Material Properties and Selection
Key Takeaways
- Mechanical properties include strength (yield, ultimate), stiffness (modulus), ductility (% elongation), hardness, and toughness.
- Thermal properties include conductivity, specific heat, coefficient of thermal expansion, and melting point.
- Electrical properties include conductivity/resistivity — metals are conductors, ceramics are insulators, semiconductors are in between.
- Material selection balances mechanical, thermal, chemical, and economic requirements for the application.
- Hardness tests (Brinell, Rockwell, Vickers) measure resistance to indentation — correlated with strength.
- Toughness is the total energy absorbed before fracture (area under the stress-strain curve).
Last updated: March 2026
Material Properties and Selection
FE Exam Weight: Materials accounts for 6-9 questions (~7% of the exam). Focus on property definitions, phase diagrams, and material classifications.
Mechanical Properties
| Property | Definition | Test Method |
|---|---|---|
| Yield Strength (σy) | Stress at onset of permanent deformation | Tensile test (0.2% offset for metals) |
| Ultimate Tensile Strength (σu) | Maximum stress the material can withstand | Tensile test |
| Elastic Modulus (E) | Slope of linear stress-strain curve (stiffness) | Tensile test |
| Ductility | Ability to deform plastically before fracture | % elongation or % reduction in area |
| Toughness | Energy absorbed before fracture | Area under stress-strain curve; Charpy/Izod impact |
| Hardness | Resistance to surface indentation | Brinell, Rockwell, Vickers |
| Fatigue Strength | Maximum cyclic stress for infinite life | S-N curve testing |
| Creep Strength | Resistance to time-dependent deformation at high T | Creep test at constant load/temperature |
Hardness Testing
| Test | Indenter | Scale | Application |
|---|---|---|---|
| Brinell (HB) | 10 mm steel ball | HB (number) | Softer materials, castings |
| Rockwell (HR) | Diamond cone or ball | HRC, HRB, etc. | Most metals (quick test) |
| Vickers (HV) | Diamond pyramid | HV (number) | Very hard materials, thin sections |
Approximate correlation: For steel, σu (MPa) ≈ 3.45 × HB
Thermal Properties
| Property | Symbol | Units |
|---|---|---|
| Thermal Conductivity (k) | k | W/(m·K) |
| Specific Heat (c) | c | J/(kg·K) |
| Coefficient of Thermal Expansion (α) | α | 1/°C or 1/K |
| Melting Point | Tm | °C or K |
Typical Values
| Material | k (W/m·K) | α (10⁻⁶/°C) | Tm (°C) |
|---|---|---|---|
| Copper | 401 | 17.0 | 1,085 |
| Aluminum | 237 | 23.6 | 660 |
| Steel | 50 | 12.0 | 1,370 |
| Stainless Steel | 16 | 17.3 | 1,400 |
| Concrete | 1.0 | 12 | N/A |
Electrical Properties
| Classification | Resistivity (Ω·m) | Examples |
|---|---|---|
| Conductors | 10⁻⁸ to 10⁻⁶ | Copper, aluminum, silver, gold |
| Semiconductors | 10⁻⁴ to 10⁴ | Silicon, germanium, GaAs |
| Insulators | 10⁸ to 10²⁰ | Rubber, glass, ceramics, polymers |
Material Categories
Metals and Alloys
- High strength, stiffness, and ductility
- Good thermal and electrical conductivity
- Examples: steel, aluminum, copper, titanium
- Ferrous (iron-based): carbon steel, stainless steel, cast iron
- Non-ferrous: aluminum, copper, titanium, nickel alloys
Ceramics
- High hardness and compressive strength
- Brittle (poor tensile strength)
- High melting points, chemical stability
- Poor thermal shock resistance
- Examples: alumina, silicon carbide, concrete, glass
Polymers
- Low density, good chemical resistance
- Low strength and stiffness compared to metals
- Thermoplastics: soften when heated, can be remolded (PE, PP, PVC, nylon)
- Thermosets: permanently crosslinked, cannot be remolded (epoxy, polyester)
- Elastomers: rubber-like, large elastic deformation
Composites
- Combine two or more materials for superior properties
- Fiber-reinforced polymers (FRP): carbon fiber, fiberglass
- Concrete: cement matrix with aggregate reinforcement
- Metal matrix composites: aluminum with ceramic particles
Material Selection Factors
| Factor | Considerations |
|---|---|
| Mechanical requirements | Strength, stiffness, fatigue, impact |
| Environmental conditions | Temperature, corrosion, UV exposure |
| Manufacturing | Machinability, weldability, formability |
| Cost | Raw material, processing, lifecycle |
| Weight | Strength-to-weight ratio for aerospace, automotive |
| Regulations | Building codes, safety standards, environmental |
Test Your Knowledge
Which material property represents the total energy a material can absorb before fracture?
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Test Your Knowledge
What type of material cannot be remolded after initial curing?
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B
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D
Test Your Knowledge
A material has high strength but fractures with very little plastic deformation (< 2% elongation). This material is best described as:
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B
C
D
Test Your Knowledge
Which material has the highest thermal conductivity?
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B
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D