computational analysis of aircraft impact on …computational analysis of aircraft impact on...

Computational Analysis of Aircraft

Impact on Concrete Panel

Joon-Ki Hong and Thomas H.-K. Kang Dept. of Architecture & Architectural Engineering, Seoul National University

E-mail: [email protected]

Abstract

The primary objective of this paper is to confirm the results of an experiment performed by Sandia National Laboratories (SNL) by conducting a computational analysis and comparing the results drawn from the experiment and analysis. With respect to aircraft impact accidents, extremely large deformations occurred over a very short period of time. Therefore, the use of LS-DYNA, a computational fluid dynamics (CFD) finite element software, is reasonably perceived to conduct such a computational analysis, as it is the finite element software for analyses related to transient dynamic response, large deformation and/or extreme loading. In this study, the computational analysis resulted in several interesting findings.

Keywords: Computational analysis, LS-DYNA, CFD, aircraft impact, concrete

1. Introduction

As one of the nation’s crucial infrastructures, Nuclear Power Plants (NPPs) are conservatively designated as high-risk facilities regardless of how improbable accidents may occur. However, aircraft crushes had not been a force considered in concrete building design prior to the World Trade Center (WTC) tragedy in 2001. Since this accident, the assessment of the aircraft impact has now been considered in the code. Also, the studies into impact accidents on NPPs have been widely involved.

Formerly, Riera [1] suggested an equation to calculate impact force on the target in 1968. It has been widely used because of its ease to evaluate simply approximate impact force. Additionally, Kar [2] adopted and revised the Riera function including the effective mass coefficient. This coefficient in particular should be defined through the real impact experiment. In 1988, a full scale of F-4 Phantom impact test was conducted by Sandia National Laboratories (SNL) using their facilities. The test results were detailed in a paper published in 1993 [3]. In this paper, the appropriate coefficient value was clearly determined and the accuracy about the Riera function was verified by comparison with test results. Nonetheless, considering variable circumstances, actual testing would be significantly difficult to be done and thus recent studies have been on the rise using computational analysis software such as AUTODYN, ABAQUS, and ANSYS LS-DYNA. Wilt et al. [4] conducted numerical simulations about the F-4D Phantom and F-15E aircraft using Smooth Particle Hydrodynamics (SPH) method in LS-DYNA. The simulation for the F-4D Phantom

International Journal of Pure and Applied MathematicsVolume 118 No. 9 2018, 279-289ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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crush experiment was described at first. The results showed good agreement with those obtained from the test. Furthermore, an additional simulation about the impact on reinforced concrete building was performed. The concrete damage model (MAT_072) in the finite element program was applied to concrete, while MAT_003 was used for reinforcement steel. It was revealed that the response of the concrete subjected to impact forces can be predicted using LS-DYNA and be affected by the engine attached to aircrafts. In addition, Lee et al. [5] conducted a computational analysis, considering the F-4D crash experiment, and verified the validity of Missile-Target Interaction Analysis (MTIA) using LS-DYNA. The MAT_159 was used for a concrete material, and MAT_003 was for reinforcement steel. The impact simulation was conducted under different aircraft modeling techniques including the Lagrangian model, Smooth Particle Hydrodynamics (SPH), and Hybrid (Lagrangian & SPH) model. Liquid fuel was also considered. Furthermore, Lee et al. [6] performed impact simulations on a prestressed concrete containment building using both LS-DYNA and AUTODYN. The effects of the prestressing force, different impact points, and incidence angle were well summarized in their study.

The primary interest of this paper is to confirm the results of an experiment performed by Sandia National Laboratories (SNL) by using the results drawn from a computational analysis. With respect to aircraft impact accidents, it generates extremely large deformations spanning a very short time. Therefore, the use of LS-DYNA is feasible for a computational analysis because it is finite element software based on large deformation and/or short duration. In this study, typical concrete materials in LS-DYNA are used along with Smooth Particle Hydrodynamics (SPH) method. 2. Previous impact experiment

A full-scale experiment of aircraft impact on a concrete panel was performed by Sandia National Laboratories (SNL) in 1988. The primary interest of this experiment was to reveal uncertainties in analytical methods and to evaluate impact force time histories against the aircraft impact on a rigid target. A representative F-4 Phantom with a weight of 469 ton crashed into a rigid reinforced concrete block at a speed of 215 m/s. For the test, some equipment in the F-4D were removed or added. For instance, while the gears and flaps at the main wings were removed, but sled and rockets were added. As a result, the total impact weight of the aircraft was 19 tons.

