# The Flexural Fatigue Behavior of Honeycomb Sandwich Composites Following Low Velocity Impacts
## 1 Introduction  
Composites are fundamental materials in today's engineering applications. The reason for their popularity is because they combine high strength with low weight. In particular, sandwich composites are preferred in applications requiring low weight [1; 2; 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; 4; 5; 6]. Honeycomb sandwich composites are formed by putting thick core structures in between thin and thick plates. The bonding between the plates and the honeycomb cells is achieved by structural adhesives that have numerous application areas [7; 8]. Honeycomb structures are defined by low weight and high flexural rigidity characteristics, but they are also conveniently used to counter tensile, compression and flexural loads [9; 10; 11]  
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 delamination [12; 13; 14]. [PERSON] et al. investigated fatigue after impact behavior of carbon epoxy composites. Fatigue tests are conducted under tensile-tensile loading. Results showed that tensile strength and the fatigue life of samples decreased with low velocity impact [15]. [PERSON] et al. investigated the fatigue behavior of woven carbon-fiber-reinforcedplastic after low velocity impact. Impact damage decreased the fatigue life of samples [16]. [PERSON] et al. reported that the compression fatigue life of impacted samples reduced according to non-impacted samples. The damage growth of samples was inspected with a pulse-echo system. Results showed that the initial delamination growth occurred in the width direction and the delamination extended in the loading direction at the end of the fatigue life [17]. [PERSON] et al. investigated the compressive cyclic behavior of polymer matrix launch vehicle hardware. They reported that in most cyclic compression load cases, damage growth is not observed before ultimate failure or before the growth is very unstable [18]. [PERSON] et al. reported that stable damage propagation is perpendicular to the loading direction in CFRP composites under tensile loading conditions. [19] [PERSON] et al. presented an energy-based method to determine the delamination propagation under a compression load. They reported that stable damage propagation observed in the loading direction [20]. The most common types of damage observed during honeycomb sandwich composite production are \"malformation and adhesive failures\" [21]. [PERSON] 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 results of the study revealed that the optimum temperature and curing time for the film adhesive was 110 \({}^{\circ}\)C and 2 h, respectively [22]. In another study performed by [PERSON] 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 [23]. [PERSON] 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 specimens. It is reported that honeycomb sandwich panels' fatigue life is more effected by drilling a hole than a Brinell defect. [24]. [PERSON] et al. stated that difficult-to-detect damage like delamination formed in sandwich composites after impact caused significant reductions in the strength of specimens [25]. [PERSON] et al.