Autodyn Blast

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<p>Computer Modeling of Blast Loading Effects on BridgesGreg Black Lafayette College Easton, Pennsylvania Advisor: Dr. Jennifer Righman University of Delaware Newark, Delaware Submitted to NSF-REU 11 August 2006</p> <p>AbstractThe goal of this report is to evaluate a hydrocode, which is a type of computer program, called AUTODYN for the use of modeling blast loads on bridge sections. Blast modeling is necessary due to the threats posed by terrorist attack and current technology makes computer simulations cheaper than experimental testing. It discusses various options presented by AUTODYN which set it apart from other hydrocodes and other available software. These include the benefits of it graphical interface, modeling options and remapping capabilities. Meanwhile, its large demand on memory for complex models creates issues in the modeling phase, before the models can actually be analyzed. Yet if the user can get past the quirks of the program and work within the memory limits it is possible to obtain fairly accurate results from carefully made models.</p> <p>Table of ContentsABSTRACT .................................................................................................................................................. 2 1 INTRODUCTION ..................................................................................................................................... 5 2 INTRODUCTION TO BLASTS .............................................................................................................. 5 2.1 EXPLOSIONS ......................................................................................................................................... 6 2.2 CONWEP............................................................................................................................................... 7 3 INTRODUCTION TO HYDROCODES ................................................................................................. 8 3.1 MODELING TECHNIQUES ...................................................................................................................... 9 3.1.1 Structured vs. Unstructured Solvers .......................................................................................... 11 3.1.2 Lagrange Solvers ....................................................................................................................... 13 3.1.3 Euler Solvers.............................................................................................................................. 14 3.1.4 Other Solvers ............................................................................................................................. 16 3.2 INTRODUCTION TO AUTODYN.......................................................................................................... 17 3.2.1 Material Models......................................................................................................................... 18 3.2.2 Parts........................................................................................................................................... 20 4 MODELING AND RESULTS................................................................................................................ 23 4.1 AUTODYN MODELS ......................................................................................................................... 23 4.2 RESULTS AND DISCUSSION ................................................................................................................. 27 4.2.1 Pressure in the Slab ................................................................................................................... 28 4.2.2 Deflection in the Slab................................................................................................................. 32 4.2.3 Effective Strain in the Slab......................................................................................................... 33 4.2.4 Pressure in the Air ..................................................................................................................... 34 4.2.5 Conclusions and Suggestions for Future Investigation.............................................................. 37 5 ACKNOWLEDGEMENTS .................................................................................................................... 38 6 REFERENCES ........................................................................................................................................ 38 APPENDIX MODELING NOTES ........................................................................................................ 40</p> <p>List of FiguresFigure 2.1 Charge and Blast Wave...............................................................................................6 Figure 2.2 Standard Pressure vs. Time Curve for an Explosion...................................................7 Figure 3.1 Example Grid.............................................................................................................10 Figure 3.2 Typical Calculation Sequence...................................................................................11 Figure 3.3 Structured Grid..........................................................................................................12 Figure 3.4 Unstructured Grid......................................................................................................13 Figure 3.5 Example Lagrange Grid...13 Figure 3.6 Example of Normal Mesh and (a)-(d) Examples of Problematic Mesh Distortion..14 Figure 3.7 Stationary Euler Grid Example15 Figure 3.8 Example SPH Node Dispersal17 Figure 3.9 Example of Erosion..19 Figure 4.1 Standard Slab, Air and Charge Model...24 Figure 4.2 Standard Slab, Air and Charge Model...24 Figure 4.3 (a) Moving Gauges in Slab (b) Fixed Gauges in Air...26 Figure 4.4 Pressure vs. Time for Gauge #1, Same Air Element Size.........................................28 Figure 4.5 Pressure vs. Time for Gauge #2, Same Air Element Size.........................................29 Figure 4.6 Pressure vs. Time for Gauge #1, Same Slab Element Size......................................30 Figure 4.7 Pressure vs. Time for Gauge #2, Same Slab Element Size......................................31 Figure 4.8 Deflection Comparison for the Back Center of the Slab............................................32 Figure 4.9 Effective Strain vs. Time for Gauge #2, Same Air Element Size...............................33 Figure 4.10 Effective Strain vs. Time for Gauge #2, Same Slab Element Size..........................34 Figure 4.11 Initial Pressure Results for 1000mm from Charge Center.......................................35 Figure 4.12 Remap of Wedge onto 20mmel Air and 20mmel Slab.............................................36 Figure 4.13 Comparison of 10mmel Wedge, ConWep and Remapping Results at 1000mm from Center of Charge...........................................................................................................................37</p> <p>List of TablesTable 4.1 Models........................................................................................................................27 Table 4.2 Gauge #1 Initial Peak Pressure, Same Air Element Size............................................29 Table 4.3 Gauge #2 Initial Peak Pressure, Same Air Element Size...........................................29 Table 4.4 Gauge #1 Initial Peak Pressures, Same Slab Element