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 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, 3M 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, 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 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