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Unformatted text preview: Chapter 6 Notes Mechanical Properties of Metals Introduction- Important mechanical properties are strength, hardness, ductility, and stiffness- Mechanical properties of materials are ascertained by performing carefully designed laboratory experiments- The role of structural engineers is to determine stresses and stress distributions within members that are subjected to well-defined loads. This may be accomplished by experimental testing techniques and/or theoretical and mathematical stress analyses. Concepts of Stress and Strain- There are three ways in which a load may be applied: tension, compression, and shear- One of the most common mechanical stress-strain tests is performed in tension- The tension test can be used to ascertain several mechanical properties of materials that are important in design. - A specimen is deformed with gradually increasing tensile load that is applied uniaxially along the long axis of a specimen - The output of a tensile test is recorded as load or force vs. elongation- To minimize these geometrical factors, load, and elongation are normalized to the respective parameters of engineering stress and engineering strain.- Engineering stress, = F / A - The units of engineering stress are MPa- Engineering strain, = l i l / l i - L is the original length before any load- Compression stress/strain tests may be conducted if in-service forces are of this type- The specimen contracts along the direction of the stress- Tensile tests are more common because they are easier to perform- Compressive tests are used when a materials behavior under large and permanent strains is desired, as in manufacturing applications, or when the material is brittle in tension- Shear strain = tan - Shear stress t = F/A- GEOMETRIC CONSIDERATIONS OF STRESS STATE (need to know this?) Stress-Strain Behavior- The degree to which a structure deforms or strains depends on the magnitude of an imposed stress- For most metals that are stressed in tension and at relatively low levels, stress and strain are proportional to each other through = E - Modulus of elasticity = E- Deformation in which stress and strain are proportional is called elastic deformation, a plot of stress vs. strain results in a linear relationship- Modulus = stiffness or a materials resistance to elastic deformation- Elastic deformation is nonpermanent, which means it returns to its original shape- On an atomic scale, a macroscopic elastic strain is manifested as small changes in the interatomic spacing and stretching of interatomic bonds - The magnitude of the modulus of elasticity is a measure of the resistance to separation...
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- Spring '08