Materials Lab
Compression and Structural Bending of Wood
Nolen Gerke
February 14, 2006
February 21, 2006
Dr. Jim Brenner
Introduction:
The wood lab is conducted in order to get a general idea of what range of
pressures wood will be capable of withstanding. Wood is a material that is widely used in
the construction of buildings, bridges, and other structures. This lab will use a load
apparatus machine (see picture 1) and apply a force to different types and geometries of
wood until they break. After breakage or shearing, the location of the break will be
recorded along with the load pressure. By measuring and comparing these values, we will
be able to better understand the properties of wood such as the modulus of elasticity and
the modulus of rupture. The modulus of elasticity (MOE) measures the stiffness of the
wood and the modulus of rupture (MOR) refers to the strength of the wood under
bending.
The wood lab will consist of 3 different samples being pressed until breaking
occurs. One sample will have knots, one sample will not have knots, and one sample will
be cylindrical in shape and a different type of wood. The 4th sample will be a piece of
long wood with 4 pressures applied, basically breaking the wood in half (see picture 3).
Some Expectations/Assumptions may include:
 Breaking along the grain of the wood.
 Breakage should be around the knot if present.
 Data should roughly follow the given standards for wood.
 Air conditioning unit in room may cause electrical fluctuations.
 Extensometer (measures the length that the wood changes) does not reach its
maximum (2 inches) before end of test.
 Machine is properly calibrated for use and operator is fully trained.
 The cut of the wood is proper (for flexural bending test).
 Moisture content in wood is normal.
Experimental Procedure:
Beginning this lab, Dr. Brenner Set up the load apparatus. The Load apparatus
was set to take the first 3 samples of wood and apply a pressure to them. Dr. Brenner
provided the samples of wood and his assistant made all the dimension calculations using
a vernier caliper (see picture 2). For all physical attributes, view Wood Lab Data Sheet.
For all data recorded see Data Sheets, Data Fit Graph, Fit Information and Data table.
Four samples of wood were tested over 2 types of test. For the compression test, 2
blocks of wood (one with point deflects (knots), one without) were pressed perpendicular
to the grain, and 1 cylinder of wood was pressed parallel to the grain. For the 4th sample,
a 4 point roller was used to bend the long wood until it fails. (see figure 1)
Figure 1
Each piece of wood is loaded into the load apparatus and then force is applied
until the wood breaks. The breakage point on all of the specimens is based off of the
audible loud “crack” the wood makes, not the splintering.
For the first test, a sample of wood with no knots was placed into the load
apparatus. The wood was lain so that the longest part of the wood was horizontal. The
load apparatus was turned on and set to apply pressure until failure.
The second test was the same as the first using a block of wood with nearly the
same dimensions and grain as the first. However, this block had point defects in the
center, which caused the failure directly down the center of the wood, through and around
the point defect.
The third test included a cylindrical piece of Brazilian wood, which was placed
into the load apparatus with the longest dimension in the vertical stance. This caused the
wood to be compressed parallel to the grain seeing as how the grain follows it’s longest
axis.
The fourth test was somewhat different from the rest of the tests because it used
what is referred to as a 4-point roller test. This test means that the applied force was
condensed down into 2 rollers that were positioned at equal lengths from the center of the
wood. Rather then the wood being on a flat surface, it was elevated by 2 more rollers.
This distance between the rollers is recorded on the WoodlabDataSheet. The wood was
loaded into the load apparatus with the longest axis horizontally.
Results:
During the first three experiments, only the rupture load was recorded rather then
the stresses at each 100 lb interval like experiment four. Here is a comparison of actual
data of the first three experiments to the textbook solutions:
Type
Ex. Strength
Pine With Knots (Perpendicular)
4100 psi
Pine Without Knots (Perpendicular)
4598 psi
Pine Dowel (Parallel)
7270 psi
Text Strengths (theoretical)
N/A
N/A
5,370 psi
Some errors involved in the recordings may have to do with the certain type of wood
used (seeing as how we are not exactly sure it is pine), moisture content of the wood may
be out of specs and the load apparatus may not be calculated correctly.
