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Unformatted text preview: ME 222: Deformation of Materials Objective The exercises in laboratory will demonstrate the fundamental deformation behavior of materials and the effects of geometry on deformation. Experiments will be conducted on various samples that are cut from rubber sheet. The deformation of these rubber test samples will be measured using precision measuring devices. The specific objectives of this laboratory are: 1. To observe deformation of materials under load for samples having various shapes, 2. To introduce the concepts of stress, strain, Youngs modulus Poissons ratio, and 3. To practice properly citing graphs and tables in written documents. Background Most of you are presently taking Statics and Dynamics. In that course you have learned to determine the forces in engineering structures and components using the concept of equilibrium combined with vector mathematics. It is assumed in this course that the components of the structure or mechanism are rigid, i.e. do not change shape. However, we are all aware that all structures under load do deform or change shape. Deformation of materials will be addressed in great depth in Mechanics of Materials, but at this point it is valuable to demonstrate the deformation behavior of materials and to give you the chance to develop some understanding of this important material behavior. In reality, all engineering structures and mechanisms deform. In most cases, this deformation is so small that it cannot be seen visually. There are, however, many structures and mechanisms where deformation is easily observed. Some examples are airplane wings deflecting due to wind loading, the mast of a sailboat bending, the shape change in tires as they roll, and the stretching of membranes (balloons) as pressures are applied. Interestingly, the deformation in a body (structure or mechanism) changes with position. The deformation from location to location depends on the geometry of the body and the location of applied loads, moments, and pressures. The experiments conducted in this laboratory explore the deformation characteristics of some simple geometries under uniaxial (one dimensional) loading. Stress, Strain, and Youngs Modulus Suppose I pull 1 on an elastic cord, like a rubber band, with some load and observe how much the cord stretches. If I pull harder the cord stretches more 2 . In addition to how hard I pull on the cord, this stretching is a function of the length of the cord: given two cords of the same material, cross sectional area, and applied load, the longer cord will stretch more than the shorter one. The elongation of each will be proportional to the lengths of the two cords. However, I can use the initial length of each cord to normalize the deflection and obtain a useful engineering concept, the normal strain , , given by Equation 1. Now although the two cords in this example stretch differently because of their differing lengths, they will show the same strain for the same load. Note that this quantity is...
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- Fall '07