flexural analysis of balsa core sandwich composite- failure mecha
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Wayne State UniversityDigitalCommons@WayneState
Wayne State University Theses
1-1-2012
Flexural analysis of balsa core sandwich composite:failure mechanisms, core grain orientation andpadding effectAvinash S. PhadatareWayne State University,
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Recommended CitationPhadatare, Avinash S., "Flexural analysis of balsa core sandwich composite: failure mechanisms, core grain orientation and paddingeffect" (2012). Wayne State University Theses. Paper 164.
FLEXURAL ANALYSIS OF BALSA CORE SANDWICH COMPOSITE: FAILURE MECHANISMS, CORE GRAIN
ORIENTATION AND PADDING EFFECT
by
AVINASH S. PHADATARE
THESIS
Submitted to the Graduate School
of Wayne State University,
Detroit, Michigan
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
2012
MAJOR: MECHANICAL ENGINEERING Approved by: _________________________________ Advisor Date
© COPYRIGHT BY
AVINASH S PHADATARE
2012
All Rights Reserved
ii
ACKNOWLEDGMENTS
I would like to thank the professor Dr. Golam Newaz for the opportunity to
work on Composite materials. I am very grateful for his continuous support,
inspiration and advice in these two years during my Master’s program. I would
also like to thank Dr. Mohammad Hailat for his guidance and help to perform all
the experimental work.
This research work was funded through DOE’s Lightweight Automotive
Materials Program (LAMP) administered by NCMS (Mr. Steven Hale – Program
Manager). I acknowledge Jim Dallam and Daniel Allman of MAG-ias for the
composite skin prepreg’s that were provided. Also, I thank Robert Graybill of
Nimbis Services for providing access to OSC HPC portal for the FE analysis.
Lastly, I want to thank to all those who supported me in any respect during
the completion of the project.
AVINASH S. PHADATARE
iii
TABLE OF CONTENTS
Acknowledgments ...…...……………………………………………………… ii
Nomenclature …...……………………………………………………………… vi
List of Tables ….……………………………………………………………….. vii
List of Figures …………………………………………………………………. viii
CHAPTER 1: INTRODUCTION ……………………………………………….. 1
1.1 SANDWICH COMPOSITE ………………………………………… 1
1.1.1 SKIN ……………………………………………………… 2
1.1.2 CORE …………………………………………………….. 3
1.2 MANUFACTURING ………………………………………………… 6
1.3 FLEXURE TESTS ………………………………………………….. 8
1.4 FINITE ELEMENT ANALYSIS …………………………………….. 9
1.5 POTENTIAL APPLICATION ……………………………………….. 10
1.6 RATIONALE …………………………………………………………. 11
1.7 RESEARCH OBJECTIVE ………………………………………….. 12
CHAPTER 2: LITERATURE REVIEW ………………………………………... 13
2.1 INTRODUCTION ……………………………………………………. 13
2.2 SANDWICH BEHAVIOR …………………………………………… 14
2.3 FINITE ELEMENT ANALYSIS …………………………………….. 16
iv
CHAPTER 3: MATERIAL AND SANDWICH CONSTRUCTION …………. 17
3.1 MATERIAL DESCRIPTION ……………………………………….. 17
3.2 PRELIMINARY TESTS …………………………………………….. 19
3.2.1 TENSILE TESTS …………………………………………. 19
3.2.2 COMPRESSION TESTS ………………………………… 24
3.2.3 SHEAR TESTS …………………………………………… 28
3.3 CURING SANDWICH COMPOSITE .…………………………….. 35
CHAPTER 4: EXPERIMENTAL WORK ..……………………………………. 37
4.1 THREE-POINT BENDING TEST SETUP ………………………... 37
4.2 FAILURE MODES OF SANDWICH COMPOSITE
UNDER FLEXURE ……………............................................... 40
4.2.1 INDENTATION OF SKIN ………………………………… 41
4.2.2 SKIN FAILURE ……………………………………………. 41
4.2.3 PROGRESSIVE CORE COLLAPSE ….………………... 42
4.2.4 CORE SHEAR FAILURE ………………..………………. 42
4.2.5 DELAMINATION OF SKIN-CORE INTERFACE …….... 43
4.3 SPECIMEN DETAILS ………………………………………………. 44
4.4 TEST DATA: CORE GRAIN ORIENTATION ……………………. 46
4.5 TEST DATA: PADDING COMPARISON ..……………………….. 48
v
CHAPTER 5: FINITE ELEMENT ANALYSIS ………………………………... 50
5.1 INTRODUCTION …………………………………………………… 50
5.2 SANDWICH COMPOSITE FE MODEL ...………………………… 50
5.3 LOAD-DISPLACEMENT DATA …………………………………… 54
CHAPTER 6: RESULTS AND DISCUSSION ……………………………….. 58
6.1 COMPARISON OF DIFFERENT CORE GRAIN ORIENTATION
IN SANDWICH COMPOSITE …………………………………. 58
6.2 COMPARISON OF PADDING ON SANDWICH COMPOSITE ... 64
CHAPTER 7: CONCLUSIONS ……………………………………………….. 70
FUTURE SCOPE OF WORK ………………………………………………….. 72
Appendix: Statistical Data ……………………………………………………. 73
References …...………………………………………………………………….. 82
Abstract …………………………………………………………………………… 88
Autobiographical Statement …………..………………………………………... 90
vi
NOMENCLATURE
ρ Density
E Elastic modulus of material
G Shear modulus of material
X, Y Failure strength in X, Y-axis
S Failure shear strength
σ, SIG Normal stress
τ Shear stress
f Yielding in material model
∆ Flexural displacement
Subscript
X, Y, Z Global co-ordinate system
1, 2, 3 Local co-ordinate system
T Tensile
C Compressive
║ Parallel to the grain
┴ Perpendicular to the grain
vii
LIST OF TABLES
Table 1.1 Typical skin materials ………………………………………………. 3
Table 1.2 Typical core materials ………………………………………………. 5
Table 3.1 Material properties of 0°/90° E-glass/epoxy laminate …………… 34
Table 3.2 Material properties of balsawood …………………………………. 34
Table 4.1 Sandwich composite beams – core grain orientations …………. 44
Table 4.2 Sandwich composite beams – padding comparison ……………. 45
Table 6.1 Comparison balsa grain-orientation in sandwich composite …… 59
viii
LIST OF FIGURES
Figure 1 Sandwich composite …………………………………………………. 1
Figure 2 Sandwich panel and I-Beam ………………………………………... 2
Figure 3 Core materials (a) Honeycomb; (b) Balsa wood; and (c) Cellular foam ……………………………………………………… 4
Figure 4 (a) Resin Transfer Molding; (b) Continuous lamination ………….. 6
Figure 5 (a) Schematic diagram; (b) Autoclave vacuum press ……………. 7
Figure 6 Three-point bending test …………………………………………….. 8
Figure 7 (a) E-glass/Epoxy pre-preg; (b) Fully cured laminate …………….. 17
Figure 8 (a) Balsa wood; (b) Schematic representation of grain direction … 18
Figure 9 MTS Tensile testing machine with 100kN capacity ……………….. 20
Figure 10 (a) Tensile test coupon; (b) coupon secured in tensile machine; (c) damaged test coupon …………………………………………. 21
Figure 11 Tensile test data of typical 0°/90° E-glass/epoxy laminate ……... 22
Figure 12 Tensile test data of Balsa wood in fiber direction ………………... 23
Figure 13 Tensile test data of Balsa wood in transverse direction ………… 23
Figure 14 (a) Compression test coupon; (b) Compression fixture; (c) Damaged coupon ……………………………………………… 25
Figure 15 Compression test data of typical 0°/90° E-glass/epoxy laminate. 26
ix
Figure 16 Compression test on balsa wood (a) Undamaged End-grain balsa; (b) Compression test; (c) Damage in End-grain balsa along thickness as seen in SEM ………………………….. 27
Figure 17 Compression test data of Balsa wood in fiber direction ………… 27
Figure 18 Compression test data of Balsa wood along transverse to fiber . 28
Figure 19 Shear test fixture ……………………………………………………. 29
Figure 20 V-notched beam test coupon dimensions ……………………….. 29
Figure 21 Shear test data of 0°/90° E-glass/epoxy laminate across fiber … 30
Figure 22 Short-beam shear test (a) Setup; (b) after damage …………….. 30
Figure 23 Short-beam shear test data ………………………………………... 31
Figure 24 Shear test of Balsa wood (a) Setup; (b) damaged specimen ….. 32
Figure 25 Shear test data of Balsa across fiber ……………………………… 32
Figure 26 Shear test data of Balsa along fiber ………………………………. 33
Figure 27 (a) Autoclave vacuum press machine; (b) Post-curing oven; (c) Fully cured sandwich composite ……………………………… 36
Figure 28 Three-point bending test setup …………………………………… 38
Figure 29 MTS testing machine with three-point bending fixture …………. 38
Figure 30 Typical load vs. displacement curve from three-point bend test . 40
Figure 31 Indentation in regular balsa core sandwich composite …………. 41
