# Unidirectional Lamina Micromechanics > Use when you must predict the engineering constants of a unidirectional composite lamina from the fiber and matrix constituent properties and the fiber volume fraction: compute the longitudinal modulus E1 and the major Poisson ratio nu12 by the rule of mixtures, the transverse modulus E2 by the Halpin-Tsai closed form with the xi equal to 2 shape factor on the fiber transverse modulus, the in-plane shear modulus G12 by the Halpin-Tsai closed form with the xi equal to 1 shape factor on the fiber shear modulus, and the density by the rule of mixtures. Produces the lamina engineering constants in the material axes and the Voigt-Reuss and Hashin-Shtrikman bound bands on E2 and G12 as the verification envelope predictions fall inside. Constituent properties are inputs; no property tables are reproduced. Trigger: unidirectional lamina micromechanics, fiber volume fraction, rule of mixtures, halpin tsai transverse and shear moduli, hashin shtrikman bounds. - Skill: `ashfordeou/unidirectional-lamina-micromechanics` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/unidirectional-lamina-micromechanics` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/unidirectional-lamina-micromechanics/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/unidirectional-lamina-micromechanics --- # Unidirectional Lamina Micromechanics (structures/composites/unidirectional-lamina-micromechanics) Use when the task is predicting the four lamina engineering constants, E1, nu12, E2 and G12, of a unidirectional composite ply from the fiber and matrix constituent properties and the fiber volume fraction, the producer side of the lamina-constant chain that structures/composites/laminate-stiffness takes as a given input at its workflow step 1. This leaf implements the rule of mixtures for the longitudinal modulus, major Poisson ratio and density, and the Halpin-Tsai closed form for the transverse modulus and in-plane shear modulus, with the Voigt-Reuss and Hashin-Shtrikman bound bands as the verification envelope. Pure Python, stdlib only. It pairs with structures/composites/laminate-stiffness for the downstream ply stiffness and laminate A matrix, and with structures/materials/material-selection for constituent property context. ## Domain quick reference - Rule of mixtures (exact for the composite cylinder assemblage): E1 = V_f E_f + V_m E_m and nu12 = V_f nu_f + V_m nu_m, with V_m = 1 - V_f. The fiber axial modulus E_f and major Poisson ratio nu_f enter the longitudinal arm directly. - Matrix shear modulus (isotropic matrix): G_m = E_m / (2 (1 + nu_m)). - Halpin-Tsai shared form: eta = (Mr - 1) / (Mr + xi) with Mr the constituent property ratio; P / P_m = (1 + xi eta V_f) / (1 - eta V_f). - Transverse modulus E2: the Halpin-Tsai closed form against the fiber TRANSVERSE modulus ratio E_fT / E_m with the circular-fiber shape factor xi_E2 = 2.0. A carbon fiber is transversely isotropic; using the fiber axial modulus in this arm overpredicts E2 by roughly a factor of two. - In-plane shear modulus G12: the Halpin-Tsai closed form against G_f / G_m with the circular-fiber shape factor xi_G12 = 1.0, which equals exactly the composite-cylinder-assemblage longitudinal shear solution. - Density: rho_c = V_f rho_f + V_m rho_m, when both constituent densities are given. - Voigt/Reuss band: E2_Voigt = V_f E_fT + V_m E_m (iso-strain upper bound) and E2_Reuss = 1 / (V_f / E_fT + V_m / E_m) (iso-stress lower bound); the same two forms with G_f and G_m give the G12 band. - Hashin-Shtrikman G12 band (anti-plane, circular-cylinder problem): the lower endpoint equals the xi_G12 = 1 Halpin-Tsai value exactly. - Hashin-Shtrikman E2 band (in-plane, Hashin 1965 plane-strain bulk modulus k* and transverse shear modulus m* bounds): converted through the exact