The specimens were loaded parallel to the grain such that one part slides along another part adjacent to it. The crosshead speed was 0.6 mm/min. Both displacement and load at failure were recorded. Shearing strength is a measure of the ability of wood to resist an external force that tends to move the fibers of the part past each other along the
P A
P BL
grain but in opposite directions. The maximum shearing strength was obtained from an average value of six specimens.
Strength in shear parallel to grain (or axial) is determined by dividing a force per unit area from the relationship
σmax= = ( 4.7)
where σmax = maximum shear strength (N/mm2 or MPa)
P = maximum compressive load (N)
A = B x L = cross-sectional area of the specimen (mm2) B = breadth of test specimen (mm)
L = length of test specimen (mm),(see Figure 4.10).
All of the test results of strength in shear parallel to grain were calculated and are given in Tables I.5.1 to I.5.7 of Appendix I.
4.4.6 Hardness test
The hardness test is where a ball with a diameter of 10 mm is pressed halfway into wood tissue. The load was applied with a crosshead speed of 6 mm/min throughout the
P A
P BL
The hardness is measured the maximum load at a displacement of 5 mm of the steel ball embedded on the wood surface. The maximum load of hardness was obtained from an average value of six specimens.
All of the test results of maximum load in hardness were calculated and are given in Tables I.6.1 to I.6.7 of Appendix I.
4.4.7 Specific gravity and density
For this test, all the wood specimens were placed in a conventional oven at a temperature of 103±2ºC and heated for 48 h. Then the wood specimens were removed from the oven and weighed (see Figure 4.18). The moisture content and specific gravity were calculated using equations (4.8) and (4.10), respectively. The specimens in tension and in shear were weighed on a balance with a 200 g capacity, whereas a balance with a 500 g capacity was used for others test specimens.
Specific gravity is the ratio of weight of an oven-dried wood specimen to the weight of water. As with density, the higher the specific gravity, the heavier the wood, and the stronger it tends to be. At a moisture content of 12±2 percent, the seven types of woods have a specific gravity of 0.6 to 0.9 (water has a specific gravity of 1).
Density is weight per unit volume. Density is expressed as grams per cubic centimeter (the oven-dry weight divided by the final volume of the specimen). Density is an important parameter for strength in wood: a wood that is heavier will normally tend to be stronger than a lighter one. The specific gravity and density were obtained from an
average value of six specimens. The results of tests were calculated and are given in Table I.7.1 to I.7.14 of Appendix I.
The formulae used for determining the moisture content, weight density and specific gravity are as follows [22] [23] [24].
Moisture content (MC) = x 100 (4.8)
where MC = moisture content WI = initial weight of the specimen WF = final weight of the specimen
Weight density (ρ) = (4.9)
where W = weight of the specimens V = volume of the specimen
Specific gravity ( sp.gr) = k (4.10)
or sp.gr =
where k = a constant (= 1 when weight is in g and volume is in cm3) WI = initial weight (g) of the specimens
WI - WF
WF
W V
WI
(1 + MC/100)*VI
WF VI
MC = moisture content (at oven-dry weight) VI = initial volume (cm3) of the specimens
4.4.8 Static bending test
The load was applied perpendicular to the grain ( or the axis) continuously at the mid-span of the beam specimen at a rate of a crosshead speed of 2.5 mm/min, causing it to bend and deflect. Three fundamental stresses will occur within the sample in manner of compression, tension and shear, all acting in a direction parallel to the grain. In this experiment, all of the specimens failed on the tension side as can be seen of the sample failure of May Khen heau e1 in Figure 4.19. For these tests, the deflection and load at failure were recorded. From this test the maximum tensile strength and modulus of elasticity (or stiffness) can be determined. An average value of strength was calculated from six tested specimens. The modulus of elasticity was determined by evaluating the slopes of the straight line of the load-deflection curves. Shear strengths were not determined by this test as the mode of failure was tension, not shear [25] [26] [27].
The results are related to loads, deflection, and sizes of the specimens, and are calculated as follows:
Maximum bending strength is the maximum bending load a specimen can resist prior to failure. Modulus of rupture at maximum load in static bending is calculated using the following relationship
σmax= ( 4.11 )
where Mmax = PL/4, ie. the maximum bending moment (N.mm) Sx = BH2/6, ie. the section module in bending (mm3)
The bending strength is given by:
σmax= ( 4.12)
σmax = Maximum bending strength (MPa) P = maximum load (N)
L = length of beam (mm) B = Breadth of test piece (mm) H = Depth of test piece (mm)
For example, bending strength for May Deng a1 in static bending was calculated using equations (4.12) as follows
B = 48.0 mm, H = 48.5 mm, L = 700 mm
Max load, P = 1220 kgf ≈ 12200 N (from the test in Table I.8.1)
σmax = = 113.46 N/mm2 = 113.46 MPa 3PL
2BH2 Mmax Sx
3 x 12200 x 700 2 x 48 x 48.52
The modulus of elasticity is the ratio of stress to strain, and when used in reference to a beam (or long column), is a measure of its ability to sustain deformation or bending. It is expressed in term of the “stiffness” and applied only below the proportional limit. The modulus of elasticity and stiffness are calculated from the following relationship
E = ( 4.13)
where I = BH3/12, ie. second moment of area of the beam cross-section
For calculating the modulus of elasticity in static bending, the equation of the elastic line to load-displacement curves is used to obtain the slope, ie.
