Interlaminar Behavior of Paulownia Wood Sandwich Composites with Grooves

Abstract

Some grooves were arranged on the surface of the paulownia wood core, with the resins fulfilled. After solidifying, the resins left in the grooves strongly bonded the faces and core together. Therefore, the adhesive capacity between the faces and the core was enhanced. VARIM process was used to manufacture the specimens with different groove widths, depths and distances, and the DCB method was used to test the interfacial strength of the sandwich beams. Based on the energy release ratios from the test data, some factors are analyzed. It was found that the grooves can improve both the processing and interfacial properties. The present analysis provides the basis for the wide applications of paulownia wood sandwiches in future.

Full text

Interlaminar Behavior of Paulownia Wood Sandwich Composites
with Grooves
Wan Li (sloman@126.com ), Liu Weiqing, Fang Hai, Zhou Ding
Advanced Engineering Composites Research Center, Nanjing University of Technology, Nanjing, China
ABSTRACT Some grooves were arranged on the surface of the paulownia wood core, with the resins fulfilled.
After solidifying, the resins left in the grooves strongly bonded the faces and core together. Therefore, the adhesive
capacity between the faces and the core was enhanced. VARIM process was used to manufacture the specimens with
different groove widths, depths and distances, and the DCB method was used to test the interfacial strength of the
sandwich beams. Based on the energy release ratios from the test data, some factors are analyzed. It was found that the
grooves can improve both the processing and interfacial properties. The present analysis provides the basis for the
wide applications of paulownia wood sandwiches in future.
KEY WORDS
1 INTRODUCTION
A sandwich structure is a three-layer structure comprising
a low density and low modulus core material between
two high modulus face sheets. This arrangement provides
a structure with a high bending stiffness[1]. The sandwich
structures have been applied among aircraft, ships,
automobiles, rail cars, wind energy systems, and bridge
construction.
Face sheets and core are bonded by some adhesive. In
adhesive joints, the thickness of the adhesive is typically
much smaller than the thickness of the adherents. But
adhesive is very relevant to sandwich’s mechanical
behavior. Zenkert (2009) studied foam sandwich under
circular loads, and the crack started from the interface[2].
Somers (1992) found sandwich’s buckling and post-
buckling behavior were very sensitive to its interlaminar
property[3]. In Suvorov’s research (2005), the low impact
behavior could be improved by higher interfacial energy
release ratio[4].
Due to the high price of balsa wood, Advanced
Composites Research Center of Nanjing Univ. of Tech.
started the research of paulownia wood at 2006, which
was a proper replacement of balsa wood[5]. To improve
the mechanics of paulownia wood sandwich and expand
its application, the interface behavior was studied.
2INTERFACE OF PAULOWNIA
SANDWICH
2.1Test method
Srinivas (1994) used cantilever beam to measure the
energy release ratio of aluminum face sheets and PVC,
PMI foam sandwich[6]. ASTM also proposed a method for
mode I interlaminar fracture toughness of unidirectional
fiber-reinforced polymer matrix composites. With a
standard DCB pre-cracked specimen, after the crack
propagated 3-5mm, the interfacial energy release ratio
should be the mode I interlaminar fracture toughness.
But there were grooves on the surface in the grooved
paulownia wood sandwich, standard DCB method
cannot satisfy the facial structures. Pan (2007) used the
DCB method to test the interfacial properties of the
honeycomb sandwich[12].
In this paper, sandwich specimens comprise paulownia
wood core(25.4mm) and 2 face sheets(four 800g glass
fibers [0嘙/90嘙, f45嘙, 0嘙/90嘙, f45嘙], vinyl ester resin).
The dimension of the specimens was 200mm (length) ×
60mm (width). The core thickness was 25mm and face
sheet thickness was 3.5mm. The overall thickness was
32mm. In Figure 1, a 50mm pre-crack was between the
top face sheet and paulownia wood core. With two
perforated metal solid, the cracks propagates along the
pre-crack. The interlaminar properties were analyzed by
the propagation speed and propagation energy.




