Blast wave propagation in the air and action on rigid obstacles

106 Pages·2011·3.37 MB·Polish

Poznań University of Technology
Faculty of Civil and Environmental Engineering

Master’s Thesis

Wojciech Mamrak

Blast wave propagation in the air and
action on rigid obstacles

Supervisors:
Marcin Wierszycki, PhD
Piotr Sielicki, MSc

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Analiza propagacji ciśnienia w powietrzu i obciążenia wybuchem
sztywnej przeszkody

Obciążenia wyjątkowe odgrywają kluczową rolę w projektowaniu wielu obiektów, w
szczególności obiektów użyteczności publicznej. Na przestrzeni ostatnich lat uwzględnienie
wpływu obciążenia wybuchem stało się nierzadko wymogiem w krajach Europy Zachodniej ze
względu na rosnące i coraz częstsze groźby ataków terrorystycznych. W obliczu tak poważnych
wyzwań projektanci wciąż nie zdołali doczekać się norm prawnych opisujących proces
projektowania obiektów zdolnych przeciswstawić się, w możliwym stopniu, wspomnianym
zagrożeniom. Ta sytuacja, wespół z obecną tendencją do wznoszenia konstrukcji lekkich i
delikatnych, w związku z czym także bardziej podantych na obciążenia dynamiczne i lokalne
zniszczenia, dodatkowo potęguje zagrożenia grożące użytkownikom tychże obiektów.
Niniejsza praca stanowi próbę zmierzenia się z wyzwaniami oceny siły fali uderzeniowej
powstałej w skutek wybuchu jak i złożonymi zjawiskami wpływającymi na jej propagację.
W pracy omówiono rodzaje obciążeń wyjątkowych ze szczególnym naciskiem na
obciążenie wybuchem. Przedstawiono definicję i klasyfikację materiałów wybuchowych oraz
precyzyjnie omówiono proces propagacji fali uderzeniowej w powietrzu. Dodatkowo,
zaprezentowano aktualny stan prawny i regulacje, zarówno europejskie jak i amerykańskie,
dotyczące projektowania konstrukcji narażonych na obciążenia wybuchem. Przywołano prace
badawcze z całego świata dotyczące rozpatrywanych zagadnień.
W celu wykonania obliczeń numerycznych posłużono się programem komputerowym
Abaqus. Do tworzenia modeli wykorzystano język programowania Python wraz z Interfejsem
Programowania Aplikacji programu Abaqus dla tegoż języka.
Przeprowadzono zespół analiz dotyczących trzech głównych zagadnień:
- określenia odpowiedniego rozmiaru elementu skończonego do dyskretyzacji modeli,
określenia energii właściwej trotylu do wykorzystania w późniejszych analizach oraz
określenia zgodności wyników dla prostego modelu,
- weryfikacji ciśnień wewnętrznych powstałych w wyniku eksplozji zewnętrznej,
- wpływu zjawiska osłaniania na wartości ciśnień i impulsów wraz z porównaniem dwóch
programów komputerowych wykorzystujących odmienne metody numeryczne i
weryfikacją koncepcji odległości skalowanej.

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Table of contents
1. Introduction .................................................................................. 5
1.1. Preface .................................................................................................. 5
1.2. Thesis.................................................................................................... 7
1.2.1. Motivation ......................................................................................................... 7
1.2.2. Objectives .......................................................................................................... 7
2. Accidental loading ....................................................................... 8
2.1. Introduction ........................................................................................... 8
2.2. Types of accidental loading ................................................................... 8
2.3. Explosives ............................................................................................. 9
2.3.1. Definition and classification ............................................................................... 9
2.3.2. Blast wave propagation .................................................................................... 10
2.4. Explosive loading in Civil Engineering ................................................18
2.5. Previous researches on the topic ...........................................................20
3. Tools and methods ..................................................................... 22
3.1. Abaqus FEA .........................................................................................22
3.2. Explicit dynamics .................................................................................22
3.3. Blast modelling approaches ..................................................................24
3.4. Abaqus Scripting Interface ...................................................................30
4. Python modelling script ............................................................. 32
4.1. Description ...........................................................................................32
4.2. Main script ...........................................................................................34
4.3. Configuration file .................................................................................37
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4.4. Modules ...............................................................................................38
5. Analyses..................................................................................... 43
5.1. Mesh size study ....................................................................................43
5.1.1. Description ...................................................................................................... 43
5.1.2. Analytical solution ........................................................................................... 44
5.1.3. Numerical solution ........................................................................................... 53
5.1.4. Discussion ....................................................................................................... 68
5.2. Interior pressure due to external burst ...................................................71
5.2.1. Description ...................................................................................................... 71
5.2.2. Analytical solution ........................................................................................... 73
5.2.3. Numerical solution ........................................................................................... 75
5.2.4. Discussion ....................................................................................................... 78
5.3. Comparative analysis ...........................................................................79
5.3.1. Description ...................................................................................................... 79
5.3.2. Original study description ................................................................................ 80
5.3.3. Numerical solution ........................................................................................... 83
5.3.4. Discussion ....................................................................................................... 95
6. Conclusions ................................................................................ 99
6.1. Final remarks .......................................................................................99
6.2. Future tasks ........................................................................................ 100

