Numerical and experimental modeling for bird and hail impacts on aircraft structure

Proceedings of the IMAC-XXVIII February 1 4, 2010, Jacksonville, Florida USA 2010 Society for Experimental Mechanics Inc. Numerical and experimental modeling for bird and hail impacts on aircraft structure M.-A. Lavoie*, A. Gakwaya*, Marc J. Richard*, D. Nandlall**, M. Nejad Ensan*** and D.G. Zimcik*** *Department of Mechanical Engineering Laval University 1065 avenue de la Médecine Québec, QC, G1V 0A6 **Defense Research Development Canada Valcartier, QC ***Institute for Aerospace Research Building U66A, Uplands Ottawa, ON, K1A 9R6 Abstract Aircraft bird-strike events are very common and dangerous. Hailstone impacts represent another threat for aircraft structures. As part of the certification process, an aircraft must demonstrate the ability to land safely after impact with a foreign object at normal flight operating speeds. Since experimental studies can be cost prohibitive, validated numerical impact simulation seems to be a viable alternative. Modelling of these soft body impacts still represents a challenge, involving modelling of both the target and the projectile. Here the smooth particle hydrodynamics method (SPH), which has been used successfully in ballistic applications involving bird strike scenarios, is extended to hail impact. The paper thus presents the meshless SPH numerical method as a novel modeling approach. The method is applied to model bird and hail impacts which are problems that traditional FEM based modeling methods typically struggle to solve because of involved mesh distortion problems. The numerical results are then evaluated by comparing with the data collected during recent experimental tests. The data acquisition methods are also described and evaluated for applications where the short duration of the impact presents a challenge. The accuracy of the numerical results allows us to conclude that the models developed can be used in the certification and/or design process of moving (aircraft) and stationary (wind turbines) composite structures subject to bird and hail impact. Keywords: bird & hail impact, SPH method & experimental validation 1. Introduction Nowadays, predictive numerical methods are an intrinsic part of aircraft and other high performance design. This has been strongly motivated by the relatively low cost of simulating events before conducting destructive tests and by the always increasing accuracy and efficiency of numerical methods. When modeling an impact event, the components are typically classified into two categories: the projectile and the target. A great deal of finite element work has been performed to develop element and material models that can accurately predict the behaviour of metallic and composite materials under large deformations induced by high velocity loadings. Although less frequent, impacts with soft bodies also pose a threat to structures. However, the amount of information and number of tools available to produce an accurate numerical model of such projectiles are usually limited. Because of its low computational cost, its reasonable precision and stability compared with finite elements method (FEM) and, more importantly, because of its ability to handle large distortions by avoiding the need for intensive FEM remeshing, SPH is a competitive approach compared to FEM and is increasingly being used in

some fast-transient dynamics problems [1]. Several authors have proposed to couple FE and SPH which seems a reasonable approach in order to benefit from the advantages of both formulations [2,3]. A simple way to describe the SPH method is to imagine a structure finely meshed with solid elements where the elements themselves are discarded and only the nodes are kept. The connectivity between the nodes no longer depends upon the mesh but rather on the proximity of the neighbouring nodes, now called particles. All information such as stress, displacement, mass and density are now computed at each node. It also includes the contribution of each neighbouring particle which is proportional to the proximity of each given neighbour. Suitable information on the mathematics of meshless methods can be found in Nguyen et al. [4]. In a first attempt to model birds and hail as projectiles, it was observed that the experimental information was relatively limited. Hence, bird strike and hail impact modeling was sponsored by the Consortium for Research and Innovation in Aerospace Quebec (CRIAQ) and bird and hail tests were respectively conducted at the Defence Research and Development Canada (DRDC) and the National Research Council Canada (NRC) air gun facilities. This paper deals with the experimental studies of bird and hail projectiles that have been performed in order to increase the experimental data available as well as provide validation with an efficient numerical method, the smoothed particles hydrodynamics (SPH). 2. Bird impact tests In recent years, efforts were increased to model the bird impact event and predict the viability of aircraft structures prior to the mandatory expensive destructive certification procedures. Different modeling techniques were studied [5] as well as ways to evaluate the reliability of the obtained numerical results. Moreover, since the available experimental data were collected over thirty years ago with the available instrumentation [6], new tests were conducted and results were published by Lavoie et al. [7]. This paper describes briefly the developed numerical SPH bird model and compares the obtained results with those from new experimental data. During the experimental set-up, a 1 kg gelatine bird substitute impacted a rigid 12.7 mm thick steel plate at a velocity of 95 m/s (185 knots). The plate was of 0.3048 m by 0.3048 m side dimension with an elevated edge of 12.7 mm wide by 6.35 mm thick. A high-speed video camera was used to capture the behaviour of the projectile during the impact and frames were taken at a frequency of 3000 frame per second (fps). The numerical model was created in LS-DYNA and included a steel target modeled with solid elements and the bird, modeled with approximately 4500 SPH particles. An automatic nodes-to-surface contact controls the interaction between the projectile and the target. The developed numerical model is shown in Figure 1. Since theory tells us that the expected behaviour of the bird during impact is similar to a fluid, an elastic material model is used for the plate and an elastic-plastic-hydrodynamic material model with a polynomial equation of state is used to model the bird. The physical properties of the gelatine are a density of 950 kg/m 3, a shear modulus of 2 GPa, a yield stress of 0.02 MPa and a plastic hardening modulus of 0.001 MPa. Figure 1 Numerical model for the SPH impact on a rigid flat plate

