Abstract
This study experimentally investigated the flexural fatigue behaviors of
honeycomb sandwich composites subjected to low velocity impact damage by
considering the type and thickness of the face sheet material, the cell
size and the core height parameters. First, the static strength of
undamaged specimens was determined by three-point bending loads, and
calculating their fatigue lives; secondly, the fatigue lives of the
specimens damaged by low velocity impact tests were then determined.
Increasing the face sheet thickness and core height increased the
flexural strength, while increasing the cell size decreased the flexural
strength. Low velocity impact damage decreased the flexural strength and
fatigue lives but increased the damping ratio for all specimens.
Keywords : Honeycomb Sandwich Composites; Bending Test; Low
Velocity Impact; Fatigue
Introduction
Composites are one of the fundamental materials in today’s engineering
applications. The reason for that popularity is because they combine
high strength with low weight. In particular, sandwich composites are
preferred in applications requiring low weight 1–3.
Usually, aluminum and Nomex honeycomb are used as a core material to
produce sandwich composites; on the other hand, carbon fiber-reinforced
composites (CFRP) and auxetic materials in various geometries have also
been used in recent years 3–6.
Honeycomb sandwich composites are formed by putting thick core
structures in between thin and thick plates. The bonding between plates
and honeycomb cells is achieved by structural adhesives that have
numerous application areas 7 Honeycomb structures are
defined by low weight and high flexural rigidity characteristics, but
they are also conveniently used to counter tensile and flexural loads8–10
Unlike metals, composites do not have a homogeneous and isotropic
structure. Damage is not seen due to local crack propagation. Fiber
damage occurs through the formation of different damage mechanisms like
matrix damage and delamination11–14. The most common
types of damage observed during honeycomb sandwich composite production
are “malformation and adhesive failures” 15. Jen et
al. numerically and experimentally investigated the amount of adhesive
used between an aluminum honeycomb core and a face sheet on the flexural
fatigue behavior of composites. They used three different amounts of
adhesive (0.4, 0.7 and 1 kg/m2) and determined that
increasing amounts of adhesive increased the flexural strength, and that
debonding between the honeycomb core and the face sheet caused permanent
fatigue damage 16. Subhani investigated the effect of
the curing temperature of the film adhesive – used for bonding
honeycomb cores and face sheet material together – on the flexural
strength of the composite. The result of the study revealed that the
optimum temperature and curing time for the film adhesive was 110°C and
2 hours, respectively 17. In another study performed
by Jen et al., their numerical and experimental investigation of
four-point bending fatigue behaviors of honeycomb sandwich composites
with different face sheet thicknesses, showed that the reason for the
resulting damage formed in the specimens was delamination between the
core and the face sheet materials 18. Abbadi et al.
investigated the fatigue behaviors of undamaged and damaged specimens by
using four-point bending loads. The results obtained indicated that the
damage formed had no effect on the static strength of specimens19. Schubel et al. stated that difficult-to-detect
damage like delamination formed in sandwich composites after impact
caused significant reductions in the strength of specimens20. Belingardi et al. investigated the effect of
adhesive failure between cores and face sheet material on the fatigue
life of honeycomb sandwich composites and reported that core crushing
was observed in the regions of adhesive failure 21.
Shi et al. investigated the effect of aramid fiber-reinforcement of the
interface to increase bonding between the honeycomb cores and face sheet
material and revealed that aramid fiber-reinforcement increased the
strength of the specimen under bending and compression loading22.
In the aviation industry, where honeycomb sandwich composites are
primarily used, airplanes are subject to impact damage due to external
factors like birds, stone and surface modes. Akatay et al.
experimentally investigated the behavior of repetitive low velocity
impact on 10 mm-thick sandwich composites produced using an Al 5052
alloy honeycomb core. Results revealed that a 110 Joules of impact
energy crushed the specimen at the first impact. More collisions were
necessary to crush the specimen as impact energy decreased. A maximum of
81impacts was found to produce the same damage at an impact energy of 3
Joules 23. Galehdari et al. experimentally,
analytically and numerically analyzed static and low velocity impact
behaviors of reinforced honeycomb sandwich composites and showed that
damage was usually observed in the shape of a “V”; their analysis
results were in accordance with experimental results24. Baba experimentally investigated low velocity
impact behaviors of curved sandwich composites with a foam core and
revealed that curved sandwich composites were stronger than flat
sandwich composites under low velocity impact loading25. He et al. investigated the effect of design
parameters on the impact behaviors of honeycomb sandwich composites. The
parameters affecting impact loading the most were found to be face sheet
thickness and honeycomb cell wall thickness, while core height was
determined to be the most ineffective parameter 26.
To calculate changes in the damping ratio of materials under fatigue
loading, hysteresis curves drawn using data obtained from tests can be
used 27. Another method used in determining the change
in damping ratio of a specimen is “damping change”28. Damping is defined as the amount of energy that
materials absorb under cyclic stresses 29.
In the present study, the damage behaviors of honeycomb sandwich
composites were experimentally investigated with three-point bending,
low velocity impact, and fatigue before and after impact. The static
strengths of specimens were determined by three-point bending loads, and
then they were subjected to fatigue tests to determine the fatigue lives
of undamaged specimens. Impact damage – a type of damage commonly
observed in applications where honeycomb sandwich composites are used –
was investigated by low velocity impact testing, and damaged specimens
were subjected to fatigue test, and then the effect of impact damage on
fatigue lives were investigated.
Material and Methods
The core material of the honeycomb used was Al-3003 alloy. Honeycomb
cores were placed in between two different face sheet materials, one
Al-5754 and the other a carbon fiber-reinforced composite plate.
Adhesion between the face sheet materials and the honeycomb cores was
provided by 3M 2216 epoxy-based adhesive.
The honeycomb cores had cell sizes of 6.35 mm and 9.525 mm in diameter
(D) and 10 mm, 15 mm and 20 mm in height (T). Face sheet materials had
various thicknesses: 0.5 mm, 1 mm and 1.5 mm. These materials produced a
composite sample of 80 mm in width and 135 mm in length. The honeycomb
sandwich composite layers are shown in Figure 1, while a specimen
labeling list and specimen properties for each label are shown in Table
1.