Composites in Construction 2005 – Third International Conference
Lyon, France, July 11 – 13, 2005
MECHANICAL CHARACTERIZATION OF SANDWICH STRUCTURE COMPRISED OF GLASS
FIBER REINFORCED CORE: PART 1
S.V. Rocca and A. Nanni
Department of Civil, Architectural and Environmental Engineering, University of Missouri – Rolla
Rolla, USA
[email protected]
[email protected]
ABSTRACT: This paper aims at demonstrating the feasibility of Glass Fiber Reinforced Polymer
(GFRP) systems for low-profile bypass roadways, in particular, sandwich panels comprised of
GFRP facings and a Fiber Reinforced Foam (FRF) core. With the purpose of investigating the
performance of the sandwich construction, an experimental program, including static and dynamic
fatigue tests, was performed. The same type of specimens tested under static loading, were
cyclically conditioned in order to determine its residual strength. The investigation focused on the
ultimate capacity and stiffness (compressive and flexural) of the sandwich structure, as well as its
residual strength and rigidity after fatigue conditioning. The mechanical properties resulting from
the experiments on virgin and fatigue-conditioned samples constitute the basis for the assessment
and validation of the material system for the intended application.
1. INTRODUCTION
A structural sandwich is a special form of a laminated composite comprised of a combination of
different materials that are bonded to each other so as to utilize the properties of each separate
component to the structural advantage of the whole assembly [1]. The sandwich structure is
composed of two faces and a core. Usually the faces are identical in material and thickness. The
variety of types of sandwich constructions basically depends upon the configuration of the core, not
to mention the material constituents. The most common types of core are: foam, honeycomb and
web core truss [2]. The faces that must be stiff, strong and thin; are separated and bonded to a
light, weaker and thick core. The adhesion of both materials is very important for the load
transferring and therefore the functioning of the sandwich as a whole.
Regarding the behavior of a sandwich structures, an analogy with an “I” cross section is
appropriate. In this efficient type of section, as much as possible of the material is placed in the
flanges which are situated farthest from the neutral axis (center of bending), and enough material
is left in the web to make the flanges work together and to resist shear and buckling. Likewise, in a
sandwich, the faces act as the flanges and the core takes the place of the web, in other words, the
faces carry in-plane and bending loads, while the core resist transverse shear forces and keeps
the facings in place [2]. Compared to a single laminate structure, by using the sandwich concept,
the flexural rigidity and flexural strength can be significantly increased. For structural applications in
civil engineering, low cost sandwich panels can be used. The combination of a Fiber-Reinforced
Foam (FRF) with glass fiber polymer composites facings allows the construction of a low cost
composite structure [3].
The sandwich panel object of this study is produced by WebCore Technologies (Ohio). It has a
total height of 89 mm (3.5 in) and it is characterized by a new type of core referred to as FRF. It is
composed of closed-cell foam combined with dry fibers. The foam keeps in position different
structural fiber forms that at the end become Fiber-Reinforced Polymer (FRP) members once
infused with resin during molding. These members take the form of angled struts oriented in truss
or flat continuous webs [3]. In this particular case, the thickness of the foam is 76.2 mm (3 in), and
glass reinforcement has been used creating a hybrid stitched/webbed FRF core, which is
characterized by stitches in the x-direction (longitudinal) and by continuous webs in the y-direction
(transverse). The material constituent of the stitches and the webs is Glass Fiber Reinforced
Polymer (GFRP). The skin reinforcement consists of the pre-attached GFRP fabric plus several
layers of bidirectional E-glass fabrics added during molding. The sandwich panel characterized in
this research was composed of GFRP facings of 6.35 mm (0.25 in) thick.
Regarding the objective of this research project, since in the last few years, the availability of
lightweight FRP sandwich panels, not only for bridge decks but also for temporary bypass
roadways, has been identified as a very interesting alternative to traditional methods of
construction, an investigation whose starting point is the mechanical characterization of a sandwich
panel structure comprised of a FRF core was conducted. The determination of the sandwich
mechanical properties as well as its performance under different loading conditions provides part of
the information necessary to decide whether the material is suitable or not for construction use.
