tensile strength properties of tropical hardwoods in structural size testing-z ahmad

International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 10 No: 03 1

Abstract-In Malaysia, there is no research done on the

tensile strength of structural size timber. The tension

stress found in the Malaysian code of practice, MS 544:

Part 2: 2001, was not determined directly and it was

taken as 60% of the bending strength values of small

clear specimens. This paper presents the results of the

tensile test on structural size timber specimens from

selected Malaysian Tropical hardwoods namely

Kedondong (Canarium spp), Keruing (Dipterocarpus

spp) and Bintangor (Calophyllum spp) under the

strength group SG5. A special tensile grip was fabricated

using the model suggested in ASTM D198 and the tests

were also conducted according to ASTM D198. The

tensile strength characteristics evaluated include tensile

strength, modulus of elasticity and Poisson’s ratio, and

the data were analyzed statistically. In this study it was

found that the grade stresses for structural size

specimens were higher than that published in the

Malaysian code of practice.

Keywords— tensile strength, structural size, Malaysian

tropical timber

INTRODUCTION

imber is the oldest and widely used construction

material. It is used in various structural forms such as

beams, columns, joists, trusses and many others. To

apply timber as structural components such as roof trusses,

the tension properties of the timber are particularly

important. The tensile strength of the lower truss chord or the tension flange of an I-beam or box-beam is considered the

critical design parameter [1]. The tensile strength parallel to

grain is the highest strength property of wood [2]. However

the knots, cross grain and/or any other irregularity in growth,

considerably reduce the tensile strength [3]. The high tensile

strength of wooden parts cannot be utilized in construction

for several reasons. The shearing strength along the grain is

Manuscript received April 2, 2010. This work was supported by

Research Management Institute, Universiti Teknologi Mara, Malaysia.

Z.A and Y.C.B are with Faculty of Civil Engineering, Universiti

Technologi Mara Malaysia, 40450 Shah Alam, Selangor, Malaysia, phone:

60355435236, Fax: 60355435275, e-mail: [email protected]

E.S.A.W is with Faculty of Civil Engineering, Universiti Malaysia

Pahang, Lebuhraya Tun Razak, 26300, Kuantan, phone: 6095492939, Faz:

6095492998, e-mail: [email protected].

extremely low (only about 6 to 10%) in comparisons to the

tensile strength along the grain. Therefore the wood tends

towards shear failures or cleavage at the fastening or joints”

[3]. Timber is grown naturally. Thus it is a difficult material

to characterize and partly accounts for the wide variation in

the strength of timber, not only between different species but

also between timber of the same species and even from the

same log [4].

The need to classify timber species by evaluating the

physical and mechanical properties of small clear specimens

has always existed. Because of sensitivity to irregularities of gain, edge knots, notches and other stress risers, it is difficult

to realize this superior strength in structural members of

commercial lumber if it is only based on small clear

specimen. The need for precise design criteria for the tensile

strength of structural timber and composite lumber is

important for the effective design and utilization of timber. It

is therefore important to have direct measurements of the

actual tensile strength of the lumber. Structural uses of solid

wood and composite wood products require the knowledge

of physical and mechanical properties of timber as a basis for

design criteria. The research and the actual records of tensile strength properties of Malaysian tropical timber are not

sufficient. In the past, testing for the tensile strength of

timber was seldom carried out directly, but was often taken

to be numerically equal to the bending strength. The code of

Practice for the Structural use of Timbers, MS 544:2000 for

example, recommends the use of the same design value for

tension and for bending. The necked-down shaped tensile

specimens are very difficult to prepare accurately. This could

be the reason for the relatively little information available on

the tensile strength of tropical timbers.

So far, in Malaysia, there is no research done on the tensile strength of structural size timber. The tension stress

on structural size is not determined directly but is taken as

60% of the bending stress of the same grade.

The code of practice MS544: Part 2: 2001 gives

recommendations for the structural use of Malaysian timbers.

The grade stresses such as tension, compression and bending

stresses, given in Tables 1 and 2 of the code were obtained as

a factor of clear timber stresses. For testing small clear

specimens, it is assumed that the specimens tested are

entirely free from defects such as decay, knots, sloping grain, compression failure, and brittle heart, shakes, checks, wanes

and borer holes. This is the reason why the values obtained

from such tests are reduced by a certain factor of safety in

Tensile Strength Properties of Tropical

Hardwoods in Structural Size Testing

Z. Ahmad, Y. C. Bon, E.S. Abd Wahab

T

mailto:[email protected]


the design. According to Hankinson‟s formula, a small clear

tension specimen would experience over 20 percent loss in

strength with as little as 5 degrees of grain angle [5]. The two

most significant growth characteristics affecting lumber

strength are knots and variations in grain orientation. The

ratio of parallel-to-grain tensile strength to perpendicular–to-grain tensile strength of clear wood for a structural softwood

species can be 40 to 1 [6]. In addition, timber dimension and

sawing pattern affect the structural properties of timber. With

increasing length, tension strength is reduced due to an

increase in material defect and density variation within the

member [7]. In fact based on the classical brittle fracture

theory, there are direct effects of length and width to the

strength properties of the structural timber. The tensile

strength parallel to grain is the highest strength property of

wood [8]. Sensitivity of the strength properties to

irregularities of grain, edge knots, notches, and other stress

risers makes it difficult to realize this superior strength in structural members of commercial lumber if is only based on

the data available small clear specimens.

