Determination of tensile shear strength in Scot pine
# Rajendra K.C.
Student; Tropical and International Forestry
Abstract:
The lab test conducted to determine the tensile shear strength of Scot pine against
longitudinal shear according to EN20-1. The tensile shear strength of Scot pine with two
different adhesives i.e. semi structured adhesive such as PVAC (Poly Vinyl Acetate) and
structured adhesive i.e. PF (Phenol Formaldehyde) was tested in bonded specimens of Scot
pine (Pinus sylvestris L.) sapwood. Altogether 40 specimens (10/10) for each adhesive (and
state) were tested in two different (dry and wet) states. The test was conducted in fully
automated computer programmed machine named Zwick Z010 / Zwick Roell using testXpert
II software.
The lab results show that there is similar tensile shear strength between PVAC and PF
bonded Scot pine in dry state whereas they differ highly in wet state.
Key words: Stress, Shear strength, Poly Vinyl Acetate (PVAc), Phenol
Formaldehyde (PF), Adhesives, Bonded wood, Scot pine, Wet state, Dry state.
1.0 Introduction:
Strength properties of wood can be described as the ability to resist the applied stress. The wood,
or timber or glulam is said stronger while it has the greater resistance against stress. The stress is
simply the amount of force for a given unit of area. It is typically measured in pound per square
inch (psi) or in Newton (www.woodbin.com).
The tensile stress elongates or expands an object. Tensile stress perpendicular and/or parallel to
grain differs largely. Wood is generally weak in tension perpendicular to the grain but it is very
strong in tension parallel to the grain.
Different adhesives used in preparation of the glulam might have different strength properties.
The strength properties of bonded wood largely depend on the physical properties of itself, and
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also the quality, quantity, and application of the adhesives. For the effective utilization of
bonded wood, the binding capacity of adhesives should be greater than the natural strength of
wood. If the bonding is weaker, the bonded wood might have glue failure hence could cause
serious damage, and economic loss. Therefore, before mass production of the glulam, their
strength properties should be tested and guaranteed.
2.0 Objective:
ƒ
To determine the tensile shear strength of Scot pine with PVAc and PF bonding in dry
and wet states.
3.0 Methods and Materials used:
3.1 Test equipments and materials:
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The computer programmed machine named
Zwick Z010 / Zwick Roell.
ƒ
The software : testXpert II
ƒ
The dry and wet specimens of Scot pine (PVAc
bonded) sapwood.
ƒ
The dry and wet specimens of Scot pine (PF
bonded) sapwood.
Fully computer programmed Zwick Z010 / Zwick Roell at Institute.
The wood specimens used were made of Scot pine (Pinus sylvestris L.) sapwood. This pine
species is native to Scotland. Wood is pale brown to reddish brown and used for general
construction works. It has a dry density of around 470 Kg/m3 (Wikipedia).
Schematic pictures of tested specimen
3.2 Test Methods:
The size of specimen was 15 cm x 2 cm =30 cm2. But
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the bonded area was 200 mm (20 mm x 10 mm). Two
different adhesives such as Poly Vinyl Acetate (PVAC)
and Phenol Formaldehyde (PF) were tested in two
different states i.e. wet and dry. 10 specimens for each
adhesive and each state were tested in the lab.
Altogether 40 specimens prepared to test longitudinal
tensile strength of bonded wood for two different states:
i.e. the dry state and wet state. The test at A1 (Dry
20 mm x 10 mm= 200 mm
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state) and A2 (wet state) conditions required specimens prepared in different manner. The
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bonded specimens required for dry state testing were prepared in 7 days at 20 degree Celsius in
65% Relative Humidity whereas for wet state specimens, apart from 7 days at 20° C and 65 %
RH, they were soaked in water for 4 days at 15 ± 5 °C. The Scot pine sapwood was tested
against longitudinal stress with PVAc and PF bonding adhesives.
Readymade specimens were given to us for testing. The specimen were labeled, then kept
vertically among the clamps and lightly tightened in computer programmed machine called
Zwick Z010 / Zwick Roell. Afterward, the longitudinal tensile stress were regularly applied and
increased with computer command. As the increase of stress, the bonded wood started to exert
resistance in the beginning but while the stress crossed the limit, the failure has been noticed and
the graphs dropped down reaching after the climax. This maximum stress (in Newton) point
were taken into note and by dividing the Force Max force with the bonded area (i.e. 200 mm2),
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we calculated the tensile shear strength of wood in N/mm . After that the failed specimen took
out and studied the failure types.
4.0 Findings and Discussions:
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The following statistical data recorded during the test in the lab.