Figure 32 Skin failure in regular balsa core sandwich composite …………. 41
Figure 33 Core shear failure in regular balsa core sandwich composite …. 42
x
Figure 34 Interface delamination in end-grain balsa core sandwich composite …………………………………………………………… 43
Figure 35 Sandwich composite beam ………………………………………… 44
Figure 36 Test data of sandwich with balsa core grain in X-axis ………….. 47
Figure 37 Test data of sandwich with balsa core grain in Y-axis ………….. 47
Figure 38 Test data of sandwich with balsa core grain in Z-axis ………….. 48
Figure 39 Test data of sandwich with Teflon pad (Balsa grain in Y-axis) … 49
Figure 40 Test data of sandwich with Rubber pad (Balsa grain in Y-axis) .. 49
Figure 41 Finite element analysis model …………………………………….. 51
Figure 42 Failure model used in Mat 59 of LS Dyna ……………………….. 51
Figure 43 Experimental and FEA data of sandwich composite beam with balsa core grain in X-axis …………………………………… 54
Figure 44 Experimental and FEA data of sandwich composite beam with balsa core grain in Y-axis ……………………………………. 55
Figure 45 Experimental and FEA data of sandwich composite beam with balsa core grain in Z-axis ……………………………………. 56
Figure 46 Experimental and FEA data of sandwich composite beam with balsa core grain in Y-axis and with Teflon padding ………. 57
Figure 47 Experimental and FEA data of sandwich composite beam with balsa core grain in Y-axis and with Rubber padding ……... 57
Figure 48 Test data comparison and failure mode of sandwich beam with different balsa core grain orientations ……………………… 58
xi
Figure 49 Progressive damage under the loading pin observed experimentally and FEA for sandwich beam with different balsa core grain orientations ……………………………………… 60
Figure 50 Stress concentration of skin around loading pin just before skin failure/peak load ……………………………………………… 63
Figure 51 Comparison of load-displacement plots of sandwich composite beam without and with padding (balsa grain oriented in Y-axis). 64
Figure 52 Failure modes in sandwich composite without padding, with Teflon pad and with rubber pad …………………………………… 65
Figure 53 Stress contour at peak load on top skin of sandwich composite without and with rubber padding …………………………………. 67
Figure 54 ZX-shear stress contours at peak load in the core of sandwich beam without and with rubber padding ………………………….. 68
1
CHAPTER 1: INTRODUCTION
1.1. SANDWICH COMPOSITE
Sandwich construction consists of two thin stiff laminates bonded with a
low density material in between. ASTM defines a sandwich structure as follows
“A structural sandwich is a special form of a laminated composite comprising 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”.
Figure 1: Sandwich composite
In a sandwich composite, skin will be adhesively bonded to the core for
transferring the load between the components, thereby one skin acts in
compression as the other skin acts under tension and the core resists the shear
loads. This provides high stiffness, bending rigidity, strength-to-weight ratio and
energy absorbing capability to the structure. The bond must be strong enough to
resist shear and tensile stresses in the sandwich panel.
The sandwich panel can be compared to I-beam, as the skin corresponds
to flange of I-section beam and the core corresponds to the web, as represented
in figure 2.
2
Figure 2: Sandwich Panel and I-Beam. (Source [36])
Also, the thermal and acoustic insulation can be incorporated functions of
sandwich panel. For application of sandwich composite panel’s factors such as
flexural rigidity, bending strength, skin-core thickness ratio, span length, core
density plays a significant role. The increase in core thickness provides higher
flexural rigidity and bending strength to the sandwich composite.
The material configurations for a sandwich system are unlimited with wide
range of skin and core materials. For structural purposes, materials are selected
considering factors such as strength, stiffness, adhesive performance,
environmental behavior and economic availability.
1.1.1. SKIN
Skin is the major component that provides flexural stiffness and impact
resistance to the sandwich system. High performance materials are considered
for the skin of sandwich panels. The skin materials are mainly categorized into
metallic and non-metallic materials and some of them are listed in table 1.1.
3
Table 1.1: Typical skin materials. (Source [1])
Material ρ (kg/m3) E (GPa) u (MPa)
Metals:
Stainless steel
Aluminum Alloy
Titanium alloy
7900
2700
4500
196
73
108
200
300
980
FRP:
Carbon/Epoxy (Unidirectional)
E-glass/Epoxy (Bi-directional)
Kevlar/Polyester (Bi-directional)
1600
1800
1300
180/10
20
17.5
1500/40
550
375
Most of composite materials offer low density along with higher strength
properties than metals, but the stiffness is generally lower. For this reason, fiber-
composite laminates are preferred over the metals for sandwich construction.
Fiber reinforced plastics have several advantages over other materials, as fibers
provide stiffness and strength and the resin offer rigidity to the laminate. The
anisotropic behavior of composites can be used to tailor the properties as per the
loading conditions. This assists in stressing the component to its ultimate limit,
thereby utilizing the material in a more efficient way.
1.1.2. CORE
As the core in sandwich panel is mainly subjected to shear, the primary
interests in the material are low density, higher shear modulus, strength and
4
stiffness perpendicular to faces. Balsa wood core (figure 3(b)) was the first
material used for sandwich composite panels and is still used in Marine Industry.
Under a microscope, balsa wood shows a high-aspect ratio closed cell structure.
The properties are high in the direction of growth which can be used depending
on loading conditions. End-grain balsa is the most used balsa wood as a core.
Concept of Honeycomb was developed, that provide good shear strength,
stiffness-to-weight ratio and were primarily used in aerospace industry.
Honeycomb (figure 3(a)) can be manufactured in variety of cell shapes and sizes,
but required proper bonding to the face due to lesser bonding area. Aluminum,
Nomex and Kraft paper are some of the materials used for manufacturing
Honeycomb. The high cost of honeycomb material, made the advent of cellular
foams such as polyvinyl chloride (PVC) and polyurethane (PU) foam materials
(figure 3(c)). Cellular foams are easy to shape and offer high thermal insulation
and acoustic damping. Metallic foam cores are also developed to provide higher
stiffness to the sandwich structure. Table 1.2 gives some of the common core
materials used in the composite industry.
Figure 3: Core materials (a) Honeycomb; (b) Balsa wood; and (c) Cellular foam.
5
Table 1.2: Typical core materials. (Source [1])
Material (Density, kg/m3) Gc (MPa) u (MPa)
Balsa wood (96) 72.85/12.5 10.1/0.81
Honeycomb:
Aluminum alloy (92)
Nomex (80)
Paper (56)
620/260
69/44
141/38
3.1/2.0
2.2/1.0
1.3/0.48
Cellular Foam:
Polyurethane (40)
Polystyrene (60)
Polyvinyl chloride (80)
4
20
31
0.25
0.6
1.0
6
1.2. MANUFACTURING
Sandwich composite panels can be manufactured from several
manufacturing techniques such as adhesive bonding, Liquid molding, continuous
lamination, vacuum bag and autoclave molding. These manufacturing techniques
are discussed below.
Adhesive bonding is the simplest manufacturing process for Sandwich
composite. The adhesive layers are interleaved between the skin and core and
the whole system is subjected to high temperature and pressure depending on
requirements of adhesive material. To acquire better adhesion, bonding surface
should be rugged or abraded.
