MATERIALS SELECTION IN MECHANICAL DESIGN �

DEPARTMENT OF MECHANICAL ENGINEERING

GANDHI INSTITUTE FOR EDUCATION AND TECHNOLOGY, BANIATANGI, BHUBANESWAR

Relationship To Course�

• A key concept throughout this course is

how to select among technology choices

• Economic Analysis
• Cost Modeling
• Life Cycle Assessment
• Focus has been on economic assessment

of alternatives

• How does this fit into larger technology

choice problem?

Approach Changes as Design Evolves

What parameters define material selection?

Example: SUV Liftgate

Attractive Options

May Be Found Outside of Expertise

SMC: sheet molding compound

Need Method for Early Material Selection:�Ashby Methodology*��Four basic steps

1. Translation: express design requirements as constraints & objectives

2. Screening: eliminate materials that cannot do the job

3. Ranking: find the materials that do the job best

4. Supporting information: explore pedigrees of top-ranked candidates

*M.F. Ashby, Materials Selection in Mechanical Design, 3rd Ed., Elsevier, 2005

First Step: Translation�“Express design requirements as constraints and objectives”

• Function: What does the component do?
• Do not limit options by specifying implementation w/in function
• Objective: What essential conditions must be met?
• In what manner should implementation excel?
• Constraints: What is to be maximized or minimized?
• Differentiate between binding and soft constraints
• Free variables: Which design variables are free?
• Which can be modified?
• Which are desirable?

Using design requirements, analyze four items:

Identifying Desirable Characteristics�Example: Materials for a Light, Strong Tie

• Function:
• Support a tension load
• Objective:
• Minimize mass
• Constraints:
• Length specified
• Carry load F, w/o failure
• Free variables:
• Cross-section area
• Material