Machinery's Handbook, 31st Edition
Time-Related Properties of Plastics 571 the expected time and temperature. This value E app can be used instead of E in predicting the maximum beam deflection from the traditional equation given earlier.
Unfilled 30%, glass-reinforced 20%, glass-reinforced
PBT
150
Nylon
High T g , unfilled Low T g , unfilled
PBT Nylon
100
Modified PPO Polycarbonate
Acetal
Polycarbonate Acetal Nylon 6 PBT Rigid PVC
23
Modified PPO
Polypropylene High-density polyethylene
600
800
200
400
0
Creep Modulus, 10 3 psi
Fig. 10. 1000-Hour Creep Moduli of Several Plastics at Room Temperature and Higher Manufacturers’ data often include curves of creep modulus (or log creep modulus) ver sus log time at either constant stress or constant strain derived from creep data. The bar graphs of Fig. 10 show 1000-hour creep moduli for several engineering resins at 73°F (23 ° C). This kind of information may also be provided as tables of values at constant stress and temperature for various time periods. Creep rupture data are obtained in the same manner as creep data except that higher stresses are used and time is measured to failure. Such failures may be brittle or ductile with some degree of necking. Results are generally plotted as log stress versus log time to failure. Stress relaxation occurs when plastics parts are assembled into a permanent deflected condition, as in a press fit, a bolted assembly, or some plastics springs. Under constant strain over a period of time the stress level decreases due to the same internal molecular movement that produces creep. Stress relaxation is important with such applications as bolt preloading and springs, where loading must be maintained. The relaxation can be assessed by applying a fixed strain to a sample and measuring the load over time. A relaxation modulus similar to the creep modulus can be derived from the relaxation data. Relaxation data are not as readily available as creep data, but the decrease in load due to stress relaxation can be approximated by using the creep modulus E app calculated from the creep curves. Plastics parts often fail due to imposition of excessive fixed strains over extended periods of time, for example, a plastics tube that is a press fit over a steel shaft. No relaxation rupture equivalent to creep rupture exists, so for initial design purposes, a strain limit of 20 percent of the strain at the yield point or yield strength is suggested for high-elongation plastics. For low-elongation brittle plastics that have no yield point, 20 percent of the elongation at break is also recommended. These figures should be regarded only as guidelines for development of initial design concepts; prototype parts should be thoroughly tested under end-use conditions to confirm the suitability of the design. Higher or lower property limits may also be indicated in manufacturers’ data on specific materials. Extrapolating creep and relaxation data must be done with caution. When creep and relaxation data are plotted as log property against log time, the curves are generally less pronounced, facilitating extrapolation. This procedure is common practice, particularly with creep modulus and creep rupture data. Extrapolation should not exceed one unit of
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