A Composite Sandwich Plate With a Novel Core Design
Anil Uzal,1 Fazil O. Sonmez,*1, Fatih E. Oz1, Kenan Cinar,1,2 Nuri Ersoy1
1Department of Mechanical Engineering, Bogazici University, Istanbul, Bebek 34342, Turkey
2Department of Mechanical Engineering, Namik Kemal University, Tekirdag, Corlu 59860, Turkey
In this study, a new core design is introduced for sandwich composite structures. Its strength
and failure behavior are investigated via three-point bending tests. E-glass-fiber-reinforced epoxy
resin is selected as the material for both the core and the face sheets. The core has an egg-crate
shape. Acoustic emission (AE) method is used to detect the progression of damage. Signals due to
elastic waves caused by activated damage mechanisms are investigated in order to identify the
corresponding failure modes. A finite element model of the sandwich structure is developed to
predict the failure behavior of the specimens under the loading conditions in the tests. A promising
agreement between the results of the finite element model and the experiments is observed. The
force-deflection- relation, the failure load as well as the region where damage initiates are accurately
Keywords: Sandwich panels; Composite materials; Acoustic emission; Out-of-plane loading;
Three-point bending; Finite element modeling
In this study, the failure behavior of a sandwich plate with an egg-crate shaped core design is
investigated. The structure is manufactured by vacuum infusion method. The resin used is epoxy and
the reinforcement is non-crimp fabric made of E-glass-fibers. In order to understand the failure
behavior under out-of-plane loading, the sandwich plates are tested using a three-point bending
setup under real-time acoustic emission monitoring. The collected AE data are post-processed and
classified according to their dominant frequency characteristics. It is found out that under transverse
forces, the sandwich plates can withstand loads up to 5 kN; first-ply failure occurs due to local
buckling of side walls at about 4.1 kN. In the deflection-controlled experiments, beyond the peak
load level, the structure is observed to withstand further deformation without catastrophic failure.
After some point, extensive fiber failures are observed; still the part can carry 1.8 kN load even after
the transverse deflection exceeds three times the plate thickness.
In order to predict the failure behavior of the sandwich plate, a Python Script code is29
developed to model the structure and simulate three-point bending tests. As the failure measure,
Tsai-Hill failure criterion is adopted. The failure load is found iteratively using the secant algorithm
and the outcome is compared with the experimental results. The region of failure, mode of failure,
and the load level at which failure initiates are predicted within reasonable accuracy. The FE element
model can predict the first-ply-failure load level within 5% error margin.
In this study, a procedure is proposed to determine the strength properties of the laminates
reinforced by non-crimp fabrics, using tension test and AE results for specimens with [0/45/−45/
90]𝑠 and [0/90]2𝑠 layup sequences and a progressive failure model. Using this procedure,
longitudinal and transverse tensile strengths, 𝑋𝑡 and 𝑌𝑡, and shear strength, 𝑆, can be obtained.
Structural design optimization of the core and comparison of the new sandwich design with
conventional sandwich designs like corrugated and foam filled sandwiches are left as future studies.
This paper is based on the study supported by Bogazici University Research Fund with grant
number 11168-16A06P2. The authors would also like to thank to Istanbul Development Agency,
which supported the development of the infrastructure of Composites Laboratory in Bogazici
University under project code ISTKA/BIL/2012/58.
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