PSI - Issue 5

Sebastian Heimbs et al. / Procedia Structural Integrity 5 (2017) 689–696 Sebastian Heimbs, et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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loads was implemented into the connector element by defining maximum loads, after which it is removed. The hinge models were verified in dedicated simulations to check for their correct rotational and failure behavior.

4.4. Bird modeling

When a bird hits the target structure at the velocities of interest, it flows in a fluid-like manner over the structure with the large deformations of the spreading material being a major challenge for finite element simulations. Different numerical methods exist to model this fluid behavior, which are described in detail by Heimbs (2011). In this study, the smoothed particle hydrodynamics (SPH) approach was used, which is a meshless Lagrangian technique with the fluid being represented by a set of discrete interacting particles that can exhibit large deformations without the numerical problem of mesh distortion. The projectile geometry that is typically used is a cylinder with hemispherical ends that represents the torso of the bird. The 1 kg bird of this study was represented by a part with a length of 186 mm and a diameter of 93 mm and 60,736 SPH particles. Since real birds are mostly composed of water, a water-like hydrodynamic response can be considered as a valid approximation for the constitutive modeling of the bird projectile. A Mie-Grüneisen-type equation of state (EOS) of the type u s -u p was used in this study. For a separate validation of the bird projectile model independent of the target structure, impact test results of birds being shot against rigid plates or rigid edges as presented by Liu et al. (2014) and Allaeys et al. (2017) are typically used, where the pressure or force vs. time response is compared to ensure an accurate loading and stationary flow behavior. In this study, in-house test data from bird strike tests of 1 kg birds on rigid plates and rigid edges were used to verify both the correct load-time history and the correct splitting behavior of the SPH bird. As mentioned before, the critical bird impact location, which leads to the largest amount of structural damage, was initially unknown. Therefore, different impact locations on the searchlight pod and attachment strut (see Fig. 5) as well as on the searchlight itself (see Fig. 6) were simulated. This can be performed numerically in a very efficient way, while it would be irrationally expensive to perform such a study experimentally in terms of testing efforts and prototype costs. The initial impact velocity of the 1 kg bird was 85.9 m/s with a pitch angle of 0° in all simulations. It needs to be mentioned that in impact simulations on the searchlight pod, the searchlight was simplified as a point mass to reduce the number of elements for the sake of efficiency. For each individual load case the following damage indicators were assessed and compared: 1. the detachment of any parts, 2. the number and location of failed bolted joints, 3. the state of fracture and residual plastic deformation, 4. the energy plots to identify how much of the initial bird’s kinetic energy is absorbed by the structure through elastic deformation, plastic deformation and damage. From all the load cases shown in Fig. 5 and Fig. 6 it turned out that the impact position 5, which is the central impact on the searchlight pod, is the most critical one with the highest number of failed joints and the highest amount of damage. 50% of the initial kinetic energy of the bird projectile is absorbed by the structure through deformation and damage. The state of residual plastic deformation and damage to the inner housing, which exhibits significant cracking, is shown in Fig. 7. Furthermore, this is the only load case that leads to noticeable damage to the inner components of the searchlight pod. Indeed, as shown in Fig. 8, two of the major electrical components are detached by the impulse of the hitting bird and are accelerated towards the rear part of the structure. However, no further critical state of damage is generated as they do not penetrate the rear wall of the housing. Although this is the load case with the highest amount of damage, the structural response is still not critical as no detachment of critical parts occurs that could lead to a critical flight situation. This is also true for all the other load cases, where structural integrity of the impacted searchlight is maintained. Worth mentioning is impact position 4 in Fig. 5, where the side wall of the housing is torn open to a large extent and a major part of the bird intrudes into the searchlight pod housing, without having any further critical consequences. The impact on the attachment strut (position 1) is also not critical as the load level is far below the failure load. With the meticulous development, verification and validation of simulation methods and models based on the building block approach and successful demonstration of these methods in previous studies (see e.g. Heimbs et al. (2015) or Heimbs et al. (2017)), this numerical simulation study was accepted by the airworthiness authority as means of compliance to demonstrate the bird strike resistance of these external helicopter components. 5. Simulation results