The mechanical properties of polymers are specified with many of the same parameters that are used for metals which are, modulus of elasticity, and yield and tensile strengths. For many polymeric materials, the simple stress–strain test is employed for the characterization of some of these mechanical parameters. These properties of polymers are the features that distinguish them from small molecules and give us information about how a polymer can be used. In this article, the tensile properties of polymers have been briefly investigated. Keywords: mechanical properties, tensile test, stress-strain curve.
Most thermoplastics (molten and solid) exhibit a non-Newtonian and viscoelastic behavior. The behavior is non-Newtonian (i.e., the stress and strain are not linearly related for most parts of the stress-strain curve). The viscoelastic behavior means when an external force is applied to a thermoplastic polymer, both elastic and plastic (or viscous) deformation occurs. The mechanical behavior and deformation (strain) are closely related to how the polymer chains move relative to one another under load (stress). The most basic test to determine the mechanical properties of a material is the tensile test. In the tensile test, materials are stretched uniformly. Since force acts over a cross-sectional area, stress defines as the force per unit area.
Where σ is the stress, F is the applied force, and A is the cross-sectional area. Similarly, strain defines as the percent change in length. The total displacement is the change in length, so the strain is the change in length divided by the original length.
where Ɛ is the strain, ∆ L is the change in length, and L is the length. In general, when a sample is subjected to tension, its length increases, and its cross-sectional area decreases. But most engineers would prefer to ignore the changing sample size. To do this, they simply divide by the sample’s original dimensions instead of the constantly changing dimensions. The stress and strain are affected by changing temperatures or long times (creep), and also, they will change if you repeat the stress many times (fatigue). Tensile properties are evaluated by placing a dumbbell-shaped specimen between two clamps of a tensile apparatus that move away from each other at a constant speed using hydraulic or mechanical force until the specimen breaks. In this case, the applied force (stress) as well as the elongation (deformation or strain) induced in the sample are measured and since the cross-sectional area of the polymer sample is considered constant, the stress-strain curve can be drawn (Fig. 1). The stress-strain curve can provide information about a material’s strength, toughness, stiffness, ductility, etc.
As previously mentioned, the mechanical characteristics of polymers are much more sensitive to temperature changes. Fig. 2 shows the stress–strain behavior for poly (methyl methacrylate) (Plexiglas) at several temperatures between 4 and 60 °C.
Also, decreasing the rate of deformation has the same influence on the stress–strain characteristics as increasing the temperature; that is, the material becomes softer and more ductile.
The polymer’s stress-strain curves consist of different regions, which are explained below. Fig. 3 shows different regions of the stress-strain curve of a polymer sample under tensile test:
These regions include:
When the material is subjected to stress for the first time, it shows elastic behavior. Elastic behavior means that while the material is deformed under stress, it returns to its initial state after removing the stress. This phenomenon occurs because of the stretching of the atomic bonds and their return to their initial state after removing the stress. Therefore, elastic behavior, even in small amounts, occurs in all materials. Young’s modulus is obtained through the slope of the elastic region.
The local maximum in the stress-strain curve is called the yield point and indicates the permanent deformation. The Corresponding stress and elongation are called yield strength and elongation at yield. Beyond the yield point, the material stretches out considerably and this region is called the plastic region. Further elongation leads to strain hardening. In this area, at first, the polymer chains are out of their elastic state (yielding) and then the sample undergoes transverse contraction and longitudinal stretching and takes a neck shape. The necking phenomenon proceeds until the chain’s deformation is aligned. In the plastic region, due to the chain orientation in the tension direction, the strength increases. Finally, the necking phenomenon spreads along the sample and causes the sample ruptures. Fig. 4 (a-e) shows the mechanism of polymer chains in the tensile test and the phenomenon of strain hardening for a semi-crystalline polymer sample:
Ultimate tensile strength (UTS) is the highest engineering stress that a material can endure. In an engineering stress-strain curve, this is the maximum point. It marks the point where necking overtakes strain hardening. The stress-strain behavior of a polymeric material depends on the molecular characteristics and the microstructure of the polymers. Polymers can have many different stress-strain behaviors, especially because certain polymer interactions can change at different stress levels. Fig. 5, indicates the types of stress/strain relationships obtained for different groups of polymers.
From the above image, we deduce that rigid materials such as brittle polymers have high Young’s modulus, but at the same time will undergo fracture when subject to stress and associated elastic compression much earlier (in terms of total elongation strain) than the other classes of polymer materials. Polystyrene (PS) is one of these polymers. At the other extreme, we find the mechanical behavior of a highly elastic and rubbery class of polymers known as elastomers, that have both low values of the Young’s modulus and are capable of enduring large amounts of recoverable stretching, before ultimately breaking. The second curve is related to semi-ductile materials such as polymethyl methacrylate (PMMA) and finally, the third curve is related to ductile materials whose modulus is lower than rigid and semi-ductile materials, comparing the area of these groups with other groups, their toughness is higher, and they withstand more energy until they reach the rupture point. Polycarbonate is one of these materials.
The tensile property is one of the most important mechanical properties of polymers. Studying the mechanical properties is one of the most important parts of the manufacturing of polymer products, and before other properties such as thermal, optical, and rheological properties, it provides the manufacturer with the possibility of comparing materials and controlling their quality to improve the properties of the manufactured product. The tensile tests are used to determine the mechanical properties of a polymeric material. These characteristics show the behavior of polymers under tension. The tensile test is one of the destructive tests that the tested specimen is subjected to tensile force until the rupture point, and meanwhile, the increase in length is recorded simultaneously with the applied force The results of this test are used to control quality and predict how a material will react under the influence of forces. The results are in the form of a stress-strain curve, which characterizes the behavior of the material under tension.
By: Samin Saleki
Edition by: Marzieh Shams Harandi
1.https://insights.globalspec.com/article/7810/how-to-perform-tensile-testing-on-polymers 2.https://msestudent.com/stress-strain-and-the-stress-strain-curve/ 3.https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_(Materials_Science)/Polymer_Chemistry/Polymer_Chemistry%3A_Mechanical_Properties 4. https://polymerinnovationblog.com/characterization-thermosets-part-21-tensile-testing-polymers-molecular-interpretation