P
P
3UHFUDFN
Figure 1Double cantilever beam specimens
CICE 2010 - The 5th International Conference on FRP Composites in Civil Engineerin
g
September 27-29, 2010, Beijing, China
L. Ye et al. (eds.), Advances in FRP Composites in Civil Engineering
© Tsinghua University Press, Beijing and Springer-Verlag Berlin Heidelberg 2011

Proceedings of the 5th International Conference on FRP Composites in Civil Engineering
124
Figure 2 were the apparatus and specimen sample. An
electric servo universal testing machine was used to pull
the metal blocks bonded on the face sheets. The tensile
force P and load displacement were recorded at 50Hz.
At the same time, a camera was used to record the
process of propagation. On the specimen, a paper staff
was pasted to indicate the crack length in the test video.
Figure 2Apparatus and specimen of DCB test
2.2Paulownia wood sandwich
The energy release ratio could be calculated by the
Eq.(1)[9].
1d
d
U
Gba
 (1)
In Eq.(1), U is the total elastic strain energy, b is the
width of the specimen and a is the crack propagation
length. U could be integrated from the P-¨ curve, while a
could be measured with the paper staff on the specimens.
Figure 3P-¨ curve of the hand lay-up specimens
In Table 1, the energy release ratios of four specimens
were listed. NRD indicated the specimen type which was
a Non-Reinforced Double cantilever specimen. There was
little difference among the four specimens. DCB test
method showed good reliability.
Table 1Interfacial energy release ratio of handy lay-up paulownia
wood sandwich
NRD-1
(N/m)
NRD-2
(N/m)
NRD-3
(N/m)
NRD-4
(N/m)
Average
(N/m)
Coefficient
of Variance
Energy
Release
Ratio
380 364 426 379 374 2%
3GROOVED PAULOWNIA WOOD CORE
SANDWICH
DIAB’s ProBalsa and Alcan’s BALTEK are typical balsa
wood core with grooves. The balsa wood was cut into
pieces and bonded along its transverse direction. The
grooves are 0.8mm width, 3mm height, with the distance
of 20mm, which provides flow path for the resins[10],[11].
According to the usual saw blade, three typical groove
widths were provided. Orthogonalization and nondimen-
sionalization method were used in the experiment design.
Eight dimension combinations were proposed in Table 2
with the 1mm, 3mm, 5mm in width, and 10mm, 20mm,
Table 2Interfacial energy release ratio of grooved paulownia wood
sandwich
Grooves
Width b
(mm)
Height h
(mm)
Distance d
(mm)
Energy
Release Ratio
G (N/m)
GCD-1 0.8 1.0 40 395
GCD-2 0.8 3.0 40 783
GCD-3 1.3 3.0 20 745
GCD-4 1.3 5.0 20 797
GCD-5 2.2 3.0 20 803
GCD-6 2.2 5.0 20 805
GCD-7 1.3 5.0 10 864
GCD-8 2.2 5.0 10 1002
GD-X 0.8 3.0 20 791
b d
h
Figure 4Grooved paulownia wood sandwich

