O.F.I. OCCASIONAL PAPERS

NO. 31

INTRA-TREE VARIATIONS OF STRENGTHPROPERTIES IN PLANTATION GROWN TEAK(TECTONA GRANDIS L F) AND TECHNIQUES

FOR THEIR SYSTEMATIC SAMPLINGby

S.K. Sanwo*

1986

*Department of Forest Resources ManagementUniversity of Ibadan

Ibad anNigeria

ISBN 0 85074 092 4ISSN 0269-5790

Oxford Forestry InstituteDepartment of Plant Sciences

University of Oxford

1

SUMMARY

This investigation involved an assessment of the intra-tree variation of somestrength properties in teak (Tectona grandis L. f) grown in Nigeria: nominalspecific gravity, modulus of rupture (MOR) , modulus of elast\city (MOE), totalwork done (TWO) and maximum compressive strength parallel-to-grain (MCS).

Nine trees were examined from three canopy classes: two dominants, five co­dominants and two sub-dominants. Within each tree, systematic sampling wascarried out (a modification of Richardson's (1961) methods). Twenty sampleswere removed from each tree in positions common to all the trees. Non-standardsamples were tested, following the procedure of Wood (1970). Results wereanalysed by graphical analysis by the method used by Duff and Nolan (1953).The results show that within each tree, the pattern of variation of these woodproperties is systematic.

This study highlights the techniques of systematic sampling in tropicalplantation species, the use of non-standard, small, clear specimens in strengthevaluation, and the justification for applying these techniques to fast growntropical hardwoods.

In the Duff and Nolan oblique sequence Specific Gravity, Modulus of Elasticity,Modulus of Rupture, Total Work Done, and Maximum Compressive Strength increasedfrom tree apex to the base. In the horizontal sequence SG, MOR and MCSincreased from the pith outwards at different distances from the pith fordifferent properties and then decreased slightly. In the vertical sequence SG,MOR and TWD increased with year of wood formation.

2

CONTENTS

SUMNARY

Section 1. INTRODUCTION

1.1 Purpose and Plan of Present Study

Section 2. MATERIALS AND METHODS ••

2.1 Selection of Material2.2 Preparation of Test Specimens2.3 Evaluation of Wood Properties

2.3.1 Nominal Specific Gravity2.3.2 Static Bending Tests2.3.3 Compression Tests

Section 3. RESULTS AND DISCUSSION

3.1 Patterns of Variation

3.1.1 Nominal Specific Gravity3.1.2 Modulus of Rupture (MOR)3.1.3 Modulus of Elasticity (MOE)3.1.4 Total Work Done (TWD) ••3.1.5 Max Compressive Strength

Parallel-to-Grain (MCS)

3.2 Sampling and Testing Techniques

Section 4. CONCLUSIONS

4.1 Suggestions for Further Study

ACKNOWLEDGEMENTS

REFERENCES

TABLES

FIGURES

APPENDICES

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38

3

TABLES

2.1 Table of length and testing speeds for specimens of varying depth(width) for compression parallel to grain and static bending testscovering the range of specimen sizes used in the present investigation.

2.2 Distribution of specimen size by squared cross section, andcorresponding mean strength values.

2.3 Result of the calculated F-ratios and the relationship between F-ratioand depth (specimen size) examined by linear regression, f-or fourmechanical strength properties, of teak.

2.4 Multiplying factors for correcting for slope of grain employed in thisstudy, derived from estimates on figure 2.3.

FIGURES

i.lA

1.1B

2.1

Map of Nigeria showing the location of Ibadan.

Map of University of Ibadan showing the location of the studied plot.

Schematic diagram of the partitioning of sample trees into comparablezones application to all sample trees.

Wi thin-tree variation of strength properties.method presented schematically indicating theforest to the testing.

Summary of samplingvarious stages from

2.3 Graph of grain slope against multiplying factor for clear strength withregards to four strength properties.

3.1 Illustration of the zones adopted within the tree for data analysis,showing associated notations of the Duff and Nolan graphs.

3.2 Variation of nominal specific gravity (SPG) in the oblique, horizontaland vertical sequences.

3.3 Variation of modulus of rupture (MOR) (N/mm2 ) in the oblique,horizontal and vertical sequences.

3.4 Variation of modulus of elasticity (MOE) (N/mm2 ) in the -oblique,horizontal and vertical sequences.

3.5 Variation of total work done (TWO) (mm.N/mm3 ) in the oblique,horizontal and vertical sequences.

3.6 Variation of maximum compressive strength parallel-to-grain (MCS)(N/mm2 ) in the oblique, horizontal and vertical sequences.

APPENDICES

1 Static bending test formulae used in the study.

2 Formula for compression test parallel to the grain.

4

1.0 INTRODUCTION

This paper presents some of the results of an investigation into the variationin wood characteristics of plantation teak (Tectona grandis L.f.) grown inNigeria. The paper summarises the results of a study made for a higher degreeof the Department of Forestry and Wood Science, University College of NorthWales) Bangor) Wales, UK.

In Nigeria, as in many parts of the tropics, natural stands of mixed forestsare being replaced by fast growing monoculture of species, such as teak andGmelina arborea Roxb. The properties of wood produced from trees in these'new' forests are different from those of the original forests (Einspahr, 1976;Hillis, 1980; Zobel) 1980; Kromhout and Bosman, 1981; Kellison, 1981).

Teak has been accepted in Nigeria as a potential source of all purpose woodsuitable in both structural and non-structural end uses. This acceptance hasbeen gained not from experience with its use in Nigeria, but from the generalworld-wide knowledge that teak is a good quality, naturally durable (heartwoodonly), attractive and easy to work, all-purpose wood.

Considering the importance of teak to the Nigerian economy, there has beenremarkably little previous work on the wood properties of the species. Studieswere carried out on some samples of teak logs grown in the north and south ofNigeria by the Forest Products Research Laboratory, England (Lavers, 1969).These results, which were the first detailed assessment of the strength andquality of teak grown in Nigeria, did not cover intra-tree variation.

1.1 Purpose and plan of Present Study

The present investigation examines the intra-tree variation of some woodcharacteristics of teak, mechanical and non-mechanical, which bear directrelevance to its potential end uses as sawn timber in Nigeria. In addition, asystematic approach to sampling within the tree suggested by Richardson (1961),has been attempted and this has been followed up with graphical analysis, basedon the systematic pattern of variation of wood properties within the treeestablished by Duff and Nolan (1953). This paper is presented in foursections. Section (1) deals with the introduction Section (2) explains thematerial anc methods used in the investigation, Section (3) discusses theresults and Section (4) concludes the paper and gives relevant suggestions for~urther stedy on this important subject.

5

Section 2. MATERIALS AND METHODS

2.1 Selection of Material

The site chosen for this study was in the University of Ibadan teak plantation,(see figures 1. lA and 1.1B). The area lies within the drier part of thetropical rainforest of southern Nigeria. It is an area with two distinctseasons; wet (April - October) and dry (November - March). In this type ofcondition, teak is able to produce clear annual growth rings (Rudman and DaCosta, 1959; Renes, 1978) which are visible to the naked eye and under lightmicroscope.

The present investigation was carried out in a 27 year old stand. The sampleplot measured 40m x lOOm giving a size of 0.4 hectares. All trees in the plotwere of the same age and 208 trees were available for sampling. From these,nine trees were selected from three canopy classes; employing similarprocedures to Elliott (1966) and recently Tsoumis and Panagiotidis (1980).

The following canopy classification based on dbh (o.b.) was employed:

Dominant •••••••••••••••••• >30 cm (d.b.h.o.b.)Co-dominant •••••••••••••••• 20 cm to 30 cm (d.b.h.o.b.)Sub-dominant •••••••••••••• ,20 cm (d.b.h.o.b.)

There were 44 dominants, 134 co-dominants and 30 sub-dominants. In percentageterms, the dominants were roughly 21%, co-dominants 64% and sub-dominants 15%.With this distribution, and considering the time and funds available for thesampling, it was decided to select for examination two trees from the dominantcanopy class, five from the co-dominant class and two from the sub-dominantclass. This made the total of nine trees examined.

All the sample trees were selected on the basis of stem and crown qualities,following the procedure of Burley and Wood (1976). The nine sample treespresented the best trees in the stand. It was clear from the start of theexperiment that owing to the cutting problems anticipated in the removal ofsmall clear samples from bolts, such as high proportion of knots and graindefects, complete internodal sampling might be difficul t to attain. It wastherefore necessary to extract the maximum nwnber of comparable samples betweentrees in such a way that the description of the pattern of variation of studiedproperties would not be seriously jeopardised or biased. An additionalconstraint was the need to procure bolts of at least 60 cm length at eachsampling position in each tree in order to ensure that a clear specimen of 30cm maximum length could be obtained at each within-bolt sampling position.

In the event an approximation to sequential sampling was carried out enablingtrees to be compared with one another in the sample plot. This was achieved bydividing each tree radially and lengthwise into 10 predetermined positionscommon to all the nine sample trees. These positions identified as zones, arelabelled from apex to base with the letters A to J and shown in Figure 2.1. In~he vertical direction, the 27 internodes from apex (IFA) have been dividedinto four zones; to IFA (because the last la rings to "the bark were verysmall), then 15 IFA, 20 IFA, and finally the remaining 27 IFA (Figure 2.1).

In each tre~, therefore, one sampl(~ for testing was t.aken from each of theseten zones to give a modified "Duff and Nolan" sampling system (see figure 3.1).

