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A. Masmoudi et alii, Fracture and Structural Integrity, 73 (2025) 41-58; DOI: 10.3221/IGF-ESIS.73.04 
 

41 
 

 

 
 
  
Experimental investigation on mechanical behavior of sandwich 
structures using Digital Image Correlation (DIC) 
 
 
Amina Masmoudi,  Abdelhak Khechai, Ahmed Bouaziz 
Laboratory of Research in Civil Engineering, Mohamed Khider University of Biskra, Algeria. 
amina.masmoudi@univ-biskra.dz , https://orcid.org/0009-0003-3212-7743  
a.khechai@univ-biskra.dz, ahmed.bouaziz@univ-biskra.dz 
 
Guerira Belhi  
Mechanical Engineering Laboratory, University of Biskra, Algeria. 
b.guerira@univ-biskra.dz 
 
Afaf Zeroual, Yassine Adimi 
Laboratory of Research in Civil Engineering, Mohamed Khider University of Biskra, Algeria. 
Afaf.zeroual@univ-biskra.dz, yassine.adimi@univ-biskra.dz 
 
 

 
  
KEYWORDS. Sandwich structure, PU, GFRP, Behavior, DIC.  
 

 

 
 

Citation: Masmoudi, A., Khechai, A., 
Bouaziz, A., belhi, G., Zeroual, A., Adimi, Y., 
Experimental investigation on mechanical 
behavior of sandwich structures using Digital 
Image Correlation (DIC), Fracture and 
Structural Integrity, 73 (2025) 41-58. 
 
Received: 25.02.2025  
Accepted: 10.04.2025 
Published: 13.04.2025  
Issue: 07.2025  
 
Copyright: © 2025 This is an open access 
article under the terms of the CC-BY 4.0, 
which permits unrestricted use, distribution, 
and reproduction in any medium, provided 
the original author and source are credited. 

 

 
INTRODUCTION  
 

andwich structures are extensively used as structural materials across diverse industries, including civil structure and 
construction. This increased use is due to their remarkable strength-to-weight ratio, corrosion resistance, and 
durability. The design of sandwich structures, with thin, strong skins and a thick lightweight core, provides stiffness 

in compressive, tensile, bending, and buckling loadings [1]. The incorporation of fiber-reinforced polymers (FRP) as skins 
S 

https://youtu.be/IinfxOwQT7w


 
 
 

