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Tensile Testing of Material

Tensile Testing of Material - Tensile Testing of Materials...

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Tensile Testing of Materials Jenna Puckett North Carolina State University Department of Materials Science and Engineering MSE 210 Lab Report Section 202 Zach Lampert 1/20/08 Abstract Tensile testing is common materials practice that gives insight into a material’s properties. This experiment performed tensile tests on cold rolled 1018 steel, annealed 1018 steel, and low density polyethylene. The LPDE polymer was very ductile (441 % elongation), while the cold rolled and annealed steels were less ductile with 21.8 and 33.1 % elongation. The modulus of elasticity for the steel (1.78e5 and 2.88e7 MPa) was much greater than that of the polymer (173 MPa). Metallic bonding in metals is much stronger than the secondary Van der Waals bonding in polymers; this bond strength resists elastic deformation and increases the steel’s elastic modulus. Strength in steel was much greater than the strength of the LDPE. The polymer exhibited a fibrous break, while the cold rolled steel showed a cup and cone break. Both failures were ductile failures.
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Introduction Tensile testing is a widely used process to determine the strength and ductility of materials. Knowledge of these properties is essential to the application of materials science to real world problems. This experiment investigates the information about materials properties derived from tensile tests. In a tensile test, tensile load is applied to a sample, and the subsequent elongation is measured as a function of the load. A common tensile test machine will separate its cross heads at a constant rate and then measure the load required to do so. A stress-strain curve is created to analyze the data. Engineering stress is defined as load divided by original cross sectional area. Engineering strain is a measure of elongation; it is the change in gauge length divided by the original gauge length. Figure 1, below, shows a typical stress-strain curve. Figure 1: Typical Stress-Strain Curve In the linear section of the stress-strain curve, the dependence of strain on stress is given through Hooke’s Law, σ = E *ε . The proportionality constant relating stress and strain is known as the elastic modulus. During this part of the tensile test, the sample exhibits elastic behavior. Atomic bonds are stretched but not broken. Once the load is removed, the atomic bonds relax to their original position. Physical properties of the materials are not permanently altered. The elastic modulus is a material dependent constant; materials with stronger atomic bonding will have a higher elastic modulus. The material behaves elastically up to the yield point (shown in Figure 1). After the yield point, the material experiences permanent, plastic deformation. Atomic bonds are broken and reformed, uniformly elongating the material. With experimental data, the specific
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yield point is difficult to determine. Instead, a line with the same slope as the elastic
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