Snap-shots are compared for the obtained experimental and numerical results in terms of deformations from the beginning of the impact in Figure 2 at time intervals of 0.0066 s. A very good correlation can be observed between the two sets of data. Figure 2 Impact at 0 at time intervals of 0.66 ms, video camera and SPH method Moreover, the deceleration of the end of the projectile and the increase of the diameter of the projectile were measured to provide a more quantitative approach. Those are plotted in Figure 3 where an experimental curve is given for three typical tests conducted. Although the time intervals are relatively large for the experimental data, the trend of both numerical and experimental results is very similar. Figure 3 Variation of velocity (left) and diameter of the projectile (right) for perpendicular test The accuracy of the SPH bird model can be assessed in other ways. For instance, it is possible to incline the target and measure the diameter and velocity of the projectile. It is also quite current to compare the pressure reading at the center of impact and the radial pressure distribution with the analytical and experimental values. This was done using carbon gages. However, the results were very inconclusive and are not presented here. For

more information on the analytical approach and the experimental approach the reader is referred to Lavoie et al. [5] and Lavoie et al. [7], respectively. The general conclusion drawn from the results presented here is that the SPH method yields results that are sufficiently reliable for it to be used in a more complex numerical simulation where the unknown are related to the structure and not the projectile. Moreover, there is no mass loss or significant decrease in the timestep as the deformation of the projectile increases. These properties make the SPH method very well suited for secondary impacts where damage caused by rebound on nearby structures is of concern. 3. Hail impact Another important threat to aircraft is when they are stationary and hail storms occur. In such instances, the velocity of hail impacting is approximately of 25 m/s, which is the velocity of free fall for a 0.04 m hail ball. So the minimal velocity at which the tests/simulations should be performed is 25 m/s [8]. Although hail is an easier projectile to manufacture when compared to a gelatine bird, very little theory is found in the literature as to the expected behaviour during impact. One interesting point is that in general, the density of hail is lower than that of ice. The nominal density of fresh water ice is 917 kg/m 3 whereas the average density of hail is 846 kg/m 3 [9-12]. As opposed to bird impact, no theory was found regarding the impact behaviour of hail. However, it is reasonable to assume that the behaviour is brittle upon impact. According to the literature [10,11], the elasticplastic with failure model is suitable for such an application. The purpose of the hail tests was to conduct preliminary tests. Moreover, given the difficulties encountered using the carbon gages to measure the pressure during the bird impact, it was suggested to use pressure sensitive film instead. The hail tests made it possible to evaluate the performance of pressure sensitive film for pressure acquisition. The hail itself is made by compressing snow until it turns into ice and reaches the desired density. This allows a better control of the density and it is easier to obtain the proper shape. The concept of the 0.04 m die is shown in Figure 4. Figure 4 0.04 m die to create hail The pressure sensitive film used to measure the impact was purchased from Fuji Film. Microballoons containing dye explode at a given pressure and the concentration of the coloration indicates the pressure reached. Only one pressure is obtained for the whole event and corresponds to the peak pressure of the impact. The pressure was determined by comparing the color concentration over an area of 0.5 mm by 0.5mm to a provided scale.