The square-shaped concrete panel had 7 m of height or width, 3.66 m of thickness, and 469 tons of the total weight which was approximately 25 times the F-4D aircraft. Through five instrumentations, the impact responses of the target were recorded including velocity, acceleration, and displacements. Furthermore, the accuracy of the Riera function was verified to be reliable in the event of impact accidents. The acceptable effective coefficient for mass was found to be 0.9. In other words, 10% of the projectile mass would be reduced during impact. 3. Methodology

To evaluate damage caused by crash, one of three methods specified by International Atomic Energy Agency (IAEA) should be used [7, 8]. Among these

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methods, Force-Time History Analysis (FTHA) and Missile-Target Interaction Analysis (MTIA) are typically used for evaluation. 3.1. Force-Time History Analysis

The typical method for force-time history analysis (FTHA) is the Riera function suggested by Riera in 1968, which focuses on calculating aircraft impact force [1]. This function has been widely used in research to better evaluate the impact force of crushing projectiles. The basic principle of the Riera method lies on the crushing strength of aircrafts and conservation principles in impulse which are derived from the kinematic theorem [9].

The essential advantage of FTHA is that impact effects are relatively easy to adopt through a dynamic time history analysis and using an indirect method such as the Riera equation. However, several assumptions should be postulated in the use of FTHA, as it is an indirect approach. First, target elements on which the aircraft impacts on is rigid. Also, the Riera formula is based on the perpendicular impact coinciding with progressive axial destruction of the aircraft. However, typical disadvantages of the indirect method are that it has several limits and neglects secondary effects that might occur in realistic aircraft impact. It was figured out that it has some discrepancy between the analysis and reality [8]. In order to supplement the aforementioned disadvantages and assumptions, Missile-Target Interaction Analysis (MTIA) was developed.

3.2. Missile-Target Interaction Analysis

The MTIA is a method using finite element analysis. The aircraft and the target constructed as a finite element would crush directly into each other. It is a problem that the initial velocity should be assumed to conduct dynamic analysis [10]. The direct method of MTIA considers additional effects, not included in the indirect method. One of the prime advantages of using MTIA is eliminating the need to calculate the crushing area of the aircraft. Additionally, it is possible to consider secondary effects such as fragmentation. The information of exact mass distribution; however, is required so that the accuracy of results drawn from analysis would be considerably improved. Results obtained from the analysis and Riera approach should be verified and compared to ensure accurate results.

4. Numerical Model Using LS-DYNA

4.1. F-4 Phantom

Smooth Particle Hydrodynamics (SPH) is considerably applied in analysis with large deformations without any mesh. Due to disconnected particles between each other, the particles are spread out at impact. Therefore, it is possible to consider secondary effects of debris or fuel [12].

The basic concept in aircraft modeling in this study is that F-4 Phantom only consists of same material with SPH element. In other word, there are no classifications about material among engine, fuel, or fuselage. Mass for each length is differentiated to combine multiple mass distribution of several parts. First of all, SPH


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elements are generated via LS- PrePost and divided into several parts along with the unit length (1 m) of the F-4 Phantom model. To match with mass distribution for an actual test, different values for mass are introduced at particles in each part. Fig. 1 represents comparison of mass distribution between an experiment and analysis in this study. Mass distribution per unit length is calculated using Excel. Consequently, the even mass distribution of the test validates the aircraft model used in this research.

(a) (b)

Fig. 1 (a) Comparison of Mass Distribution with Experiment, (b) Numerical SPH

Model

Particles consisting of the F-4D model are defined using the keyword: *DEFINE_BOX. Using this keyword, particles located in the outside of the specified box are not considered so that it can decrease computational time during the analysis. Also, the keyword: *CONTACT_AUTOMATIC_NODE_TO_SURFACE is applied for impact between the SPH modeling and the target. Aircraft SPH elements are defined as slave parts, while master parts are set to the reinforced concrete target. Velocity of the aircraft is 215 m/s by using the keyword: *INITIAL_VELOCITY. Aluminum is assumed as a material for the aircraft using plastic kinematic model (MAT_003) in LS-DYNA. The specific properties are summarized as shown in Table 1. In this study, 1.8 of failure strain (FS) is applied based on Lee et al. [5] even though contact conditions, components, properties are quite different from the previous study.

