Abstract: Sandwich composites with glass fabric and epoxy matrix as the skin and rigid polyurethane foam (PUF) as the core were prepared in the shape of panels. Three panels were fabricated with the rigid foam densities of 125, 250 and 500 kg/m 3 , separately. The Flexural properties were compared and detailed analyses done, on the influence of different foam densities on otherwise identical sandwich composite beams, prepared from the panels. All the panels were fabricated with the skin weight maintained to about 40% of the foam weight

&amp

Bh N S Kaushik Varma

Ch Surya

Chava Uday

M Ramya

Teja Varma

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International Journal of ChemTech Research
                                                                  CODEN (USA): IJCRGG      ISSN : 0974-4290
                                                            Vol.6, No.6, pp 3314-3317,     Aug-Sep 2014
ICMCT-2014 [10th – 12th March 2014]
International Conference on Materials and Characterization Techniques
The Influence of Rigid Foam Density on the
Flexural Properties of Glass Fabric/Epoxy-Polyurethane
Foam Sandwich Composites
Chava Uday, Ch. Surya Teja Varma, Bh. N.S. Kaushik Varma,
 M Ramya & K. Padmanabhan*
Composites Laboratory, School of Mechanical And Building Sciences, VIT-University,
Vellore, India-632014.
*Corres.author: padmanabhan.k@vit.ac.in.
Abstract: Sandwich composites with glass fabric and epoxy matrix as the skin and rigid polyurethane foam
(PUF)  as  the  core  were  prepared  in  the  shape  of  panels.  Three  panels  were  fabricated  with  the  rigid  foam
densities of 125, 250 and 500 kg/m3, separately. The Flexural properties were compared and detailed analyses
done, on the influence of different foam densities on otherwise identical sandwich composite beams, prepared
from the panels. All the panels were fabricated with the skin weight maintained to about 40% of the foam
weight.
Key words: Rigid Foam Density, Rigid Poly Urethane Foam, Sandwich Composites, Flexural Properties.
Introduction
        The sandwich composites are used in various applications and there are different techniques employed
to fabricate the sandwich composites. In this investigation, the sandwich composite panels were fabricated by
the vacuum bagging technique for rigid foam densities of 125, 250 and 500 kg/m3 separately. The employed
technique is suitable for small batch production of complex shapes and structures that cannot be obtained
through normal temperature and pressure. The flexural test was performed on the sandwich composite beams of
different foam densities for a complete mechanical characterisation in order to evaluate their flexural properties.
The obtained results show that there is an influence of different foam densities on sandwich properties that are
fabricated by vacuum bagging technique. For the same weight ratio between the skin and the core, the obtained
mechanical properties were compared and the results analysed.
Experimental Method
       To  fabricate  the  sandwich  specimens  for  three  different  rigid  foam  densities,  Glass  fabric,  Poly
Urethane Foam, Epoxy Resin and Hardener were used. The glass/epoxy skin and PUF were used as the skin and
core materials. For getting good stiffness the weight of the glass fabric skin should be nearly half of the PUF
weight. The epoxy resin and hardener were mixed and applied to the glass fabric & PUF. The sandwich
specimen had three, five and nine glass fabric skin layers on either side of the core for 125, 250 and 500 kg/m3
K. Padmanabhan et al /Int.J. ChemTech Res.2014,6(6),pp 3314-3317. 3315
panels respectively. The core used in sandwich panels for the three different rigid foam densities was of 50mm
thickness. For a detailed understanding of the poisson’s ratio of foams refer to reference [1]. The vacuum
bagging technique was used to fabricate the sandwich panels. The part was compacted by the atmospheric air
applying pressure on the evacuated part and good bonding was formed between the PUF core and glass
fabric/epoxy skins. In all the panels a volume fraction of 0.3 Vf was maintained for the fabric against the epoxy
resin. Froud, Gibson, Srikant et al and Hemnath and Padmanabhan have earlier investigated the effect of skin:
core weight ratios on the flexural properties of sandwich composites [2, 3, 4, 5].  Here, in all the three panels
the skin: core weight ratio was maintained at ~ 0.4:1. Figure 1 shows the vacuum bag process. Once the vacuum
bagging process was completed the sandwich panels were removed from the bag and cut into specimens for the
bend test in a Universal Testing Machine.
       Bending tests were carried out in order to calculate the flexural rigidity for the sandwich specimens and
shear stresses in the core for comparing the influences of three different rigid foam densities. The bending test
for the three different panels was carried out in a Universal Testing Machine as per the ASTM standard guide
lines [6,7]. Four specimens from each panel were tested in the UTM & the load deflection plots were noted. The
UTM consists  of  two supports  and a  central  load is  applied from the top.  The support  span to thickness ratio
was maintained at   16:1 for each sandwich composite beam specimen as shown in figure 2. A strain rate of 0.01
s-1 was maintained for the testing of the beams.
Fig 1: The vacuum bagging technique                      Fig 2: Specimen under three point bending test
