Mechanical Properties of Small Clear Wood Specimens of <i>Pinus patula</i> Planted in Malawi

doi:10.4236/ojf.2014.41002

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Open Journal of Forestry

2014. Vol.4, No.1, 8-13

Published Online January 2014 in SciRes (http://www.scirp .o rg /journal/ojf) http://dx.doi.org/10.4236/ojf.2014.41002

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Mechanical Properties of Small Clear Wood Specimens of

Pinus patula Planted in Malawi

Felix Dalitso Kamala1, Hiroki Sakagami2, Junji Matsumura2*

1Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, Fukuoka, Japan

2Faculty of Agriculture, Kyushu University, Fukuoka, Japan

Email: *[email protected]

Received September 21st, 2013; revised October 29th, 2013; accepted November 20th, 2013

Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,

provided the original work is properly cited. In accordance of the Creative Commons Attribution License all

Pinus patula is one of the major exotic species grown in Malawi mainly for saw-timber production. It is

native to Mexico. Little has been reported about the mechanical properties of the wood. The objective of

this study was to investigate the mechanical properties of Pinus patula in more detail, in order to provide

a basis for utilizing this resource. The mechanical properties of small clear wood specimens of Pinus pa-

tula were evaluated using 40 cm logs from 1, 3, 5, 7 and 9 m above the ground. Small clear wood speci-

mens were selected and subjected to a bending test in accordance with Japan Industrial Standards (JIS)

air-dry conditions. The growth rate did not affect the mechanical properties measured. There were signifi-

cant correlations at 1% level between air-dry density and Modulus Of Elasticity (MOE) (R = 0.85) and

between air-dry density and Modulus Of Rupture (MOR) (R = 0.83). There was also a significant correla-

tion between MOE and MOR at 1% level (R = 0.90). At about 12% moisture content, the tested five Pi-

nus patula families have average MOR and MOE of 105.17 MPa and 10.93 GPa, respectively.

Keywords: Pinus patula; Modulus of Elasticity; Modulus of Rupture; Malawi; Air-Dry Density; Wood

Introduction

In Malawi, high demand for wood coupled with high defore-

station rates has led to the increase in the adoption of exotic trees

and introduction of plantation forestry. Although wood is natu-

rally variable, fast grown trees produce wood, which may be

significantly different in properties, compared with wood from

slow grown trees. Furthermore, for trees grown as exotic, the

wood produced may have different properties from wood of the

same species in the original environment.

Pinus patula is one of the major exotic species grown in

Malawi. It is planted about 80% of Malawi’s 74,000 ha of

softwood plantation. It is native to Mexico. Tree height of Pinus

patula ranges from 30 m to 35 m and the diameter at breast

height ranges from 50 cm to 90 cm (Stanger, 2003). Under

favorable conditions, it may attain a hei ght of 15 m after 8 yea rs

and 35 m after 30 years. The species are mainly used for saw-

timber. Despite their wide use for structural purposes among

other uses, no detailed mechanical properties research has been

done to determine the strength of the species under Malawi

growth conditions. This is because, in Malawi, just like many

other species, research on Pinus patula has concentrated on the

height growth, volume and form. Against this background, a

study on the wood properties of Pinus patula wa s carried out

using the seed orchard established by the Forest Research In-

stitute of Malawi (FRIM).

The broad objective of the study was to find out and analyze

genetic factors on wood properties of Pinus patula in Malawi, to

recommend the possibility of the improvement of wood quality

and to contribute to sustainable management for plantation

forests. Specifically, the general study looked at the extent of

family and within family wood property variation of Pinus

patula in order to find out if selection for wood property inclu-

sion in tree breeding is possible. Kamala et al. (2013) looked at

the growth characteristics and wood density because of their

large influence on many other wood properties. A total of 15

trees from five families were studied. The growth rate for the

five families was significantly different. The present study looks

at the mechanical properties namely: Modulus of Rupture

(MOR), Modulus of Elasticity (MOE) and wood density. Mod-

ulus of rupture and modulus of elasticity are important properties

for the use of wood as a structural material. MOR is an indication

of the bending strength of a board or structural member, and

MOE is an indication of the stiffness. When analyzed among

trees and within a tree, mechanical property variation tends to

follow similar patterns to those observed in wood density. The

determination of MOR and MOE together with density is im-

portant to better understand their relationships. The relationships

among these properties are species specific, stronger in others

but weaker in other species. These relationships allow prediction

of effect of selecting one property to breed on the other proper-

ties. The relationships are also important in machine stress

grading.

