Balsawood Structure Design Essay Research Paper Balsawood — страница 2

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spaces with the smallestcross-sectional areas, where imprecisions in cutting took place or thematerial was simply weaker. It took many tests to get breaks thatoccurred in the center section instead of at the ends, perhaps with aneven smaller center section this would have been easier. It should alsobe noted that two different batches of balsa were tested and there was anotable discrepancy between the results. Table 1: Tension Tests ResultsSpecimen # Strength (psi)1 11542 13163 18304 1889Specimens 3 and 4 were from a different batch of balsa and were thickerpieces in general, although thickness should have had no effect onmaximum stress, it is assumed that the second batch simply has agreater density than the first one, or perhaps that it had not beenaffected by air humidity as

much as the first batch. (See the designconcepts section for more discussion of moisture content in thespecimens.)Compression Compression testing was also performed parallel to the wood s grain(See Figure 2). The specimen used must be small enough to fail undercompression instead of buckling. For analysis of compression tests,failure was defined as occurring when little or no change in load causedsudden deformations. This occurs when the yield strength is reached andplastic behavior starts. Figure 2: Compression Testing Setup Failure was taken at the yield strength because the material is nolonger behaving elastically at this point and may be expanding outsideof the design constraints. It should be noted that original specimensproved to be too tall and they failed in buckling

(they sheared to oneside), instead of failing under simple compression. Table 2: Compression Test ResultsSpecimen # Strength (psi)1 4642 3803 397Average 414Under tension, the pieces all had similar strength values. This tookmany tests, but in every other test, the material exhibited buckling aswell as compression. The three tests which ran the best were used forTable 2. Since the test of the design will be under compression, this data isvery relevant for the final design. Apparently balsa can withstandapproximately 3 times more load under tension than under compression.However, much like in these test, buckling is likely to occur in thefinal design. This fact should be of utmost consideration whendesigning 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, thuscreating shear stress throughout the member. Much like the tensiontest, the wood will deform and then break at a critical stress. Figure3 shows how this test was setup. The data obtained form this test canbe 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 theloading cap. Unfortunately, the data obtained from these tests was not conclusive ofmuch. The test was flawed due to a bolt which stuck out and restrictedthe material s bending behavior in each test. The two sets of data takenfor this test varied greatly (as much as 300%), and therefore this datais likely to be very error prone. Figure

3: Three Point Bending SpecimenTable 3: Bending DataSpecimen # Rupture Load (lb) Elastic Modulus (lb/in)1 26.6 120,0002 62.5 442,000 Included in the Appendix is a graph of load versus displacement for thefirst test, it shows how the experiment was flawed at the end when thematerial hit the bolt which was sticking out of the machine, thuscausing stress again. It also shows the slope from which the elasticmodulus of the material was taken. Ideally, four point bending tests should have been performed, where thematerial is subject to pure bending, and not just shear forces. Furthertests need to be performed using this test, on materials ranging fromplywood style layered balsa, (with similar grains, perpendicular grains,etc.) This would have been a more useful test if stronger pieces

ofbalsa had been tested. 3. Glue Testing The final structure will consist of only balsa wood and glue, thus thechoice of glue is a crucial decision. Glue is weakest in shear, but asbefore and to simplify the testing process, specimens will be tested intorsion, normal to the glue surface. In the actual design, the gluewill mostly be under shear, notably when used to ply several layers ofwood together. However this test yields comparative results for eachglue and has an obvious best solution. It is assumed that the resultswould be similar for testing in shear.Sample specimens were broken in two, and then glued back together, seeFigure 4. Next, the specimen were tested under tension to determinewhich glue was the strongest. Three glues were tested, 3M SuperStrength Adhesive,