Resin Transfer Molding or Vacuum-Injection moldings are some of the
techniques followed in Liquid molding process, which are more economical. In
this process, fabric and core are arranged in the mold and resin is infiltrated into
the mold with the vacuum assistance. For bulk manufacturing, Continuous
lamination process is preferred. This consists of double-belt press, where it is
possible to heat and cool the material while applying pressure at the same time.
Figure 4: (a) Resin Transfer Molding; (b) Continuous Lamination.
7
In this research work, Autoclave type vacuum press molding is used for
constructing sandwich panels. The prepreg’s are layered directly onto both sides
of the core in the mold and is placed in a vacuum chamber with heat and
pressure applied. The temperature and pressure are controlled for significant
amount of time for resin cross-linking and temperature is gradually reduced after
curing. Fabricating the sandwich composite using this process, gives better
mechanical properties with low void content in the laminate.
Figure 5: (a) Schematic diagram; (b) Autoclave vacuum press.
8
1.3. FLEXURE TESTS
Sandwich panels are best suited for structures subjected to bending and
impact loads. Three and four point bending tests are good to study the flexural
behavior observed practically. From these tests, bending and shear stiffness can
be measured. Figure 6, illustrates three-point bending test setup. Sandwich
panels subjected to bending are sensitive to loading and support conditions,
structure geometry and other sandwich parameters.
Figure 6: Three-point bending test
The experiments are done as per the ASTM standards and are explained
further in the report. Similarly, four point bending can be tested which acts with
decreased stress magnitude on the beam.
9
1.4. FINITE ELEMENT ANALYSIS
With the advent of FEA, the theoretical research has shifted to
optimization of laminates [book]. The FEA are generally more accurate than
existing analytical solutions that require several approximations. Complex
structures and loading conditions can be analyzed to obtain detailed and
accurate stress distribution results in the structure. Advanced development and
understanding of composite material behavior gives an advantage for
implementing finite element analysis to analyze composite structures. Non-linear
analysis and failure mode study can also be done using several FE packages
available commercially. Several composite material models along with several
failure criterions are developed. The three-point bending test scenario can be
easily simulated using this package.
10
1.5. POTENTIAL APPLICATION
Earlier the composites were limited to Aerospace industry due to its high
cost and manufacturing difficulties. Invent of new low cost material and its
understanding of behavior under various conditions allows the composite
materials for several applications in Industries such as Automotive, Marine and
more. Environmental Protection Agency (EPA) requires that automotive industry
reach Corporate Average Fuel Economy (CAFÉ) of at least 35mpg by 2020, for
new passenger cars and light trucks combined. This emphasizes the importance
of light-weight structures in automotive, giving wide scope of applications for
sandwich composites. Sandwich composites with end-grain balsa core are used
for its high-strength and light-weight properties along with good fire resistance of
balsa in its end-grain plane.
To cite few applications in automotive industry, sandwich composite
panels can be implemented for sunroof panels, hard tops, luggage floor, front
hood, and also structural components such as B-Pillar.
11
1.6. RATIONALE
Research work on failure modes of sandwich composites are done by
several researchers such as Daniel and Gdoutos (2009), Gdoutos and Daniel
(2008), Zenkert and Burman (2011), Lim et. al. (2004) and Avila (2007). The
work was mainly focused for aerospace grade materials. Daniel and Gdoutos
studied failure modes of sandwich beams with uni-directional prepregs of carbon
fabric/epoxy and glass fabric/vinyl ester for skin and PVC foam, balsawood for
core. Failure mode transition of sandwich composite was studied by Gdoutos and
Daniel. Zenkert and Burman presented work on failure mode shift during fatigue
of GFRP/foam core sandwich beams. For applications in automotive industry
along with weight savings, high strength, other factors such as cost, curing
process are vital. Curing process in automotive industry requires less steps, easy
preparation and faster curing cycle. The work on glass fiber/epoxy laminate and
balsa core sandwich composite with no external adhesive is not studied with
focus to the automotive industry. Understanding failure modes with reference to
material behavior is needed for full structure analysis. This correlation of failure
mode with the flexural behavior of sandwich composite such as load-
displacement curve is not found in the literature.
12
1.7. RESEARCH OBJECTIVE
For applications in the automotive industry weight savings, strength of
structure influenced the work with respect to light-weight sandwich composite
materials. This research work focuses on the analysis of GFRP/balsawood
sandwich composite panels subjected under flexure. One of the objectives is to
cure sandwich panels without any external adhesive between skin and core. The
failure modes are analyzed and are also correlated with the load-displacement
curve.
The balsa core is chosen to study the effect of grain direction in the core
of sandwich panels. These sandwich composite beams with different core grain
orientations were compared with respect to failure modes, energy absorption,
flexural stiffness and strength. Finite element analysis is done using a non-linear
analysis package to understand the behavior and to study the failure mechanism
of sandwich panels.
Due to indentation failure of sandwich panels with soft core, American
Society for Testing and Materials (ASTM) suggest the use of padding. No
detailed specification for padding is given and the effect of padding on sandwich
composite beam is not discussed. In this work, the effect of padding under the
loading pin in three-point bending test has also been analyzed and discussed.
13
CHAPTER 2: LITERATURE REVIEW
2.1. Introduction
The concept of sandwich construction was first discussed in the 19th
century, even though commercial applications started in the World War II era.
Structures of small aircrafts such as de Havilland Mosquito used sandwich
panels made of veneer faces with balsa core. Several new techniques and
understanding of sandwich panels, has advanced its applications into wide range
of industries. Improvements include invention of structural adhesive, research on
core materials, theoretical work, and more.
In this section, only few material characteristic with distinct behavior are
discussed. The bimodular behavior of composite laminates is discussed and
theoretical analysis is developed by Yi-Ping Tseng (1995), He et al. (2009) and
Patel B.P et al. (2004). Patel et al. (2004) split the composite laminate into
tension and compression layers at neutral axis. Work on convergence of bi-
moduli material model by He et al. (2009) is helpful for predicting mechanical
behavior. Khalil (2003) studied the interlaminar shear stress response of stacking
sequence in E-glass/epoxy composite using three-point bending tests. This study
showed that due to interlaminar shear stress, first ply failure occurred at neutral
axis of the sample. Bendtsen (1964) discussed the work of Dohr on low-density
wood, Yagrumo hembra (a substitute of balsa). Dohr, concluded that toughness
was greater when the load was applied on the radial face than on the tangential
face. Similar behavior is observed in balsa wood. Cesim and Sevim (2010)
investigated sandwich composite panels for impact response and showed that
14
sandwich panels with balsa wood core absorbed energy better than panel with
PVC foam core.
2.2. Sandwich behavior
The fundamentals of sandwich construction along with theory and failure
modes are discussed by Zenkert (1997) in his book. Through series of tests,
Thomson et al. (1998) found abrupt decrease in strength when the interfacial
crack length exceeded a critical size. Also the impact damage has strong
influence on its residual shear properties. Dai and Hahn (2003) have calculated
the critical span length of beam where failure shifts from core shear to skin
failure. They also found that, higher shear strength of core leads to higher load
carrying capacity for shorter beams; however this effect disappears for longer
span. Theoretical analysis predictions from classical sandwich theory, high-order
sandwich panel theory (HSAPT) have been compared with the experimental
work by Thomsen and Frostig (1997), Sokolinsky et al. (2003). Classical
sandwich theory underestimates vertical displacement and also fails to predict
bending deformation of soft-core sandwich beams under concentrated load
because of the inability to model core indentation. The work suggests the use of
HSAPT, which can predict the vertical displacement of soft-core sandwich with
great accuracy.
Indentation or localized bending of skin and analytical analysis has been
discussed by Shuaeib and Soden (1997), Thomsen and Frostig (1997). Shuaeib
and Soden (1997) showed that with increase in skin thickness or core density, a
15
higher indentation failure load can be achieved. However, increase in core
thickness or indenter size did not show significant effect on failure load.
