Fatigue Behaviour of Polyurethane Core Sandwiched Glass
Reinforced Epoxy Composite Material
Sandeep1 , Dr. D Choudhary2 ,
¹ Mechanical Engineering Department, GNDEC, Bidar, Karnataka,India
² Mechanical Engineering Department, GNDEC, Bidar, Karnataka,India
Abstract— In this paper flexural fatigue behavior of Polyurethane core sandwiched in glass/ epoxy composite
laminate structures is studied. The effect of face sheet thickness and core thickness on the flexural fatigue is studied
by testing the sandwich structures in computer controlled universal fatigue testing machine. The effect of core
thickness and facesheet thickness on flexural strength of sandwich structures is also studied. From the experimental
results of fatigue testing, it is observed that, Polyurethane core sandwiched in glass/ epoxy composite laminate
structures have more than 225000 cycles of fatigue life at 80 % of flexural loading. The result of ANOVA on flexural
strength reveals that, face sheet thickness is most significant parameter.
Keywords— E-glass fiber, epoxy resin, Sandwich structures, ANOVA, Fatigue life
I. INTRO DUCTIO N
Polymer matrix composite materials are finding its applications in the areas like aerospace for
making wings of the plane, blades of the helicopter and in naval applications as boat hulls as a
structural material. In civil engineering as a structural material, in automobiles they are used for
making shafts, chassis for F1 cars, roofs, side panels. We must use, Because of similar mechanical
and thermal properties as of metals with greater strength to weight ratio and high stiffness. One of the
composite structures is of sandwich type. A sandwich panel is an uncommon form of a laminated
shell structure including of three dissimilar layers which are joined together utilizing resin bonds to
form a firm stiff load carrying assembly or simply the basic sandwiched structure consists of two
facesheets between which a core material is sandwiched.
Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and
localized structural damage that occurs when a material is subjected to cyclic loading. If the loads are
above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the
surface, persistent slip bands, and grain interfaces. Inevitably a crack will reach an incredulous size,
the crack will propagate suddenly, and the structure will fracture. The shape of that structure will
basically influence the fatigue life.
Nitin kulkarni et al. [1] investigated a fatigue crack growth of glass/epoxy PVC foam core sandwich
beams under flexural load condition. Authors found that, the fatigue life is grater for low density core
materials. Mohammad mynul Hossain et al. [2] studied compression-compression fatigue on Ecocore
synthetic material under a sinusoidal cyclic load. Flexural fatigue tests were conducted on crosslinked PVC foams. The density is considered as an major parameter and it is varied from 75 to 300
kg/m3 and he considered frequency constant of 3Hz and at a stress ratio, R=0.1 by Krishnan kanny et
al. [3] Authors found that, the fatigue life decreases with increasing stress level. Mohammad mynul
Hossain et.al.[4] conducted flexural fatigue tests under an sinusoidal cyclic load at a frequency 2Hz
and stress ratio R=0.1on Eco-core syntactic foam. R. Vijayalakshmi Rao et.al. [5] Have made an
attempt to study the flexural and fatigue behaviour of E- glass/Vinyl ester/Polyurethane foam
sandwich composites. The results of the experiment revealed that, the cyclic load and test frequency
are significant factors in fatigue strength. Nitin Kulkarni et.al. [6] Investigated the flexural fatigue
response of foam core sandwich beams. The failure phenomenon during fatigue was united
encompassing the homogeneous low-density core material. A.M. Harte et al. [7] Measured the
tension-tension and compression-compression cyclic properties for an open cell “Duocel'' foam of
composition Al 6101-T6, and a closed cell ``Alporas'' foam of composition Al- 5Ca-3Ti (wt%).
Authors concluded that, the magnitude of fatigue ration is less depend on mean stress and is not
depend on relative densities of the foam. The flexural test is conducted on sandwich beam and the
NCADOMS-2016
Special Issue 1
Page 336
failure modes of face fatigue, core shear and core indentation are plotted in S-N curves by A.M.Harte
et al.[8] The numerical study of Fatigue crack growth of foam core sandwich beams loaded in
flexure is done by E.E.Theotokoglou et al.[9] . Pizhong Qiao et al.[11] studied the tension-tension
fatigue behaviour of recently created pultruted e-glass/polyurethane composites. The test result shown
that those examined composite is a fatigue delicate material compared to different polymer
composites. Hence, in this paper the effect of facesheet thickness and core thickness on the fatigue life
of the polyurethane core sandwiched glass reinforced in epoxy matrix is studied using Taguchi’s
orthogonal array technique.