6

2.2 Preparation of Test Specimens

The method of preparing and testing small clear specimens of wood are specifiedin both British Standard No 373:1957 "Method of testing small clear specimensof timber" and the American Society of Testing Materials Standard D143-5Z(1952), reapproved (1965). The former is in general use because itaccommodates samples of small metric sizes of 20 mm standards.

It was clear from the start of the study that it would be necessary to usespecimen sizes below the standard sizes of 20 mm recommended (B. S. 373: 1957).It was therefore decided to work within the size range of 4 mm to 20 mm, squaresections.

Since the number of rings encountered in a bolt is equivalent to its positionin terms of number of internodes from apex of the tree, it was possible tolocate each sample with the appropriate internode by cross-cutting the samplebolt so as to gain (cut at the top) or lose (cut at the base) some rings; untilthe entire bolt fell within the required internode from the tree apex.

Following the work of Wood (1970), it was decided to test samples three ringsin width radial1y. This was necessary because the cutting procedure adoptedrequired that test samples must be square in cross-section and at least tworings apart to allow for saw kerf. Thus from each bolt, rings number 4, 9, 14and 19 from the pith were chosen. Specimen size was consequently determined bythe width of the rings at each sample location so that each sample containedthe complete sample ring and sufficient material from rings on either side togive a square cross section. Specimen length was then determined on the basisof !::= 14, for static bending tests and - = 3, for maximum compressive strengthparJilel-to-grain, where L = length of ~ecimen and D = depth of specimen. Themethod of within-tree sampling is summarized in Figure 2.2.

In all there were 20 samples from each tree, 10 on either side of the pithexcept in tree 7 where there were only 18 samples. In all the trees it waspossible to extract standard sized specimens (20 mm x 20 mm x 300 mm)(B.S.373:1957), within the first 4 to 10 rings from the pith, at the base ofthe tree. These samples were valuable subsequently in calculating the F-ratio(the ratio of the strength value of standard-sized specimen to that of non­standard specimen), in order to find out the relationship between the two typesof samples.

The removal of the test stick from the sample bolt, (Figure 2.2) followedvarious stages of cutting and planing. The first stage involved the removal ofa strip which was planed on one side. The ring continuity from base waschecked and a suitable position was selected on the 60 cm strip which wasdefect-free and which could give the length of the sample required by the ratioof the width of specimen to length. of 1 : 14. (Span/depth ratio of 14 : 1).The longitudinal position of the sample determined on this basis was then drawnon the planed surface of the strip. The other face of the strip was thenmachined until the entire sample was extracted, with the grain parallel to theedge of the stick and the sampled ring running through the entire length of thestick. Whenever a clear sample could not be removed, the sample size wasstepped downwards still keeping the sampled ring until a defect-free sample wasachieved.

7

2.3 Evaluation of Wood properties

Nominal specific gravity (SPG), modulus of rupture (MOR), modulus of elasticity(MOE) , total work done (TWD) and maximum compressive strength parallel-to-grain(MCS) were determined as follows:

2.3.1 Nominal Specific Gravity

Gravimetric density was measured by measuring the dimensions of the testsamples in three planes, using a dial gauge accurate to 0.01mm, calculatingvolume and weighing oven dry. This gave basic density (volume green and ovendry, dry weight.)

2.3.2 Static Bending

Static bending tests were carried out on an rnstron Universal testing machine,series TT-CM 1190. The stress/strain curve was recorded on a chart recorderand the integrator calculated the area under the curve as the latterprogressed. For each test, and each specimen, the load was sustained until thespecimen failed to support one tenth of the maximum load recorded or deflectedmore than 60 mm under stress, whichever occured first.

The load was applied centrally to each specimen, the largest being 20 mm x 20mm x 300 mm, which was supported over a span of 280 mm (B.S.373:1957). Thespecimen sizes used decreased in steps of one millimetre from the standard 20mm to 4 mm. For all the specimens, the span:depth ratio was 14:1. The rangeof specimen sizes and corresponding length are shown on Table 2.1. The testjig was specially designed to accommodate the varying sample sizes and the verysmall spans. Two loading heads were used for the entire range of sizes.Samples 20 mm to 15 mm in size were loaded with the large head recommended inB. S. 373: 1957. This head had a curvature with radius of 30 mm. Smallerspecimens, from 14 mm to 4 mm, were loaded with the small non-standard headwi th a curvature of 15 mm. This loading head was successfully used in asimilar study, Wood (1970). The load at full scale deflection varied withspecimen size from 500 kg for size 20 mm, to 50 kg for sizes 7 mm to 4 mm.The annual rings in the test sticks were oriented parallel to the direction ofloading, that is the test sticks were loaded on the radial face.

Deflections of the specimens during the tests were derived from the chartrecord and not by using an extensometer. The chart speed used for all thetests was 2 cm/min. (2Omm/min).

The speed of testing, that Is descent of the loading head, was calculated foreach specimen size; it varied from 6.6mm/min for the 20 x 20mm samples to0.66mm/min for the 2 x 2mm samples.

In a previous investigation (Wood, 1970), which considered non-standard sizedspecimens, comparison with standard sized samples was accomplished by usingadditional material outside the scope of the main experiment. The presentinvestigation incorporated some 16 standard sized samples in the total of 178samples examined. The total distribution of samples by size is shown in Table2.2, together with the number of samples and their mean strength values. Byplotting and calculating the regression of F-ratio (the ratio of the strengthof a non-standard specimen to that of a standard sized specimen) against thespecimen size, the variation of F-ratio with the size of specimen wascalculated for each strength property shown in Table 2.2.

8

A summary of the regression equations for F-ratio against specimen size foreach of these strength parameters is shown in Table 2.4. The importance ofthese equations is in the low values recorded for the slope of each of thelines, regression coefficient, b in the general formula F = a + bD, where F isF-ratio, a and b are coefficients and D is specimen depth. Although, thecalculated regressions are not statistically significant, they all approximateto a low value of 'b' indicating that the slope of the line approaches a zerovalue. Thus it may be concluded from the evidence of the relationship of F­ratio against specimen size that in the present investigation, where a commondepth:span ratio of 1:14 is used and the testing speed is adjusted toaccommodate variations in the depth of the test pieces, the effect of specimensize on strength in static bending is neg1igiblefor the range of sizes tested.

The slope of grain on each test stick was measured in both tangential facesbefore testing, using a scribe to trace the grain. The angle of the grain lineto the edge of the piece was measured with a protractor. The larger of the twomeasurements were recorded and used in subsequent analysis. The result of eachstatic bending test was corrected for slope of grain following the approach ofWilson (1921).

The method of assessing and correcting the influence of slope of grain onstrength was based on a graphical representation.

The specimens in the present study were grouped into six classes of slope ofgrain based on severity of the defect:

1. Low slope (L) 0° 1.50 (mid-class, 0.750)

2. Moderate Slope (M) 2.00 3.50 (mid-class, 2.75°)3. Fairly High Slope (FH) 4.00 5.50 (mid-class, 4.750

)

4. High Slope (H) 6.00 7.50 (mid-class, 6.75°)5. Very High Slope (VH) 8.90 9.5° (mid-class, 8.75°)6. Extremely High Slope (EH) 10.0° - 11.50 (mid-class, 10.750

)

Strength property multiplying factors calculated by Wilson (1921) based onthese classes are presented on Table 2.3 and were used for this study. Thestrength recorded for each test specimen was corrected using the multiplyingfactors shown in Table 2.3. The corrected values are used throughout theinterpretation of strength data in this report.

The· present investigation was conducted with green wood samples, hence theinfluence of moisture content between samples was not critical. The influenceof temperature variation on the tests results was however monitored. The testswere carried out in a room having a controlled temperature and relativehumidity of 20 + 30 C (68 + 6°F) and 65 + 2 per cent respectively asspecified in B.s.373:1957.

2.3.3 Compression Tests

Samples used for the compression parallel-to-grain tests were extracted fromthe static bending sticks after the tests were completed. Consequently, thesizes of specimens employed in this test also corresponded with those used inthe static bending tests (4 mm x 4 mm to 20 mm x 20 mm cross sections). Thedepth:length ratio was kept constant at 1:3 as stipulated in B.S.373:1957.This ratio has been found to reduce the tendency for the specimen to buckleduring test (WOod, 1970). The range of sizes is shown on Table 2.2.

9

For the 20 mm x 60 mm standard, the B.S.373:1957 prescribes that the loadingplates should approach each other at a constant rate of 0.025 in/min (0.01mm/sec). To use sizes smaller than the standard, it was necessary to keep thedepth:length ratio constant and to adjust the speed of loading according to agiven constant rate of fibre stress common to all specimen sizes. SeeAppendix 2.

After each test, the maximum compressive strength parallel-to-grain was readdirectly from the recorded chart. The testing machine automatically dividedthe load sustained during the test with the cross sectional area of thespecimen, before recording it as a stress value on the chart. . Maximumcompressive strength parallel-ta-grain is normally exressed in N/mm2 •

The influence of sample size on the strength in compression parallel-to-grainwas also examined as with the static bending tests, using the F-ratio.