For the fourth experiment, modulus of elasticity and rupture are shown:
MOE:
MOE = ((S) x (L^3/4))/ (12I)
L = length between the supports = 16 1/8
S = slope of graph of load versus deflection (elastic region) = P/ changeX
I = moment of inertia = (1/12) x W x h^3
I = (1/12) x (1.491) x (1.486^3)= 0.408
MOE = (3300 x 1048.188)/ (12 x .408) = 7.06x10^6
MOR:
σfs = (4Ff)/ (2Wh3)
Ff = load at failure
W = width of sample
h = height of sample
σfs = (4 x 1390)/ (2 x 1.491 x 1.486^3) = 5.68x10^2
For all experiments view Figures A-H in the Appendix: Picture Listing.
Discussion:
By comparing the differences in the loads that wood could take, I found that
compression parallel to the grain was much more complicated and took more effort to
split the wood. One can conclude that if using a material such as wood, they should build
using the grain as a directional. The first three tests showed how brittle (MOE) the object
was when compressed perpendicular to the grain vs. parallel to the grain. These results
were confirmed by the book that wood would take more of a load when compressed
parallel to the grain. The fourth experiment tested the wood’s ductility (ability to bend
without breaking). Wood has a very low ductility (MOR) and would not be suggested for
use when something needs to bend. A graph of Load Vs Stress of the long wood is
included in appendix 2, which confirms how the wood broke (Figure Z). However, wood
does flex and should not be taken lightly because the experiment shows that the wood
becomes weaker the more it bends. Wood is also a composite of layers, which is where
the grain comes from. These experiments tested the wood’s delaminating failure, which is
the separation between those layers.
Some plausible sources of error include the electrical error in calculation of
course, which has proved a problem in earlier experiments. Moisture in the wood can
cause the wood’s properties to change. Also, before experiment four was started, all
students pointed out that the wood sample was slightly warped (within an 1/8 of an inch
at the center). I believe Dr. Brenner set it in the machine so that this was negligible.
Appendix 1: Picture Listing
Figure A – Brazilian Wood with Cracks Labeled
Figure B – Long Wood cracked in center with labeled lines for 4 roller points.
Figure C – Long Wood (side) cracked in center with labeled lines for 4 roller points.
Figure D – Close up of fracture from Long Wood test.
Figure E – Side View of Block of Wood with No Knots
Figure F – Close up of labeled fracture on “No Knots” wood block
Figure G – Side view with labeled cracks of wood with knots
Figure H – Close-up of fracture on wood with knots
Picture 1 – Load Apparatus (With a sample of dowel wood)
Picture 2 – Venier Caliper Used for measurement
Picture 3 – Example of a 4 point roller test on Long Wood.
Appendix 2: Data Visual
DataFit version 6.1.10
Results from project "Untitled1"
Equation ID: a*x+b
Number of observations = 15
Number of missing observations = 0
Solver type: Nonlinear
Nonlinear iteration limit = 250
Diverging nonlinear iteration limit =10
Number of nonlinear iterations performed = 11
Residual tolerance = 0.0000000001
Sum of Residuals = 3.97903932025656E-13
Average Residual = 2.65269288017104E-14
Residual Sum of Squares (Absolute) = 25866.2061236236
Residual Sum of Squares (Relative) = 25866.2061236236
Standard Error of the Estimate = 44.6061449057825
Coefficient of Multiple Determination (R^2) = 0.9907159585
Proportion of Variance Explained = 99.07159585%
Adjusted coefficient of multiple determination (Ra^2) = 0.9900018015
Durbin-Watson statistic = 0.64456609131857
Regression Variable Results
Variable
Value
Standard Error
a
4921.23193 132.1283812
b
19.5471626 21.58143632
t-ratio
37.24583534
0.90573965
68% Confidence Intervals
Variable
Value
68% (+/-)
a
4921.23193 136.6207462
b
19.5471626 22.31520516
Lower Limit
Upper Limit
4784.611185 5057.85268
-2.768042585 41.8623677
90% Confidence Intervals
Variable
Value
90% (+/-)
a
4921.23193 233.9861503
b
19.5471626 38.21856558
Lower Limit
Upper Limit
4687.245781 5155.21808
-18.67140301 57.7657282
95% Confidence Intervals
Variable
Value
95% (+/-)
a
4921.23193 285.4501548
b
19.5471626 46.62453503
Lower Limit
Upper Limit
4635.781776 5206.68209
-27.07737246 66.1716976
Prob(t)
0.0
0.38155
99% Confidence Intervals
Variable
Value
99% (+/-)
a
4921.23193 398.0103228
b
19.5471626 65.00976063
Variance Analysis
Source
DF
Regression 1
Error
13
Total
14
Sum of Squares
2760227.127
25866.20612
2786093.333
Lower Limit
Upper Limit
4523.221608 5319.24225
-45.46259806 84.5569232
Mean Square F Ratio
Prob(F)
2760227.127 1387.25225 0
1989.708163
Figure Z – Graph of Load Vs Stress of Long Wood. Last Point is fracture of wood. The
drop off is visible.
