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.