| name | material-selector |
| description | Engineering material selection — steel grades, aluminum alloys, polymers, composites, ceramics. Ashby charts, property indices, failure mode matching, cost, machinability, weldability, corrosion. |
| metadata | {"priority":8,"promptSignals":{"phrases":["material selection","what material","choose material","steel grade","aluminum alloy","material properties","Ashby","specific strength"],"minScore":4}} |
Material Selection — Complete Skill
Selection Strategy (Ashby Method)
- Define function (what it does) + objective (minimize/maximize) + constraints (limits)
- Derive performance index M from analysis
- Screen on constraints, rank on index
- Consult local conditions (availability, cost, processing)
Common performance indices:
| Function | Objective | Index M |
|---|
| Tie (tension) | Min weight | σ_y/ρ (specific strength) |
| Beam (bending) | Min weight | σ_y^(2/3)/ρ |
| Column (buckling) | Min weight | E^(1/2)/ρ |
| Thermal insulation (panel) | Min heat flux | 1/(k·√α) |
| Spring | Max energy stored | σ_y²/E (resilience) |
| Fatigue-limited | Max cycles | S_e/ρ |
Steels
Carbon steels (AISI/SAE):
| Grade | Sy (MPa) | Sut (MPa) | Condition | Application |
|---|
| 1020 | 210 | 380 | HR | General, weldable |
| 1045 | 310→580 | 570→700 | HR→Q&T | Shafts, bolts |
| 1080 | 420→620 | 615→770 | HR→Q&T | Springs, rail |
| 1340 | 470→830 | 670→940 | HR→Q&T | High strength structural |
| 4130 | 360→835 | 560→1020 | N→Q&T | Aircraft, pressure vessels |
| 4140 | 415→1100 | 655→1170 | N→Q&T | Gears, shafts, dies |
| 4340 | 470→1470 | 745→1620 | N→Q&T | High-performance shafts |
| 8620 | 385→830 | 530→930 | HR→case | Gears (case hardened) |
| 52100 | 515 | 1000 | Ann. | Bearings |
Weldability: C < 0.30%, CE < 0.40 (good); C > 0.45% → preheat required
ASTM structural steels:
A36: Fy=250MPa, Fu=400MPa (general structural)
A572 Gr50: Fy=345MPa, Fu=450MPa (high-strength structural)
A514: Fy=690MPa, Fu=760MPa (T-1 steel, very high strength, limited weldability)
A992: Fy=345MPa, Fu=450MPa (W-shapes, controlled Fy/Fu ratio)
Stainless steels:
304: Sy=215MPa, Sut=505MPa — general corrosion, food, chemical
316L: Sy=170MPa, Sut=485MPa — chloride resistance, marine, medical
316SS: Higher Sy with cold work, better pitting resistance
410: Sy=275MPa, martensitic, hardenable, less corrosion resistant
17-4 PH: Sy=1000-1170MPa — precipitation hardened, aerospace
Aluminum Alloys
| Grade | Sy (MPa) | Sut (MPa) | Condition | Application |
|---|
| 1100-H18 | 150 | 165 | Cold worked | Electrical, chemical |
| 2024-T3 | 345 | 483 | T3 | Aircraft structure |
| 2024-T351 | 325 | 470 | Plate | Fuselage, wings |
| 6061-T6 | 275 | 310 | T6 | General structural |
| 6063-T6 | 215 | 240 | T6 | Extrusions, architectural |
| 7050-T7451 | 490 | 550 | T7 | Aircraft (thick section) |
| 7075-T6 | 503 | 572 | T6 | High strength aircraft |
| 7075-T651 | 462 | 524 | T6 plate | Structural |
Note: Aluminum does NOT have a true endurance limit (S-N curve continues to drop)
Fatigue strength typically at 5×10⁸ cycles ≈ 0.4Sut
Titanium Alloys
Ti-6Al-4V: Sy=880MPa, Sut=950MPa, ρ=4430kg/m³ — aerospace, biomedical
Ti-3Al-2.5V: Lower strength, better formability — tubing
Advantages: corrosion resistance, high specific strength, biocompatible
Disadvantages: expensive, difficult to machine (use carbide tools, low speeds)
Copper Alloys
C11000 (ETP copper): k=388W/mK — electrical conductors
C26000 (brass 70/30): Sy=95→340MPa — good formability, decorative
C51000 (phosphor bronze): Sy=130→480MPa — springs, bearings (good fatigue)
C93200 (bearing bronze): — plain bearings, bushings
Polymers
| Material | Sy (MPa) | E (GPa) | Max Temp | Note |
|---|
| HDPE | 25-30 | 0.7-1.4 | 90°C | Chemical resistance |
| Nylon 6/6 | 55-90 | 3.0 | 100°C | Gears, bearings |
| PTFE | 14-25 | 0.4 | 260°C | Low friction |
| PEEK | 100 | 3.6 | 250°C | High-performance |
| Acetal (POM) | 60-70 | 3.1 | 90°C | Gears, cams |
| Polycarbonate | 55-70 | 2.4 | 130°C | Impact resistant, optical |
| Epoxy | 35-100 | 3-6 | 120-200°C | Adhesive, matrix |
Composites (CFRP/GFRP)
CFRP (unidirectional): E_fiber=230GPa, Sut=3500MPa (fiber direction only)
CFRP laminate (quasi-isotropic): E=70GPa, Sut=600MPa
GFRP: E=20-45GPa, Sut=250-450MPa — cheaper than CFRP
Density: CFRP=1550kg/m³ (vs. Al=2700, steel=7850)
Specific strength: CFRP >> Ti > Al > steel
Ceramics
Al₂O₃ (alumina): E=380GPa, σ_c=2600MPa (compression), K_IC=3-4 MPa√m — brittle
Si₃N₄: E=310GPa, higher toughness — cutting tools, bearings
SiC: E=410GPa — very hard, abrasion resistant
WC-Co (cemented carbide): E=600GPa — cutting tools
Material Property Charts (Ashby)
E vs. ρ: metals cluster, polymers lower E, ceramics high E
σ_y vs. ρ: CFRP and Ti top right
K_IC vs. σ_y: metals have good fracture toughness, ceramics poor
Thermal shock resistance: σ_f·k/(E·α) — CFRP and Si₃N₄ best
Output
Provide top 3 candidate materials with: Sy, Sut, E, ρ, cost factor, machinability rating, weldability, corrosion resistance, and recommendation with reasoning.
