Lab number/title:
Introduction
Purpose of the lab experiment was to gain knowledge about structural properties of wood. In this lab experiment, a species of wood was tested. Four parameters namely Modulus of rupture, compressive strengths parallel to the Grain in longitudinal Orientation as well as perpendicular to the Grain in radial Orientation were found out by carrying out the experiment. Wood is an important construction material for possessing some outstanding structural properties. Application of wood can be seen as a compressive member. Being an anisotropic material, its properties change with direction with respect to wood's grain structure. Engineering properties of this material are different in each of the three directions namely Longitudinal, Radial, and Tangential. It is the strongest structural material in the longitudinal direction. Tensile strength in the longitudinal direction is much higher than the compressive strength in the same direction as well as about 40 times higher than that in either the radial or tangential directions. Compressive strength in longitudinal direction is about 6 times greater than that in either of the other directions.
Procedure:
4 Point Bend test for each direction of grain -
This is test of Modulus of Rupture. At first the tangential direction of the specimen was determined. Then the span length (L), distance (/), width (b), and height (h) of the specimen were measured. The midpoint of the beam along the length of the member was marked. After this, the specimen was loaded in the testing apparatus, making sure to center the block at the mid-span of the specimen. Deflection Δx at every 100 pound load was recorded. The beam was loaded to failure till the failure mode was clearly visible. Then Modulus of Rupture, MOR was determined by following equation-
MOR = Mc/I, where: M = moment at location of failure; c = distance from neutral axis to extreme fiber; I = moment of inertia
Procedure mentioned above was repeated for the radial direction also.
Compression test parallel and perpendicular to the grain -
A. Stresses parallel to the grains (longitudinal)
In this part of the experiment length L, and cross-section of the given test piece were measured and test piece was placed upright in the testing machine. Then the specimen was loaded and changes ΔL was recorded at every 1000 pound load. Using the relationship σ = P/A and e = ΔL/L a stress-strain curve was plotted and E was computed. Experimental value of E was compared with that obtained through flexural tests.
B. Stresses perpendicular to the grains (radial & tangential)
In this part widths (w) and (b) of the steel block and sample were measured respectively. Loading area i.e. (w x b) was computed. Then load was applied through the steel block, until the dial indicator read 0.1" change in length. This was the ultimate for this case. Finally, σult = Pult/loading area was determined. Stress verses strain relationships was plotted on the same graph as in part A and the values of E and the ultimate strengths obtained in the various directions were compared.
Apparatus:
Result:
Dimensions of specimen have been shown in Table no-1.
Data of Load vs Deflection has been tabulated in following table.
Graphs of load vs deflection have been plotted using the experimental data mentioned in Table no. 2.
Graph no.1: Load vs Deflection - Tangential
Graph no.2: Load vs Deflection - Radial
Following graph shows the moment diagram for 4 point flexure test. The graph is based on Table no.3 where moment has been calculated.
Graph no. 3: Moment Diagram
Discussion:
Moment of Rupture i.e. MOR = Mc/I,
where: M = moment at location of failure = 5402 lb-in;
c = distance from neutral axis to extreme fiber = 1.533/2 = 0.7665 in;
I = moment of inertia = ( bh^3)/12 = 0.46 in4
So, MOR = 5402 x 0.7665 / 0.46 = 9001.4 psi.
Compressive strength of wood specimen was determined.
For Compressive Test Parallel to the Grain - Longitudinal Orientation,
For Compressive Test Perpendicular to the Grain - Radial Orientation,
Compressive Strength (psi) = Ultimate load / A= 1000.325 psi.
Conclusion:
The lab experiment was carried out safely. There were two portions of the experiment. By performing first part knowledge about Modulus of Rupture and the procedure to evaluate Modulus of Rupture of a wood sample were gained. By experimental value of Modulus of Rupture it can be said that the species of wood may be Silver Maple whose Modulus of Rupture is about 8900 psi. One more parameter, we have obtained by carrying out this lab experiment, is the compressive strength. Compressive strength of Silver Maple wood is about 750 psi which is closer to the experimental value of compressive strength. So, by performing this lab experiment two parameter of an important construction material were known. Not only this, procedure of determining these two parameters were also known. Here lies the success of this lab experiment.
