5. Materials Engineering

What Is Materials Engineering?

Materials engineering is the study and application of materials so that they can perform particular jobs safely, effectively, and economically.

Engineers do not simply ask:

“Which material is strongest?”

Instead, they ask:

“Which material has the best combination of properties for this application?”

A material might need to be:

• Strong.
• Lightweight.
• Hard.
• Tough.
• Ductile.
• Malleable.
• Conductive.
• Corrosion resistant.
• Heat resistant.
• Affordable.

The choice depends on what the material needs to do.

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Structure, Properties, and Performance

A central idea in materials engineering is:

composition → structure → bonding → properties → performance → application

The atoms present in a material and the way they are arranged determine its bonding and structure.

These microscopic features determine observable properties.

Those properties determine whether the material performs well in a particular application.

For example:

metallic bonding → delocalized electrons → high electrical conductivity → useful for electrical wiring

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Material Properties

Engineers consider many properties when choosing materials.

Strength

Ability to withstand forces without failing.

Hardness

Resistance to scratching, indentation, or wear.

Toughness

Ability to absorb energy and resist fracture.

Ductility

Ability to undergo significant deformation, especially stretching, before breaking.

Malleability

Ability to be pressed, hammered, or rolled into shape.

Density

Mass per unit volume.

density = mass / volume

Electrical conductivity

Ability to allow electrical charge to move.

Thermal conductivity

Ability to transfer thermal energy.

Corrosion resistance

Ability to resist chemical deterioration.

Melting point

Temperature at which a solid becomes a liquid at a specified pressure.

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No Material Is Best at Everything

Materials usually involve trade-offs.

A material may be:

very strong but heavy

or:

lightweight but relatively weak

or:

highly conductive but expensive

or:

hard but brittle

Therefore, engineers rarely select a material based on only one property.

They must decide which properties are most important for the application.

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Comparing Metallic Materials

Metals can have very different properties.

Consider:

• Steel.
• Aluminum alloys.
• Copper.
• Titanium alloys.

Each is useful because it provides a different combination of properties.

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Steel

Steel is an iron-based alloy containing carbon and often other elements.

Depending on its composition and processing, steel can provide:

• High strength.
• High stiffness.
• Good toughness.
• Good durability.
• Relatively low cost.

Steel is widely used in:

• Buildings.
• Bridges.
• Vehicles.
• Railways.
• Machinery.
• Tools.

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Why Steel Is Used in Buildings

A structural material must support large forces.

Steel can provide high strength and stiffness while also being manufactured into:

• Beams.
• Columns.
• Reinforcement.
• Plates.
• Fasteners.

Steel is also widely available and can be joined by techniques such as welding and bolting.

Its combination of properties makes it extremely important in construction.

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Limitations of Steel

Steel is not ideal for every application.

Possible disadvantages include:

• Relatively high density.
• Corrosion in some environments.
• Need for protective coatings in some applications.

Engineers may therefore choose another material when low mass or exceptional corrosion resistance is more important.

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Aluminum and Aluminum Alloys

Aluminum has a much lower density than steel.

It also has:

• Good corrosion resistance.
• Good thermal conductivity.
• Good electrical conductivity.
• High malleability.
• High ductility.

Pure aluminum is relatively soft, so stronger aluminum alloys are commonly used in engineering.

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Why Aluminum Alloys Are Used in Aircraft

Aircraft must be strong enough to withstand significant forces while remaining as light as practical.

Reducing mass can reduce the energy required for flight.

Therefore, an important property is:

strength-to-weight ratio

Aluminum alloys can provide:

useful strength + relatively low density

This makes them useful for many aerospace structures.

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Copper

Copper is particularly important because of its excellent:

• Electrical conductivity.
• Thermal conductivity.
• Ductility.

Copper is widely used for:

• Electrical wiring.
• Motors.
• Generators.
• Electronics.
• Plumbing.
• Heat-transfer applications.

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Why Copper Is Used for Electrical Wiring

Electrical wiring requires a material that:

• Conducts electricity efficiently.
• Can be drawn into thin wires.
• Can bend without easily breaking.
• Remains reliable during use.

Copper meets these requirements very well.

Its metallic structure contains mobile delocalized electrons.

Therefore:

metallic bonding → delocalized electrons → electrical conductivity

Its ductility allows it to be manufactured into long wires.

