Balsawood Structure Design Essays - Mechanics, Materials Science

Balsawood Structure Design Essays - Mechanics, Materials Science Balsawood Structure Design 1. Introduction: This report is the first stage of the design, construction and testing of a balsa wood structure. In April, the design will be tested against classmates designs, where the design with the highest load/weight ratio wins. The information gained from this report will be used in the construction of the structure. The report is composed of two sections. The first is an evaluation of material properties of balsa, glues and different joint configurations. The second section consists of a discussion on a preliminary design that is based on conclusions drawn from the testing section. Common material tests of tension, compression and bending were performed and analyzed. The qualities of three different adhesives were tested and evaluated, and finally, three different joint configurations were tested. Illustrations of each test setup are included. Whenever possible, qualitative results will be given as opposed to strictly quantitative values. A qualitative result is much more useful in general design decisions. Experimental results from the testing stage combined with experiences is working with the materials offered clues for the preliminary design. The design section mixes both practical and experimental experience together to present the best possible solution for the structure. It also offers additional insights that were not considered in the initial material testing procedure. The design presented in the this section, is likely to be similar the final model, however modifications may be needed for the final design that were unforeseeable at the time of this report. This report generally functions as a guide for the construction stage of the project. Its role is to provide useful information and a basis for the final design. Before the final design is tested, prototypes will be constructed to test the principles discussed in this report. The goal of this report is to combine the results from testing and experience to produce a working preliminary design. 2. Material Testing All standard testing was performed on the Applied Test System located in room XXXXXXXXXXXXXX. The goal of this section is to determine the material strengths of balsa, and how balsa responds to different loading. Before testing, the basic structure of balsa needs to be considered. Wood grain is composed of bundles of thin tubular components or fibers which are naturally formed together. When loaded parallel to this grain, the fibers exhibit the greatest strength. When loaded perpendicular to the grain, the fibers pull apart easily, and the material exhibits the least strength. Generally, for design considerations, the weakest orientation should be tested. However, testing procedure called for testing of the material in the greatest strength orientations; torsion and compression, parallel to the grain, and bending with the shear forces perpendicular to the grain. Testing the materials for their "best direction" characteristics can produce results that are not representative of real behavior. To expect uniform stress distributions and to predict the exact locations of stresses prior to testing prototypes is generally not a good idea. However the values obtained from these tests can give a general idea of where the structure may fail, and will display basic properties of the material. Tension Test In tension testing, it is important to have samples shaped like the one in Figure 1, or the material may break at the ends where the clamps are applied to the material. Failure was defined to occur when the specimen broke in the center area, and not near the clamps. The machine records the maximum load applied to the specimen and the cross sectional area was taken of the central area prior to testing. These two values are used to compute the maximum stress the material can withstand before failure. Figure 1: Sample Torsion Specimen In general, the 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