Glossary
Every symbol and abbreviation used across mpolyco, in plain language — and why each letter was chosen for its job.
Force spread over an area — how hard a material is being pushed or pulled internally.
Why this symbol: Greek sigma — the standard symbol for normal (direct) stress since 19th-century elasticity theory; pairs with τ for shear.
How much a material stretches, as a fraction of its original length.
Why this symbol: Greek epsilon, for 'extension' — the conventional symbol for normal strain; pairs with γ for shear strain.
Stiffness: how much stress it takes to stretch a material a given amount. Higher = stiffer.
Why this symbol: E for elasticity (the modulus of elasticity); named after Thomas Young.
Stiffness of a composite ply in the strong, fiber direction.
Why this symbol: Subscript 1 = the fibre direction. In composites, axes 1-2-3 mean along-fibre, in-plane transverse, through-thickness.
Stiffness of a ply sideways to the fibers — much softer than along them.
Why this symbol: Subscript 2 = in-plane, transverse to the fibres — the soft direction.
Resistance to being twisted or skewed sideways, as opposed to stretched.
Why this symbol: G for the shear (rigidity) modulus, the conventional partner to E.
A ply's resistance to being skewed within its own plane.
Why this symbol: Shear modulus in the 1-2 plane of the ply (fibre / transverse).
How much a material thins sideways when you stretch it lengthwise.
Why this symbol: Greek nu — the long-standing symbol for Poisson's ratio.
Sideways thinning of a ply when pulled along the fibers.
Why this symbol: Major Poisson ratio: contraction in direction 2 when loaded along the fibres (1).
The share of a composite that is fiber rather than resin. 60% is typical.
Why this symbol: V for volume, subscript f for fibre — the fibre volume fraction.
The most pulling stress a ply can take along the fibers before breaking.
Why this symbol: Tsai strength notation: X = longitudinal (fibre, 1) strength; subscript t = tension.
The most squeezing stress a ply can take along the fibers.
Why this symbol: X = longitudinal (fibre) strength; subscript c = compression (quoted positive).
The most pulling stress a ply can take sideways to the fibers — usually weak.
Why this symbol: Y = transverse (2-direction) strength; subscript t = tension.
The most squeezing stress a ply can take sideways to the fibers.
Why this symbol: Y = transverse strength; subscript c = compression (quoted positive).
The most shear (skewing) stress a ply can take before it fails.
Why this symbol: S for the in-plane shear strength of the ply.
The temperature where a plastic changes from hard-and-glassy to soft-and-rubbery.
Why this symbol: T for temperature, subscript g for the glass transition.
The temperature at which a crystalline plastic melts into a liquid.
Why this symbol: T for temperature, subscript m for melting.
How much mass is packed into a given volume — how heavy a material is for its size.
Why this symbol: Greek rho — the conventional symbol for mass density.
How far a resin has hardened, from 0 (liquid) to 1 (fully set).
Why this symbol: Greek alpha — used here for the degree of cure, a fraction from 0 (uncured) to 1 (fully cured).
How much a material damps vibration — turns motion into heat instead of springing back.
Why this symbol: The tangent of δ, the phase-angle lag between stress and strain in cyclic loading; equals loss ÷ storage modulus.
The springy, energy-returning part of a material's stiffness under vibration.
Why this symbol: The prime marks the real, in-phase part of the complex modulus E* = E′ + iE″ — energy stored.
The gooey, energy-absorbing part of a material's response under vibration.
Why this symbol: The double-prime marks the imaginary, out-of-phase part of E* — energy lost as heat.
How many times a load can be applied and removed before a part cracks from fatigue.
Why this symbol: N for the number of load cycles to failure in fatigue.
The largest straight pull or push at a point, once you rotate to the natural axes.
Why this symbol: Principal stresses are ordered; σ1 is the largest (most tensile).
The stress at right angles to σ1 on the natural axes.
Why this symbol: The second (smaller) principal stress.
Stress that tries to slide one layer of material past the next.
Why this symbol: Greek tau — the conventional symbol for shear stress, partner to σ.
How much stronger a part is than it strictly needs to be — a margin for the unknown.
Why this symbol: Safety factor — the margin between capacity and applied load.
A ply with all fibers running the same way — very stiff along them, soft across.
Why this symbol: Unidirectional — all fibres aligned in one direction.
The table of numbers that tells how a whole laminate stretches, bends, and twists.
Why this symbol: The three laminate stiffness sub-matrices: A (extensional), B (extension-bending coupling), D (bending).
The standard math for turning a stack of plies into one predictable plate.
Why this symbol: Classical Laminate Theory — the thin-plate model that stacks plies into one plate stiffness.
A formula linking how a polymer behaves at different temperatures and speeds.
Why this symbol: Williams-Landel-Ferry — the three authors of the time-temperature shift equation (1955).
A test that wiggles a sample to measure its stiffness and damping vs. temperature.
Why this symbol: Dynamic Mechanical Analysis — measuring modulus and damping under oscillating load.