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Titanium Alloys

Titanium alloys can combine:

• High strength.
• Relatively low density.
• Excellent corrosion resistance.
• Good performance at elevated temperatures.

These properties make titanium alloys useful in demanding applications.

Examples include:

• Aircraft components.
• Aerospace systems.
• Chemical processing equipment.
• Some medical devices.

However, titanium can be significantly more expensive to produce and process than common steels or aluminum alloys.

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Comparing Common Engineering Metals

The most suitable material depends on the requirements of the application.

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Strength Is Not the Same as Hardness

These terms are often confused.

Strength describes a material's ability to withstand applied stress without failing or permanently deforming beyond acceptable limits.

Hardness describes resistance to:

• Scratching.
• Indentation.
• Surface wear.

A material can be strong without being exceptionally hard.

A hard material can also be brittle.

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Strength Is Not the Same as Toughness

Toughness describes the ability of a material to absorb energy before fracturing.

Consider two materials.

Material A

Very strong but brittle.

It withstands a large force but then fractures with relatively little deformation.

Material B

Strong and tough.

It can absorb considerable energy and deform before fracture.

For many engineering structures, toughness can be extremely important.

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Why Brittleness Matters

A brittle material undergoes relatively little plastic deformation before fracture.

Examples include many:

• Ceramics.
• Glasses.
• Ionic crystalline materials.

Brittle materials can still be very strong under particular types of loading.

However, their tendency to fracture with limited warning must be considered during design.

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Ductility and Safety

Ductile materials can undergo substantial deformation before breaking.

This can be valuable in engineering because deformation may occur before complete failure.

For example, a metal structure might:

bend or stretch before fracturing

rather than failing suddenly.

Engineers therefore consider ductility when designing structures exposed to large or changing loads.

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Density

Density is particularly important when mass must be minimized.

density = mass / volume

or:

ρ = m/V

Suppose two components have identical volumes.

Material A:

density = 2.7 g/cm³

Material B:

density = 7.8 g/cm³

For the same volume, Material B will have much greater mass.

This can matter enormously in vehicles and aircraft.

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Worked Example: Comparing Mass

Two components each have a volume of:

500 cm³

Component A is made from a material with density:

2.7 g/cm³

Mass:

m = ρV

m = 2.7 × 500

m = 1350 g

Component B has density:

7.8 g/cm³

m = 7.8 × 500

m = 3900 g

For the same volume, Component B is:

2550 g heavier

This shows why density matters when engineers are trying to reduce mass.

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Strength-to-Weight Ratio

Sometimes strength alone is not the most useful comparison.

Engineers may want a material that provides high strength without excessive mass.

This leads to the idea of:

strength-to-weight ratio

A lightweight material with sufficient strength may outperform a much stronger but much heavier material for certain applications.

This is particularly important in:

• Aircraft.
• Spacecraft.
• Vehicles.
• Bicycles.
• Portable equipment.

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Electrical Conductivity

Some designs require electrical current to move efficiently.

In metals, electrical conductivity results mainly from mobile delocalized electrons.

Materials such as copper and aluminum are therefore important electrical conductors.

An engineer designing an electrical cable might consider:

• Conductivity.
• Resistance.
• Density.
• Strength.
• Ductility.
• Cost.

The best conductor is not automatically the best overall engineering choice.

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Copper Versus Aluminum Conductors

Copper has excellent electrical conductivity.

Aluminum has lower conductivity for the same cross-sectional area but also has much lower density.

Therefore:

copper is widely used in building wiring and electronics.

aluminum-based conductors are widely used in overhead power transmission where low mass can be particularly valuable.

The engineering choice depends on the complete set of requirements.

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Thermal Conductivity

Some applications require thermal energy to move rapidly.

Examples include:

• Cookware.
• Heat exchangers.
• Electronic heat sinks.

Metals such as copper and aluminum have high thermal conductivity.

Other applications require the opposite.

For example, handles on cooking equipment should reduce thermal energy transfer.

Therefore, insulating materials may be chosen instead.

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Corrosion Resistance

Materials interact with their environments.

Water, oxygen, salts, acids and other chemicals can damage some metals through corrosion.

Engineers must therefore consider the environment in which a material will operate.