II. MATERIAL SELECTION
The materials and their properties selected for making the face sheet and core material are shown in
table 1.
T ABLE 1.
MAT ERIAL AND T HEIR PROPERT IES
Reinforcement
Matrix material
Hardner
Core material
E-glass of 300
GSM bidirectional
woven roving
Epoxy
resin(LY556)
[Araldite]
Hardner
HY951 [Aradur]
Polyurethane core
1.M ethyl
di-isosynate(M DI)
2.polyether
polyol(PEP) At 60:40
Density of fiber
ῥf=2.54 g/cc
Density of Epoxy
resin at 25°C
ῥm =1.15-1.20g/cc
At 20°C 1:1 in
water
Boiling
point>200°C
Density=1g/cc
Density 80kg/m³.
III. DEVELOPM ENT OF EXPERIM ENTA L PLAN
In this project the parameters selected for the study are face sheet thickness and the core thickness
at constant density of core at 80 kg/m³. The list of parameters and their levels are shown in tab le
number 2.
T ABLE 2
PARAMET ERS AND T HEIR LEVELS CONSIDERED FOR T HE ST UDY
Levels
Parameters
Face
sheet
thickness, mm
Core
thickness,
mm
1
2
3
1
2
3
10
20
30
Using design of experiment (DOE) approach, the minimum number of experiments to be conducted
are 5. For this the nearest orthogonal array is L9. The experimental plan according to L9 array is
shown in table number 3.
T ABLE 3
EXPERIMENT AL PLAN
Experiment
no.
1
NCADOMS-2016
Facesheet
thickness,
mm
1
Special Issue 1
Core
thickness,
mm
10
Page 337
2
1
20
3
4
1
2
30
10
5
6
2
2
20
30
7
8
9
3
3
3
10
20
30
IV. FATIGUE TESTING OF SANDWICH STRUCTURE
Fatigue test is done as per ASTM standard C393 standard test method for flexural properties
of polymer matrix sandwich structure. The fatigue test is conducted at constant frequency of 5Hz and
at 80% of peak load. The fatigue testing is done on10 KN capacity fatigue machine in IISE Bangalore.
Span length is varied for different specimens. The setup is shown in fig 1. The specimen size and
specifications are similar to flexural specimen as shown in table 4.
Fig 1 Fatigue test setup for sandwich material
T ABLE 4
T EST SPECIMEN GEOMET RY
Sl.no
Core
Thicknes
,mm
10
Span
Length,
mm
110
Overall
Length,
mm
130
Thickness
mm
1
Facesheet
Thickness,
mm
1
2
3
4
5
6
7
8
9
1
1
2
2
2
3
3
3
20
30
10
20
30
10
20
30
210
310
120
220
320
130
230
330
250
370
140
260
380
150
270
390
22
32
14
24
34
16
26
36
12
V. RESULTS AND DISCUSSIONS
A. Fatigue behaviour of sandwich structure
The fatigue behaviour of sandwich structures of different Facesheet thickness and core thickness is
studied at 80% flexural loading condition. The flexural strength verses number of cycles is plotted for
all the nine specimens and is shown in figure number 2 - 10. From the graphs, the fatigue life of the
sandwich structures is more than 225000 cycles. The Flexural strength and endurance limit for all the
NCADOMS-2016
Special Issue 1
Page 338
specimens is shown in table 5. From the table, the maximum endurance limit is 34.7 for 3mm
Facesheet and 10 mm core sandwich structure.
Fig 2 Fatigue life of sandwich structure of 1mm face sheet and 10 mm core thickness
Fig 3 Fatigue life of sandwich structure of 1mm face sheet and 20 mm core thickness
Fig 4 Fatigue life of sandwich structure of 1mm face sheet and 30 mm core thickness
Fig 5 Fatigue life of sandwich structure of 2 mm face sheet and 10 mm core thickness
NCADOMS-2016
Special Issue 1
Page 339
Fig 6 Fatigue life of sandwich structure of 2mm face sheet and 20 mm core thickness
Fig 7 Fatigue life of sandwich structure of 2mm face sheet and 30 mm core thickness
Fig 8 Fatigue life of sandwich structure of 3mm face sheet and 10 mm core thickness
Fig 9 Fatigue life of sandwich structure of 3 mm face sheet and 20 mm core thickness
NCADOMS-2016
Special Issue 1
Page 340
Fig 10 Fatigue life of sandwich structure of 3mm face sheet and 30 mm core thickness
The calculated values of flexural strength and endurance limit is given in table 5
T ABLE 5
FLEXURAL ST RENGT H AND ENDURANCE LIMIT OF SANDWICH ST RUCT URE
Specimen
number
Peak load
in N
Flexural
strength
In N/mm²
Endurance
limit
900
20.625
16.11
525
6.833
3.4
400
3.63
1.5
1625
29.84
22.1
675
7.734
4.5
725
3010
6.020
45.855
3.3
34.7
725
9.696
6.1
825
6.302
3.4
1
2
3
4
5
6
7
8
9
B. Anova for endurance limit
The analysis of variance is performed for the endurance limit of all the sandwich structures to find the
most significant material parameter which corresponds to the fatigue life. The results of ANOVA are
shown in table 6 and response plots in figure 11. From the results of ANOVA it is found that the most
significant parameter for endurance limit is core thickness contributing 89.68 % followed by
Facesheet thickness of 9.23 %. The optimum parameter combination for higher endurance limit is
Facesheet thickness at third level (3mm) and core thickness at level 1 (10 mm).The table no.6 shows
the results of ANOVA for endurance limit.
