 Open Access
 Total Downloads : 181
 Authors : Sathyanarayana V., Sharath N., Dr. Irfan G., Swetadri Srinivasan
 Paper ID : IJERTV5IS030641
 Volume & Issue : Volume 05, Issue 03 (March 2016)
 DOI : http://dx.doi.org/10.17577/IJERTV5IS030641
 Published (First Online): 22032016
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A Theoretical and Experimental Approach for Sandwich Composites
Sathyanarayana V.1* , Sharath N2*, Dr. Irfan G3, Swetadri Srinivasan4*,
1,2,3,4 Dept. of Mechanical Engineering, Akshaya Institute of Technology, Tumkur
Abstract – Sandwich composites are becoming more and more popular in structural design, mainly for their ability to substantially decrease weight while maintaining mechanical performance. This weight reduction results in a number of benefits, including increased range, higher payloads and decreased fuel consumption.
It has long been known that separating two materials with a lightweight material in between increases the structures stiffness and strength. So Macro mechanical analysis of sandwich composites is done, Theoretically, modified classical lamination theory (CLT) and mechanics of material (MOM)approach has been used to determine inplane elastic properties of sandwich composites, Experimentally, sandwich composites were tested to determine inplane elastic properties, Experimental results are in good agreement with the theoretical values obtained modified CLT and MOM approach.
Keywords: Composites, Elastic Properties, Sandwich
INTRODUCTION
The use of composite sandwich structures in aerospace and civil infrastructure applications has been increasing especially due to their extremely low weight that leads to reduction in the total weight and fuel consumptions. High flexural and transverse shear stiffness and corrosion resistance. In addition, these materials are capable of absorbing large amounts of energy under impact loads which results in high structural crash worthiness. In its simplest form a structural sandwich, which is a special form of laminated composites, is composed of two thin stiff face sheets and a thick light weight core bonded between them. A sandwich structure will offer different mechanical properties with the use of different types of materials because the overall performance of sandwich structures depends on the properties of the constituents. Hence, optimum material choice is often obtained according to the design needs. Various combinations of core and face sheet materials are utilized by researchers worldwide in order to achieve improved crash worthiness.
In a sandwich structure generally the bending loads are carried by the force couple formed by face sheets and the shear loads are carried by the light weight core material. The face sheets are strong and stiff both in tension and compression as compared to the low density core material whose primary purpose is to maintain a high moment of inertia. The low density of the core material results in low panel density; therefore under flexural loading sandwich panels have high specific mechanical properties relative to the monologue structures. Therefore, sandwich panels are
highly efficient in carrying bending loads. Under flexural loading, face sheets act together to form a force couple, where one laminate is under compression and the other under tension. On the other hand, the core resists transverse force sand stabilizes the laminates against global buckling and local buckling. Additionally, they provide increased buckling and crippling resistance to shear panels and compression members.
Modeling composite Sandwich Structures
Mechanics can be divided into three major areas
a) Theoretical b) Applied c) Computational
Theoretical mechanics is concerning about fundamental laws and principles of mechanics. Applied mechanics uses this theoretical knowledge in order to construct mathematical models of physical phenomena and to constitute scientific and engineering applications. Lastly, computational mechanics solves specific problems by simulation through numerical methods on computers.
According to he physical scale of the problem, computational mechanics can be divided into several branches:
a) Nano mechanics and micromechanics b) Continuum mechanics c) Systems
Nanomechanics deals with phenomena at the molecular and atomic levels of matter and microcechanics concerns about crystallographic and granular levels of matter and widely used for technological applications in design and fabrication of materials and microdevices. Continuum mechanics is used to homogenize the microstructure in solid and fluid mechanics mainly in order to analyze and design structures. Finally systems are the most general concepts and they deal with mechanical objects that perform a noticeable function.
As it is the issue of this study, the modeling of composite materials is more complex that of traditional engineering materials. The properties of composites, such as strength and stiffness, are dependent on the volume fraction of the fibers and the individual properties of the constituent materials. In addition, the variation of layup configurations of composite laminates allows the designer greater flexibility but complexity in analysis of composite structures. Likewise, the damage and failure in laminated composites are very complicated compared to that of conventional materials. Due to these aspects, modeling of composite laminates is investigated as macromechanical modeling
Macromechanical modeling
Classical lamination theory for thick laminates
In the classical lamination theory, it was assumed that the laminate is thin compared to its lateral dimensions and that straight lines normal to the middle surface remain straight and normal to that surface after deformation. As a result,
xy
yx
A12 A22
A12 A
the transverse shear stress xz , yz and shear strains 11
xz ,yz are zero. These assumptions are not valid in the case of thicker laminate and laminates with low stiffness central plies undergoing significant transverse deformations. In the theory discussed below, referred to as first order shear deformation laminated plate theory, the assumption of normality of straight lines is removed, and that is, straight lines normal to the middle surface remain straight but not normal to that surface after deformation.
Figure 5.4 thick sandwich composite plates and Eglass epoxy laminate
Figure 5.5 shows a section of a laminate normal to the y axis before and after deformation, including the effects of transverse shear. The result of the latter is to rotate the crosssection A by an angle x to a location A, which is not normal to the deformed middle surface.
xy = Longitudinal Poisons ratio of sandwich composite
yx = Transverse Poisons ratio of sandwich composite
G A66
xy h
Gxy = In plane shear modulus of sandwich composite
Table 5.6 Macro mechanical properties of sandwich composites
Properties 
Values 
Youngs modulus (GPa) Ex 
6.246 
Youngs modulus (GPa) Ey 
6.246 
Major Poissons ratio (inplane) xy 
0.1539 
Minor Poissons ratio yx 
0.1539 
In plane shear modulus (GPa) Gxy 
1.6737 
Characterization of sandwich composite laminate using mechanics of material approach
Tensile modulus of sandwich composite can be determined by using strength of materials approach.
Fig 5.6. Sandwich composite in Mechanics of material approach
2Ef Af Ec Ac
Esandwich
Or
2Af Ac
Fig 5.5: The relationship between displacements through the thickness of a plate to midplane displacements and curvatures.
Esandwich EFs VFs ECore Vcore
A A2 Ex 11 22 12
hA22
A A A2 Ey 11 22 12
hA11
Ex = Longitudinal Youngs modulus of sandwich composite
Ey = Transverse Youngs modulus of sandwich composite
Ef = youngs modulus of fiber Em = youngs modulus of matrix Af = Area of fiber
Ac = Area of core
VFs = Volume fraction of face sheet Vcore = Volume fraction of core
Table 5.7 properties of sandwich composites by mechanics of materials
Properties 
Values 
Youngs modulus (GPa) E 
6.228 
Poissons ratio (inplane) 
0.1846 
Properties 
Values 
Youngs modulus (GPa) E 
6.228 
Poissons ratio (inplane) 
0.1846 
approach
EXPERIMENTAL
3 COMPOSITE SANDWICH STRUCTURE