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or facesheets is commonly applied in fabricating sandwich structures owing to their high specific strength/stiffness 
properties and improved fatigue life, and because of the complexity of their microstructure, the damage mechanisms should 
be fully studied [2,3]. One of the reasons of the wide application of sandwich panels is their lightweight nature, which 
facilitates the transportation and assembly[1]. This low weight is typically gained from the core material. Polyurethane (PU) 
foam appears as the most frequently core material used thanks to their good insulation properties, having low density, good 
impact, and shock absorption. As highlighted by Khan et al [4], polyurethane foam (PUF) cores have become a staple in the 
construction industry for sandwich structures. These structures are widely used in commercial, industrial, and residential 
buildings, functioning as both structural walls and non-structural elements. These papers [5–7] studied the application of 
sandwich structures in civil engineering demonstrating their versatility to various difficulties in bridges and building 
constructions such as floors, roofs or walls.  
Due to the complexity of sandwich structures, the requirement of studying and understanding their performance and failure 
characteristics have raised. Current research  have illustrated that the application of fiber reinforced composite sandwich 
structure in construction can be efficiently and economically [8]. Experimental and numerical investigations on the 
mechanical behavior of sandwich structures have been discussed by several researchers. Tuwair et al. [9] investigated 
different core alternatives for GFRP foam-infill sandwich bridge deck panels. Three polyurethane foam core designs were 
tested: a high-density foam, a grid filled with low-density foam, and a special trapezoidal design with GFRP reinforcement. 
They assessed compressive and tensile strengths through the flatwise compressive and tensile tests. In addition, Xie et.al[10] 
examined the mechanical behavior of fiber-reinforced polymer sandwich structures subjected to three-point bending and 
double-cantilever-beam tests. They used polyurethane foam of different densities, infused with galvanized metal tooth nails 
as core. Results showed the improvement of both shear and compressive strength with the increase of foam density. Also 
Cui et al [11] studied the mechanical behavior of sandwich panel under flexural and edgewise compression loadings. The 
panels were made of fiber-reinforced geopolymer composite faces and PU foam core. They had found that the failure modes 
differed with varying thickness-to- length ratio when the sandwich panels are subjected to edgewise compression.  
In order to comprehend and study the deformation and failure mechanisms of composites and sandwich structures, full 
field measurement is widely used. DIC is one of the methods used. DIC is a non-contact, optical measurement technique 
based on computer vision. Unlike the classical methods for displacement and deformation measuring such as pointwise 
strain gauge that only provide local strain at a selected point, DIC detects full field deformation on all over the surface of 
the sample[12]. Recently, numerous researchers have employed DIC to study the deformation of composite plates and 
sandwich structures[13,14]. Khechai et al [2] used DIC technique to obtain full-field strain of notched composite plates and 
found the results obtained were  roughly similar to the numerical outputs determined by finite element analysis. Also, 
Hosseini-Toudeshky and Navaei [15] characterized the interphase elastic modulus of glass/epoxy composites using DIC 
and FEM.  
The failure mechanisms of sandwich structures are determined by several factors: loading conditions, structure geometry, 
mechanical properties of the skin and core, and the interface between them. The use of sandwich structures exposes them 
to flat and edgewise compressive loads, resulting in complex failure modes [11]. Some of those modes are skin wrinkling, 
Euler macro buckling and macro shear buckling when the structure is under edgewise compression load, and core crushing 
and/ or densification when the structure is under flatwise compression load. Thus, it is required to monitor and measure 
the failure evolution to understand sandwich failure modes under those loadings.  
The objective of this study is to investigate the mechanical response of sandwich structures. These structures consist of 
GFRP skins enclosing a core of PU. The paper is divided into two main parts. The first part is to investigate the mechanical 
and morphological characterization of GFRP skin where tensile and compression tests were conducted, while the second 
part is focused on the mechanical characterization of sandwich structure subjected to flatwise and edgewise compression 
loadings. The compression tests are performed on the sandwich panels, following ASTM C365 for flatwise and ASTM C364 
for edgewise loading standards. Various sandwich structure lengths are tested under edgewise compression to understand 
the effect of using different geometries. During all the tests full -field displacement and full-field deformation were obtained 
using DIC 2D-Ncorr. 
 
 
MATERIALS AND METHOD 
 
Materials 

he sandwich structure studied in this work is composed of GFRP skins and PU foam as core. The GFRP skins are 
made by hand lay-up technique on a 50x50 cm² plate of glass after applying mold release on it. The materials used 
for the skins are Polipol 353 unsaturated polyester resin as the matrix with a density of 1.121 g/cm3 and chopped T 



 
 
 

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strand mat glass fiber; four layers of mat, outer layer of 300 gr/m2 glass fiber and three layers of 600 gr/m2 glass fiber. Tab. 
1 presents the mechanical properties of the used resin obtained through tensile test according to ASTM D638-14 [16]. The 
fiber weight fraction was determined by following the standard ISO 1172 [17] and it is estimated to be 36.55%. A CNC 
machine was used to cut the specimens from a 45x45 cm2 plate into the desired shapes, 5 cm were removed from all the 
edges of the plates to avoid discrepancy in thickness (Fig.1a). The core is made of open-cell polyurethane foam with a 
density of 50 kg/m3. To prevent the cutting effect, the skin and core of each specimen were cut separately and then 
assembled under cold pressure using epoxy resin as an adhesive (Fig.1b). All the materials used were commercially available. 
To create a random speckle pattern for DIC analysis, the specimens were coated with white paint and subsequently sprinkled 
with black paint. 
 

Tensile properties σmax (MPa) εmax (mm/mm) E (MPa) 

Unsaturated polyester resin 29.88 0.0132 3620.02 
 

Table 1: Mechanical properties of resin used. 
 

