Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 

Correspondance: biekopbertin@yahoo.fr 
ISSN 2235-9362 Online ©2022 University of Sri Jayewardenepura.                            

 

22 

Physical and mechanical properties of Ayous wood (Triplochiton 

scleroxylon) from Cameroon 
 

Bertin. Biekop1, 2, *, Paul. William. Mejouyo Huisken1, 2, Didier. Fokwa1, 2, Ebenezer 

Njeugna1, 2 

  
1 Laboratory of Mechanic, University of Douala-Cameroon 

2 Mechanic Laboratory and Adapted Materials (LAMMA), ENSET – University of 
Douala-Cameroon 

      *biekopbertin@yahoo.fr 
 

Date Received: September 14, 2022                          Date Accepted: October 27, 2022                           

 

Abstract 

The present study deals with Ayous wood from the East Cameroon forest reserve in the locality 
of Abong-Mbang. The water absorption rate of Ayous wood and the absorption kinetics were 

evaluated. Ayous wood reached its absorption saturation around 28 days. The primary diffusion 

coefficient was found to be 1.51 x 10-11 m2/s with a standard deviation of 0.23x10-11 m2/s while 
the saturation absorption rate is 144% with a standard deviation of 16.3%. About the modeling 

of kinetic absorption, many models were tested, and (Sikame, 2014) was the best model for our 
experiments. In order to determine the mechanical properties, four point bending and 

compression test were done through the three orthotropic directions.  It is found that the 

modulus of elasticity value is 8792.75 MPa with a standard deviation of 527 MPa and the 
fracture stress (σl) value is 53.6 MPa with a standard deviation of 8 MPa. Longitudinal, radial 

and tangential compressive stress are 31.51 MPa, 29.15 MPa and 31.4 MPa respectively, with 
standard deviations of 4.34MPa, 4.52 MPa and 4.23 MPa. 

 

Keywords: Absorption kinetics, diffusion coefficient, orthotropic directions, modulus of 
elasticity, modulus of rupture.  

 
1. Introduction  

Wood is a material derived from trees. Its production constitutes a great wealth for central 

African economy (Kisito, 2008). It is a biological composite, constituted of fibers as 
reinforcement, embedded in lignin as matrix. (Pot, 2012). The fibers orientation leads to three 

orthotropic directions: radial, longitudinal and transversal. It is a natural material that has 
several advantageous characteristics such as its lightness, ease of absorbing and releasing water. 

The water absorption by wood assumes great importance, especially in the structure uses of 

wood (Baronas, 2001), but a technological problem that limits its valuation resides in its 
mechanical variability. Indeed, unlike other common building materials, wood is a living 

material whose properties are governed by nature. There may therefore be a variability of 
properties between trees of the same species. This is due to the presence of singularities such 

as knots, slots, and wire slope. It is therefore important to first determine the performance of 

each wood species before it is used in a construction application.  
 

The aim of the present work is to characterize Ayous (Tripochiton scleroxylon), a species 
intensively used in building construction as formwork for reinforced concrete, or structure 

elements. When this wood is used as formwork, it is influenced by climatic variations (rain and 
heat) due to its exposure on construction sites. It is therefore subjected to sorption phenomena 

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Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 23 

(Saifouni, 2014). The knowledge of its absorption kinetics is fundamental for the understanding 
of its behavior under various conditions.  

 
2. Materials and methods 
2.1 Sampling  

      The wood we have worked on, is Ayous (Tripochiton scleroxylon), a tropical wood of the 
Eastern region of Cameroon in the locality of Abong-Mbang. In order to obtain results that 

validate the methods used, a set of 1205 specimens were tested. Three trees have been chosen 
in this species that have a diameter between 30 cm and 43 cm. They are straight and have few 

defects. This sampling allows a certain representativeness of the species studied. 

 
                                                                                                                                                                                                    

                                                                                                
 

 

 
 

 
 

 

 
 

 
 

 

 
 

 
Figure 1: Tree trunks and sawing along longitudinal, radial and transverse directions 

 

                                                                                                       
 

 
 

 

 
 

 
 

 

Figure 2: Test specimens 
 

2.2 Hygroscopic behavior 
      The absorption rate is determined by gravimetric method. This test was conducted by 

immersing sample in the water, removing it every two days and deducting the amount of water 

absorbed. If Mi is the initial weight of the sample, and Mt the actual value measured, then the 
equation 1 gives the absorption rate (Scida, 2010). 

 

TA(t) =
𝑀𝑡−𝑀𝑖

𝑀𝑖
× 100                                                                                                                             (1)    

Where TA(t) is the absorption rate.   



