elastomer-design

name	elastomer-design
description	Elastomer design — rubber compound selection (NBR/EPDM/FKM/Silicone/HNBR), shore hardness, modulus-hardness correlation, Mooney-Rivlin hyperelastic model, fatigue, compression set, ASTM D412/D624.
metadata	{"priority":7,"promptSignals":{"phrases":["elastomer design","rubber design","rubber compound","hyperelastic model","Mooney-Rivlin","shore hardness rubber","rubber fatigue"],"minScore":3}}

Elastomer Design — Complete Skill

Elastomer Types and Selection

NBR (Nitrile Butadiene Rubber)

T_range: -40 to +120°C (standard); -40 to +150°C (peroxide-cured) Media resistance: petroleum oils, fuel, mineral hydraulic fluid, water Limitations: aromatic solvents (toluene), ketones, ozone, UV Hardness range: 40–90 Shore A Applications: O-rings for petroleum service, seals, gaskets, hose

HNBR (Hydrogenated NBR): T_max to 150°C; better aging; automotive transmission

EPDM (Ethylene-Propylene-Diene Monomer)

T_range: -50 to +150°C; steam service to 175°C (peroxide) Media resistance: water, steam, ketones, alcohols, phosphate ester hydraulic fluid (Skydrol) Limitations: petroleum oils, fuel — CANNOT use with hydrocarbons Applications: HVAC seals, water pipe gaskets, automotive weather-stripping, brake fluid service

FKM (Fluoroelastomer, Viton)

T_range: -20 to +200°C (standard); -40 to +200°C (specialty) Media resistance: petroleum, aromatic solvents, acids, hot water Limitations: ketones, amines, glycol brake fluid, low-temperature sealing below -20°C Applications: chemical processing, aerospace fuel seals, exhaust systems High cost: 5–15× NBR; justified for critical chemical service

FFKM (Perfluoroelastomer, Chemraz, Kalrez)

T_max = 300°C; chemical resistance to almost all media Extremely high cost; semiconductor processing, aggressive chemical service

Silicone (VMQ, PVMQ)

T_range: -60 to +200°C; special grades to -100°C (PVMQ) Media resistance: ozone, UV, oxygen, dry heat Limitations: petroleum oils, hydrocarbons (very poor); low mechanical strength (S_u ≈ 5–10 MPa) Applications: medical, food grade, electrical insulation, static seals in extreme temperature

CR (Chloroprene/Neoprene)

T_range: -35 to +100°C; ozone/weather resistant; moderate oil resistance Applications: outdoor gaskets, weather seals, bridge bearings, bearing pads

PTFE (Thermoplastic, not true elastomer)

Not viscoelastic; hardness ~65 Shore D; very low friction (μ = 0.04–0.1) Filled PTFE (glass, graphite): higher stiffness; improved wear Applications: dynamic seals (rotary), bushings, diaphragms

Mechanical Properties

Shore A Hardness → Approximate Young's Modulus

Shore A	E [MPa]
30	1.0
40	1.4
50	2.0
60	3.0
70	4.5
80	7.5
90	15–20

Gent-Thomas approximation: E ≈ 0.0981 × 10^(0.0235 × (Shore_A - 100)) [MPa; rough formula]

Shear modulus: G = E/(2(1+ν)); ν ≈ 0.499 for unfilled; G ≈ E/3

Compression Set

Test: ASTM D395 Method B; compress 25% for 24hr; release; measure recovery Compression set CS: CS = (t₀ - t₂)/(t₀ - t₁) × 100% [t₀ initial; t₁ compressed; t₂ recovered] Good seal: CS < 25%; poor: CS > 40% FKM: CS = 15–25% at 200°C; NBR: 20–35% at 100°C

Tensile Properties (ASTM D412)

Dumbbell specimen; elongation at break = 200–600% typical Tensile strength S_u = 5–25 MPa depending on compound Tear strength (ASTM D624) = 15–60 kN/m