Figure 1 - Sandwich Panel Cubic Specimen
2. TEST MATRIX & TEST SETUP
2.1. Compressive Test
This test was accomplished according to ASTM C365 [4]. According to the standard, cubic nominal
specimens of 102 mm (4 in) were cut out of two panels, namely panel A and B. This panels were
apparently the same, but it was later observed that panel B’s performance was slightly more
deficient than panel A’s. Mechanical properties such as ultimate compressive capacity,
compressive strength and compressive modulus were determined.
2.1.1. Static Compressive Test
Testing of six specimens per panel was conducted in the laboratory. The equipment utilized for this
test was a MTS880 universal testing machine. Regarding the instrumentation, since the core of the
sandwich specimens is composed of foam, it was not possible to fix an extensometer to it and
obtain vertical displacement readings; instead, a potentiometer was attached to the side
corresponding to the longitudinal direction of the panel on the specimens, otherwise the widening
in the transversal direction would have avoided accurate readings. Some silicon was placed
between the foam and potentiometer (Figure 2a). For the actual test, the specimen was centered
under the load plate to ensure a correct pressure distribution. The crosshead displacement was set
up at the loading rate of 0.1075 mm/min (0.0043 in/min) and the individual average time to failure
of the specimens was 5.45 minutes.
2.1.2. Fatigue Compressive Test
The purpose was to determine the performance of the sandwich panel under repeated loading.
After conditioning of the specimens to 0.5, 1, 1.5, and 2 million cycles at preset load levels and the
subsequent up to failure test, it was possible to study the behavior of the material comparing it to
the one observed in the Static Compressive Test for both panels A and B.
Four different levels of load were imposed in this test. For the minimum level (Load Level 1) it was
decided to take the value of 30 percent of the ultimate load determined from the static test. Such
value is higher than the “Sustained plus Cyclic Stress Limit” for Glass FRP suggested by ACI 440
[5]: 20 percent. The maximum level (Load Level 4) corresponds to 60 percent of the ultimate
compressive capacity. The levels of load shown in the following table were calculated based on the
ultimate compressive capacity of Panel A. In Table 1 the ranges of the imposed loads for the
different levels are presented. A minimum of 5 percent of the ultimate compressive capacity was
considered suitable to maintain the stability of the specimens and equipment during the
conditioning.
Table 1 - Levels of Load
Load
Values
Minimum
Maximum
Load Level 1
30%PU (kN)
4.23
25.27
Load Level 2
35% PU (kN)
4.23
29.50
Load Level 3
40% PU (kN)
4.23
33.73
Load Level 4
60% PU (kN)
4.23
50.55
Note: Specimens from panel A were fatigue conditioned under levels 1 and 4; specimens from panel B were
conditioned under levels 1, 2 and 3.
The MTS880 was also suitable for the conduction of the cyclic test, since it is capable of applying
defined amplitude loading, and monitoring the variation of the load during conditioning. The load
levels shown above correspond to the maximum values applied to the specimens. With a
frequency of 5 Hz, the duration of the test for each imposed level of load was approximately five
days, due to the fact that the four series of four specimens were stacked and it was removed one
series at a time after each 0.5 million cycles until the last series accumulated 2 million cycles.
For safety reasons, avoidance of horizontal displacement of the specimens, and to guarantee a
uniform load distribution from the crosshead over each sample during conditioning, a steel frame
was built and placed in the MTS880. The biggest plate was properly fixed to the bottom plate of the
machine and the rest of them were stacked and aligned. The specimens were set up on the plates
in four series of four (Figure 2b).
(a) Static Compressive Test
(b) Fatigue Compressive Test
Figure 2 - Compressive Test Setup
2.2. Flexural Test
Testing of sandwich beams was conducted under static and cyclic load condition. This test was
performed according to ASTM C393 [6]. The objective was to determine the flexural mechanical
properties of the sandwich construction. The significance of the influence of the core shear
modulus and its contribution to the overall deflection of a member subjected to bending moment is
analyzed. The evaluation of the experimental results is accomplished through the application of
ordinary beam theory [7].
The specimens can be described as having rectangular cross section, with a depth equal to the
thickness of the sandwich panel: 89 mm (3.5 in); a width of 203.2 mm (8 in) and a length of 1.47 m
(58 in). Due to the anisotropy of the material, two types of specimens were considered for testing:
transversally-cut (y-direction) and longitudinally-cut (x-direction).
