ashfordeOU Material Selection (structures/materials/material-selection)
Use when the task is structural material selection for an aerospace component: material families, selection indices, stiffness-to-weight and strength-to-weight ranking, temperature limits, corrosion, cost and availability.
Domain quick reference
- Material families in aerospace structures: aluminum 2xxx and 7xxx alloys, titanium alloys (6Al-4V the common grade), low-alloy steels (4340 class), and carbon-epoxy laminates.
- Selection indices (Ashby, Materials Selection in Mechanical
Design) rank materials per unit weight for a fixed stiffness or
strength target:
- tension stiffness per weight: E / rho
- beam bending stiffness per weight: E^(1/2) / rho
- panel bending stiffness per weight: E^(1/3) / rho
- tension strength per weight: sigma / rho
- beam bending strength per weight: sigma^(2/3) / rho
- panel bending strength per weight: sigma^(1/2) / rho with E the modulus, sigma the yield strength (tension allowable for a laminate), and rho the density.
- The exponent changes the winner: on tension stiffness per weight (E/rho) aluminum, titanium, and steel are close; on beam bending stiffness per weight (E^(1/2)/rho) aluminum beats steel by a wide margin; on strength per weight titanium beats aluminum.
- Representative band values (verify against MMPDS, AMS, or CMH-17
before design use; MMPDS design-value tables are never reproduced
here):
- 2024-T3: rho 2.78 g/cm3, E 71.7 GPa, Fty 345 MPa, Tmax 150 C
- 7075-T6: rho 2.80 g/cm3, E 71.7 GPa, Fty 503 MPa, Tmax 130 C
- Ti-6Al-4V: rho 4.43 g/cm3, E 113.8 GPa, Fty 880 MPa, Tmax 315 C
- 4340 steel: rho 7.85 g/cm3, E 200 GPa, Fty 1240 MPa, Tmax 370 C
- carbon-epoxy laminate: rho 1.60 g/cm3, E 140 GPa, tension allowable 800 MPa, Tmax 120 C
- Design drivers beyond the index: temperature limits (aluminum loses strength above roughly 150 C, epoxy composites above roughly 120 C, titanium retains strength to roughly 315 C), corrosion and galvanic coupling (carbon fiber drives galvanic corrosion of adjacent aluminum), stress-corrosion cracking of 7xxx alloys in the short-transverse direction, fatigue behavior, damage tolerance, lightning protection for non-conductive composites, cost, and availability or lead time.
- Cost reality: 4340 steel is cheap, aluminum is the baseline, titanium runs roughly 5-10 times aluminum per kg, carbon-epoxy raw material runs higher still. Indices ignore cost; cost screens after the ranking.
Workflow
- Define the load case and the failure mode. Tension stiffness selects on E/rho, beam bending on E^(1/2)/rho, panel bending on E^(1/3)/rho, strength-limited parts on sigma/rho (or the strength exponent for bending).
- Gather property data for the candidates and verify the values against MMPDS or CMH-17.
- Compute indices with selection_index for each candidate.
- Rank with rank_materials.
- Screen on temperature, corrosion, galvanic coupling, cost, and availability.
- Confirm the deterministic checks with the contract test scripts/test_material_selection.py.
Pitfalls
- Ranking on the wrong index: a bending-critical part must use E^(1/2)/rho, not E/rho; the ranking changes.
- Mixing units across candidates (E in GPa against rho in kg/m3). Keep rho in g/cm3, E in GPa, sigma in MPa.
- Using ultimate strength for one candidate and yield for another.
- Treating a unidirectional lamina modulus as isotropic plate stiffness.
- Ignoring the temperature limit: aluminum and epoxy parts derate well below engine or high-speed surface temperatures.
- Ignoring galvanic corrosion at carbon-epoxy to aluminum joints.
- Copying design-value tables from MMPDS; verify property values at the source and reproduce only representative bands.
- Using an index ranking as an allowable; indices screen, they do not certify.
Behavior contract (gate 3)
The selection index, ranking, temperature, corrosion, cost, and family logic is exercised by the gate 3 contract test: scripts/test_material_selection.py against scripts/material_selection_logic.py (stdlib unittest, offline). Run: python3 skills/structures/materials/material-selection/scripts/test_material_selection.py
Compliance
- Standards referenced, not reproduced: MMPDS is proprietary (SAE, successor to public-domain MIL-HDBK-5); name + paraphrase + link only per standards-map.yaml; never reproduce design-value tables.
- Selection index methodology is common engineering knowledge (Ashby), summary-only.
- compliance: STANDARDS-REF, gated: false.