A material used:

indoors in dry conditions

may face very different requirements from one used:

in seawater

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Preventing Corrosion

Several approaches can reduce corrosion.

These include:

• Protective coatings.
• Painting.
• Galvanizing.
• Using corrosion-resistant alloys.
• Careful material selection.
• Appropriate design.

Stainless steel, for example, contains chromium that allows a protective oxide layer to form.

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Material Selection for a Bridge

Imagine selecting a material for a bridge.

The material must withstand:

• The bridge's own weight.
• Vehicles or pedestrians.
• Wind.
• Temperature changes.
• Repeated loading.
• Environmental exposure.

An engineer may therefore consider:

• Strength.
• Stiffness.
• Toughness.
• Fatigue resistance.
• Corrosion resistance.
• Cost.
• Ease of construction.
• Maintenance requirements.

No single property is enough.

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Material Selection for an Aircraft

An aircraft presents different requirements.

Low mass is extremely important.

Materials must also withstand:

• Aerodynamic forces.
• Repeated loading.
• Temperature changes.
• Vibration.
• Environmental exposure.

Therefore, engineers may consider:

strength + low density + fatigue performance + corrosion resistance

Materials may include aluminum alloys, titanium alloys, steels and composite materials depending on the component.

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Material Selection for a Saucepan

A saucepan presents another set of requirements.

The base should transfer thermal energy effectively.

The material should also:

• Withstand cooking temperatures.
• Resist corrosion.
• Be durable.
• Be practical to manufacture.

The handle has different requirements.

It should reduce thermal energy transfer to the user's hand.

Therefore, one product may contain several materials because different components perform different jobs.

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Material Selection for Electrical Wiring

Consider the requirements of an electrical wire.

The conducting core should have:

• High electrical conductivity.
• Low electrical resistance.
• Good ductility.
• Adequate mechanical strength.

The outer coating should have:

• Low electrical conductivity.
• Flexibility.
• Durability.

Therefore, a typical cable combines:

metal conductor + polymer insulator

This demonstrates that engineers often combine materials rather than searching for one material that performs every function.

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Bonding and Engineering Performance

Different types of bonding help explain different material properties.

Metallic bonding

Positive ions surrounded by delocalized electrons.

Helps explain:

• Electrical conductivity.
• Thermal conductivity.
• Malleability.
• Ductility.

Ionic bonding

Strong electrostatic attractions between oppositely charged ions.

Helps explain:

• High melting points.
• Hardness.
• Brittleness.
• Conductivity when molten or dissolved.

Covalent structures

Strong covalent bonds can produce very different properties depending on structure.

For example, giant covalent structures can be extremely strong or hard.

Therefore:

bonding alone is not enough — structure also matters.

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Structure Matters

Consider carbon.

Diamond and graphite are both made entirely of carbon atoms.

However, their structures differ dramatically.

Diamond has a three-dimensional covalent network.

Graphite contains layers and delocalized electrons.

As a result, their properties are very different.

This demonstrates:

same element + different structure → different properties

Materials engineering depends heavily on understanding these structure-property relationships.

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Processing Matters Too

Material properties are not determined only by chemical composition.

The way a material is processed can also change its structure and properties.

Processing methods include:

• Heating.
• Cooling.
• Rolling.
• Forging.
• Casting.
• Heat treatment.
• Mechanical working.

For example, two pieces of steel with similar compositions can have different properties if they have undergone different processing.

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Heat Treatment

Controlled heating and cooling can change the internal structure of some metals and alloys.

This can alter properties such as:

• Hardness.
• Strength.
• Toughness.
• Ductility.

Engineers can therefore modify a material after its chemical composition has already been chosen.

This provides another important relationship:

composition + processing → structure → properties

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Manufacturing Requirements

A material may have excellent final properties but still be unsuitable if it is extremely difficult to manufacture.

Engineers may ask:

• Can it be cast?
• Can it be welded?
• Can it be machined?
• Can it be rolled into sheets?
• Can it be drawn into wire?
• Can it be shaped economically?

Manufacturing considerations can strongly influence material selection.

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Cost

Engineering designs must usually operate within a budget.

A material with outstanding properties may be technically suitable but economically impractical.

Engineers consider:

• Raw material cost.
• Manufacturing cost.
• Installation cost.
• Maintenance cost.
• Expected lifetime.
• Replacement cost.