2.2.1. Static Flexural Test
The MTS880 was used for this experiment, which was conducted under four point bending
configuration. A total of four specimens (two per type) were tested over a clear span of 1.4 m (54
in) with equal loads applied at 0.46 m (18 in) from each support. The instrumentation utilized in this
test consisted in two Linear Variable Differential Transformers (LVDT) transducers placed at each
support point of the beam (top faces), five string transducers, two of them located at mid-span in
both sides of the beam, and the rest at the loading points (Figure 3).
Loading block
LVDT
Sandwich beam
String Transducers
Figure 3 - Static Flexural Test Setup
2.2.2. Fatigue Flexural Test
The specimens were fatigue conditioned to 0.5, 1, 1.5 and 2 million cycles at a single load level of
20 percent of the ultimate compressive capacity (PU) corresponding to each type of beam. An
appropriate steel frame was built and fixed to the MTS880 for the conditioning.
After reaching the preset numbers of cycles, each sandwich beam was statically tested to failure.
The load was applied in displacement control at a rate of 6.35 mm/min (0.25 in/min), and the failure
was reached in between five to six minutes. The instrumentation consisted in four Linear Variable
Differential Transformers (LVDT) transducers located at mid-span, at the loading point, at the
support of the sandwich beam (top face) and a fourth LVDT was attached to the steel beam to also
measure its deformation.
3. TEST RESULTS
3.1. Compressive Test
The results corresponding to panel A and B are presented in the following tables. The ultimate load
is represented by PU, U the ultimate displacement, HU the ultimate compressive stress, and E the
compressive modulus.
3.1.1. Static Compressive Test
About the behavior, the load-deformation curves were essentially linear but showed some ductility
beyond the peak load. Concerning the mode of failure, it was observed in all the specimens (Panel
A and B), buckling of the continuous webs and the subsequent creation of gaps in between the
foam and the webs. Due to the longitudinal configuration of the stitches, the specimens popped out
in that direction generating horizontal cracks in the foam (Figure 4).
Table 2 - Static Compressive Test Results; Panel A & B
Panel
A
B
Specimens
PU (kN)
>U (mm)
?U (MPa)
E (MPa)
Mean
Standard Deviation
Coefficient of Variation
Mean
Standard Deviation
Coefficient of Variation
84.26
3.65
4.33%
68.10
7.73
11.35%
1.533
0.069
4.53%
1.407
0.091
6.46%
8.65
0.49
5.64%
6.49
0.68
10.53%
568.90
41.60
7.31%
455.50
32.87
7.22%
Vertical crack
Horizontal crack
(a) Buckling of the webs
(b) Popping out of the stitches
Figure 4 - Mode of failure - Compressive Tests
Under compressive vertical stress, the continuous webs buckled originating vertical cracks or gaps
and bending of the foam in the transverse direction, and since the foam had negligible tensile
resistance, the appearance of a horizontal crack was the consequence. This crack development
was also influenced by the fact that the lattice created by the stitches did experience deformation in
its longitudinal direction. Contrary to the vertical cracks, the horizontal crack was not expected
under compression, but it was the result of the configuration of the material that affects the mode of
failure.
3.1.2. Fatigue Compressive Test
The residual mechanical properties resulting from the fatigue conditioning and subsequent static
up to failure test for the specimens under different levels of load are summarized in the following
figures. The residual ultimate compressive strength is presented as a function of the number of
cycles at which the different sets of specimens were subjected to. Figure 5 shows that there is not
appreciable reduction in the compressive capacity of the material. The minimum measured
residual ultimate compressive strength corresponds to 80 percent of its ultimate value, fatigue
conditioned to 2,066,263 cycles. Figure 6 presents a summary of the data in the form of a S-N
diagram. The letter in the label of each point designates the panel they belong to, and the number
references the conditioning load level. The points with an arrow indicate the set of specimens that
did not fail during the conditioning, therefore for a certain number of cycles, their residual strength
(hollow points) was determined. The colored dots without an arrow represent the series of samples
that failed during the fatigue conditioning. Additionally, the points are labeled according to the
panel where they were obtained from (A or B), and the levels of load to which they were
conditioned to (1, 2 and 3).