If using small clear specimen, the stresses have to be

modified by certain reducing factors to reflect the defects

permitted in the structural size member. In other country

such as United Kingdom, the concept of basic stresses has

been abandoned and the new approach for assessing the

strength of timber appears to be in line with „limit states‟

design philosophy. The process would first involve grading

structural size timber into various stress grades. The graded timber is then subjected to short-term load tests. The results

are used to determine the characteristic stress, which is taken

to be the value below which not more than 5% of the test

results fall. Lastly, the grade stress is obtained by dividing

the characteristic stress by a reduction factor which includes

adjustments for a standard depth of specimen of 300 mm,

duration of load and a factor of safety [4]. In general, the

stresses based on structural size shown that some of the

earlier assumptions were conservative [9,10].

Therefore it is important to determine the strength of timber

in structural size in order to have more reliable data for design timber structural member. This study investigates the

tensile strength properties of selected Malaysian tropical

timber based on structural size specimens and compared the

results with the data from small clear specimens. In order to

obtain data for the tensile strength properties of timber in

structural size, a special tension grip was fabricated. The

device must have the end fastening secure enough for the full

tensile strength to be brought into play before the fastening

shear off longitudinally. Therefore in this study, a special

tensile grip was fabricated and the reliability of the

equipment had been validated through testing large number of specimens and the investigation of failure mode.

I. EXPERIMENTAL PROCEDURES

A. Materials

Three timber species, Keruing, Kedondong, and Bintangor

were used in this experimental study. All timber materials

used in this project were selected on one occasion in order to

obtain a test material without too high a variation in strength

which could be arisen from different growth condition. They

are common types of timber available from the local saw-

mill. Based on MS544 Part 2, these species are in the

strength group. The total number of specimens was 60.

B. Preparation of specimens and experimental methods

Specimen preparation

The specimens were prepared according to ASTM D198.

The specimens were planned on four sides to the size of 50

mm x 50 mm x 1500 mm. These specimens were air dry at

room condition for about two months to attain a moisture

content less than 19% or dry condition. Then the specimens

were visually stress graded and the common grade was

selected for the study. Common grade means that the timber

contains a defect which reduces its strength by 50%. To prevent crushing on the specimen by the grips, four pieces of

timber capping, each of length 250 mm were glued to the

ends of the specimen using phenolic resorcinol formaldehyde

resins as shown in Fig. 1.

(a)

(b)

Figure 1: (a) Schematic diagram of the tensile specimen (b)

preparation of the end capping.

Fabrication of tensile clamping jigs

Figure 2a shows the schematic drawing of the test set-up

of tension parallel to grain. The wedge-shape jaw grip (see

Figure 2b) was designed to provide firm grip on the

specimens without causing crushing on the end specimens.

This part of the grip transmit tensile load from the moveable

Capping

1000 mm 250 mm 250 mm

1500 mm


drive mechanism (Figure 2d) to the other end of the tension

specimen where another grip was placed. Load cell

(2000kN) (Figure 2c) was attached to record the loading

capacity. The side view of the test set-up is as shown in

Figure 2e.

(a)

(b)

(c)

(d)

(e)

Fig. 2: Jigs for tensile test. (a) Schematic diagram of test set-

up (b) Wedge-shape grip (c) Load cell (d) The drive

mechanism (e) Side view of structural size tension specimen.

Testing procedures

The tensile tests were carried out in accordance with ASTM

D 198(Tension parallel to grain test). After physical

measurements had been taken and recorded, the specimen

was placed in the grips and securely clamped with special

care taken to have the longitudinal axis of the specimen and

the grip coincides (see Figure 2e) so that the tensile forces

should be axial and generally uniformly distributed

throughout the cross sections without flexure along its

length. A small preload was applied to ensure that all jaws moved an equal amount and maintained axial alignment of

specimen and grips. The load was applied at a constant rate

(0.0006 mm/mm.min) for that maximum load is achieved in

about 10 minutes but not less than 5 minutes. Two strain

gauges were placed perpendicular to each other at the mid-

span of the test section to measure strains for determination

of Poisson‟s ratio and extensometer was also attached to the

specimens The extensometer and strain gauges were

connected to data loggers for data recording. After testing,

the failed specimens were cut for moisture content

determination

III. RESULTS AND DISCUSSION

Tensile Failure Characteristics

It was found that the test set-ups used in this study was

able to produce tension failures with few specimens that

being rejected. 85% (51) of the specimens failed in tension.

