center for Aerospace Systems Analysis (CASA)

Georgia Tech


Course Materials

• Primary Text, Dieter, “Engineering Design: A Materials and Processing Approach”, 3rd Edition, McGraw Hill, 2000

• Secondary Text,”Systems Engineering Fundamentals” Defense Systems Management College, 1998

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The Product Design Process(Chapter 1, Dieter)

• Introduction and Importance of Product Design• The Design Process – A Simplified Approach• Considerations of a Good Design• Detailed Description of Design Process• Marketing• Organization for Design• Computer-Aided Engineering• Designing to Codes and Standards• Design Review• Technological Innovation and the Design Process

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Some Important Concepts• Design: “ to fashion after a plan” (Webster Dictionary)

leaves out the essential fact that to design is to create something that has never been

• Synthesis: “pulling together”• Ability to design is both a science and an art

The science “can be learned” through techniques & methods

The art is best “learned by doing” design• Discovery: “getting the first sight of, or the first knowledge of

something”, as when Columbus discovered America• Invention: requires the design be a step beyond the limits of

existing knowledge (beyond the state of the art). Some designs are truly inventive, but most are not

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Aerodynamics Economics

Propulsion

Safety

Aerodynamics

S&C

Propulsion

Performance

Manufacturing

Economics

Safety

Structures

Manufacturing

Structures Performance

Conceptual Design Tools (First-Order Methods)

Synthesis & Sizing

Preliminary Design Tools (Higher-Order Methods)

Geometry

Mission

Increasing Sophistication and

Complexity

Approximating Functions Direct Coupling of Analyses

Integrated Routines Table Lookup

Integrated Synthesis and AnalysisVarying Fidelity of Synthesis, Sizing& Analysis

S&C

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Definition of Design(per Dieter)

• Design establishes and defines solutions to and pertinent structures for problems not solved before, or new solutions to problems which have previously been solved in a different way

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Good Design requires both Synthesis & Analysis• Typically, we approach complex problems like design by

decomposing the problem into manageable parts or components– Because we need to understand how the part will perform in

service we must be able to calculate as much about the part’s behavior as possible by using the appropriate disciplines of science and engineering science and the necessary computational tools

– This is called Analysis and usually involves the simplification of the real world through models

– Synthesis involves the identification of the design elements that will comprise the product, its decomposition into parts, and the combination of the part solutions into a total workable system

• In the typical design you rarely have a way of knowing the correct answer. Hopefully, your design works, but is it the best, most efficient design that could have been achieved under the conditions? Only time will tell

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The Four Challenges (C’s) of the Design Environment

• Creativity– Requires creation of something that has not existed before

or not existed in the designer’s mind before• Complexity

– Requires decisions on many variables and parameters• Choice

– Requires making choices between many possible solutions at all levels, from basic concepts to smallest detail of shape

• Compromise– Requires balancing multiple and sometimes conflicting

requirements

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Product Design Process• Engineering design process can be applied to several

different ends

– Design of Products, whether they be consumer goods and appliances or highly complex products such as missile systems or jet planes

– Another is a complex engineered system such as an electric power generating station or a petrochemical plant

– Yet another is the design of a building or bridge

• The principles and methodology of design can be usefully applied in each of these situations. However, the emphasis in Dieter’s book is on product design and in this course is complex product design, specifically Aerospace Systems

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Dieter’s Book Goal

• Provide insight into the current best practices for doing product design

• The design process should be conducted so as to develop quality cost-competitive products in the shortest time possible

• Is necessary, but insufficient for Aerospace Systems Design

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Japanese Auto Industry and The U.S. Auto Industry

90% Total Japanese

Changes Complete

U.S. Company

Japanese Company

20

-24

M

on

ths

14

-17

M

on

ths

1-3

M

on

ths

Job

#1

+3

M

on

ths

Nu

mb

er

of

En

gin

eeri

ng

Pro

du

ct

Ch

an

ges

Pro

cess

ed

Japanese/U.S. Engineering Change Comparison

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The Quality Engineering Process provides Recomposition Methods & Tools

CustomerQuality Function

Deployment

Off-Line

Quality Function

Deployment

Off-Line

Seven Managementand Planing

ToolsOff-Line

Seven Managementand Planing

ToolsOff-Line

Statistical Process Control

On-Line

Statistical Process Control

On-Line

RobustDesign Methods(Taguchi, Six -Sigma, DOE)

Off-Line

RobustDesign Methods(Taguchi, Six -Sigma, DOE)

Off-Line

Knowledge Feedback

•Needs• Identify Important Items

•Variation Experiments

•Make Improvements

•Hold Gains

•Continuous Improvement

Having heard the “voice of the customer”, QFD prioritizes where improvements are needed; Taguchi provides the mechanism for identifying these improvements

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Traditional Design & Development Using only a Top Down Decomposition Systems Engineering Process

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IPPD Environment for System Level Design Trades and Cycle Time Reduction

SYSTEMPROCESS

RECOMPOSITION

SYSTEMFUNCTIONAL

DECOMPOSITION

COMPONENTFUNCTIONAL

DECOMPOSITION

COMPONENTPROCESS

RECOMPOSITION

PARTPROCESS

RECOMPOSITION

PARTFUNCTIONAL

DECOMPOSITION

ProductTrades

ProcessTrades

ProductTrades

ProcessTrades

PRELIMINARYDESIGN

(PARAMETER)

PRELIMINARYDESIGN

(PARAMETER)

DETAILDESIGN

(TOLERANCE)

DETAILDESIGN

(TOLERANCE)

MANUFACTURINGPROCESSES

CONCEPTUALDESIGN

(SYSTEM)

ProcessTrades

INTEGRATEDPRODUCTPROCESS

DEVELOPMENT

ProductTrades

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School of Aerospace EngineeringCC04264506.ppt

Typical System Life Cycle Cost

CumulativePercentof LCC

Production, Deployment, Operations and SupportE&MDPD & RRCon Exp

•

• •

• • •

100%

75%

50%

25%

0%

Life Cycle CostActually Expended

Life Cycle CostEffectively Rendered

Unchangeable fora Given Design

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Ramifications of the Quality Revolution

• Decisions made in the design process cost very little in terms of the overall product cost but have a major effect on the cost of the product

• Quality cannot be built into a product unless it is designed into it

• The design process should be conducted so as to develop quality cost-competitive products in the shortest time possible

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Design Process Paradigm Shift(Research Opportunities in Engineering Design, NSF Strategic Planning Workshop Final

Report, April 1996)

100%

50%

0%

Today’s Design Process Future Design Process

Kno

wle

dge

Abo

ut D

esig

n D

esig

n Fr

eedo

m

Cos

t Com

mitt

ed

Con

cept

Pre

limin

ary

Des

ign

Analysis and Detail

Design

Prototype Development

Redesign Product Release

• A paradigm shift is underway that attempts to change the way complex systems are being designed

