![. MOE and Poisson's ratio of Guadua strips pre and post THM modification [3]](https://www.researchgate.net/profile/Hector-Archila/publication/264043427/figure/tbl1/AS:614295473774598@1523470862605/MOE-and-Poissons-ratio-of-Guadua-strips-pre-and-post-THM-modification-3_Q320.jpg)
Content uploaded by Hector F Archila
Author content
All content in this area was uploaded by Hector F Archila on Jul 19, 2014
Content may be subject to copyright.
9
MECHANICAL CHARACTERIZATION OF
ENGINEERED GUADUA-BAMBOO PANELS USING
DIGITAL IMAGE CORRELATION
Hector F. Archila
Department of Architecture and Civil Engineering, University of Bath, Bath, UK.
H.F.Archila.Santos@bath.ac.uk
Martin P. Ansell
Department of Mechanical Engineering, University of Bath, Bath, UK.
Pete Walker
Department of Architecture and Civil Engineering, University of Bath, Bath, UK.
ABSTRACT: Engineered panels were manufactured by cross laminating heat and
pressure treated strips of Guadua angustifolia Kunth (Guadua). Cross laminated Guadua
(CLG) panels comprised of three and five layers glued with a high performance epoxy
resin. Large specimens of these panels were tested in compression and their elastic
mechanical properties evaluated. The digital image correlation (DIC) method was
applied to measure strain variations in the X, Y (in-plane) and Z (out of plane) axes on
the surface of 600 mm2 CLG panels. Strain results were analysed using VIC 3D
software and used to calculate the elastic values of the panels. Moduli of elasticity
(MOE) values from DIC for three and five ply panels were 13.50 GPa and 22.59 GPa in
the main direction (E0) and 5.28 GPa and 12.54 GPa in the transversal direction (E90).
While predicted MOE values for three and five ply panels were 21.43 GPa and 19.51
GPa in the main direction (E0) and 11.83 GPa and 13.75 GPa in the transversal direction
(E90). This study is part of a research project that aims to develop standardised industrial
structural products from Guadua and to measure and predict their mechanical behaviour.
Keywords: Bamboo, cross laminated panels, digital image correlation, Guadua
angustifolia Kunth, thermo-hydro-mechanical modification.
INTRODUCTION
Guadua has remarkable eco-credentials when compared to conventional building
materials and exceptional advantages when compared to wood forest products. As with
other bamboo species, Guadua is a fast growing non-wood forest resource that renews
itself; it also has a high yield per hectare and captures and fixes more carbon than most
softwood trees [1]. Vogtländer et al [2], identified bamboo as one the best renewable
resource in terms of yield when used in durable applications. Comparison of the annual
yield of Guadua in m3/ha for products such as MDF is very similar to that of Eucalyptus
10
and Radiata Pine [2] which makes Guadua a competitive alternative material.
Furthermore, bamboos in general have a considerable strength to weight ratio which is
comparable to mild steel. Average values of elastic modulus per unit density (specific
modulus) of bamboo are very similar to those of steel (25 x 106 m2/s2) [3]. However,
factors such as the bamboo species, its variation in density across and along the culm
and anisotropic mechanical properties hinder its use in stiffness-driven applications
where steel has been widely used. Engineered bamboo products are scarce and require
complex manufacturing processes. For instance, fabrication of laminated Guadua
products results on an energy intensive process due to the machining of round culms
into rectangular strips that produces high amounts of waste [2, 4]. Therefore, the
development of engineered Guadua products needs to exploit its remarkable features,
tackle the issues regarding manufacture and improve durability.
Studies on heat treatments applied to bamboo have shown improvements on the
mechanical properties and resistance to termites and fungal decay [5, 6]. These studies
and primary experimentation with THM treatments showed that temperatures below
160oC had a positive effect on the mechanical properties of Guadua and provided a
dimensionally stable flat sheet material with a densified profile [3]. Thus, THM
modifications were used as a way of reducing machining and producing durable
engineered Guadua panels with improved mechanical properties. The panels were
subjected to a testing programme with the aim of characterising their mechanical
properties and results from compression tests of large CLG panels were assessed using
the DIC method. This paper reports on the development of CLG panels at the University
of Bath, manufactured using straightforward densification and assembly methods that
could be easily applied industrially.
MANUFACTURE OF THE PANELS
Preparation of the Material
Round culms of Guadua were cut to the required length and its outermost layer was
removed using a professional burnisher fitted with a 100 mm x 289 mm x 40 grit
Zirconium cloth belt. This highly abrasive belt was used to remove about 100µm of the
tough cutinized layer that covers the cortex of Guadua. Subsequently the peeled lengths
of cylindrical Guadua were radially cut into six to eight strips (depending on the
diameter) and the inner pith cavity membrane was also removed.
