Material Selection Process

James Stuart, Professor of

Engineering at Cambridge

1875 - 1890

No polymers – now over 45,000

• Today: > 160,000 engineering materials

• Creates two needs:

1 Education – how best teach materials?

2 Industry – how manage materials

information ?

No light alloys – now several thousand

No composites – now hundreds ……

• Solutions:

1 The CES EduPack – Education

2 Granta mi – Industry

• In his day: a few hundred materials

Today

General criteria for selection

• Performance characteristics (properties)

• Processing

• Environmental profile

• Cost (Business consideration)

• First task in materials selection to

determine which material properties are

relevant to the situation.

• Yield strength ( easy to obtain and cheap

to measure, )

• Hardness

SELECTION PROCESS

Translation

Screening

Eliminates candidates that cannot do the job at

all because one or more of their attributes lies

outside the limits set by the constraints.

As examples, the requirement that ‘‘the

component must function in boiling water’’, or

that ‘‘the component must be transparent’’

imposes obvious limits on the attributes of

maximum service temperature and optical

transparency that successful candidates must

meet. We refer to these as attribute limits.

Ranking

Attribute limits do not, however, help with

ordering the candidates that remain. To do this

we need optimization criteria. They are found in

the material indices.

Objective What is to be maximized or

minimized?

MATERIAL INDICES

Performance is sometimes limited to single property, sometimes

combination of them. Maximizing single property maximize the

performance of material.

Best material for buoyancy those are with low density; those

best with for thermal insulation the ones with the smaller density.

Best material light stiff tie-road , with the specific stiffness E/rho,

The best material for spring sigma_f^2/rho.

The property group that maximize performance for a given design

is called its material index.

Attribute limits and material indices

Constraints set property limits. Objectives define

material indices, for which we seek extreme values.

When the objective in not coupled to a constraint, the

material index is a simple material property. When,

instead, they are coupled, the index becomes a group

of properties.

Example: Heat sinks for hot

microchips

Example2: Overhead transmission

line

Material indices when objectives are

coupled to constraints.

L and F fixed area is free variable, so minimizing the area minimize the

mass.

A must be sufficient to carry load Eliminating A between

two equation above gives ;

Deriving indices — how to do it

Ranking: indices on charts