• Emphasis has shifted from design for performance to design for affordability, where affordability is defined as the ratio of system effectiveness to system cost +profit

• System Cost - Performance Tradeoffs must be accommodated early

• Downstream knowledge must be brought back to the early phases of design for system level tradeoffs

• The design Freedom curve must be kept open until knowledgeable tradeoffs can be made

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Static vs Dynamic Products

• Some products are static, in that the changes in their design concept take place over a long time period; rather, incremental changes occur at the subsystem and component levels (most air vehicles are static)

• Other products are dynamic, like telecommunications systems and software, that change the basic design concept fairly frequently as the underlying technology changes (avionics and mission equipment & software are dynamic)

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Simplified Design Process

• Definition of the Problem

• Gathering Information

• Generation of Alternative Solutions

• Evaluation of Alternatives

• Communication of the Results

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Georgia Tech Generic IPPD Methodology

COMPUTER-INTEGRATED ENVIRONMENT

PR

OD

UC

T D

ES

IGN

DR

IVE

NP

RO

CE

SS

DE

SIG

N D

RIV

EN

REQUIREMENTS & FUNCTIONAL

ANALYSIS

SYSTEM DECOMPOSITION &

FUNCTIONAL ALLOCATION

SYSTEM SYNTHESIS THROUGH MDO

SYSTEM ANALYSIS &

CONTROL

ESTABLISH THE NEED

DEFINE THE PROBLEM

ESTABLISH VALUE

GENERATE FEASIBLE ALTERNATIVES

EVALUATE ALTERNATIVE

7 M&P TOOLS AND QUALITY FUNCTION DEPLOYMENT (QFD)

ROBUST DESIGN ASSESSMENT & OPTIMIZATION

ON-LINE QUALITY ENGINEERING &

STATISTICAL PROCESS

MAKE DECISION

SYSTEMS ENGINEERING METHODS

QUALITY ENGINEERING METHODS

TOP-DOWN DESIGN DECISION SUPPORT PROCESS

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Detailed Description of Design Problems(Morris Asimow’s Morphology of design)

• Phase I. Conceptual Design

• Phase II. Embodiment Design (Preliminary Design)

• Phase III. Detail Design

• Phase IV. Planning for Manufacture

• Phase V. Planning for Distribution

• Phase VI. Planning for Use

• Phase VII. Planning for Retirement of the Product

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Discrete Steps in Engineering Design Process

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Design Depends on Individual Who Defines Problem

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Classification of Products Based on Market

• Platform Product – Is built around a preexisting technological subsystems, e.g. Apple

Macintosh operating systems

– Is similar to a technology-push product

• Process-Intensive Products– Manufacturing process places strict constraints on the properties of

the product

– Examples are automotive sheet, steel, food products, semiconductors chemicals and paper

• Customized Products– Variations in configuration and content created in response to a s

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The Total Materials Cycle

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The Systems Engineering ProcessProcess Input• Customer Needs/Objectives/ Requirements - Missions - Measures of Effectiveness - Environments - Constraints• Technology Base• Output Requirements from Prior Development Effort• Program Decision Requirements• Requirements Applied Through Specifications and Standards

Requirements Analysis• Analyze Missions & Environments• Identify Functional Requirements• Define/Refine Performance & Design Constraint Requirement

Functional Analysis/Allocation• Decompose to Lower-Level Functions• Allocate Performance & Other Limiting Requirements to All Functional Levels• Define/Refine Functional Interfaces (Internal/External)• Define/Refine/Integrate Functional Architecture

Synthesis• Transform Architectures (Functional to Physical)• Define Alternative System Concepts, Configuration Items & System Elements• Select Preferred Product & Process Solutions• Define/Refine Physical Interfaces (Internal/External)

System Analysis& Control(Balance)

Verification

Requirement Loop

Design Loop

• Trade-Off Studies• Effectiveness Analysis• Risk Management• Configuration Management• Interface Management• Performance Measurement - SEMS - TPM - Technical Reviews

Process Output• Development Level Dependant - Decision Data Base - System/Configuration Item Architecture - Specification & Baseline

Related Terms: Customer = Organization responsible for Primary Functions Primary Functions = Development, Production/Construction, Verification, Deployment, Operations, Support Training, DisposalSystems Elements = Hardware, Software, Personnel, Facilities, Data, Material, Services, Techniques

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Systems Engineering, Its Purpose

To satisfy a mission need with a system

that is cost effective, operationally

suitable, and operationally effective.

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Systems Engineering Objectives• Translate customer needs into balanced system/subsystem

design requirements and product• Integrate technical inputs of the entire development

community and all technical disciplines into a coordinated program effort

• Transition new technologies into product and abatement program

• Ensure the compatibility of all functional and physical interfaces

• Verify that the product meets the established requirements• Conduct a formal risk management and

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What Is a System?• A system is a collection of components

(subsystems) that– Interact with one another– Have emergent capabilities - capabilities above

and beyond what the same collection of components would if they did not interact

– Interacting components implies architecture

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Elements of a System• Elements

– Equipment Hardware– Software– Facilities– Personnel– Data

• All elements are interrelated

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System Element Constituents• Equipment Hardware

– Mission hardware

– Ground equipment

– Maintenance equipment

– Training equipment

– Test equipment

– Special equipment

– Real Property

– Spares

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System Element Constituents (cont.)• Software

– Instructions

– Commands

– Data

• Facilities– Industrial

– Operational

– Training

– Depot

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Systems Engineering Principles Apply to All Acquisition Phases at All Levels of the Engineering Hierarchy

Levels in the System Hierarchy

CED - Concept Exploration/DefinitionPDRR - Program Definition & Risk Reduction

System ofsystems

System

Segment

Subsegment

ItemCED

PDRR

EMD

P/D

AcquisitionPhasesEMD - Engineering/Manufacturing Definition

P/D - Production/Deployment

Require

men

ts

anal

ysis

Syste

m a

nalys

is/

contro

l/eva

luat

ion

Configura

tion

synth

esis

Functio

nal

anal

ysis

Pre-CED

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SystemsEngineeringProcess

Systems Engineering In IPD

ProductTeamsProductTeams

IPD

ConcurrentDevelopmentConcurrent

Development

SystemsEngineering

Process

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Ability to Influence CostHigh

Low

Time

CED

PDRR

EMD

Production.Deployment

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System Element Constituents (cont.)