The strips were stored under controlled temperature (27o C ± 2o C) and relative
humidity (70 ± 5 %) in a conditioning room, enabling them to reach equilibrium at 12%
moisture content (MC). By following the above mentioned process a reduction of 27%
of wasted material was achieved [3].
Densification
Following immersion in water for 24 hours, the strips were subjected to an open thermo-
hydro-mechanical (THM) treatment for 20 minutes using a daylight opening hot press
with 1000 square mm oil heated platens. Pressure on the hydraulic press was computer
controlled using PressMAN software and applied across the radial direction. Maximum
pressure, temperature and compression set were fixed to 50 kg/cm2, 150oC and 45%
11
respectively. The compression set C is defined as C = (Ro-Rc)/Ro where Ro and Rc
are the thickness of the samples before and after compression respectively.
(a) (b)
Fig. 1: (a) THM diagram (b) Image and diagram of the heat and pressure process.
As can be seen in Figure 1, THM modification occurred in two stages; the first is a
plasticisation stage where temperature and pressure on the strips of Guadua is increased
for 10 minutes. The second is the densification stage, where maximum pressure and
temperature were maintained for 10 minutes. This densification process provided
densified flat Guadua strips (FGS) with improved mechanical properties (Table 1), as
previously reported on [3]. A slight reduction in the dry weight of the strips was recorded
post-THM treatment; however the MC was not markedly affected (-0.5%).
Table 1. MOE and Poisson's ratio of Guadua strips pre and post THM modification [3]
(a) Pre THM (control
sample)
(b) Post THM
(FGS)
Fibre fraction area
25.53 %
47. 78 %
EL
16.21 GPa ± 0.76
31.04 GPa ± 0.47
ET
-
2.22 GPa
Density (ρ)
540 kg/m3
830 kg/m3
Specific stiffness (L)
(average)
29.84 x 106 m2/s2
37.58 x 106 m2/s2
Lamination
FGS were used to form the individual plies of three and five layer cross laminated
Guadua panels, which were labelled CLG-3, CLG-5 respectively. The plies were glued
with a mix of wood epoxy resin (Sicomin SR 5550) and wood gap filler (Wood fill 250),
which also increased the viscosity of the mix. The content of resin by total weight of the
composite was 4% and the spreading rate was 215 g/m2. Cold pressure of about 35
kg/cm2 was applied to the panels until the resin was set (Figure 2b) and then left to cure
in a conditioning room for about 20 days before machining.
(a) (b) (c)
12
Fig. 2: a) PressMAN hot press b )Lamination of the CLG panel c)Cross section of a CLG-5 panel.
The symmetrical composition of the laminate was determined by the odd number
of layers (three and five) and the lamination of alternating layers with regular
thicknesses (5.5±0.3) at 0o and 90o. For structural analysis the manufactured CLG-3 and
CLG-5 panels are considered as shell elements under plane stress conditions and their
orthotropic elastic properties need to be evaluated (MOE, shear modulus and Poisson’s
ratio).
The longitudinal orientation of the CLG panels corresponds to its load bearing
direction and is defined by the orientation of Guadua fibres in the outer layers (Figure
3a). This also represents the highest number of layers orientated in X1 and a ratio of 2
to 3 for CLG-3 and 3 to 5 for CLG-5.
EXPERIMENTS AND ANALYSIS
Compression test
CLG-3 and CLG-5 panels of 600 mm x 600 mm were tested in compression in the X1
(longitudinal) and X3 (transversal) directions as illustrated in Figure 3. The panels were
tested according to the BS EN 789:2004 [7] standard for structural timber elements.
Compression tests of the panels were carried out in a 200 kN Mayes universal test
machine (Figure 4a) at a rate of 0.5 mm/min. Ten loading series below the elastic limit
were undertaken per panel and special test fixtures were used to anchor the panels to the
test machine.
a) b) c)
Fig. 3: a) Geometric (X1, X2, X3) and orthotropic (L, R, T) axis of the CLG panel b) Diagram of the
compression test on the longitudinal direction of the panel c) Diagram of the compression test on the
transversal direction of the panel.
Digital Image Correlation Method
During mechanical testing, two monochrome high speed cameras (Fast Cam SA3)
recorded simultaneous images of a speckle pattern painted on the surface of the panel at
a rate of one frame per second (Figure 4). These sets of paired images were analysed
using VIC3D-2009 software and 3D strain maps were produced (Figure 4b and 4c). A
virtual extensometer (A-B) was placed on the face of the panel with the speckle pattern
and the axial strain variations per image were recorded. Figures 4b and 4c illustrate the
area analysed and the location of the extensometer.