Typical failures are as shown in Figs 3a, 3b and 3c.

Strain

gauges Stationary

grip Load cell

Data

logger

Drive mechanism


(a) (b)

(c)

Fig. 3: Typical tension failure characteristics of timber

specimens (a) Keruing (b) Bintangor (c) Kedondong

The other 15% of the specimens did not failed in tension but slipped at the grip ends. This was due to improper gluing

of the capping as shown in Fig. 4.

(a)

(b)

Fig. 4: Failure due to slipping at the capping (a) side view

and (b) front view.

Besides that, the choice of timber species to be used as

capping also influenced failure type. It was found that in

order for the specimens not to slip at the grip, the species for

the capping must be of higher density than the species tested.

Performing tensile test is difficult. Kollmann [3] mentioned

that tensile tests parallel to the grain are difficult: the

manufacturer of the test specimens needs much skilled

manual labor, and clamping the samples in the machine implies the possibility of torque or of compressive stresses

perpendicular to the grain which are too high. The failure in

the sample is often not entirely tensile. Tensile tests along

the grain are therefore scarce in comparisons to compression

or bending tests.

When the specimens failed in tension the line of the failure

were observed to be zigzag type along the depth of the

specimens. The failure time for Kedondong was generally earlier than Bintangor and Keruing respectively. This

indicated that Keruing was tougher and this could also be

seen from the failure type of Keruing as shown in Figure 3a.

As for Bintangor, true tensile failure started at the bottom of

the test specimen near the center of the specimen through its

thickness but not to the fullness of the width as shown in

Figure 3b. The different behaviour amongst the three

hardwood species during tension was caused by the different

nature of wood grain arrangement i.e., straight grain in

Keruing and Bintangor and wavy grain in Kedondong. The

wood density was also seen as the major influence on the

failure characteristics especially in Bintangor where the vessels arrangement was less dense and the cell structure was

larger thus thinner cell wall, than the other two species.

Tensile Strength properties of timber in structural size

Fig. 5 shows typical stress-strain graphs for tensile strength

of each species. The graph shows that strain increases with

load and is approximately linear until the point the specimen

failed. The slope of the graph represents the MOE. The ultimate stress then occurs at the highest point when the load

reaches its maximum value. The samples broke at that final

stage and the reading indicated an abrupt drop of strength.

Therefore the solid timber is a brittle material.

Fig. 5: Stress-strain relationship

The summary statistics for the tensile strength of different

species is given in Table 1. An analysis of variance

(ANOVA) was performed to determine if there were

differences in mean tensile strength values among the species

tested. F-test indicated that there were no significant

differences in the mean tensile strength (p-value = 0.464) at

5% significance level. This also showed that these species

are in the same strength group (MS 544: Part 2, 2001). The coefficient of variation of these species is within 15 to 20

percent in comparison with values of 25 to 35 percent

reported for visually graded lumber. This indicates that the


properties of solid timber by mechanical testing are more

uniform than by visually graded lumber.

Table 1 Summary statistics for tensile strength

Species

Tensile Strength For Solid (MPa)

Mean Tensile Strength

1 Percentile*1 MS 544 Part 2*2

Grade stress Common grade

stress

Keruing 41.2 ± 6.2 10.2 6.6

Bintangor 38.8 ± 5.8 8.3 5.9

Kedondong 38.7 ± 6.2 10.2 5.9

*1Grade stress for structural size timber

*2Grade stress based on small clear specimens, MS544 Part2

In order to compare this result with MS 544 Part 2, grade

stress was computed base on 1st percentile as shown in Table 1. It was also assumed that the tensile stress distribution was

normal. The 1st percentile of grade stresses is given by :

FOS

σ.TT Nmean

%1

1

332 [1]

where,

meanT = The average tensile strength

2.33 = 1st percentile coefficient

1Nσ = Standard deviation

FOS = Factor of Safety = 2.5

It was also assumed that the tensile stress distribution was

normal. By comparing the grade stresses, the grade stresses

from the experiment were higher than the grade stresses from

MS 544: Part 2. This indicates that when using MS 544 Part

2 the design could be over-designed. This would lead to

bigger section and higher cost.

