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

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.

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

How to Use Material Indices

Objective: 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

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

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.

Case Studies

Case 1: Automotive B-Pillar

• 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

• 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

Case 3: Bicycle Frame

• 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)

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