Material Selection Methods
Knowing material properties is necessary but not sufficient. The real engineering challenge is selecting the right material for a specific application — balancing performance, manufacturability, cost, weight, and environmental requirements. This lesson covers the systematic approaches used in industry.
The Material Selection Process
Material selection follows a funnel: start with all possible materials (~160,000 engineering materials) and narrow down through screening, ranking, and detailed evaluation.
- Translation — Define the function, constraints, objectives, and free variables
- Screening — Eliminate materials that can't meet non-negotiable constraints (Tmax, corrosion environment, conductivity requirement)
- Ranking — Use material indices and Ashby charts to rank survivors by performance-per-cost or performance-per-weight
- Supporting information — Manufacturability, availability, recycling, company experience, supply chain
Ashby Material Property Charts
Developed by Professor Michael Ashby at Cambridge, these charts plot one material property against another on logarithmic axes. Each material class occupies a characteristic region (a "bubble"), making it easy to see trade-offs and identify candidates.
Key Ashby Charts
Young's Modulus vs. Density (E–ρ)This chart reveals specific stiffness. Lines of constant E/ρ (specific modulus) are diagonal — materials above and to the left are stiffer per unit weight.
- Steels, Ti, Al all have roughly the same E/ρ (~25 MN·m/kg)
- CFRP is 2–4× better in specific stiffness
- Wood and foams appear in the low-density, moderate-stiffness region
Reveals specific strength. CFRP and high-strength titanium dominate the upper-left corner.
- Mild steel: high density, moderate strength
- Al 7075-T6: moderate density, high strength
- Ti-6Al-4V: moderate density, very high strength
- CFRP: low density, very high strength
Reveals the strength-toughness trade-off. Most materials follow a falling trend: higher strength means lower toughness. Materials in the upper-right corner (high strength AND high toughness) are premium — steels like 300M, titanium alloys.
Strength vs. Temperature (σy–T)Shows which materials maintain strength at temperature. Polymers drop off above 200°C. Aluminum drops above 200°C. Titanium is useful to ~540°C. Nickel superalloys carry load to ~700°C (with cooling, higher).
Material Indices
A material index is a combination of properties that maximizes performance for a specific structural function. Derived from the objective function and constraints.
Common Material Indices
| Function | Objective | Constraint | Material Index (maximize) |
|---|---|---|---|
| Tie rod (tension) | Minimize mass | Fixed load capacity | σy/ρ |
| Beam (bending) | Minimize mass | Fixed stiffness | E^(1/2)/ρ |
| Beam (bending) | Minimize mass | Fixed strength | σy^(2/3)/ρ |
| Panel (bending) | Minimize mass | Fixed stiffness | E^(1/3)/ρ |
| Spring | Maximize stored energy | No yielding | σy²/E |
| Thermal insulation | Minimize heat loss | Fixed thickness | 1/λ (thermal conductivity) |
How to Use Material Indices
Example: Lightest stiff beamObjective: minimize mass for a beam of given length carrying a bending load with a maximum deflection constraint.
The derivation (from beam bending theory) gives: Material Index = E^(1/2)/ρ
| Material | E (GPa) | ρ (g/cm³) | E^(1/2)/ρ | Rank |
|---|---|---|---|---|
| Steel | 200 | 7.85 | 1.80 | 5 |
| Aluminum | 70 | 2.70 | 3.10 | 3 |
| Titanium | 110 | 4.43 | 2.37 | 4 |
| CFRP (QI) | 70 | 1.55 | 5.40 | 2 |
| CFRP (0° dominant) | 140 | 1.55 | 7.63 | 1 |
| Wood (spruce) | 12 | 0.45 | 7.70 | 1 |
Guidelines for Material Index Lines on Ashby Charts
On a log-log plot of E vs. ρ, lines of constant E^(1/2)/ρ are straight lines with slope 2. Moving the line to the upper-left captures better materials. The best candidates are those closest to the upper-left corner, above the line.
Weighted Decision Matrix (Pugh Matrix)
After screening and ranking narrow the field to 3–5 candidates, a weighted decision matrix makes the final selection by incorporating factors that Ashby charts don't capture: cost, manufacturability, availability, environmental impact, and company experience.
How to Build One
- List criteria — All factors that matter for the decision
- Assign weights — Each criterion gets a weight (0–10 or percentage) reflecting its importance. Weights must be agreed upon by the design team BEFORE scoring.
