Balsawood Structure Design Essay Research Paper 1 — страница 2

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material failed at the spaces with the smallest cross-sectional areas, where imprecisions in cutting took place or the material was simply weaker. It took many tests to get breaks that occurred in the center section instead of at the ends, perhaps with an even smaller center section this would have been easier. It should also be noted that two different batches of balsa were tested and there was a notable discrepancy between the results. Table 1: Tension Tests Results Specimen # Strength (psi) 1 1154 2 1316 3 1830 4 1889 Specimens 3 and 4 were from a different batch of balsa and were thicker pieces in general, although thickness should have had no effect on maximum stress, it is assumed that the second batch simply has a greater density than the first one, or perhaps that it had

not been affected by air humidity as much as the first batch. (See the design concepts section for more discussion of moisture content in the specimens.) Compression Compression testing was also performed parallel to the wood?s grain (See Figure 2). The specimen used must be small enough to fail under compression instead of buckling. For analysis of compression tests, failure was defined as occurring when little or no change in load caused sudden deformations. This occurs when the yield strength is reached and plastic behavior starts. Figure 2: Compression Testing Setup Failure was taken at the yield strength because the material is no longer behaving elastically at this point and may be expanding outside of the design constraints. It should be noted that original specimens

proved to be too tall and they failed in buckling (they sheared to one side), instead of failing under simple compression. Table 2: Compression Test Results Specimen # Strength (psi) 1 464 2 380 3 397 Average 414 Under tension, the pieces all had similar strength values. This took many tests, but in every other test, the material exhibited buckling as well as compression. The three tests which ran the best were used for Table 2. Since the test of the design will be under compression, this data is very relevant for the final design. Apparently balsa can withstand approximately 3 times more load under tension than under compression. However, much like in these test, buckling is likely to occur in the final design. This fact should be of utmost consideration when designing the legs

of the structure. Three Point Bending This test is performed by placing the specimen between two supports, and applying a load in the opposite direction of the supports, thus creating shear stress throughout the member. Much like the tension test, the wood will deform and then break at a critical stress. Figure 3 shows how this test was setup. The data obtained form this test can be used in design of the top beam in the final design. This part of the structure will undergo a similar bending due to the load from the loading cap. Unfortunately, the data obtained from these tests was not conclusive of much. The test was flawed due to a bolt which stuck out and restricted the material?s bending behavior in each test. The two sets of data taken for this test varied greatly (as much as

300%), and therefore this data is likely to be very error prone. Figure 3: Three Point Bending Specimen Table 3: Bending Data Specimen # Rupture Load (lb) Elastic Modulus (lb/in) 1 26.6 120,000 2 62.5 442,000 Included in the Appendix is a graph of load versus displacement for the first test, it shows how the experiment was flawed at the end when the material hit the bolt which was sticking out of the machine, thus causing stress again. It also shows the slope from which the elastic modulus of the material was taken. Ideally, four point bending tests should have been performed, where the material is subject to pure bending, and not just shear forces. Further tests need to be performed using this test, on materials ranging from plywood style layered balsa, (with similar grains,

perpendicular grains, etc.) This would have been a more useful test if stronger pieces of balsa had been tested. 3. Glue Testing The final structure will consist of only balsa wood and glue, thus the choice of glue is a crucial decision. Glue is weakest in shear, but as before and to simplify the testing process, specimens will be tested in torsion, normal to the glue surface. In the actual design, the glue will mostly be under shear, notably when used to ply several layers of wood together. However this test yields comparative results for each glue and has an obvious best solution. It is assumed that the results would be similar for testing in shear. Sample specimens were broken in two, and then glued back together, see Figure 4. Next, the specimen were tested under tension to