*Corresponding author.

F. D. KAMALA ET AL.

OPEN ACCESS

The main objective of this research was to look at the me-

chanical properties of Pinus patula in more detail, in order to

provide a basis for utilizing this resource in Malawi. In general,

this research seeks to provide reliable data for comparing the

mechanical properties of Pinus patula grown in Malawi with

various species. It also provides data, which may be used to

analyze the influence on the mechanical properties of such

factors as: distance of timber from the pith of the tree, height of

timber in the tree, etc. The study further seeks to provide data,

which can be used to classify the strength of Pinus patula grown

in Malawi for grading purposes.

Materials and Methods

Sampling

The samples used in this research were collected from Dedza

in Central Malawi (altitude 1737 meters above sea level). The

orchard used in this study was planted with forty-two families

without routine silvicultural treatments. Families were rando-

mized within each block. The total number of treatments (fami-

lies) was 42 in 4 replications. Plot size was 7 × 7 trees at a

spacing of 9 × 9 feet (2.74 × 2.74 m). The plantation was 30

years old, but for unknown reasons, some of the trees studied

were less than 30 years old.

This study looked at five of the 42 families. One tree per

family was used representing a total of five trees with repre-

sentative grow th rates . Table 1 shows the growth information o f

the five trees in terms of height, Diameter at Breast Height (DBH)

and height at 15 cm diameter. The 15 cm diameter is the mini-

mum merchantable diameter in Malawi. The mechanical prop-

erties of Pinus patula were evaluated using 40 cm logs from 1, 3,

5, 7 and 9 m above the ground. The logs were further cut into 20

mm × 20 mm × 32 cm specimens. The total number of small

clear wood specimens was 86. A lot of care was taken to avoid

any defect on the specimens. The specimens were selected and

subjected to a bending test in accordance with Japanese Indus-

trial Standards (JIS) air-dry conditions. The moisture content for

the specimens was about 12%.

Data Analysis

In order to find out the relationship between the mechanical

properties, scatter plots with regression line were produced

using Minitab statistical software. The relationship was com-

pared at individual tree and family levels. Analysis of variance

was run at family and height above the ground levels for Mod-

ulus Of Elasticity (MOE), Modulus Of Rupture (MOR) and

air-dry density. This was done in order to find out the extent of

variation of the properties among the families.

Table 1.

Volume growth information of f ive trees (five fa milies) of Pinus patula

at 30 years old.

Family DBH (cm) Height (m) Height at 15 cm

diameter (m)

1 30.0 26.0 18.9

2 34.0 27.7 20.2

3 28.0 23.2 16.7

4 31.0 23.5 19.3

5 27.0 23 15.1

Mechanical Properties of Juvenile Wood versus

Mature Wood

Data from a previous research (K amala et al., unpubl. dat a) on

tracheid length for the same sample trees was used to determine

juvenile and mature wood boundary. The study of the tracheid

length of the five families showed that there was a rapid increase

of tracheid length up to ring number 10 and then a stable pattern

towards the bark. Therefore, ring number 10 was assumed to be

the boundary between the juvenile wood and mature wood for

Pinus patula grown in Malawi. The data for the juvenile wood

and mature wood mechanical properties was subjected to an

analysis of variance to find out if the variation of the mechanical

properties was significant or not.

Grade Yield

Grading standard of physical and mechanical properties of

timbers from Southeast Asia and Pacific regions by For estry and

Forest Products Research Institute (FFPRI) in Japan and the

South African national standards were used to check the grade

yield using MOE and MOR sep ar atel y. Tables 2 and 3 show the

grading standard of mechanical properties of timbers from

Southeast Asia and Pacific regions by Forestry and Forest

Products Research Institute and the grades according to the

characteristic stress for South African Pine respectively.

Results and Discu ssion

Mechanical Properties

Table 4 shows the average mechanical properties for the five

trees from five families. The average air-dry density was uni-

form among the families with family five registering a slight

increase at 0.54 g·cm−3. MOE was also comparatively uniform

with family five producing the highest MOE and MOR of 11.90

GPa and 111.47 MPa respectively.

Table 2.

Grading standard of mechanical properties of timbers from Southeast

Asia and Pacific regions by Forestry and Forest Products Research

Institute (FFPRI) (1975).

Grade MOE (GPa) MOR (MPa)

I - 7.35 - 58.8

II 7.45 - 10.3 58.9 - 82.4

III 10.4 - 13.2 82.5 - 107

IV 13.3 - 16.2 107 - 130.4

V 16.3 - 131 -

Table 3.

