Ashby selection chart: for a member sized by a strength floor (axial tension/tie), the mass needed to carry a fixed load scales as mass ∝ density / strength = 1 / (specific strength). Maximizing specific strength σ/ρ therefore minimizes structural mass — the classic Ashby "material index" for a strength-limited tie.
- Screening line: the slider is a hard requirement (with margin already folded in). Any material below the line physically cannot carry the load at any thickness worth flying — it's disqualified outright, not just "worse".
- Why specific stiffness barely moves between metals: E/ρ for aluminum, titanium and steel all sit near 25–26 GPa/(g/cm³) — swapping metals buys almost no stiffness-per-weight gain. That's why stiffness-critical redesigns reach for CFRP (≈84 GPa/(g/cm³)), not a different metal.
- Why not always CFRP: real selection also weighs cost, damage tolerance and manufacturing — this chart isolates the mass-minimization axis only, which is why aluminum still covers low-stress brackets and access panels.