• Personnel– Training

– Tasks

– Number

– Types and skills

• Data– Parts Manuals

– Maintenance Manuals

– Operating Manuals

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Systems Thinking

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Roles of Systems Engineers*• Requirements Owner• System Designer• System Analyst• Validation/Verification Engr• Logistics/Ops Engineer• Glue Among Subsystems

• Customer Interface• Technical Manager• Information Manager• Process Engineer• Coordinator• Classified Ads SE

*Source: Twelve Roles of Systems Engineers, Sarah SheardURL: www.software.org/pub/externalpapers/

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What Is a System?• A system is a collection of components

(subsystems) that– Interact with one another– Have emergent capabilities - capabilities above

and beyond what the same collection of components would if they did not interact

– Interacting components implies architecture

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Examples of Systems• Aircraft engine vs a collection of parts

• Aircraft with engines and avionics

• Air traffic control with aircraft, airfields, radars, controllers, CCS

• Air transportation with air traffic control, airlines, passengers, cargo, maintenance, pickup and delivery

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More Complex SystemsSystems of Systems*

• Individual systems can operate on their own• Systems of systems not owned and controlled as a whole by single entity

*Mark Maier, “Architecting Principles for Systems-of-Systems”, Journal of the International Council on Systems Engineering, Vol I, 1998

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Examples of Systems of Systems

• Internet• Auto and truck transportation• Air Defense System – maybe• National Airspace System (NAS)• Future Combat Systems (FCS) for the Objective Force

Brigade (Unit of Action)

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Technical Director Is the Systems Thinker

• If not, objectives, approaches, and decisions will not reflect systems thinking

• Technical Directors who don’t think systems inhibit systems thinking on their project

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Why Is Systems Thinking Good?

• Intractable problems often have solutions in the design space of the larger system

• Solutions in the larger systems space are often less costly or less risky

• Integration with external systems are addressed early in the development

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A Community Example• The Problem (or so they thought)

– Trees, fuels and other natural resources are being used up, so we need to recycle them

• The Solution (or so they thought)– Collect selected trash separately and sell it to

recycling facilities

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A Dose of Reality• Separate trash collections for recycleable

would double the cost

• Market for recycled newspaper and aluminum cans was saturated

• Unsold recycleables would have to be stored -- at additional cost

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Starting to Think Systems• Who currently collects trash?• From whom?• What is done with the trash?

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Answers and More Questions• Two trash collectors

– One collects from homes– One collects from businesses

• Does the collector from businesses separate the recycleables?

• Both put trash in land fills• Both pay to put trash in land fills• How much does it cost to put trash in a land

fill?

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The Land Fills as Part of the System

• $17* per ton to dump trash in the land fill

• Expected to reach $30 per ton in 15 years

• Land fills charge $150 per ton in New York

Gee, maybe we should think about

conserving the land fills?

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A Systems Solution• Two collections per week

– One for recycleables– One for non-cycleable trash

• Slight increase in fees for storing recycleables

Market demand of recycled paper and aluminum increase soared in

5 years

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Consequences of Systems Thinking

• The original objective (saving resources) was satisfied

• Current costs were contained• Future cost containment made the

slight increase saleable to the public

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Dieter Chapter 2:Need Identification and Problem Definition

• Of all the steps in the engineering design process, problem definition is the most important

• Before the Problem-Definition Step: Design projects commonly fall into one of five types:– Variation of an existing product– Improvement of an existing product– Development of a new product for low-volume production run– Development of a new product for mass production– One-of-a-kind- design

• Identifying Customer Needs• Gathering Information from Customers

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Dieter Chapter 2:Need Identification and Problem Definition

• Constructing a Survey Instrument• Benchmarking• Customer Requirements• Quality Function Deployment• Product Design Specification

– The basic control and reference document for the design and manufacture of the product

– In-Use Purposes and Market– Functional Requirments– Corporate Constraints– Social, Political and Legal Requirements

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Presentation Outline

• Synthesis and Sizing of Aerospace Vehicles

• Maneuverability and Agility Considerations for Aerial Vehicles

• Autonomous Vehicle Considerations

• Summary and Conclusions

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Synthesis and Sizing of Aerial Vehicles• For Aerial Vehicles Synthesis and Sizing provides the

Closure between Mission Requirements and Geometric Configuration Solutions

• A Fuel and Thrust/Power Balance Approach is used which allows for analytical design optimization (min. GW, etc.) through the coupling of a few critical design parameters (FW~aspect ratio, wing loading; RW~disk loading)

• Maneuverability and Agility can be related to Energy Principles (differences between Thrust/Power Available and Thrust/Power Required), Handling Qualities and the design

of the Flight Control System

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Maneuverability and Agility Considerations for Aerial Vehicles

• Fixed Wing Fighter Aircraft normally have a good high speed capability, good maneuverability at normal combat speeds (medium to high subsonic and transonic speeds), high specific excess power, good to excellent avionics, and the ability to employ guns and a wide range of air-to-air missiles. To achieve these capabilities, their optimum maneuvering speeds are usually rather high, impacting on low speed maneuverability

• Rotary Wing Aircraft have excellent low speed capability due to the rotor hub control moments which provides excellent control power in any axix. This allows rotary wing aircraft to fly Nap-of-the-Earth and stress aggressive concealed movement to take full advantage of masking provided by trees and terrain and attacking from a position of advantage at maximum standoff range

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Summary and Conclusions• Aerial Vehicle Design and Performance is highly

dependent on the Mission identified and use of a Fuel and Thrust/Power Synthesis Approach

• For high speed, high altitude, high maneuvering attack missions, such as Suppression of Enemy Air Defense (SEAD), Fixed Wing Aerial Vehicle are the Choice

• For low speed, low altitude, high agility(along with vertical takeoff and landing (VTOL)capability) reconnaissance and attack missions, such as Urban Warfare, Rotary Wing Aerial Vehicles are the Choice

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Technological Innovation and The Design Process

• The advancement of technology has three phases:– Invention: The creative act whereby an idea is conceived

– Innovation: The process by which an invention or idea is brought into successful practice and is utilized by the economy

– Diffusion: The successive and widespread initiation of successful innovation

• The technological innovation activity can considered to be:

Ident. OfMkt Need

Productidea

Pilotlot

Trialsales

CommercExploitation

Development

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Successful products delineate four factors that lead to success

1. Product planning and research: Products where adequate time was spent in problem definition & concept development

2. Product superiority: Having a superior high-quality product that delivers real value to the customer makes all the differences between winning and losing

3. Quality marketing: High in importance is how well the marketing activities were executed from concept of the idea to the launch of the product in the marketplace

4. Proper organizational design: Successful products are most often developed by a cross-functional team, led by a product champion, supported by top management, and accountable for the entire project from beginning to end

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Product and Process Cycles

• Product Life Cycle and Cash Flow Analysis

• Technology Development Cycle and S- Curves

• Process Development Cycle

– Uncoordinated development

– Segmental development

– Systematic development

• Producition and Consumption Cycle

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Societal Considerations in Engineering

• Characteristics of an Environmentally Responsible Design

• Five roles of government in interacting with technology

• Technology Identification, Evaluation and Selection (TIES)

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Dieter: Chapter 3 Team Behavior and Tools

• A team is a small number of people with complementary skills who are committed to a common purpose, performance goals, and approach for which they hold themselves mutually accountable