 
Figure 1: a) Molded GFRP plate, b) Manufactured sandwich structure. 

 
Mechanical testing: GFRP skin 
In order to investigate the mechanical performance of GFRP skin, experimental studies were carried out using an Instron 
5659 Universal testing machine. The mechanical properties of GFRP skin were investigated through compressive and tensile 
tests, in accordance with ASTM D695-15 [18]  and ASTM D638-14 [16] respectively. Fig.2 displays the specimens studied 
and their dimensions. Compression test methods for the skin require application of a compressive load with preventing 
buckling of the material [19]. Fig.3 shows the fixture for the test. Because the interface of the specimen is covered by the 
compressive fixture, the speckle pattern for DIC is painted on the side of the specimen.  
 
Mechanical testing: PU core 
Three specimens of 50x50x20 mm2 were used to acquire the mechanical properties of the used foam. Flatwise compression 
test was carried out according to ASTM C265 [20]. The load was applied on using cylindrical loading blocks with 
displacement rate of crosshead of 1 mm/min.  
 
Mechanical testing: Sandwich structure 
To study the mechanical behavior of the sandwich structure, flatwise compression test was carried out in accordance with 
ASTM C365 [20]. The load was applied on the specimens using cylindrical loading blocks (Fig.4a and Fig.4d). The edgewise 
compression test was performed according to ASTM D364 [21]. The compression load was applied on the specimens using 
an edgewise fixture and cylindrical loading blocks (Fig.4b, Fig.4c and Fig.4e). Two different geometries of samples were 
studied under edgewise compression load: w=L (S-60) and w<L (S-200). Tab. 2 summarizes the geometrical properties of 
sandwich structure studied. The crosshead displacement rate for both tests was 0.5 mm/min. During compression test for 

b 

Plate of glass 50x50 cm² 

GFRP plates of 45x45 

GFRP skin 

PU foam 

a 



 
 
 

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GFRP skin and both flatwise and edgewise compression tests the bottom plate was fixed and the load was applied using 
the top plate. 
 

 

Figure 2: Geometry of GFRP skin samples used for characterization a) compressive test. b) tensile test, dimensions are in mm. 
 
 

 
 

Figure 3: Compression test fixture for GFRP skin. 
 
Scanning electron microscopy 
The purpose of this study was to understand the failure mode and assess the efficiency of the fiber-matrix and skin-core 
bonding in samples. To achieve this, a microscopic examination was conducted using the Scanning Electron Microscopy 
technique at the Scientific and Technical Research Center in Physical-Chemical Analysis located in Biskra, Algeria. 
 
Digital image correlation 
DIC is an optical, full-field measurement technique that it based on computer vision. It is used in engineering and materials 
science to analyze the deformation and movement of objects or materials by tracking patterns or markers on their surfaces 
using digital image processing. DIC is widely used in areas such as structural mechanics and materials testing, to provide 
information on the behavior of materials and structures under different conditions. The methodology involves acquiring 
digital images of a surface at different stages of deformation, the analysis and comparison of these images to determine the 
movement of surface points, and finally the calculation of deformation maps [22]. The analysis of DIC and the subsequent 



 
 
 

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determination of strain involved the use of an open-source tool for Digital Image Correlation (2D) Ncorr [23] , which is a 
post-processing tool from MATLAB. To measure the displacement and strain, DIC-Ncorr code uses subsets of pixels; 
subset radius which refers to the size of a circular or square subset or region of interest (ROI) that is used to track and 
analyze the displacement or deformation of an object or material in an image sequence; subset spacing refers to the distance 
between the centers of adjacent subsets. The primary objective is to choose the smallest subset possible that avoids 
generating noisy displacement data [24]. In this study 43 pixels and 3 pixels were chosen for subset radius and subset spacing 
respectively. High-resolution images were recorded during the tests using camera CANON 1300D. A led source was fixed 
in front the machine to improve the illumination of the analyzed specimens. Fig.5 demonstrates the DIC setup used for all 
the tests for GFRP skin and sandwich specimens. 

