Posted: February 19th, 2015

Material Lab

Material Lab

Introduction

In this experiment, different samples of wood are subjected to specific type of tests in order to determine their properties that help in the classification of these wood materials. There are three kinds of wood samples used for three kinds of tests: a long wood sample for flexural bend test, a parallel to grain compression test on an oak sample, and short wood samples used for perpendicular to the grain compression test. The long wood sample is a long piece of Yellow Pine sample with knots, the Oak sample is the cylindrical Oak Dowel, and short wood samples used are the Douglas Fir, Yellow Pine with knots, and Yellow Pine without knots. All the tests on the samples are done on the Baldwin Universal Testing Machine (elaborated in experimental procedure), and the wood materials’ properties evaluated from these tests are flexural modulus of elasticity (FMOE), modulus of rupture (MOR), and ultimate compressive strength (UCS).

Fig 1 Baldwin Universal Testing Machine (Brenner 2011; Ref 1)

Experimental Procedure
The physical characteristics of each sample of wood such as length, width, height and mass were measured before carrying out any of the tests. These physical quantities (refer Appendix) are taken down because they will be required later for finding FMOE, MOR, and UCS of the samples after collection of raw data when the experimental tests have been carried out.

For the short wood samples (Douglas fir, Yellow Pine w/knots, Yellow Pine w/o knots), the test involved applying compressive load perpendicular to the grain of the wood sample through a small metal block. The load applied is made to increase until the wood sample under test ruptures by making a ‘snapping sound’, and the load at this instance is recorded from the Universal Testing Machine, which is the force at failure. This is repeated for the remaining short wood samples left to be tested.

Fig 2 (From left to right) Side views of Douglas Fir, Yellow Pine w/knots, Yellow Pine w/o knots before test (Brenner 2011; Ref 2)

Fig 3 (From left to right) Top views of Douglas Fir, Yellow Pine w/knots, Yellow Pine w/o knots before test (Brenner 2011; Ref 2)

For the test on the oak dowel, the sample was positioned on the Universal Testing Machine such that it was subjected to the increasing compressive loads parallel to its directional strength or the grains of this cylindrical wood sample. The procedure carried out was the same as for the previous short wood samples, whereby the force at the instant of failure was recorded after hearing of the ‘snapping sound’.

Fig 4 View of Oak dowel sample before test (Brenner 2011; Ref 2)
For flexural bending test, the long wood sample of yellow pine with knots was used, and load was applied through four contact points on the sample that caused bending moment. Two contact points on each surface were used, with the bottom pair separated by 16.125 inches and top pair separated by 1.25 inches, with the midpoint of the sample equidistant from all contact points on its surface. A dial gauge with deflection arm was positioned at middle of this setup, and as the Universal Testing Machine increased the magnitude of load on the sample, the dial gauge showed greater deflection, with deflection readings recorded for every 100 pounds of increment in the load. This procedure continued until, due to excessive bending caused by the load, the long piece of wood finally ruptured at the force of failure, whose value was noted down along with the final deflection reading. The results from these were then used for the plotting of a load v/s deflection graph for finding the slope, and other values which would be required later for the calculation of FMOE, MOR, and UCS.
(For all calculations related to the entire experimental procedure, refer Appendix)

Fig 5 View of long piece of Yellow Pine sample w/knots before test (Brenner 2011; Ref 2)
Results

The data obtained from the flexural bending test is first plotted, and by noting down key values from the graph such as the slope of the graph and the load at which the graph ends (or at the failure point when sample ruptures), properties of FMOE and MOR are found out through calculations with final results along with errors displayed on graph.

Graph 1 Flexural Bending test graph (Black markers – X error bars; Red markers – Y error bars)
After all the properties are found out for all the samples through the necessary calculations, the final results are tabulated for ease of comparison between the materials themselves.

Table 1 Final table of results
Note: For wood abbreviations, refer Appendix

The aftermath of wood samples subjected to different tests are shown, indicative of the values presented in the final table of results.

Fig 5 (From left to right) Side view of Douglas Fir, top view of Yellow Pine w/knots, side view of Yellow Pine w/o knots after test (Brenner 2011; Ref 3)

Fig 6 (From left to right) Front view of long piece of Yellow Pine w/knots, side view of oak dowel after test (Brenner 2011; Ref 3)

The properties of these wood samples are now compared to literature data from the text for further investigations and interpretations in the discussion section.

Table 2 Experimental v/s Literature values for wood samples (Brenner 2011; Ref 4)
Note: For wood abbreviations, refer Appendix
Discussion

The plot of flexural bending test shows that the long wood sample of yellow pine with knots was a ductile sample, since it undergoes little plastic deformation after the linear deflection and soon reaches the rupture point as observed from the graph. In addition, based on the comparison between the experimental and literature data, the experimental values are more in agreement with the literature data for the 12 % moisture content than the values for the green or wet content. For all the wood samples, its physical quantities of properties are on the higher side, such as FMOE and MOR of the long piece of yellow pine sample with knots has very less percent difference when compared with its 12 % moist counterpart, but significant difference exists between the experimental and literature data when comparison done with wet content. Higher values of UCS, FMOE, and MOR indicate a stronger and stiffer material, which is suggestive of the fact that the wood samples subjected to different tests for this experiment had 12 % H2O and was kiln-dried, therefore the samples must be relatively stronger and stiffer.

Another reason for the high UCS values is that for perpendicular compressive loads, the force applied might not have been exactly perpendicular to the grain directions of the wood, thus, there would be a parallel component to stress, and hence the load applied would have to be increased to reach the desired ultimate compressive stress for causing rupture. Besides that, couple of wood samples used for tests had knots, which affected the final values of the properties of those materials. For example, in the perpendicular compression of Yellow Pine short wood sample that had knots at center of tree, compression took place directly on top of the knots, and in this case, knots infact provided a beneficial effect by making the material more resistant to compressive loads, hence leading to a greater ultimate compressive strength. This explains for the UCS of Yellow Pine with knots to be significantly higher than that of Yellow Pine devoid of any knots. However, in bending, the presence of knots often limit the amount of stress and strain you can put on the wood sample, which is the reason for relatively lower UCS value of Yellow Pine with knots in flexural bending test. For parallel compression test, which the Oak Dowel was subjected to, the sample experienced rupture on the opposite side of the material where there was no knot, and not on the side of knots, as depicted by the picture of the Oak sample where there is a crack at the extreme upper corner, and the bottom without any signs of crack.

During the flexural bending test, the dial gauge had a systematic error, or more specifically, a ‘zero error’ that had to be kept note of while performing the experimental procedure, because at zero load, the dial gauge had an initial deflection of 0.3 inches that had to be subtracted from all deflection readings recorded from the dial gauge.
Appendix I (Sample Calculations)

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