compression test wood

COMPRESSION TESTEM 327: MECHANICS OF MATERIALS LABORATORY

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EXPERIMENT: COMPRESSION TEST

OBJECTIVES:

(1) To conduct a compression test on three types ofwood and obtain material properties for the testedsamples.

(2) To perform statistical analysis on the results toobtain sample means, standard deviations, confidenceintervals, and other appropriate statistics.

INTRODUCTION:

In this experiment, three types of wood will be testedto failure in compression. Several material propertieswill be determined for each specimen.

BACKGROUND:

When a simple compressive load is applied to aspecimen, the following types of deformation maytake place: elastic or plastic shortening in ductilematerials, crushing and fracture in brittle materials, asudden bending deformation called buckling in long,slender bars, or combinations of these. Ductilematerials, such as mild steel, have no meaningfulcompressive strength. Lateral expansion and thus anincreasing cross-sectional area accompany axialshortening. The specimen will not break: excessivedeformation rather than loss of strength oftencharacterize failure. Brittle material, such as the woodspecimens that are to be tested in this lab, commonlyfracture along a diagonal plane which is not the planeof maximum compressive stress, but rather one ofhigh shear stress which accompanies the uniaxialcompression.

Strain is a measure of the intensity of deformation(deformation per unit length). Normal strain, ε,measures the contraction (or elongation) of a bodyduring deformation.

εavg = δn/L

In cases where the deformation is non-uniform theaverage strain may be significantly different than thetrue strain at a given point.

WOOD:Wood consists of tube-like cells which are tightlycemented together to form a basically homogeneousmaterial. The cells, which mostly run in the samedirection, form fibers, which constitute the grain.Important physical properties are moisture contentand density. These properties are related to themechanical properties of the wood.

Factors that affect the properties of wood include thearrangement of the grain and the amount ofheartwood (the dark core wood of the tree).Irregularities also affect material properties. There arethree important classes of defects: 1.) knots, 2.)checks, and 3.) shakes. Knots are the areas of thetrunk in which the wood surrounds the base of thebranch as the tree grows. Checks are longitudinalcracks that run normal to the growth rings and shakesare cracks that run parallel to the growth rings.

Wood is anisotropic which means that properties willbe different in different directions. When wood isloaded in compression parallel to the grain direction,it will resist large forces. However, if it is loadedtransverse to the grain direction it can be quite weak.Wood, when loaded in compression parallel to itsgrain, is one of the strongest structural materials inproportion to its weight. The compression strength toweight ratios of some woods exceed that of structuralsteel by as much as 80 percent and exceeds 2024-T4aluminum alloy by more that 50 percent.

Wood is relatively weak in shear parallel to the grain,and will often fail in this mode.

THEORY:Figure 1 illustrates the test specimen undercompression loading.


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FIGURE 1

L

d

b

P

P

The test consists of uniaxial loading, and thereforethe stress is calculated by:

σ = PA

Where: P is the applied loadA is the cross-sectional area

Specific gravity is defined as the density of amaterial, ρ, divided by the density of water.

SG= ρ ρwater

Where: ρwater= 1.94 slugs/ft3 [ 1 g/cm3]

Note: specific weight is defined as: γ = ρ g

MATERIALS TO BE TESTED:Three types of wood will be tested; red oak, yellowbirch, and ponderosa pine. Specimens have been pre-cut into blocks of approximately 1-3/4" x 1-3/4" x

8". Exact measurements must be made for eachspecimen prior to testing.

EQUIPMENT TO BE USED:MTS Testing Machine 55,000-lb capacity.

SAFETY:!! USE SAFETY PLEXIGLASS SHIELD ONMTS MACHINE AT ALL TIMES WHENTESTING WOOD !!

Never operate the MTS machine when someone'shands are between the grips. Make sure all labparticipants are clear of equipment before beginningor resuming testing.

PROCEDURE:

TEST PREPARATIONS:The weight, length, and cross-sectional dimensions ofeach specimen must be measured prior to testing.

TEST DATA:The student will need to produce a stress-versus-strain diagram for each of the three tests, similar tothe one shown in Figure 3.

Note that the load versus stroke curve may notcontain an initial straight-line portion. If not, you willneed to estimate the best fit tangent to the curve toobtain the Modulus of Elasticity, E. In doing this youmay find that the tangent line intercepts the horizontalaxis to the left of the curve. If this is the case, thepoint where the tangent line intercepts the horizontalaxis should be selected as the location for the origin.Thus, it is advisable to zero the plotter pen about aninch from the left-hand side of the graph paper.


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FIGURE 3

Note that Load vs Stroke (Pv.δ) will be recorded at the MTS.These values can be converted to stress-strain (σv.ε)

ProportionalLimit

CompressiveStrength

Best-FitEstimate

ε(δ)

σ(P)

Several other calculations will be required based onthe results of the test. For additional information,please consult the tension test experiment write-up.