 

 
 

Figure 4: Sandwich compressive setup a) flatwise compression. b) edgewise compression S-60. c) edgewise compression S-200. d) 
Flatwise compression setup. e) Edgewise compression setup. 

 
Dimensions Flatwise compression 

test 
Edgewise compression 

test 
S-60 S-200 

w: width (mm) 50 60 60 
L: length (mm) 50 60 200 

tt: Total thickness (mm) 28.62±2.3 29±1.12 27.83±3 
tc: Core thickness (mm) 21.3±2.1 21.8±1.08 20.51±3 
ts: Skin thickness (mm) 3.67±0.35 3.8±0.15 3.62±0.15 

 

Table 2: Geometrical properties of sandwich structure. 

 
a 

c 

b 

Edgewise 
compression 

fixture 

Compression 
loading blocks 

Specimen 

Load application 

 

Compression 
loading blocks 

Specimen 

Load application 

 

d 

e 



 
 
 

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Figure 5: Experimental setup for DIC analysis adopted, a) GFRP skin compression test, b) Flatwise compression for sandwich. 

 

 
 

Figure 6: a) Stress-strain curves for tensile test of GFRP skin. b) Specimens after tensile test. 
 
 

RESULTS AND DISCUSSION 
 
Tensile test for GFRP skin 

ig.6a shows the stress-strain curves from the tensile test for GFRP skin. It demonstrates that the curves behave 
identically and approximately overlay one another, which confirms the repeatability and reliability of the GFRP skin 
behavior under tensile load. It is observed that this behavior is divided into three parts as demonstrated. Firstly, the 

behavior is mainly linear up to the point where the elastic stress is about 12.57 % of the ultimate tensile stress. Then the 
curves lose their linearity and the damage starts, it is defined by the elastic modulus. At the end of the elastic part, the curves 
exhibit a bend that could be related to prior fiber damage caused by fragile bonding between fiber and matrix. The final 

F 

a 

b 

a b 



 
 
 

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phase is when tensile stress increases nonlinearly with longitudinal strain until final damage occurs. As illustrated in Fig.6b, 
the failure positions of the specimens after testing, were almost close to the center. Fig.7 demonstrates SEM image of 
fractured zone from the specimen under tensile test. One can see an irregular distribution of glass fibers, a debonding of 
fiber-matrix, fiber breaking and pull out. The maps in Fig.8 represent the in-plane strain fields of the GFRP skin under 
tensile loading for Sp 03. The DIC images were taken just before final failure. The chromatic map displays the maximal 
value of strain εy in red 0.03, which confirming the experimental results with high accuracy. The concentration of longitudinal 
strain indicates the location of the fracture in the specimen. The results obtained from the tensile test are summarized in 
Tab. 3. All samples have approximately the same strength, and the average value is 151.12 MPa with corresponding strain 
of 2.7%. 
 

 

 
 

Figure 7: SEM of fracture surface of GFRP skin under tensile failure. 

 
 

Figure 8: DIC engineering strain fields for tensile test of GFRP skin. 

Fibers breaks 

Debonding of 
fiber‐matrix 

x 

y εy εx εxy 



 
 
 

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Pmax 
(N) 

Displacement 
(mm) 

Elastic stress 
(MPa) 

Corresponding 
strain 

(mm/mm) 

Ultimate 
tensile stress 

(MPa) 

Tensile 
strain 

(mm/mm) 

Young’s 
Modulus 

(MPa) 

Sp 01 7331.99 4.47 15.54 0.0017 145.36 0.027 9656.84 

Sp 02 7645.72 4.34 20.04 0.002 150.80 0.026 9899.19 

Sp 03 8172.43 4.73 22.39 0.0023 162.86 0.029 9806.83 

Sp 04 7430.43 4.35 18.07 0.0018 145.44 0.026 9406.89 

Average 7645.14 4.47 19.01 0.0019 151.12 0.027 9692.44 

SD 8.23 0.18 2.91 2.65E-04 8.23 1.0E-03 214.97 
 

Table 3: Mechanical parameters obtained through tensile test of GFRP skin. 
 