Appendix 3: Woodlab Data Sheet
1st sample:
Wood Type: Unknown – Assumed Idaho Timber or Pine
Length – 6.125 inches
Width – 1.493 inches
Height – 1.515 inches
Wblock – 4.961 inches
Cross Sectional area – Width X Wblock – 7.407
Note: Error in lengths – 0.001
Weight – 100.3 grams
Rupture load - 4,598 lb (+-) 1 lb
By visible inspection of the rings in the wood, this sample was close to the center of the
tree and the grain travels along the longest axis.
There is a small knot on one end, which was taken as negligible.
2nd sample
Wood Type: Unknown – Assumed Idaho Timber or Pine
Length – 6.05 inches
Width – 1.495 inches
Height – 1.482 inches
Wblock – 4.961 inches
Note: Error in lengths – 0.001
Cross Sectional Area – Width X Wblock – 7.417 inches ^2
Weight – 98.9 grams
Rupture load – 4100 (+-) 1 lb
By visible inspection of the rings in the wood, this sample was close to the center of the
tree and the grain travels along the longest axis.
This sample has a knot in the middle of the wood.
3rd sample
Wood Type: Brazilian Wood – Dowel Form
Length – 3.057 inches
Diameter – 0.884 inches
Note: Error In Lengths – 0.001
Cross Sectional area – (pi)(Diamter)^2/4 – 1.928
Weight – 25.30 Grams
Rupture load – 7270 lb
By visible inspection, no knots are visible in the wood and the grain travels along its
longest axis.
4th sample
Wood Type: Unknown – Assumed Idaho Timber or Pine
Length – 25.75 (+-) .125 inches
Width – 1.491 inches
Height –1.486 inches
Weight – 462.8 grams
Actual Length where breakage may occur – 16.125
Rupture load – 1390 lb
4th Sample Load Vs. Deflection
Load (lb)
Deflection (in)
0
0
100
0.019
200
0.040
300
0.061
400
0.089
500
0.096
600
0.114
700
0.133
800
900
1000
1100
1200
1300
1390
0.151
0.172
0.190
0.209
0.233
0.265
0.300 <<<Breaking Point
Appendix 4: Sample Calculations – taken from Mohammed Alamri’s
Lab Report
Flexural Modulus of Elasticity:
MOE = ((S) x (L3/4))/ (12I)
L = length between the supports = 16 1/8
S = slope of graph of load versus deflection (elastic region) = P/ΔX
I = moment of inertia = (1/12) x W x h3
I = (1 x 1.523 x 1.5193)/12 = 0.445
MOE = (3300 x 16.1253)/ (12 x 4 x 0.445) = 6.6 x 105psi
Flexural Strength/ Modulus of Rupture (σfs)
σfs = (4Ff)/ (2Wh3)
Ff = load at failure
W = width of sample
h = height of sample
σfs = (4 x 1290)/ (2 x 1.523 x 1.5192) = 1.11 x 103
Density = Mass/ Volume
Density = 420.2/ (1.523 x 1.519 x 26) = 6.99g/in3 = 0.43g/cm3
Units = g/cm3
(39.37)3 = (100)3cm3
(Using dimensions for flexural bending test wood sample)
Ultimate Compressive Strength:
UCS = Failure load/ (C/S Area)
Sample calculation using wooden dowel: 5420/ (3.1416 x 0.8472/4) = 9619.3psi
Appendix 5: Sources
Alamri, Mohammed Lab report fall 2003
Callister, William D. Materials in Science and Engineering an Introduction Fifth Edition.
DataFit software. Provided by Dr. Brenner.
Experimental photographs. Email from Dr. Brenner.
Leahy, Edward Lab report spring
Microsoft excel. Graphs and tables.
New York. John Wiley and Sons, Inc. Copyright 2000. 2002
WWW3265 CD. Course CD. Given by prof. J Brenner