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A fundamental principle of viscoelastic behavior is the equiva- lence of time and temperature in determining the behavior of a material. Since all polymers are visco- elastic in nature, this principle applies to the mechanical properties of all things plastic. Unfortunately, these principles are typically addressed only in textbooks that provide a very rigorous mathematical treatment of the subject. The practical significance tends to get lost in the math; consequently, engineers who design plastic parts and specify raw materials remain unaware of this very important principle. Simply put, under load, the effects of increased tempera- ture are also observed by the effects of extended time without increasing the temperature. For example, the modulus of a poly- propylene may decrease from 1600 MPa (232 kpsi) at room temper- ature to 800 MPa (116 kpsi) at 60 C (140 F). Modulus is a measure of stiffness and is defined as stress divided by strain. If we applied a stress of 4 MPa (580 psi) to a beam of this PP at room temperature, we would instantly observe a strain of 0.25%. If we performed the same operation on the material at 60 C we would observe a higher strain of 0.50% reflected in the reduced modulus caused by the Understanding Strain-Rate Sensitivity In Polymers increase in temperature. However, if we applied the same 4 MPa stress at room temperature and then maintained that stress for a period of 10 hr, we would observe the same total strain of 0.50%. The first 0.25% would occur the instant we applied the stress and the remaining 0.25% would occur over the ensuing 10-hr period as a phenomenon we call creep. In this example, the time-temperature equivalence is that an increase of 40° C (104° F) equals 10 hr. The specifics of this equivalence vary from material to material and can even vary for a given material depending upon the temperature range of interest. Strain rate is the reciprocal of time. Longer times are related to lower strain rates while shorter times are related to higher strain rates. Therefore, lower strain rates mimic the behavior observed at higher temperatures while higher strain rates reflect the behavior at lower temperatures. Figure 1 shows an example of this principle. Tensile tests were performed on a PP material at the same tempera- ture using three different strain rates, indicated by the speed at which the crosshead of the tensile tester moved. At the slowest crosshead speed, the yield strength and the modulus of the material are at their lowest value. As the speed increases, the yield strength and modulus also Slower strain rates mimic the behavior observed at higher temperatures while higher strain rates reflect the behavior at lower temperatures. Material behavior is fundamentally determined by the equivalence of time and temperature. But that principle tends to be lost on processors and designers. Here's some guidance. Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS By Mike Sepe Effect of Strain Rate on Stress-Strain Properties of a Polypropylene 40 35 30 25 20 15 10 5 0 -5 -2 0 2 4 6 8 10 12 14 16 Strain, % Stress, MPa 500 mm/min 50 mm/min 5 mm/min FIG 1 Slower strain rates mimic the behavior observed at higher temperatures, while higher strain rates reflect the behavior at lower temperatures. 26 AUGUST 2016 Plastics Technology PTonline.com K now How MATERIALS