MODELING AND EVALUATION OF LOADS IN VEHICLES SUBJECTED TO MINE BLAST Artur Iluk Mechanical Department, Wrocław University of Technology, Łukasiewicza 7/9, 50-371 Wrocław, Poland
[email protected] Keywords: blast load, mine resistant vehicle, impulse measurement, numerical simulation Abstract. In the paper, the numerical and experimental approach to the blast load evaluation was presented. The evaluation process is done in two steps. In the first, the full scale blast test is carried out with a simply mock-up resembling geometry of the bottom part of a vehicle. The pressure impulse is measured indirectly with the use of high speed camera and the motion analysis software. In the second step, a numerical simulation with the use of Multi Material Arbitrary LagrangianEulerian method is utilized to assess the influence of the fluid-structure interaction and the real vehicle mass on the value of the applied pressure impulse. In the paper the application of method was presented and the results were discussed. Introduction In the design process of the mine resistant vehicles, one of the main problem is reliable evaluation of the pressure impulse applied to the vehicle by detonating explosive device. The knowledge of the value if this impulse play the important role in design process of the vehicles resistant to the blast load. The most dangerous case is the detonation below the center of the vehicle, because the blast wave the products of detonation acts on the maximal area of the vehicle’s bottom. The second common place of the mine detonation is the position below front or rear wheel. Both cases are covered by NATO standardization agreement STANAG 4569 [1] and the corresponding instruction AEP-55 [2], describing in details the test methods and conditions. All described tests should be carried out experimentally, as a verification of the vehicle’s ability to safe the crew. At the design stage of the vehicle life cycle, the reliable value of the blast impulse load applied to the structure is obligatory. State of the art The blast pressure impulse can be measured experimentally or determined by numerical simulations. Commonly used numerical methods of the blast modeling are the Kingery-Bulmash model and the MM-ALE blast simulation. The Kingery-Bulmash model is based on experimental pressure and impulse measurements [3]. The time and space variable pressure field is defined directly on the loaded surface of structure as a function of the charge mass, distance from the charge center, and the angle of blast wave incidence. In this approach, the charge itself is not modeled and the blast wave propagation is neglected. Moreover, the reflected impulse doesn’t depend on the loaded structure motion, which is not true, especially for the relatively light or elastic structures because of the fluid-structure interaction (FSI). The FSI effects influence significantly the total transferred impulse [4] and cannot be neglected. This model has been implemented in the program CONWEP [5] and the commercial codes like ABAQUS and LS-DYNA [6]. The explosion can be defined as air blast or surface (hemispherical) blast, when charge is detonating near to the rigid, undeformable surface. In the second case, the blast wave is amplified by the coincidence of the direct and reflected wave. The assumption of rigidity of the surface makes
unreliable the usage of Kingery-Bulmash model for the charges buried in a soil experiencing large deformations and interacting with the products of detonation. The second method of the blast simulation is the Multi Material Arbitrary LagrangianEulerian method [7]. In this approach, the charge, soil and air are modeled with the use of the regular mesh of the solid elements (MM-ALE mesh). During the simulation of the explosion, all three phases are transferred through the stationary element boundaries avoiding the mesh deformations. The processes of the charge detonation, the explosion crater formation and blast wave propagation can be successfully simulated. The loaded structure, usually modeled as the Lagrange mesh, is placed in the space filled out by the MM-ALE air mesh. The interaction of the blast wave and the loaded surface (FSI) is realized by application of the appropriate pressure fields to the fluid and structure parts of the model, with the conservation the energy and momentum [8]. The MM-ALE simulations are very time consuming, because of big number of the finite elements necessary to fill out all area of the soil and air affected by the blast phenomenon. It makes this method not suitable on the design stage, when a number of concepts is requiring the numerical verifications. Moreover, the modeling of the detonation process and dynamic deformation of the soil requires determination of many model parameters, so the results of such simulations should be verified by the experimental tests.
Fig 1. The typical horizontal ballistic pendulum [9] (left) and Vertical Impulse Measurement Facility – VIMF [10] (right) The last method of blast impulse determination are the experimental, field tests. In tests, the Hopkinson-Cranz scaling law can be used to decrease size of charge and structure, but for buried charges, the load is not scaled correctly [11]. The reason is the lack of possibility of scaling the material properties of the structure and soil. For reliable results, the full-scale test should be conducted. The typical measurement device is the horizontal ballistic pendulum (Fig. 1). The transferred impulse is converted into the pendulum arm kinetic energy and subsequently in potential energy of lifted arm. The directly measured variable is the angle of the arm rotation. The usability of the pendulum is limited to a relatively small charges, because of possible plastic deformation of the pendulum structure. The impulses produced by bigger charges of the order of 10kg TNT are measured with the use of large scale measurement facilities, like Vertical Impulse Measurement Facility (VIMF) used in the US Army Research Laboratory [10]. Method and results The proposed method of the blast impulse measurement takes under consideration all effects associated with the ground influence, soil type and moisture and fluid-structure interaction. For the
calibration of the model, the field test with the simple mock-up resembling the vehicle’s bottom geometry is required. No special equipment or test facility is needed for the test, so experiment is relatively simply to conduct, even with the full scale blast load. The algorithm of the presented method was depicted in figure 2.
Fig. 2. Algorithm of the blast impulse measurement method In the first step, two equivalent numerical models of the mock-up are prepared, one loaded with the use of CONWEP air blast, and second designed with MM-ALE technique. To avoid FSI effects in MM-ALE model, the mass density and stiffness of the both mock-ups should be significantly scaled up.
