| 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 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, Carpenter s Wood Glue, and standard Epoxy. Figure 4: Glue Test SpecimenTable 4: Glue Testing ResultsIronically, the cheap Carpenters Wood Glue is the best glue to use.Both the Wood Glue and the Epoxy both were stronger then the actualwood, and the wood broke before the glued joint did. The so called, 3MSuper Strength Adhesive proved to give the worst results, and gave off anoxious smell both in application and in failure. Since price is alsoan important design consideration, and drying time is not of the utmostimportance, the Carpenters Wood Glue was used in joint testing, andwill most likely be used in the final design. Another factor thatwasn t considered is that the Wood Glue is also easy to sand, whichmakes shaping the final design much easier. 4. Joint TestingAt first, basic joint testing was done, three different connections wereglued together using carpenters wood glue as shown in Figure 5 andloaded until failure of either the joint or the material. Figure 5: Joints Tested The finger joint (Figure 5-c) was the only of the above joints found tofail before the actual wood. This is simply a continuation of the gluetest. The finger joint is likely to have failed because it has the mostarea under shear force and as stated earlier, glue is weaker in shearthan in normal stress. Thus a more advanced form of joint testing wasneeded. Figure 6: Advanced Joint TestingLoad was applied evenly along the horizontal section of the joint,creating a moment and vertical force at the joint. Failure wasdetermined to occur when the joint either snapped or would not hold anymore load. Each joint s performance was rated in accordance with themaximum load it held.Table 5: Joint Testing ResultsJoint Type Load Performance Results of Test6-a good glue peeled off6-b better reinforcement crushed6-c best joint crushed The scarf joint held the most load, and therefore was rated as best.This may be because the scarf joint has the highest amount of surfacearea that is glued. Therefore requiring more glue and reinforcing thejoint more. In general joint construction this should be kept in mind,while not all joints will occur at 90 degree angles, it should be notedthat there was a definite relationship between surface area glued andstrength of joint. Discussed in the design section are special selfforming joints that occur only under load, these special type of jointsshould be kept in mind for the design as well. 5. Design Concept Among issues not previously discussed in this report is the effect ofbaking the structure. Since balsa, like most woods, is high in watercontent, and the goal of this project is to win a weight versus loadcarrying capacity competition, the effects of baking out some of thewater were tested. It was apparent that a decent percentage of thedesign s weight could be removed using this method without seriouslyeffecting the strength of the material. Another issue to consider is the appearance of “self forming” jointsduring testing. Often a vertical piece of balsa would bite in to ahorizontal piece, thus creating a strong joint that was better than mostglued joints simply because the material had compressed to form a sortof socket for the joint. Although it is doubtful that this would be apart of the design, it is important to take this in to consideration inthe design, and hopefully take advantage of this type of behavior. The use of plywood-style pieces of balsa was not tested, but it needsto be considered. Where the load and stresses are known it would bebest to form the plys in a unidirectional grain orientation, where thestrongest orientation is used. However, where the stresses are unknownit would be better to use a criss-cross pattern in the balsa plys toproduce a strong, general purpose material in these regions. Now to discuss the initial design. Figure 7 shows a basic design. Thegrain representations are accurate for the lower portion. However, inthe top section where the arch is horizontal, and the load will beapplied, this section will be in bending and therefore requires ahorizontal grain. (This inaccuracy is due to limitations in the graphicspackage used for the figure.) Note that the bottom support piece isthick at the ends to encourage the self forming joints previousdiscussed, and since the bottom piece is believed to be subject totension, the middle section is made thinner to cut down on materialweight. The loading cap will need to be constrained so it will not slide downthe side of the structure, so added material needs to be place in thosepoints. In testing prototypes, the effects of the grain orientationneeds to be observed. In the top most sections, strictly horizontalgrains will be used, but as the arch curves to a vertical orientation,vertically oriented grains need to be used. This gradual change ingrain will be possible with plywood style layering of the balsa. Until further testing of prototypes is possible, this is all of therelevant information available. Ideally, a structure such as this oneshould perform well, but that remains to be seen. Figure 7: Basic Design (Code name: Arch) 6. AppendicesFigure 8: Bending Test Results
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