Differences between a working group and a teamWorking Group Team

-Strong, clearly focused leader -Individual & mutual accountability-The group,s purpose is the - Specific team purpose that the teamSame as the broader org.msn. Itself develops- Individual work products - Collective work products- Runs efficient meetings - Encourages open-ended discussion

and active problem-solving meetings- Measures its effectiveness - Measures performance directly by

indirectly by its influence assessing collective work products-Discusses,decides and delegates- Discusses, decides and does real work

together

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• What It Means to be an Effective Team Member– Take responsibility for the success of the team– Be a person who delivers on commitments– Be a contributor to discussions– Give your full attention to whomever is speaking and demonstrate this by asking

helpful questions– Develop techniques for getting your message across to the team– Learn to give and receive useful feedback

• The following are characteristics of an effective team:– Team goals are as important as individual goals– The team understands the goals and is committed to achieving them– Trust replaces fear and people feel comfortable taking risks– Respect, collaboration and open-mindedness are prevalent– Team members communicate readily; diversity of opinions are encouraged– Decisions are made by consensus and have the acceptance and support of the members

of the team

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• TEAM ROLES: Within a team members assume different roles in addition to being an active team member

• TEAM DYNAMICS:Students of team behavior have observed that most teams go through five stages of development

• EFFECTIVE TEAM MEETINGS: Students who complain about design projects taking too much time often are really expressing their inability to organize their meetings and manage their time effectively

• PROBLEMS WITH TEAMS: A well-functioning team achieves its objectives quickly and efficiently in an environment that induces energy and enthusiam

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• PROBLEM SOLVING TOOLS

• TIME MANAGEMENT

• PLANNING AND SCHEDULING

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Dieter: Chapter 5 Concept Generation and Evaluation

• With a clear product design specification developed in Chap. 2 we have arrived at the point where we are ready to generate design concepts, evaluate them, and decide which one will be carried forward to a final product

• The principle that grades this work is that put forth by the American architect-engineer Louis Henri Sullivan, “form follows function”

• By this we mean, if the functions of the design are clearly understood, then its appropriate form or structure will be easier to determine

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Dieter: Chapter 5 Concept Generation and Evaluation

• A design concept is an idea that is sufficiently developed that it can be evaluated in terms of physical realizability, i.e., the means of performing each major function has been determined

• The process that is applied in this chapter will result in the generation of multiple design concepts

• Then, with a set of design concepts we will subject them to an evaluation scheme to determine the best concept or small subset of best concepts

• Finally, a decision process will be used to decide on the best concept to develop into the final design

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Dieter: Chapter 5.2 -Creativity and Problem Solving– Creative thinkers are distinguished by their ability to synthesize new combinations of

ideas and concepts into meaningful and useful forms– A characteristic of the creative process is that initially the idea is only imperfectly

understood– Usually the creative individual senses the total structure of the idea but initially

perceives only a limited number of the details– The creative process be viewed as moving from an amorphous idea to a well-structured

idea, from the chaotic to the organized, from the implicit to the explicit– Engineers, by nature and training, usually value order and explicit detail and abhor

chaos and vague generality– To achieve a truly creative solution to a problem a person must utilize two thinking

styles: vertical or convergent thinking and lateral or divergent thinking– Vertical thinking is the type of analytical though process reinforced by most

engineering courses where one moves forward in sequential steps after a positive decision has been made about the idea

– In lateral thinking your mind moves in may different directions, combining different pieces of information into new patterns (synthesis) until several solution concepts appear

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Dieter: Chapter 5.3 -Creativity Methods5.4 – Creative Idea Evaluation

• Mental Blocks: Perceptual blocks, Emotional blocks, Cultural blocks, Environmental blocks, Intellectual blocks

• Brainstorming: Carefully define the problem at the start; Allow 5 minutes for each individual to think the problem on their own before starting the group process; SCAMPER checklist to aid in brainstorming

• Synectics: technique for creative thinking which draws on analogical thinking – Direct analogies, Personal analogies, Symbolic analogies, Fantasy analogies

• Force-Fitting Methods: SCAMPER is one of most widely used methods

• Mind Map: Concept map

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Dieter: Chapter 5.5: Theory of Inventive Problem Solving (TRIZ)

• Developed in Russia, starting around 1946, Genrich Altshuller,etc. Studied over 1.5 million patents

• They organized the problem solutions from the patent literature into five levels:– Level 1: Routine design solutions (~30%)– Level 2: Minor corrections to an existing system (~45%)– Level 3: Fundamental improvements which resolve contradiction

(~20%) This is where creative design solutions would appear– Level 4: Solutions based on appln of new scientific principle to

perform the primary functions of the design (~4%)– Level 5: Pioneering inventions based on rare scientific discovery

(<1%)

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Dieter: Chapter 5.6: Conceptual Decomposition• Two chief approaches to conceptual decomposition:

– Decomposition in the physical domain– Decomposition in the functional domain – the great advantage of

functional decompostion is that the method facilitates the examination of options that most likely would not have been considered

• Decomposition in the Physical Domain: an important emerging design consideration is product architecture – scheme by which the functional elements of the product are arranged into physical building blocks

• Functional Decomposition: system’s functions are described as a transformation between an initial state and a desired final state; originated with the German school of design methodology

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Dieter: Chapter 5.7: Generating Design Concepts

• Concept Development

• Morphological Chart

• Combining Concepts

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Dieter: Chapter 5.8: Axiomatic Design

• Axiom 1: The independence axiom– Maintain the independence of functional

requirements (FRs)

• Axiom 2: The information axiom– Minimize the information content

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Dieter: Chapter 6Embodiment (Preliminary) Design

• Many U.S. writers divide the design process into 3 phases:– Conceptual Design– Preliminary (Embodiment) Design– Detail Design

• Others call embodiment design “analytical design” because it is the design phase where most of the detailed analysis and calculation occurs

• Dieter adopts the terminology conceptual design, embodiment design, and detail design because they seem to be more descriptive of what takes place in each of these design phases

• Moving the setting of dimensions and tolerances into embodiment design (from detail design) is in keeping with the current trend for utilizing CAE so as to move the decision making as early as possible in the desing process to compress the product development cycle

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Dieter: Chapter 6

• Three different forms of design:– Routine design: the attributes that define the design and the

strategies and methods for attaining them are well known– Innovative design: not all attributes of the design may be known

beforehand, but the knowledge base for creating the design is known

– Creative design: neither the attributes of the design nor the strategies for achieving them are known ahead of time

• The Conceptual design phase is most central to innovative design

• At the opposite pole is selection design or catalog design, which is more central to routine design

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Dieter: Chapter 6Product Architecture

• Product architecture is the arrangement of the physical elements of a product to carry out its required functions

• It is in the Embodiment design phase that the layout and architecture of the product must be established by defining what the basic building blocks of the product should be in terms of what they do and what their interfaces will be between each other. Some organizations refer to this as system-level design

• There are two entirely opposite styles of product architecture, modular and integral:

– Modular: components (chunks) implement only one or a few functions and the interactions are well defined

– Integral: implementation of functions uses only one or a few components (chunks) leading to poorly defined interactions between components (chunks)