Mechanical grades accordin g to the characteristic stress for South Afri-

can Pine (SANS 10163, 2003).

Grade MOE (GPa)

xxx 7.79

Five 9.59

Seven 11.99

Ten 18

F. D. KAMALA ET AL.

OPEN ACCESS

Table 4.

Average mechanical properties for the five families (Total five trees).

Family Air dry density

(g∙cm−3) MOE

(GPa) MOR

(MPa)

1 0.50 (0.11)* 11 (3.42)* 101.34 (35.83)*

2 0.50 (0.07)* 11.54 (3.28)* 107.92 ( 28.25)*

3 0.50 (0.12)* 10.84 (3.23)* 110.77 ( 34.15)*

4 0.51 (0.11)* 10.47 (3.51)* 105.97 ( 32.95)*

5 0.54 (0.09)* 11.90 (3.59)* 111.47 ( 39.91)*

Note: *S tand ar d de vi a tio n.

Table 5 shows the summarized mechanical properties

among the five trees from fives families. The maximum MOE

and MOR were 16.72 GPa and 185.19 MPa respectively. The

maximum air-dry density was 0.69 g∙cm−3. At about 12%

moisture content, the tested five Pinus patula families have

average MOR and MOE of 105.17 MPa and 10.93 GPa, re-

spectively.

Tables 6 and 7 show the analysis of variance (Anova) results

for MOE and MOR respectively. The Anova results show that

stem height had no significant effect on the mechanical proper-

ties. The results also show that despite the differences in growth

rate among the five trees from the five fami lies, the mec hanical

properties were not significantly different among them. This

shows that growth rate had no effect on the mechanical proper-

ties of the species.

This result is comparable to the results by Anon (1947) for

South African Pinus patula. He also reported no correlation

between timber strength and rate of growth. Craib (1939) also

concluded that the rate of growth did not affect lumber strength

in Pinus patula. The absence of influence of growth rate on

mechanical properties is an advantage to forest managers who

prefer higher growth rate to increase the volume yield of plan-

tations because the higher growth rate will not affect the

strength of the lumber produced. However caution should be

taken because although the density of softwoods is generally

not related to growth rate, density is directly related to the per-

centage of latewood in a growth ring (Shmulsky & Jones, 2011).

There is typically a large difference in density between early-

wood and latewood. For this reason, relatively wide latewood

zones indicate relatively high density. Because the strength of

wood increases with density, wide growth rings exhibiting a

low proportion of latewood may be of concern in products

where strength is important (Shmulsky & Jones, 2011). As long

as the increased growth rate does not decrease the proportion of

latewood within growth rings, there should be no problem in

applying silvicutural practices that increase the growth rate.

After applying these silvicultural practices, it will be also

important to find out the effect of increase of growth rate on the

wood properties. The lack of significant strength properties

variation among the five families is positive news for house and

building construction where there is a need to develop uniform

wood products from sustainable sources. P. patula has proved

to be an ideal timber for construction in South Africa. More

research on lumber strength and variation in Malawi should

allow further development of the species for these high value

products.

Table 8 shows mechanical properties of Pinus patula from

Table 5.

Summary of the mechanical properties (86 specimens).

Air dry density (g∙cm−3)

MOE (GPa) MOR (MPa)

Minimum

0.33 4.14 51.78

Maximum

0.69 16.72 185.19

Std 0.09 3.19 31.46

Average 0.51 10.93 105.17

CV 17.87 29.88 29.53

Table 6.

MOE analysis of variance summary for family and stem height (86

specimens).

Source DF Type III SS Mean Squar e

Family 4 1827.11 456.77 NS

Stem hei ght 4 1171.92 292.97 NS

Error 78 93292.23 1196.05

Note: NS: Not Significant at 1 % level.

Table 7.

MOR analysis of variance summary for family and stem height (86

specimens).

Source DF Type III S S Mean Square

Family 4 19.10 4.88 NS

Stem

height

4 22.15 5.33 NS

Error 78 875.36 11.22

Note: NS: Not Significant at 1 % level.

Table 8.

Mechanical properties of Pinus patula from other countries (as cited by

Wright, 1994).