Table 1 Aircraft Element Properties (MAT_003) [4]

Density

(kg/m3)

Modulus of

Elasticity

(GPa)

Yield Stress

(MPa) Poisson’s Ratio

Aluminum 2770 69 95 0.33

4.2. Target

In this analysis, foundation mat and platform is neglected so that only a concrete block directly collided with the aircraft is established in LS-DYNA. As a boundary condition, the target is assumed that it can only move to the direction corresponding to the impact way. It consists of 7 m in height and width, and depth of 3.66 m. All element size is 0.07 m for each side. In LS-DYNA, it provides several material types


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to establish concrete modeling. Typically, MAT_024, MAT_072 (Karagozian and Case Concrete), MAT_084 (Winfrith Concrete), MAT_111 (Johnson-Holmquist Concrete), and MAT_159 (CSCM) are frequently used in analysis corresponding to the concrete material. To observe various characteristics for concrete models, MAT_159 of concrete material models are employed in this study. However, no detail information about target properties was described in the experiment. Therefore, compressive strength of the concrete is basically assumed as 30 MPa. In accordance with Wu et al. [13], MAT_159 has elastic-softening behavior steps for stress-strain relationship. It can demonstrate peak strength under no confinement. Also, post-peak softening and brittle ductile transition are illustrated herein, but shows low accuracy. It has a tendency to be ductile in the event of pressure loading and least influenced by effects of the strain rate. Additionally, it is stiff under blast, although favorable results can be derived from impact loading. The detail properties for CSCM concrete model are summarized in Table 2.

Table 2 Material Properties for CSCM Concrete Model (MAT_159)

Density

(kg/m3) ERODE

Compressive

Strength (MPa)

Maximum

Aggregate Size (mm)

CSCM Concrete 2615 1.05 30 19

MAT_003 (PLASTIC_KINEMATIC) material model is selected as the reinforcement. Additionally, it is modeled as a truss element, considered as a conservative approach [11]. Analysis time can also be reduced using the truss element for reinforcement steel because it only resists axial deformation. Basically, D-13 steel is assumed in the analysis so that cross sectional area of the steel is 126.7 mm

2.

Density of reinforcement steel is 7850 kg/m3, 0.3 of Poisson’s ratio, 200 GPa of modulus of elastic, and 490 MPa of yield strength with 0.2 failure strain. The keyword: *CONSTRAINED_LAGRANGE_IN_SOLID is used to apply reinforcement steel (slave) embedded into concrete element (master). The specific properties for MAT_003 are described in the table below.

Table 3 Material Properties for Reinforcement Steel (MAT_003)

Density

(kg/m3)

Modulus of

Elasticity

(GPa)

Poisson’s

Ratio

Yield Stress

(MPa) Failure Strain

Reinforcement 7850 200 0.3 420 0.2

5. Results

5.1. Riera Function

The cardinal objective of using Riera function is to calculate impact force for aircrafts. At first, impact force drawn from Riera method is conducted based on the given mass distribution condition. As Lee et al. suggested [5], Eq. (8) neglecting crushing force as the baseline condition. As a result, Fig. 3 shown below is derived.


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Fig. 2 Impact Force and Impulse of Riera Function

In Fig. 2, solid lines are impact force of an aircraft in LS-DYNA derived from modifying the Riera function, while dashed lines indicate results of the experiment. Aircraft impact force derived experimentally was calculated from Riera function including crushing force from Eq. (7), while the target force was directly derived from recorded data from the test. On the other hand, impact force for the aircraft in this study shows identical tendency along with the mass distribution. Also, total duration of impact calculated by Riera function is identical for both the experiment and mass distribution in this study because Riera function is an approximate solution. Moreover, the differences in magnitude are due to the crushing force.