Hyperelastic Material Models

Mooney-Rivlin (2-parameter)

Strain energy density: W = C₁₀(Ī₁ - 3) + C₀₁(Ī₂ - 3)

Ī₁, Ī₂ = first and second deviatoric invariants C₁₀, C₀₁ = material constants (from biaxial tension test data)

Engineering constants: G = 2(C₁₀ + C₀₁) [shear modulus]

Good for: moderate strains (< 100–150%); typical for seals and mounts

Neo-Hookean (1-parameter)

W = C₁₀(Ī₁ - 3) G = 2C₁₀ Simpler: valid for small strains (< 50%); no C₀₁ term

Ogden Model (N-parameter)

W = Σ μ_n/α_n × (λ₁^α_n + λ₂^α_n + λ₃^α_n - 3) More accurate for large deformations; typically N = 3 parameters fit well

Polynomial (Rivlin) Models

W = Σ C_ij(Ī₁ - 3)^i × (Ī₂ - 3)^j [i+j ≥ 1] Many variations; include Mooney-Rivlin and Neo-Hookean as special cases

Arruda-Boyce (8-chain model)

Physics-based; limited extensibility network; good for high stretch W = μ Σ C_i(Ī₁^i - 3^i) / λ_m^(2i-2) [Cr, λ_m = network parameters]

ABAQUS/ANSYS hyperelastic material input: Provide test data (uniaxial tension + equibiaxial + planar shear) → FEA fits C₁₀, C₀₁ automatically Or specify C₁₀, C₀₁ directly if known

O-Ring Design

Diametral squeeze (radial compression): Squeeze% = (W_groove - d_wire) / d_wire × 100% Static seal: 15–20% squeeze (minimum 0.6 mm) Dynamic rotary: 10–15% Dynamic reciprocating: 10–12%

Groove dimensions (AS568 O-ring dash sizing): d_wire = 1.78 mm (100-series), 2.62 mm (200-series), 3.53 mm (300-series), 5.33 mm (400-series) Groove depth = d_wire × (1 - squeeze/100) ± tolerance Groove width = 1.4 × d_wire (sufficient for squeeze + thermal expansion + swelling)

Back-up rings: for P > 35 bar (500 psi) in reciprocating service; prevent extrusion into clearance gap

Rubber Fatigue

Crack initiation: generally governed by ozone attack and stress concentration at surface defects Crack propagation: tearing energy T = dU/dA [J/m²] T_c ≈ 1000–10,000 J/m² (critical tearing energy for crack growth)

Thomas-Lake crack growth: dc/dN = A × T^m [m ≈ 2 for many elastomers; A = material constant]

Fatigue life (simple estimate): N_f ≈ 10^6 cycles at ε_a = 10%; N_f → 10^3 at ε_a = 30% (very rough guidance) FKM and HNBR significantly better than NBR at equivalent strain amplitude

Ozone cracking: perpendicular cracks on stressed elastomer surfaces exposed to ozone Threshold: as low as 0.1 ppm ozone + static or dynamic strain EPDM and CR have excellent ozone resistance; NBR poor

Thermal Aging

Arrhenius prediction: t_life(T₂) = t_life(T₁) × exp[E_a/R × (1/T₂ - 1/T₁)] E_a ≈ 0.8–1.4 eV for NBR/EPDM; 0.6–1.0 eV for FKM

ASTM D573: hot air aging; measure retention of tensile strength, elongation, hardness Acceptance: retention > 70–80% of initial for most specifications

Output

Provide: elastomer type selection (NBR/EPDM/FKM/Silicone/HNBR) with temperature and media justification, Shore A hardness, approximate E [MPa] and G [MPa], Mooney-Rivlin C₁₀ and C₀₁ estimates (or direct from supplier data), compression set CS [%] at service temperature, O-ring dash number and groove dimensions, fatigue life estimate (cycles at design strain amplitude), aging life at service temperature [hours to 50% property retention], applicable standard (ASTM D412, D395, D573, AS568B for O-rings).