Modulus of Elasticity (MOE)

An analysis of variance (ANOVA) was performed to

determine if there were differences in mean MOE values

among the species tested. F-test indicated that there were

significant differences in the mean MOE of different species

(p-value = 0.01) at 5% significance level. By DUNCAN

multiple comparisons, it was found that there was significant

different between Kedondong and Bintagor and between

Kedondong and Keruing but there was no significant

different between Bintagor and Keruing. The summary

statistics for the MOE of different species is given in Table

2.

The values of MOE for both Bintangor and Keruing

showed little difference either in the mean or minimum

values. However, Kedondong has wavy grain fibers that

could make it stiffer and higher MOE. Whilst, a straight

grain timber such as Bintangor or Keruing might not have

significant effect on stiffness.

Table 2: Summary statistics of MOE values for all species

Species

MOE (GPa)

Mean Minimum MS 544 Part 2

Mean Min.

Kedondong 13.4 ± 3.26 9.6 ± 3.20 12.0 7.5

Bintangor 11.9 ± 4.28 8.2 ± 5.17 14.0 9.6

Keruing 11.2 ± 4.25 8.2 ± 2.49 11.9 8.7

For design, the mean value of MOE is used for load

sharing factor, while the minimum value of MOE is for non-

load sharing for the structural design purposes. Load sharing

factor is used in the design for joist, or joint timber and etc., while non-load sharing factor is used in the design of column

and beam.

From Table 2, it can be seen that the mean and minimum

values for MOE for structural size specimens are not in the

same order as in MS544 Part2. These results further enhance

the need to revise the mechanical properties of tropical

timber based on structural size specimens.

Poisson’s Ratio

ANOVA was performed to determine if there were differences in mean Poisson‟s ratio values among the species

tested. F-test indicated a p-value = 0.063 at 5% significance

level, which indicates that there was no significant

differences in Poisson‟s ratios among the different species

and it was also found that the Poisson‟s ratios vary within

species.

Table 3: Summary statistics of mean and coefficient of

variation of Poisson‟s ratio

Species Poisson‟s ratio

mean COV

Kedondong 0.63 0.90 Bintagor 0.60 0.28

Keruing 0.54 0.10

After testing, the failed specimens were cut for moisture

content determination. It was also found that the moisture

content does not vary within species. This is because the

average of moisture content for the three species is less than

19% which is considered dry.


IV. CONCLUSION

The properties of tensile strength of solid timber made from Kedondong (Canarium,spp), Bintangor

(Calophyllum,spp) and Keruing (Dipterocarpus,spp) were

investigated. The following conclusions were derived:

i. There is no significant difference in tensile strength

for the investigated species; Kedondong, Keruing and

Bintangor. This investigation confirmed that these

species are in the same strength group.

ii. The grade stresses for timber in structural size are

relatively higher than the grade stresses based on

small clear specimen.

iii. The species which has the highest value of MOE is

Kedondong followed by Bintangor and Keruing.

There is no significant difference in the MOE of

Bintangor and Keruing since the timbers are of

straight grains.

iv. The MOEs based on structural size specimens were

not in the same order as the MOEs based on small

clear specimens.

ACKNOWLEGMENT

The work reported here was financially supported by the the

Institute of Research, Development and Comercialization,

Universiti Teknologi Mara, Malaysia.

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[2] S.E. Taylor and D.A. Bender, “Stochastic model for localized tensile strength and modulus elasticity in lumber”. Wood and Fiber Science, vol. 23, No. 4, 1991, pp.501-519.

[3] F.P. Kollman. Mechanics and rheology of wood. In F. P. Kollman and W.A. Cote, Jr., Principles of wood science and technology, vol. 1. Solid wood, Springer Verlag, New York, 1968,pp. 192-419.

[4] C. Arya. Design of Structural Elements. E & FN SPON,

London, 1994. [5] ASTM D 198-197 (1998), Standard Test Methods of Static

Tests of Lumber in Structural, 1998 [6] J.D. Wellons and R.L. Kramer. “Self bonding in bark

composites”. Wood Science, vol. 6, No.2, 1973. pp.112-121. [7] J.C.S. Chin, “Properties of Laminated Veneer Lumber (LVL)

from low density and lesser-used wood species”. Degree of Science, Project Report, Faculty of Forestry, UPM, Serdang,

70 pp, 1997. [8] Q. Wang, H. Sasaki, S. Kawai and R. Abdul Kader,

“Utilization of Thinnings from Sabah (Malaysia) Hardwood Plantation: Properties of Laminated Veneer Lumbers and the Application to Flanges of Composite Beams with a Particleboard Web”. Conference on Processing and Utilization of Low-grade Hardwoods and International Trade of Forest-Related Products, 11-13 June 1990.

[9] H.J. Hansen. Modern Timber Design. 2nd edition John Wiley & Sons Inc., 1968, pp.151-171

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tensile strength properties of tropical hardwoods in structural size testing-z ahmad

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