• In integral product achitectures components perform multiple functions• Products designed with high performance as a paramount attribute often have

an integral architecture

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Dieter: Chapter 6Product Architecture• A modular design makes it easier to evolve the design over time, to adapt it to

the needs of different customers, to replenish components as they wear out or are used up, and to reuse the product at the end of its useful life by remanufacture

• Modular design may even be carried to the point of using the same component in multiple products, a product family

• Integral desing is often adopted when constraints of weight,k space, or cost require that performance be maximized

• There is a natural tension between component integration to minimize costs and product architecture

• The best approach is to consider integration of components only within a single chunk (set of components) of the product architecture

• Thus, the product architecture has strong implications for manufacturing costs• A modular architecture tends to shorten the product development cycle becasuse

module can be deveolped independently provided there is not coupling of functon betgween modules, and provided that interfaces are well laid out and understood

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Dieter: Chapter 6Product Architecture

• Four step process for establishing the product architecture– Create a schematic diagram of the product (FFBD, Schematic

Block Diagram)

– Cluster the elements of the schematic (DSM, DeMAID)

– Create a rough geometric layout (3-view drawing)

– Identify the fundamental and incidental interactions (Interrelationship Diagraph, Compatibility Matrix)

• SEE EXAMPLES FROM TEXT

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Dieter: Chapter 6Configuration Design

• In configuration design we establish the shape and general dimensions of components. Exact dimensions and tolerances are established in parametric design

• The term component is used in the generic sense to include special-purpose parts, standard parts, and standard assemblies or modules

• A part is a designed object that has no assembly operations in its manufacture• A standard part is one that has a generic function and is manufactured routinely

w/o regard to a particular product (bolts, washers, etc.)• A special-purpose part is designed and manufactured for a specific purpose in a

specific product line• An assembly is a collection of two or more parts• A subassembly is an assembly that is included within another assembly or

subassembly• A standard assembly or standard module is an assembly or subassembly which

has a generic function and is manufactured routinely (electric motors, pumps, etc.)

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Dieter: Chapter 6Configuration Design

• Steps in starting Configuration design:– Review the PDS– Establish the spatial constraints that pertain to th product or the

subassembly being designed. Most have been set by the product architecture

– Create and refine the interfaces or connections between components– Maintain functional independence in the design of an assembly or

component– Answer the following questions:

• Can the part be eliminated or combine with another part?• Can a standard part or module be used

• Generally, the best way to get started with configuration design is to just start sketching alternative configurations of a part

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Dieter: Chapter 6Parametric Design

• In configuration design the emphasis was on starting with the product architecture and then working out the best form for each component

• In parametric design the attributes of parts identified in configuration design become the design variables for parametric design

• A design variable is an attribute of a part whose value is under the control of the designer

• Robustness means achieving excellent performance under the wide range of conditions that will be found in service

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Dieter: Chapter 6Parametric Design

• Read Table 6.2: Questions for revealing part configuration design risks

• Failure Modes and Effects Analysis (FMEA)

• Design for Reliability

• Robust Design

• Tolerances

• Design Guidelines for Best Practices

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Dieter: Chapter 7Modeling and Simulation

• The Role of Models in Engineering Design– Descriptive model

– Predictive model

– Static or dynamic

– Deterministic or probabilistic

– Iconic-analog-symbolic

– Simulation

– The Prototype

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• Mathematical Modeling– The components of a system are represented by idealized elements that

have the essential characteristics of the real components and whose behavior can be described by mathematical equations

– Techniques for treating large and complex systems by isolating the critical components and modeling them are at the heart of the growing discipline called systems engineering

– Important simplification results when the distributed properties of physical quantities are replaced by their lumped equivalents.

– A system is said to have lumped parameters if it can be analyzed in terms of the behavior of the endpoints of a finite number of discrete elements

– Once the chief components of the system have been identified, the next step is to list the important physical and chemical quantities that describe and determine the behavior of the system

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• Dimensional Analysis– Buckingham Pi Theorem

• Similitude and Scale Models– Scale models

– Geometric similarity• Model dimension = scale factor x prototype dimension

– Static similarity-same portion as geometric dim under cons. stress

– Kinematic similarity- ratio of time proportionality

– Dynamic similarity- fixed ratio of forces

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• Simulation– Finite-Difference Method

• A method of approximate solution of partial differential equations

– Monte Carlo Method• A way of generating information for a simulation when events occur in

a random way

– Geometric Modeling on the Computer• From it initiation,CAD has promised 5 important benefits to the

engineering design process– Automation of routine design tasks– Ability to design in 3D– Design by Solid Modeling– Electronic transfer of the design db to manuf (CAD/CAM)– A paperless design process

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• Surface Modeling

• Methods of Generating Solids

• Constraint-Based Modeler and Features

• Finite-Element Analysis– Types of Elements

– Steps in the FEA Process• Preprocessing: Geometry, Matl constit reln, FE mesh, Bndy Conds

• Postprocessing: Data interpret., Error estim., Design optim

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• Computer Visualization– Dynamic Analysis

– Interactive Product Simulation

• Rapid Prototyping

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Dieter: Chapter 8Materials Selection and Materials in Design

• The selection of the correct materials for a design is a key step in the process because it is the crucial decision that links computer calculations and lines on an engineering drawing with a working design

• Materials, and the manufacturing processes which convert the material into a useful part, underpin all engineering design

• The adoption of concurrent engineering methods has brought materials engineers into the design process at an earlier stage, and the importance given to manufacturing in present day product design has reinforced the fact that materials and manufacturing are closely linked in determining final product performance

• The extensive activity in materials science worldwide has created a variety of new materials and focused our attention on the competition between six broad classes of materials: metals, polymers, elastomers, ceramics, glasses, and composites

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• Relation of Materials Selection to Design– An incorrectly chosen material can lead not only to failure of the

part but also to unnecessary life-cycle cost– Selecting the best material for a part involves more than selecting a

material that has the properties to provide the necessary performance in service; it is also intimately connected with the processing of the material into the finished part (Fig. 8.1)

– As design proceeds from concept design, the material and process selection becomes more detailed

– Figure 8.2 compares the design methods and tools used at each design stage with the materials and processes selection

– Thus, material and process selection is a progressive process of narrowing from a large universe of possibilities to a specific material and process selection

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• General Criteria for Selection: Materials are selected on the basis of four general criteria:– Performance characteristics (properties)– Processing characteristics– Environmental profile– Business considerations

• The chief business consideration that affects materials selection is the cost of the part that is made from the material

• This considers both the purchase cost of the material and the cost to process it into a part. A more rational basis for selection is life cycle cost (LCC), which includes the cost of replacing failed parts and the cost of disposing of the material at the end of its useful life

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• Performance Characteristics of Materials– The performance or functional requirements of material usually is

expressed in terms of physical, mechanical, thermal, electrical, or chemical properties