Tensile test
Inplane tensile tests were conducted to determine the tensile strength and modulus characteristics of the
Where the 1 and 2 is the strain measured by the +450 gauge and the strain measured by the 450 gauge. The average shear stress is then determined by dividing the applied load P by the area of the cross section between the notches.
P
composite sandwich panels. Forth is purpose, tensile test specimens were sectioned from larger composites and wich panel and tests were performed using them echanical
avg 2 A
(4.12)
(UTM) test machine at a crosshead speed of 0.5 mm/min. Figure 4.18 shows the geometry and test configuration of tensile test specimen Load versus deformation values was recorded during testing.The tensile strength and modulus
The apparent shear modulus is then calculated by dividing the average shear stress by the average shear strain:
avg
values were obtained by equations 3. 3 and 3.5 similarly with the face skin material.
Gavg
avg
(4.13)
(a)
(b)
Figure 4.18 Sandwich test specimen (a) Geometry of the specimen (b) Actual specimen
Shear block as shown in figure 4.17(a)
Was used for the testing of sandwich composite specimens
(a)

Shear test
The shear tests were conducted to determine the in plane and out of plane loading effects on shear properties of sandwich composite specimens. The details of the specimen for shear test are shown in figure 4.18. The specimen was loaded in a universal testing machine by shear test xture at a constant head speed of 1 mm/min. Three for each specimen type were provided with resistance strain gauges oriented at Â±450 to the loading axis and bonded in the middle of the specimen to determine its shear response during the entire loading regime. The average shear strain is then determined from the strain gauges using the relation
(b)
Figure 4.18 Sandwich shear test specimen (a) Geometry of the specimen
(b) Actual specimen
RESULTS AND DISCUSSION
Tensile test of Sandwich composites
The Youngs modulus and poisons ratio are obtained from the slope of the initial portion of stressstrain plots and lateral and linear strain plots as shown in figure 6.5 and
6.6. The predicted and experimental values of elastic properties are shown in Table 6.4 and 6.5. The elastic moduli predicted by modified CLT and mechanics of
avg 45 45
(4.11)
material showed good agreement.
50
Stress, MPa
Stress, MPa
40
30
20
10
0
0 0.005 0.01
Strain
SF SFK20 SFK15 SFK10 SFK5
Table 6.6 comparing the theoretical results with experimental results of sandwich composites
Sandwich composite (SF) 
MCLT Approach 
Strength of material approach 
Experimental 
Longitudinal Youngs modulus 
6.246 Gpa 
6.228 Gpa 
5.747 Gpa 
Transverse Youngs modulus 
6.025 Gpa 
6.228 Gpa 
5.747 Gpa 
Poissons ratio 
0.1539 
0.1846 
0.1588 
Sandwich composite (SF) 
MCLT Approach 
Strength of material approach 
Experimental 
Longitudinal Youngs modulus 
6.246 Gpa 
6.228 Gpa 
5.747 Gpa 
Transverse Youngs modulus 
6.025 Gpa 
6.228 Gpa 
5.747 Gpa 
Poissons ratio 
0.1539 
0.1846 
0.1588 
Figure 6.5 Stress versus strain plot of sandwich composite
600
Lateral strain
Lateral strain
500
400
300
200
100
0
0 1000 2000 3000 4000
Linear strain Ã—106
Figure 6.6 Linear versus Lateral strain plot
6.3.1 Failed specimen of Sandwich composite
Specimen code 
Dimension (mm) 
Max Load (kN) 
Max stress (Mpa) 
Max. strain 
Youngs modulus E GPa 

