Engineering Materials Guide: Steels, Aluminum, Alloys & Plastics
Compare mechanical properties (yield strength, elastic modulus, density, machinability) for carbon steel, alloy steel, stainless, aluminum, brass, cast iron, and polymers.
Featured Snippet Summary
Mechanical material selection balances yield strength (Sy), elastic modulus (E), density, and corrosion resistance. Compare carbon and alloy steels, 6061/7075 aluminum, stainless steels, brass, and engineering plastics with verified Machinery's Handbook properties.
Key Mechanical Property Definitions
- **Modulus of Elasticity ($E$)**: Also known as Young's Modulus, $E$ defines elastic stiffness (resistance to elastic deflection). Structural steels share a nearly identical stiffness ($E \approx 200\text{ GPa}$ / $29\text{ Mpsi}$), whereas aluminum alloys are roughly one-third as stiff ($E \approx 69 - 72\text{ GPa}$ / $10\text{ Mpsi}$). - **Yield Strength ($S_y$)**: The stress threshold beyond which permanent plastic deformation occurs. Sizing ductile machine elements under static load is governed by $S_y$ divided by a design factor of safety ($N_{sf}$). - **Ultimate Tensile Strength ($S_u$)**: The maximum engineering stress sustained prior to necking and tensile fracture. - **Mass Density ($\rho$)**: Dictates component inertial weight and structural mass (Steel $\approx 7,850\text{ kg/m}^3$, Aluminum $\approx 2,700\text{ kg/m}^3$, Titanium $\approx 4,500\text{ kg/m}^3$). - **Poisson's Ratio ($\nu$)**: The ratio of transverse contraction to axial extension under uniaxial load ($\nu \approx 0.28 - 0.33$ for structural metals).
Structural Metals and Engineering Polymers Taxonomy
## 1. Carbon Steels (AISI 1018, 1045, ASTM A36) - **Characteristics**: Low cost, excellent weldability, high modulus ($E = 205\text{ GPa}$), and broad availability. - **Yield Strength**: $S_y \approx 250\text{ MPa}$ (A36 hot-rolled) to $370\text{ MPa}$ (1018 cold-drawn). - **Applications**: Machine frames, structural weldments, baseplates, non-critical shafts.
## 2. Alloy Steels (AISI 4140, 4340, 8620) - **Characteristics**: Chromium-molybdenum and nickel-alloyed steels formulated for high hardenability, fatigue strength, and impact toughness. - **Yield Strength**: Heat-treated/quenched and tempered yield strengths range from $S_y = 650\text{ MPa}$ to over $1,100\text{ MPa}$. - **Applications**: Transmission shafts, power gears, heavy-duty studs, connecting rods. See our [Shaft Design Basics](/knowledge/articles/shaft-design-basics) guide.
## 3. Stainless Steels (Austenitic 304/316, Martensitic 410, Precipitation-Hardening 17-4 PH) - **Characteristics**: Chromium content $> 10.5\%$ forms a self-healing passivated chromium oxide layer delivering outstanding corrosion resistance. - **Key Caveat**: Austenitic grades (304, 316) exhibit galling tendencies during fastener assembly; always apply anti-seize lubricants as detailed in [Bolt Preload Explained](/knowledge/articles/bolt-preload-explained). - **Applications**: Chemical processing, food and beverage equipment, marine hardware, medical devices.
## 4. Aluminum Alloys (6061-T6, 7075-T6, 2024-T3) - **Characteristics**: Approximately one-third the density of steel ($\rho \approx 2,700\text{ kg/m}^3$), non-magnetic, excellent machinability, and naturally corrosion-resistant. - **Yield Strength**: 6061-T6 provides $S_y \approx 276\text{ MPa}$; high-strength aerospace 7075-T6 provides $S_y \approx 503\text{ MPa}$ (comparable to medium-carbon steel). - **Applications**: Aerospace structures, automotive chassis, robotics brackets, machined optical mounts.
## 5. Cast Irons (Gray Iron Class 30, Ductile Iron 65-45-12) - **Characteristics**: High compressive strength, superior vibration damping capacity, and excellent wear resistance. Gray iron has low tensile ductility; ductile iron incorporates nodular graphite for enhanced tensile toughness. - **Applications**: Engine blocks, machine tool beds, brake rotors, heavy pump housings.