In addition, a high speed video camera was used during these tests. Issues encountered with the digital camera made it impossible to present results of the hail impacting. This was mainly due to the lack of resolution of the camera used. Since the impact only lasts 1.2 ms, a minimum acquisition speed of 10,000 fps would be required to have about 10 frames during the impact. However, the camera available was limited to 5,000 fps and at such a speed, the actual window captured was very small. It was nevertheless possible to observe that the hail is brittle upon impact and does not spread the way gelatine does. It is interesting to note that the zone of impact for which a pressure was captured by the pressure film is less than the diameter of the projectile. For the various tests, the diameter of the area of impact varied between 1.8 mm to 2.3 mm which can be compared later with the numerical results. Pressure results are shown for a 35 gr hail projectile impacting the rigid target at a velocity of 45 m/s. A high pressure sensitive film was used since the medium film seemed to be saturated and did not indicate the proper maximum pressure. Figure 5 shows, on the left, the actual pressure sensitive film after impact and on the right, the dye concentration measured by the computer. The pressure along the horizontal and vertical centerlines is plotted in Figure 6. Figure 5 Pressure sensitive film and measurement Figure 6 Horizontal & Vertical line profile The SPH model created to simulate the hail uses the elastic plastic with failure material model. The density was set to 846 kg/m 3, the elastic shear modulus to 3.46 GPa, the yield strength to 10.30 MPa, the hardening modulus to 6.89 GPa, the bulk modulus to 8.99 GPa, the plastic failure strain to 0.35 and the tensile failure pressure to 4.00 MPa [10,11]. A hail stone of 40 mm. was modeled using 587 particles. It was impacted on a target 0.2 by 0.2 m and 6.35 mm thick. The target had three elements in thickness and the mesh was refined to a size of 2.5 by 2.5 mm in the zone of impact. The properties of aluminum were used to model the plate with an elastic

material model. The resulting mesh includes 600 SPH elements for the projectile and 5000 solid elements for the target. The projectile and the point of impact on the plate are illustrated in Figure 7. Figure 7 Refined mesh for target To compare the numerical results with the experimental results, the maximum interface pressure was plotted along the centerlines. In order to obtain a valid comparison with the experimental pressure, for which the area used was of 0.5 mm by 0.5 mm, the mesh in the impact area was made as small as possible for the simulation to work properly and lowered to 2.5 mm by 2.5 mm. This results in the area over which the pressure is calculated being 25 times larger for the numerical simulation, and this will have to be taken into consideration when analyzing the results. The pressure along the centerline is plotted in Figure 8. The right order of magnitude is reached for the value of the pressure. The fact that it is lower is explained by the fact that the pressure is calculated over a larger area which has an averaging effect when compared with the experimental data. Moreover, the diameter of the area where the pressure larger than the cut-off pressure of 50 MPa is 15 mm, which compares very well with the experimental results. Figure 8 Maximum pressure reading across centerline Snapshots of the impact at time intervals of 0.25 ms are given in Figure 9. Although it is impossible to compare with the snapshots from the video camera, it is in agreement with the fact that the hail is brittle and it correlates very well with observations from Kim et al. [13].

Figure 9 Hail impact at time intervals of 0.25 ms Overall, the performance of the pressure sensitive film proved satisfactory and reliable. The provided data is limited to the maximum pressure and does not provide any information with respect to time, but it is relatively simple and inexpensive to use. Depending on the results expected from various tests, it can prove very adequate. Future tests would obviously require a better high speed camera and could include impacts on metallic and composite deformable structures. The obtained numerical results are in good agreement with the experimental data in terms of the pressure reached and the size of the zone of impact. Although it was impossible to correlate the deformations of the hailstone with experimental data, the numerical simulation results tend to agree with the current understanding and observation of the phenomenon by other authors [13]. The SPH method has the additional advantage that the modeling of multiple projectiles requires less computational and time efforts. However the mesh refinement for the hailstone and zone of impact is required for a more accurate reading of the pressure, which could become computationally expensive. 4. Conclusions We have shown in this paper how the SPH method can be used to obtain accurate results for bird and hail impact simulations. Many of its features make the method suitable to simulate complex impact events. Given the success of SPH method in the current project, it should be used in subsequent work involving more complex fluid-solid interaction as in aircraft ditching simulation. Future work would be to perform complex simulations and hopefully conduct experiments in parallel to evaluate the performance of the SPH projectile and the performance of the metallic/composite material model used for the structure. In such cases, additional valuable data would be found in the final deformations of the structure. 5. Acknowledgements We would like to thank the CRIAQ for their financial support for this project as well as Laval University, DRDC Valcartier and NRC for their close collaboration. Thanks also go to our industrial partners and Mr. Jacques Blais and Ron Gould for their expertise during the test programs. 6. References 1. Marie-Anne Lavoie, Soft body impact modeling and development of a suitable meshless approach, Université Laval, 2008, 131 pages 2. Alastair F. Johnson, Martin Holzapfel, Modeling Soft Body Impact on Composite Structures, Composite Structures, Vol. 61, pp. 103-113, 2003 3. M. A. McCarthy, R. J. Xiao, C. T. McCarthy, A. Kamoulakos, J. Ramos, J. P. Gallard, V. Melito, Modeling Bird Impacts on an Aircraft Wng- Part 2 Modeling the impact with and SPH bird model, International Journal of Crashworthiness, Vol. 10, n.1, pp. 51-59, 2005 4. Vinh Phu Nguyen, Timon Rabzuk, Stéphane Bordas, Marc Duflot, Meshless methods: A review and computer implementation aspects, Mathematics and computers in simulations, 79, 208, 763-813

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