– Material properties are the link between the basic structure and composition of the material and the service performance of the part (Figure 8.3)

– We can divide structural engineering materials into metals, ceramics, and polymers; Further division leads to the categories of elastomers, glasses, and composites; Finally, there is the technology driving class of electronic, magnetic, and semiconductor materials

– The chief characteristics of metals, ceramics, and polymers are given in Table 8.1

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• Performance Characteristics of Materials– The ultimate goal of materials science is to predict how to improve the

properties of engineering materials by understanding how to control the various aspects of structure

– Figure 8.4 relates various dimensions of structure with typical structural elements

– The first task in materials selection is to determine which material properties are relevant to the situation

– Figure 8.5 shows the relations between some common failure modes and the mechanical properties most closely related to the failures

– The material properties usually are formalized through specifications: Performance and Product specifications

– Table 8.2 provides a fairly complete listing of material performance characteristics

– Figure 8.6 illustrates the generic tree that is developed by expanding the category of fatigue properties

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• The Materials Selection Process– The problem is not only often made difficult by insufficient or

inaccurate property data but is typically one of decision making in the face of multiple constraints without a clear-cut objective function

– A problem of materials selection usually involves one of two different situations

• Selection of the materials for a new product or design• Reevaluation of an existing product or design to reduce cost,

increase reliability, improve performance, etc.– It generally is not possible to realize the full potential of a new

material unless the product is redesigned to exploit both the properties and the manufacturing characteristics of the material

– In other words, a simple substitution of a new material without changing the design rarely provides optimum utilization of the material

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• Materials selection for a new product or new design: The steps that must be followed are:

– Define the functions that the design must perform– Define the manufacturing parameters– Compare the needed properties and parameters with large database– Investigate the candidate materials in more detail– Develop design data and/or a design specification

• Materials substitution in an existing design– Characterize the currently used material in terms of performance, manufacturing

requirements, and cost– Determine which characteristics must be improved for enhanced product function– Search for alternative matls & processing routes – Compile a short list of matls & processing routes and use these to estimate the costs

of manufactured parts– Evaluate the results of Step 4 & make a recommendation for a replacement material

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• Design Process and Materials Selection– There are two approaches to determing the material-process

combination for a part• Material first approach: the designer begins by selecting a material

class and narrowing it down

• Process first approach: the designer begins by selecting the manufacturing process

– While materials selection issues arise at every stage in the design process, the opportunity for greatest innovation in materials selection occurs at the conceptual design stage

– Ashby Charts: Figure 8.7a: Young’s modulus vs density; Figure 8.7b: Strength vs density

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• Materials Selection in Embodiment (Preliminary) Design– Detailed materials selection is typically carried out in the

embodiment design phase using the process illustrated in Fig. 8.8

– When the material process selection is deemed adequate for the requirements, the process passes to a detailed specification of the material and the design

– Once the component goes into production, the early runs will be used to fine tune the manufacturing process and to gauge the market receptivity to the product

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• Sources of Information on Materials Properties– The purpose of this section is to provide a guide to material

property data that are readily available in the published technical literature

– Scatter or variability of material property results is considerable, however, it is rare to find a property data presented in a proper statistical manner by a mean value and the standard deviation (See Chap. 10)

– Obviously, for critical applications in which reliability is of great importance, it is necessary to determine the frequency distribution of both the material property and the parameter that describes the service behavior

– Figure 8.9 shows that when the two frequency distributions overlap, there will be a statistically predictable number of failures

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• Sources of Information on Materials Properties– Conceptual Design

• Typical material selection references, such as Ashby scheme

– Embodiment (Preliminary) Design• Design decisions are being made on the layout and size of parts and components• Design calculations require materials properties for a narrower class of materials but

specific to a particular heat treatment or manufacturing process• These data are typically found in handbooks and computer dbs.

– Detail Design• Very precise data is required• This goes beyond just material properties to include information on

manufacturability, cost, the experience in other applns, avail in the sizes and forms needed, and issues of repeat. of properties & QA

• Two often overlooked factors are whether the manufacturing process will produce different properties in different directions in the part, and whether the part will contain a detrimental state of residual stress after manufacture

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• Economics of Materials– Ultimately the decision on a particular design will come down to a trade-off

between performance and cost– Where performance doesn’t dominate the manufacturer must provide a value to

cost ratio that is no worse, and preferably better, than the competition

– By value we mean the extent to which the performance criteria appropriate to the application are satisfied. Cost is what must be paid to achieve that level of value

– Because cost is such an overpowering consideration in material selection we need to give this factor additional attention

- Scarcity - Cost & amount of energy required to process- Basic supply & demand for the material

- Increases in properties, like yield strength, beyond those of the basic material are produced by changes in structure brought about by compositional changes and additional processing steps

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• Methods of Materials Selection– There is no method or small number of methods of materials selection that has

evolved to a position of prominence– Since the final choice is a trade-off between cost and performance (properties),

it is logical to attempt to express that relation as carefully as possible– Figure 8.10 shows the costs of substituting lightweight magterials to achieve

weight saving (fuel economy) in automobiles– It is important to realize that the cost of a material expressed in dollars per

pound may not always be the most valid criterion– Total LLC is the most appropriate cost to consider– Consideration of factors beyond just the initial materials cost leads to relations

like the relation shown in Figure 8.11– A classic situation regarding cost is the choice between two or more materials

with different initial costs and different expected lives. This is a standard problem in the field of engineering economy (See Chap. 13)

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• Selection with Computer-Aided Databases– Use of a Merit Factor approach similar to an OEC

• Material Performance Indices– A materials performance index is a group of material properties

which governs some aspect of the performance of a component

• Decision Matrices– Pugh Selection Method

– Weighted Property Index

• Materials Selection by Expert Systems

• Value Analysis

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• Design for Brittle Fracture: An important advance in engineering knowledge has been the ability to predict the influence of cracks and crack-like defects on the brittle fracture of materials through the science of fracture mechanics

• Design for Fatigue Failure: Materials subjected to a repetitive or fluctuating stress will fail at a stress much lower than required to cause fracture on a single application of load

• Infinite-life design• Safe-life design• Fail-safe design• Damage-tolerance design

– Design for Corrosion Resistance– Designing with Plastics– Designing with Brittle Materials

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Dieter: Chapter 9Materials Processing and Design

• Role of Processing in Design– Producing the design is a critical link in the chain of events that starts

with a creative idea and ends with a successful product in the marketplace

– A serious problem has been the tendency to separate the design and manufacturing functions into separate organizational units

– More conventional manufacturing is divided into (1) process engineering, (2) tool engineering, (3) work standards, (4) plant engineering, and (5) administration and control

– We ordinary think of modern engineering in terms of the automotive assemble line, but mass production manufacturing systems account for less than 25 percent of metal pars manufactured

– The major opportunity for greatly increasing manufacturing productivity in small-lot manufacture