Compression test for GFRP skin 
Fig.9a presents the stress-strain curves from the compressive test for GFRP skin. Similar to the tensile test, it is illustrated 
that the curves exhibit an identical behavior, which also approves the consistency of the GFRP skin behavior under 
compressive loading. Three distinct phases are noted. Initially, the behavior remains predominantly linear, reaching a point 
where the curves deviate from linearity and damage initiates; this phase is primarily characterized by the elastic modulus. 
Elastic stress is estimated by 2.17% of the ultimate compressive stress. At the end of the elastic phase, the curves exhibit a 
noticeable bend, possibly due to prior fiber damage resulting from weak bonding between the fibers and the matrix. The 
final phase involves a nonlinear increase in compressive stress with longitudinal strain until eventual failure or damage is 
observed. As it is shown in Fig.9b and Fig.10, the failure modes that occur in the specimens are localized at the center. It 
was evident that the observed failure involved a combination of longitudinal splitting, interlaminar cracking, fiber breakage, 
and the formation of kink bands, specifically manifested as fiber micro-buckling. Kim et al. had detected a comparable 
failure mode of GFRP composites subjected to compression [25]. Fig.11a illustrates the region of interest studied in DIC 
and Fig.11b presents full-field strain maps of Sp 03. The DIC images were captured just before final failure. The chromatic 
maps display the highest absolute values of strain εy 0.031 at the top, where the load is applied. On the other hand, it can be 
seen that the values obtained through DIC are similar to the those obtained experimentally (see Tab. 4). The maximum 
values of εx appeared in the middle of the specimen is 0.02, demonstrating the region of the local buckling happened to the 
sample. The results obtained from the compressive tests are listed in Tab. 4. The samples have an average strength of 226.70 
MPa and 3.4 % strain. 
 

 
 

Figure 9: a) Stress-strain curves for compression test of GFRP skin. b) Specimens after compression test. 
 

a b 



 
 
 

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Figure 10: SEM of fracture surface of GFRP skin under compressive failure. 
 

 
Figure 11: a) Region of interest studied in DIC, b) DIC engineering strain fields for compression test of GFRP skin. 

 



 
 
 

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 Pmax (N) Displacement 
(mm) 

Elastic 
stress (MPa) 

Corresponding 
strain 

(mm/mm) 

Ultimate 
compressive 
stress (MPa) 

Compressive 
strain 

(mm/mm) 

Young’s 
Modulus 

(MPa) 
Sp 01 12575.44 2.60 4.22 5.17E-04 229.74 0.032 9524.71 

Sp 02 11680.25 2.82 5.67 7.75E-04 215.89 0.035 8643.35 

Sp 03 11562.19 2.49 4.89 4.32E-04 220.44 0.031 9285.39 

Sp 04 13055.18 2.77 5.53 1.03E-03 240.74 0.035 9701.55 

Average 12218.27 2.67 5.08 6.88E-04 226.70 0.034 9288.75 

SD 718.31 0.15 0.66 2.69E-04 10.99 1.9 E-03 462.83 
 

Table 4: Mechanical parameters obtained through compressive test of GFRP skin. 
 

 
 Figure 12: Compressive Load-Displacement curve for PU foam. 

 
 
 
 
 
 
 
 
 
 

 
 

 

Table 5: Mechanical properties of PU foam. 
 

Flatwise compression test 
The results of the flatwise compression of PU foam are summarized in Fig.12 and Tab. 5. Load-Displacement curves of the 
studied sandwich under flatwise compression are shown in Fig.13. As it is marked, the zoomed-in region of the curves 
exhibits a similar behavior to polyurethane foam under compression test (Fig.12), which can be explained by that the core 
is the first part of sandwich to experience damage. It is noticed that the curves behave in a comparable performance; an 
elastic region (OA), followed by a plateau phase where the specimen starts to condense (AB), then a densification phase 
was noted where a rapid increasing of load with small displacement (BC). Fig.14 depicts the final failure of the specimens.  
Fig.15 illustrates the SEM micrograph of flatwise compression samples. The samples were cross-sectioned to obtain the 
SEM images Fig.15a. It is noted that no debonding between skin and core induced by testing is recorded in Fig.15b. This 
can be attributed to the efficient stress transfer facilitated by the strong interfacial adhesion between the GFRP skin and the 
PU foam core under flatwise compression. This loading mode promotes foam densification, which limits interfacial failure 

