The Stress-Strain Curve of Polymers under the Effect of Tensile Loading - Essay Example

?REPORT Experiment Aim The experiment was conducted with the objective of analyzing the stress-strain curve of polymers under the effect of tensile loading (i.e. tensile force application). The testing was done on four different specimens: PE (polyethylene – from a shopping bag), PP (polypropylene – from plastic folder), Rubber (natural rubber – from a rubber band) and Acetate (cellulose acetate – from an overhead transparency). Introduction The method of tensile testing involves testing a material specimen for failure by the application of a tensile force. The tensile force application results into the change in shape (elongation) of the specimen followed by failure i.e. rupture. The method of tensile testing is conducted by a ‘tensile testor’ in which the test specimen is clamped and loaded and subjected to a tensile force until the point of failure. ‘Stress-strain curves are an extremely important graphical measure of a material’s mechanical properties’ (Roylance, 2001). Stress is defined as the force of resistance offered against the deformation and Strain is defined as the ratio of the change in length to the original length of the member (Ramamrutham, 2003). The stress-strain curve for the specimen is an important method of characterizing the behavior of the material and adjudging its suitability as a material for any function. Almost all the materials, obey Hooke’s law in the early portion of the curve i.e. at low strain which states that stress is proportional to strain with the constant of proportionality being the Young’s modulus, E: Stress = Strain ? Young’s Modulus. As the strain increases, the linear proportionality comes to an end at a point termed as the proportional limit and marks the beginning of the plastic phase rearrangement of the specimen. Plasticity requires molecular mobility and materials lacking this mobility are usually brittle rather than ductile. “Polymeric materials behave both as viscous fluids as well as elastic solids. They are viscoelastic materials” (Koustos, 2002). The stress-strain curve of a polymer is different from those of other materials. The critical point in the stress-strain curve is the yield point beyond which the material enters the plastic deformation state. Experimentation The experiment was conducted to obtain the stress-strain curve of polymer samples and study their behavior under the application of tensile force. For this test, polymer samples were loaded and clamped in the tensile testing machine. The tensile testing machine that was used was Instron 1026. The tensile testing machine pulls the sample from both ends and measures the force required to pull the specimen apart and how much the sample stretches before breaking. The testing was done on four different specimens: PE (polyethylene – from a shopping bag), PP (polypropylene – from plastic folder), Rubber (natural rubber – from a rubber band) and Acetate (cellulose acetate – from an overhead transparency). The specimens were obtained by cutting the polymer samples into appropriate lengths. The thickness and width of the samples were measured before stretching each one of them and putting them in the tensile tester. The dimensions of the specimens are taken with the aid of calipers for precision. Results and Discussion The stress-strain curves for the various test specimens are as follows: Figure 1: Stress-Strain Curve of Rubber (Poly-Isoprene) Figure 2: Stress-Strain Curve of Acetate (Cellulose Acetate) Figure 3: Stress-Strain Curve of PE (Polyethylene) Figure 4: Stress-Strain Curve of PP (Polypropylene) S. no. Test name Tensile Strength (MPa) Elongation at failure 1 PE 11.1 26% 2 PP 23.5 738% 3 Rubber 4.1 575% 4 Acetate 1 181.9 131% 5 Acetate 2 297.3 114% 6 Acetate 3 166.9 93% It can be observed from the graphs that the stress-strain curve of acetate was obtained thrice. The possible explanation for this can be that cellulose acetate exhibits different tensile strengths at different states – dry, wet & after being boiled (Stadlinger). The difference in tensile strength is due to presence of moisture in the strand of cellulose acetate fibers which add to the cohesive strength of the fiber and makes it strong. The utility of polyethylene is augmented by the fact that it has a lower tensile strength and thus two purposes are fulfilled which is of attaining durability as well as recyclicity. (Polyethylene (PE) Plastic n.d.) Due to the low tensile strength the fabrication and manufacturing process of polyethylene is also very easy. A material like polyethylene can be used as a substitute for paper and wood as packing material. Glass being a brittle material, the stress-strain curve tensile testing is supposed to show the following results: Minimal or no elongation Very low Ultimate Tensile Strength (< 50 MPa) Failure at Ultimate Tensile Stress Absence of plastic zone in the stress-strain curve Nearly liner relation between Stress and Strain The advantage rubber has over polypropylene is the fact that rubber being a viscoelastic material exhibits the property of an elastomer i.e.it has a low Young’s Modulus and very high yield strain as can be seen from the results (575%). It means that rubber can exhibit elasticity and can change back to its original shape if the force being applied on it is removed. This property is absent in propylene which is not an elastomer. In fact propylene is a thermoplastic where as rubber is thermosetting. As can be seen from the graph, the stress-strain curve proceeds with not much change in the slope or with almost a constant Young’s Modulus (Sebrell, Park & Martin, 1925). Thus, after testing while rubber regains its shape, polypropylene has achieved plasticity beyond the yield point and can’t change back to its original shape or form. Conclusion From the results obtained from the experiment, one can say that different polymers exhibit different yield strength at different yield strains. While rubber showed less tensile strength but good elasticity, acetate had different tensile strengths for different states that it was in i.e. wet, boiled & dried. While specimens like Polyethylene failed at a stress lesser than their Ultimate Tensile Strength, there specimens that failed at their Ultimate tensile Strength. Also, the stress-strain curves obtained in each case provided us insight with the suitability of that polymer for a particular function and helped us in guessing the behavior and physical properties of other polymers of similar structure. References Koustos V, May 2002, ‘Introduction to polymers’, Lecture Notes, University of Edinburgh, Edinburgh. Ramamrutham S, 2003, Strength of Materials, Dhanpt Rai Publishing Company, New Delhi. Roylance D, August 2001, ‘Stress-Strain Curves’, Lecture Notes, Massachusetts Institute of Technology, Cambridge. Stadlinger, H, ‘Comparative Studies on Cellulose Acetate’, Charlottenburg, Germany. Sebrell, L B, Park C R, Martin Jr., S M, November 1925, The Physical Properties of Rubber,GoodYear Tire & Rubber Co., Akron, Ohio. Polyethylene (PE) Plastic, Viewed 19th November 2011, < http://www.ides.com/info/generics/27/Polyethylene-PE> Read More