Plastics Strength and Modulus Machinery's Handbook, 31st Edition
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(17) This is the basis for ASTM Test D790 for the flexural modulus and strength of plastics. The flexural modulus reported is usually the initial modulus from the load/deflection curve. Most plastics parts undergo bending, so use of flexural values, which tend to be higher than tensile values for many molded plastics, should give more conservative and hence trustworthy results in product design. Rate Dependence of Mechanical Properties: Tensile and flexural data in manufacturers’ literature are measured at specific displacement rates. These rates are usually not consis tent with the loading environment encountered in use of the product. The same plastics material, under differing rates or in other environmental conditions, can produce different stress-strain curves. Designers should be aware of the loading rates in specific applications and request the appropriate data. End-use testing must always be considered, particularly when adequate data are not available. Time-Related Mechanical Properties.— Mechanical properties discussed previously were related to loads applied gradually and applied for short periods. Long-term and very short-term loading may give somewhat different results. With high-performance thermo plastics it is important to consider creep, impact, fatigue, and related issues. Even the best laboratory test methods do not always predict structural response of production parts accurately, and other factors may also affect results. Creep is defined as increasing strain over time at constant stress. The rate of creep for a given material depends on applied stress, temperature, and time. Creep behavior of a material is an important, even crucial, issue with plastics when parts are to be subjected to loads for extended periods and the maximum allowable deflection is critical. To determine creep behavior, test samples may be loaded in tension, compression, or flexure in a constant-temperature environment. Under constant loads, deflection is recorded at regular intervals over suitable periods. Results are generally obtained for four or more stress levels and recorded as creep curves of strain versus loga rithmically scaled time. In general, crystalline materials have lower creep rates than amorphous plastics. Glass reinforcement substantially improves the creep resistance. Fig. 9 shows the great differences in creep-rupture behavior that exist among different plastics at several temperatures. E ybh FL 4 3 3 =
8
A-SAN (23 deg. C) B-Epoxy MC (120 deg. C) C-30% glass-reinforced nylon (dry) (120 deg. C) D-30% glass-reinforced PBT (150 deg. C) E-Mineral-filled phenolic MC (120 deg. C) F-Acetal (80 deg. C) G-Impact polystyrene (23 deg. C) H-Alkyd (120 deg. C)
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Fig. 9. Plots Showing Diminishing Strengths of Several Plastics as Creep Period is Extended (Curves are Steeper at Higher Temperatures)
Apparent or Creep Modulus: If the deflection of a part subjected to continuous loading is calculated using the short-test modulus of elasticity E, results are likely to be disappointing or even disastrous because the effects of creep have not been considered. If the stress level and temperature are known and creep curves are available for the temperature in question, an apparent, or creep, modulus E app can be calculated from the creep curves by the formula: E app = σ / ε c where s is the calculated stress level and ε c is the strain from the creep curve at
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