Experimental study of debonded sandwich panels under compressive loading by
Aviles and Carlsson (2006), showed failure occurred by local buckling of
debonded face sheet followed by rapid debond growth. The debond shape in
sandwich panels also play a role, as the square debond failed at lower loads than
those with circular debond of same area. In the past decade, failure modes of
sandwich beams under bending are investigated in several research works by
Lim et al. (2004), Avila (2007), Gdoutos et al. (2008), Daniel et al. (2009) and
Zenkert et al. (2011). Lim et al. (2004) constructed failure mode transition
equations using non-dimensional parameters such as strength ratio, modulus
ratio, normalized thickness and normalized span. Detailed investigation of failure
mechanism of composite sandwich beams with glass fabric/vinylester and balsa
wood material under bending was done by Daniel and Gdoutos (2009). Zenkert
and Burman (2011) presented the transition of core shear failure to face laminate
failure as function of load amplitude in fatigue loading. For high loading
amplitudes and few cycles to fatigue, beams failed by core shear and as the load
amplitude decreased, the failure mode shifted to face sheet failure. Analytical
predictions are made for three-point bending collapse mechanisms of sandwich
beams by Steeves and Fleck (2004). Also, minimum weight design and failure
mechanism maps were also constructed for optimization of sandwich beams with
composite faces.
16
Several works on sandwich composite has been done over the years to
increase resistance to failure. Research on sandwich composite structure with
high skin-core debonding resistance has been developed by Vuure et. al., (2000)
by weaving part of the pile fibers into the core. Along with debonding, this type of
sandwich construction provides better compression and shear resistance. Daniel
(2000) developed special techniques to prevent premature failure under loading
pins by reinforcing the outer core sections with epoxy. Avila (2004) illustrated the
beam configuration with functionally graded core. Best performance of sandwich
beam was noticed when highest density core is layered below the upper face-
sheet and the density reduces as it reaches lower skin. In balsa core sandwich
composite, improved shear properties were achieved by Bekisli and Grenestedt
(2004) through better design and utilizing the anisotropy property of balsa.
Another concept developed by Kepler (2011) improves the shear stiffness of
sandwich panels up to fourfold. This is obtained by tailoring several lay-ups with
different grain orientation of balsa core.
17
2.3. Finite Element Analysis
Tolson and Zabaras (1990) developed computational model to determine
ultimate strength under complex loading conditions and determining first ply
failure and last ply failure by progressive stiffness reduction technique.
Schweizerhof et al. (1998) presented the merits and limits of several composite
material models available in LS-Dyna and discussed the failure criterion used.
LS-Dyna theory manual (2006) published by LSTC provides details about the
model along with damage characteristic and the formulation used. Vaidya and
Deka (2009) confirmed the use of wood material model for Balsa wood from
Impact response study on Balsa wood core sandwich composite. In this study
fracture energy, strain softening parameter and rate effect were not considered,
but good correlation was found with experiments.
Several research works can be found in the literature on finite element
analysis of sandwich composite beam under bending. Mines and Alias (2002)
studied the progressive collapse mechanism using experimental and 2D
numerical simulation for sandwich beams with low or high density foam core.
Czichon et al. (2011) analyzed ultra-thick laminates for progressive failure on
macro scale using UMAT in Abaqus. Fan et al. (2011) investigated sandwich
panels under quasi-static impact for different failure mechanism such as fiber
breakage, matrix or core cracking, and interfacial delamination.
18
CHAPTER 3: MATERIAL AND SANDWICH CONSTRUCTION
3.1. MATERIAL DESCRIPTION
For the skin of sandwich composite, E-glass/epoxy laminate were used.
The laminate is cured from E-glass/epoxy prepreg layers. Each layer of the
prepreg, shown in figure 7(a) is a cross-ply of two plies stitched that are oriented
in 0° and 90°. The resin employed in this prepreg is Epon 202 epoxy. The 0°/90°
E-Glass/Epoxy prepreg is provided by MAG-ias, Cincinnati. The prepreg rolls are
stored in the freezer until it is processed for curing. Fully cured laminate is shown
in figure 7(b).
Figure 7: (a) E-glass/Epoxy pre-preg; (b) Fully cured laminate.
Balsa wood is a light-weight material, with high stiffness in the grain
direction and softer perpendicular to grain. A 9.5 mm thick balsa wood of density
96 ± 5 kg/m3 is used as core for sandwich construction. End-grain and regular
balsa wood are the two types of balsa sheets available commercially. Regular
balsa wood has the grain oriented along the length of the sheet and the grain is
maintained along thickness in case of End-grain balsa (ref. figure 8(b)). Regular
19
balsawood of 915mm X 101mm and End-grain balsawood of 305mm X 305mm
were bought from Balsawood Inc.,
Figure 8: (a) Balsa wood; (b) Schematic representation of grain direction.
Even though these materials are being used for a long time, yet there is a
lack of knowledge concerning the effect of damage and failure mode due to
different grain orientation in core of sandwich composite panels.
20
3.2. PRELIMINARY TESTS
Finite element analysis requires all the basic properties of material and its
behavior under various loading conditions. Hence, preliminary tests are
conducted on the material to evaluate the properties under tests such as Tensile,
compression and shear. These tests are done on both skin and core materials in
different directions as the materials are considered to be anisotropic. The
experimental setup and process for these tests are described below.
3.2.1. Tensile tests
Tensile tests on E-glass/Epoxy laminate are done as per ASTM D3039
test method. E-glass/Epoxy laminate of 250mm X 250mm with 4 plies of
0°/90°/90°/0° are cured for tensile tests. Laminates are cut into 200mm X 25mm
test coupons with 38mm grip length on each side. Rectangular cross-section of
test coupons is maintained to avoid failure near grip. Grit paper is used on the
laminate in the gripping area to avoid direct pressure on the coupon.
Extensometer is used to obtain strain from the coupon. A MTS tensile testing
machine (figure 9) with load capacity of 100kN is used for tensile tests.
21
Figure 9: MTS Tensile testing machine with 100kN capacity.
22
Figure 10: (a) Tensile test coupon; (b) coupon secured in tensile machine;
(c) Damaged test coupon.
Properties in X and Y axis are considered to be same, as equal plies are
oriented in 0° and 90°. From the load-displacement data of machine, stress-strain
curve is plotted. Figure 11 shows a stress-strain curve of typical tensile test of
0°/90° E-glass/Epoxy laminate. Elastic modulus, Ultimate tensile strength and
strain can be obtained from the results.
23
0
100
200
300
400
500
600
0 0.005 0.01 0.015 0.02 0.025 0.03
Strain
Stre
ss (M
Pa)
Figure 11: Tensile test data of typical 0/90 E-glass/Epoxy laminate
Similarly, tensile tests are done on balsa wood material. Tests are
repeated in parallel and perpendicular to the grain direction, as balsa wood is
considered to exhibit transverse isotropic behavior. Coupons of 100mm X 13mm
with constant rectangular cross-section are cut from the balsa sheets of 5mm
thickness. Grip length on each side of coupon is 25mm. These tests are done on
smaller tabletop MTS tensile testing machine with 12kN load capacity.
Figure 12 shows the tensile test plot of a typical balsa wood in the grain direction
and tensile test results across the fiber is shown in figure 13.
24
Figure 12: Tensile test data of Balsa wood in fiber direction
Figure 13: Tensile test data of Balsa wood in transverse direction
25
3.2.2. Compression tests
Compression tests on E-glass/Epoxy laminate are done as per ASTM
D3410 test method. Thick E-glass/Epoxy laminate of 250mm X 250mm with 16
plies of [0°/90°/90°/0°]4 are cured for compression test coupons. Laminates are
cut into 150mm X 25mm test coupons with 25mm gauge length at the middle.
End-tabs are used in the gripping area of the coupon. Compression fixture is
employed to reduce failure due to global buckling. A MTS testing machine with
load capacity of 200kN is used for these tests. Load and displacement data are
obtained from the machine, which are converted into stress-strain curve shown in
figure 15.
26
Figure 14: (a) Compression test coupon; (b) Compression fixture;
(c) Damaged coupon
27
0
50
100
150
200
250
300
350
0 0.02 0.04 0.06 0.08 0.1 0.12
Strain
Stre
ss (M
Pa)
Figure 15: Compression test data of typical 0/90 E-glass/Epoxy laminate
Compression test data shows that E-glass/Epoxy laminate gives different
response under tension and compression, as the modulus and strength of
compression data are different from tensile tests.
Balsa wood is also tested under compression and similar bimodulus
behavior is observed. For compression tests along fiber direction, End-grain
balsa of 25mm X 25mm are cut and tested along thickness. Regular balsa wood
of same dimensions is used for compression tests in transverse to fiber direction.
