Introduction to Engineering Materials
Every engineered product begins with a material choice. The wing spar of a commercial aircraft, the crumple zone of a sedan, the turbine blade of a jet engine — each demands a specific combination of properties that only certain materials can deliver. A 10% reduction in vehicle weight can improve fuel efficiency by 6–8%, which is why automotive OEMs spend billions annually on material innovation.
Why Materials Matter
Material selection directly impacts four critical factors in automotive and aerospace design:
Weight — A Boeing 787 uses ~50% composites by weight, saving roughly 20% fuel compared to an equivalent all-aluminum design. In automotive, the Ford F-150 switched from steel to aluminum body panels in 2015, shedding ~315 kg (700 lb). Safety — Modern car bodies use hot-stamped boron steel (22MnB5) in the A-pillar and B-pillar at 1,500 MPa tensile strength to protect the passenger cell, while softer HSLA steels in the crumple zones absorb crash energy through controlled deformation. Cost — Mild steel costs roughly 1–3 USD/kg. Aluminum is 3–5 USD/kg. Aerospace-grade titanium runs 20–60 USD/kg. Carbon fiber composite parts can reach 230–310 USD/kg for aerospace applications. Material cost often determines whether a design is viable. Performance — Jet engine turbine inlet temperatures exceed 1,500°C. The materials in the hot section must maintain strength and resist creep at temperatures that would melt most metals.The Four Classes of Engineering Materials
Metals
Metals are crystalline materials with metallic bonding. They offer high strength, stiffness, ductility, and thermal/electrical conductivity. They can be shaped by casting, forging, rolling, and machining.
- Automotive: Steel (body structure, chassis, powertrain), aluminum (hoods, doors, engine blocks), magnesium (steering wheel armatures)
- Aerospace: Aluminum alloys (fuselage skins), titanium (fan blades, bulkheads), nickel superalloys (turbine discs)
Polymers
Polymers are long-chain organic molecules. They are lightweight, corrosion-resistant, and easily moldable, but have lower strength and temperature limits than metals.
- Automotive: Polypropylene (bumper fascias, ~20% of car plastics), nylon PA66 (intake manifolds, radiator tanks), EPDM rubber (weather seals)
- Aerospace: PEEK (bushings, seals rated to 250°C), polyimide (wire insulation), silicone (gaskets)
Ceramics
Ceramics are hard, brittle compounds with very high melting points and excellent wear/corrosion resistance. They are poor in tension but excellent in compression.
- Automotive: Alumina spark plug insulators, silicon nitride (Si₃N₄) turbocharger rotors, ceramic brake discs (SiC on high-performance vehicles)
- Aerospace: Thermal barrier coatings (yttria-stabilized zirconia on turbine blades), SiC/SiC ceramic matrix composites (CMCs) in jet engine hot sections
Composites
Composites combine two or more materials to achieve properties neither can alone. Typically a reinforcing fiber (carbon, glass, aramid) in a matrix (epoxy, thermoplastic).
- Automotive: SMC/BMC body panels (Corvette), GFRP leaf springs, CFRP passenger cells (BMW i3)
- Aerospace: CFRP fuselage and wings (Boeing 787: 50%, Airbus A350: 52%), Nomex honeycomb sandwich panels (floor panels, control surfaces)
Key Material Properties at a Glance
| Property | What It Measures | Typical Units |
|---|---|---|
| Density (ρ) | Mass per unit volume | kg/m³, g/cm³ |
| Young's Modulus (E) | Stiffness — resistance to elastic deformation | GPa |
| Yield Strength (σy) | Stress at which permanent deformation begins | MPa |
| Ultimate Tensile Strength (σu) | Maximum stress before fracture | MPa |
| Elongation (%) | Ductility — how much it stretches before breaking | % |
| Hardness | Resistance to indentation | HRC, HV, HB |
| Thermal Conductivity (k) | Ability to conduct heat | W/m·K |
| CTE (α) | How much it expands with temperature | ×10⁻⁶/°C |
Material Selection Drivers
Engineers rarely optimize for a single property. Instead, they balance competing requirements:
Specific Strength (σ/ρ) — Strength per unit weight. Critical in aerospace where every gram counts. CFRP has ~4× the specific strength of steel. Specific Stiffness (E/ρ) — Stiffness per unit weight. Aluminum and steel have nearly identical specific stiffness (~25 GPa·cm³/g), which is why aluminum saves weight without sacrificing panel stiffness. Cost — Steel dominates automotive because it's cheap, strong, and recyclable. CFRP dominates aerospace because the fuel savings over decades justify the upfront cost. Manufacturability — Can it be stamped, cast, forged, or injection molded at production volume? Magnesium is lighter than aluminum but harder to form and prone to corrosion. Recyclability — Steel recycling rate is ~90%. Aluminum is ~95%. CFRP recycling is still below 5%, a growing concern as composite use increases.Materials in a Modern Car (by weight)
A typical mid-size sedan contains roughly:
- Steel & iron: ~55–65% (BIW, chassis, powertrain)
- Aluminum: ~8–12% (engine block, wheels, closures)
- Plastics & polymers: ~8–10% (interior trim, bumpers, under-hood)
- Rubber: ~4–5% (tires, seals, hoses)
- Glass: ~3% (windshield, windows)
- Other (copper, fluids, textiles): remainder
Materials in a Commercial Aircraft
The Boeing 787 Dreamliner material breakdown:
- Composites (CFRP): 50%
- Aluminum: 20%
- Titanium: 15%
- Steel: 10%
- Other: 5%
Compare this to the older Boeing 777: 50% aluminum, 12% composites, 7% titanium. The shift toward composites and titanium reflects the industry drive for lighter, more fuel-efficient, and more corrosion-resistant airframes.
Industry Trends
Lightweighting — Every major OEM has a multi-material strategy. The Audi A8 uses an aluminum space frame with steel, magnesium, and CFRP in a single body structure. Multi-Material Design — Mixing materials (e.g., steel floor + aluminum closures + CFRP roof) optimizes each component for its specific loads, but creates joining challenges (riveting, adhesive bonding, friction stir welding). Sustainability — EU regulations require 25% recycled plastic content in new vehicles. Steel and aluminum are highly recyclable; composite recycling remains a significant challenge. Additive Manufacturing — 3D-printed titanium (Ti-6Al-4V) components are now flying in GE jet engines. AM enables complex geometries that reduce part count and weight.Key Takeaways
- Material selection is driven by the balance of weight, strength, cost, manufacturability, and environmental impact
- Engineering materials fall into four classes: metals, polymers, ceramics, and composites
- Automotive is steel-dominated but shifting toward multi-material bodies
- Aerospace leads in composite and titanium adoption due to the high value of weight savings
- Specific strength (σ/ρ) and specific stiffness (E/ρ) are more important than absolute values when weight matters