(see Figure 4.20).
y = ax ± b (4.14)
where, y = load = P x = Δ = deflection a = slope
b = intercept P L3 Δ 48I
For example, modulus of elasticity and stiffness for May Deng a6 in static bending were calculated using equations (4.13) and (4.14), and from the straight line (elastic line) in Figure 4.20 as follows
I = 45 * 483/12 = 414720 mm4
L 3 = 7003 = 343000000 mm3
From equation in the graph (Figure 4.20)
Y = 69.059x-0.6669
Therefore, Y/x = P/Δ = 69.059 = slope
From equation (4.13)
E = =
E = = = = 11899.21N/mm2 = 11.9 GPa
E = 11.9 GPa (For May Deng a6) P L3
Δ 48 I
69.059L3 Δ 48I
69.059 x 10 x 343000000 48 x 414720 P L3
Δ 48 I
Y L3 x 48 I
For stiffness,
From the test values in Table I.8.1
Stiffness = EI = PL3/48Δ = 69.059 x 10 x 7003/48 = 4934841042 N.mm2 = 4934.84 N.m2 EI = 4934.84 N.m2 (For May Deng a6)
The bending strength (MOR), modulus of elasticity (MOE) and stiffness are obtained from average values of six specimens. The test results were calculated and are given in Table I.8.1 to I.8.7 of Appendix I.
Table 4.1. General information on the sample trees
Species Origin Location No.of
tree Dia cm
tree age 1. May Deng
2. May Tai 3. May Dou 4. May Nhang 5. May Khen Hine 6. May Khen Heua 7. May Khe Foy
N N N N N N N
Vientiane province Vientiane province Vientiane province Vientiane province Vientiane province Vientiane province Vientiane province
1 1 1 1 1 1 1
80-85 80-85 80-85 65-70 65-60 60-65 60-65
90-95 90-95 90-95 70-75 75-80 65-70 65-70
N- Naturally-grown trees
Figure 4.1 Methods of sawing logs for lumber or beam: flat-sawn and quarter-sawn
Figure 4.2 Method of flat (or plain) sawing wood in Laos
Figure 4.3 The samples of the flat-cut woods in different dimensions
May Dou
Figure 4.5 Wood specimen and its dimensions, and direction of applied force for tension parallel to grain test. (dimensions in mm)
25
100 95 63 100
W=10
95
452
t=5.0 30
6.3
6.3
444 Radius
Region for making wood samples in flat cut from board as shown in Fig 4.3
Figure 4.4 Rectangular pieces for making wood specimens
Figure 4.6 Wood specimen and its dimensions, and direction of applied force for tension perpendicular to grain test. (dimensions in mm)
Figure 4.7 Wood specimen and its dimensions and direction of applied force for compression parallel to grain test. (dimensions in mm)
L=200
B=50
H=50
B=50 L
Figure 4.8 Wood specimen and its dimensions and direction of applied force for compression perpendicular to grain test. (dimensions in mm)
Figure 4.9 Wood specimen and its dimensions and direction of applied force for static
L=760 B=50
H=50
H=50
L=150
B=50
L
Figure 4.10 Wood specimen and its dimensions and direction of applied force for shear parallel to grain test. (dimensions in mm)
Figure 4.11 Tension parallel to grain test (May Khen Heua e1)
20 30
B=
L=
Figure 4.12 Graph of load vs displacement for calculating modulus of elasticity in axial tension (May Khe Foy g6)
Figure 4.13 Tension perpendicular to grain (May Nhang d6) Tension parallel to grain (May Khe Foy g6)
Figure 4.14 Compression parallel to grain test (May Khen Heua e1)
Figure 4.15 Compression perpendicular to grain test (May Dou c1)
Figure 4.16 Shear parallel to grain test (May Khe Foy g5)
Figure 4.17 Hardness test (May Khe Heua e2)
Figure 4.18 The performance of wood dry specimens in a conventional oven
Figure 4.19 Static bending test (May Khen Heua e1)
Figure 4.20 Load vs deflection under the proportional limit in static bending (May Deng) Static bending test (May Deng a6)