With regards to the mode of failure, crushing of the specimens did occur during cycling. Under load
level 1, specimens from panel A were conditioned to the designated number of cycles without any
failure or visible damage, on the contrary of specimens from panel B that, all of them survived the
first 0.5 million cycles, so the top series was removed and the three remaining were left to continue
the conditioning. After 83374 cycles (total of 583,374) the top two series failed. The samples in the
bottom layer were apparently not damaged, so that cycling was continued. This last series was
able to resist 2,083,374 cycles without failure. The top two layers of specimens from only panel B
failed under load level 2, after 66,263 cycles. From the two remaining series, one was removed
and one was left to continue the cycling, the latter reached 2,066,263 cycles without any visible
damage. Regarding the conditioning load level 3 (panel B), after 65340 cycles, the third series
starting from the top failed. The top layer was removed and the two remaining series continued the
cycling. There is no record of the exact number of cycles at which these two last layers crashed.
The MTS880 was set up for 0.5 million cycles, but since no limits were set up in the stroke, the
machine continued the cycling until it reached the present count. About conditioning under load
level 4 (panel A), during the definition of the load range, after less than 100 cycles, the second
layer of specimens starting from the bottom, failed. No static test was performed in the remaining
samples.
Residual Ultimate Compressive
Strength
160%
140%
120%
B2
100%
B1
B1
B3
B2
A1
A1
80%
A1
A1
60%
40%
20%
0%
0
0.5
1
1.5
2
2.5
Number of Cycles (millions)
Figure 5 - Residual Ultimate Compressive Strength
1.6
1.4
S
1.2
1.0
B1
0.8
A1
0.6
B3
B2
0.4
A1
B1
B2
A1
B2
B1
A1
0.2
A1
B1
B1
A1
A1
A1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
N
Figure 6 - Conditioning Load S vs. Number of Cycles in millions N.
3.2. Flexural Test
The ultimate load is represented by PU. Regarding the bending stiffness of the sandwich beams,
this property was computed in three ways (D1, D2, and D3). D1 and D3 were computed based on
the deflection given by the elastic beam theory (deflection due to bending only) at the loading
points and at mid-span, respectively. D2 was computed not only considering the bending deflection
but also the deformation due to shear in the total deflection at mid-span and at the loading points.
This was considered in order to investigate the influence of the shear deformation. The ultimate
facing bending stress is represented by Ufacing, and the ultimate core shear stress by U. The
values of core shear modulus G for each beam are not reported in the corresponding tables since
the sensitivity of the test setup, instrumentation and the equipment did not allow the assessment of
values that could be useful for technical purposes. For both the static and fatigue flexural test, the
observed modes of failure were the following (Figure 7):
(a) Wrinkling
(b) Wrinkling and Delamination
(c) Delamination and Shear Failure
Figure 7 - Failure Modes - Flexural Tests
3.2.1. Static Flexural Test
Table 3 - Mechanical Properties; Longitudinally-cut
Specimen
PU (kN)
D1 (kN.m)
D2 (kN.m)
D3 (kN.m)
?Ufacing (MPa)
Mean
St. Deviation
Coef. of Variation
70.44
8.14
11.58%
437.65
0.16
0.04%
429.63
18.76
4.37%
462.40
38.99
8.42%
173.28
33.83
19.53%
U
(MPa)
2.07
0.21
11.48%
Table 4 - Mechanical Properties; Transversally-cut
Specimen
PU (kN)
D1 (kN.m)
D2 (kN.m)
D3 (kN.m)
?Ufacing (MPa)
Mean
St. Deviation
Coef. of Variation
61.09
0.44
0.71%
458.78
13.45
2.93%
588.39
161.70
27.49%
470.31
25.54
5.42%
141.04
0.96
0.69%
U
(MPa)
1.79
0.01
0.54%
3.2.2. Fatigue Flexural Test
Table 5 - Residual Mechanical Properties; Longitudinally-cut
Specimen
PU (kN)
D1 (kN.m)
D2 (kN.m)
D3 (kN.m)
?Ufacing (MPa)
1L0.5
2L1.0
4L2.0
69.55
70.44
91.84
555.06
503.87
559.35
589.18
626.92
432.90
559.12
516.98
538.45
160.61
162.67
212.01
U
(MPa)
2.07
2.14
2.76
Note: The subscript of the specimens represents the number of cycles in millions to which they were fatigue
conditioned.
