Modulus of elasticity (Young’s modulus).

How to use this

Stiffness ≠ strength.

• All steels are roughly the same modulus.

  4140, 4340, mild steel, stainless, tool steel — all hover around 200 GPa. Heat treatment, alloying, hardness — none of those significantly change Young’s modulus. They change strength and hardness; stiffness stays the same.

• Titanium is half-stiff for half-weight.

  Ti-6Al-4V: 114 GPa, 4.43 g/cc. Steel: 200 GPa, 7.85 g/cc. Specific stiffness (E/ρ) is ~25.7 vs ~25.5 — essentially the same. So titanium isn’t stiffer per pound, just lighter for the same stiffness.

• Aluminum is one-third the stiffness of steel.

  ~69 GPa vs ~200 GPa. For deflection-critical parts (machine bases, optical mounts, structural brackets) you need ~3× the moment of inertia in aluminum to match steel deflection — that’s why aluminum structural sections are dimensionally larger.

• Plastics are 50–60× softer than metal.

  PEEK at 3.6 GPa is one of the stiffer engineering plastics; still ~55× softer than steel (and ~19× softer than aluminum). For load-bearing plastic parts, design for plastic stiffness — don’t scale a metal design.

• Beryllium-copper is the stiffness sweet spot in copper alloys.

  131 GPa vs 117 GPa for pure copper. BeCu’s combination of conductivity, non-magnetic property, and stiffness makes it valuable for electrical springs and contacts.

• Inconel and steel: same stiffness, different operating temperature.

  Inconel 718 = 200 GPa, same as steel. The reason to pay 10× for Inconel is high-temperature creep resistance and corrosion — not stiffness.