September 27–29, 2010, Beijing, China
125
40mm in distance. The fiber plies were the same with
the handy lay-up specimens. The thickness was 31mm,
slightly thinner than the handy specimens because of the
manufacture process.
To ensure the impregnation of the resins in the
grooves, vacuum assisted resin infusing molding (VARIM)
process was used to manufacture all the specimens in
Figure 5. By the vacuum pump, the air was driven out of
the specimens, while the resin were infused under
atmospheric pressure.
Figure 5Manufacture process of grooved paulownia wood sandwich
Figure 6The P-¨ curve of grooved paulownia wood sandwich
specimens
Figure 7Face sheets and grooved wood after delamination
Figure 7 was the GCD-7 after delamination. It was
found that the resins had fulfilled all the grooves
completely. Additionally, there were some white fibers
pulled out of the matrixes in the grooves. This kind of
fiber-resin interfacial failure could be easily observed in
all the grooved specimens. Because the interface between
the glass fibers and resins was much more available than
the interface of wood and face sheet. Therefore, the
adhesion of grooved sandwich was enhanced by the
resins left in the resins. And different energy release
ratio in Table 2 showed the accurate reinforcing effect of
different grooves.
In Table 2, G in GCD was short for “Grooved”, while
D was short for “DCB test”. And C indicated this was
the 3rd group. Because the VARIM method was very
sensitive and unstable for the small specimens, three
groups of specimens were manufactured. Only in group
C, all the grooves were fulfilled with resins. The energy
release ratios had been listed in Table 2. GCD-1 showed
the worst interfacial property. The energy release ratio
was only 395N/m, far below other specimens. The 1mm
groove height should be responsible for this. The lowest
height made the grooves much easier to be blocked by
the woven fibers and the resins could not fulfill all the
grooves. Most grooves were empty. While, comparing
with the handy lay-up specimens, the other 8 specimens
improved 100% above.
4DISCUSSION
4.1Parameter analysis
GCD-1 was ignored, because of its blocked resin paths
and imperfect resin grooves. The left eight specimens
were analyzed. In Figure 6, P-¨ curves of all the specimens
were printed. With the same groove width and distance,
the groove height gave little influence to the energy
release ratio. The P-¨ curves of GCD-3 and GCD-4 or
GCD-5 and GCD-6 were almost the same. The most
improvement of energy release ratio was less than 1%.
There was no additional effect when the groove height
increased from 3mm to 5mm. To increase the groove
width from 1.3mm to 2.2mm, the energy release ratio
grew more effectively (GCD-3 and GCD-5, GCD-4 and
GCD-6, GCD-7 and GCD-8). The least improvement
was 1%, while the most could be 16%. Decreasing the
distance between grooves was the most effective method
(GCD-2 and GD-X, GCD-4 and GCD-7, GCD-6 and
GCD-8). The most remarkable improvement was 24%,
least was 1% and average was 11%. Although the
improvement was instable, it was figured that the
interfacial properties were affected only by the groove
width and groove distance.
Eq.(2) was the fitting equation, with related coefficient
factor r = 0.9857.
2
790 454( ) 6476( )
bb
Gdd
  (2)
4.2Manufacture process and grooves
According to Eq. (2), it was found that the real factor
was the surface area of the grooves in the core. The
grooves played two roles in the sandwich. First, they
were the necessary paths for resins. Second, the grooves

Proceedings of the 5th International Conference on FRP Composites in Civil Engineering
126
could be interfacial reinforcing structures. The resins
left in the grooves could bond the face sheets and core
strongly.
The constant in Eq. (2) was very similar to the energy
release ratio of GCD-2. And in GCD-2, the area of grooves
was only 4% of the total core surface, so the constant
could be recognized as the energy release ratio without
any grooves. These improvements were the effect of the
VA R I M .
Tot al G could be rewritten as Eq.(3). The total energy
release ratio was divided into two parts. In Eq.(3), Gv
was the energy release ratio because of the VARTM
process, and Gg was the energy release ratio because of
the resins left in grooves.
In Eq.(4), (bd-4b2)/d2 was percent of the groove area
in the core. Then Gg and Gv could be expressed in Eq.(4)
and Eq.(5). And Gg/G was the percent of the resin grooves’
contribution. In GCD-8, grooves’ was 25%.
vg
GG G  (3)
2
2
4
790(1 )
v
bd b
Gd

 (4)
2
329 5693
g
bb
Gdd
§· §·

¨¸ ¨¸
©¹ ©¹
(5)
Additionally, the delamination caused some white
areas on the face sheet, in Figure 7. And the white areas
were just the position of the grooves. That meant the
delamination between face sheet and paulownia was
partly because of the microscopical debonding of glass
fibers and resins left in the grooves. It needed further
proof. If there was another way improve the interfacial
properties of the glass fibers and resins, the interlaminar
behavior might be better.
5CONCLUSION
Grooves in the paulownia wood could improve the
interlaminar behavior of sandwich effectively. Both the
process and interfacial structure would be benefitted. The
energy release ratio would be promoted as the increase
of the resin groove width and decrease of the groove
distance. So groove could be considered as a reinforcing
method to improve the interlaminar properties of paulownia
wood sandwich.
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