A more expensive material may sometimes be worthwhile if it lasts much longer or requires less maintenance.

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Environmental Impact

Modern materials engineering also considers environmental effects.

Questions can include:

• How much energy is required to produce the material?
• Can it be recycled?
• How long will the product last?
• How much material is required?
• What environmental effects occur during extraction?
• Can the material be reused?

Material selection therefore involves more than simply mechanical performance.

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Life-Cycle Thinking

Engineers can consider the entire life cycle of a product:

raw material extraction

↓

material production

↓

manufacturing

↓

transport

↓

use

↓

maintenance

↓

reuse, recycling, or disposal

A material that initially seems environmentally preferable may have disadvantages elsewhere in its life cycle.

Engineers therefore try to examine the whole system.

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A Materials Selection Process

A simplified engineering materials-selection process is:

Identify the purpose

↓

Determine the forces and environmental conditions

↓

Identify required properties

↓

Identify possible materials

↓

Compare their properties

↓

Consider manufacturing

↓

Consider cost and availability

↓

Consider durability and environmental impact

↓

Select and test a suitable material

Material selection is therefore an evidence-based decision.

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Worked Example: Bicycle Frame

Suppose an engineer is selecting a material for a bicycle frame.

Important properties could include:

• Strength.
• Low mass.
• Toughness.
• Fatigue resistance.
• Corrosion resistance.
• Manufacturability.
• Cost.

Possible materials might include:

• Steel.
• Aluminum alloys.
• Titanium alloys.
• Composite materials.

There is no universal "best" material.

The choice depends on the intended bicycle, manufacturing method, performance requirements and cost.

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Worked Example: Cooking Pan

Suppose an engineer needs to design a cooking pan.

The material should:

• Conduct thermal energy.
• Remain stable at cooking temperatures.
• Resist corrosion.
• Be sufficiently strong.
• Be practical to manufacture.

Aluminum may provide excellent thermal conductivity and low mass.

Stainless steel may provide excellent durability and corrosion resistance.

Some cookware combines layers of different metals to obtain advantages from several materials.

This demonstrates an important strategy:

combine materials to obtain complementary properties.

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Worked Example: Suspension Bridge Cable

A suspension bridge cable must support very large tensile forces.

Important properties include:

• High tensile strength.
• Toughness.
• Fatigue resistance.
• Durability.

High-strength steel is commonly suitable because it can be manufactured into strong wires that are combined into large cables.

The material is chosen because its mechanical properties match the forces it must withstand.

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Worked Example: Electrical Transmission Line

Suppose an overhead electrical cable must carry current over a long distance.

Important requirements include:

• Good electrical conductivity.
• Low mass.
• Adequate mechanical strength.
• Durability.
• Reasonable cost.

Copper has excellent conductivity.

Aluminum has lower density.

Therefore, aluminum-based conductors can be advantageous when cable mass is especially important.

Again:

the highest value for one property does not automatically determine the engineering choice.

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Using Data to Select Materials

Engineers often compare quantitative data.

Suppose three hypothetical materials have these properties:

If designing an electrical cable, conductivity may receive high priority.

If designing a lightweight aircraft component, density and strength may receive greater priority.

If designing equipment for a corrosive environment, corrosion resistance may become critical.

The application determines which properties matter most.

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Failure and Safety

Engineers must also consider how a material could fail.

Possible failure mechanisms include:

• Fracture.
• Excessive deformation.
• Fatigue.
• Wear.
• Corrosion.
• Overheating.

A material that performs well initially may still be unsuitable if repeated use causes dangerous failure.

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Fatigue

Fatigue is damage caused by repeated cycles of loading and unloading.

For example, an aircraft wing experiences repeated loading during:

• Takeoff.
• Flight.
• Turbulence.
• Landing.

A material may eventually develop cracks even when individual loads are below the level that would cause immediate failure.

Fatigue resistance is therefore important in many engineering designs.

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Safety Factors

Engineers normally do not design a component to operate exactly at its predicted failure limit.

A factor of safety provides a margin between normal operating conditions and failure conditions.

This helps account for:

• Unexpected loads.
• Material variation.
• Wear.
• Manufacturing imperfections.
• Uncertainty.

Material properties therefore contribute directly to safe engineering design.

