PSI - Issue 14

Akhilesh Kumar Jha et al. / Procedia Structural Integrity 14 (2019) 416–428 Akhilesh Kumar Jha et al. / Structural Integrity Procedia 00 (2018) 000–000

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fully or intermittently autonomously, by on-board computers. There is a wide variety of UAV shapes, sizes, configurations, and characteristics. UAVs perform a wide variety of functions. The majority of these functions are remote sensing, this is central to the reconnaissance role, that most UAVs fulfill, other functions include transport, research and development, to search for and rescue people in perilous locations etc. However, as UAVs tend to be smaller than conventional aircraft and with a limited fuel capacity, their flight times tend to be significantly lower than those of their manned counterparts. This issue becomes even greater when considering the payload of the vehicle, which can range from a set of surveillance payloads to a small store for based on user requirement. In order to improve this, a reduction of weight in the UAVs is paramount and the use of conventional aerospace materials might not be a feasible design option in the construction of UAVs. As a result, composite materials take a central role in the design and manufacture of drones. Composite materials have been extensively used in defense, automotive and aerospace applications attributed to their high stiffness and low weight. In these applications, they play a key role in absorbing the energy against impact loading. An impact event could range from a dropped tool, travelling at a low velocity (<10m/s), to high speed projectiles travelling at a few hundred meters per second. The structural response of the composite can be very different in these two scenarios, from minor damage to complete penetration of the composite. One of possible and very critical accidents is a bird strike on the aircraft/UAV during the flight mission. A collision with a bird during flight can lead to serious damage to the aircraft/UAV. All forward facing components, i.e. Engine propeller blades, Radome, forward fuselage skin as well as the leading-edge wings, Horizontal and Vertical stabilizers are prone to bird strike. Various certification requirements are set by the certification authorities depending on the parts. The final design and acceptance of the bird resistant components are typically dependent on the testing. The typical method of bird proofing an airplane is to build and test, then redesign and test again. The bird strike test is carried out in accordance with Federal Aviation Regulations (FAR) parts 23, 25 and 33. Earlier the tests used to involve a live chicken of an appropriate weight shooting against a structure that needs to be certified. For the simplicity, the sanitary and the repeatability reasons, the synthetic bird of an appropriate size and weight is now used. The GAS- GUN type shooting canon is used for the certification testing having 5-10 inches of diameter. High speed cameras, usually 10,000-20,000 frames per seconds are placed inside and outside the cockpit to capture the details of any failure. During the certification process, an aircraft must demonstrate its ability to land safely after being struck by a bird anywhere on the structure, at normal operating speeds. Impacted components must maintain structural integrity during the large transient loading resulting from bird strike loads. In order to comply this requirement, aircraft manufacturer needs to test the aircraft structure experimentally. This physical validation is time consuming and very costly as this involves number of iterations. Hence in order to optimize the number of experiments required for certification, FEM methodology is implemented. Finite element numerical simulations of bird impact on composite plates have been carried out to study the performance of progressive material damage models in predicting the structural response. The impact loading simulations are performed by using ABAQUS/Explicit. An existing constitutive material model capable of progressive damage modeling was used to describe material behavior of the laminate in the ABAQUS environment. The material model is implemented as an in-built ABAQUS VUMAT subroutine. Different methodologies available for fluid structure interactions are Lagrangian, Coupled Eulerian and Lagrangian (CEL) and the Smooth particle Hydrodynamics (SPH) methods. In the current study, CEL formulation with general contact in Abaqus/Explicit are used. 2. Bird impact theory There are three major categories of the impact event, i.e. Elastic Impact, Plastic Impact and Hydrodynamic Impact. These impacts are categorized based on the impact velocity, and the stresses generated in a projectile due to impact. The elastic impact is typically a low speed impact, and the stresses generated due to collision are lower than the materials yield stress. Therefore, the nature and duration of impact depends on the elastic modulus and elastic wave velocities of the materials. In case of a high impact speed, the produced stresses cause plastic deformation of the targeted materials. For this impact, materials strength is still a dominating factor, and hence such impact falls under the plastic impact category. Finally, for a high impact velocity, the stresses generated by the deceleration of the