Appendices
Appendix A. Lagrangian and Eulerian descriptions
Appendix B. Model generating script UML class diagram

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1. Introduction
1.1. Preface
Exceptional loads play an important and increasing role while designing engineering structures,
especially in the case of public facilities. These should be designed to resist not only the typical
loads, like permanent (self-weight of the structure and all its permanent elements, fixed equipment,
actions caused by uneven settlements) and variable loads (imposed loads, wind actions, snow
loads etc.), but also exceptional loads, such as fire, earthquake and explosion, since the collapse or
some serious damage of the structure may cause death or severe injuries to hundreds or thousands
of people. To such buildings one can include museums, theatres, cinemas, shopping centres,
stadiums, hospitals, bridges, but also structures of high importance like power plants, dams and
others.
The existing structures are highly vulnerable to explosives. In many cases the extent of the
collapse of a building would be disproportionate to the cause, if the explosive material were
detonated at significant location, for example not well-protected steel column supporting the roof
of a large span. This state is the resultant of both lack of regulations and the assumption, that the
probability of occurrence is negligible, and conviction, that in the case of terrorist attack there is
not much the designer can do. In general, the latter is true, since people with bad intentions will
always find a way to realize their plans, but it shouldn’t be the reason to resign from all
protections. Many disasters and tragic events are a consequence of lack of a very basic security.
Another factor increasing the above mentioned susceptibility is a tendency to design and
construct lighter, more delicate, slender and attractive structures, which are more sensitive to
dynamic loads and local damages. This applies especially to public facilities such as shopping
centres, stadiums and bridges, which should be not only functional, but also impressive and
breathtaking. The latter often affects negatively the safety of the structure and its users.
Except the highly dynamic phenomena caused by a blast (such as already mentioned blast
pressures, flying fragments of a structure and shock loads transmitted through air or ground),
usually other types of loading, inter alia fire, occur as its result. The destructive power of both
shock wave and high temperature complicates already complex issue and protection methods, as
blast wave and objects carried by it can damage the fire protection.
Nowadays, engineers take into account the resistance of the structure to natural phenomena,
for example resistance to dynamic loads caused by earthquake, if there is a risk of occurrence of it.
Similarly, each significant structure is designed to withstand fire for given period and prevent its
further propagation. This is because of existence of design codes and regulations, which instruct
the designers on how to take into consideration and deal with these complex issues.
In contrary, explosive loads are almost completely ignored in the current European
standards. Hence, Unified Facilities Criteria (UFC) [23] document from the Department of
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Defense of the United States of America is used in this thesis as a background for analytical
calculations.
Explosive loads, despite the fact they originate from various sources, are mostly associated
with terrorist attacks. In recent years, due to tragic events of this type, the awareness of the society
about the safety of structures increased considerably. Yet, the observations prove, that the majority
of public facilities is susceptible and vulnerable to explosive loading, and even a small charge
placed near crucial structural elements can cause a serious damage or even total destruction of the
whole building. Except of military facilities, only high-rise buildings, nuclear power plants and
dams are presently designed to resist explosive loads.
The influence of exceptional loading can be analysed in two ways: by means of destructive
and non-destructive tests. The former requires sophisticated permissions, measuring devices and
experience. Both experiments and analyses take a lot of time and are expensive. The latter
approach requires a special numerical software, able to compute these highly dynamic phenomena.
Here, only a verification to experimental data is required. Numerical approach has been chosen
with use of Abaqus Finite Element Analysis (FEA) computer code for modelling and simulation of
explosion and shock wave propagation for the purposes of this thesis.
Since many models were required for each analysis, the decision was made to use Python
programming language together with Abaqus Application Programming Interface (API), which
provides means for interaction with Abaqus and use of its functionalities and capabilities. The
computer script has been written to reduce the modelling time and effort. This way the creation of
model, which varied in such parameters as mesh element size, charge weight, size and location of
obstacles and other parameters, has been automated. What is more, generation of new models to
reflect the individual needs requires little user code and programming knowledge.
To model blasts in Abaqus, use of the explicit solver and Coupled Eulerian-Lagrangian
(CEL) formulation can be used. In this formulation air volume is modelled by Eulerian elements,
and obstacles are modelled by Lagrangian elements. The Lagrangian, Eulerian and CEL
descriptions are broadly presented in this thesis and in Appendix A.
In the thesis three tests are presented. The first one is a mesh size study, which aims in
finding the proper size of the finite element, yet computationally efficient and producing reliable
results. Obtained results are verified for correctness based on analytical solution from the UFC
[24].
In the second analysis the blast wave flow through holes in the front face is simulated.
Pressures inside the structure are compared with each other and with outside, reflected pressure.
Again, results are verified with help of UFC [24].
In the final simulation, the numerical research performed by Remennikov and Rose [17]
regarding the blast wave propagation in complex city geometries is repeated. A comparative
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analysis between Computational Fluid Dynamics (CFD) code used by Remennikov and Rose with
CEL code is performed, and scaled distance concept is verified.