## 6. Copper Alloys (C36000 Free-Cutting Brass, C93200 SAE 660 Bronze) - **Characteristics**: High thermal/electrical conductivity, spark resistance, and low coefficient of friction against steel. - **Applications**: Journal bearing bushings, sleeve wear plates, electrical contacts, fluid valves.
## 7. Engineering Plastics (Acetal/Delrin POM, Nylon 6/6, PEEK, PTFE) - **Characteristics**: Low weight ($\rho \approx 1,140 - 1,420\text{ kg/m}^3$), chemical inertness, electrical insulation, and self-lubricating wear properties. - **Limitations**: Lower modulus ($E \approx 1.5 - 4\text{ GPa}$) and operating temperature limits ($< 100 - 150^\circ\text{C}$ for standard polymers; up to $250^\circ\text{C}$ for PEEK).
System & Design Schematics
Figure 4: Mechanical properties spectrum and Ashby chart showing modulus vs density and strength across metals and polymers.
Engineering Equations & Formulas
Hooke's Law (1D Elastic Stress)
Relates stress directly to strain within the elastic deformation limit of materials.
Mass and Volume Density
Calculates total component mass from material density and geometric volume.
Worked Sizing Examples
An aluminum 6061-T6 structural rod ($E = 70\text{ GPa}$, length $L = 500\text{ mm}$, diameter $d = 20\text{ mm}$) supports a tensile load of $44\text{ kN}$ ($\sigma \approx 140\text{ MPa}$). Calculate the resulting elastic elongation ($\Delta L$) and the total mass of the rod ($\rho = 2,700\text{ kg/m}^3$).
- 1. Identify inputs: E = 70,000 MPa, σ = 140 MPa, L = 500 mm, d = 20 mm, \rho = 2,700 kg/m³.
- 2. Calculate elastic strain: \epsilon = \sigma / E = 140 / 70,000 = 0.002.
- 3. Calculate rod elongation: \Delta L = \epsilon * L = 0.002 * 500 mm = 1.00 mm.
- 4. Calculate volume: V = (\pi * d² / 4) * L = (\pi * 20² / 4) * 500 = 314.16 * 500 = 157,080 mm³ = 1.5708 * 10^-4 m³.
- 5. Calculate total mass: m = \rho * V = 2,700 kg/m³ * 1.5708 * 10^-4 m³ = 0.424 kg (424 grams).
Design Guidelines & Best Practices
- Match Stiffness to Geometry: Remember that all steels share $E \approx 200\text{ GPa}$. Switching from mild A36 steel to high-strength 4140 alloy steel will not reduce elastic deflection unless cross-sectional dimensions are changed.
- Account for Thermal Expansion: Aluminum expands roughly twice as fast as steel ($\alpha \approx 23 \times 10^{-6}/\text{K}$ vs $12 \times 10^{-6}/\text{K}$). Specify dowel slot clearances to prevent binding in thermal environments.
- Verify Densities and Weights: Use the [Material Weight Calculator](/calculators/material-weight) to evaluate shipping, inertial, and material costs across alloys.
Common Engineering Mistakes
- Confusing Strength with Stiffness: Selecting expensive heat-treated alloy steels to solve deflection or vibration problems without increasing moment of inertia ($I$).
- Overlooking Galvanic Corrosion: Fastening aluminum plates with unplated steel screws in moist environments without insulating washers.
- Neglecting Plastic Creep: Subjecting engineering plastics to continuous high mechanical loads at elevated temperatures.
Applicable Standards & Textbook References
| Standard / Source | Reference Title | Description |
|---|---|---|
| ASTM A36 / ASTM A108 | Standard Specification for Structural and Carbon Steels | Mechanical property definitions for carbon steels |
| MMPDS-14 | Metallic Materials Properties Development and Standardization | Authoritative aerospace handbook for metallic material properties |
| Machinery's Handbook 31st Ed | Industrial Press | Standard properties of engineering metals, plastics, and hardness conversions |
Frequently Asked Questions
Q:What is the difference between yield strength and ultimate tensile strength?
Q:Why do all carbon and alloy steels have roughly the same elastic modulus?
Senior Mechanical Engineer — Power Transmission
PhD Mechanical Engineering, Stanford. 12+ years shaft design, fatigue analysis, and ASME code compliance for aerospace and industrial drivetrains.
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