Results and Discussion
The results obtained for the vacuum bagging technique were compared for the mechanical properties.
Figure 3a shows the load-defection plots of the sandwich beam specimens. The parameters like flexural rigidity,
bending stress, shear stress, normal stress, shear deflection and shear strain were calculated and these properties
compared  for  the  sandwich  specimens  with  three  different  densities  of  foams.  The  average  values  of  all  the
parameters were taken and compared with the average values for the other densities. A rigid foam 500 kg/m3
panel takes the highest load. The shear deflection and the shear strains show a decreasing trend for the 250
kg/cu.m  panel but  their values are  slightly higher for the 500 kg/cu.m panel. As we gather from [1] that light
density rigid foams have a higher poisson’s ratio than the medium density foams, the same is reflected here in
the shear deflection and shear strain values for the three densities. The flexural rigidity and the bending strength
per unit width increase with an increase in the rigid foam densities. Table I gives the bending stress, shear stress
and the other flexural properties comparisons for the three sandwich composites investigated here. A
comparison has been made for the shape factor designs also. As more layers of skin are required to maintain the
same weight ratio for denser foams, the shape factors decrease with an increasing foam density.  The flexural
rigidity of a sandwich beam can be calculated from the elastic considerations  at a long span or a fixed span.
Figure 3b shows that the actual fixed span flexural rigidities of the beams are much lower than those  for  long
span considerations, more so  for higher foam densities.
K. Padmanabhan et al /Int.J. ChemTech Res.2014,6(6),pp 3314-3317. 3316
Table-I: Comparison of Mechanical Properties of Three Sandwich Panels
Fig 3a: Load vs Displacement, thickness                         Fig 3b: A Comparison of flexural rigidity
              (t)=2x Width(2W).                                                             considering long span (Red) and 16:1 span (Blue)
Conclusions
       The properties for the three rigid foam density sandwich panels were calculated and obtained from
three-point bending test in a UTM. The bending test results show that with an increase in foam density,
parameters like bending stress, shear stress and normal stress decrease. Parameters like flexural rigidity,
bending strength and centre deflection increase with increase in the foam density of sandwich panels. The  shear
deflection and the shear strain decrease for the 250 kg/cu.m density beam  and increase marginally for the 500
kg/m3 foam, which is attributed to the poissons ratios of the foams.
Acknowledgements
       We are grateful to the Aeronautical Research and Development Board, India, for having given us the
opportunity to conduct studies on sandwich composites through a project funding. The authors thank Dr.Mohan
Ganesh and Dr. AS Shanthi of the Strength of Materials Lab of SMBS, VIT University, for the UTM facilities
extended.
Sandwich Description G/E- PUF 125 kg/cu.m G/E-PUF 250 kg/cu.m G/E- PUF 500
kg/cu.m
Maximum Bending Stress
(N/mm2)
231.30 142.12 112.72
Flexural Rigidity( N-mm2) 6.5807×108 2.10753×109 9.77586×109
Flexural Rigidity(N-
mm2/mm) per unit width
6.32×106 2.07×107 8.89×107
Shear Deflection(mm) 18.23 10.22 12.31
Shear Strength in Core, At
y=0 mm distance(N/mm2)
3.286 6.051 16.783
Shear Strain at maximum load 0.044 0.024 0.028
Maximum Normal Stress
(N/mm2)
231.3 142.1 112.50
Bending Shape Factor for
Stiffness
121.13 33.58 11.16
Bending Shape Factor for
failure
5.25 1.43 0.41
Bending Strength(N-mm) 2.01×105 4.49×105 1.36×106
Bending Strength(N-mm/mm)
per unit width
2.01506×103 4.34935×103
1.242318×104
K. Padmanabhan et al /Int.J. ChemTech Res.2014,6(6),pp 3314-3317. 3317
References
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scientific-research institute of synthetic resins, January-February, 1973, 1, 45.
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3. Gibson LJ,”Optimization Of Stiffness in Sandwich Beams with Rigid Foam Cores", Materials Science
and Engineering, 1984, 67,  125-135.
4. Murthy O, Munirudrappa N, Srikanth L, Rao RMVGK, “Strength and Stiffness Optimization Studies
on Honeycomb Core Sandwich Panels”, Jl. Of Reinforced Plastics and Composites, 2006, vol. 25, No:
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5. Hemnath T and Padmanabhan K, ,”Finite Element Modeling, Analyses and Experimental Evaluation Of
Design Optimization for Flexural Strength in Rigid Foam Core-Fabric/Epoxy Skin Based Sandwich
Composite,  Intl Jl. Of Contemporary Science Engg and Technology, January-December 2011, Vol:2,
No:1 and 2,103.
6. ASTM D-790M-93 Standard Test Methods for Flexural Properties of unreinforced and reinforced
plastics and electrical insulating Materials [Metric], West Conshohocken, PA,USA.
7. ASTM D7250/D7250 M-06, Standard practice for determining sandwich beam flexural and shear
stiffness. West Conshohocken, PA, 19428-2959.
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The Influence of Rigid Foam Density on the Flexural Properties of Glass Fabric/Epoxy-Polyurethane Foam Sandwich Composites

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The Influence of Rigid Foam Density on the Flexural Properties of Glass Fabric/Epoxy-Polyurethane Foam Sandwich Composites

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