- Score each material — Rate each candidate on each criterion (1–5 or 1–10)
- Calculate weighted scores — Multiply score × weight for each cell, sum across all criteria
- Compare totals — Highest weighted total is the recommended material
Example: Automotive Hood Panel Material Selection
Criteria and weights:| Criterion | Weight |
|---|---|
| Specific strength | 8 |
| Formability | 7 |
| Material cost ($/kg) | 9 |
| Part cost (including tooling) | 8 |
| Corrosion resistance | 5 |
| Joinability (to steel body) | 6 |
| Recyclability | 4 |
| Dent resistance | 6 |
| Criterion | Wt | Mild Steel | AHSS (BH210) | Al 6016-T4 | CFRP |
|---|---|---|---|---|---|
| Specific strength | 8 | 2 | 3 | 4 | 5 |
| Formability | 7 | 5 | 4 | 3 | 2 |
| Material cost | 9 | 5 | 4 | 3 | 1 |
| Part cost | 8 | 5 | 4 | 3 | 2 |
| Corrosion | 5 | 2 | 2 | 4 | 5 |
| Joinability | 6 | 5 | 5 | 3 | 2 |
| Recyclability | 4 | 5 | 5 | 5 | 2 |
| Dent resistance | 6 | 3 | 4 | 3 | 4 |
| Weighted Total | 213 | 205 | 178 | 158 |
In this scenario, mild steel wins on cost-driven criteria despite being heaviest. For a luxury or performance vehicle where weight matters more, the weights shift and aluminum or CFRP can win.
The weights reflect the design intent, not absolute truth. A cost-focused program (economy car) weights cost heavily; a performance program (sports car) weights specific strength heavily.Case Studies
Case 1: Automotive B-Pillar
Function: Protect occupants in side impact (absorb energy, maintain survival space) Constraints:- Must resist intrusion under FMVSS 214 / Euro NCAP pole impact
- Must connect to roof rail and rocker panel
- Production volume: 200,000+ units/year
Case 2: Jet Engine Fan Blade
Function: First rotating stage — ingests air, accelerates it into the compressor. Must survive bird strike (4 lb bird at takeoff speed). Constraints:- Maximum tip speed ~450 m/s (centrifugal loads proportional to density)
- Bird impact tolerance (FAA certification requires continued operation after ingesting a 4 lb bird)
- Fatigue life: 20,000+ flight cycles
- FOD (foreign object damage) tolerance
- Operating temperature: ambient to ~150°C
| Property | Ti-6Al-4V | CFRP + Ti LE |
|---|---|---|
| Weight per blade | ~25 kg | ~15 kg |
| Bird strike tolerance | Excellent | Good (with Ti LE) |
| FOD tolerance | Excellent | Moderate |
| Fatigue | Excellent | Excellent |
| Manufacturing cost | High | Very high |
| Weight savings (full set) | Baseline | ~40% |
Case 3: Bicycle Frame
Function: Primary structure carrying rider loads through pedaling, road vibrations, and impacts. Constraints:- Rider weight up to ~120 kg
- Fatigue life: 10+ years of varied loading
- Stiffness for efficient power transfer
- Comfort (vibration damping)
- Production volume varies: 100 (high-end) to 100,000+ (mass market)
| Property | 4130 Steel | Al 6061-T6 | Ti-3Al-2.5V | CFRP |
|---|---|---|---|---|
| Frame weight | ~2.0 kg | ~1.4 kg | ~1.5 kg | ~0.9 kg |
| Ride quality | Excellent | Harsh | Excellent | Tunable |
| Fatigue | Infinite life (below Se) | No endurance limit — must overdesign | Infinite life | Excellent if designed properly |
| Repairability | Weldable | Weldable (but weakens HAZ) | Weldable (specialized) | Difficult |
| Cost (frame) | ~200 USD | ~300 USD | ~1,500 USD | ~500–5,000 USD |
Common Material Selection Mistakes
- Optimizing one property in isolation — Selecting the strongest material without considering formability, joinability, or cost. Strength is necessary but not sufficient.
- Ignoring manufacturing constraints — A material that's perfect in a datasheet but can't be formed, welded, or machined to required tolerances is useless.
- Not accounting for the full system — Replacing a steel bracket with aluminum saves weight on that part but may require thicker sections (lower modulus), new fasteners (galvanic corrosion), and new joining processes.
- Applying room-temperature data to elevated-temperature applications — Aluminum loses 50% of its strength by 200°C. Polymers lose stiffness above Tg. Always check properties at the actual service temperature.
- Ignoring fatigue for cyclic applications — Static yield strength is irrelevant if the part fails by fatigue at 50% of yield after 10⁶ cycles.
Key Takeaways
- Ashby charts are the screening tool — plot property combinations on log-log axes to identify candidate material classes
- Material indices (σy/ρ, E^(1/2)/ρ, etc.) quantify which material gives the best performance for a specific structural function
- Weighted decision matrices incorporate cost, manufacturability, and other non-technical factors for final selection
- The "best" material depends entirely on the weighting of criteria — there is no universally optimal material
- Always consider the full system: joining, corrosion compatibility, manufacturing process, supply chain, and end-of-life
