Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies
The analysis of stiffness and the identification of rupture mechanisms during and after static tests of sandwich panels and their components have been investigated. The sandwich panels, having cross-ply laminates skins made of glass fibre and epoxy resin were manufactured by vacuum moulding an...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
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irk-123456789-480412013-08-13T20:06:54Z Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies Bezazi, A. El Mahi, A. Berthelot, J.-M. Bezzazi, B. Научно-технический раздел The analysis of stiffness and the identification of rupture mechanisms during and after static tests of sandwich panels and their components have been investigated. The sandwich panels, having cross-ply laminates skins made of glass fibre and epoxy resin were manufactured by vacuum moulding and subjected to three-point bending tests. Two PVC cores of similar type but with differing densities were investigated. The effect of core density and its thickness on the behavior and the damage was highlighted. In terms of stiffness and load at failure, the sandwich structure has better mechanical characteristics compared to its components. Експериментально досліджено зміну жорсткості та проаналізовано механізми руйнування при статичних випробуваннях багатошарових композитних пластин і їх компонентів. Багатошарові композитні пластини з перехресними шарами зі скловолокна та епоксидної смоли, що виготовлені методом вакуумної відливки, піддавали навантаженню триточковим згином. Досліджували два варіанти пластин з однотипними наповнювачами з полі- вінілопласта різної щільності. Розглянуто вплив щільності і товщини внутрішнього шару наповнювача на поведінку та пошкодження композита. Показано, що композит із наповнювачем великої щільності має більш високі характеристики статичної міцності і стійкості порівняно з композитом із наповнювачем меншої щільності. Экспериментально исследовано изменение жесткости и проанализированы механизмы разрушения при статических испытаниях многослойных композитных пластин и их компонентов. Многослойные композитные пластины с перекрестными слоями из стекловолокна и эпоксидной смолы, изготовленные методом вакуумной отливки, подвергали нагружению трехточечным изгибом. Исследовали два варианта пластин с однотипными наполнителями из пеновинилопласта различной плотности. Рассмотрено влияние плотности и толщины внутреннего слоя наполнителя на поведение и повреждение композита. Показано, что композит с наполнителем большей плотности обладает более высокими характеристиками статической прочности и устойчивости по сравнению с композитом, имеющим наполнитель меньшей плотности. 2007 Article Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies / A. Bezazi, A. El Mahi, J.-M. Berthelot, B. Bezzazi // Проблемы прочности. — 2007. — № 2. — С. 88-98. — Бібліогр.: 29 назв. — англ. 0556-171X http://dspace.nbuv.gov.ua/handle/123456789/48041 539.4 en Проблемы прочности Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| topic |
Научно-технический раздел Научно-технический раздел |
| spellingShingle |
Научно-технический раздел Научно-технический раздел Bezazi, A. El Mahi, A. Berthelot, J.-M. Bezzazi, B. Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies Проблемы прочности |
| description |
The analysis of stiffness and the identification
of rupture mechanisms during and after static
tests of sandwich panels and their components
have been investigated. The sandwich panels,
having cross-ply laminates skins made of glass
fibre and epoxy resin were manufactured by
vacuum moulding and subjected to three-point
bending tests. Two PVC cores of similar type
but with differing densities were investigated.
The effect of core density and its thickness on
the behavior and the damage was highlighted.