1.2. Thesis
1.2.1. Motivation
A significant factor was an increasing danger of terrorist attack proceeded by increasing awareness
of the society about the issue. Lack of both Polish and European standards regarding the
exceptional loading, especially explosive loading, that could be used by engineers while designing,
was another motive. From personal point of view, a desire to experience and explore topics, that
are not covered during the studies, a desire to improve Abaqus skills, especially in terms of writing
user’s scripts (Abaqus Scripting Interface) and to improve the knowledge about Finite Element
Method in general and Coupled Eulerian-Lagrangian in particular were the most significant
reasons.

1.2.2. Objectives
The major objective is to perform a set of analyses regarding the blast wave propagation arisen
from the explosion. Another goal is to prepare a script allowing for model creation based on
certain parameters. As a result, the pressure-time curve at certain significant points should be
returned. The pressure, that can be later used to specify the loading of the structure or to estimate
the level of its destruction.
Generated models will depend on the following parameters:
- size of examined area of wave propagation,
- size of finite elements used for discretization of the model,
- number of explosives, their weight and location,
- number, geometry and location of obstacles,
- location of the analysed structure.
Models can be summarized in three words: 3D, parameterised, scalable.
Except the air pressure changes, the size of finite elements used for discretization, the
pressure change inside a structure as a result of an external explosion and the influence of
surrounding structures on incident wave propagation will be examined.

Thesis structure
The thesis consists of 6 chapters, bibliography, appendices and a CD containing the Python script.
This introductory chapter contains abstract and description of the thesis. Later in this
document the types of exceptional loadings are presented (Chapter 2). Explosive loading in field
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of civil engineering is described. The current state of knowledge and regulations are discussed.
Numerous researches on the topic are recalled. Chapter 3 describes the tools and methods that
were used while working on the thesis. Abaqus FEA, explicit dynamics and modelling in Abaqus
are presented. Moreover, Python programming language, Abaqus Scripting Interface and basics on
interacting with Abaqus FEA are described. In Chapter 4 the Python script used to create models is
shown. Its structure, possibilities, limitations and usage are discussed. Exemplary script is
provided. In subsequent Chapter 5 the description, results and discussion about three types of
simulations are presented. The ultimate chapter includes final remarks related to explosive loading
and blast modelling in Abaqus FEA. Fields of future development are indicated.
Appendix A contains the theory of Lagrangian and Eulerian descriptions. Appendix B
contains UML class diagram with class’ relationships of the model generating script. On the
attached CD the Python script and short technical notes about its usage are provided.

2. Accidental loading
2.1. Introduction
Two European standards are devoted to the issue of accidental actions. [1] defines the basis of
structural design. It contains primary rules and basic concepts regarding the proposed calculation
method (the limit state concept in conjunction with a partial factor method), design situations,
modelling and others. It defines different types of actions loading the structure, inter alia
accidental actions. [2] is fully devoted to the issue of accidental loading, and contains, among
others, provisions about vehicles’ impact, internal explosions of dusts and gases, risk analysis and
some indications on limits of admissible damage. UFC [24] document describes blast loading and
its effects on structures in details. More about these two can be found in subsection Explosive
loading in Civil Engineering.

