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Plastics Engineering 467

FIGURE 2 Typical creep and recovery behavior of a plastic.

the five-year modulus should be used in the calculations, have an inherent energy-absorbing mechanism, but the
not the short-term modulus quoted in Table I. This design long-term fractures occur without warning and sometimes
approach usually results in a considerable over-design at with catastrophic results. Figure 4 shows the type of de-
short time scales, but this can be accommodated in any sign chart that should be used for this type of situation.
safety factors used. Figure 3 shows the type of creep mod- Note in particular the “knee” in the curve, which should
ulus vs. time design graph that can be obtained for plastics. alert one to the danger of extrapolating short-term data if
that is all that is available.
From the foregoing it is clear that creep can result in Another aspect of viscoelasticity that the designer must
unacceptably high deformations of a product if it is not be aware of is relaxation. If a plastic product is in a sit-
designed correctly. In some cases, creep fracture may oc- uation where the strain is not permitted to increase (or
cur. Thus, although a plastic product is able to withstand decrease), the stress necessary to maintain this strain will
an applied stress at short times, it may fracture under the decrease with time. Although it might appear that this re-
same stress at long times. A situation where this can occur laxation of stress is not a concern, it can lead to problems
is in gas- or water-pressurized plastic pipes. The problem in some situations—for example, in cases where the stress
is also made worse by the fact that, whereas short-term level must be maintained to prevent leakage of a fluid.
creep-fracture failures are normally ductile, the long-term As plastics are used in more and more demanding engi-
failures are often brittle. Thus, the short-term fractures neering applications, it is inevitable that they will be sub-
provide a visual warning that failure is imminent and they jected to fluctuating stresses as well as to static stresses.

2
FIGURE 3 Creep modulus for plastics. Modulus values in MN/m .