In terms of stiffness and load at failure, the
sandwich structure has better mechanical
characteristics compared to its components. |
| format |
Article |
| author |
Bezazi, A. El Mahi, A. Berthelot, J.-M. Bezzazi, B. |
| author_facet |
Bezazi, A. El Mahi, A. Berthelot, J.-M. Bezzazi, B. |
| author_sort |
Bezazi, A. |
| title |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies |
| title_short |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies |
| title_full |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies |
| title_fullStr |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies |
| title_full_unstemmed |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies |
| title_sort |
experimental analysis of behavior and damage of sandwich composite materials in three-point bending. part 1. static tests and stiffness degradation at failure studies |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| publishDate |
2007 |
| topic_facet |
Научно-технический раздел |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/48041 |
| citation_txt |
Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 1. Static tests and stiffness degradation at failure studies / A. Bezazi, A. El Mahi, J.-M. Berthelot, B. Bezzazi // Проблемы прочности. — 2007. — № 2. — С. 88-98. — Бібліогр.: 29 назв. — англ. |
| series |
Проблемы прочности |
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2025-07-04T08:12:02Z |
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2025-07-04T08:12:02Z |
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| fulltext |
UDC 539.4
Experimental Analysis of Behavior and Damage of Sandwich
Composite Materials in Three-Point Bending. Part 1. Static Tests and
Stiffness Degradation at Failure Studies
A. Bezazi,ab A. E l M ah i,c J.-M . B erthelo t,d and B. B ezzazie
a Laboratoire de Mécanique et Structure (LMS), Université de Guelma, Algeria
b Department of Aerospace Engineering, University of Bristol, Bristol, UK
c Université du Maine, Le Mans, France
d ISMANS, Institute for Advanced Materials and Mechanics, Le Mans, France
e Laboratoire des Matériaux Minéraux et Composite (LMMC), Université de Boumerdes,
Boumerdes, Algeria
УДК 539.4
Экспериментальное исследование прочности и повреждения
многослойных композитных материалов при испытаниях на
трехточечный изгиб. Сообщение 1. Исследование разрушения и
понижения жесткости при статических испытаниях
А. Б езази аб, А. Э ль М ахив, Дж.-М . Б ертелог, Б . Б еззазид
а Университет г. Гуэльма, Алжир
6 Университет г. Бристоль, Великобритания
в Университет г. Ле Манс, Франция
г Институт перспективных материалов и механики, г. Ле Манс, Франция
д Университет г. Бумэрди, Алжир
Экспериментально исследовано изменение жесткости и проанализированы механизмы раз
рушения при статических испытаниях многослойных композитных пластин и их компо
нентов. Многослойные композитные пластины с перекрестными слоями из стекловолокна и
эпоксидной смолы, изготовленные методом вакуумной отливки, подвергали нагружению
трехточечным изгибом. Исследовали два варианта пластин с однотипными наполнителями
из пеновинилопласта различной плотности. Рассмотрено влияние плотности и толщины
внутреннего слоя наполнителя на поведение и повреждение композита. Показано, что
композит с наполнителем большей плотности обладает более высокими характеристиками
статической прочности и устойчивости по сравнению с композитом, имеющим наполни
тель меньшей плотности.
К л ю ч е в ы е с л о в а : пеновинилопласт, механизм разрушения, многослойные
композитные пластины с перекрестными слоями, статический трехточечный
изгиб, наполнитель.
© A. BEZAZI, A. EL MAHI, J.-M. BERTHELOT, B. BEZZAZI, 2007
88 ISSN 0556-171X. Проблемы прочности, 2007, № 2
Experimental Analysis o f Behavior and Damage
In tro d u c tio n . A sandwich composite material results from the bonded
assembly (or welding) o f two thin skins typically m ade o f materials having good
characteristics in tension (high strength and high Young’s modulus) and a much
thicker core w ith low density possessing good compression properties [1]. The
obtained sandwich structures combine lightness and stiffness. In the case of
bending, the stiffness and resistance increase quickly with the structure thickness.
Since only the external layers are taking m ost o f the imposed loads on the
structure, considerable benefit can be obtained by replacing the inner part (i.e.,
between the outside layers) with a very light core to obtain a sandwich material.
Sandwich structures are known to possess good resistance to weight ratios
compared to conventional materials. However, these structures can present
complicated failure mechanisms [2-4]. A lthough qualitatively it is well known
from the classical work o f A llen [5] that compressed sandwich panels sometimes
fail by a combination o f overall (Euler) buckling and local buckling (wrinkling)
of face plates, it is only recently that this has been formulated in a geometrically
non-linear framework. The interaction can lead to extremely unstable localized
buckling which is highly sensitive to initial imperfections in the geometry [6].
Triantafillou and Gibson [7, 8] studied the various modes o f degradations o f a
sandwich subjected to bending and classified them as follow:
- plastic deformation o f the skin;
- buckling o f the skin in compression (or wrinkling);
- rupture in shear, tension or compression o f the foam;
- indentation of the foam by the upper roller;
- rupture o f the interface core/skin.
It is found in the literature that the greater proportion of sandwich panels
tested to failure tend to fail when the face plate comes apart from the core surface
[9-11]. This type o f delamination was also noticed by Wadee and Blackmore
[12].
In the last decade, sandwich structures have appeared as ideal candidates in
m echanical applications where weight saving is of param ount importance to
ensure a m aximum effectiveness. It is undoubtedly in the fields of launchers,
shuttles and satellites that the problem o f w eight saving is m ost crucial. For
example, each kilogram saved on the launcher represents for ARIANE E.S.A.