X1 (L)
X3 (T)
X3 (T)
X1 (L)
X1 (L)
X3 (T)
X2 (R)
13
a) b) c)
Fig. 4: a) Panel mounted on the test machine b) Strain map in X2 (z)of a CLG tested on the transversal
axis c) in z of a transversal CLG Typical strain-stress graph of the compression test carried on CLG
panels
Determination of MOE
Strain values from the DIC method were used to calculate the MOE of CLG-3 and
CLG-5 panels in the longitudinal (E0) and transversal direction (E90). Typical stress-
strain response obtained from the compression test of three and five layers CLG panels
was plotted and a linear regression analysis performed (Figure 5). The longest portion of
the graph with a correlation coefficient ≥ 0.60 was used to determine MOE.
Table 2. MOE results
Fig. 5: Typical strain-stress graph for CLG-5 panels tested in the longitudinal direction.
Results for E0 and E90 from the compression test of CLG-3 and CLG-5 panels are
presented in Table 2 together with predicted values using data from previous tests
reported in [3]. E0 and E90 values depend on the number of layers and the stiffness’s of
the individual layers (EL and ET).
Predicted MOE values were generally higher than the MOE values obtained
through the DIC method. CLG-3 and CLG-5 panels longitudinally oriented presented a
load capacity between 1.5 and 2.5 times their transversal orientation in both predicted
and test results. No permanent deformation (post-test) in any axis was recorded by the
DIC; however, 3D strain maps showed areas prone to deformation in the X2 (R)
direction that presented gaps or fabrication defects.
MOE
Predicted
values
MOE
Test values
Correlat.
Coeff. (R2)
E0, CLG-3
21.43 Gpa
13.50 GPa
0.94
E90, CLG-3
11.83 GPa
5.28 GPa
0.84
E0, CLG-5
19.51 GPa
22.59 GPa
0.95
E90, CLG-5
13.75 GPa
12.54 GPa
0.63
A
B
X1
X3
A
B
X3
X1
14
CONCLUSIONS
Flat cross-laminated Guadua (CLG) panels for use in construction were manufactured
using a simplified process that reduced the wastage produced during conventional
lamination processing by 27%.
The elastic mechanical properties of engineered bamboo panels were characterised
using digital techniques and compared to predicted values using data from previous tests
[3]. Similar MOE values (E0,5ply = 14 GPa) have been reported by [8] for cross laminated
bamboo products using different techniques. However, validation of the obtained results
will require further testing using physical strain measurement systems. DIC methods
allowed for a qualitative assessment of the structural behaviour of the panels but
difficulties were encountered on the quantitative analysis of their mechanical properties.
ACKNOWLEDGEMENT
The first author is grateful to AMPHIBIA GROUP LTD and COLCIENCIAS, sponsors of
his studies at the University of Bath, UK.
REFERENCES
[1] Lou, Y. Li, Y. Buckingham, K. Giles, H. Zhou G. Bamboo and Climate Change
Mitigation Bamboo: a comparative analysis of carbon sequestration. INBAR
Technical Report No. 32. Beijing, 2010.
[2] Vogtländer J, van der Lugt P, Brezet H. The sustainability of bamboo products
for local and Western European applications. LCAs and land-use. J. Clean. Prod.
Elsevier Ltd; 2010 (September) 18(13):1260–9.
[3] Archila-Santos, H., Ansell, M., Walker, P. Elastic properties of thermo-hydro-
mechanically modified bamboo (Guadua angustifolia Kunth) measured in
tension. Accepted to Key Engineering Materials, 2013.
[4] Flander K, Rovers R. One laminated bamboo-frame house per hectare per year.
Constr. Build. Mater. 2009 (January) 23(1):210–8.
[5] Zhang YM, Yu YL, Yu WJ. Effect of thermal treatment on the physical and
mechanical properties of phyllostachys pubescen bamboo. Eur. J. Wood Prod.,
2012 (November) 71(1):61–7.
[6] Manalo RD, Garcia CM. Termite Resistance of Thermally-Modified
Dendrocalamus asper (Schultes f.) Backer ex Heyne. Insects. 2012 (March)
3(4):390–5.
[7] British Standards Institution. Timber structures. Test methods. Determination of
mechanical properties of wood based panels. BS EN 789:2004. BSI, London,
2004.
[8] Verma CS, Chariar VM. Development of layered laminate bamboo composite
and their mechanical properties. Compos. Part B Eng. Elsevier Ltd; 2012 (April)
43(3):1063–9.
