MME/06/8201, LOTO OLUWASAYO I. 1 Contents 1.0 PRINCIPLE OF MATERIAL SELECTION .................................................................................................. 2 1.1 Material Selection for a New Design .............................................................................................. 2 1.2 Material Substitution for an Existing Design ................................................................................... 3 1.3 Design Limiting Properties ............................................................................................................. 4 1.4 Functions, Objectives, Constraints And Variables ........................................................................... 4 1.5 Process of Material Selection in relation to Design ......................................................................... 5 2.0 PERFORMANCE CHARACTERISITICS OF MATERIALS ............................................................................ 5 2.1 TOOLS IN THE SELECTING PROCEDURE............................................................................................... 8 2.1.1Material Selection Chart........................................................................................................... 8 2.1.2 Material Indices .................................................................................................................... 11 3.0 CASE STUDIES .................................................................................................................................. 12 3.1 SPRINGS....................................................................................................................................... 12 3.2 Con-Rods For High-Performance Engines ..................................................................................... 19 3.3: THE AUTO COOLING FANS........................................................................................................... 23 3.4: Materials for Flywheels ............................................................................................................... 26 3.5: Brake Disc ................................................................................................................................... 30 3.6: Engine Cylinder ........................................................................................................................... 31 3.7: Automobile Exhaust System ........................................................................................................ 31 3.8: Automobile Chassis- Body in Weight ........................................................................................... 31 3.9: The Dash board ........................................................................................................................... 32 3.10: Automobile Engine block .......................................................................................................... 32 4.0 Conclusion: ...................................................................................................................................... 32 5.0 REFERENCES .................................................................................................................................... 33
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MME/06/8201, LOTO OLUWASAYO I.
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Contents 1.0 PRINCIPLE OF MATERIAL SELECTION .................................................................................................. 2
1.1 Material Selection for a New Design .............................................................................................. 2
1.2 Material Substitution for an Existing Design ................................................................................... 3
Another example of a materials selection chart would be a plot of yield strength (failure
strength) σy versus density ρ which would allow selection of materials for light, strong
structures. Charts with axes which are combinations of properties (axes of E / ρ and σy / ρ, for
instance), or which measure relative corrosion resistance or wear resistance can also be helpful.
2.1.2 Material Indices
The design of a mechanical component is specified by three things: the functional requirements
(the need to carry loads, transmit heat, store elastic or thermal energy, etc), the geometry, and
the properties of the material of which it is made, including its cost. The performance of the
element can be described by an equation with the general form
,
where p describes the aspect of performance of the component that is to be optimized: its
mass, or volume, or cost, or life for example; and f( ) means 'a function of'. Optimum design can
be considered to be selection of the material and geometry which maximize (or minimize) p.
The optimization is subject to constraints, some of them imposed by the material properties.
The three groups of parameters in equation above are said to be 'separable' when the equation
can be written
where f1, f2 and f3 are functions. When the groups are separable, the optimum choice of
material becomes independent of the design details. The optimum material is the same for all
geometries G, and all values of the functional requirements F. Then the optimum material can
be identified without solving the complete design problem, or even knowing all the details of F
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and G. This enables enormous simplification: the performance for all F and G is maximized by
maximizing f3(M), which is called the 'performance index'.
Following these principles of material selection, the task of selecting a material may be very
cumbersome, however there is a simplification of the process by the use of already developed
softwares which contain a database of most of the common materials encountered in process
of selecting a material for a component or process. The most common and widely used is the
CES selector developed by Ashby and Granta. It makes the selection process less cumbersome
as different material properties can easily be compared over a wide range of material types.
Depending on the functional requirements, objectives and constraints the best suitable
material can be obtained.
In the remaining aspect of this paper, we will look into some case studies explain the process of
material selection.
3.0 CASE STUDIES
We are going to consider a typical automobile car component. An automobile car has a lot of
component in the design therefore we will select some part for our studies. Generally, any
automobile component design will want to minimize the weight of the car in order to reduce
fuel consumption and invariably gas emissions. These are the basic considerations for design
and there are various standards requirement and organizations that sees to environmental
issues in the design. It should however be noted that each of the component study will be
summarized so that we can discus at least ten of the parts in an automobile design. Hence, it
only gives a basis of what we would expect in designing of such component parts.
3.1 SPRINGS
The best material for a spring is that which can store the greatest elastic potential energy per unit mass
(or volume), without failing. In an automobile we have springs in the shock absorber (helical) and also in
the valves and some other reciprocating part. Springs for vehicle suspensions must resist fatigue (the
selection should then be made with the endurance limit, σe, replacing the modulus of rupture, σMOR).