Properties PU foam 

Compressive stress (MPa) 0.44 

Compressive strain (mm/mm) 0.74 
Elastic stress (MPa) 0.1 

Corresponding strain (mm/mm) 0.4 
Compressive Modulus (MPa) 4.19 

Shear modulus (MPa) 1.49 
Poisson ratio 0.40 



 
 
 

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and enhances the structural response. The presence of epoxy resin at the interface further improves mechanical integrity 
due to its high compressive strength and stiffness, enabling effective load distribution between the skin and the core. 
Furthermore, significant compression of the PU foam led to extensive cell collapse, making most of the foam cells no longer 
distinguishable. It is worth noting that the localized gaps observed at the skin–core interface are not indicative of debonding 
initiated during testing, but rather result from the inherent surface porosity of the foam structure and minor surface 
imperfections of the skin. The zoomed-in part of the sample in Fig.15c exposes microcrack in the skin parallel to the loading 
fixture. This microcrack may have formed as a result of continuous micro-voids. DIC results of SP 02 are shown in Fig.16. 
The images were taken just before losing correlation, and the map faded caused by foam compression (point F in Fig.13).  

 
 

Figure 13: Flatwise compression Load-Displacement curves of sandwich structure. 
 
It is notable that the maximum displacement along the load direction (V) is corresponding to the top plate where the load 
is applied, colored in red, and it gradually decreases to zero towards the bottom fixed plate. However, the displacement 
traverse to the loading (U) indicates the expansion of the foam on both sides in opposite directions and tends to zero in the 
rest of points. Furthermore, the concentration longitudinal strain εy in blue signifies the densification of the PU foam. The 
results obtained from the test are listed in Tab. 6. The average ultimate flatwise compression strength and compressive strain 
are 20.99 MPa and 0.69 respectively. The total displacement of the specimens attains about 70% of its initial thickness. 
According to ASTM C-365M the ultimate flatwise compression strength was calculated using Eqn. (1) 
 

  max
fc

P

A
           (1)   

 
where  fc is the ultimate flatwise compressive strength (MPa). Pmax is the ultimate force prior to failure (N) and A is the 

area of the of the surface of facing subjected to compressive load (mm2).  
 
 



 
 
 

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Figure 14: Specimen after Flatwise compression. 

 

Figure 15: SEM micrographs for flatwise compression sample. 

 
 
 

Figure 16: DIC engineering displacement and strain fields for flatwise compression test of sandwich structure. 

Compression 
loading blocks 

Specimen 

εy εx εxy 

x 

y 

           U 
                                             mm   

              V 
                                           mm   



 
 
 

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 Pmax (kN) Displacement 
(mm) 

Ultimate 
flatwise 

compressive 
stress (MPa) 

Compressive 
strain 

(mm/mm) 

Sp 01 52.49 20.41 20.997 0.68 

Sp 02 52.43 18.31 20.993 0.70 

Sp 03 52.47 20.30 20.990 0.68 

Average 52.48 19.67 20.99 0.69 

SD 9.00E-03 1.18 3.50E-03 0.011 
 

Table 6: Mechanical parameters obtained through flatwise compressive test of sandwich structures. 
 

