Material Testing
The materials allowed in the project consisted of 3mm and 6mm plywood as well as wood glue. To understand how the wood would perform under load we conducted a series of tests using an Instron Testing Machine.
Uniaxial Tension Test
Sample 1: 3mm Plywood, Parallel Grain Orientation
Ultimate Strength: 72.9536 MPa
Modulus of Elasticity: 6.6258 Gpa
Sample 2: 3mm Plywood, Cross Grain Orientation
Ultimate Strength: 80.0375 MPa
Modulus of Elasticity: 7.5849 GPa
Conclusion: Plywood is stronger when the load is parallel to the grain.
3 Point Bending Test
Conclusion: Plywood is stronger when a vertical load is applied perpendicular to the grain.
Dimensions
Deflection Predictions
Measured deflection prediction (at the center point at the bottom of the truss) with 1kN applied is 0.136mm
Predicted Maximum Load
From Lab 2, we found the 6mm plywood's maximum bending stress to be about 95 MPa. At a 9.250 kN load, the platform experiences a bending stress of 94 MPa, just below the maximum.
Predicted Buckling at Max Load
Under the same load of 9kN, the Max Von Mises Stress Factor of Safety was 0.67, and the Buckling Factor of Safety was 9.5398. With a higher Factor of Safety in the Buckling FEA than the Force FEA, our bridge will fail by force, not by buckling.
Design Choice!
Because of the asymmetry caused by the roller pin support configuration, we opted for an asymmetrical Cannpoy to compensate for the disproportionate buckling on the roller side.
Final Product
Ultimately, the design weighed 1.6 lbs & was able to support 3235 lbs!