Fig. 2. Model set-up (left), MM-ALE model view (right) The impulse obtained from CONWEP model for the air blast exhibits very good correlation with experimental data [12] and can be used for the verification of the MM-ALE model outcomes. The example of both models and obtained results were presented in figures 2 and 3.
Fig. 3. Velocity field, t=1.1ms, air blast MM-ALE model (left), comparison of resultant impulse values for CONWEP and MM-ALE model (right) In the next step, the simple full-scale experiment with the buried charge is carried out to measure the impulse transferred to the light mock-up (Fig. 4). The trajectory of the mockup flight is captured by camera and analyzed with the image processing software to find the initial velocity and total applied impulse (Fig. 5). The optical measurement is possible, because the mock-up is relatively light and after detonation quickly leaves the non-transparent area of the fireball and the ground particles ejected by the explosion. The experiment is resembled in the verified MM-ALE model supplemented by the soil domain. On the basis of the experimental results, the MM-ALE model is calibrated to capture reliable influence of the soil domain on the impulse value.
Fig.4 . Experimental mock-up (left), the detonation of the buried charge under the mock-up, MM-ALE simulation, t=7ms (right) Due to the FSI effects, the transferred impulse strongly depend on the mass and stiffness the loaded object. In the third step of the presented algorithm, the model of mock-up is modified to have the full mass and stiffness of the real vehicle. The calibrated MM-ALE model containing the charge, soil, air and the mock-up is used to calculate the total impulse for the real vehicle. The final mock-up model and resultant values of the impulse was shown in figure 6.
Fig. 5. The full-scale blast test: the view of the flying mock-up and the measured trajectory.
Fig. 6 . Model for MM-ALE simulation, buried charge, real mass and stiffness of structure (left), transferred impulse (right) The calibrated MM-ALE model can be used directly for the simulation of the full vehicle structure subjected to the blast load. The simulation of the detonation, ejection of the ground and propagation of the blast wave in the air in the space surrounding the body if vehicle is very time consuming. The knowledge of reliable value of the total transferred impulse makes possible, as the last step of algorithm, the usage of the more simple CONWEP load model. The surface, hemispherical blast CONWEP load should be scaled up to apply on the vehicle equivalent total pressure impulse. The amplified CONWEP blast load simulation is much faster and suitable for the multiple design verification purposes. Conclusions Presented method allows to identify the impulse acting on the vehicle without a need to build a complete structure of the vehicle. The measurement can be performed with the use of lightweight mock-up, without the need to build a special test bench. Using the experimental procedure allows to take into account, without the use of the Hopkinson-Cranz scaling laws, most uncertain interaction phenomena associated with an explosive charge size, shape and depth of burial and the ground properties. Implementation of experimental studies in small-scale is also possible, however, increases the measurement uncertainty.
Other effects associated with the loaded structure are included in the numerical part. Unlike the properties of the ground and charge, the parameters related to the stiffness and mass of structure are much easier to reliable define in the numerical model. Presented original method can be used to evaluate and modify the geometry of the body of tracked and wheeled vehicles exposed to an underbelly explosion. Usage of this method to determine the impulse transmitted by the explosions of improvised explosive device (IED) on the roadside may also be recommended in situations where there is a significant deformation of the ground, changing the geometry of the space in which the shock wave propagates. A great advantage of the method is that a result of the procedure is a reliable numerical model of vehicle and its load is, which can be used to optimize the geometry and internal structure, or by attaching a human body model, directly to assess the level of passive safety and effectiveness of the protection means. References [1] STANAG 4569: Protection Levels for Occupants of Logistic and Light Armoured Vehicles, Ed. 1, 2004. [2] Procedures for Evaluating the Protection Level of Logistic and Light Armoured Vehicles, AEP-55 Volume 2 Edition 1. 2006. [3] C. N. Kingery, G. Bulmash: Airblast parameters from TNT spherical air burst and hemispherical surface burst. Technical Report ARBRL-TR-02555,” 1984. [4] W. Peng: Modeling and simulation of interactions between blast waves and structures for blast waves mitigation, University of Nebraska, 2009. [5] Conventional Weapons Effects program, Version 2.00. US Army Engineer Waterways Experimental Station, Vicksburg, USA, 1991. [6] J. O. Hallquist: LS-Dyna. Theoretical manual. California Livermore Software Technology Corporation, 1998. [7] M. Souli, I. Shahrour: Arbitrary Lagrangian Eulerian formulation for soil structure interaction problems, Soil Dynamics and Earthquake Engineering, vol. 35, pp. 72–79, 2012. [8] M. Souli, A. Ouahsine, L. Lewin: ALE formulation for fluid–structure interaction problems, Computer Methods in Applied Mechanics, vol. 190, no. 5–7, pp. 659–675, 2000. [9] M. Grujicic, B. Pandurangan, N. Coutris, B. Cheeseman, W. Roy, R. Skaggs: Derivation and Validation of a Material Model for Clayey Sand for Use Inlandmine Detonation Computational Analyses, Multidiscipline Modeling in Materials and Structures, vol. 5, no. 4, pp. 311–344, Oct. 2008. [10] M. Grujicic, B. Pandurangan, Y. Huang, B. Cheeseman, W. Roy, R. Skaggs: Impulse loading resulting from shallow buried explosives in water-saturated sand, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, vol. 221, no. 1, pp. 21–35, Jan. 2007. [11] A. Neuberger, S. Peles, D. Rittel: Scaling the response of circular plates subjected to large and close-range spherical explosions. Part II: Buried charges, International Journal of Impact Engineering, vol. 34, no. 5, pp. 859–873, May 2007. [12] D. Bogosian, J. Ferritto, Y. Shi: Measuring uncertainty and conservatism in simplified blast models, 30th Explosives Safety Seminar, 2002, no. August, p. 26.
