Beatriz Esteban’s research while affiliated with University of the Bundeswehr Munich and other places

Throughout the last decades, great attention has been given to model the dynamic behaviour of concrete. As such, numerous material models have been conceived in hydrocodes to describe the complex behaviour of this composite material subjected to large deformations leading to damage and failure. Concrete shows, for instance, strain rate dependency as well as hydrostatic pressure dependency. This article focuses on the comparison of two material models conceived for such purpose. These are the RHT model (RHT - Riedel, Hiermaier, Thoma) developed at the Fraunhofer Ernst-Mach-Institut and the HPG (HPG - Hartmann, Pietzsch, Gebbeken) model developed at the University of the Bundeswehr Munich, both of which have been proven to be successful in a variety of scenarios. They are macro-level models, i.e., they regard concrete as a homogeneous material. Both models incorporate the traditional splitting in hydrocodes of the hydrostatic and the deviatoric response. On the one hand, both material models have three pressure-dependent limit surfaces in the stress space to characterise concrete strength. These are the elastic limit surface, the failure surface and the residual strength surface, the latter being related to a damage model. Strain rate effects are inherent in the models. On the other hand, both material models use the p–α formulation in the Mie–Grüneisen equation of state. The parameters of this equation are, however, differently calculated. A final comparison of these two models is done by conducting numerical simulations where the RHT and the HPG models are used as material models. The scenario is a contact detonation as an example of a high strain rate problem. Such an in-depth and detailed comparison has not been done to date. It aims to help the user decide which material model to choose as well as to provide ideas for future development in the field of material modelling.

Bauwerke können durch Luftstoßbelastung nach Explosionen extrem belastet werden. Insbesondere wenn Gebäude nicht explizit für Explosionsszenarien bemessen wurden, führt diese Belastungsart häufig zu einer Schädigung der exponierten Bauteile, die ihre jeweilige Tragfähigkeit mindert oder gar zum Verlust dieser beiträgt. Der Schädigungsgrad eines Bauteils unter stoßwellenartiger Belastung kann mit Hilfe von Ingenieurverfahren oder mittels numerischer Si-mulationen erfasst werden. Dadurch wird jedoch noch keine Aussage über die Schädigung des Gesamtgebäudes getroffen. Das Ausmaß des strukturellen Ge-samtschadens – und damit die Resttragfähigkeit des Gesamtgebäudes – hängt vom Zusammenspiel der lastabtragenden Bauteile und damit dem Einfluss ihrer jeweiligen Bauteilschädigungen ab. Dieser Beitrag beschreibt, wie die Schädi-gung einzelner Bauteile durch Ingenieurmodelle oder numerische Simulationen bestimmt und klassifiziert werden kann. Anschließend wird ein Ingenieurverfahren vorgestellt, das den zu erwartenden Schaden am Gesamtgebäude auf Grundlage der Einzelschädigungen nachvollziehbar klassifiziert. Eventuell durchführbare Verstärkungsmaßnahmen an kritischen Bauteilen können so gezielt und effizient geplant werden, so dass der erwartbare Gebäudegesamtschaden minimiert wird.

The structural behaviour of reinforced concrete is governed significantly by the transmission of forces between steel and concrete. The bond is of special importance for the overlapping joint and anchoring of the reinforcement, where rigid bond is required. It also plays an important role in the rotational capacity of plastic hinges, where a ductile bond behaviour is preferable. Similar to the mechanical properties of concrete and steel also the characteristics of their interaction changes with the velocity of the applied loading. For smooth steel bars with its main bond mechanisms of adhesion and friction, nearly no influence of loading rate is reported in literature. In contrast, a high rate dependence can be found for the nowadays mainly used deformed bars. For mechanical interlock, where ribs of the reinforcing steel are bracing concrete material surrounding the bar, one reason can be assumed to be in direct connection with the increase of concrete compressive strength. For splitting failure of bond, characterized by the concrete tensile strength, an even higher dynamic increase is observed. For the design of Structures exposed to blast or impact loading the knowledge of a rate dependent bond stress-slip relationship is required to consider safety and economical aspects at the same time. The bond behaviour of reinforced concrete has been investigated with different experimental methods at the University of the Bundeswehr Munich (UniBw) and the Joint Research Centre (JRC) in Ispra. Both static and dynamic tests have been carried out, where innovative experimental apparatuses have been used. The bond stress-slip relationship and maximum pull-out-forces for varying diameter of the bar, concrete compressive strength and loading rates have been obtained. It is expected that these experimental results will contribute to a better understanding of the rate dependent bond behaviour and will serve for calibration of numerical models.

The current contribution presents and evaluates a series of experimental results of shaped charge jets impinging concrete targets. The purpose of this study is threefold. Firstly, the dimensions of the damaged areas, namely the crater, spalling and perforation/penetration regions were measured. Crater dimensions are found to be independent of target thickness whilst spalling dimensions are not. Secondly, the depths of penetration of Normal Strength Concrete (NSC) and Ultra High Performance Concrete (UHPC) were compared. In these experiments the use of UHPC does not provide, on average, a significant reduction in the total depth of penetration, whilst being three times more expensive than NSC. Thirdly, the following three jet properties inside and behind the target were examined: jet velocity, jet mass and jet kinetic energy. The amount by which these properties are reduced in concrete and in air is quantified and compared. As a long term goal, these experiments aim to contribute to the assessment of existing protective structures and, likewise, to the design of future ones.

This paper compares two sets of equations that can be employed to transform the parameters of the shock Equation of State into those of the polynomial Equation of State used in hydrocodes. The first set is taken from literature, whilst the second set is derived by the authors, which can be seen as a correction to the first set of equations. Furthermore, the consequences on the numerical simulations of using the corrected set of equations is discussed. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)