(EUR) rocket a profit o f 30,000 US dollars in payload [13]. The first industrial
interest in sandwich composites occurred at the Second World War (during the
late 1940’s), when the use o f sandwich materials (laminated w ith a balsa core) as
structural elements for the ‘M osquito’ aircraft [14, 15] was first investigated in
Great Britain. Since then, the development of core materials continued in an effort
to reduce the weight of the sandwich laminates. The late 1940’s saw the arrival of
honeycomb core materials, developed mainly for aerospace industry. Honeycomb
cores currently offer the greatest shear strength and stiffness to w eight ratios; but
require care in ensuring a strong bond to the skin. The core materials have been
produced in various forms and developed for a range of applications, generally
using the hexagonal cell shape for an optimal effectiveness. The high cost of the
cores in honeycomb limited their application m ainly to aerospace industry. The
late 1950’s and early 1960’s saw the arrival o f the polyvinyl chloride (PVC) and
ISSN 0556-171X. Проблемы прочности, 2007, № 2 89
A. Bezazi, A. El Mahi, J.-M. Berthelot, and B. Bezzazi
polyurethane (PURE) core materials generally employed today in low and
medium cost applications [15]. PVC found widespread applications: in medium
and high velocity ocean liners [16, 17], in freezer trucks [18] and in numerous
sandwich structures for civil and military applications. However, effective
exploitation o f these cores require proper understanding o f their mechanical
behavior and properties. For m arine applications, the sandwich structures are
often used in hulls where high local stiffness exists to maintain the structural
integrity and hydrodynamic effectiveness. The late 1960’s were the first time
sandwich techniques were applied in m inesweepers o f the Swedish Royal Navy
such as the ‘V ikste’. Sandwich structures have been used m ainly because o f their
nonm agnetic properties and their resistance to underwater explosions [19].
Several researchers have indicated the insufficient reliability o f the standard
available procedures used to characterize foam core materials, even for the basic
properties such as shear strength and elasticity modulus [20, 21]. Moreover, very
little is known about the behavior o f foam core under im pact loading, particularly
significant for applications in mine counter-measure vessels and surface effect
ships. Only fracture mechanics was applied to the core [22] in order to model
delamination phenomena, frequent in the sandwich structures w ith thin coatings.
The fundamental principles o f sandwich m anufacturing and the investigation of
experimental and analytical methods have been originally described by A llen [5],
while Zenkert [14, 15] and Clark et al. [23] undertook further w ork in this subject.
Gibson and Ashby summarize in [24] the basic mechanical properties o f polymeric
foam core and in the description o f their specific cellular structure.
Because o f the critical applications o f composite sandwich materials,
understanding o f damage mechanisms and the prediction o f the fatigue life in
service are o f particular interest. Due to their constitution, the mechanical
properties o f these materials can be adapted by using various materials for the
skins (identical or not) and the core, and acting on the thickness o f each phase.
Accordingly, our contribution consists o f an experimental investigation of
composite sandwich materials behavior and the damage modes under three-point
bending tests, both in static and fatigue. The m aterial investigated consists o f two
cores o f expanded PVC foam, o f the same composition and o f different densities;
the skins being a cross-ply laminate glass/epoxy. This type o f sandwich material,
which associates good m echanical properties at a relatively low cost, is
particularly adapted to a w ide range o f industrial applications. Our analysis is
within an industrial context for w hich the means o f characterization and analysis
o f materials m ay be easily implemented.
1. M ateria ls and E x p erim en ta l Technique.
1.1. M a te r ia ls . Two types o f sandwich materials w ith different kinds o f cores
were investigated. The skins were cross-ply laminates (02/9 0 2 ) s consisting of
unidirectional glass fibre fabric with a surface density o f 300 g /m 2 and epoxy
resin SR 1500/SD whose principal characteristics are given in [25]. This lay-up’s
stacking sequence is chosen for this work since it was found to have strong
fatigue resistance compared to other stacking sequences investigated by the author
[26, 27]. Two similar types o f PVC foam o f different density were used. Herex
C70 55 and a C70 75 foams were expanded polyvinyl chloride (PVC) provided
by the Airex company and m arketed by SICOM IN company in panels o f 15 and
90 ISSN 0556-171X. Проблемы прочности, 2007, N2 2
Experimental Analysis o f Behavior and Damage
3
25 mm thickness. The two foams used were different in density: 60 kg/m for the
Herex C70 55 and 80 kg /m 3 for the Herex C70 75. The diameter o f the pores
varied between 620 and 880 /im for the C70 55 and between 280 and 500 /im for
the C70 75. These same types o f core were tested in shear, indentation and in
tension by Lolive [28]. The principal characteristics o f these foams provided by
SICOM IN are given in Table 1.