Impulse results are also demonstrated in Fig. 3, which is calculated by integrating impact forces multiplied by each time. Parts of delayed impact force are first reflected in the impulse near 0.04 second as shown in the Fig. 3. In addition, impulse of the experiment using Riera function illustrates similar pattern to those of the mass distribution in this study. In other word, it means that the mass distribution established in LS-DYNA is significantly analogous to the F-4D model used in the experiment. As Sugano et al. [3] concluded, 0.9 of the effective mass coefficient was considerably matched with the response of the target. Also modified formula with =0.9 is identical to the results of the experiment. It is concluded that Riera formula is accurate to evaluate impact force for the projectile, and it is verified that 90% of the mass (=0.9) would be reflected during the impact. However, key results from the experiment are not response of the aircraft, but that of the target because impact force of the aircraft evaluated by Riera function distributes the mass during the impact. This is a reason why the effective mass coefficient () was introduced. Therefore, it is necessary to compare results from the analysis with response of the target from the experiment and results for mass distribution with 0.9 of effective mass coefficient. 5.2. LS-DYNA Results

As shown in Fig. 3, impact force of LS-DYNA is analogous to the Riera function. However, Riera evaluation shows exactly the same pattern of mass distribution of the SPH model. The reason being the Riera method assumed that the projectile crushes into the rigid target and remains perpendicular incidentally. On the other hand, for reality and during the analysis, crushing parts can be affected by surrounding situations. Nevertheless, it exemplifies a similar graph between analysis result and


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Riera function. At 0.06 seconds, it is dramatically decreased. This incident may be due to reflected and penetrated particles during the simulation. The numerical analysis in this study estimates impact force correctly and the result is reliable compared with the Riera approach.

Fig. 3 Analysis Results of Impact Force and Impulse

Additionally, CSCM concrete model in LS-DYNA shows assurance with the target response in this study. It is continued along with the aircraft behavior with 0.9 of the effective mass coefficient. In this point, the numerical model with the SPH method is similar to the aircraft behavior drawn from the test. Therefore, 1.8 of the failure strain (FS) is appropriate to be used in numerical simulations.

Fig. 4 Velocity and Displacement Comparison


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0.02 sec 0.04 sec 0.06 sec 0.08 sec

Fig. 5 SPH Particles during Impact

The velocity and displacement of the concrete target are demonstrated in Fig. 4. It shows considerably similar patterns for both categories between the experiment and the analysis. Around 0.04 seconds, when the maximum impact force is recorded, both velocity and displacement results turn into considerable increasing tendency. In summary, numerical analysis using LS-DYNA can illustrate impact accidents with accurate results. The numerical analysis using LS-DYNA can therefore be verified and be reliable.

Fig. 5 shows changes at each step during the impact. Particles consisting of the aircraft are spread out at a crushing moment. Furthermore, at 0.08 seconds, some particles are not spread out significantly. It is considered due to reaction of the reflected particles. In addition, it is speculated that it causes significant drop in the impact force graph.

Fig. 6 Shape Comparison for Test and Analysis (Photo: Sugano et al., 1993)

At the end of the simulation, concrete elements are deleted as shown in the Fig. 6. With comparison to the test target [3], penetrate shape is analogous to the results sectional area of the test. However, there were slight differences due to discrepancies in conditions between analysis and the test, especially at the bottom of the impact area. According to the Riera method, total area of the impact was regarded as twice the impact area of the projectile. For the experiment, the crushed area of fuselage of F-4D aircraft area was approximately 4.6 m

2, while the impact area of the target was

10 m2. In this study, the measured impact area of the target is approximately 10.14 m

2

which is considerably close to that of the experiment. At this point, the results from LS-DYNA are more believable and reliable compared to the prediction of Riera method.

6. Conclusion

This paper aims to verify F-4D Phantom crash experiment performed by Sandia National Laboratories (SNL) using LS-DYNA, a finite element software specialized in simulations with large deformation. In addition, the validity of Missile-Target


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Interaction Analysis (MTIA) method is confirmed by comparison with Riera formula and results of the experiment. In LS-DYNA, there are several types of concrete modeling with different characteristics. In this study, MAT_159 is selected as the prominent concrete material. As such, the primary conclusions drawn from this research are listed below.

(1) Riera function is a typical approximate solution to evaluate impact forces. It is based on the mass distribution of the projectile. In this study, a SPH aircraft model is separated by several parts with unit length. Based on this mass distribution, impact force derived from Riera formula resembles the results of the test.