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Valve springs for the engines must cope with elevated temperatures; here strength-at-temperature is
needed. The performance indices derived below can be used to identify materials which satisfy the
design specification summarized. Note that a generalized form of equation using MOR was used
however parameters can be changed to determine other characteristics.
FIG 3: Springs have many shapes, but all perform the same function: that of storing elastic
energy.
FUNCTION Elastic Spring
OBJECTIVE (a) Maximum stored elastic energy/unit volume
(b) Maximum stored elastic energy/unit mass
CONSTRAINTS No failure by yield, fatigue or fracture (whichever is more restrictive)
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Adequate toughness (Gc > 1 kJ/m2)
Reasonable cost per unit weight (Cm < 100 GBP/kg)
The primary function of a spring is that of storing elastic energy and releasing it again when required. The elastic energy stored per unit volume in a block of material stressed uniformly to a stress σ is:
,
where E is the Young's modulus. It is this that we wish to maximize. The spring will be damaged if the stress σ exceeds the yield stress or failure stress σf. So the constraint is σ ≤ σf. The maximum energy density is therefore:
,
Torsion bars and leaf springs are less efficient than axial springs because some of the material is not fully loaded: the material at the neutral axis, for instance, is not loaded at all. For solid torsion bars
,
and for leaf springs loaded in pure bending the maximum energy storage is
.
But, as these results show, this has no influence on the choice of material. The best material for springs, regardless of the way in which they are loaded, is that with the biggest value of
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.
If mass matters rather than volume, we must divide this by the density ρ (giving energy stored per unit mass), and seek materials with high values of
.
The Selection
The selection of materials for springs of minimum volume is shown in below. Here the modulus
of rupture, σMOR, has been used as the measure of the failure strength σf. The chart shows σMOR
plotted against modulus, E. A family of lines of slope 1/2 link materials with equal values of M1
= σf2/E. Those with the highest values of M1 lie towards the top left. The heavy line is one of the
family. It is positioned at 10 MJ/m3 such that a small subset of materials is left exposed. They
include high-strength steel (spring steel, in fact) lying near the top end of the line, and, at the
other end, rubber. But certain other materials are suggested too: GFRP (now used for truck leaf
springs), titanium alloys (good but expensive), glass fibers (used in galvanometers) and —
among polymers — nylon (children's toys often have nylon springs). The procedure identifies a
candidate from almost every material class: metals, glasses, polymers, elastomers and
composites. A protective stage, limiting the values of the toughness Gc (Gc = KIC2 / E) and the
cost Cm to the those listed in the design requirements, has been added .
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Figure 4: A chart of the modulus of rupture, σMOR, against Young's modulus, E. The diagonal
line shows M1.
Figure 5: A 'protective' chart of the toughness, Gc, against cost per unit weight, Cm. The box
restricts the selection to materials with Gc > 1 kJ/m2 and Cm < 100 GBP/kg.
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MATERIAL
COMMENT
Spring Steel
15 – 25 The traditional choice: easily formed and heat treated.
Ti Alloys 15 – 20 Expensive, corrosion-resistant.
CFRP 15 – 20 Comparable in performance with steel; expensive.
GFRP 10 – 12 Almost as good as CFRP and much cheaper.
Glass fibers 30 – 60 Brittle in tension, but excellent if protected against damage; very low loss factor.
Nylon 1.5 – 2.5 The least good; cheap and easily shaped, but high loss factor.
Rubber 20 – 60 Better than spring steel; but high loss factor.
Materials for efficient springs of low volume
Materials selection for light springs is shown in Figure 4. It is a chart of σMOR/ρ against E/ρ,
where ρ is the density. Lines of slope 1/2 now link materials with equal values of
.
One is shown at the value M2 = 2 kJ/kg. Composites, because of their lower densities, are better
than metals. Elastomers are better still (you can store almost 8 times more elastic energy per
unit weight in a rubber band than in the best spring steel). Elastomeric springs are now widely
used in aerospace because of their low weight and high reliability. Wood — the traditional
material for archery bows, now appears in the list.
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Figure 6: A chart of σMOR/ρ against E/ρ. The diagonal line is a contour of M2.
The mechanical loss coefficient is important in springs which are loaded dynamically: polymers
have high loss factors and therefore dissipate energy when they vibrate; metals, if strongly
hardened, do not. Polymers, because they creep, are unsuitable for springs which carry a
steady load, though they are good for catches and locating-springs which spend most of their
time unstressed. Springs made from unprotected carbon-steel fail rapidly in a chemically
corrosive environment. The least expensive solutions to this problem is to plate or polymer-
coat them to provide a corrosion barrier, but if the coating is damaged, failure can follow. The
more expensive solution is to make the spring from an intrinsically corrosion-resistant material: