A buckling-critical strut redesign — rule-of-mixtures property estimation for carbon- and glass-fiber composites, a worked Euler buckling sizing calculation against a fixed envelope, layup and fiber-orientation decisions, and a final material selection with justification.
The project brief: a lightweight equipment frame's diagonal compression strut needs a material substitution to cut weight without changing the frame's existing 16 mm clamp-fitting envelope, carrying a 2200 N buckling-critical design load at a 2.5x factor of safety. Three candidates are compared at that fixed envelope — 6061-T6 aluminum, unidirectional carbon-fiber/epoxy, and unidirectional glass-fiber/epoxy — using the rule of mixtures to estimate the composite properties an off-the-shelf datasheet wouldn't hand you directly.
This module walks the whole comparison the way a working materials engineer would: why specific stiffness (E/ρ), not specific strength, is the property that actually governs a buckling-critical design, a worked sizing calculation carried through to a real wall thickness and mass for every candidate, why the carbon-fiber tube's real design ends up gauge-driven rather than buckling-driven, and the genuinely counterintuitive result that glass fiber comes out heavier than the aluminum it was meant to replace. The full worked numbers, material comparison table, and finished selection reasoning are part of the unlocked module below.