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• Classification of Manufacturing Processes– Solidification (casting) processes– Deformation processes– Material removal or cutting (machining) processes– Polymer processing– Powder processing– Joining processing– Heat treatment and surface treatment– Assembly processes

• Types of Process Systems– Job shop - Assembly line– Batch - Continuous flow

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• Factors Determining Process Selection– Cost of manufacture and life cycle cost– Quantity of parts required– Complexity – shape, form, size– Material– Quality of part– Availability, lead time, and delivery schedule

• Design for Manufacturability (DFM)– DFM Guidelines (Min tot # of parts; Standardize comps; Use

common parts across product lines; Design parts to be multifcnl; Design parts for ease of fab.; Avoid too tight tolerances; Avoid secondary opns; Utilize the special characteristics of processes)

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• Design for Assembly (DFA)– The assembly process consists of two operations,

handling followed by insertion– There are three types of assembly, classified by the level

of automation– A list of DFA guidelines are:

• Min. the tot. no. of parts• Min. the assembly surfaces• Avoid separate fasteners• Min. assembly direction• Max. compliance in assembly• Min handling in assembly

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• Early Estimation of Manufacturing Cost– The decisions about materials, shape, features and tolerances that

are made in the embodiment phase of design determine the manufacturing cost of the product

– It is not often possible to get large cost reductions once production has begun because of the high cost of change at this stage of the product life cycle

– Therefore, we need a way of identifying costly designs as early as possible in the design process

– One way is to include knowledgeable manuf psnl on IPT

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Dieter: Chapter 10Engineering Statistics

• Statistics and Design– Since in engineering design we typically deal with poorly defined

situations or are forced to use data that have low precision, it is easy to appreciate how the proper application of statistical analysis can help greatly with engineering design

– At least four major aspects of statistical analysis are important in engineering design

• Hypotheses tests

• Confidence limits

• Analysis of variance

• Statistical design of experiments

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Dieter: Chapter 10Engineering Statistics - Probability

• A basic underlying assumption of probability theory is that it deals with random events

• A random event is one in which the conditions are such that each member of the population N has an equal chance of being chosen

• A special and precise system of language and notation is used in probability theory

• Two events A and B are said to be independent if the occurrence of either one has no effect on the occurrence of the other

• Two events that have no elements in common are said to be mutually exclusive events

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Dieter: Chapter 10Engineering Statistics – Errors and Samples & Frequency

Distribution

• The act of making any type of experimental observation involves two types of errors: – Systematic errors (which exert a nonrandom basis)

– Experimental,or random, errors

• When a large number of observations are made from a random sample, a method is needed to characterize the data– Histograms,

– Frequency Distribution

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Dieter: Chapter 10Engineering Statistics – Measures of Central Tendency &

Dispersion

• A frequency distribution can be described with numbers that indicate the central location of the distribution and how the observations are spread out from the central location (dispersion)

– Arithmetic mean, or average

– Mode and Median

– Standard Deviation

– Range

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Dieter: Chapter 10Engineering Statistics – Types of Distributions

• Normal and Lognormal Distributions– Many physical measurements follow the symmetrical, bell-shaped curve of

the normal, or Gaussian, frequency distribution• Weibull Distribution

– Widely used for many engineering problems because of its versatility, since many random variables follow a bounded, nonsymmetrical distribution, such as fatigue life of components

• Gamma Distribution– Used to describe random variables that are bounded at one end

• Exponential Distribution– Special case of the gamma distribution for η = 1

• Distributions for Discrete Variables– The normal and other distributions discussed deal with continuous random

variables;however, there are important engineering problems in which the random variable takes on only discrete values

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Dieter: Chapter 10Engineering Statistics – Sampling Distributions

• The central problem in statistics is relating the population and the samples that are drawn from it

• This problem is viewed from two perspectives:– What does the population tell us about the behavior of the samples

– What does a sample or series of samples tell us about the population form which the sample came

• Distribution of Sample Means

• t Distribution

• Distribution of Sample Variances

• F Distribution

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Dieter: Chapter 10Engineering Statistics – Statistical Tests of Hypotheses

and Statistical Intervals

• The statistical decision-making process can be put on a rational, systematic basis by considering various statistically based hypotheses – Null hypothesis Ho: μ = μo

– Alternative hypothesis H1: μ < μo

• Interval estimation is commonly used to make probability statements about the population from which a sample has been drawn or to predict the results of a future sample from the same population– Confidence Interval - Tolerance Interval– Prediction Interval - Rejection of Outliers

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Dieter: Chapter 10Engineering Statistics – Analysis of Variance

• When we have three or more samples treatments we can use a statistical procedure call the Analysis of Variance (ANOVA) which is important in design of experiments

• With ANOVA we determine:– The total spread of results between the different treatments

– The spread of results within each treatment

• One-Way Classification

• Two-Way Classification

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Dieter: Chapter 10Engineering Statistics – Statistical Design of Experiments

• The greatest benefit can be gained from statistical analysis when the experiments are planned in advance so that data are taken in a way that will provide the most unbiased and precise results commensurate with the desired expenditure of time and money

• This can best be done through the combined efforts of a statistician and the engineer during the planning stage of the project

• Probably the most important benefit from statistically designed experiments is that more information per experiment will be obtained than with an unplanned experimentation

• A second benefit is that statistical design results in an organized approach to the collection and analysis of information

• Still another advantage of statistical planning is the credibility that is given to the conclusions of an experimental program when the variability and sources of experimental error are made clear by statistical analysis

• Finally, an important benefit of statistical design is the ability to discover interactions between experimental variables

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Dieter: Chapter 10Engineering Statistics – Statistical Design of Experiments

• In general, there are three classes of statistically designed experiments– Blocking designs use blocking techniques to remove the effect of

background variables from experimental error– Factorial designs are experiments in which all levels of each factor

in an experiment are combined with all levels of every other factor– Response surface designs are used to determine the empirical

functional relation between factors (independent variables) and the response (performance variable). The central composite design and rotatable designs are frequently used for this purpose

• Factorial Design• Fractional Factorial Designs

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Dieter: Chapter 10Engineering Statistics – Regressional Analysis

• Regression analysis is the statistical technique for establishing such relationships between two or more variables– Functional relation: emphasis is on prediction

– Association: correlation between variables, which vary jointly

• Method of Least Squares

• Linear Multiple Regression Analysis

• Nonlinear Regression Analysis

• Linearization Transformation

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Dieter: Chapter 10Engineering Statistics – Response Surface Methodology

• A powerful statistical procedure that employs factorial analysis and regression analysis has been developed for the determination of the optimum operating condition.