Size.......................................31 Table 4.5 Gauge #2 Initial Peak Pressures, Same Slab Element Size.......................................31</p> <p>1 IntroductionThe events of 9/11 continue to have a lasting effect on the US and the world. Everyone on the planet has been affected in some way or another. The implications of the vulnerability of the nations infrastructure to terrorist attack are a concern that should be shared by all engineers. If bridges and other structures may be subjected to severe loads from explosions or other sources, then it is the engineers responsibility to prepare for them. However, before design codes can be better developed or adequate protections can be created it is necessary to gain a better understanding of the complex interactions between structures and explosions. Yet methods for explosive testing are limited due to cost and permissions for experimental results. Therefore, with modern advances in computing technology called hydrocodes may be a better option. This paper will evaluate a hydrocode program called AUTODYN for the use of blast simulation on complex structures, with a focus on hydrocodes as a technology and user interactions with the program as well as the accuracy of the several simulations run in the program. It will also provide the reader with a brief overview of blasts or explosions in order to provide some background on the subject as well as a basis for the comparison of test results.</p> <p>2 Introduction to BlastsThis section will discuss a few of the basic properties of explosions. Once these ideas are understood, the interactions between explosions and structures can be more easily discussed. It will also discuss ConWep, a blast calculation program distributed by</p> <p>the United States government (Robert, 2007), which will be used to evaluate the performance of AUTODYN.</p> <p>2.1 ExplosionsFigure 2.1 depicts a few of the basic characteristics of a simple explosion in air. There is the charge (a), the pressure wave (p) and the standoff distance (r). The main component that any explosion requires is some type of fuel or charge such as TNT. When ignited, this charge rapidly releases energy in the forms such as heat, sound or pressure waves (Robert, 2007). The pressure wave expands out from the charge. The leading edge of this wave is sometimes called the shock front and will generally have the highest pressure in the wave at any given point in time (Wilkinson et al., 2003). The standoff distance is basically the distance from the center of the explosion to any object or point of interest.</p> <p>P r</p> <p>a Spherical Air Blast</p> <p>Figure 2.1 Charge and Blast Wave (Robert, 2007)</p> <p>The pressure at a specific point in air in the path of an explosion over time will follow the same general pattern, so long as there isnt any reflection from nearby objects. This pattern, called an overpressure curve (Wilkinson et al., 2003) can be seen in Figure 2.2, below. The main components of the overpressure curve are the detonation (a), arrival time (b), peak pressure (c), and time duration (c to e). The detonation can be</p> <p>considered as time 0, while the arrival time is the time that it takes for the pressure wave to reach the point of interest (Robert, 2007). Once the peak pressure is reached, it immediately starts to decay and the time it takes the pressure to return to normal is called the time duration (Wilkinson et al., 2003). As the material in the blast wave expands outward it can leave a void, creating a region with pressure lower than normal atmospheric pressure (Robert, 2007). The size, shape and material of the charge, as well as the stand off distance will all determine the magnitude and shape of this curve. In addition to the above factors, the blast wave and the pressure involved can reflect off of surfaces in various directions, and cause further fluctuations in pressure at a single point.</p> <p>1.0 c 0.9 SoD = 1000 mm CONWEP Calculations Explosive: TNT Quantity: 1 kg</p> <p>Blast Over Pressure, P, MPa</p> <p>0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 a 0 0.2 0.4 b 0.6 0.8 1.0 d</p> <p>e 1.2 1.4 1.6 1.8 2.0</p> <p>Time, t, ms.</p> <p>Figure 2.2 Standard Pressure vs. Time Curve for an Explosion (Robert, 2007)</p> <p>2.2 ConWepAs mentioned above, ConWep is a simple blast program distributed by the United States government. Users can input a charge size and standoff distance and receive</p> <p>pressure for that point in relation to time as output. It also allows users to receive pressure data after interaction with simple structures such as plates and shells. ConWep is not guaranteed to give a 100% accurate result, but it has been compared to hand calculations and found to be generally correct (Robert, 2007). For phenomena as complex as explosions a generally correct answer may be the best one. A main disadvantage of ConWep comes from the fact that pressure curves can only be obtained for one point at a time. Also ConWep has limited structural interaction capabilities, and certainly cannot evaluate failure or deformation in a structural component.</p> <p>3 Introduction to HydrocodesWhile the damage level produced by blasts is what makes them so critical for examination, it is this same nature that makes experimental studies expensive and difficult. In addition, the dynamic, time dependent nature of the loads produced by blasts increases the complexity further, especially when compared to simple static loads. Large scale tests can require millions of dollars in investments (Zukas, 2004). Therefore, anyone doing experimental blast research needs an almost bottomless source of funding. For the purposes of this project, the costs of destroying a bridge or even simple structure, as well as having the permission to do so, make such testing out of reach. Therefore, anyone interested in examining the effects of an explosion on a structure needs to look into alternatives to experimental testing. One such alternative has been made possible through advanced computer programs called hydrocodes. What is a hydrocode and what is it used for? Zukas defines</p> <p>a hydrocode as a computer program for the study of very fast, very intense loading on materials and structures(2004). Developed in the 1960s, hydrocodes originally performed calculations by assuming hydrodynamic behavior in the materials, and therefore ignoring material strength, which is the origin of the term hydrocode. This method was used because the pressures generated by experiments often greatly surpassed the strength of the materials (Zukas, 2004). Also, while many of the calculations performed by hydrocodes could be done by hand or even with the use of a calculator, the shear number of calculations involved in even simple problems makes the use of powerful computers invaluable. Modern hydrocodes, including AUTODYN do a great deal more than model the hydrodynamic behavior of materials, but the name has stuck. They can make use of a variety of methods to model different material behaviors. In addition to their use in blast modeling, hydrocodes have been used to evaluate structures for aircraft impacts, to simulate vehicle crashes and even design sports equipment (Zukas, 2004). The following sections will detail some of the methods hydrocodes use in modeling, with a focus on Lagrangian and Eulerian models in particular.</p> <p>3.1 Modeling TechniquesThe systems used for modeling in hydrocodes are based on finite element and finite difference techniques. How these techniques ar...</p>

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