strain_new - University of Texas at Arlington MAE 3183...

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University of Texas at Arlington MAE 3183, Measurements II Laboratory Strain Measurement 1 Experiment #6 Strain Measurement
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University of Texas at Arlington MAE 3183, Measurements II Laboratory Strain Measurement 2 Introduction A short lived engineer would be one that never considered stress or strain. Stress is present in all structures either static or dynamic, at least on earth anyway. Stress is typically a key element that dictates every design. Strain is the resulting deformation that a structure experiences under a given stress. For most materials considered in mainstream engineering, stress is directly proportional to strain. Within the design process a preliminary survey produces likely candidates for use as materials and the expected stress-strain quantities are calculated. At this point two key problems exist. These problems are (1) How is the stress calculated for complex shapes such as curved areas? And (2) How are the strains measured once the structure has been built? To attack the first problem, analytical solutions are available for almost every type of structure imaginable. These equations produce equations normal or shear stress as a function of loading and geometry. The equations are based entirely upon geometric quantities and don't depend on empirical data for a solution. In this experiment, we challenge those equations. Given various structures, loads are applied and measurements are made that will either coincide or diverge from values produced using theoretically derived equations. Now all that's left to find is a means to measure strain. Most people, with the aid of a ruler, can measure 1/32 of an inch within +/- 1/64. That's fine for extremely high loadings or extremely weak materials, but many materials might fail at say 3500 μ strain, which would be well below 1/20 of the resolution of a ruler. To solve this problem several instruments can be used, one of which is the strain gage/wheatstone bridge. This instrument combination allows measurements, in some cases, as low as +/- 1 μ strain. With resolution this good the question arises "Is this minute quantity significantly useful in most applications?" Typically, this resolution is not needed and the closest desired resolution may be +/- 5 μ strain. In this experiment the student loads actual structures then examines the strains produced by the loading. Later, the student calculates the strains expected by geometrically derived equations and compares the difference in the two values. In the end, the student can provide evidence if the equations can accurately predict strains present in a structure and also if the strain gage/strain box combination is useful in the measurement of these strains. Theory Foil strain gages are essential elements in the measurement of small displacements of materials. Strain gages are constructed from a thin plastic film coated with a copper film layer. The copper film layer is etched away in a certain pattern to give the strain gage lines of conductive material. The direction of these conductive lines is the direction in which the strain is to be measured.
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This note was uploaded on 06/15/2009 for the course MAE 3183 taught by Professor Staff during the Spring '08 term at UT Arlington.

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strain_new - University of Texas at Arlington MAE 3183...

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