• Response surface methodology (RSM) has two objectives:– To determine with one experiment where to move in the next

experiment so as to continually seek out the optimal point on the response surface

– To determine the equation of the response surface near the optimal point

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Dieter: Chapter 11Risk, Reliability, and Safety

• A hazard (unsafe condition) is the potential for human, property, or environmental damage

• A risk is the likelihood, expressed either as a probability or as a frequency, of a hazard’s materializing

• Risk assessment has become increasingly important in engineering design as the complexity of engineering systems has increased

• Reliability is a measure of the capability of a part or a system to operate without failure in the service environment. It is always expressed as a probability

• Safety is relative protection from exposure to hazards. A thing is safe if its risks are judged to be acceptable

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Dieter: Chapter 11Risk, Reliability, and Safety

• Regulation as a Result of Risk– In a democracy when the public perception of a risk reaches sufficient

intensity, legislation is enacted to control the risk

• Standards– Standards are one of the most important ways in which the engineering

profession makes sure that society receives a minimum level of safety and performance

• Risk Assessment– The assessment of risk is an imprecise process involving judgment and

intuition– Three classifications of level of risk

• Tolerable risk• Acceptable risk• Unacceptable risk

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Dieter: Chapter 11Risk, Reliability, and Safety: Probabilistic Approach to

Design

• Conventional engineering design uses a deterministic approach

• It disregards the fact that material properties, the dimensions of the components, and the externally applied loads are stochastic in nature

• In conventional design these uncertainties are handled by applying a factor of safety

• In critical design situations, such as aircraft, space, and nuclear applications, however, there is a growing trend toward using a probabilistic approach to better quantify uncertainty and thereby increase reliability

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Design

• There are three typical approaches for incorporating probabilistic effects in design– The use of a factor of safety

– The use of the absolute worst case design

– The use of probability in design

• The use of probability in designPf = P(σ > Sy)

The reliability R is defined as

R = 1-Pf

See Example in Text

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Design

• Variability in Material Properties– The mechanical properties of engineering materials exhibit

variability

– Fracture and fatigue properties show greater variability than do the static tensile properties of yield strength and tensile strength

– Conservative design values for material properties are required in the design of minimum weight

• Probabilistic Design– Review the illustrated example of a crank that must support a

single static load

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Design

• Safety Factor– The use of a safety factor is far simpler but with much less

information content

– Using a safety factor is a form of “derating” but the extent of reduction from the true capacity is not known

• Absolute Worse Case Design– In absolute worse case (AWC) design the variables are set at either

the lowest or largest expected values

– AWC design, like the use of safety factor, is an approach that accounts for the statistical nature of the design environment in a deterministic way

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Dieter: Chapter 11Risk, Reliability, and Safety: Reliability Theory

• Reliability is the probability that a system, component, or device will perform without failure for a specified period of time under specified operating conditions

• The discipline of reliability engineering basically is a study of the causes, distribution, and prediction of failure

• Definitions– Mean life: The average life of the number of components put on test or

in service, measured over the entire life curve out to wearout– Mean time between failures (MTTF): The sum of survival time (up

time) for all of the components divided by the number of failures– Mean time between failures (MTBF): The mean time between two

successive component failures. MTBF is similar to MTTF, but it is applied for components or systems that are repaired

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• Constant Failure Rate– The probability distribution of reliability is a negative exponential

distribution– Although an individual component may not have an exponential

reliability distribution, in a complex system with many components the overall reliability may appear as a series of random events and the system will follow an exponential reliability distribution

• Variable Failure Rate– Mechanical failures and some electronic components, e.g. relays and

thermionic devices, do not exhibit a period of constant failure rate– The most common practice is to consider that failure is distributed

according to the Weibull function

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• System Reliability– Most mechanical and electronic systems comprise a collection of

components

– The overall reliability of the system depends on how the individual components with their individual failure rates are arranged

– It is obvious that if there are many components exhibiting series reliability, the system reliability quickly becomes very low

– A system in which the components are arranged to give parallel reliability is said to be redundant; there is more than one mechanism for the system functions to be carried out

– In a system with full active redundancy all but one component may fail before the system fails; See Aircraft Example for partial redun.

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• Maintenance and Repair– If a failed component can be repaired while a redundant

component has replaced it in service, then the overall reliability of the system is improved

– If components subject to wear can be replaced before they have failed, then the system reliability will be improved

– Preventive maintenance is aimed at minimizing system failure– Repairing a failed component in a series system will not improve

the reliability, since the system is not operating– However, decreasing the repair time will shorten the period during

which the system is out of service– Maintainability is the probability that a component or system that

has failed will be restored to service within a given time

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Dieter: Chapter 11Risk, Reliability, and Safety: Design for Reliability

• The design strategy used to ensure reliability can fall between two broad extremes– Fail-safe approach– “the one-horse shay” approach– Absolute worse-case approach

• Two major areas of engineering activity determine the reliability of an engineering system– Provision for reliability must be established during the earliest

design concept stage, carried through the detailed design development, and many steps in manufacture

– Once the system becomes operational, it is imperative that provision be made for its continued maintenance during its service

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Dieter: Chapter 11Risk, Reliability, and Safety: Methods and Techniques

• Failure Mode and Effects Analysis (FMEA)– Team-based methodology for identifying potential problems with

new or existing designs

• Fault Tree Analysis (FTA)– A technique that provides a systematic description of the

combinations of possible occurrences in a system that can result in failure or severe accidents

• Defects and Failure Modes– Hardware failure– Software failure– Human failure– Organization failure

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Dieter: Chapter 11Risk, Reliability, and Safety: Design for Safety

• Safety may well be the paramount issue in product design

• There are three aspects to design for safety– Make the product safe, i.e. design all hazards out of the product

– If above not possible, then design in protective devices

– If Step 2 cannot remove all hazards, then warn the user of the product with appropriate warnings like labels, flashing lights, and loud sounds

• Guidelines for Design for Safety– Be familiar with these

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Dieter: Chapter 13Economic Decision Making

• The major engineering infrastructure that built this nation – the railroads, major dams, waterways, and air transportation – required a methodology for predicting costs and balancing them against alternative courses of action

• Mathematics of Time Value of Money• Depreciation• Taxes• Profitability of Investments• Other Aspects of Probability• Inflation• Sensitivity and Break-Even Analysis• Uncertainty in Economic Analysis• Benefit-Cost Analysis

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Dieter: Chapter 14Cost Evaluation

• An engineering design is not complete until we have a good idea of the cost required to build the design or manufacture the product

• Categories of Costs• Methods of Developing Cost Estimates• Cost Indexes• Cost-Capacity Factors• Estimating Plant Cost • Design To Cost• Manufacturing Costs• Value Analysis in Costing• Overhead Costs• Activity-Based Costing• Product Profit Model• Learning Curve• Cost Models• Life Cycle Costing

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Exam #2: Life Cycle Design Considerations

• Primary Text Chapters– Chap 6: Embodiment Design– Chap 7: Modeling and Simulation– Chap 8: Materials Selection & Materials in Design– Chap 9: Materials Processing & Design– Chap 10: Engineering Statistics– Chap 11: Risk, Reliability, and Safety– Chap 13: Economic Decision Making– Chap 14: Cost Evaluation

• Secondary Text Chapter– Chap 13: Modeling and Simulation

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