The mean values for each specimen size in compressive strength parallel-to­grain and the various sample sizes are shown in Table 2.2. The regression ofF-ratio against specimen size in this case, is shown in Table 2.4. It is clearfrom the regression coefficient 'b' that the slope of the regression in thecase of compressive strength parallel-to-grain, that the slope approximates tozero and that the correlation coefficient of the regression was notstatistically significant. This clearly demonstrates that the effect ofspecimen size on compressive strength para1lel-to-grain is negligible for thesizes tested once the appropriate adjustment is made to the speed of testingand the depth: length ratio is kept constant at 1:3 for each specimen.

The compression tests were carried out in a controlled room with temperature of20 ~ 30 C (68 + 60 F) and relative humidity of 65 ~ 2% as specified inB.S.373:1957. The tests were carried out on green timber as in the case of thestatic bending tests.

The influence of slope of grain on maximum compressive strength parallel-to­grain was also examined. As with the static bending tests, each test resultwas adjusted using the approach of Wilson (1921).

Section 3: RESULTS AND DISCUSSION

3.1 Pattern of Variation

This section presents the main test results of this investigation. Theanalysis employed is limited to the graphical analysis suggested by Duff andNolan (1953) and Richardson (1961).

Duff and Nolan (1953) established the systematic variation of ring width withthe position of the crown in Pinus resinosa Ait. They found three general mainpatterns of variation within~ree based on the factors controlling cambialgrowth. These patterns are the oblique variation, radial variation, andvertical variation.

Duff and Nolan (1953), observed that three main factors act as controls fortree growth and these are responsible for the three patterns of variation.These factors are:

(1) Systematic factors or complexes of factors, which operate to induce certainuniformity or pattern in the distribution of growth activities in the tree no

10

matter what the conditions of growth may be.

(2) Systematic factors or complexes of factors operating to induce regularityin the distribution of growth activity which are not the same under allcircumstances but vary with variation in growth conditions such as site andstand density.

(3) Randomised factors, the action of which tend to produce irregularity andfluctuations in the distribution of growth activity.

In order to separate and assess these three types of variation discussed above,Duff and Nolan suggested a consideration of the activities of the cambiumforming the wood. Thus they proposed the following three sequences teachidentifiable with the three main patterns of variation within a tree:

(1) Variation 1: sequence type 1 - oblique. A cambium operating as a singlesheath from tip to base of the tree, therefore showing the effect of increasingcambial age operating ina common calendar year. (Variation down the treewithin a particular growth ring)

(2) Variation 2: sequence type 2 - horizontal radial. A cambium increasing inage from pith to bark in a radial horizontal plane, showing the effect ofincreasing cambial age with corresponding increase in Calendar year.

(3) Variation 3: sequence type 3 - vertical. Cambium of common physiologicalage, operating in progressively increasing calendar years. This sequence isvertical.

The description of these patterns has been confirmed by many researchersworking with conifers and hardwoods; by using additional wood properties to thering width used by Duff and Nolan (1953), such as density, tracheid length andfibre length (Richardson, 1961); Dinwoodie, 1963; Kandeel and Bensend, 1969).Wood (1970), established the systematic variation of some mechanical strengthproperties:- modulus of rupture, modulus of elasticity, total work done,maximum compressive strength parallel-to-grain, maximum shear strength, andhardness in Sitka spruce (Picea sitchensis Carr.), by employing completeinternodal sampling. The present study follows the pattern established byWood, (1970).

Graphs were drawn according to the method of Duff and Nolan (1953), to show thepattern of variation in the selected wood properties associated with changes inspatial parameters within the tree. The spatial parameters were: number ofinternodes from apex (sequence type 1 - oblique sequence; number of rings frompith (sequence type 2 - horizontal sequence); and year of wood formation(sequence type 3 - vertical sequence).

The sequences of plots and associated zones and notations employed on thegraphs are as follows t with illustration in Figure 3.1 t which is based onFigure 2.1 on within-tree sampling system:

Sequence type 1: oblique sequence (Figure 3.1) Plotting details:

Zones A C F J = Outermost band, joined by notationZones B E I = Outermost band, joined by notationZones D H • Inner band, joined by notationsZone G = Innermost band, shown with

••oo

11

The vertical axis of the graph in this sequence is the property in questionwhile the horizontal axis is the approximate number of internodes from apex(IFA) •

Sequence type 2: horizontal sequence (Figure 3.1) Plotting details:

Zone AZones B CZones D E FZones G H I J

Uppermost band shown with aUpper band t joined by notationLower band t joined by notationLowermost band t joined by notation

oo••

The vertical axis of the graph is the property in question while the horizontalaxis is the approximate number of rings from pith (NRP).

Sequence type 3: vertical sequence (Figure 3.1) plotting details:

Zones A B D GZones C E HZones F IZone J

Innermost band, joined by notationInner band, joined by notationOuter band, joined by notationOutermost band shown with

••oo

The vertical axis of the graph is the property in question as in the two casesabove, while the horizontal axis is the approximate year of wood formation(YOF).

For each wood characteristic, there were 39 graphs, comprising 3 sequences x 9trees combined data for each of 3 canopy classes (dominants = 40 samples, co­dominants = 90 samples and sub-Dominants = 40 samples) and an all trees pooleddata (178 samples) (3 x (9 + 3 + 1) = 39). Since five principal propertieswere investigated, the total number of graphs produced was 5 x 3 x 13 = 195.Each graph was examined and described. It became obvious after the patternexhibited by the individual trees in most cases conformed generally with thosebased on measurements averaged over all trees and canopy classes. Moreover,the more the trees were pooled together, the simpler was the pattern exhibited.Consequently, only relevant graphs, which elucidate the inter-tree variation inwood properties examined, have been highlighted.

3.1.1 Nominal Specific Gravity

The mean nominal specific gravity value of all the nine trees examined was0.548 ~ 0.004. This value was slightly higher than those reported previouslyfor teak in Nigeria (reported in Forest Products research Bulletin No 50(1969), HMSO): .

Site

Earlier study:

No of treestested

Approxage (Yrs)

Conditionat test

Specific GravityMean SD n

Western region

Northern region

Recent study:

6

6

39-40

33

green

green

0.530

0.530

0.039

0.056

56

71

Western region 9 27 green 0.548 0.055 178

12

These results are strictly not comparable; in the recent study, the trees wereolder and fewer samples were examined. The present study showed the dominantcanopy class had the highest average nominal specific gravity (0.582) followedby co-dominant (0.545) and sub-dominant (0.523). Statistical analyses showedthat these means were not statistically significantly different from oneanother at the 95% level of significance.

(a) Variation of Nominal Specific Gravity in the Oblique Sequence

The variation of nominal gravity in the oblique sequence (Duff and Na1an(1953), sequence 1), is shown in Figure 3.2. Nominal specific gravitydecreases sharply with increasing number of internodes from the apex. However,the consistency of the pattern as shown in the combined data is matched only bythe co-dominant group when the data are parti tioned by crown· class. thecombined trees data (CID) (Figure 3.2), clearly shows that the variation ofspecific gravity within the tree is systematic. Thus, within anyone group ofzones traced downwards from the apex, specific gravity decreases sharplytowards the tree base.

(b) Variation of Nominal Specific Gravity in the Horizontal Sequence

In the horizontal sequence (Duff and Nolan (1953), sequence 2), nominalspecific gravity tends to decrease from pith to bark with increasing age,(Figure 3.2). This pattern is, however, much more complex when data arepartitioned by crown classes, and at different sampling heights, both dominantsand co-dominants show patterns which indicate an initial increase in specificgravity around the pith, followed by a decrease towards the tree periphery(Figure 3.2). It is evident that systematic variation in the horizontalsequence is less marked than in the oblique sequence, a finding which isconfirmed by the few authors who have attemped Duff and Nolan (1953) sequencialanalysis. Richardson (1961) observed that in the horizontal sequence (sequence2), variation in the outermost rings is less systematic than in the centre ofthe tree.

(c) Variation of Nominal Specific Gravity in the Vertical Sequence

The vertical sequence is normally considered to be the least uniform indistribution of the wood parameters. Calendar year and climatic influenceshave a limited influence on wood production except, in catastrophic years ofdrought, flood or frost. There is some danger in attempting to analyse thispattern from sampling systems which are not complete, for all of the years asin the present study. The trends observed here (Figure 3.2), indicate thatspecific gravity increases with tree age, until a maximum is reached around1969 and thence the specific gravity value is maintained or decreases.Clearly, there is some influence of calendar year on specific gravity (Figure3.2), at least in the early years of the tree growth (1958-1969) when thegrowth is vigorous. The trend seen in the sub-dominant crown class in thissequence, deviates from those seen in the other crown classes or the combineddata. This fact may be attributed to the suppression of the crown class withinthe stand, after canopy closure, resulting in slower growth and lower specificgravity with increasing age. .

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3.1.2 Modulus of Rupture (MOR)

The mean modulus of rupture obtained in this study was clearly higher thanthose reported previously for teak grown in Nigeria (Forest Products ResearchBulletin No 50 (1969) HMSO). Mean values were as follows:~~~----~----~--~~--~~-~--~~--~----~--~-~-~~~-----~----~-~--~-----~---------~-~-

Site/locations No of trees Approxtested age (yrs)

Conditionof test

Modulus of Ru~ture

(MOR) N/mmMean SD n

--~-~---~-~~~~~---~-~-~--~--~~--~-~----~--~---~-----~~~-~--~~-~~----------~----

Earlier study:

Western region

Northern region

Recent study:

6

6

39-40

33

green

green

90.0

83.0

9.80

9.50

27

33

Western region 9 27 green 111.59 27.07 178

These figures are not strictly comparable because in the present study, sampleswere removed systematically from the tree while the previous study, was basedon random selection of samples (probably around the tree base). Moreover,trees used in the previous study were fewer, and older. It may also berecalled that nominal specific gravity was higher in the present study than inthe earlier one. As with specific gravity, the effect of crown class onnominal specific gravity was not statistically significant. Ranked means ofthe canopy classes showed the dominant crown class to have the highest MORvalue (124.60 + 4.70 N/mm2) , followed by the sub-dominant class (108.78 +4.331 N/mm2) and the co-dominant class (107.44 ~ 2.454 N/mm2).