T a b l e 1
Characteristics of the Foams Used
Foam type Nominal Compression Bulk Tension Tension
density stress modulus stress modulus
(kg/m3) (MPa) (MPa) (MPa) (MPa)
C70 55 60 0.85 58 1.30 45
C70 75 80 1.30 83 1.95 63
Foam type Shear Shear Shear Impact Thermal Maximal
stress modulus failure resistance conductivity temperature
(MPa) (MPa) (%) (kJ/m2) (W/m- K) (°C)
C70 55 0.8 22 20 0.5 0.023 70
C70 75 1.2 30 30 0.9 0.025 75
The sandwich m anufacturing was carried out at the laboratory using a
vacuum bag m oulding technique. The m anufacture o f the sandwich, the skins and
the joining o f the core, was carried out at the same time w ith the laying up o f the
skin plies and then by interposing the core and the second skin. The sandwich
was im pregnated at room tem perature, and then was vacuum ed at a pressure o f
30 kPa for 10 hours inside the mould. Before any tests, the plates were left at
room tem perature for 2 to 3 weeks in order to allow a complete polym erization of
the epoxy resin.
As previously carried out by the authors [29], the specimens (foams, skins
and sandwich) were cut out using a diamond saw from plates o f 3 0 0 x 3 0 0 mm
according to ASTM C393-00 standard. Dimensions o f these specimens are given
in Table 2.
Sandwich SD 1 and SD 2 have the same core thickness o f 15 mm; they are
differentiated by the foam core density. The same foam (C70 75) core is used for
SD 2 and SD 3; the only difference lays in the thicknesses o f the core w hich were
respectively 15 and 25 mm.
1.2. E x p e r im e n ta l S e tu p a n d T est P ro c e d u re . Testing o f the specimens was
carried out in three-point bending (Fig. 1) using a universal hydraulic monotonic
testing m achine (INSTRON m odel 8516 o f capacity ± 100 kN) whose control and
data acquisition were perform ed by a computer. The applied load was m onitored
w ith a 5 kN load cell, the displacement by a LVDT sensor, and the deformation
using an extensometer. The supports were o f cylindrical shape o f 10 mm diameter
for the two lower supports and 15 m m for the central support. A m inimum o f five
tests were carried out for each type o f specimen, the loading rates in the static
tests being 5 m m /m in for the foam core and the sandwiches, and 2 m m /m in for
the skins.
ISSN 0556-171X. Проблемы прочности, 2007, № 2 91
A. Bezazi, A. El Mahi, J.-M. Berthelot, and B. Bezzazi
T a b l e 2
Specimen Dimensions of the Sandwiches Used
Specimen Dimension (mm)
Total
length L
Span
length l
Width b Specimen
thickness h
SD 1 (C70 55) 300 280 40 18.9
SD 2 (C70 75) 300 280 40 18.9
SD 3 (C70 75) 460 435 50 29.0
Laminated skin [(02/902)̂ ] 300 280 40 4.0
Foams C70 55 and C70 75 300 280 40 15.0
Fig. 1. Three-point bending experimental setup.
2. S ta tic Tests.
a) C ores. Foam core Herex C70 55 and C70 75 specimens were cut out with
a diamond disk or with a cutter, w ith dimensions (length and width) similar to the
sandwich specimens and were subjected to static flexural load tests. The resulting
load-displacem ent curves are given in Fig. 2. It was observed that the behavior of
the load versus displacement comprises three distinct stages:
60
15
0 ter------- ,---------1--------- ,---------1------ l_j--------
0 10 20 30 40 50 60
Displacement (mm)
Fig. 2. Influence of the 15 mm thickness cores [C70 55 (1) and C70 75 (2)] density on the
load-displacement evolution.
