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1. Objective
1. Objective
The purpose of testing is the quantify the strength of our cardboard members, both tensile and compressive. We will do so by experimentally testing them in tension and compression and analyze the results using the principle of the lever and graphs. This will help us understand bridge structures more and evaluate their strength.
2. Process
2. Process
Testing Device
The testing device we used was a wooden lever. Certain points along the lever are marked to indicate where members are placed during testing. One end has a notch to hold the bucket of sand that will add weight and force upon each member.
Member Fabrication
Member Fabrication
IMG_3666.MOVUsed the Epilog laser to cut out the designs
Finished cutouts
Tension Test Members
Assembled the members by supergluing their components together
Compression Test Members
Assembled tubes (also glued thin strip around the perimeter of each end)
Testing Procedure
Testing Procedure
Tensile & Compressive Strength:
Set up the testing machine with the specimen placed at the correct line (T-line for tension, C-line for compression)
Ensure the specimen is properly aligned so the load is applied evenly
Apply load gradually by adding sand to the bucket one scoop at a time, pausing about 5 seconds between scoops
Continue loading until failure occurs (fracture in tension or buckling in compression)
Remove the bucket immediately after failure and measure the mass of the bucket and sand
Record specimen dimensions (cross-section and length) and the corresponding mass at failure
Adjust the machine as needed for shorter specimens to keep the loading arm perfectly horizontal
Repeat the test for multiple specimens of each size and length to ensure reliable data
Convert the measured mass into force using the lever principle to calculate tensile or compressive strength
Plot strength versus member length on a graph to analyze how length and cross-section affect strength
Questions:
Describe the load-deformation curve for the material. Did the cardboard show elastic behavior?
For cardboard in tension, we would expect a load-deformation curve to be linear in the beginning, where load and deformation are proportional. As the load increases, so will the damage in the cardboard's fibers, and the load-deformation graph will begin to curve. When the cardboard material tears, the curve will end with a sudden drop.
The cardboard showed elastic behavior in the beginning, but does not remain elastic .
Is cardboard a ductile or brittle material? How do you know?
Cardboard is a brittle material in tension. We know because its failure occurs suddenly, not gradually. It shows little plastic deformation and its failure is a tear, unlike ductile materials.
Specimen Results
8 mm width test 1
8 mm width test 2
Fracture points of two 8 mm wide members
Fracture points of the three 6 mm wide members
Fracture points of the three 4 mm wide members
Questions:
How did the length of the member affect its compressive strength?
As the member length increased, the compressive strength generally decreased. Longer members were more susceptible to buckling, causing failure at lower loads compared to shorter members, which resisted higher compressive forces.
Specimen Results
Compressive strength testing setup
10 x 10 x 160 mm crushing points
10 x 10 x 100 mm crushing points
10 x 5 x 50 mm crushing/buckling points
10 x 6 x 160 mm buckling points
10 x 6 x 100 mm buckling points
10 x 5 x 50 mm crushing points
5. Conclusion & Sources of Error
5. Conclusion & Sources of Error
There were inconsistencies in the cardboard members themselves. Differences in cardboard quality could have caused some samples to fail earlier than others. The amount of glue used to form the members was not perfectly controlled, which may have affected stiffness and strength by adding extra reinforcement for some members.
The loading method also introduced discrepancies. In some trials there was a noticeable delay before failure occurred, meaning the member may have experienced slow tearing that was not consistently accounted for across tests.
For compression testing, alignment errors also affected results. The lever arm was not always perfectly perpendicular to the member, which introduced bending forces and crushing rather than pure compression. Finally, in some trials the members slipped off the support books instead of failing by buckling or crushing, reducing the accuracy of the compressive strength measurement.
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