(a) Variation of Modulus of Rupture (MOR) in the Oblique Sequence

The pattern suggests that MOR falls with increasing number of internodes fromthe apex. This pattern is clearly shown in the combined trees data (CTD) butwithin the individual canopy classes, especially in the sub-dominant class, thepattern is extremely variable. (Figure 3.3).

(b) Variation of Modulus of Rupture (MOR) in the Horizontal Sequence

Figure 3.3 shows that the variation of MOR from the pith outwards varies withcanopy classes suggesting a complex and variable pattern of MOR between trees.The combined trees data indicate that the variation of MOR across the treecross-section is systematic in that, with the exception of some regions aroundthe tree apex, MOR increases from pith (age) towards the tree periphery until amaximum value is reached near the pith before decreasing towards the bark.

(c) Variation of Modulus of Rupture (MOR) in the Vertical Sequence

In the vertical sequence, (Figure 3.3), the combined trees data (CTD), and tosome extent the dominant trees data, show a clear systematic pattern ofvariation of MOR with year of wood formation (YOF). This pattern indicatesthat MOR increases with increasing year of wood formation from year of plantingto around 1970 and thereafter MOR remains uniform or decreases with increasing

14

calendar year in a rather unsystematic pattern. The co-dominant and sub­dominant canopy classes show no evidence of systematic pattern of MOR withcalendar year (Figure 3.3).

3.1.3 Modulus of Elasticity (MOE)

The mean modulus of elasticity (MOE) value of the combined trees data in thisstudy was 11072.04 N/mm2 and this value was higher than those found in anearlier study reported in Forest Products Research Laboratory, Bulletin No SO,

(1969) HMSO:

Site/location

Earlier study:

No of trees Approximate Conditiontested age of test

Modulus of Elasticity(MOE) N/mm2

Mean SD n

Western Nigeria

Northern Nigeria

Recent study:

6

6

39-40

33

green

green

8900

8900

1360.0

1250.0

27

33

Western Nigeria 9 27 green 11072.04 2638.07 178

The reason for the higher MOE value obtained in the current study may berelated to age. The other reason is related to the intensity of within-treesampling as mentioned above.

As in the case of MOR, canopy class had no effect on MOE. The dominant crownclass had higher mean MOE value (12162.0 + 472.37 N/mm ) followed by the co­dominants (10912.32 + 241.70 N/mm ) and the sub-dominant class (10373.4 +407.33 N/mm ). The- sub-dominants had slightly more variable ~iOE values (CV24.83%) than the co-dominants (CV 21.93%) or the dominants (CV 23.84%).

(a) Variation of Modulus of Elasticity (MOE) in the Oblique Sequence

Figure :3.4 shows the variation of MOE in the oblique sequence. There is ageneral trend for MOE to decrease sharply from tree apex to base with increasein the number of internodes from apex. This trend is very clear in thedominant, co-dominant and the combined tree data, especially in the region nearthe tree periphery. In the sub-dominant trees data, the pattern is morevariable, but from the combined trees data it can be assumed that the variationof MOE in this sequence is systematic.

(b) Variation of Modulus of Elasticity (MOE) in the Horizontal Sequence

From Figure 3.4, the highest MOE values occur close to the pith and the lowestoccur further away from it. Similarly, the largest amount of variation in MOEoccurs near the pith, in all the data groups, suggesting that MOE may not havea systematic pattern near the pith. However, the general pattern seen in thesequence (Figure 3.4), is that MOE increases slightly from pith to about thetenth ring from pith and thereafter decreases rather sharply towards the treeperiphery with increase in age from pith.

15

(c) Variation of Modulus of Elasticity (MOE) in the Vertical Sequence

The pattern exhibited by the sub-dominant crown classes was different fromthose seen in the other crown classes or the combined trees data (Figure 3.4).The sub-dominant crown class, showed a slight increase in MOE with increase inthe year of wood formation (YOF) , while the other crown classes and thecombined data showed no apparent change in MOE with YOF (Figure 3.4).

3.1.4 Total Work Done (TWO)

The overall mean total work done (TWD) (n = 178) in this study was. 0.475 +0.018 mm N/mm3 , with a coefficient of variation of 49.9%. This mean valuewas higher than those reported earlier for teak grown in Nigeria presented inthe Forest Products Research Bulletin No 50 (1969) HMSO.

Site/location

Earlier study:

No of trees Approximate Conditionage (Yrs) of test

Total Work Done(mm N/mm3 )

Mean SD n

Western region

Northern region

Recent study:

Western region

6

6

9

39-40

33

27

green

green

green

0.360

0.263

0.475

0.128

0.100

0.237

27

33

178

The higher value of TWD in the present study can be attributed to the samereasons given above for other properties.

Between canopy classes, the average TWO values ranged from 0.42 mm N/mm3 forthe co-dominant class to 0.572 mm N/mm3 for the sub-dominant class. Thedominant crown class had a TWO value of 0.514 mm N/mm3• The influence ofcrown class on total work done was not statistically significant at the 95%confidence level.

(a) Variation of Total Work Done (TWO) in the Oblique Sequence

Figure 3.5 shows that TWD is extremely variable within the tree. In thedominant class TWO increases rapidly from apex to about the tenth internode,then decreases gradually towards the base of the tree. In the other two canopyclasses, the variation of TWO around the pith was so extensive that it almostsuggested the absence of a systematic pattern. However, when the combinedtrees data were examined, the pattern in the dominant class was apparent thoughwith a greater amount of variation around the pith.

(b) Variation of Total Work done (TWD) in the Horizontal Sequence

In the horizontal sequence, the pattern of TWO showed that at anyone level TWDvalues were low near the pith and then increased with increasing ring numberfrom pith but attained a maximum value between the IQ-15th ring from the pithbefore decreasing towards the tree periphery. This pattern is clearly shown on

16

the combined trees data and in the partitioned crown class data (Figure 3.5).

(c) Variation of Total Work Done (TWO) in the Vertical Sequence

When traced through successive internodes down the tree at fixed number ofrings from pith, TWD showed in the dominant. co-dominant and the combined treesdata, a pattern suggesting a sharp increase in TWO from year of planting withincreasing year of wood formation (Calendar year). In the sub-dominant data.the pattern was extremely variable suggesting no systematic variation betweenTWO and YOF (Figure 3.5).

3.1.5 Maximum Compressive Strength Parallel-to-grain (MCS)

The overall mean maximum compressive strength paralle1-to-grain (MCS) of thecombined data (n=178) was 41.953 + 0.568 N/mm with a coefficient ofvariability of 18.06%. This mean valUe compared favourably with previous workon teak in Nigeria (in Forest Products Research Bulletin, No 50 (1969) HMSO).These figures are close to those obtained in previous studies.

n

Site/location No of trees Approximate Conditionage (yrs) of test

Max. Comp. Str. 1/grain (MCS)

MeanN/mm2 SD

Earlier study:

Western region 6 39-40 green 37.90 5.29 71

Northern region 6 33 green 41.20 5.54 56

Recent study:

Western region 9 27 green 41.95 7.58 178------~-------~~~~-----~--~--------~--~---~~--~~~~----~-~~-~---~--~-~-~--~~-~-~

Amongst the crown classes, the dominants, had the highest mean MCS (46.54 +1.332 N/mm2) with a coefficient of variation of 18.11%. followed by the co::dominant (41.132 + 0.569 N/mm2) with a CV of 13.66% and finally the sub­dominants (39.369- + 1.423 N/mm2) with a CV of 22.85%. With this lowdifference in the mean MCS values between crown classes, it is not surprisingthat the effect of canopy class on MCS is not statistically significant.

(8) Variation of Maximum Compressive Strength Paral1el-to-Grain in the ObliqueSequence.

In the oblique sequence. (Figure 3.6). there was a general trend for MCS todecrease with increasing number of internodes from the apex. This trend waswell represented in the combined trees data (CTD). In this data group. therewas very little variation in the property. The variation of MCS around thepith was high in the crown classes, dominant and sub-dominant but considerablylower in the co-dominant crown class and the combined data.

17

(b) Variation of Maximum Compressive Strength Parallel-to-Grain in theHorizontal Sequence

The horizontal sequence (Figure 3.6), showed that within the individual crownclasses, and the combined data (CTD), MCS decreased from pith towards the treeperiphery with increasing age (number of rings) from the pith. At thedifferent heights or internodes levels on the tree trunk the patterns observedvaried considerably amongst the crown classes. An interesting obse.rvation,however, was that in this sequence, the lowest MCS value in each of the crownclasses and the combined trees data occured in the zone nearest the bark of thetree base (Figure 3.6).

(c) Variation of Maximum Compressive Strength Parallel-to-Grain in theVertical Sequence

The vertical sequence, showed that increased slightly with year of woodformation. Since this sequence allows the examination of the influence ofcalendar year on MCS, it can be assumed that the year to year variation in theenvironment was not detected in this investigation to have any influence on themaximum compressive strength parallel-to-grain (MCS).

