DEM simulation of impact processes (GRS)
Table of contents
Project data
| Titel | Title DEM-Simulation von Impaktvorgängen | DEM-Simulation of Impact Processes Förderer | Funding Bundesministerium für Wirtschaft und Technologie (BMWi) Zeitraum | Period 10/2010 – 09/2013 Leiter | Project Manager Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach Bearbeiter | Contributor Dipl.-Ing. Birgit Beckmann |
Report in the yearbook 2013
DEM simulation of concrete fracture
Strain at total collapse.
The aim of this project is to investigate the failure and fracture behaviour of concrete. The reaching of a certain maximum load or the investigation of the concrete’s behaviour within a range of safe working loads are not in the focus of the project. It is rather a question of the fracture behaviour and the crack propagation preceding the global failure. In a numerical simulation developed at the Institute of Concrete Structures, a concrete specimen with a size of 10 cm is subjected to vertical compression load up to total collapse.
Based on the Discrete Element Method (DEM), the forces and displacements of in the concrete specimen due to the load are calculated in the simulation. For a comparison with engineering quantities, macroscopic values such as stresses and strains can be determined from the known system variables. This is done using averaging approaches. The stresses and strains are postprocessing values and have no influence to on the further simulation process.
In principle, stresses and strains are continuum-mechanic values. This means the stresses and strains are assumed to be continuous or smeared, respectively, and constant within a certain range of influence. Furthermore this means that the calculation of stresses and strains presupposes steady fields and a continuous concrete body. The two figures show exemplary results of the project. The horizontal strain of concrete specimen at low load intensity can be seen in the figure on the left side. Red-coloured areas show higher tensile strains than yellow-coloured ones. Thus, ranges with future cracks can be identified before the onset of the crack initiation and the arising of a macroscopic crack. The right figure shows the horizontal strain at total collapse. It shows primarily that the strain calculation can formally be done formally, but it is not meaningful at this loading state. Due to increased crack evolution and proceeding damage accumulation, the concrete specimen can no longer be seen as a continuous body in post-peak behaviour. Along with this, the assumption of a continuous concrete body and of continuous fields becomes invalid. As a consequence it is identified that the calculation of stresses and strains is meaningful only at low loading states.
Report in the yearbook 2012
DEM simulation of concrete fracture phenomena
Compression tests in laboratory
Concrete fracture phenomena are investigated in this research project. It is not the aim to reach a certain maximum load or to investigate the concrete behaviour within a range of save working loads. It is the fracture processes and failure mechanisms, which are specifically investigated. Why does a crack rise just at this or that position? How vary the crack positions from one test to another? Is it possible to identify in advance where for example micro-cracks will emerge to a global macro-crack later on? Such questions are the matter of this research project.
In real uniaxial compression tests, slightly varying maximum loads are achieved. They are in the range of the same order of magnitude, but they are never exactly the same from test to test. The crack patterns of two tests are similar and typical, but they are never exactly the same and differ in kind and position of the actual cracks.
Such real laboratory experiments are described using a two-dimensional numerical simulation based on the Discrete Element Method (DEM) in this research project. The concrete body is discretised as an ensemble of separate particles. Otherwise than in continuum-based methods, here the single particles are cinematically independent and can move towards and away from each other. A repulsing force is calculated resulting from a virtual overlap area of two colliding particles. Doing this for all particles of the concrete body, the force flow from the loading plate to the supporting plate through the concrete body becomes visible. When two particles move away from each other, this leads – automatically – into a crack. This means, cracks are not added to the simulation using, say, special crack elements or crack width parameters, but they emerge just due to the fact, that particles separate from each other.
A random function within the particle generation leads to similar, but slightly varying particle ensembles. This is comparable to the specimen production in the laboratory, for example several concrete specimens made of the same charge. Neither in the laboratory nor in the simulation, the positions of aggregate and hydrated cement are never exactly the same for two concrete specimens. Simulating the compressive test using virtual test specimens, similar, but not identical fracture patterns arise – in simulation as well as in the laboratory experiment.
Report in the yearbook 2011
2D DEM simulation of concrete fracture
Compressive stress during post-peak behaviour
The aim of the project is to investigate phenomena of concrete fracture especially under high loading velocities and to simulate crack patterns, which appear before and during the concrete failure. The shape and size of the concrete specimen chosen in the simulation are the same as in real laboratory tests. Regarding the load cases, the focus lays on impact loads such as rock fall or vehicle impact.
The concrete behaviour is simulated with an inhouse-code using the Discrete Element Method DEM. The concrete body is discretized into separate – discrete – elements. From the kinematic point of view, each element can follow its own rigid-body motion. The load is applied onto the concrete specimen via virtual loading plates. Between the concrete elements, contact forces are transmitted, which influence the position and motion of the elements in the next time step. Doing so, the behaviour of a concrete specimen can be simulated with an assembly of many single particles. It is a great advantage of the Discrete Element Method, that cracks can evolve anywhere in principle. In contrary to continuum-based methods, the cracks are not represented by special crack elements added on potential positions, but arise automatically from the model during simulation. For different assemblies of the same charge of concrete specimen, the crack patterns resemble broadly, but differ in details and mirror the statistic character of concrete fracture – same as in real-life laboratory experiments.
In a post-processing, stresses are calculated from the forces acting on the particles. This allows the comparison to engineering variables. The graphic at the left hand side shows the stress distribution in a time step concisely after peak load of a standard compression test. The graphic at the right hand side shows the stress distribution with wider cracks during post-peak behaviour. In elements coloured in red, no (or almost no) compressive stress is transmitted. Thus, the later spalling areas can be seen, even when the cracks are still extremely thin.
Report in the yearbook 2010
DEM simulation of impact processes
Crack pattern after simulated failure of concrete specimen
The aim of the project is to increase the understanding of fracture processes, damage processes and wave propagation of concrete due to impact loading. Therefore, the concrete behaviour under impact loading, i.e. under high dynamic loading and high strain rates, is analysed in a numerical simulation using Discrete Element Method. The propagation of pressure waves and the initiation of microcracks and microstructural changes as well as the effects of defects in material structure on loading capacity is studied. A close cooperation of numerical and experimental investigation is required.
Why investigation of impact loading? The investigation of impact loading is of general interest. On the one hand, accident situations like rock fall, vehicle impact or plane crash and the high dynamic loads involved are not preventable. Therefore, the investigation of impact loading subserves the protection of buildings, especially of such buildings like hospitals, nuclear power plants requiring increased structural safety. This applies all the more, since impact loading mostly happens without precedent indication. On the other hand, the theoretical fundamentals of the rather complex processes of crack initiation, concrete damage and concrete failure due to impact loading are of interest. The occurring cracks need not lead necessarily to failure, but they induce substantially changed material behaviour.
Why numerical simulation? Although numerical simulations represent a virtual space and cannot replace real experiments, they allow the investigation of processes, which are rarely realisable in real life experiments due to high pressure, high temperature or danger of explosion, respectively. Furthermore, spatial and temporal variables are arbitarily scalable, independently if the real process takes a few seconds or several months. Moreover, variables can be varied separately and independently. A computer simulation represents a link between laboratory experiments and physical theory. The interaction of two particles – for example two solids or two molecules – can be described analytically. But already a three particle system causes analytical difficulties. Therefore, computer simulations are a suitable way to treat such problems.