Recorded tensile shear strength (Newton / mm2) of Scot pine sapwood:
PVAC bonding
Dry samples Wet samples
10.18
5.36
11.75
6.08
10.86
3.960
12.98
4.626
10.81
8.956
8.02
3.793
12.80
6.637
11.09
5.70
12.97
6.14
10.29
5.25
111.756
56.53
8.022
3.79
12.98
8.9564
11.17
5.653
1.4632
1.41
0.46
0.446
13.09 %
24.94 %
PF bonding
Dry samples
12.80
13.09
10.13
12.80
8.014
9.323
9.5371
12.62
12.51
10.17
110.66
8.014
13.09
11.06
1.734
0.548
15.67 %
Wet samples
7.48
6.75
7.37
6.97
9.03
7.21
5.957
7.6092
8.0149
8.24
74.6
5.957
9.03
7.46
0.805
0.254
10.7 %
Remarks
Total
Minimum
Maximum
Mean
Standard deviation
Standard error
Coefficient of
Variations
Note: Standard deviation is the deviation of sample observations from mean, standard error is
calculated by dividing standard error with the square root of sample number, and coefficient of
variation is calculated by dividing standard deviation by mean.
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ƒ
The PVAc and PF bonding both have
the similar strength properties during
dry treatment. PVAc bonding has
slightly greater shear strength than
the PF bonding on in average. This is
due to the similar room temperature
and dry condition provide, at which
condition both of adhesives has
almost
similar
strength properties.
Shear strenth of PVAc bonding
Shear strength (Newton/mm2)
ƒ
14
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10
Dry specimens
Specimens
The wood specimen tested under the
Wet specimens
dry condition has clearly demonstrated the higher tensile shear strength rather than wet
one in both PVAc and PF bonded wood.
Shear strength of PF bonding
It is because the moisture weakens
Shear strength (Newton/mm2)
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bonding strength and lowers the shear
strength of the wood.
ƒ
The PF bonded wood has been found
stronger against the shear state than
PVAc bonded wood in wet condition.
12
10
8
6
4
2
0
1
This is because of the properties of
2
3
4
5
6
7
8
9
10
Dry samples
Specimens
Wet samples
adhesives. PVAc is the thermoplastic
adhesives which will lose more strength with increase of moisture whereas PF is the
thermosetting adhesives which sustain high strength even when exposed to heat or
moisture. Also, the PVAc adhesives set with the loss of water whereas PF adhesive sets
with chemical reaction.
Shear strength of PVAC and PF bonding
ƒ
The Scot pine in PVAc bonding
has higher variability in wet state
than dry state. This is because of
the thermoplastic properties of
PVAc adhesive. Moisture content
heavily and irregularly lowers the
strength properties of PVAc.
The PF bonding has greater shear
strength than PVAc bonding at
wet
condition.
Hence,
while
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shear strength (N/mm2)
ƒ
12
10
8
PVAC Dry
specimens
6
PVAC Wet
specimens
4
PF Dry
specimens
2
PF Wet
specimens
0
1
2
3
4
5
6
7
8
9
10
Specimens
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using the bonded timber in exterior condition the chance of failure is higher in PVAc
bonding than the PF bonding however it is not the cases in interior uses.
ƒ
Also, the PF bonded Scot pine specimens has the least standard error and least standard
deviation. All these mean the PF bonded Scot pine gives more consistent results against
tensile shear while using in exterior conditions.
ƒ
While increasing the forces constantly, it has been noticed that the failure were from
woods in most of the cases. These explain that the bonding were quite effective and have
the better strength properties than the wood itself. However, in some cases the failure
noticed were in bonding that is due to the weaknesses in bonding. This means the setting
of adhesives did not occur perfect due to some reasons. That might be due to
inappropriate, inhomogeneous thickness of glue line.
ƒ
The regular and denser grained specimen required more stress to occur failure. This is
due to the denser and regular grained wood has more tensile shear strength than the
lighter one.
5.0 Conclusions:
ƒ
The bonding is an essential element in manufacturing of almost all kind of
engineered wood. It is probably the most important one in manufacturing of bonded /
laminated wood.
ƒ
Bonded wood has various uses. Some are supposed to use in exterior condition in
uncontrolled and open environment. They might have been reacting with moisture
(rainfall and humidity) that can degrade the bonding properties of adhesives.
Therefore the manufacturer should be very careful in selecting the adhesives
according to their expected uses and environment reactivity. In this perspective, the
PF bonding has better resistance than PVAC bonding in wet condition; however for
dry condition it has not much difference. Therefore for the exterior uses, the PF
bonded wood should be chosen rather than the PVAc bonded wood.
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Schematic pictures of tested specimen ƒ
The bonding quality and resistance against tensile force also depends on the glue line
thickness, homogeneity and the properties of wood. Therefore, due attention should
be given while preparing the woods and glue and also in their proper application.
ƒ
The use of bonded wood has been part and parcel of present modern life. To get the
longer life span and better use of money spent. We should be careful in selecting best
product made of best adhesives / and techniques as per the need of application.
References:
http://en.wikipedia.org/wiki/Scots_Pine
http:// www.woodbin.com
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