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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)…
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Download file to see previous pages 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 ...Download file to see next pagesRead More
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Lab report
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