Lecture 01,2,3,4.pdf - Lecture 01 Mechanical Properties of...

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Lecture 01: Mechanical Properties of Metals Why do we need to know about the properties of materials? Not only do we need to know this for identification of materials but the information is crucial in determining the usefulness of a material. We would need to analyze both the application environment ( i.e. service conditions) and the properties of materials considered for application in order to find out what material to select for any specific condition. The following queries could arise should the material be strong, or stiff, or ductile? Will it be subjected to repeated application of a high force, a sudden intense force, a high stress at elevated temperature, or abrasive conditons? But before we proceed, we would first need to know what the properties mean and how they are determined. Tensile testing is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. Procedure for conducting tensile test The tensile test measures the resistance of a material to a slowly applied force. The specimen is placed in the testing machine and a force F, called the load , is applied. A strain gage or extensometer is used to measure the amount that the specimen stretches between the gage marks when the force is applied. The results of a tensile test can be tabulated and used to plot a graph. (Refer to Table 6-1 and Figure 6-2 from Askeland. D. The Science and Engineering Stress-Strain Diagram
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of Materials). This is later used to plot stress-strain diagrams. (It is important to note that the results of the test are specific to the diameter. If a larger diameter specimen is used, the force required to produce a given amount of stretching would be greater.) Ref: Askeland. D. The Science and Engineering of Materials pg 100,101 Engineering stress and strain The results of a single test apply to all sizes and shapes of specimens for a given material if we convert the force to stress and the distance between gage marks to strain. Engineering stress and engineering strain are defined by the following equations:
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Elastic vs Plastic Deformation When a force is first applied to the specimen, the bonds between the atoms are stretched and the specimen elongates. When we remove the force, the bonds return to their original length and the specimen returns to its initial size. Stretching of the metal in this elastic portion of the stress- strain curve is recoverable. At higher forces the materials behaves in a plastic manner. As the stress increases, dislocations begin to move, slip occurs, and the material begins to plastically deform. Removal of the force permits the elastic deformation to be recovered, but the deformation caused by slip is permanent. The stress at which slip begins is the dividing point between elastic and plastic behavior.
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Yield Strength The yield strength is the stress at which slip becomes noticeable and significant.
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