Advanced Joint Testing Load example essay topic

1. Balsa Balsa wood 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. 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 imprecision's 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 wood's 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 material's 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, 3 M Super Strength Adhesive, Carpenter's Wood Glue, and standard Epoxy. Figure 4: Glue Test Specimen Table 4: Glue Testing Results Ironically, the cheap Carpenters 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, 3 M 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 Carpenters Wood Glue was used in joint testing, and will most likely be used in the final design. Another factor that wasn t 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 carpenters 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 joint's 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 design's 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.