Seal design theory basics from PSP, large o-ring distributor

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)

O-ring and Seal Design Theory

Three Considerations