Edgewise compression test 
Fig.17a shows the failure modes of sandwich structures under edgewise compression loading for S-60. It is observable that 
Sp 03 behaved differently from the other specimens, this variability is a known characteristic of compression tests for 
sandwich structures, as slight differences in specimen alignment, material properties, or initial defects can significantly 
impact the failure mode and load response. An abrupt failure caused by skin buckling, succeeded by skin-core debonding 
on both sides for Sp 03 with a shear sliding damage of the skin on the left side. The other three specimens experienced the 
same behavior until final failure; localized buckling in the mid-section of the specimens followed by skin cracking on both 
sides and progressed throughout the width of the specimen and a small skin-core debonding on one side. Fig.17b depicts 
the load-displacement curves obtained from the test for S-60. The curve for Sp 03 demonstrates a decent linearity to ultimate 
load then the load decreased gradually to total failure. Meanwhile, the curves of the other three specimens exhibited a similar 
pattern and performed an initial phase of linear increase of the applied load, followed by a non-linear phase until the ultimate 
load is reached, then the load decreased gradually to final failure. The average ultimate load for those three is about 37.86 
KN, which is roughly twice compared of Sp 03. Fig.18a illustrates the collapse modes of S-200, where a significant difference 
in failures mechanisms is noticed. For S-200, different behavior was detected, the specimens collapsed in the global buckling 
mode or as it known Euler buckling. A densification of the foam was observed during the test for both S-60 and S-200. 
Fig.18b represents the load-displacement curves obtained from the test for S-200. The curves exhibit a small linear pattern 
succeeded by a non-linear phase until the ultimate load reached its maximum. Followed by softening phase of post-buckling. 
The divergence in failure modes of S-60 and S-200 samples under edgewise compression load signifies that the lengths of 
samples have a crucial impact. When the length is higher than the width, Euler buckling becomes the dominant failure 
mode, and the load capacity of the samples decreases. The ultimate load decreased from 39.76 kN to 5.1 kN with the of the 
length of specimens increased. DIC engineering displacement and strain fields for edgewise compression test for Sp 02 for 
both S-60 and S-200 are shown in Fig.19. Point A in Fig.17a and 18a indicates where the images were taken, before the 
correlation is lost.  The collapse modes of the specimens during this test are further clarified by the displacement along the 
horizontal direction U. The displacement U of S-60 is small compared to S-200. The maximum displacement for S-60 is 3.5 
mm and it is concentrated in the region, where it first buckled and damaged. While the maximum displacement for S-200 is 
14 mm in the middle part of the sample, with the top and bottom ends have minor displacement. Summarized results are 
listed in Tab. 7, for S-60 the average values of the test were calculated without including Sp 03. According to ASTM C-
364M the ultimate flatwise compression strength was calculated using the Eqn. (2) 
 

   max
EC

s

P

w t2. .
               (2)  

 
where EC  the ultimate edgewise compressive strength (MPa). Pmax is the ultimate force prior to failure (N), w  width of 

specimen (mm) and st thickness of a single face sheet (mm). 
 



 
 
 

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Figure 17: a) Specimens after edgewise compression test for S-60. b) Edgewise compression load-displacement curves for S-60. 
 

 



 
 
 

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Figure 18: a) Specimens after edgewise compression test for S-200. b) Edgewise compression load-displacement curves for S-200. 
 

 
 

Figure 19: DIC engineering displacement and strain fields for edgewise compression for sandwich; upper row S-60, lower row S-200. 
 
 



 
 
 

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  Pmax 
(kN) 

Displacement 
(mm) 

Ultimate 
edgewise 

compressive 
stress (MPa) 

Compressive 
strain 

(mm/mm) 

S-60 Sp 01 39.77 4.16 87.21 0.069 

 Sp 02 40.16 5.04 95.63 0.084 

 Sp 04 33.65 2.99 73.79 0.049 

 Average 37.86 4.06 85.54 6,73E-02 

 SD 3.65 1.03 11.01 1,76E-02 

 Sp 03 20.08 0.98 44.87 0.016 

S-200 Sp 01 5.10 14.78 11.34 0.074 

 Sp 02 8.18 13.38 17.93 0.067 

 Sp 03 6.18 16.30 14.11 0.082 

 Average 6.49 14.82 14.46  0.074 

 SD 1.56 1.46 3.31 7.00E-03 
 

Table 7: Mechanical parameters obtained through edgewise compressive test of sandwich for S-60 and S-200. 
 