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Common Mistakes

Choosing the Strongest Material Automatically

Strength is only one requirement.

Density, cost, corrosion resistance, conductivity and manufacturing requirements may be equally important.

Confusing Hardness and Strength

Hardness describes resistance to surface deformation.

Strength describes resistance to failure or excessive deformation under applied stress.

Confusing Strength and Toughness

A very strong material can still be brittle.

Assuming Lightweight Means Weak

Some materials provide excellent strength-to-weight ratios.

Ignoring the Environment

A material that works well indoors may corrode or degrade rapidly in another environment.

Ignoring Manufacturing

A material must be practical to shape, join and process.

Assuming Pure Metals Are Always Better

Alloys can provide combinations of properties that pure metals cannot.

Looking Only at Initial Cost

Maintenance, lifetime and replacement costs can also matter.

Forgetting About Structure

Material properties arise from atomic bonding and microscopic structure.

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Check Your Understanding

1. What is materials engineering?

2. Name five properties engineers might consider when selecting a material.

3. Why is there rarely one "best" material for every application?

4. Distinguish between strength and hardness.

5. Distinguish between strength and toughness.

6. Why can ductility be important for safety?

7. Why is density important in aircraft design?

8. Explain the meaning of strength-to-weight ratio.

9. Why is copper suitable for electrical wiring?

10. Why can aluminum be useful for overhead electrical cables?

11. Give two advantages of steel as a structural material.

12. Give one possible disadvantage of steel.

13. Why are aluminum alloys useful in transportation?

14. Why might titanium alloys be selected for demanding engineering applications?

15. Explain how metallic bonding contributes to electrical conductivity.

16. Explain how metallic structure contributes to ductility.

17. Why can alloying change mechanical properties?

18. How can processing change a material's properties?

19. Why must engineers consider corrosion?

20. What is fatigue?

21. Why might a material with excellent properties still not be selected because of manufacturing considerations?

22. Why is cost important in material selection?

23. What is meant by life-cycle thinking?

24. Explain why different materials might be used for the body and handle of a cooking pan.

25. Describe the main steps engineers could use to select a material for a new product.

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Key Terms

• Materials engineering – study and application of relationships between material structure, properties, processing and performance.
• Strength – ability to withstand applied stress without unacceptable failure or deformation.
• Hardness – resistance to scratching, indentation or wear.
• Toughness – ability to absorb energy before fracturing.
• Ductility – ability to undergo substantial deformation, especially stretching, before fracture.
• Malleability – ability to be pressed, hammered or rolled into shape.
• Density – mass per unit volume.
• Electrical conductivity – ability to allow electrical charge to move.
• Thermal conductivity – ability to transfer thermal energy.
• Corrosion resistance – ability to resist chemical deterioration.
• Strength-to-weight ratio – comparison of strength with material weight or density.
• Fatigue – progressive damage caused by repeated loading.
• Factor of safety – design margin between expected operating conditions and failure conditions.
• Life cycle – stages from raw material extraction through manufacture, use and eventual recycling or disposal.
• Trade-off – situation in which improving or prioritizing one characteristic may require accepting a disadvantage in another.

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Key Takeaways

• Engineers choose materials according to the requirements of a particular application.
• No single material has the best value for every property.
• Strength, hardness, toughness, ductility and malleability describe different mechanical properties.
• Density is especially important when mass must be minimized.
• Copper is valuable when high electrical conductivity is required.
• Aluminum alloys combine low density with useful strength.
• Steel provides useful combinations of strength, stiffness, toughness and cost.
• Titanium alloys can combine strength, relatively low density and corrosion resistance.
• Bonding and microscopic structure determine many material properties.
• Metallic bonding explains conductivity, malleability and ductility.
• Alloying can modify material structure and therefore change its properties.
• Processing can also alter structure and performance.
• Engineers must consider operating conditions, manufacturing, cost, durability and environmental impact.
• Material selection usually involves trade-offs.
• Different materials can be combined when different parts of a product require different properties.
• Engineers use measurements and material data rather than simply choosing materials by appearance.
• Long-term failure mechanisms such as corrosion and fatigue must be considered.
• Safe designs include margins for uncertainty and unexpected conditions.
• The central idea of materials engineering is to connect structure and bonding to properties, then select materials whose properties match the demands of the design.