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O-ring and Seal Design Theory

Three Considerations

Incompressibility

A material is incompressible if it exhibits zero volumetric change (isochoric) under hydrostatic pressure. Theoretically, Poisson's ratio is exactly one-half (0.5) and the bulk modulus is infinite (and det f = 1).

Near incompressibility means that Poisson's ratio is slightly less than 0.5.

Viscoelasticity

Rubber exhibits a rate-dependent behavior that can be modeled as a viscoelastic material whose properties change with temperature and time. Features of viscoelastic materials are:

• Under constant stress leads to creep
• Under constant strain leads to stress relaxation
• During loading/unloading leads to hysteresis
  1. Internal friction-rearrangement of molecular structure under load.
  2. Strain-induced crystallization-formation and melting of crystallized regions.
  3. Stress softening (Mullin's effect).
  4. Structural breakdown-the breakdown of reinforcing filler/ polymer bonds.
  5. Domain deformation-dispersed inclusions contribute to hysteresis.

Thermomechanical

• Temperature change causes thermal strains.
• Material properties change. Heat flow may occur.

Next Topic Nonlinear Finite Element Analysis (FEA)

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