3.2 Sampling and Testing Techniques

The present study has applied two relatively new approaches to the testing andanalysis of intra-tree pattern of variation of strength properties withintropical broadleaf species as follows:

1. the use of internodal sampling on teak, a tropical broadleaf that doesnot display internodes as in coniferous species but has regular annualrings, and

2. the use of the Duff and Nolan (1953) graphical analysis method ofstudying and describing' the nature of variation of wood propertieswithin trees.

This paper is, therefore, not complete without reviewing the difficulties andadvantages of these new approaches in the study of variation of woodcharacteristics within trees and its biological basis.

Teak in the study area (southern Nigeria) displays distinct rings which aremore or less annual. The determination of internodal positions was easilyachieved by counting the number of rings at any given height level. Thisnumber at any level is equivalent to the number of internodes from the apex ofthe tree in which that segment is removed.

Ring counting in the field was found to be extremely difficult and timeconsuming, so this was not done until the segments were moved to thelaboratory. This subsequently created more problems because the internodes didnot match one another between trees. This problem was satisfactorily solved bygrouping the internodes along the tree stem into la pre-determined positions,and by this comparison between trees was made possible between these zoneswhich were common to all the sample trees.

18

This approximation severely reduced the number of samples available within thetree to describe the variation. There were 10 sample zones» each zonereplicated twice (i.e. opposite sides of the pith) resulting in 20 test sticksin each tree, except co-dominant tree 7 with 18 test sticks. Undoubtedly,future studies of this nature should attempt to count the rings in the fieldand to reduce the size of the sampling zones within the tree as much aspossible. The best internodal sampling system within the tree, as Wood (1970)has shown» is the complete sampling where samples are removed from everyinternode from apex to base. It must however be stated that any increase inprecision deriving from such improved sampling techniques is only justifiableif it compensates for the increase in time, effort and cost spent in obtainingit, (Snedecor and Cochran, 1967, page 531).

The advantage of systematic sampling within the tree has been highlighted byRichardson (1961). When it is necessary to describe the pattern of variationwithin the tree and isolate the influences of the physiological age of thecambium in the variation of wood properties, it is of utmost necessity toselect samples within the tree on the basis of biological growth pattern withinit. This is what sequential sampling and analysis is all about.

Theoretically» tree means derived from samples selected systematically withinthe tree may be different from those derived from samples selected randomly.This is because the systematic selection considers all the different 'types' ofwood corewood and outer wood in the tree while, with random selection, there isa tendency for a bias towards the outer wood. This is why some investigationsweight values of samples for position of the sample on the tree circumference(Kandeel and Bensend, 1969).

The second aspect of this investigation is the assessment of strengthproperties by the use of non-standard specimens. This deviation from standardwas possible in this investigation mainly because of the inspiration andknowledge gained from the work of Wood (1970) who used non-standard samples ondetermining the strength properties in Sitka spruce. The influence of thisapproach was assessed after the experiments were completed. This was done byaveraging the individual strength value for each specimen size and thenrelating this to the standard size 2 cm x 2 cm stick and calculating F-ratio(the strength ratio of the non-standard stick, to the standard stick). Thesample sizes used in this investigation range from 0.4 cm to 2 cm (17 sizes).On the basis of these 17 sizes, the relationship of F with depth of stick wasthen determined by simple linear regression analysis. It was noted and isdiscussed in Section 2.3 that the overall regression F against D for each ofthe properties MOR, MOE, TWO and MCS was significant at the 5% level in onlyMOR and TWO, and in ~fOE and MCS was not significant. In all the fourregressions, the influence of 'D' (depth) ranges only from 14.2% for MCS to38.4% for MOR. These 'R' values are considered quite low (Wood, 1970) and itwas not considered necessary to correct further for specimen size, mainlybecause this will also introduce more errors. Since the size of test specimenswas related to ring width and, therefore, position in the stem, there is likelyto be some systematic variation introduced and small correlations can beexpected.

The knowledge available on the effect of specimen depth or volume on strengthproperties is still very scanty, hence it is suggested that in future more workshould be done on this aspect especially relating to hardwoods.

19

The samples used in this investigation were aimed to study one single ring, butbecause of the difficulties of extracting good samples, 3 rings, one on eitherside of the sample, were taken. In some cases, when the rings were very smallas in the last 10 growth layers to the bark, more than 3 rings were containedin the smallest possible size. In this situation the sample ring was kept inthe centre of the specimen. To facilitate the use of non-standard sizes, thespan: depth ratio was kept at 14 as suggested in B.S.373:1957 (Table 2.1). Foreach specimen size, the speed of testing was adjusted to give a constant rateof fibre stress. These speeds were derived from equations explained inAppendix 1. These series of equations explore the relationships betweenstrength, depth, speed of testing, and span: depth ratio for static bending andmaximum compressive strength tests. There is no reason to doubt the validityof the approach of testing used here as these methods are similar to those usedin many previous investigations, and are fully explained in Section 2.3 of thispaper. Moreover, the results for the present investigation compare favourablywith those published in many previcus reports.

In all the samples tested, the slope of grain was assessed and corrected for,using procedures suggested by Wilson (1921) also explained in Section 2.3 inthis paper. The quality of the trees was slightly low, in terms of slopes ofgrain and amount of knots. This is the main reason why the smaller non­standard sizes are invaluable for this type of investigation on such lowquality timber. The smaller the samples for test, the better the qualitybecause defects are easily removed by cutting procedures.

Section 4: CONCLUSIONS

The main objectives of this study as stated in the introduction were, in verybroad terms, to examine the variation of five wood properties of teak grown inNigeria, within trees, selected on the same site. Based on the 9 treesexamined, the following conclusions were arrived at:

1. Based on the combined trees data, all the wood properties examined, thoughto varying extent, show systematic variation within the trees.

2. To compare the effects of crown classes, it was desirable to partition thedata by crown classes for graphical analysis, along with the unpartitionedcombined trees data. In all cases, the combined data showed a simpler andclearer pattern and on this basis the pattern of variation of each of thefive wood properties can be described according to the Duff and Nolan(1953) terminologies as follows:

(a) Oblique Sequence (Sequence type 1)

Nominal specific gravity and the four strength properties (MOE, MOE,TWO and MCS), were generally found to decrease from tree apex to basewith increasing number of internodes from the apex, but TWD ofdominant trees increased slightly from apex to about the 10thinternode from apex before decreasing towards the tree base withincreasing number of internodes from apex.

(b) Horizontal Sequence (Sequence type 2)

Nominal specific gravity and some strength properties (MOR and MCS)were found generally to increase initially from pith towards the treeperiphery, attain a maximum value, sometimes near the pith as in the

20

case of specific gravity or further away from it, as in the case ofTWO before decreasing gradually towards the bark, with increase innumber of rings from the pith.

(c) Vertical Sequence (Sequence type 3)

Nominal specific gravity and some strength properties (MOR and TWO)were found to increase generally with increasing year of woodformation. Sampling constraints did not make it possible to observeany special influence of calendar year on MOE and MCS as would bepossible if complete internodal sampling were employed on the trees.On the whole it appears that faster tree growth and wider ring widthwithin trees give higher wood density and strength.

3. The use of non-standard small clear samples for mechanical strengthtesting in this investigation was successful and this method is advocatedfor future research, on fast grown tropical re-growth hardwoods.

4. The advantages of the systematic (internodal) sampling within the treehave been highlighted and this method is strongly recommended for tropicalbroad leaf species with regular annual growth rings as teak, even thoughthey do not show visible internodes as in coniferous species.

4.1 Suggestions for Further Study

This work has discussed one of the major aspects necessary for the successfulutilization of teak grown in Nigeria, especially from the fast grownplantations. Some other areas however, need to be investigated in future tocomplement the present and earlier investigations. The following pertinentareas of research are suggested:

1. Wood quality should be monitored from time to time. It is necessary thatsilvicultural practices such as pruning and thinning should be appliedwith caution and monitored to find out the influences of these activitieson cambial activity and xylem differentiation (Kromhout and Bosman, 1981).

2. The desirability of systematic intra-tree sampling has been highlighted(Duff and Nolan, 1953; Richardson, 1961; Elliott, 1970; Wood, 1970). Thismethod of sampling has been successfully used in the currentinvestigation. It is necessary that this sytem is intensified for teakand other tropical broadleaf trees.

3. In earlier investigations on the mechanical and physical properties ofwood, standard samples were normally used. In the current study, non­standard samples have been successfully used. This investigation was ableto show that where a common depth:span ratio of 1: 14 (in the case ofstatic bending) and depth:length of 1:3 (in the case of compressivestrength), are used and the speed of testing is adjusted to accommodatevariation in the depth of the test pieces, the effect of specimen size onstrength is negligible. It would be desirable in future to develop aclearer picture of the effect of sample depth on strength of wood, withspecial reference to hardwoods for the use of non-standard (smaller) testsamples. This is necessary in tests of strength in quality timber stands.This method could possibly develop into a 'non-destructive' method,whereby only small samples are removed from the tree without sacrificing

21

the tree. Another advantage of non-standard samples is that the smaller thesamples, the fewer are the strength-reducing defects. Hence it becomes easy totake testable samples from lower quality fast grown tropical trees.