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 24 

   
      

 
 

 

 
 

 
 

 
 

Figure 3: Hygroscopic measurement: (a) precision balance, (b) soaking tank 
 

If 𝑀∞ is the weight at saturation, the absorption ratio is calculated using equation 2. 
 

𝑀𝑅(𝑡) =
𝑀𝑡 − 𝑀𝑖
𝑀∞ − 𝑀𝑖

× 100                                                                                                                    (2) 

 

Where 𝑀𝑅(𝑡) is the absorption ratio. 
 

From the mathematical point of view, the problem of water absorption by wood can be 
treated as a diffusion problem based on the Fick’s second law of diffusion. The  absorption 

kinetics of this wood species and the mathematical model of this absorption kinetics are then 
determined by testing several empirical models from the literature under Matlab software. 

 

The Fick diffusion coefficient gives a linear relationship between water flow through the 
porosity and the concentration gradient (Crank, 1956). 

 

𝐽𝑗 = −𝜌𝐷𝑖𝑗∇𝑐𝑗                                                                                                                                            (3)  

 

where Jj is the diffusion flux, of which the dimension is the amount of substance per unit area 

time in kg.m-2s-1 and 𝜌 the mass per unit volume in kg m-3  

 
Dij is the binary diffusion coefficient or diffusivity in m-2s-1 

∇ is the symbol for the mathematical gradient. 
cj is the mass fraction. 

 
In this expression the diffusion coefficient Dij is given by equation 4 

𝐷𝑖𝑗 = 𝜋 (
𝐾

4𝑀∞
)

2

                                                                                                                                       (4)  

 

Where K is the slope of the linear part of the curve defined by equation 5 

𝑤𝑡 = (
√𝑡

ℎ
)                                                                                                                                                  (5)  

 

𝑤𝑡 is the water content at time t. 
 

 
 

 



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 25 

2.3 Mechanical behavior 
      Bending test 

      In order to determine the modulus of elasticity as well as the bending stress, four-point 
bending test is carried out according to the standard NF B 51-016 1987(Seram, 1987), as shown 

in Figure 4. The test is carried out using prismatic specimens of 360 mm length and 20 mm x 

20 mm square cross-section, a compression press, a test stands and two pressure gauges. For 
this purpose, we made a support and loading device that was adapted to the existing press. 

 
 

 

 
 

 
 

 

Figure 4: four points bending test 

 
Figure 4: Bending moment 

 

The modulus of elasticity and the bending tensile stress are derived by equation 6. 
 

MOE =
𝑝𝑙3

48𝐼𝛥𝑚𝑎𝑥
[

3(𝑙−𝑎)

2𝑙
−

(𝑙−𝑎)3

𝑙3
]                                                                                                         (6)       

 

MOE : modulus of elasticity 
I: quadratic moment of the section of the sample 

Δ: maximal deflexion obtained from the test.  

P maximal load 
a: dimension of the section of the sample (a= 20 mm) 

l: the distance between supports (320 mm) 
 

The maximum bending moment is given by equation 7 

 

 𝑀𝑚𝑎𝑥 =
𝑝

2
𝑎                                                                                                                                              (7)   

 

    𝑀𝑚𝑎𝑥: the maximum bending moment. 

320

80
40P

240

150 350
x mm

100 30050 250

M(X)  N.mm

0
200



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 26 

Equation 8 gives the maximal static bending stress. 
 

 𝜎𝑚𝑎𝑥 =
3𝑝𝑎

𝑏3
                                                                                                                                              (8)  

 

𝜎𝑚𝑎𝑥 : the maximal static bending stress 
 

Compression test 
The compression test is conducted by standard NFB 51-007 (Seram, 1987). The test method 

is based on the following principle: the chosen test (compression) aims to impose a uniform 

uni-axial stress state on a prismatic specimen and, by measuring the resulting deformations, to 
evaluate the elastic compliances expressed in the reference frame linked to the specimen. The 

samples are taken through the three orthotropic directions according to the Figure 5.  
 

 

 
 

 
 

 

 
 

 
 

 

 
 

 
Figure 5: The three orthotropic directions of wood 

 

The practice is to use three specimens as shown in Figure 6 below. 
 

(a)                                                                       (b) 
 

 

 
 

 
 

 
 

Figure 6: The three specimens in the three directions (a), Test machine and specimen loading 

mode in compression (b) 
 

For each test, if P is the maximum force applied to the specimen, the stress is given by equation 
9 

 

𝜎 =
𝑃

𝑏2
                                                                                                                                                      (9)  

 



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 27 

Where  𝑏2 is the section of the sample. (20x20mm2). The modulus of elasticity is then given 
by. 