transverse-isotropy identity E2 = 4 k m E1 / (E1 (k + m) + 4 k m nu12^2) at the exact CCA E1 and nu12. - Module constants XI_E2 = 2.0 and XI_G12 = 1.0 are the standard circular-fiber Halpin-Tsai shape factors (Halpin and Tsai, AFML-TR-67- 423, 1969); the model is closed-form arithmetic in SI units (Pa, kg/m^3) on the unidirectional lamina in its material axes. ## Workflow 1. Collect the fiber constituent properties (axial modulus E_f, major Poisson ratio nu_f, transverse modulus E_fT, longitudinal shear modulus G_f), the matrix constituent properties (modulus E_m, Poisson ratio nu_m) and the fiber volume fraction V_f. 2. Derive the matrix shear modulus with shear_modulus_isotropic(E_m, nu_m). 3. Predict the longitudinal modulus E1 and major Poisson ratio nu12 by the rule of mixtures with e1_longitudinal and nu12_major. 4. Predict the transverse modulus E2 with e2_halpin_tsai, using the fiber transverse modulus E_fT and the xi_E2 = 2 shape factor, never the fiber axial modulus. 5. Predict the in-plane shear modulus G12 with g12_halpin_tsai, using the fiber shear modulus G_f and the xi_G12 = 1 shape factor. 6. Compute the density with rho_composite when both constituent densities are given. 7. Verify E2 against the Voigt-Reuss bound band (e2_voigt_upper, e2_reuss_lower) and the tighter Hashin-Shtrikman bound band (e2_hashin_shtrikman_bounds). 8. Verify G12 against the Voigt-Reuss bound band (g12_voigt_upper, g12_reuss_lower) and the Hashin-Shtrikman bound band (g12_hashin_shtrikman_bounds), noting the xi_G12 = 1 identity with the lower endpoint. 9. Run the one-shot report unidirectional_lamina_constants for the full engineering-constants and bound-band dict. 10. Confirm the deterministic checks with the contract test scripts/test_unidirectional_lamina_micromechanics.py. ## Worked example A T300 carbon / epoxy 5208-class unidirectional ply at V_f = 0.60. Constituent inputs: E_f = 230.0 GPa, nu_f = 0.20, E_fT = 20.0 GPa, G_f = 27.0 GPa (fiber); E_m = 3.5 GPa, nu_m = 0.35 (epoxy matrix); rho_f = 1760.0 kg/m^3, rho_m = 1230.0 kg/m^3. - Matrix shear modulus: G_m = 3.5 / (2 (1 + 0.35)) = 1.296296 GPa. - Rule of mixtures: E1 = 0.60 * 230 + 0.40 * 3.5 = 139.400000 GPa; nu12 = 0.60 * 0.20 + 0.40 * 0.35 = 0.260000. - Halpin-Tsai E2: eta = (20 / 3.5 - 1) / (20 / 3.5 + 2) = 0.611111, and E2 = 3.5 * (1 + 2 * 0.611111 * 0.60) / (1 - 0.611111 * 0.60) = 9.578947 GPa. - Halpin-Tsai G12: G12 = 4.402037 GPa, exactly the composite-cylinder- assemblage longitudinal shear solution. - Density: rho = 0.60 * 1760 + 0.40 * 1230 = 1548.000000 kg/m^3. - E2 band (GPa): Reuss 6.930693 < Hashin-Shtrikman 8.821709 < 9.578947 < Hashin-Shtrikman 10.939799 < Voigt 13.400000, with the magnitude gate E1 in [135, 145] GPa, E2 in [8, 12] GPa and G12 in [4, 6] GPa (Daniel and Ishai textbook T300/5208 values). - G12 band (GPa): Reuss 3.023033 < 4.402037 = Hashin-Shtrikman lower 4.402037 < Hashin-Shtrikman upper 12.608295 < Voigt 16.718519, the Halpin-Tsai prediction attaining the Hashin-Shtrikman lower endpoint exactly. - Degeneracies: at V_f = 0 the E2 band closes to E_m (3.500000 GPa) exactly; at V_f = 1 it closes to 20.758123 GPa, 3.8 percent above E_fT because of the cross-section idealization in the k* and m* bounds, while the Halpin-Tsai E2 limit at V_f = 1 is exactly E_fT. ## Verification - Confirm e1_longitudinal(230e9, 3.5e9, 0.6) is within 1e-6 relative of 139.4e9 and nu12_major(0.2, 0.35, 0.6) is within 1e-9 of 0.26. - Confirm e2_halpin_tsai(20e9, 3.5e9, 0.6) is within 1e-6 relative of 9578947368.421053 and g12_halpin_tsai(27e9, 1296296296.2962961, 0.6) is within 1e-6 relative of 4402037250.138515. - Confirm the nested-band ordering at V_f = 0.6: E2 Reuss < E2 Hashin-Shtrikman lower < E2 Halpin-Tsai < E2 Hashin-Shtrikman upper < E2 Voigt, and the analogous G12 ordering with the Halpin-Tsai and