92 ISSN 0556-171X. Проблемы прочности, 2007, N 2
Experimental Analysis o f Behavior and Damage
- stage 1: for low values o f flexural displacements, the foam core has an
elastic linear behavior;
- stage 2: flexural stiffness decreases gradually in a nonlinear way;
- stage 3: the curve reaches a pseudo steady state beyond w hich the force
does not vary significantly, until catastrophic failure o f the specimen.
The comparison between the two foams shows that C70 75, the densest, is
m ost rigid w ith a larger displacement and load at failure. The presence o f the
oscillations can be interpreted by the viscoelastic characteristic behavior o f the
foam core.
b) S k in s . The two sandwich’s skins laminates were bonded to each other
using their curing resin, creating a new [(02/9 0 2 ) s ]s lay-up. A num ber of
specimens o f 4 mm thick and 40 mm long were made up o f this lay-up and then
subjected to a three-point bending static test w ith a distance between test rig
supports o f 285 mm. The load-displacem ent behavior o f the two skins laminate
o f sandwich elements SD 1 and SD 2 is presented in Fig. 3. It can be noticed that
the specim ens’ load-displacem ent behavior is linear until a displacement of
nearly 42 mm. Beyond this displacement, the behavior becomes nonlinear; this
was more likely due to a slip o f the specimens under the lower supports. No
catastrophic failure o f these specimens occurres during these tests.
800
600
i 400
o _i
200
0
0 20 40 60
Displacement (mm)
Fig. 3. Load-displacement behavior of the laminated skins [(0^902)s ]s.
c) S a n d w ic h e s . C o re T ype In flu e n c e . F igure 4 represents the load
displacem ent behavior for both sandwich specimens SD 1 and SD 2, obtained
from the three-point bending static tests. This evolution proceeded in various
stages: at the beginning o f the test, the load F increased linearly with
displacement, and then the behavior became nonlinear up to m aximum loading
where it decreases non-linearly for a short period before linearly and gradually
decreasing until the rupture o f the specimen. The latter occurred after an abrupt
fall o f the load. The initial linear behavior corresponds to that o f the skin laminate
in tension, whereas the nonlinear behavior depends on the properties o f the foam
core under the effect o f the indentation and shearing forces. Sandwich SD 2
having the densest core C70 75 was m ost rigid and had the largest rupture load
(Fig. 4).
ISSN 0556-171X. Проблемы прочности, 2007, № 2 93
A. Bezazi, A. El Mahi, J.-M. Berthelot, and B. Bezzazi
1500
1200
<2 900
T3
03
.3 600
300
0
Fig. 4. Influence of the sandwich [SD 1 (/) and SD 2 (2)] core type on the load-displacement
evolutions.
C ore T h ick n ess In flu e n c e . In order to highlight the influence o f the core
thickness on the static behavior o f the sandwich, two thicknesses o f foam core (15
and 25 mm) o f the same type C70 75 were investigated. Figure 5 represents the
evolution o f the load versus the displacement for the sandwich specimens SD 2
and SD 3. From the obtained load-displacem ent curves, it can be possible to
derive the following observations:
2000
1600
£ 1200
~o
03
° 800
400 - / '// i
0 \£.------------!------------- !------------- 1------------- !-----
0 5 10 15 20
Displacement (mm)
Fig. 5. Influence of the sandwich [SD 2 (/) and SD 3 (2)] core thickness in the load-displacement
evolutions.
As noted earlier, the load-displacement evolution o f sandwich specimen SD 2
w hich has a 15 m m foam core thickness, proceeded as follows: linear then
nonlinear passing by the m aximum loading followed by a small reduction in the
force and finally it suddenly and catastrophically fails. W hereas in the case of
sandwich specimen SD 3, which has a 25 mm foam core thickness, the load
displacement curve behaves linearly until around 7 mm, then it becomes slightly
non-linear until it reaches the m aximum load. Then the load-displacem ent curve
decreases non-linearly for a shorter period compared to SD 2 behavior before a
sudden drop in load and specimen rupture occurs. However, it can be noted that
the specimen still withstands load at around 800 N until a displacement o f 25 mm
where the test is then terminated.
94 ISSN 0556-171X. Проблемы прочности, 2007, № 2
Experimental Analysis o f Behavior and Damage
In addition, it was observed that the rigidities o f sandwiches SD 2 and SD 3
were practically identical for lower displacements (below approximately 7 mm).
The effect o f the thickness allowed an increase o f 37% o f the load at the rupture
o f sandwich specim en SD 3 com pared to that o f sandwich specim en SD 2
(Table 3).