(2) In addition to Riera function, to get more accurate solutions the effective mass distribution factor () was introduced. From previous reports, it was concluded that 0.9 is the suitable coefficient value for effective mass distribution. As a result, its reliability is confirmed from this study.

(3) With respect to concrete material properties, variable failure strain (FS) values can be determined in various ways. For this analysis, FS=1.8 is adopted because it describes more reliable results than the others.

(4) Lastly, aforementioned conditions help to evaluate impact force so that it can be illustrated using LS-DYNA with relatively accurate results. As well as aircraft impact accidents, it can be extensively expanded into wide accident simulations.

In future explorations, same simulations with different concrete models in LS-DYNA will be performed. Results can also be compared in terms of material characteristics. Further studies will provide significance to confirm material usage, to understand each material feature, and how to use related keywords.

7. Acknowledgements

This study was supported by the Innovation Center for Engineering Education at Seoul National University, Korea, sponsored by the Ministry of Trade, Industry and Energy of Korea (Grant No. N0001337).

8. References

[1] J. D. Riera, “On The Stress Analysis of Structures Subjected To Aircraft Impact Forces”, Nuclear

Engineering and Design, Vol. 8, No. 4, pp. 415-426, 1968.

[2] A. K. Kar, “Impactive Effects of Tornado Missiles and Aircraft”, Journal of the Structural

Division, Vol. 105, pp. 2243-2260, 1979.

[3] T. Sugano, H. Tsubota, Y. Kasai, N. Koshika, S. Orui, W. A. Von Riesemann, D. D. Bickel, and M. D. Parks, “Full-Scale Aircraft Impact Test for Evaluation of Impact Force”, Nuclear Engineering and

Design, Vol. 140, No. 3, pp. 373-385, 1993.

[4] T. Wilt, A. Chowdhury, and P. A. Cox, Response of Reinforced Concrete Structures to Aircraft

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[5] K. Lee, J. W. Jung, J. W. Hong, “Advanced Aircraft Analysis of an F-4 Phantom on A Reinforced

Concrete Building”, Nuclear Engineering and Design, Vol. 273, pp. 505-528, 2014.

[6] K. Lee, S. E., Han, and J. W. Hong, “Analysis of Impact of Large Commercial Aircraft on A

Prestressed Containment Building”, Nuclear Engineering and Design, Vol. 265, pp. 431-449, 2013.


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[7] IAEA, Safety Standard Series: External Events Excluding Earthquakes in the Design of Nuclear

Power Plants, Safety Guide No. NS-G-1.5, 2003.

[8] H. Jiang, and M. G. Chorzepa, “Aircraft Impact Analysis of Nuclear Safety-Related Concrete

Structures: A Review”, Engineering Failure Analysis, Vol. 46, pp. 118-133, 2014.

[9] Rajesh, M. & Gnanasekar, J.M. Wireless Pers Commun (2017) 97: 1267.

https://doi.org/10.1007/s11277-017-4565-9

[10] K.-S. Lee, H.-K. Kim, and S.-E. Han, “Aircraft Impact Theory and Mass Model for the Missile-Target Interaction Analysis of Large Aircraft”, Journal of the Architectural Institute of Korea:

Structure & Construction, Vol. 29, No. 300, pp. 27-34, 2013 (in Korean).

[10] NEI, Methodology for Performing Aircraft Impact Assessments for New Plant Designs, NEI 07-

13, NEI, Washington, DC, 2009.

[11] D. W. Seo, and H. C. Noh, “Aircraft Impact Analysis of Steel Fiber Reinforced Containment

Building”, Journal of Computational Structural Engineering Institute of Korea, Vol. 26, No. 2, pp.

157-164, 2013 (in Korean).

[12] M. Kostov, M. Miloshev, A. Nicolov, and I. Klecherov, “Non-Structural Mass Modeling in Aircraft Impact Analysis Using Smooth Particle Hydrodynamics”, 10th European LS-DYNA

Conference 2015, 2015.

[13] Y. Wu, J. E. Crawford, and J. M. Magallances, “Performance of LS-DYNA Concrete Constitutive

Models”, 12th International LS-DYNA Users Conference, 2012.


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