Location Mechanical property

Reference

Colombia MOE—10.8 GPa (Anon, 1980)

New Zealand MOE—7.65 GPa

MOR—70.6 MPa (Bier, 1983)

Mexico MOE—11.5 GPa

MOR—91.5 MPa (Ordonez et al., 1989)

South Africa MOE—10.8 GPa

MOR—71.6 MPa (Otto & Van Vuren, 1976)

Malawi MOE—10.93 Gpa

MOR—105.17 MPa This study

other countries at about 12% moisture content. In terms of

MOE, at 11.5 GPa, the Mexican Pinus patula had slightly

higher values than that of Malawi. The MOE values were al-

most the same as that of Colombia and South Africa at 10.8

GPa. Malawi Pinus patula bending strength and stiffness com-

pare favorably with the same species grown in other countries

listed in Table 8. Thus, its wood products (such as lumber,

composite panels, and structural composite lumber products)

can compete successfully with same products in the huge con-

struction markets of Southern Africa. This is especially true for

Oriented Strand Board (OSB), laminated veneer lumber (LVL),

and structural composite lumber (such as parallel strand lumber

F. D. KAMALA ET AL.

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(PSL), and laminated strand lumber (LSL).

Relationships among Wood Propertie s

Research has shown increased density to be strongly linked

to favorable strength, stiffness, hardness and working properties

of sawn timber, as well as wood pulp yield and paper-making

quality. The relationships among the mechanical properties in

this study were linear and positive.

Figures 1-3 show the relationships among air-dry density,

Figure 1.

Relationship between specific MOR and MOE among the five families.

Figure 2.

Relationship between specific MOR and density among the five fami-

lies.

Figure 3.

Relationship between specific MOE and density among the five families.

MOR and MOE. There were significant correlations at 1% level

between air-dry density and MOE (R = 0.85) and between air-

dry density and MOR (R = 0.83). There was also a significant

correlation between MOE and MOR at 1% level (R = 0.90).

These results are also comparable with results for East African

grown Pinus patula where wood density correlated significant-

ly with all green and dry lumber strengths (FFPRI, 1975).

The correlation of MOR and MOE with specific gravity is

typical ly very strong, as reported by Shottafer et al. (1972) and

Shepard & Shottafer (1992) for red pine, Wolcott (1985) for red

spruce, and Han (1995) for red maple. However, in some con-

iferous species, such as Abies fabri, Picea asperata, and Pinus

massoniana, the relationship of MOE to specific gravity is

weaker than the relationship between MOR a nd specific gravity

(Zhang, 1997), and this was also found to be true in fast-grow-

ing red pine (Deresse, 1998). It has been reported that wood

samples having similar specific gravity can a lso exhibit signifi-

cantly different strength values due to factors that may be asso-

ciated to other factors to which specific gravity is less sensitive

(Perem, 1958; Zhang, 1995; Deresse, 1998). Correlations in this

study indicate that controlling density would have a positive

effect on some mechanical properties.

Mechanical Properties and Grade Yield of Juvenile

Wood versus Mature Wood

Tables 9 and 10 show the analysis of variance results for

juvenile and mature woods among the five trees for MOE and

MOR respectively. Significant variation was noted between

juvenile wood and mature wood in terms of mechanical proper-

ties.

Figures 4-7 show the grade yield for both juvenile and ma-

ture wood according to MOE and MOR using grading standard

of mechanical properties of timbers from Southeast Asia and

Pacific regions (FFPRI, 1995). The grade yield for MOE in

juvenile wood was highest for Grade II followed by Grade I.

Grade III and Grade IV was lower. There was no grade yield

for Grade V according to MOE in juvenile wood (Figure 4).

The grade yield for MOR in juvenile wood was highest for

Grade III followed by Grade II and Grade I. Grade IV was

lower and the grade yield for Grade V was the lowest (Figure

5).

Table 9.

MOE Analysis of variance summary for juvenile and mature wood (86

specimens).

Source DF Type III S S Me an Square

MOE Juv/MAT 1 565.99 565.99**

Error 86 314.25 3.65

Note: **Significant at 1 % level.

Table 10.

MOR Analysis of variance su mmary for juvenile and mature wood (86

specimens).

Source DF Type III SS Mea n Square

MORJuv/MAT 1 40618.47 40618.47**

Error 86 44476.93 517.17

Note: **Significant at 1 % level.

F. D. KAMALA ET AL.

OPEN ACCESS

Figure 4.

Specimen grade allocation in terms of MOE of juvenile wood according

to Grading standard of mechanical pro perties of timbers fro m Southeast

Asia and Pacific regions by FFPRI (1975). See Table 2.

Figure 5.