T a b l e 3
Summary of the Sandwiches Mechanical Characteristics
and Their Components Determined from the Experimental Static Tests
Composite Mechanical characteristics
Stiffness (N/mm) Load at failure (N) Displacement
at failure (mm)
Foam
C70 55 1.01 28.86 47.38
C70 75 1.67 49.31 50.40
Laminate [(°2/9°2)s ]s 11.60 - > 60
Sandwich
SD1 C70 55 168.31 1051.11 11.60
SD2 C70 75 185.01 1400.60 11.51
SD3 C70 75 193.05 1929.79 12.22
d) C o m p a r iso n b e tw e e n th e S a n d w ic h S p e c im e n s a n d T h e ir C o m p o n e n ts .
Table 3 combined with the superposition o f the load-displacem ent evolution
curves o f the sandwich specimens and those o f their components (Fig. 6) m ake it
possible to compare their characteristics as detailed below:
S ti f fn e s s : the load at rupture and the mass o f the foam core were negligible
comparative to those o f the skins and those o f the sandwich specimen because of:
- the stiffness o f sandwich SD 1 was 166 times higher than that o f foam core
C70 55;
- the stiffness o f sandwich SD 2 was 110 times higher than that o f foam core
C70 75.
2000
1600
~ 1200
"O
° 800
400
0
0 20 40 60
Displacement (mm)
Fig. 6. Comparison of the sandwich load-displacement behaviors and their various components:
(/) skin; (2) C70 55; (3) C70 75; (4) SD 1; (5) SD 2; (6) SD 3.
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A. Bezazi, A. El Mahi, J.-M. Berthelot, and B. Bezzazi
S tiffn e s s va lu es o f the sandwich specimens were m uch higher than those of
the skins although the difference o f the masses was not significant. Consequently:
- stiffness and the load at rupture o f sandwich specimen SD 1 increased
respectively by factors o f 15 and 1.5 times compared to those o f the skins;
- stiffness and the load at rupture o f sandwich specimen SD 2 increased
respectively by factors o f 16 and 2 times compared to those o f the skins.
2.1. O b se rv a tio n s o f th e F r a c tu r e T o p o g ra p h ies . The analysis o f optical
m icroscopic observations o f the failed specimens under static tests shows that the
rupture o f the sandwich strongly depends on the type o f foam core. Indeed, the
failure o f sandwich specimens SD 1 was obtained essentially by the rupture o f the
upper skin and compression with an indentation o f the foam core in the vicinity o f
the central support (upper roller) as shown in Fig. 7a. On the other hand, the
failure o f sandwich specimens SD 2 was prim arily by shearing o f the foam
followed by a delamination between the two skins and the core (Fig. 7b).
a b
Fig. 7. Fracture topographies of Sd 1 (a) and SD 2 (b) sandwich specimens after static tests.
C onclusions. The behavior and damage propagation under static loading in
three-point bending o f three sandwich composite materials in one hand and their
components (skins and cores) on the other hand have been presented. The static
tests were able to determine the characteristics necessary for determining the
types o f fatigue tests and to identify the resulting mechanisms o f damages. The
load-displacem ent behavior o f the sandwich panels during static loading revealed
three distinct phases, and the final failure was not obtained until a sudden fall of
the load. Compared to its components, the sandwich structure possessed much
m ore desirable m echanical characteristics in terms o f stiffness and load at failure.
In addition to static studies, fatigue tests o f sandwich specimens were performed,
their results will be presented in Part 2.
Р е з ю м е
Експериментально досліджено зміну жорсткості та проаналізовано меха
нізми руйнування при статичних випробуваннях багатошарових композит
них пластин і їх компонентів. Багатошарові композитні пластини з пере
хресними шарами зі скловолокна та епоксидної смоли, що виготовлені
методом вакуумної відливки, піддавали навантаженню триточковим згином.
96 ISSN 0556-171X. Проблемы прочности, 2007, № 2
Experimental Analysis o f Behavior and Damage
Досліджували два варіанти пластин з однотипними наповнювачами з полі-
вінілопласта різної щільності. Розглянуто вплив щільності і товщини внут
рішнього шару наповнювача на поведінку та пошкодження композита. Пока
зано, що композит із наповнювачем великої щільності має більш високі
характеристики статичної міцності і стійкості порівняно з композитом із
наповнювачем меншої щільності.
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Received 10. 02. 2006
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