T ABLE 6
ANOVA T ABLE FOR ENDURANCE LIMIT
Sl.no
Parameters
DOF
SS Factor
M S Factor
F
% Influence
1
Face Sheet Thickness
2
60.724198
30.362099
12.819122
9.235428625
2
589.68389
294.84194
124.48464
89.68390902
2
Core Thickness
NCADOMS-2016
Special Issue 1
Page 341
4
5
Error
Total
3
7.1055018
9
657.51359
2.3685006
1
0.010806624
138.30376
Fig 11.Response plots
VI. CONCLUSION
From the experimental results of fatigue life of PU core sandwiched in glass/epoxy laminates,
the following conclusions are drawn.
 The fatigue life of PU core sandwiched in glass/epoxy laminates, is found to be infinite
and is more than 225000 cycles at 80 % of flexural loading.
 The endurance limit is maximum for greater thickness values of face sheet of sandwich
structure.
 The most significant factor for fatigue life is face sheet thickness.
REFERENCES
[1] Nitin Kulkarni, Hassan Mahfuz, Shaik Jeelani, Leif A. Carlsson, “Fatigue crack growth and life prediction
of foam core sandwich composites under flexural loading”, Elsevier, composite structures 59, 2003.
[2] Mohammad Mynul Hossain, Kunigal Shivakumar, “Compression fatigue performance of a fire resistant
syntactic foam”, Elsevier, composite structures 94, 2011.
[3] Krishnan Kanny, Hassan Mahfuz, Leif A. Carlsson, Tonnia Thomas, Shaik Jeelani, “Dynamic mechanical
analyses and flexural fatigue of PVC foams”, Elsevier, composite structures 58, 2002.
[4] Mohammad Mynul Hossain, Kunigal Shivakumar, “Flexural fatigue failures and lives of Eco-core sandwich
beams”, Elsevier, Materials and Design 55, 2014.
[5] R.Vijayalakshmi Rao, Manujesh B J, “Behavior of sandwich composites under flexural and fatigue loading:
Effect of variation of core density”, IJEST, ISSN: 0975-5462, Vol.3 No.10 October 2011.
[6] Nitin Kulkarni, Hassan Mahfuz, Shaik Jeelani, Leif A. Carlsson, “Fatigue failure mechanism and crack
growth in foam core sandwich composites under flexural loading”, SAGE, Journal of reinforced plastics and
composites, Vol.23,No.1/2004.
[7] A. M. Harte, N. A. Fleck, M. F. Ashby, “Fatigue failure of an open cell and a closed cell Aluminium alloy
foam”, Pergamon, Acta mater. Vol. 47, No.8, pp. 2511-2524, 1999.
[8] A. M. Harte, N. A. Fleck, M. F. Ashby, “THE fatigue strength of sandwich beams with an aluminium alloy
foam core”, Elsevier, International journal of fatigue 23, 2001. (499-507)
[9] E. E.Theotokoglou, D. Hortis, L. A. Carlsson, H. Mahfuz, “Numerical study of fractured sandwich
composites under flexural loading”, SAGE, Journal of sandwich structures and materials, Vol. 10 Jan (2008) 75.
[10] Dan Zenkert, Magnus Burman, “Tension, compression and shear fatigue of a closed cell polymer foam”,
Elsevier, Composites science and technology, 2008.
[11] Pizhong Qiao and Mijia Yang, “Fatigue life prediction of pultruted Eglass/ Polyurethane composites”,
SAGE Publications, Vol. 40 (2006).
NCADOMS-2016
Special Issue 1
Page 342