ACKNOWLEDGEMENTS

I gratefully acknowledge the assistance of my supervisor, Dr G K Elliott, ofthe Department of Forestry and Wood Science, University College of North Wales,Bangor, Wales UK, where this work was carried out and that of many otherpersons who are too numerous to mention here: Professor L Roche is speciallythanked here.

I acknowledge the financial assistance of FAO, Inter-University Council (IUC)and the University of lbadan, Ibadan, Nigeria.

Without the financial and moral assistance of Sanwo family of Iwasi, Ijebu ­Imushin, Ogun State, Nigeria, this work would not have been completed. MrsAnike Sanwo and children Wale, Bose and Kayode are also gratefully thanked.

I acknowledge with thanks the encouragement of the staff and members of theCommonwealth Forestry Institute, South Parks Road, Oxford, England, especiallythat of Mr R A Plumptre, without whose interest in the work this paper wouldnever have been published.

22

REFERENCES

A.S.T.M., 1952. "American Society of Testing Material, DI43-52.Method of testi.ng small clear specimens of timber".

Standard

Bohannan, B. 1966. "Effect of size on bending strength of wood members". U.S.Forest Service Research, F.P.L. 56, May 1966.

B.S.373:1957. "Methods of testing small clear specimens of timber". BritishStandards Institution, UK. 1957.

Burley, J. and Wood P.J., 1976. "A manual on species and provenance researchwith particular reference to the tropics (1)". Department ofForestry, Commonwealth For. Inst. Oxford (2). "Specialappendices", by Hughes, J.F. and Plumptre, R.A., Dept. of For.Comm. For. Inst. Oxford.

Dinwoodie, J.M., 1963. "Variation on tracheid length in Picea sitchensis Carr."D.S.I.R. For. Prod. Res. Special Report No ~963).

Duff, G.H. and Nolan, J.J., 1953. "Growth and morphogenesis in the Canadianforest species (1)". The control of cambial and apical activity inPinus resinosa Ait. Canad. Jour. Bot. 31(4) (1953) 471-513 pp.

Elliott, G.K., 1966. "Tracheid length and specific gravity distribution inSitka spruce". Ph.D. Thesis. Dept. of For. and Wood Se., U.C.N.W.,Bangor, Wales, UK. 1966.

Einspahr, D. W., 1976. "The influence of short rotation forestry on pulp andpaper quality". Paper presented at the XV IUFRO World CongressDiv. 5, Oslo, 1976.

Garratt, G.A. 1931. "The mechanical properties of wood".Inc., Lond. Chapman & Hall Ltd., 1931.

John Wiley & Sons

Hillis, W.E., 1980. "The efficient use of wood resource". Keynote Addressproceedings, IUFRO, Div. 5. Congress, Oxford, 1980.

Kandeel, S.A., Bensend, D.W., 1969. "Structure, density and shrinkage variationwithin a silver maple tree". Wood Science, vol. 1, No 4: 227-238.E.P.

Kellison, R.C. 1981. "Characteristics affecting quality of timber fromplantations, their determination and scope for modification".Proceedings of the XVII IUFRO World Congress Div. 5 1981: 77-88 pp.

Kromhout, C.P. and Bosman, D.L., 1981. "The influence of short rotationforestry on wood production for sawnwood and veneer". Proceedingsof the XVII IUFRO World Congress Div. 5 1981: 60-75 pp.

Lavers t G., 1969. "The strength properties of timbers". Bulletin No 50 (2ndEd. Metric. Unit). Dept. of Environment, For. Prod. Lab. HMSO,London, 1969.

23

Renes, G.J.B. 1978. "An investigation of yield and productivity of teakplantations in south-West Nigeria (Unpublished).

Richardson, S.D. (1961). A biological basis for sampling in studies of woodproperties. TAPPI 44(3), pp 170-73.

Rudman, P. and Da Costa, E.W.B., 1959. "Variation in extractive content anddecay resistance in the heartwood of Tectona grandis L. f." Jour.Inst. Wood Sci. No 3, Sept. 1959, 33-43 pp. (34).

Snedecor, G.W. and Cochran, W.G., 1976. "Statistical Methods" 6th Edition. TheIowa State University Press, Ames, Iowa, USA. 285-296 pp.

Tsoumis, G., and Panagiotidis, N., 1980. "Effect of growth condition on woodquality characteristics of black pine (Pinus nigra Arn.)". Paperpresented to the IUFRO Div. 5 conference~o~K. April 1980.

U.S.D.A., 1974. "Wood Handbook: Wood as an engineering material". US Dept. ofAgric. For. Prod. Lab. For Ser. Agric. Handbook. No 72.

Wilson, T.R.C., 1921. "The effect of spiral grain on the strength of wood".Jour. For. 19: 740-747 pp. 1921.

Wood, D.C., 1970. "Within-tree variation in the strength properties of Sitkaspruce". Ph.D. Thesis (Wood Science) Dept. of For. and Wood. Sci.,UCNW, Bangor, Wales, UK. 1970.

Zobel, B.J. (1980). "Inherent differences affecting wood quality in fast-grownplantations". Paper presented to the IUFRO Div. 5 meeting, 6-8April, Oxford, England. 1980.

24

TABLE 2.1

Table of Length and Testing speeds for Specimens of Varying Depth (Width) forCompression Parallel-to-Grain and Static Bending Tests Covering the Range ofspecimen Sizes used in the Present Investigation

D 3D 140 + 2 3D.Z

Width of Cross Length of Length of Testing TestingTest Stick Sectional Stick Compl/ Stick Static Speed for Speed for

Area Grain Bending CompllGrain Static Bending---~~---~-----~~-~~~--~--~--~~~--~-------~~--~--~~----~------------~~~--~~~--~-

mm mm2 mm mm mm/min mm/min---~~---------~~-~~-~---~-~--------~-~--~---~-----~------~--~~~--~~-~--~~~--~~-

2.0 0.400 6.0 48.0 .061 .6603.0 0.900 9.0 62.0 .091 .9904.0 1.600 12.0 76.0 .122 1.3215.0 2.500 15.0 90.0 .152 1.6516.0 3.600 18.0 104.0 .183 1.9817.0 4.900 21.0 118.0 .213 2.3118.0 6.400 24.0 132.0 .244 2.6429.0 8.100 27.0 146.0 .274 2.972

10.0 10.00 30.0 160.0 .302 3.26911.0 12.10 33.0 174.0 .335 3.63212.0 14.40 36.0 188.0 .366 3.96213.0 16.9 39.0 202.0 .396 4.29314.0 19.6 42.0 216.0 .427 4.62315.0 22.5 45.0 230.0 .457 4.95316.0 25.6 48.0 244.0 .488 5.28317.0 28.9 51.0 258.0 .518 5.61318.0 32.4 54.0 272.0 .549 5.94419.0 36.1 57.0 286.0 .579 6.274

S 20.0 40.0 60.0 300.0 0.610 6.604

NOTE: Z for Compression//grain = 0.01016Z for Static Bending = 0.010108Z is constant rate of straining (loading) derived

from standard rate of loadingS Standard specimen size (B.S.373:1957).

25

TABLE 2.2 Distribution of Specimen Size by Squared Cross Section andCorresponding Mean Strength Values.

Mean Strength Value

Size No of

mm2 speci­mens

MOE TWD MCS

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

15

3

4

7

5

10

6

14

16

10

12

22

11

16

11

10

6

98.96 + 4.412

94.63 + 6.161

115.12 + 8.609

109.87 +14.570

112.29 + 7.223

116.63 + 8.907

96.25 + 6.094

111.65 + 6.480

106.78 + 4.680

115.08 + 5.740

109.07 + 6.776

112.70 + 5.759

107.06 + 13.16

131.96 + 7.908

118.67 + 9.539

123.63 + 6.436

83.71 + 8.571

9441.31 + 468

8188.88 + 1031

12322.18 + 565

11117.31 + 1043

10499.16 + 801

12442.96 + 767

10976.60 + 514

11487.60 + 621

11155.65 + 574

9942.41 + 1068

11091.36 + 847

10963.80 + 475

10171.04 + 1236

13287.26 + 689

10799.33 + 862

11792.03 + 671

8418.32 + 1190

0.378 + 0.046

0.586 + 0.046

0.467 + 0.083

0.318 + 0.085

0.433 + 0.093

0.363 + 0.043

0.252 + 0.035

0.405 + 0.055

0.385 + 0.029

0.409 + 0.022

0.402 + 0.027

0.486 + 0.041

0.628 + 0.111

0.644 + 0.067

0.726 + 0.107

0.640 + 0.059

0.463 + 0.074

40.06 + 1.5

40.06 + 1.5

49.16 + 2.8

43.61 + 3.0

45.11 + 3.3

43.81 + 1.9

42.61 + 2.0

43.81 + 2.4

42.40 + 1.8

43.08 + 2.0

40.31 + 1.6

41.39 + 1.5

38.13 + 2.9

45.30 + 1.9

39.85 + 2.5

42.05 + 1.7

37.64 + 4.8

TABLE 2.3

26

Multiplying Factors for Correcting for slope of GrainEmployed in this Study, derived from estimates of Figure 2.3

Strength Property/Multiplying FactorSlope ClassificationEmployed

(0°)

ClassMid-point

(0°) MOR MOE TWD MCS

1. Low (L) 0.75 1.01 1.00 1.04 1.00

2. Mlderate (M) 2.75 1.06 1.03 1.21 1.00

3. F. High (FH) 4.75 1.16 1.10 1.62 1.01

4. High (H) 6.75 1.32 1.19 2.07 1.02

5. v. High (VH) 8.75 1.51 1.30 2.54 1.04

6. E. High (EH) 10.75 1.75 1.44 3.01 1.06-----~--~------~~--~----~--------~-----~---~---~---~-~----~-~~~-~~--~~~~--~-~-~

27

TABLE 2.4 Result of the Calculated F-ratios and the Relationship between F­ratio and Depth (Specimen Size) examined by Linear Regression for4 mechanical strength properties of Teak.

