Balsawood Structure Design Essay Research Paper Balsawood

Balsawood Structure Design Essay, Research Paper

Balsawood Structure Design1. Introduction:This report is the first stage of the design, construction and testingof a balsa wood structure. In April, the design will be tested againstclassmates designs, where the design with the highest load/weight ratiowins. The information gained from this report will be used in theconstruction of the structure. The report is composed of two sections.The first is an evaluation of material properties of balsa, glues anddifferent joint configurations. The second section consists of adiscussion on a preliminary design that is based on conclusions drawnfrom the testing section. Common material tests of tension, compression and bending wereperformed and analyzed. The qualities of three different adhesives weretested and evaluated, and finally, three different joint configurationswere tested. Illustrations of each test setup are included. Wheneverpossible, qualitative results will be given as opposed to strictlyquantitative values. A qualitative result is much more useful ingeneral design decisions. Experimental results from the testing stagecombined with experiences is working with the materials offered cluesfor the preliminary design. The design section mixes both practical and experimental experiencetogether to present the best possible solution for the structure. Italso offers additional insights that were not considered in the initialmaterial testing procedure. The design presented in the this section,is likely to be similar the final model, however modifications may beneeded for the final design that were unforeseeable at the time of thisreport. This report generally functions as a guide for the construction stage ofthe project. Its role is to provide useful information and a basis forthe final design. Before the final design is tested, prototypes will beconstructed to test the principles discussed in this report. The goalof this report is to combine the results from testing and experience toproduce a working preliminary design. 2. Material TestingAll standard testing was performed on the Applied Test System located inroom XXXXXXXXXXXXXX. The goal of this section is to determine thematerial strengths of balsa, and how balsa responds to differentloading. Before testing, the basic structure of balsa needs to beconsidered. Wood grain is composed of bundles of thin tubularcomponents or fibers which are naturally formed together. When loadedparallel to this grain, the fibers exhibit the greatest strength. Whenloaded perpendicular to the grain, the fibers pull apart easily, and thematerial exhibits the least strength.Generally, for design considerations, the weakest orientation should betested. However, testing procedure called for testing of the materialin the greatest strength orientations; torsion and compression, parallelto the grain, and bending with the shear forces perpendicular to thegrain. Testing the materials for their “best direction” characteristicscan produce results that are not representative of real behavior. Toexpect uniform stress distributions and to predict the exact locationsof stresses prior to testing prototypes is generally not a good idea.However the values obtained from these tests can give a general idea ofwhere the structure may fail, and will display basic properties of thematerial. Tension TestIn tension testing, it is important to have samples shaped like the onein Figure 1, or the material may break at the ends where the clamps areapplied to the material. Failure was defined to occur when the specimenbroke in the center area, and not near the clamps. The machine recordsthe maximum load applied to the specimen and the cross sectional areawas taken of the central area prior to testing. These two values areused to compute the maximum stress the material can withstand beforefailure. Figure 1: Sample Torsion Specimen In general, the material failed at the 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