Samples are compressed between the jaws of compression testing machine as
shown in figure 16.
28
Figure 16: Compression test on balsa wood
(a) Undamaged End-grain Balsa; (b) Compression test;
(c) Damage in End-grain balsa along thickness as seen in SEM
Figure 17, shows compression test results of a typical balsawood material
in the fiber direction and stress-strain data of compression test in transverse to
fiber is shown in figure 18. Similar bi-modulus behavior is found in balsa wood
under tension and compression.
Figure 17: Compression test data of Balsa wood in fiber direction
29
Figure18: Compression test data of Balsa wood along transverse to fiber
3.2.3. Shear tests
ASTM D5379 is referred as a standard test procedure for shear tests on
E-glass/Epoxy laminate. Shear test fixture is employed on the MTS compression
testing machine for the V-notched test (figure 19). Thick test specimens are cut
into given dimensions using band-saw (figure 20).
30
Figure 19: Shear test fixture
Figure 20: V-notched beam test coupon dimensions. (Source [43])
31
Figure 21: Shear test data of typical E-glass/Epoxy laminate across fiber
Interlaminar shear properties of E-glass/Epoxy laminate is obtained from
Short-Beam tests. Laminate with 16 plies (similar to compression coupon) are cut
into 25mm X 25mm coupons for short-beam shear testing. Figure 22 shows the
setup and failure due to shear under 3-point bending. In three-point bending test,
the specimen is subjected to both bending and shear stresses. Shear stress is
dominated in specimens with small span-to-thickness ratio.
Figure 22: Short-beam shear test (a) Set-up; (b) after damage.
32
The load-displacement data (figure 23) obtained from the machine is
plotted and interlaminar stress is calculated from the Equation 1 (Khalil, 2003).
σ = hb
P**4
*3 Equation (1)
Figure 23: Short-beam shear test data of typical E-glass/epoxy laminate.
Shear tests on balsa wood are done using a different fixture shown in
Figure 24(a). Samples of balsa of 10mm thickness are cut 25mm X 25mm and
initial cracks were cut at middle all around to ensure the failure at the middle.
Failed sample is shown in figure 24(b). The stress-strain data is plotted for all the
tests done across and along fiber of balsa wood. A typical shear test curve
across fiber is shown in figure 25 and along fiber is presented in figure 26.
33
Figure 24: Shear test of Balsa wood (a) Setup; (b) damaged specimen.
Figure 25: Shear test data of Balsa across fiber
34
Figure 26: Shear test data of Balsa along fiber
The material properties such as Elastic modulus, Ultimate strength, strain
are calculated from the obtained results shown above. Each test is repeated with
six to eight samples to populate and validate the results data. The standard
deviation is calculated from each property and found to be within 10% deviation
to mean. These properties are utilized as input parameters for the numerical
modeling in LS-Dyna.
The experimentally established properties of skin and core materials are
tabulated in the table 3.1 and table 3.2 respectively.
35
Table 3.1: Material properties of 0°/90° E-glass/epoxy laminate
Density (Kg/m3) 1926.3
Tensile modulus (In-plane), EXt, yet (GPa) 19.88, 19.88
Compressive modulus (In-plane), EXc, EYc (Gpa) 7.42, 7.42
Poisson’s ratio, ν21, ν31, ν32 0.11, 0.18, 0.18
Shear modulus, GXY, GYZ, GZX (Gpa) 4.04, 3.37, 3.37
In-plane tensile strength, XT, YT (MPa) 545.8, 545.8
In-plane compressive strength, XC, YC (MPa) 288.8, 288.8
Shear strength, SXY, SYZ, SZX (MPa) 31.64, 71.96, 71.96
Table 3.2: Material properties of Balsa wood
Density (Kg/m3) 96 ± 5
Parallel normal modulus, ELt, ELc (MPa) 1683.8, 371.5
Perpendicular normal modulus, ETt, ETc, (MPa) 56, 23.5
Parallel shear modulus, GLT (MPa) 72.85
Perpendicular shear modulus, GLR (MPa) 12.5
Parallel tensile strength, XT (MPa) 10.12
Perpendicular tensile strength, YT (MPa) 0.82
Parallel compressive strength, XC (MPa) 8.05
Perpendicular compressive strength, YC (MPa) 0.707
Shear strength, SXY, SYZ (MPa) 0.82, 0.83
36
3.3. CURING SANDWICH COMPOSITE
Flat steel mold plates of 305mm X 305mm are used for curing composite
laminates. Mold plates are prepared by applying release agent and left over-
night. E-glass/epoxy laminates are cured in the vacuum press [figure 27(a)] using
prepreg’s provided by MAG, Cincinnati. Sandwich composites are fabricated
similarly from the vacuum press. In the mold plates, two layers of 0°/90° cross-ply
E-glass/Epoxy prepreg’s are layered as skin on both sides of the balsa core. The
whole system is placed in the press and cured all together to get good adhesion
between skin and core. No external adhesive has been applied between skin and
core as the epoxy from prepreg adheres to the balsa core. The curing process
includes, treating the layered prepreg’s under vacuum and 344.7kPa pressure
applied on the laminate at 135°C for 20 minutes. The laminate is then cooled by
passing mist, followed by water over the platen for 15 minutes each. The cured
laminate is then removed from the mold plate.
37
Figure 27: (a) Autoclave vacuum press machine; (b) Post-curing oven; (c)
Fully-cured sandwich composite.
After curing, the laminates are post-cured in an oven [figure 27(b)] at 80°C
for 5 hours. Fully cured sandwich composite is shown in figure 27(c).
For three-point flexure tests, sandwich composite panels were cut using a
band-saw machine, to get 25.4 mm width and total length of 254 mm. The skin
and core thicknesses were measured to be 0.88±0.05mm and 9.45±0.1mm
respectively. All the dimensions of test specimen were maintained according to
the ASTM standards.
For study on padding over sandwich composite under three-point flexure,
Teflon sheet and rubber pads are used. Two layers of Teflon sheet with
thickness of 0.15mm each and one layer of rubber pad with 2mm thickness is
used separately over sandwich beam for case study. The padding of 25mm X
25mm is fixed on top-face of sandwich composite using epoxy adhesive.
38
CHAPTER 4: EXPERIMENTAL WORK
4.1. Three-point bending test setup
ASTM test procedures are referred for the process and test setup. Test
procedures for flexure on composite materials are ASTM C393 which is a
standard test method for flexural properties of sandwich constructions and ASTM
D790 is a standard test method for flexural properties of unreinforced and
reinforced plastics.
Three-point bend fixtures were employed for conducting flexure tests on
sandwich composite beams. The fixture from Wyoming test fixtures Inc., was
secured onto MTS compression testing machine having a load capacity of
200kN. Fixture consists on two parts, bottom supports and loading pin. Loading
pin with same diameter is fixed rigidly onto the machine. Bottom supports
consists of two support pins of diameter 12.7mm and distance between the two
could be adjusted to get maximum span length of 254mm. Bottom fixture is
adjusted so that loading pin is exactly over the mid-span of the beam.
Span length was kept equal to 203mm, which exceed 16 times the total
thickness of laminate, as recommended by ASTM test procedure. The higher
span length produces greater moments without exceeding the allowable limit for
core shear stress. Total length of the test specimen is maintained to be 254mm,
allowing 25mm of overhanging beyond the supports on each side. Three-point
bending test fixture is shown in the figure 28 and complete setup including the
compression testing machine is shown in figure 29.
39
Figure 28: Three-point bending test setup
Figure 29: MTS testing machine with three-point bending fixture.
40
Quasi-static flexural tests were done in displacement control mode by
pushing the bottom supports against the sandwich composite beam at a rate of 3
mm/min. The load transducer on the top, records the load taken by the beam to
deflect. Load, displacement and time data were recorded for every 0.5s by the
computerized controlled machine. The results are analyzed by considering the
load vs. displacement plots. Also, pictures and videos were captured during the
three-point bend tests. Later the videos were analyzed for critical points of failure
mode and were correlated with time from the plot obtained.