Model: F = a + bD, where F = F-ratio, D = Depth and a,b are constants

PROPERTY OVERALLMEAN F

REGRESSION EQUATION SIG.

1. MOR 0.896 F = 0.768 + 0.010 D + 0.619 38.4 *+ 0.020-

2. MOE 0.871 F 0.760 + 0.009 D + 0.402 16.2 n.s+ 0.026-

3. TWO 0.870 F 0.533 + 0.027 D + 0.501 25.1 *+ 0.064-

4. MCS 0.940 F 1.008 + 0.005 D - 0.377 14.2 n.s+ 0.016-

R2 = coefficient of determination (R2 x 100 = %), that is the amount ofvariation in F-ratio explained by depth or the amount of variation accountedfor by the regression line.

* = statistically significant at the 5% probability level.

n.s. = significant at the 5% level.

28

Key

o Basemen t Campi ex~Cretaceous-Recent

sedimen ts~Lake Chad

FIGURE 1.1A Map of Nigeria showing the location of Ibadan

t

~--~--.....,.l.--........~--~ .f.~ I ,\~ , ,, ,.

.. , I" I I====== ....... -... ,

10........ --", ,I ,

s.a. t; ...

11 STUDCD ....

FIGURE 1.18 Map of University of Ibadan showing the location ofthe studied plot.

290

1978 ,1971 2

1976 3

1975 4

197t. 5

1973 6

fI1Z o· 7

1971 8

n 9

1969 -1)

~... - '1

~ fti7 12u. )(

81966 ~13

j 1965 §14....~ "-15c

1%~ I::s

)It

~

196' ~18

1960 19

1959

1958 21

11'S1

1956

VJ5

1951.

1953

1952

1 2 :3 4' 5 6 7 8 9 1) 11 13 ~ 15 16 18 W20 21 22 23 2Ie 2S 3'JNl ~ R1NiS FROM PlTH (K£, FRa1 PITH J

1952 W59 1964 1969 nYEAR a: wooo ~TION

FIGURE 2.1 Schematic diagram of the partitioning ofsample trees into comparable zones applic­able to all sample trees.

30

STYUSBl STEM ANAlYSlSt

STF(NjTH ....PRCJ'ER~......TY~

ISTFI:NGlH TESTl~

(STATIC BE~~ )

. StAN<AU£r-:lSSlONE'ALl"TION TEST

IDENSITY E~TION

•

•

I

II(((I})IJ)

1l£ ~t4j METHOO

3 RIN:i ~E ElJXKS (OEPTH I SPAN RATIO),: 14

FIGURE 2.2 Within-tree variation of strength properties.Summary of sampling method presented schematica!lyindicating the various stages from forest to thetesting.

31

3-0 - - - - - - - - - ~ - - - - - - - --

:r:t-~2·5·L.LJet::l­V)

et::«I.J.J-JU

c:: 2- 0ou...Cl:ot-u<{LL.

l:J

~ 1-5'~a.......t­...J::::>L

4° 0 6° 7° 8° 9°10°SLOPE OF GRAIN 0°

~ TOTAL WORK DONE (TWO)

MODULUS OF RUPTURE (MOR)

MODULUS OF ELASTICITY (MOE)

. 1:251: 20

1: 15 1:10 1:5SLOPE OF GRAIN (RATIO)

L M. FH H VH EH SLOPE OF GRAINCLASSI F ICATION USED IN TH ISSTUDY

FIGURE 2.3 Graph of grain slope against multiplying factor for clearstrength with regards to 4 strength properties; modulus ofrupture (MOR), modulus of ela.sticity (HOE), total work done(TWO) and maximum compressive strength parallel to grain:L=Low slope of grai n M= Moderate slope of grai nFH:Fairly high slope of grain H= High slope of grainVH=Very high 'slope of grainEH = Extremel y hi gh slope of gr ai n

32

ABDGInnermost band of zones in the vertical sequence. •

CEHInner band· of zones In the vertical scquenc~.. •

o

ooo

••

••

OHInner band •. oblique scq. 0

GInnermost band, oblique scq. 0

GHIJLowcrmost band in thehorizontal sequence.

DEF"'--...-..j~- Lower band, horizontal

sequence.

I

F

BC"----.....- ......-........ Upper band. horizontal

scqucnc-e.

H

E

FtOuter· band of zon es in the vertical scq.

JOutermost band, vertical· s:eq. .

A---+----+---+.... Uppermost band, horizontal

scqUlZN%~

1

G

o

o

o

27 '-_.......l~_........l.-_---_~---..

~ ACFJOutermost band in theoblique sequence.BEl

Outer: band, oblique scq

IFA

FIGURE 3.1

III ustration of the zones adopted withi n the tree for dataanalysis, showing associated notations on the Duff andNo{an graphs.

33

~O~:69

0-521

H

Key to within-tree sampli ng anddistribution of nominal specific gravity

OBLIQUE SEQUENCEDOMINANT CO-DOMINANT SU B.:-DOMI NAN T COMB INED TREES

0·65

0·61

~. • ,0·57~.

.~ Q~ ......-0

0·53 0 • o~\. .o~.."",• ~.~.'...

10·49

0·450 10 15 20 0 5 10 1S 20 0 5 10 15 20 0 5 10 1S 20

N° INTERNODE FROM APEX

~ HORIZONTAL SEQUENCE

uu::uUJ0..Vl

...J«zs:oz

0·65 DOMINANT

0·61 ?-\0·57 •

/.~.0·53

0·49

0·430 5 10 1S 20

CO-DOMINANT

o 10 15 20

sua-DOMINANT

:"-",o~

o •

o 10 15 20

COMBINED TREES

o 10 15 20

N° OF RINGS FROM PITH

VERTICAL SEQUENCE0·65 DOMINANT CO-DOMINAN T SUB- DOMINANT COMBINED TREES

0·61- ~

Q~\:\ ---. .~0·57 ---.~. ~./ --. .~c""- ~. ~ .~!

""-.0·53 • 0----0

c 0

0·490 o~~

0.45 ........................------78 74 70 56 62 58 18 74 70 65 52 58 78 74 70 55 52 58 78 74 10. 65 52 58

YEAR OF WOOD FORMATION

FIGURE 3.2 Variation of nominal specific gravity (SPG) in the oblique, horizontal and vertical sequencesfor the dominant class, co-dominant}sub-dominant and combined trees data.

34

'"Key ~o w; ~hi n· free sampti ng anddistribution of modulus of ru ture (HOR)

OBLIQUE SEQUENCE

80'--......_ ......-....._........--o 5 10 15 20 25 0 5

~\

•

o

COMBINED TREES

o 5 10 1S 20 2S

SUB-DOMINANT

10 15 20 25 0 5 10 15 20 25N° I NTERNODE FROM APE X

CO-DOMINANT

•o

90

DOMINANT170

160

150

140

130

120 o.-..<J

110

100

HORIZONTAL SEQUENCE110 -

DOMINANT CO-DOMINANT SUB-DOMINANT COMBINED TREES

u...o

150

150

1400

1300

120:~

110

~ /~90 •80

0 5 10 15 20 2S a 5 10 1S 20 25 0 S 10 1S 20 2S 10 lS 20 25

N° OF RI NGS FROM PITH

VERTICAL SEQUENCE170 DOMiNANT CO-OOMIN ANT SUB-DOMtNANT COM 81NED TREES

o

160

L150

140

130

/120

110

90

80 .............................. ......................_

o

o \~~\

•o

78 74 70 ~6 62 58 78 74 70 66 62 58 78 74 70 56 52 58 78 74 70 66 62 58

YEAR OF WOOD FORMATIONFIGURE 3.3 Variation of modulus of rupture (MGR) (N/mm2) in the oblique,horizontal and vertical sequences

for the dominant 'class, co-dominan t, sUb-domi nant and combined trees data.

35

1im;'2.0

12.~ " g.Og-O .

g.O g·o

Key to wi thi n-tree sampli n9 anddistribution of modulus of elasticity

OBLIQUE SEQUENCEDOMINANT CO-DOMINANT SUB-DOMINANT COMBI NEO TREES

15000

14000

13000

12000

11000

10000

9000

o

a

-.\o

10 15 20 25 0 5 10 15 20 25 0 5 10 15INTERNODE FROM APEX

HORIZONTAL SEQUENCEN

z:L.........Z

8000 __........_...1.---&._-'-----'o

DOMINANT CO-DOMINANT SUB-OOMI NANT

20 25 o S 10 15

COMBINED TREES

20 25

20 251S105o10 15 20 25 0 5 10 15 20 25

N° OF RINGS FROM PI TH

o

o20 2510 158000'----'-------&.--'-......

o

9000

10000

11000 ~

LI­oVl::::>...J::::>CloL

~ 15000L

>- 14000......u 13000

......~ 12000...JI.LJ

vDOMINANT ca-OOMINANT SUB-DOMINANT

ICOMBINED TREES

15000

o

74 70 66 62 58

oo100j

9000

8 --~....' ........"""--~ .........-'-.......78 74 70 66 62 58 78 74 70 66 62 58 78 74 70 66 62 58 78

YEAR OF WOOD FORMATION

11000

12000

13000

14000

FIGURE 3.4 Variation of Modulus of elasticity (MOE)(N/mm2) in the oblique, horizontal and vertical sequencesfor the domi nant class, co-domi nant , sub-domi nant and combi ned trees dat a.