E = σ/εσpl = Ppl/Aσc = Pmax/A

UR = 1/2(σpl)(εpl) = σpl2

2ESG= ρ ρwater

MTS SET-UP:1.) Follow Start- up Procedures

Station Manager compressMPT compress.000

2.) Turn hydraulics on.3.) Make sure 'MANUAL OFFSET' = 0 for Stroke.4.) Adjust 'SET POINT' to 0.05.) 'AUTO OFFSET' Load.6.) Set-up Scope to plot a/b.

Load 5000 lbf -10,000

Stroke 0.02 in -0.08Time 15 min

TESTING PROCEDURE:1.) Create specimen file comp*.2.) Center specimen on lower loading platen.3.) Lock MPT and select specimen.4.) Start scope.5.) CLOSE SAFETY SHIELD6.) Press `RUN' and let test proceed until rupture.

The load will drop off at this point. It is notdesired to crush the specimen beyond the firstmajor rupture.

7.) Press `STOP'.8.) Unlock MPT.9.) Adjust SET POINT to 0.0.10.) Remove specimen11.) Repeat procedure for each remaining specimen.12.) Turn hydraulics `OFF' .13.) Copy data files to diskette.

c:\em327data\comp*\specimen.dat14.) Delete specimen comp*.

REPORT:

The report outline found in Appendix A should beused.

REPORT REQUIREMENTS:(1) Stress versus strain plots from MTS data file

complete with:Curve Fitting to obtain Tangent ModulusLabels and Title

(2) Determine the following properties for eachspecimen:

a. Proportional Limit, σplb. Compressive Strength, σcc. Modulus of Elasticity, E

d. Modulus of Resilience, UR


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e. Specific Gravity, SG(3) Compare a,b,c, and e to reference values(4) Tabulate all values calculated in (3) and (4) and

neatly show one set of sample calculations.(5) Your instructor will provide you with a data

sheet from previous compression tests done onwood samples like the ones you tested. Add yourresults to the appropriate data and performstatistical analysis to determine the mean,median, mode (if applicable), standard deviation,and confidence interval for a 90% confidencelevel for each data set. (A brief review ofstatistical terms and calculations can be found onthe next page) Show one set of samplecalculations and tabulate your results. Discussthe statistical results and their implications.Examine the mean, median, and mode values anddiscuss which of the three may be morerepresentative for the different data sets andwhy. What does the standard deviation tell youabout the scatter in the data of each set? Whatkind of conclusions can you make about thematerial properties of wood based on yourstatistics?The EXCEL worksheet is located at:http://www.iastate.edu/~em327/experiments/compression/database

(6) Discuss sources of error as well as their impacton the design process.

(7) Describe the types of failure observed for eachspecimen. SKETCHES OF THE FAILURESARE REQUIRED.

(9) Answer the questions your instructor assigned.

QUESTIONS:(1) Are the compressive strength and the specific

gravity related? If so, what trends do the dataindicate.

(2) Strain calculations based on the measured strokemay not be very accurate when prematurecrushing occurs at the ends of the specimens.Discuss how this would effect the experimentalvalues determined in (3).

http://www.public.iastate.edu/~em327/


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STATISTICS:A measure of central tendency refers to a measureindicating the value to be expected of a typical datapoint in a data set. Three measures of centraltendency will be considered:

ARITHMETIC MEAN: The central tendencymeasure representing the arithmetic average of a setof observations.

MEDIAN: The middle point of an ordered set of data.(If an even number of data exists the median is theaverage of the middle two)

MODE: The value(s) most often repeated in the dataset. May or may not be applicable.

Units for each of these are the same as the data units.In a large enough sample size, which does not violatestatistical assumptions of randomness andindependence one would expect these values to be thesame. Thus, one can use these values to get aqualitative estimate of the true measure of centraltendency by examining how close they are to eachother, or if one is much different. In the latter case thedata set can be examined for outlying data pointswhich can strongly affect the mean. Outliers muchless impact the mode and median.

The STANDARD DEVIATION, σ, is the positivesquare root of the variance and represents the scatter(dispersion) in the data set. The units of the standarddeviation are the same as the data set. The standarddeviation of a normally distributed data set is givenby the following equation:

( ) ( )( )1

22

−−

= ��nn

xxn iiσ

where n is the number of observations.The figure illustrates the shape of a normaldistribution. Within one standard deviation from themean value one would expect to find 68% of theobservations, two standard deviation within the mean

would include 95% of the observations, and within 3standard deviations one would expect to find 99% ofthe observations. Typically, engineering applicationsconsider a 3 standard deviation limit. When morescatter exists in the data, the bell shape will be flatter,where a data set with little scatter would have a steepbell shape. Note that the tails of the curve approachthe x-axis, but will never reach it, thus, it is possibleto obtain any value for a given data set, in theory.

Normal Distribution

0.00

0.02

0.04

0.06

0.08

0.10

0 40 80 120

compression test wood

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