41
4.2. FAILURE MODES OF SANDWICH COMPOSITE UNDER FLEXURE
Failure modes in a sandwich composite include Indentation failure under
localized loading, tensile or compressive failure of skin, core shear failure,
debonding at skin-core interface and wrinkling of compression skin. The failure
mode observed in the present work are characterized and explicated with load-
displacement behavior (see figure 30).
Figure 30: Typical Load vs. Displacement curve from three-point bend test.
Load-displacement curve shown in figure 30 is used only for explaining
the flexural behavior of sandwich composite due to different failure modes.
Displacement of loading pin in the region till point 1 is purely due to global beam
deflection. No failure is observed in this region.
42
4.2.1. Indentation of skin
Figure 31: Indentation [Point 1-2] in regular balsa core sandwich composite
Sandwich panels (particularly, with soft core) are sensitive to application of
localized load. Under three-point bending, localized skin bending (indentation) is
observed under loading pin. This compresses the loaded skin into the core
material of sandwich panel, thus giving rise to local stress concentration [11-13].
Indentation is caused due to the comparatively weaker stiffness of balsa core in
radial and tangential direction, which is the case with regular balsa core. From
the load-displacement curve, local skin bending can be noticed when the curve
shifts from the linear to plateau region. Therefore, elastic limit of load-
displacement curve is a good indication for initiation of localized bending.
4.2.2. Skin failure
Figure 32: Skin Failure [Point 2] in regular balsa core sandwich composite
43
Depending on several factors, skin failure is gradual or sudden and
observed in both sandwich composites. For most of the composite materials, the
compressive strength is less than tensile strength. For this reason, failure is
observed on the top-face of sandwich composite. When the compressive stress
in the top face reaches failure stress, it initiates crack in fibers and matrix. As the
top-skin fails a sudden drop in load is noted (region 2-3, in fig. 4). This causes
indentation in the core along with skin-core interface separation. Further, core
and bottom skin carries the load till complete sandwich beam failure.
4.2.3. Progressive core collapse
This failure looks similar to Indentation, but with severe compression of
skin into the core. This failure is observed after skin failure or when the beam
reaches peak load. This is due to application of direct load from the loading pin
onto the core, as the top skin reaches its compressive limit. From the load-
displacement curve in figure 30, the progressive core collapse is the region after
point 3.
4.2.4. Core shear failure
Figure 33: Core shear failure [Similar to Region 2-3 in fig 4] in regular balsa core
sandwich composite
44
Core in sandwich composite generally resists shear load applied on the
beam. Balsawood core is observed to have good resistance to shear when
loaded parallel and radial to the grain direction, but core failure is found in
sandwich composite beam when loaded in the tangential direction. This can be
followed by delamination with thin layer of balsa attached to the skin. A sudden
load–drop to about 20%, similar to region 2-3 in figure 4 is observed in the load-
displacement curve.
4.2.5. Delamination of skin-core interface
Figure 34: Interface delamination (after skin failure) in end-grain balsa core
sandwich composite
Delamination or debonding may be caused by external loading such as
impact, in-plane compression [14, 18]. Under quasi-static flexural load on balsa
core sandwich composite (with adequate span length), skin-core interface
debonding is not observed as the first failure criterion but can follow after either
skin or core failure.
45
4.3. SPECIMEN DETAILS
Sandwich composite beams (figure 30) of 25mm width and total length of
254mm are cut from the sandwich panels for testing. Skin and core thicknesses
in sandwich composite are 0.88±0.03mm and 9.45±0.15mm respectively. Total
thickness of sandwich composite beam is 11.2±0.2mm.
Figure 35: Sandwich composite beam
Different core grain orientations of balsa in sandwich composite beams
are fabricated for this study. The core grain orientations with respect to loading
for three different cases are shown in table 4.1.
Table 4.1: Sandwich composite beams – core grain orientations (figure 30)
Core Grain Orientation X-axis Y-axis Z-axis
Type of Balsa used Regular End-grain
Loading w.r.t grain
Radial
Tangential
Parallel
46
Padding of 25mm X 25mm is fixed using epoxy adhesive on top face of
the skin at mid-span.. The details on test specimens for study on effect of
padding on sandwich beams under the loading pin are given in table 4.2.
Sandwich composite with core grain orientation in Y-axis is chosen for padding
comparison.
Table 4.2: Sandwich composite beams – padding comparison
Padding Type Without Pad Teflon Pad Rubber Pad
Padding specifications No Pad
2 layers of
0.15mm thick
Teflon pads
1 layer of 2mm
thick rubber pad
Total thickness [at Mid-span] (mm)
11.2 ± 0.05 11.5 ± 0.1 13.3 ± 0.1
Elastic modulus of pad material (MPa)
- 7100 5
Three-point bending tests were repeated with six samples of each type of
sandwich beam to check the validity of the results. A typical load-displacement
curve for each type of sandwich composite beam described above is shown in
the following sections.
47
4.4. TEST DATA: Core grain orientation
Typical load-displacement plots of flexure tests on sandwich composite
beams with balsa core grain orientation in X, Y, and Z axis are shown in figure
36, 37, and 38. The dominated failure modes in Sandwich composite with regular
balsa core (balsa grain orientation in X and Y axis) shown in figure 36 and 37,
are Indentation of skin and complete skin failure. These failures are generally
followed by progressive core collapse, as the core and bottom skin carries the
load further till complete beam failure. Core shear failure can also be observed
by the sandwich composite with regular balsa is loaded tangentially to the grain.
A sudden and complete skin failure is dominant failure mode observed in
sandwich composite with end-grain balsa core. Delamination of skin-core
interface can be observed, but mainly follows other failure mode. This shows
good interface adhesion between E-glass/epoxy and balsa wood.
48
Figure 36: Test data of Sandwich with balsa core grain in X-axis
Figure 37: Test data of Sandwich with balsa core grain in Y-axis
49
Figure 38: Test data of Sandwich with balsa core grain in Z-axis
4.5. TEST DATA: Padding comparison
A typical test data of sandwich beam with Teflon and Rubber pad are shown in
figures 39 and 40 respectively. The core of sandwich composite is balsa wood
with grain orientation in Y-axis. The below results can be compared with test data
of sandwich beam with balsa grain in Y-axis tested without any padding under
the loading pin (figure 37. From these results and experimental observations,
failure of beam detracts from progressive core collapse to core shear failure.
Detailed comparison and analysis between sandwich composite with and without
padding is made in the Results and Discussion section of the report.
50
Figure 39: Test data of Sandwich with Teflon Pad (Balsa grain in Y-axis)
Figure 40: Test data of Sandwich with Rubber Pad (Balsa grain in Y-axis)
51
CHAPTER 5: FINITE ELEMENT ANALYSIS
5.1. Introduction
Modeling and analysis of composite material is complex, as they include
several parameters. It is important to understand all the aspects of composite
behavior before choosing and implementing in the material model. A non-linear
finite element analysis package, LS-Dyna has extensive material models for
composite materials along with failure modes. Sandwich composite beams that
were investigated experimentally are analyzed using FEA for further
understanding of failure mechanisms.
5.2. Sandwich composite FE model
Sandwich composite model is constructed using a pre-processor,
Hypermesh. The material properties determined from preliminary tests are
inserted into the material model. The skin is modeled with 4-node shell elements
as the thickness is less as compared with core thickness, which has been
modeled with 8-node solid elements. Element length of 1mm is maintained in the
entire sandwich composite beam. Mat 59, Composite Failure Shell model was
assigned for skin of sandwich composite and Mat 143, Wood model was
assigned to core. Supports and loading pin were made rigid, Mat 20. Due to
complexity in composite material formulation, the computational increases
significantly. To reduce the computational time, only 1/5th of the model width has
been considered. The final load is corrected by considering 5 times the attained
result. Figure 41, shows finite element mesh of the model.
52
Figure 41: Finite element analysis model
The material model 59, which is composite failure shell model, has faceted
failure surface as shown in figure 42. Ply-by-ply orientation of skin is not available
in this model and laminate properties were directly applied.
Figure 42: Failure model used in Mat 59 of LS Dyna. (Source [28])
It is known that under bending, top face of sandwich beam acts under
compression and the bottom face acts under tension. Hence, the equivalent
(average of tension and compression) modulus was calculated and inserted as
elastic modulus to top face. The tensile modulus was considered for the bottom
face elements.
