LAB Report - Tensile strength lab comparing bras, steel and aluminum PDF

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Tensile strength lab comparing bras, steel and aluminum...

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Abstract: This experiment was conducted so as compare the mechanical properties of aluminum, brass and steel. The basics on the operation of universal testing machine were also learnt during this experiment. The Universal Testing Machine can be used to determine the tensile strengths of many engineering materials. The design of many engineering structures is based on the tensile properties of the materials used. The stress- strain relationship of various metals can be used to predict the characteristics of materials when subjected to different types of loadings. From this experiment, it can be seen that mild steel have higher tensile and yield strength than aluminum and brass. This explains the wide applications of mild steel in many constructions and other engineering applications that require high strength.

Introduction: For safe design of structural components in bridges, railway lines, marines’ ships, aircrafts, pressure vessels etc, the tensile properties of materials used should be analyzed. Hence the tensile strength of the materials should meet the strength requirements of the structural applications. The mechanical properties of the metals determine the kind of engineering application to be used for. Experiments on tensile tests can be used to predict the tensile properties and they are conducted by application of axial or longitudinal forces to a specimen with known dimensions. These forces are

applied on the specimen until deformation causes failure. The tensile load and corresponding extensions are then recorded for calculations and determination of stress-strain relationship of the material specimen. The tensile test experiment can be used to determine other mechanical characteristics of the specimen like yield strength, percentage elongation, and ultimate strength, among others. The original gauge length Lo, diameter Do or cross-sectional area also used in calculations hence should be recorded.

Procedure: To study the deformation and fracture characteristics of steel, brass, and aluminum when they are subjected to uniaxial loading. To observe the load extension and stress – strain relationships in aluminum, brass, and steel. Tensile loading on material causes the material to undergo deformations. The kind of deformation can either be elastic or plastic deformation. The elastic deformation is characterized by linear relationship between the extension and applied load. Engineering stress σ is given by the ratio of load applied to the original cross-sectional area, while engineering strain ε is given by change in length (extension) ∆ L over the original length L

σ= P A o

and σ= P A o

and σ =P / Ao and ε =∆ L / Lo Where, σ is engineering stress, P is the applied axial load, Ao, is the original cross sectional area, ε is the engineering strain, ∆ L is the extension, Lo is the original length. The engineering stress- strain relationship for elastic deformation is based on Hooke’s law. The gradient on this curve gives a modulus of elasticity called The Young’s Modulus E.E=σε Where: E is Youngs modulus is engineering stress and ε is the engineering strain. In engineering applications of materials/ metals that are subjected to deflections, Young’s modulus is of critical importance.

Discussion:

The changes encountered in cross sectional area cannot be influenced by engineering stress- strain relationships; the changes can only be possible for true stress- strain curves. Normally, true strains are of higher values than those of engineering strains. This can be explained by the fact that true strains take place in transverse directions of the gage length. High values of stress and strains in steel are attributed to strain hardening. Strain hardening or work hardening in steel occurs at higher values of stress than aluminum and aluminum is higher than brass. Engineering stress and strains were calculated after the extensometers on the Instron machine measured the strain that was applied on each sample specimen. The data on strain was obtained on the cross head after necking had occurred. The engineering stress was then calculated by dividing the applied load by the original cross- sectional area. For engineering strains, the changes in length were divided by the original length. In calculations of true stress, the load applied could be divided by the instantaneous area. True strain is calculated by dividing the change in length by the instantaneous final length.

Conclusion: Many engineering applications that require high tensile strength normally use steel than aluminum and brass. This is because of the crystalline structure of steel that allows it to withstand high axial loads before fracture can occur. Aluminum however has found many uses in designs that require low density materials like in aerodynamics and some motor vehicles. Aluminum experiences high ductility rates compared to steel and

have therefore low-level values of Young’s Modulus, a factor that determines deflections in structural components. This experiment therefore gives close relationship of tensile strength to the theoretical data....