Analysis of bird strike on metallic panels

Abstract

This thesis investigates the phenomenon of bird strikes, using a combination of literature analysis, statistical analysis, and theoretical models. The study focuses on the potential damage that bird strikes can cause to various parts of an aircraft, which are wind-facing components such as wings, stabilizers, engines, and windshields. The variety of possible outcomes from a bird strike poses a significant threat to aviation safety, as bird strikes account for 90% of Foreign Object Damage (FOD) incidents. As a result, aviation regulations require aircraft to meet specific levels of bird strike tolerance for critical components, and there are a number of certification requirements that airplanes must meet to be regarded safe to fly. To investigate the bird strikes on aircraft, the study uses numerical models, including the Smooth Particle Hydrodynamics (SPH) model, which was used to simulate sandwich plate bird impact experiments. The study concludes that the SPH model may be useful for finite element bird strike case analyses, which can help to improve aviation safety by identifying potential vulnerabilities and developing effective prevention measures. When using a new numerical approach, it is important to compare the results to experimental data to ensure that the simulation accurately reflects reality. Many research studies have included both numerical simulations and experimental data to understand how well the simulation corresponds to real-world scenarios. Experimental studies have traditionally guided aircraft designers in creating structures that are tough enough to withstand bird strikes. However, as aircraft components have become more complex, it has become necessary to develop bird strike simulation programs to design aircraft parts that are both airworthy and can be produced quickly and economically. Furthermore, the optimization process typically involves many iterative steps, which makes computer-based analyses more efficient and cheaper than experiments. However, conducting experiments with real birds, which are often dead or drugged chickens, presents a number of issues. The reproducibility of experiments, the health of researchers, and the availability of suitable bird models are all concerns. Real bird torsos vary greatly, making it difficult to obtain consistent results. While certification regulations only define the mass properties of the bird, different bird species have different densities, leading to variations in pressure loads between tests. As a result of these difficulties, researchers have begun using substitute bird materials instead of real birds. Advancements in computer technology have led to the development of cheaper and more advanced finite element software since the 1980s. This has allowed scientists to analyze bird strikes numerically due to the low cost, speed, and repeatability of the analyses. Various substitute bird models have been investigated in studies, and results have been compared with experimental data. The simple cylinder geometry is still a valuable approach to compare simulation results with experimental data. Different geometries such as spheres, cylinders with flat or hemispherical ends, and ellipsoids may also be used in simulations. When birds are struck at high speeds, their behavior is different from that of a simple elastic solid, and it is the responsibility of scientists and engineers to study the behavior of bird materials both theoretically and experimentally. Statistical data related to bird strikes is provided in the thesis, and it is emphasized that front-facing components of aircraft are the most critical as they are most likely to encounter a direct bird strike. The most frequently struck parts of an aircraft are the fuselage, nose, radome, windshield, wing, rotor, and jet engine. Approximately 70% of bird strikes occur at altitudes between zero and 152 meters, which is primarily during takeoff and landing. This information is useful in avoiding bird strike accidents. As the altitude of an aircraft increases, the natural habitats of birds become further from the plane. The velocity of the projectile has a significant impact on how it responds upon impact. The behaviour of the projectile can be divided into five categories based on the internal stresses it experiences: elastic impact, plastic impact, hydrodynamic impact, sonic impact, and explosive impact. Elastic impact occurs when the projectile material strength is well above the internal stresses caused by the low speeds and accelerations, resulting in the projectile bouncing back from the surface. As the impactor velocity increases, the projectile enters the plastic behavior region, yet the velocity is still low enough to maintain fluid-like flow behavior, causing the bird to spread in every direction parallel to the plate, and the load to expand to a larger area. The theory behind bird strike at velocities that cause the bird to act in the hydrodynamic region is investigated. When the impactor with the initial velocity hits a surface, materials in contact with the rigid plate would immediately come to rest, generating a shock wave with velocity normal to the plate and towards the impactor body. There would be a significant pressure gradient at the outer surface because there is shock load pressure on the inner side and free surface pressure on the outer side. Soft objects impacted at high velocities behave differently than at low velocities, such that even elastic solids behave like liquids. However, testing with real birds can yield scattered data and it is not ethical to kill animals for scientific purposes. Gelatine has been found to be a suitable artificial substitute material with uniform characteristics and can be shaped into simple geometries such as cylinders and spheres for easy handling. Finite element programs offer various solution methods for bird strike simulations. Lagrangian method involves nodes attached to the material while Eulerian method uses fixed nodes in a defined space where material flows through it. Arbitrary Lagrangian Eulerian method is another option that allows for the defined space to change with the material flow, leading to faster computation time. Additionally, the meshless method called smooth particle hydrodynamics allows for particles to move freely without mass distortion. Various basic shapes of birds can be examined for bird strike impacts, including a cylinder, a cylinder with hemispherical ends, an ellipsoid, or a sphere. For a bird with a mass of 1.8 kg and specific geometric parameters, the density of the bird can be determined to be 900 kilograms per cubic meter. Conversely, by using a standard density of 950 kilograms per cubic meter and entering the mass of the bird, a specific volume value can be determined and used to specify the bird's geometry. Honeycomb materials provide stiffness to the structure while not adding too much mass. Hence, honeycombs are a kind of deformable shock absorbers that is widely used in the aircraft industry. In the reference tests, they used single and double core honeycomb sandwich metal plates as specimens under bird strike. They made a correlation between test results and simulation results which can be beneficial. Modelling the material of honeycomb in LS-DYNA has a number of challenges. Firstly, honeycomb has a complex geometry which is expensive to model and simulate with shell elements. Therefore, its effective behavior can be modelled under homogenized solid elements. Out of plane stress strain curve up to crushing was given at reference. Which can be inserted as a stress strain curve to the solid elements. Particle node quantity for the bird impactor and element number for the aluminum sheets and honeycomb is limited with the computer power. Therefore, node numbers are generally about 20519 for the bird material. The simulations provide spatial displacement values and nominal strain curve values that are generally similar to the experimental results. However, there are slight differences, which may be due to errors in both the simulations and the tests. Overall, the strain values align well with the experimental data for both simulations. Therefore, the SPH method can be effectively used to simulate bird strikes on honeycomb sandwich plates, which is advantageous since experimental studies can be time-consuming and costly, especially in the initial design phase of aerospace vehicles.