Specimen grade allocation in terms of MOR of Juvenile wood according

to Grading standard of mechanical pro perties of timbers fro m Southeast

Asia and Pacific regions by FFPRI (1975). See Table 2.

Figure 6.

Specimen grade allocation in terms of MOE of mature wood according

to Grading standard of mechanical pro perties of timbers fro m Southeast

Asia and Pacific regions by FFPRI (1975). See Table 2.

The grade yield for MOE in mature wood was highest for

Grade IV followed by Grade III. Grade V and Grade II was

lower. There was no grade yield for Grade I according to MOE

in mature wood (Figure 6).

The grade yield for MOR in mature wood was highest for

Grade V followed by Grade IV and Grade III. Grade II was

lower and there was no grade yield for Grade I according to

MOR in mature wood (Figure 7).

Figures 8 and 9 show grade yield for both juvenile and ma-

ture wood according to the South African national standards.

The grade yield for MOE in juvenile wood was highest for

Grade five at followed closely by Grade XXX. Grade Seven

was lower and the lowest grade yield according to MOE in

juvenile wood was in Grade Ten (Figure 8).

Figure 7.

Specimen grade allocation in terms of MOR of mature wood ac-

cording to Grading standard of mechanical properties of timbers from

Southeast Asia and Pacific regions by FFPRI (1975). See Table 2.

Figure 8.

Specimen grade allocation in terms of MOE of Juvenile wood

according to the South African standards (XXX is the rejects

category). See Table 3.

Figure 9.

Specimen grade allocation in terms of MOE of mature wood according

to the South African standards (XXX is the rejects category). See Table 3.

F. D. KAMALA ET AL.

OPEN ACCESS

The grade yield for MOE in mature wood was highest for

Grade Ten at followed by Grade Seven. Grade II was lower and

there was no grade yield for Grade Five and XXX with both

grades yielding according to MOE in mature wood (Figure 9).

The grade yield according to MOE and MOR was different

for juvenile wood and mature wood. Some grades had high

yields in juvenile wood but low yields in mature wood. In other

cases, grades with high yield in juvenile wood were non-exis-

tent in mature wood. The results show that mature wood

yielded more grades with high values of MOE and MOR. This

clearly shows that mature wood for Pinus patula is superior in

strength and grade than juvenile wood. The implication for this

is that mature wood and juvenile wood should be used for dif-

ferent purposes to avoid underutilization. Uniform use of juve-

nile wood and mature wood for structural purposes would be

potentially dangerous because juvenile wood has inferior me-

chanical performance. To improve lumber strength, one can

process logs of older trees and minimize the use of the interior

portion of the log. Correct drying of the boards will increase the

most important lumber strength traits of modulus of rupture and

modulus of elasticity. Export of dried lumber of P. patula oc-

curs and should increase if uniformity can be maintained.

The common steps in establishing grades for lumber are:

testing of small clear specimens according to guidelines, estab-

lishing strength values and allowable properties, establishing

visual grading rules and lastly verifying grades using in-grade

testing. The contribution of this research towards creating an

accept able gr ading sy ste m is that it has clarified the variation of

mechanical propertie s. More mechanical propertie s data, through

testing of small clear wood specimens, from other areas in Ma-

lawi need to be accumulated.

This research has established first steps in assigning allowa-

ble mechanical properties for Pinus patula grown in Malawi.

After accumulating more small clear wood specimen data, test-

ing using the “in grade” approach, in which large representa-

tives samples of full size lumber can be tested to destruction is

recommended to compare the results. This will help in the as-

signment of standard grades that will ensure the efficient utili-

zation of Pinus patula structural lumber in Malawi.

Conclusion

At about 12% moisture content, the tested five Pinus patula

families have average MOR and MOE of 105.17 MPa and

10.93 GPa, respectively. There were significant correlations at

1% level between air-dry density and MOE (R = 0.85) and

between air-dry density and MOR (R = 0.83). There was also a

significant correlation between MOE and MOR at 1% level (R

= 0.90). There was no significant variation in MOE and MOR

among the five families. Stem level variation in MOE and

MOR is not significant. Mature wood of Pinus patula has more

superior mechanical performance than juvenile wood. The

growth rate did not affect the mechanical properties of the spe-

cies. This study suggests that it is potential to simultaneously

improve tree growth, density, and some mechanical properties

of the wood of this species. The results of this study are a

foundation that will provide a technical basis for the machine

grading of Pinus patula structural lumber in Malawi.

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