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- 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
- classmatesÆ 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 of moisture content in the
- specimens.)
-
- Compression
-
- Compression testing was also performed parallel to the woodÆs 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 materialÆs 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 determine
- which glue was the strongest. Three glues were tested, 3M Super
- Strength Adhesive, CarpenterÆs Wood Glue, and standard Epoxy.
-
-
-
- Figure 4: Glue Test Specimen
-
- Table 4: Glue Testing Results
-
-
- Ironically, the cheap CarpentersÆ Wood Glue is the best glue to use.
- Both the Wood Glue and the Epoxy both were stronger then the actual
- wood, and the wood broke before the glued joint did. The so called, 3M
- Super Strength Adhesive proved to give the worst results, and gave off a
- noxious smell both in application and in failure. Since price is also
- an important design consideration, and drying time is not of the utmost
- importance, the CarpentersÆ Wood Glue was used in joint testing, and
- will most likely be used in the final design. Another factor that
- wasnÆt considered is that the Wood Glue is also easy to sand, which
- makes shaping the final design much easier.
-
- 4. Joint Testing
-
- At first, basic joint testing was done, three different connections were
- glued together using carpentersÆ wood glue as shown in Figure 5 and
- loaded 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 to
- fail before the actual wood. This is simply a continuation of the glue
- test. The finger joint is likely to have failed because it has the most
- area under shear force and as stated earlier, glue is weaker in shear
- than in normal stress. Thus a more advanced form of joint testing was
- needed.
-
-
- Figure 6: Advanced Joint Testing
-
- Load was applied evenly along the horizontal section of the joint,
- creating a moment and vertical force at the joint. Failure was
- determined to occur when the joint either snapped or would not hold any
- more load. Each jointÆs performance was rated in accordance with the
- maximum load it held.
-
- Table 5: Joint Testing Results
- Joint Type Load Performance Results of Test
- 6-a good glue peeled off
- 6-b better reinforcement crushed
- 6-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 surface
- area that is glued. Therefore requiring more glue and reinforcing the
- joint more. In general joint construction this should be kept in mind,
- while not all joints will occur at 90 degree angles, it should be noted
- that there was a definite relationship between surface area glued and
- strength of joint. Discussed in the design section are special self
- forming joints that occur only under load, these special type of joints
- should be kept in mind for the design as well.
-
- 5. Design Concept
-
- Among issues not previously discussed in this report is the effect of
- baking the structure. Since balsa, like most woods, is high in water
- content, and the goal of this project is to win a weight versus load
- carrying capacity competition, the effects of baking out some of the
- water were tested. It was apparent that a decent percentage of the
- designÆs weight could be removed using this method without seriously
- effecting the strength of the material.
- Another issue to consider is the appearance of "self forming" joints
- during testing. Often a vertical piece of balsa would bite in to a
- horizontal piece, thus creating a strong joint that was better than most
- glued joints simply because the material had compressed to form a sort
- of socket for the joint. Although it is doubtful that this would be a
- part of the design, it is important to take this in to consideration in
- the design, and hopefully take advantage of this type of behavior.
- The use of plywood-style pieces of balsa was not tested, but it needs
- to be considered. Where the load and stresses are known it would be
- best to form the plys in a unidirectional grain orientation, where the
- strongest orientation is used. However, where the stresses are unknown
- it would be better to use a criss-cross pattern in the balsa plys to
- produce a strong, general purpose material in these regions.
- Now to discuss the initial design. Figure 7 shows a basic design. The
- grain representations are accurate for the lower portion. However, in
- the top section where the arch is horizontal, and the load will be
- applied, this section will be in bending and therefore requires a
- horizontal grain. (This inaccuracy is due to limitations in the graphics
- package used for the figure.) Note that the bottom support piece is
- thick at the ends to encourage the self forming joints previous
- discussed, and since the bottom piece is believed to be subject to
- tension, the middle section is made thinner to cut down on material
- weight.
- The loading cap will need to be constrained so it will not slide down
- the side of the structure, so added material needs to be place in those
- points. In testing prototypes, the effects of the grain orientation
- needs to be observed. In the top most sections, strictly horizontal
- grains will be used, but as the arch curves to a vertical orientation,
- vertically oriented grains need to be used. This gradual change in
- grain will be possible with plywood style layering of the balsa.
- Until further testing of prototypes is possible, this is all of the
- relevant information available. Ideally, a structure such as this one
- should perform well, but that remains to be seen.
-
-
-
- Figure 7: Basic Design (Code name: Arch)
- 6. Appendices
-
-
- Figure 8: Bending Test Results
-