Wood material model exhibiting transversely isotropic property was
applied to the core elements. Softening and fracture parameters are not
considered which are available in this material model, as it needs more research
53
on balsawood. Yield criteria is formulated parallel and perpendicular to the grain
from tensile, compressive and shear strength of balsa in parallel and
perpendicular direction to grain.
For parallel mode, yielding occurs when, f║ ≥ 0
f║
1)(
2||
213
212
2
211
SSIGSIG
XSIG
Eq. (2)
For perpendicular mode, yielding occurs when, f┴ ≥ 0
f┴
1)*()(
23322
223
2
233
222
SSIGSIGSIG
YSIGSIG
Eq. (3)
where,
The support and loading pins are of 12.7 mm diameter and are rigid solid
elements. The support pins are constrained in all the three-degrees of freedom.
Skin and core elements were simply connected by merging the nodes, as the
delamination failure was not a concern under quasi-static loading in this type of
Sandwich composite. Automatic surface to surface contact type was given
between the loading pin, top skin, core, bottom skin and supports. Force
transducer penalty card was activated to obtain load data from loading pin. The
loading pin in given a displacement of 10 mm at a constant rate against the
sandwich beam model.
54
For sandwich beam with rubber padding, solid elements were created and
Rubber material model, Mat 7 was assigned. For Teflon pad, simple elastic
model, Mat 1 was used. Also, purely compression modulus was inserted for top
skin elements, to compensate the softening parameter in balsa wood material.
Hourglass energy was activated for rubber pad, as the load was applied directly
on the pad.
The load-displacement data were plotted using LS-Prepost and MS-Excel.
The stress distribution on top skin was also analyzed in sandwich composite with
normal balsa core.
55
5.3. Load-displacement data
To check the correlation between the experimental and finite element
analysis, load-displacement curves from experimental and analysis data are
plotted in the same figure. Figure 43 shows good correlation between
experimental and FEA data for sandwich composite with balsa grain in X-axis.
Similarly for balsa grain in Y-axis and Z-axis, the correlation of FEA and
experimental load-displacement data are shown in figure 44 and 45 respectively.
Figure 43: Experimental and FEA data of sandwich composite beam with balsa
core grain in X-axis
56
Figure 44: Experimental and FEA data of sandwich composite beam with balsa
core grain in Y-axis
The softening parameter in the low-density wood model plays an
important role when loaded transversely to grain. This data was not found for
balsa wood in the literature. Also, the wood material model in LS-Dyna considers
the tangential and radial properties as same, whereas in case of light-weight
wood the shear and fracture toughness is known not to be the same in those
directions [28]. This can be a reason for sandwich beam model with grain in Y-
axis, for not having good correlation with experimental result.
57
Figure 45: Experimental and FEA data of sandwich composite beam with balsa
core grain in Z-axis
The sandwich composite model with balsa grain in Y-axis is used for
analysis of padding under the loading pin. Figure 46 shows the load-
displacement curve for sandwich beam with 0.4mm thick Teflon pad and for
sandwich beam with 2 mm thick rubber pad is shown in figure 47. Correlation of
FEA data with experimental results is found to be good, even in sandwich
composite beam models with padding. Comparison and detailed discussion are
made in the results section.
58
Figure 46: Experimental and FEA data of sandwich composite beam with balsa
core grain in Y-axis and with Teflon Padding
Figure 47: Experimental and FEA data of sandwich composite beam with balsa
core grain in Y-axis and with Rubber Padding
59
CHAPTER 6: RESULTS AND DISCUSSION
6.1. Comparison of different core grain orientations in sandwich composite
Sandwich composite beams with different core grain orientations are
compared and discussed in this section. The load-displacement data of 3-point
flexure tests of these sandwich composite beams is shown in figure 48. Failure
mode of each type of sandwich beam is discussed in detail. Flexure properties of
these typical sandwich composite beams are tabulated in table 6.1.
Figure 48: Test data comparison and failure mode of Sandwich beam with
different balsa core grain orientations.
60
Table 6.1: Comparison of balsa grain-orientation in Sandwich composite
Sandwich beam with Balsa grain in
X-axis Balsa grain in
Y-axis Balsa grain in
Z-axis
Strength (N) 535 267 880
Stiffness (N/mm) 154.6 56.6 140.4
Energy Absorption# 2116 1216.4 2903.9
Load after skin fails Drops by
about 20-40% No complete skin
failure Drops by
about 70-90% # measured until skin failure / peak load
From this study, we know that bending stiffness, strength and failure
modes changes with core grain orientation in sandwich composite. The bending
stiffness of sandwich beam is high with end-grain balsa core and also with
regular balsa core when loaded in radial direction. The stiffness reduces as
bending direction changes in regular balsa core. High flexural strength and
energy absorption is obtained in sandwich composite with end-grain balsa
configuration, but the failure is catastrophic in nature as the core fails instantly
after skin failure.
The sandwich behavior from experiment and FE analysis are compared at
several displacements and presented in figure 49. Similar effect of sandwich
composite is observed in both FEA and experimental pictures. The gap between
the sandwich and loading pin (in FEA) can be noticed, as it accommodates skin’s
shell thickness.
61
Figure 49: Progressive damage under the loading pin observed experimentally
and in FEA for sandwich beam with different balsa core grain orientations.
62
The flexural behavior of sandwich composite with regular balsa core (core
grain oriented in X and Y axis) is explained in this passage. In this type of
sandwich composite beam, Indentation of skin is dominant failure mode initiation.
The Indentation of skin can be followed by either skin or core failure mode. Skin
failure can be gradual or sudden, but the beam still resists load till complete
failure. The load continuously drops with increasing flexural displacement. Core
failure due to shear can occur in this type of sandwich composite, due to least
resistance of balsa wood in tangential direction.
The flexural behavior of sandwich composite with end-grain balsa core
(core grain oriented in Z axis) is described in this passage. The skin failure on top
face is prominent failure mode in this type of sandwich composite. A sudden
complete failure of skin is observed with a huge load drop. This sandwich
composite offers better energy absorption until peak load as compared to
sandwich composite with regular balsa core. Core shear failure can be the failure
mode when span length is very less. This core shear failure in sandwich beam
with end-grain balsa core is not studied, as more significance was given
comparison of core grain orientation.
Debonding of skin-core interface is not noticed in any sandwich beam with
balsa core. This shows good skin-core interface adhesion, as the pores in balsa
absorb resin during curing. With adequate span length of sandwich composite
beam, overall shear resistance of balsa core is good.
63
The finite element analysis confirms the bi-modulus behavior of E-
glass/Epoxy skin in sandwich composite beam under bending. The induced
stress concentration in skin at peak load for three different core grain orientations
are show in figure 50. Experimental and FEA data shows, skin in sandwich
composite with balsa grain oriented in Z-axis shows higher stress at failure (or
peak load). Sandwich with regular balsa shows maximum stress (compressive)
next to the loading pin and small part of skin below the pin is noted to be under
tension. This effect is due to the Indentation of skin. With end-grain balsa core,
the stress concentration is below the loading pin. Also, the stress carried by the
skin at peak load is higher in sandwich beam with end-grain balsa core. This
explains the sudden catastrophic failure of sandwich beam.
64
Figure 50: Stress concentration of skin around loading pin just before skin
failure/peak load.
65
6.2. Comparison of padding on sandwich composite
Significance of the use of padding on sandwich composite to avoid
indentation failure is discussed with the help of load-displacement and failure
mode data. Load-displacement plots of typical sandwich composite beam with
and without padding are compared in figure 51. The flexural behavior such as
stiffness and peak load of sandwich composite beam with padding are observed
to be the same as compared to sandwich composite beam without padding. After
the beam reaches peak load, failure may be different depending on sandwich
composition.
Figure 51: Comparison of load-displacement plots of sandwich composite beam
without and with padding (balsa grain oriented in Y-axis)
66
As discussed earlier, failure initiates as Indentation on top face in
sandwich composite without padding. The core is crushed under the loading pin
without padding. In comparison, failure in sandwich composite beam with Teflon
or rubber padding can be more due to core shear failure. This behavior of core
failure is not observed in finite element analysis, as fracture toughness properties
of balsa wood are not specified in the model. Also, finite element analysis is
focused mainly till the peak load or first failure of sandwich composite beam.