2.2. Types of accidental loading
Together with permanent and variable loads, accidental loads can be defined. They are a vast and
diverse group of loading arisen from various reasons and requiring individual design proceeding
and analyses. In many cases they can cause severe consequences. When the structure is subjected
to an accidental loading, one can say that exceptional conditions of the structure occur.
According to Eurocode [1], the following accidental action definition stands ([1] 1.5.3.5):
Accidental action - action, usually of short duration but of significant magnitude,
that is unlikely to occur on a given structure during the design working life
Based on [1] 3.2 and [2], major types of accidental loads are:
- seismic events (earthquakes),
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- fire,
- explosions,
- impacts from vehicles (trucks, cars, lorries, ships, trains, others),
- consequences of localised failure,
- wind,
- snow.
Yet impact, snow, wind and seismic actions (loads) may be variable or accidental actions,
depending on the statistical distributions and probability of occurrence ([1] 1.5.3.5 Note 2).
Accidental actions (loads), based on European standards, should be taken into account in
accidental design situation (see [1] 1.5.2.5) or in seismic design situation (see [1] 1.5.2.7), when
the structure is subjected to a seismic event, and shall be considered to act simultaneously in
combination with other permanent and variable actions ([2] 3.2 (5)).
To clarify, the design situation is a set of physical conditions taking place for certain time
under which the structure is required to fulfil its function, and except of two already mentioned,
the standard specifies another two: persistent and transient design situations.
As the definition of the accidental design situation claims ([1] 1.5.2.5), it is a design
situation involving exceptional conditions of the structure or its exposure, including fire,
explosion, impact or local failure. As can be seen, the resistance to explosion effects is indicated
explicitly.

2.3. Explosives
2.3.1. Definition and classification
From the Encyclopædia Britannica [22], explosive is any substance or device that can be made to
produce a volume of rapidly expanding gas in an extremely brief period. Three fundamental types
can be distinguished: mechanical, nuclear, and chemical. A mechanical explosive is one that
depends on a physical reaction (volcanic eruption, failure of a cylinder of compressed gas, mixing
of two liquids), a nuclear explosive is one in which a sustained nuclear reaction can be made to
take place with almost instant rapidity, releasing large amounts of energy, and a chemical one
depends on a chemical reaction. The rapid oxidation of fuel elements (carbon and hydrogen atoms)
is the main source of energy in this type of explosives [18].
Explosives can be classified by their various properties, such as sensitivity, velocity,
physical form, and others.
Chemical explosives account for virtually all explosive applications in engineering. Two
types of chemical explosives can be distinguished: detonating (high explosives) and deflagrating
(low explosives). Detonating explosives are characterized by extremely rapid decomposition and
development of high pressure. TNT and dynamite belong to this group. Deflagrating explosives
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involve merely fast burning and produce relatively low pressures. Black and smokeless powders
are exemplary deflagrating explosives.
Detonating explosives are usually subdivided into two categories, primary and secondary.
Primary explosives detonate when heat of sufficient magnitude is produced. Flame, spark, impact
and others can cause an ignition. Secondary explosives require a detonator and, in some cases, a
supplementary booster [22].

2.3.2. Blast wave propagation
Detonation is a very rapid chemical reaction. During the detonation, chemical explosives release
rapidly large amount of energy, which previously was stored in strong chemical bonds. Gases,
with temperature up to 4000°C and at a very high pressure are produced and expand instantly, thus
forming a layer of hot, dense, high-pressure gas called a blast wave [18]. This blast wave expands
radially outward from the surface of the explosion (Figure 1) and at a supersonic velocity in the
case of high explosives such as TNT. As the wave expands, it decays in strength, lengthens in
duration, and decreases in velocity (Figure 1 in [18]). In ideal situation (perfectly rigid ground), if
the detonation took place on the ground surface, blast wave propagates spherically and will be
identical to a free-air blast from twice the quantity of explosive.

Figure 1. USS Iowa (BB-61) firing a full of nine 410mm and six 130mm guns during a target
exercise near Vieques Island, Puerto Rico, 1 July 1984 [13][14]. Radial propagation of a shock
wave on water surface clearly seen.
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