THE ROLE OF THE

126 The Role of the Chemist in Automotive Design
the Poisson ratio is slightly less than 0.5, then the material is nearly incompressible
[14]. Viscoelasticity is rate-dependent behavior with properties that change with temperature
and time. The features of a viscoelastic material are as follows [14]:
•
•
•
•
creep under constant stress;
relaxation under constant strain;
hysteresis under loading and unloading cycles;
internal friction rearrangement of molecular structures under load;
• strain-induced crystallization formation and
melting of crystallized regions;.
• stress softening; and
• breakdown of the reinforcing filler and polymer
bonds.
FIgure 8.17 Automotive
seal in groove.
FIgure 8.18 Automotive
piston seal in groove.
O-rings are basically a round loop of material
that can be used as a mechanical seal or gasket. In
automotive design, typically a 30% compression is
maintained and the seal sits in some sort of groove,
as in Figure 8.17. O-rings are used to prevent the
transfer of fluid between or among two or more
components. The compressive force action on the
seal is relied upon to prevent the transfer of fluids.
The seal must be able to handle the stress acting
upon it [15]. For static seals, we typically think of
flange seals, radial or piston seals, and crush seals.
Dynamic seals are discussed more thoroughly in
Chapter 7. In these static types, there is no relative
motion between moving parts.
Figure 8.18 shows a flange type of static seal. The
figure shows a piston seal similar to the PEEK seals
discussed in transmission applications in Chapter 7, but without the rotating member.
Static crush seals are exactly that. They often employ a lip seal or quad seal type of
design, which is good for sealing against the surface in two points.
8.13 FaIlure and degradatIon In seal desIgn
Seal failures can be extremely detrimental to a vehicle system and customers’ enthusiasm
and willingness to purchase a product. A customer’s time and money are
invested in his or her purchase, and the failure of a minor component should be
avoided by a good design. Good material selection by understanding the physical
requirements and good design by understanding the seal interfaces can prevent failures.
From the end-user’s point of view, a seal can fail in three general ways: leaking,
contamination, and change in appearance [16]. All polymeric seals will degrade
within the environment that they are exposed to within their lifetime. A seal’s losses
in mechanical strength, aesthetic appearance, compression set, etc. are adversely
affected by certain chemical reactions, some of which are discussed here. Various