Modulus matters.
Stiff structures.
Deflection control.
Material selection for stiffness — modulus times moment of inertia equals stiffness. High-modulus materials enable thinner stiff structures.
Key principles.
Highest common
Carbon steel highest modulus among common metals. Stiffest per mass for many designs.
Light, less stiff
35% steel modulus. Larger sections needed for equivalent stiffness.
Mid-modulus
55% steel modulus. Better stiffness-per-mass than aluminum sometimes.
Light, low modulus
Lightest metal but low modulus. Bulky sections needed.
E=70-700 GPa
Composite stiffness varies wildly per fiber direction. Unidirectional carbon: extremely stiff.
E=1-12 GPa
Plastics 50-200× less stiff than steel. Larger sections needed.
E=8-15 GPa
Glass fibers boost plastic modulus 3-5×. Better than unfilled.
E=200-500 GPa
Ceramics very stiff but brittle. Specialty applications.
E=20-30 GPa
Construction material. Different application range.
FAQ
Modulus vs strength?
Different. Modulus = stiffness (deflection). Strength = failure load. Stiff materials may not be strongest.
Stiffness-to-weight optimization?
Specific modulus = E/ρ. Aluminum and steel similar specific modulus. Carbon fiber composite much higher.
Beam stiffness formula?
δ = FL³ / (3EI) for cantilever. Stiffness depends on E (material) and I (geometry, cube of dimension).
Cross-section affects stiffness more than material?
Often yes. Doubling section dimension cubes stiffness. I-beams achieve high stiffness with less material.
When to use composites?
Aerospace, racing, high-performance bicycles. Premium cost but stiffness-per-mass unmatched.
Material selection chart for stiffness?
Ashby chart: plot E vs ρ. Pareto-optimal materials in stiffness-per-mass dominated by composites and certain metals.
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