Hashin-Shtrikman lower endpoint equal. - Confirm the G12 Halpin-Tsai and Hashin-Shtrikman lower-bound identity holds within 1e-9 relative at V_f in {0.3, 0.5, 0.6, 0.7}. - Confirm the V_f = 0 and V_f = 1 degeneracies and the E2-in-band sweep over V_f in [0.3, 0.9]. - Confirm every non-positive or boolean modulus, density and shape factor, every Poisson ratio outside [0, 0.5), every fiber volume fraction outside [0, 1], and a fiber that is not the stiffer shear or cross-section phase raises ValueError. - Run the contract test offline: python3 scripts/test_unidirectional_lamina_micromechanics.py (41 tests, deterministic). ## Related leaves - structures/composites/laminate-stiffness: consumes E1, E2, nu12 and G12 as workflow step 1 inputs to build the ply stiffness, rotate it and assemble the laminate A matrix; this leaf produces those four constants, it never assembles stiffness matrices. - structures/composites/failure-criteria: consumes lamina allowables and strength values, not the elastic constants this leaf predicts. - structures/composites/laminate-hygrothermal-response: owns the moisture content, CTE and CME vein from ply-level properties; this leaf stops at density. - structures/materials/material-selection: representative constituent property bands for reference, verified against MMPDS, AMS or CMH-17 before design use; this leaf never looks up or reproduces those bands. ## Pitfalls - Using the fiber axial modulus E_f in the transverse Halpin-Tsai arm instead of the fiber transverse modulus E_fT: a carbon fiber is transversely isotropic, and this substitution overpredicts E2 by roughly a factor of two, the classic bug class here. - Reading the Voigt/Reuss band as the tight envelope: the Hashin-Shtrikman band is always tighter, with the Reuss bound at or below the Hashin-Shtrikman lower bound and the Voigt bound at or above the Hashin-Shtrikman upper bound (6.930693 GPa Reuss against 8.821709 GPa Hashin-Shtrikman lower at the worked V_f = 0.6). - Expecting the E2 Hashin-Shtrikman band to close exactly to E_fT at V_f = 1: the in-plane cross-section idealization used to keep the band closed-form gives 20.758123 GPa, 3.8 percent above the 20.0 GPa E_fT value that the Halpin-Tsai estimator itself reaches exactly. - Trusting the xi_E2 = 2 estimator below roughly V_f = 0.25: the fixed semi-empirical shape factor can dip slightly under the variational Hashin-Shtrikman lower bound there (Wall 1997); the verification sweep in this leaf deliberately starts at V_f = 0.3. - Passing engineering constants E1, E2, nu12, G12 into this leaf as inputs: they are the OUTPUTS of the constituent-level prediction here, never collected inputs; structures/composites/laminate-stiffness is the leaf that takes them as given. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_unidirectional_lamina_micromechanics.py The test covers the T300/5208 worked example (E1, nu12, E2, G12 and density within tolerance of the real module outputs and inside the magnitude gates), the matrix shear derivation, the nested Voigt-Reuss and Hashin-Shtrikman band ordering for both E2 and G12 at the worked volume fraction, the G12 Halpin-Tsai and Hashin-Shtrikman lower-bound identity across volume fractions, the V_f = 0 and V_f = 1 degeneracies, the E2-in-band sweep over V_f in [0.3, 0.9], the isotropic-fiber collapse, the one-shot report keys and determinism, and ValueError rejection of every non-physical or boolean input and of a fiber that is not the stiffer shear or cross-section phase. ## Compliance - Standards referenced, not reproduced: CMH-17 (Composite Materials Handbook, SAE International) frames the constituent and lamina property data conventions this prediction feeds, reference-only per standards-map.yaml; no CMH-17 table or design value is reproduced. - compliance: STANDARDS-REF, gated: false.