 

MOE =
𝑃𝐿

𝑏2∆𝐿
                                                                                                                                         (10) 

 

Where P is the maximum force applied to the specimen, and 𝐿 the length of the sample 
Where ∆𝐿 is the variation of the length of the sample under the maximum force P.    
                         

3. Results and discussion 
3.1 Hygroscopic behavior 

The water absorption kinetics was studied. The graph of the absorption kinetics curve is 
shown in Figure 7. This figure shows the variation of water absorption ratio with time. It appears 

that the saturation happens in twenty-eight days. The rate of absorption is 144% with a standard 

deviation of 16.3%. 
 

 
 

 

 
 

 
 

 

 
 

 
 

 

\ 
 

 
 

 

 
 

Figure 7: Absorption kinetics of Ayous wood 
 

            The modelling of the absorption kinetics was done in Matlab using mode ls from the 

literature. Figure 8 presents this modelling. 

 
 

 
 

 

 
 

 
 

 

0

0.2

0.4

0.6

0.8

1

1.2

0 4 8 12 16 20 24 28 32

W
a

te
r 

a
b

so
rp

ti
o

n
 r

a
ti

o

Time (day)

S1 S2 S3 S4 S5

S6 S7 S8 S9 S10

Time (day) 

W
a
te

r 
a
b
so

rp
ti

o
n
 r

a
ti

o
 



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 28 

 
 

 
 

 

 
 

 
 

 

 
 

 
 

 

 
 

 
 

 

 
 

 
 

 

 
Figure 8: Experimental curve of Ayous wood and curves of models 

 

To evaluate the goodness of fit of each model, two criteria were used. Coefficient of 
determination (R-square) and root mean square error (RMSE), as given in the table 1. 

 

Table 1: Distribution of the average parameter model of the absorption kinetics of Ayous wood 

Model names Goodness of fit 

 R-square Adjusted R-square RMSE 

(Sikame ,2014) 
G(x)= c-a.exp(-k.x)-b.exp(-m.x) 

0.9979 0.9962 0.01959 

(Pilosof, 1987) 

G(x)=a+ 
𝒃.𝒙

𝒄+𝒙
                                    

0.9853 0.9811 0.04378 

Experimental 0.9980 0.9974 0.01622 

(Czel ,2008) 

G(x)=a. 𝒙𝒎                                      

0.9339 0.9258 0.08672 

 

G is the model equation 
The root mean square error (RMSE) is given by equation 11 

 

𝑅𝑀𝑆𝐸 = √
∑ (𝑚𝑐𝑖−𝑚𝑒𝑖)

2𝑛
1

𝑛

2
                                                                                                                      (11)  

W
a
te

r 
a
b
so

rp
ti

o
n
 r

a
ti

o
 

0
.2

 
  

0
.4

 
  
 0

.6
  
  
  
  
 0

.8
  
  
  
  
  
1

  
  
  
  
 1

.2
  
  
  
  
 1

.4

 
 

0                5                10              15               20               25               30 

Times (days) 

Experimental curve and curves of models 



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 29 

Where 𝑚𝑐𝑖  is the theoretical mass, 𝑚𝑒𝑖  is the predicted mass, n the number of observations 
The mathematical model that best describes the absorption kinetics of Ayous wood is the 

Sikame model (2014) given by equation 12 below.         
 

G(x) = c − a . exp(−k. x) − b. exp(−m. x)                                                                               (12)
     

Where a b and c are constants and k and m are the scattering parameters. 
 

Table 2: Coefficients of Sikame Model. 

Coefficients (with 95% confidence bounds) Values 

a 0.001366 

b 0.002965 
c 1.04 

k 0.1756 

m 0.203 

 

The corrected primary scattering coefficient deduced from this absorption kinetics is 
1.51.10-11 m²/s. This value is in the order of magnitude of the diffusion coefficients of woods of 

the same characteristic in the literature. It is experimentally proven that the diffusion coefficient 

of medium-heavy tropical woods varies from 10-13 m²/s to 10-11 m²/s while that of heavy woods 
varies from 10-14 m²/s to 10-12 m²/s with a relative error of 10%. The diffusion coefficient of 

tropical woods is lower than the diffusion coefficients of temperate woods and also differs 
according to the density (Khazaei, 2008). The heavier the wood, the lower its diffusion 

coefficient, i.e. the more difficult it is to lose water (Nsouandele, 2009). 

 
3.2 Mechanical properties of wood in 4-point bending 

Its maximum deflexion value Δ is 10.73 mm with a standard deviation of 0.69 mm. The 4-
point bending test produced the curve given by figure 9. From this curve we determine the value 

of the longitudinal modulus of elasticity MOE= 8792.75 MPa. 