L 
W 
t 

SF 
130 
25.4 
15.5 
12.316 
31.28 
6355 
5.747 
SFK 20 
130 
25.4 
15.5 
12.605 
32.018 
5934 
6.065 
SFK 15 
130 
25.4 
15.5 
14.715 
37.37 
6048 
6.393 
SFK 10 
130 
25.4 
15.5 
15.009 
38.12 
6684 
6.798 
SFK 5 
130 
25.4 
15.5 
15.450 
39.24 
6275 
7.180 
Specimen code 
Dimension (mm) 
Max Load (kN) 
Max stress (Mpa) 
Max. strain 
Youngs modulus E GPa 

L 
W 
t 

SF 
130 
25.4 
15.5 
12.316 
31.28 
6355 
5.747 
SFK 20 
130 
25.4 
15.5 
12.605 
32.018 
5934 
6.065 
SFK 15 
130 
25.4 
15.5 
14.715 
37.37 
6048 
6.393 
SFK 10 
130 
25.4 
15.5 
15.009 
38.12 
6684 
6.798 
SFK 5 
130 
25.4 
15.5 
15.450 
39.24 
6275 
7.180 
Figure 6.7 Failed specimens of Sandwich composite Table 6.5 Experimental Results of Sandwich composites
From the figure 6.5 we can see that as Honeycomb cell size decreases the load taken by the sandwich composite is high. Therefore honeycomb structure and the cell size of that plays an important role in the tensile strength of sandwich composite. Failure occurs in the tensile test causes the cursing f Kraft honeycomb core resulting in delamination at the edges of the sandwich. The maximum average load which can be withstand by sandwich composite is SFK 5 15.450 KN.
In plane shear test on sandwich composite
The shear modulus is obtained from the slope of the initial portion of stressstrain plots shown in figure 6.10. The predicted and experimental values of elastic properties are shown in Table 6.7 and 6.8. The elastic moduli are predicted by modified CLT and mechanics of material showed good agreement.
25
Stress, Mpa
Stress, Mpa
20
15 SF IP
10 SFK20 IP
SFK15 IP
5 SFK10 IP
0 SFK5 IP
0 20000 40000
Strain
Figure 6.10 Stress versus strain plot of in plane shear test sandwich composite
Figure 6.11 Failed specimen of in plane shear test
Table 6.8 Experimental Results of In plane shear test on Sandwich composites
Specimen code 
Dimension (mm) 
Max Load (kN) 
Max stress (Mpa) 
Max. strain in Âµ 
Shear modulus E GPa 

L 
W 
t 

SF 
100 
40 
15.5 
25.930 
20.911 
34087 
1.547 
SFK 20 
100 
40 
15.5 
26.064 
21.019 
33000 
1.585 
SFK 15 
100 
40 
15.5 
26.795 
21.608 
32340 
1.663 
SFK 10 
100 
40 
15.5 
27.156 
21.900 
32200 
1.815 
SFK 5 
100 
40 
15.5 
27.376 
22.077 
30800 
1.921 
Table 6.9 comparing the theoretical results with experimental results
Sandwich composite (SF) 
Rule of mixture 
MCLT Approach 
Experimental 
Shear modulus 
1.649 Gpa 
1.6737 Gpa 
1.547 Gpa 
The maximum applied load under in plane shear test of composite sandwich is 27.376KN for SFK 5 shows the honeycomb structure and the cell size of that plays an important role in the shear strength of sandwich composite. Table 6.8 summarizes the predicted and the measured Shear modulus of the composite sandwich under In plane shear test. The predicted and the actual Shear modulus of the Sandwich using the inplane shear equation are nearly equal. The presence of the fibre composite skins adds to the overall strength of the specimen by preventing the widening of the crack in the core material and delayed the shear failure until all the fibres crossing the cracked core failed.
CONCLUSION

Modified classical lamination theory was developed for thicker laminate and laminates with low stiffness central plies undergoing significant transverse deformations.

Using Modified classical lamination theory, elastic properties of sandwich composites are calculated.

The experimental investigations are carried by conducting different test for the Sandwich composite, to find the elastic properties of the material.

For Sandwich composite, Experimental results are in good agreement with the theoretical values obtained Modified classical lamination theory and Mechanics of material approach.

Therefore, separating two materials with a lightweight material in between increases the structures stiffness and strength.
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