0.6 ....1&.... 0.6

0.4'- 10.t.

0.4"'- a I

\ 0·4Key to wi thi n-tree sampti ng and

distri butian of total work done (TWO)

25

20 251510

10 15 20

o

Cl

COMBINED TREES

COMBI NED TREES

o2520

•

,( 0·923)I ~SUB-DOMINANT,

5 10 15 20 25APEX

o

SUB-DOMI NAN To (0·923 II

I

10 15 20 25 0 5 la 15

N° OF RI NGS FROM PI TH

o

o

\

\

15 20 2510

DOMINANT

10 15 20 25 0 5 10 15 20 25 0N° INTERNODE FROM

HORIZONTAL SEQUENCE0---CO-DOMINANT

OBLIQUE SEQUENCECO-DOMINANT

0·35

0·40

a-50

0·30

0·25 "---.a..._""'-- ........._

o

0·45

0·55

0·65

0.50

0·70

0·70 DOMINANT

0-65

0·60

055

0·50

0..450

0·40

0·35 t 0

Q. 30

O· 25 - ......- .........--..._.......--..o

....J«l­aI-

o:3t:.UJzao

:::.:::ex:o3

o

\COMB I NED TREES

; (0·9231CO-DOMINANT

/Cl

VERTICAL SEQUENCE

rt0·50

0·55

0·45

0·65

0.40 0

0·50

tSUB-DOM! NAN T

i

I

[

:::: '------.......-....--....-. f-....-.....O~""'_~~ t--.....,~o---'--'--..1----'---"-__

o· 70DOMINANT

78 74 70 55 52 58 78 74 70 65 62 58 78 74 70 66 62 58 78 74 70 55 62 58

YEAR OF WOOD FORMATION

FIGURE 3.5 Variation of totat work done (TWO) (mm.N/mm3 ) in the oblique, horizontal and verticalseq uences for the domi nant cl ass J co dami nant, sub dami nant and com bi ned trees data.

37

~47·0-41.0- - ~·37.0

-- 37·0

Key to within-tree sampling and distribution

of maximun compressive strength paratlet tograi n (MCS).

OBLIQUE SEQUENCECOMBINED TREESSUB-DOM! NANTCO-OOMINANT

~

r ,lII\loo

l , ,

10 1S 20 2S 0 1'0 1S 20 2S 0 S 10 1S 20 2S 0 5 10 15 20 2S

o

DOMINANT

25 '---.1-_""'"-.........--"---o

55

50

30

35

45

40

N° INTERNODE FROM APE XN

1:L

~ HORIZONTAL SEQUENCEDOMIN ANT CO-DO MI NAN T SUB-DOMINANT COMBINED TREES

o 5 10 15 20 25'0 15 20 25 0 5 10 15 20 2510 1S 20 25 0

o

25 ......-"'-_.......--"'_-'--......o

30

40

ss

35

so

UJ>v;VlUJer:a..Lou

N° OF RI NGS FROM PITH

VERTICAL SEQUENCE

DOMINANT CO-DOMINANT SUB-DOMINANT COMBINED TREES

ss

50

45

40

35

o 7~o o

o30

78 74 70 66 62 58 78 74 70 66 62 58 78 74 70 ffi 62 58 78 74 70 66 52 58

YEAR OF WOOD FORMATION

FIGURE 3.6 Variation of Maximum compressive str~ngth parallel to grain (MCSH N/mm2 ) in theoblique, horizontal and vertical sequences for the dominant class,co-dominant, sub­dominant and combined trees data.

38

APPENDIX 1Static bending test Formulae used in the study

The method of calculation of loading rate established by Garrett (1931) is asfollows:

NFrom this,

Where:

z N6DIT

• • • • • • • • • • • • • • • •• 1

Z rate of fibre strain per mm of fibre length.

(This constant is derived empirically to allow enough time for thecompletion of the test).

D height of beam (depth) (mm)

L span of beam (mm)

N rate of loading which for the 20 mm standard size specimen(B.S.373:1957), is 0.26 in/min or 0.6604 cm/min or 0.11 mm/sec.(Table 1).

With a span: depth ratio of 14:1 and speed of testing of 0.6604 cm/min, Z willbecome:

Z0.6604 x 6 x 2

(28) L

7.9248784

0.0101081

Since height is equal to depth in the samples, and span: depth ratio is 14:1and L = 14D, N, speed of testing can be calculated from equation (1) asfollows*

NZ. 14 2 D

6(2)

For the 20 mm standard size (B.S.373:1957), N, speed of testing can bequantified from equation (2) as follows:

N 0.0101081 x lR x 2.0 (cm/min)6

0.6604 cm/min (B.S.373:1957)

6.604 mm/min

0.11 mm/sec (Lavers, 1969)

39

By substituting the various specimen sizes into equation (2), testing speedsfor the entire range of sizes were calculated and presented in Table 2.1.

From the static bending test data, the modulus of rupture (MOR) , modulus ofelasticity (MOE) and total work done to failure (TWD) were calculated for eachspecimen, based on the following relationships:

MOR

MOE

TWD

3 Pmax L2B D2

N/mm 2

N/mm 2

J P. df, mm N/mm 3

L.B.D.

• • • • • • • • • • • • • • • • • •• (3)

• • • • • • • • • • • • • .. • • • •• (4)

• • • • • • • •• (5)

where: Pmax

PLBDdfA

Wpmax

maximum load sustained by beam (ultimate load at fracture).

Load at proportional limit equivalent to load atthe elastic limit (position indicated by the tangentto load/deformation curve).

LoadSpan between support (mm)breadth of specimen (mm)depth of specimen (mm)deflection in centre of beam at Pmax.deflection at beam centre at proportional limit.

This was calculated on the load-deflection curve as the distancefrom start of experiment to a perpendicular line drawn fromproportional limits to the abssisca of the graph.work to maximum load, Pmax, which is calculated by integration.

It has been found that the modulus of rupture (maximum bending stress of woodbeams depends on beam size and method of loading and that the strength ofclear, straight-grained beams decreases as size increases (Bohanna, 1966; USDA,1974). According to the USDA Agriculture Handbook No 72, these effects can bedescribed by the statistical strength theory involving 'weakest line'hypothesis and can be summarised as follows:

For two beams under two equal concentrated loads applied symmetrical to themidspan points, the ratio of the modulus of rupture of beam 1 to the modulus ofrupture of beam 2 is given by the formula below (3rd point loading):

D2

L2

.,1 +

ma2

R 1L

11/

R2 ~

III1 +

D1L

1 L1 J (6)...

40

where:

beam 1 and beam 2modulus of rupturebeam depthbeam spandistance between loads placed a each side of midspan,

and 2

1 and 2RDLa

m constant, which is 18 for clear straight-grained Douglasfir

If a beam is centrally loaded, 'a' becomes zero. In a situation where the twobeams to be compared are centrally loaded, the formula (6) above becomes:

• • •• (7)~ D2L2R

2D

1L

1

If for both beams of similar size, the span,depth ratio (LID) and speed oftesting are kept constant, the relation between both beams 1 and 2 having thesame dimensions will be:

Ri [ 1 1/- "'"R2

ID

Suppose ID 2, then ~ r ] 1/rn

Therefore, sinceR

1 [ 1 J 1/R

2ID

. R1 =. .R

2

•••• (8)

(equation 8 above)

•••• (9)

This relationship has been called the F-ratio (Bohannan, 1966; Wood, 1970); theratio of the strength of a non-standard specimen to that of standard specimen.~ can only be equal to 1 if the strength recorded for both beams of

similar dimensions, depth and span is similar. This can only be achieved ifthe speed of testing and other factors affecting strength are equal for bothbeams. This is usually very difficult to achieve and it has been a cause formany investigations.

41

APPENDIX 2Formulae for Compression test parallel to the grain

The method of calculation of loading rate established by Garrett (1931) is asfollows:

Nwhere, N

L

Z

ZL units/min •••••••••• (10)testing speed in cm/min or mm/min. With the 20 mm size,B.S.373:1957, suggests 'N' of 0.025 in/min, which converts to0.06096 cm/min or 0.01 mm/sec (Lavers, 1969).Length of compressiqn test stick, which is 6 mm for 20 mmstandard samples (depth: span ratio of 3).the rate of fibre strain per unit of fibre length. This is aconstant value determined empirically.

With a metric machine, a standard compression parallel-ta-grain specimen (2.0cm x 2.0 cm x 6.0 cm) and an N of 0.06096 cm/min, specified in B.S.373:1957, Zfrom equation (10) above will become:

NL

0.060966

0.01016

• • • • • • • • •• (11)

Since L 3D, equation (10) above can be written as:

.. .N

N

3.D.Z

3 x 2 x 0.010160.06096 cm/min0.6096 mm/min0.01 mm/sec (Lavers, 1969)0.025 in/min (B.S.373:1957)

• • • • • • • • •• (12)

By substituting various specimen sizes, into equation (12), the various speedsof testing were calculated and tabulated as shown in Table 2.1.

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