 

CONCLUSION 
 

he behavior of sandwich structure subjected to flatwise and edgewise compression was investigated. Digital image 
correlation was used to study full fields measurements. Glass Fiber Reinforcement Polymer composites were used 
as skin and PU foam as core. Tensile and compression tests for GFRP skins were conducted to obtain mechanical 

characteristics. Samples of 50x50 mm2 of sandwich structure were studied under flatwise compression loads. For the 
edgewise compression test two different geometries of sandwich structures were tested 60x60 mm2 and 60x200 mm2. All 
the tests were performed according to ASTM standards. The results obtained can be summarized as follow: 

 The use of DIC throughout all tests demonstrated strong agreement with the results obtained from mechanical 
testing, confirming the accuracy and reliability of DIC measurements. 

 GFRP skins illustrated an ultimate tensile strength of 151.12 MPa and a compressive strength of 226.70MPa. the 
microscopic study shows different mechanisms; debonding of fiber-resin, fiber breaking and pull out for tensile 
samples and combination of longitudinal splitting, interlaminar cracking, fiber breakage for compressive samples.  

 Sandwich structures encountered condensation and densification of PU foam with micro cracks in GFRP skin 
under flatwise compression test and it reveals an ultimate strength of 21 MPa and 0.70 ultimate strain.  

 Different failure mechanisms were distinguished for sandwich structures under edgewise compression load. With 
length equal to the width the samples experienced skin-core debonding, shear sliding damage of the skin, and 
localized buckling in the mid-section of the specimens followed by skin cracking on both sides and progressed 
throughout the width of the samples. While when length is much greater than the width the global buckling mode 
is the dominant collapse mode. The ultimate edgewise compressive strength is 85.54 MPa and 14.46 MPa for S-60 
and S-200 respectively. 
 
 

DISCLOSURE STATEMENT  
 

o potential conflict of interest was reported by the authors. 
 
 

 
 

T 

N 



 
 
 

A. Masmoudi et alii, Fracture and Structural Integrity, 73 (2025) 41-58; DOI: 10.3221/IGF-ESIS.73.04 
 

57 
 

DATA AVAILABILITY STATEMENT  
 

he authors confirm that the data supporting the findings of this study are available within the article. 
 
 

 
 
FUNDING 
 

he author(s) received no financial support for the research, authorship, and/or publication of this article. 
 
 

 
 
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    /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke.  Stvoreni PDF dokumenti mogu se otvoriti Acrobat i Adobe Reader 5.0 i kasnijim verzijama.)
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    /NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.)
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    /ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing.  Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.)
  >>
  /Namespace [
    (Adobe)
    (Common)
    (1.0)
  ]
  /OtherNamespaces [
    <<
      /AsReaderSpreads false
      /CropImagesToFrames true
      /ErrorControl /WarnAndContinue
      /FlattenerIgnoreSpreadOverrides false
      /IncludeGuidesGrids false
      /IncludeNonPrinting false
      /IncludeSlug false
      /Namespace [
        (Adobe)
        (InDesign)
        (4.0)
      ]
      /OmitPlacedBitmaps false
      /OmitPlacedEPS false
      /OmitPlacedPDF false
      /SimulateOverprint /Legacy
    >>
    <<
      /AddBleedMarks false
      /AddColorBars false
      /AddCropMarks false
      /AddPageInfo false
      /AddRegMarks false
      /ConvertColors /ConvertToCMYK
      /DestinationProfileName ()
      /DestinationProfileSelector /DocumentCMYK
      /Downsample16BitImages true
      /FlattenerPreset <<
        /PresetSelector /MediumResolution
      >>
      /FormElements false
      /GenerateStructure false
      /IncludeBookmarks false
      /IncludeHyperlinks false
      /IncludeInteractive false
      /IncludeLayers false
      /IncludeProfiles false
      /MultimediaHandling /UseObjectSettings
      /Namespace [
        (Adobe)
        (CreativeSuite)
        (2.0)
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      /PDFXOutputIntentProfileSelector /DocumentCMYK
      /PreserveEditing true
      /UntaggedCMYKHandling /LeaveUntagged
      /UntaggedRGBHandling /UseDocumentProfile
      /UseDocumentBleed false
    >>
  ]
>> setdistillerparams
<<
  /HWResolution [2400 2400]
  /PageSize [612.000 792.000]
>> setpagedevice