Table 6 - Residual Mechanical Properties; Transversally-cut
Specimen
PU (kN)
D1 (kN.m)
D2 (kN.m)
D3 (kN.m)
?Ufacing (MPa)
1T0.5
2T1.0
3T1.5
4T2.0
66.12
86.64
75.96
67.15
485.00
534.60
583.65
525.56
485.90
598.90
600.60
877.56
485.00
542.06
585.79
554.49
152.68
200.02
175.35
155.03
U
(MPa)
2.00
2.55
2.27
2.00
Note: The subscript of the specimens represents the number of cycles in millions to which they were fatigue
conditioned.
4. DISCUSSION OF RESULTS AND CONCLUSSIONS
4.1. Compressive Tests
After the fatigue conditioning of sandwich nominal cubic samples, it was observed that the residual
compressive strength was not greatly affected, in fact, after two million cycles it was found to be 6
MPa (869 psi). No change in the failure mode of the fatigue conditioned specimens was observed.
4.2. Flexural Tests
In the flexural mechanical properties resulting from the experiments under static and fatigue cyclic
loading presented in tables 3 to 6, it is noted that the difference among the bending stiffness
denoted as D1, D2 and D3 is of consideration. It is observed in most of the cases that the values
corresponding to the fatigue conditioned beams are slightly larger than the ones obtained under
static loading. This difference could be due to the fact that the beams were cut out from different
panels. The variation among the values of the bending stiffness themselves have demonstrated
the suitability of ASTM C393 for the determination of flexural mechanical properties, but not for the
shear properties, as it is mentioned in the standard. The consistency in the values of the bending
stiffness D1 and D3, resulting not only from the fatigue test but also from the static experiments,
reveals the predominance of the deflection associated with bending. This implies that the input of
the deflection linked to the shear in the total deflection can be considered as negligible. For
technical purposes, a value of the bending stiffness of 542 kN.m (4796 kip.in) can be assumed.
It is evident that the composition of the material in terms of uniformity, which not only involves the
physical characteristics such as the presence of joints or areas of the panel where the facings are
thicker, but also individual properties of the constituents of the sandwich structure, have a direct
effect in the overall performance. In other words, it is in the manufacturing process, especially the
quality control where great attention should be focused.
5. ACKNOWLEDGMENTS
This research project was founded by the Missouri Department of Transportation (MoDOT) and the
University Transportation Center on Advanced Materials and NDT Technologies based at the
University of Missouri-Rolla. The support of WebCore Technologies, who provided the FRP panels,
is also acknowledged.
6. REFERENCES
[1] American Society of Testing and Materials. ASTM Standard Terminology of Structural
Sandwich Constructions (C274-99). West Conshohocken, PA: ASTM International 1999.
[2] Vinson, J. The Behavior of Sandwich Structures of Isotropic and Composite Materials.
Pennsylvania: Technomic Publishing Company, Inc, 1999.
[3] Stoll, F., Banerjee, R., Campbell, S., Day, S. Manufacture of Fiber-Reinforced-Foam Composite
Sandwich Structures. Virgina: ASC 16th Annual Technical Conference, September 2001.
[4] American Society of Testing and Materials. ASTM Standard Test Method for Flatwise
Compressive Properties of Sandwich Cores (C365-00). West Conshohocken, Pennsylvania: ASTM
International 2002.
[5] ACI Committee 440. Guide for the Design and Construction of Externally Bonded FRP Systems
for Strengthening Concrete Structures. (440.2R-02). Farmington Hills, Michigan: American
Concrete Institute, 2002.
[6] American Society of Testing and Materials. ASTM Standard Test Method for Flexural Properties
of Sandwich Constructions (C393-00). West Conshohocken, Pennsylvania: ASTM International
2000.
[7] Allen, Howard G. Analysis and Design of Structural Sandwich Panels. New York: Pergamon
Press, 1969.