 
It should be noted that this value of modulus of elasticity is within the range of moduli of 

elasticity of this species given in the literature 7260 MPa with a standard deviation of 1574 MPa 
(Gérard, 1998). 

 

 
 

 
 

 

 
 

 
 

 

 
 

 
Figure 9: Four point bending test, evaluation of modulus of elasticity 

y = 3188.1x - 0.9754
R² = 0.9993

0

10

20

30

40

50

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

S
tr

e
ss

 (
M

P
a
)

Strain (∆𝐿/𝐿)



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 30 

 
(a) 

 

 
(b) 

                   
Figure 10: Comparison of modulus of elasticity of Ayous wood with other species (a) 

Longitudinal breaking stress of some species (b) 

 

A static bending modulus or modulus of elasticity of 8792.75 MPa with a standard 
deviation of 527 MPa is obtained for this species, as well as a comparison with other similar 

woods (softwoods) in the literature. In this test, the bending stress parallel to the grain was 
determined to be 74.11 MPa. This value is high compared to other similar woods in the 

literature, namely Ilomba and Dibetou with similar densities (Gérard, 1998). 

 
Table 3: Fracture stress and modulus of elasticity for the four point bending test. 

Four point bending test Fracture stress in MPa (SD) Modulus of elasticity in MPa 
(SD) 

53.6  (8) 8792.75(527) 
 

 

 

 

 

 

 

 

Ayous Ilomba Dibetou

Breaking stress(MPa) 74.11 70 80

0

10

20

30

40

50

60

70

80

90

B
R

E
A

K
IN

G
 S

T
R

E
SS

WOOD SPECIES

Ayous Ilomba Dibetou

Young Modulus (MPa) 8792.75 8200 8400

7000

7500

8000

8500

9000

9500

Y
O

U
N

G
 M

O
D

U
LU

S

WOOD SPECIES



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 31 

3.2 Uni-axial compression test 
This uniaxial compression test allowed us to draw the curves below. 

 

 

 

 
 

 
 

 

 
 

 
 

 

Figure 11: Uni-axial compression test: radial compression, tangential compression, 
longitudinal compression. 

 
The values for modulus of elasticity and stress at break from this test are given in the table 4. 

 

Table 4: Stress at break and modulus of elasticity in compression 

 Compressive stress in MPa 

(SD) 

Modulus of elasticity in MPa 

(SD) 

Longitudinal 

compression 

31.51 (4.34) 6529 (510) 

Radial compression 29.15 (4.52) 4041 (309) 

Tangential 

compression 

31.49 (4.23) 4029 (301) 

 
Based on these results, we observed that the modulus of elasticity in longitudinal 

compression is greater than the moduli of elasticity in radial and tangential compression. This 
value of longitudinal modulus is also within the range of moduli of elasticity of this species 

given in the literature 7260 MPa with a standard deviation of 1574 MPa (Gérard, 1998). 

This reflects the use of this species in structures in the longitudinal direction (parallel to the 
grain). 

 
4. Conclusion 

The main objectives were to characterize the kinetics of Ayous wood and its mechanical 

behavior. Water diffusion in this species has shown that its water absorption rate is high in the 
first eight days of the test, and gradually decreases to stabilize at saturation absorption around 

28 days. This absorption rate is 144% with a standard deviation of 0.163%. The process that led 
to the determination of this value consisted of successive weighing carried out on average every 

02(two) days until saturation (mass stabilization). When we use Matlab software the model of 

Sikame is the one that best fits the absorption kinetics of this species. The correlation coefficient 
is R2= 0.9979 and the RMSE value equal to 0.01959. This model value is close to the 

experimental value which is 0.01622. The average primary diffusion coefficient is 1.15.10-
11m²/s. The four point bending test revealed a tensile stress of 53.6 MPa and a modulus of 

elasticity of 8792.75 MPa.The uni-axial test revealed a longitudinal compressive strength of 

0

5

10

15

20

25

30

35

40

0 0 . 0 0 2 0 . 0 0 4 0 . 0 0 6 0 . 0 0 8 0 . 0 1 0 . 0 1 2

S
T

R
E

S
S

 (
 M

P
A

)

ST(∆𝐿/𝐿)

Radial Compression

Tangential Compression

Longitudinal Compression



Bertin et al. /Journal of Tropical Forestry and Environment Vol. 12, No. 02 (2022) 22-32 

 

 32 

31.51 MPa, a tangential compressive strength of 31.49 MPa, and a radial compressive strength 
of 29.15 MPa. The moduli of deformation in the different directions in compression are: EL = 

6529 MPa, Er = 4041 MPa Et = 4029 MPa.  
 

In future work, it will be necessary to study a creep ratio behavior of this species (viscoelastic 

and mechano-sorptive behavior). 
 

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