Failure mode pictures are compared for sandwich composite beams with
and without padding and are represented in figure 52. This shows the amount of
indentation of skin in sandwich composite beam without padding as compared to
beam with padding. Indentation is observed to be more in beam without padding
and load is slightly distributed in beam with padding.
Figure 52: Failure modes in sandwich composite without padding, with Teflon
pad and with rubber pad
67
From the finite element analysis, detailed study on effect of failure mode
and its initiation can be done. FE analysis of sandwich composite beam with
Teflon pad doesn’t show significant differences as compared to sandwich beam
without padding. Hence sandwich beam with rubber pad is considered for
detailed study and comparison with sandwich beam without padding.
Sandwich composite with padding is observed to slightly delay the
Indentation failure of skin, but this failure mode is not avoided. Similar effect is
also observed in experimental work. That is sandwich composite beams without
padding shows non-linearity for longer flexural displacement, which represents
Indentation of skin.
Stress distribution in skin (under loading pin) and core at peak load
(similar displacement) are shown in figure 53 and 54.
68
Figure 53: Stress contour at peak load on top skin of sandwich composite without
and with padding.
69
Figure 54: ZX-Shear stress contour at peak load in the core of sandwich beam
without and with padding.
Stress concentration and maximum stress on the top skin is comparatively
less in beam with padding, as the load from loading pin is distributed. The
magnitude and stress contour is compared and shown in figure 52. From figure
53, magnitude of shear stress is found to slightly higher in case of sandwich
beam with padding. Also, stress contour in sandwich beam with padding shows
higher stress concentration over marginally more area than compared to
sandwich beam without padding.
70
With these FEA results, it is hard to conclude on the core shear failure, as
softening and fracture toughness parameters play an important role in balsa core
(which is not inserted into the model).
71
CHAPTER 7: CONCLUSIONS
Sandwich composite panels are analyzed under three-point flexure tests
for understanding flexural behavior and failure mechanisms. Study on effect of
padding to avoid locally induced stresses on skin are also analyzed with Teflon
and rubber pad.
CURING SANDWICH COMPOSITE WITHOUT EXTERNAL ADHESIVE
E-glass/epoxy prepreg’s were directly layered on the core and were cured
together. Good adhesion at the skin and core interface was found in all the
sandwich beams with balsa core, as the epoxy from the prepreg flows into the
pores in balsa wood creating a strong bond.
FAILURE MODE ASSESSMENT OF GFRP/BALSA SANDWICH
COMPOSITE
Sandwich composite with soft core such as regular balsa exhibits failure due
to Indentation of skin, which is followed by skin failure or core failure. In case
of sandwich beam with end-grain balsa core, skin failure is dominant mode of
failure. With adequate span length, core shear failure and debonding of skin-
core interface is not a concern with balsa core sandwich composite. The
failure modes were also correlated with flexural behavior of sandwich
structure (load-displacement curve) which is helpful for full structural analysis.
EFFECT OF CORE GRAIN ORIENTATION ON SANDWICH COMPOSITE
Bending stiffness, strength and failure modes are strongly influenced with
core grain orientation in sandwich composite. Sandwich panels with end-grain
72
balsa core offers better energy absorption, but catastrophic skin failure with
huge load drop is observed. Overall shear resistance of balsa is considerably
good with least resistance when loaded tangentially to balsa grain direction.
EFFECT OF PADDING ON SANDWICH COMPOSITE BEAM
Overall flexural behavior of sandwich composite is not affected with the use of
padding. Failure mode can change from skin failure to core shear failure, as
direct stress on skin is distributed
FINITE ELEMENT ANALYSIS
Good correlation in all the sandwich composite models found between FEA
and experimental results. Finite element analysis confirms the bi-modulus
behavior of E-glass/epoxy laminates. In sandwich beam with soft core
(regular balsa), before skin failure higher stress concentration is found next to
the loading pin, whereas in sandwich beam with end-grain balsa core
maximum stress is found below the loading pin. Stress contour of sandwich
beam with padding shows reduction in normal stresses on top skin with
influence of shear stress on more area in the core. Also, local bending of skin
was reduced for same flexural deflection with the use of padding.
73
FUTURE SCOPE OF WORK
1. It is difficult to conclude on core shear failure using FEA results, as more
research on balsa wood is required.
2. Develop better techniques to avoid local bending of skin (Indentation) in
sandwich composite with soft core.
3. To study progressive failure analysis of balsa core sandwich composite.
This requires complete understanding of composite and balsa wood
behavior under various loading conditions. This can be achieved with
Finite Element Analysis.
74
APPENDIX: STATISTICAL DATA
1. Tensile test data of 0°/90° E-Glass/Epoxy laminate
75
2. Tensile test data of Balsa wood in fiber direction
3. Tensile test data of Balsa wood in transverse direction
76
4. Compression test data of 0°/90° E-Glass/Epoxy laminate [8 plies]
[16 plies]
77
5. Compression test data of Balsa wood in fiber direction
6. Compression test data of Balsa wood in transverse direction
78
7. Shear test data of 0°/90° E-Glass/Epoxy laminate [across fiber]
8. Short beam shear test data of 0°/90° E-Glass/Epoxy laminate [Interlaminar]
79
9. Shear test data of Balsa wood across fiber
10. Shear test data of Balsa wood along fiber
80
11. Three-point bending test data of Sandwich composite beam with balsa core oriented in X-axis
12. Three-point bending test data of Sandwich composite beam with balsa core oriented in Y-axis
81
13. Three-point bending test data of Sandwich composite beam with balsa core oriented in Z-axis
14. Three-point bending test data of Sandwich composite beam with balsa core
oriented in Y-axis with Teflon padding
82
15. Three-point bending test data of Sandwich composite beam with balsa core oriented in Y-axis with Rubber padding
83
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ABSTRACT
FLEXURAL ANALYSIS OF BALSA CORE SANDWICH COMPOSITE: FAILURE MECHANISMS, CORE GRAIN ORIENTATION AND PADDING
EFFECT
by
AVINASH S. PHADATARE
May 2012
Advisor: Dr. Golam Newaz
Major: Mechanical Engineering
Degree: Master of Science
A comprehensive investigation was undertaken to study failure
mechanisms of sandwich composites and their influence on flexural behavior
(load-displacement curve). Sandwich composite panels were cured from
compression thermoforming of E-glass/epoxy skins and a low density balsa wood
core. Balsa core grain orientation is found to have major effect on flexural
response and failure modes. Flexural behavior, failure mode and its sequence
varies with different core grain orientations. Indentation, skin failure, core shear
failure were dominant failure modes observed for various cases. Skin-core
interface adhesion was reasonably good, as delamination was not the first failure
mode in sandwich beam with balsa core. Indentation of skin is a major concern
under localized loading, as the skin failure is premature as compared to its
normal compressive strength. Hence, the effect of padding on sandwich beam
with soft core was also completed as part of this work. Finite element analysis for
90
modeling this type of sandwich composite beam is conducted using LS-Dyna.
Several material parameters required for finite element analysis were determined
from extensive testing and data from literature. Composite failure model and
wood material model available in LS-Dyna were applied for skin and core of
sandwich beam. At the skin-core interface, nodes were merged as delamination
was not a prominent failure mode. Flexural analysis response from FEA shows
good correlation with experimental behavior.
91
AUTOBIOGRAPHICAL STATEMENT
NAME: AVINASH S. PHADATARE
PLACE OF BIRTH: BANGALORE, INDIA
EDUCATION:
MASTER OF SCIENCE in Mechanical Engineering, at
WAYNE STATE UNIVERSITY, Detroit, Michigan [2010 – 12]
BACHELOR OF ENGINEERING in Mechanical, at
VISVESWARAIAH TECHNOLOGICAL UNIVERSITY, India [2004 – 08]
EXPERIENCE:
WAYNE STATE UNIVERSITY, DETROIT, MI
Advanced Composite Mechanics Lab [2010 – 12]
SAFRAN ENGINEERING SERVICES INDIA (Formerly, SAFRAN AEROSPACE INDIA)
Mechanical Engineer at CAD Department [2008 – 10]
National Aerospace Laboratories (FRP Division), India
Volunteered for completion of final project during
Bachelor of Engineering [5 months, 2008]