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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