Concrete under impact tensile loading and lateral compression

Civil Engineering and Geoscience

The influence of moisture on the fracture behaviour of concrete loaded in dynamic tension

Dynamic tests demonstrate an extensive rate effect on the tensile strength as well as the post-peak behaviour beyond loading rates of about 50 GPa/s. One of the possible explanations for the observed rate effects on the fracture behaviour is enhanced resistance by moisture in the pores. To study the influence of the moisture content and pore structure on the rate dependency, different moisture contents and concrete types are used and tested at three loading rates. From the test results it is concluded that the moisture volume, porosity and pore structure play an important role for tensile strength as well as the fracture process. The NMR tests showed that the water in the capillary pores causes the strength increase and not the water in the gel-pores. From the analysis of the experimental results it is concluded that for loading rates < 50 GPa/s, the main cause for the observed strength increase is the viscous behaviour of concrete. For loading rates beyond 50 GPa/s, also rate effects due to limitations on crack propagation contribute to the observed strength increase for all moisture contents and concrete types. Concerning the post peak response for rates > 50 GPa/s, the additional resistance is due to additional micro cracking, the moisture in the capillary pores and the limited crack propagation velocity.Materials and EnvironmentApplied Mechanic

The dynamic fracture energy of concrete: Review of test methods and data comparison

Data on the dynamic fracture energy of concrete are scarce and also not consistent due to different test methods, data analyses and definitions. This paper intends to facilitate the discussion on dynamic fracture energy and start the standardization process for dynamic tensile testing. The response and failure mechanisms in statics and dynamics are addressed. Definitions of the fracture process zone, the fracture zone and the fracture energy are recalled. Test methods to derive strength and, especially fracture energy data for concrete in tension are summarized and reviewed. For dynamics, the uniaxial set-ups are the most suitable. To illustrate the dependency of Gf data to the applied diagnostics and data analysis, a comparison of two data sets for loading rates in the order of 1000 GPa/s is given. The paper ends with an overview of recommended test methods for uniaxial dynamic tensile testing.Structural EngineeringCivil Engineering and Geoscience

Dynamic tensile resistance of concrete-split Hopkinson bar test

The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading, on meso and macro-scale concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, ductility, and, second, there are inertia forces activated which influence the resistance and failure mode of concrete structure. The experimental and theoretical studies show that the influence of loading rate on tensile behavior of concrete is relatively strong. In dynamic testing the split Hopkinson bar (SHB) is used to measure concrete tensile resistance. The results of the experimental measurements show that after reaching some critical strain rate tensile resistance progressively increases with increase of strain rate. The questions discussed in the paper are: (i) what is the reason for progressive increase of tensile resistance ? and (ii) can the resistance be attributed only to material strength or are there some other effects ? To answer these questions the numerical analysis on a simple elastic-cohesive FE model is carried out. Moreover, simulation of the compressive pulse in a concrete bar, which is reflected from the free end-surface of the bar and causes tensile fracture, is carried out for different loading rates. The evaluation of the results clearly shows that the progressive increase of tensile resistance (apparent strength) can be attributed to structural inertia of the fracture zone, which is invoked by cracking of concrete and is not to the true material strength. It is shown that the size of the fracture process zone significantly influence apparent strength. Similar as the true strength it is also discussed that with the increase of strain rate concrete fracture energy does not increase progressively.Structural EngineeringCivil Engineering and Geoscience

Experimental techniques for design of impact-resistant material (poster)

Some polymers are not only transparent and lightweight, but also impact and ballistic resistant. Designing and preparing such polymeric materials with a high impact?resistant performance is of importance to e.g. aviation, military and windscreen applications.Structural EngineeringCivil Engineering and Geoscience

A mesoscale modelling perspective of cracking process and fracture energy under high strain rate in tension

This paper presents a numerical modelling study on the simulation of the cracking process and fracture energy in concrete under high strain rate. To capture the stress wave effect and the damage evolution at the meso-length scale, both a homogeneous model with a millimetreresolution mesh and an explicit heterogeneous mesoscale model with random polygon aggregates are employed. The tendency of development of a) discrete multiple cracks, and b) spread tensile damage across adjacent element layers, in the high strain rate tension regime is scrutinised. This phenomenon generally gives rise to an increase in the dynamic fracture energy, which is consistent with experimental observations. Relative comparison between the homogeneous and heterogeneous mesoscale simulations suggests a sensible effect of the mesoscopic heterogeneity in the dynamic fracture process.Structural EngineeringCivil Engineering and Geoscience

A viscosity regularized plasticity model for ceramics

Plasticity models are frequently used to describe ceramic materials. Well established and often used ceramic models are those by Johnson and Holmquist. These are softening plasticity models for which mesh dependency is a well known problem. A viscosity or rate dependency can be added to the material model to provide regularization and solve the mesh dependency problems. For the Johnson-Holmquist models a viscosity is proposed to work on the hydrostatic tensile strength. A consistency visco-plastic formulation is used. For the Johnson-Holmquist-2 model it is demonstrated that the proposed viscosity indeed removes the mesh dependency problems. This is shown for both quasi-static and dynamic loading. In addition it is shown that the proposed viscosity can predict an experimentally measured rate dependent spall strength of alumina ceramic, while the original model fails to do so.Applied MechanicsMaterials- Mechanics- Management & Desig

Full system response Thomassen tunnel under impact load using LS-DYNA (concept version 3)

In the Delft Cluster work package “Bijzondere Belastingen” (CT01.21) the consequences of a BLEVE and a reduced BLEVE are considered. These phenomena have a low probability of occurrence, but might have immense consequences. Therefore, a deterministic consideration seems not possible. The results of the work package must facilitate the quantitative risk analysis of the phenomena, that support the authorities in their decision of allowing transport of dangerous goods through tunnels or not. The work package focus is on the mechanical description of the loading and the response. However, it requires an interdisciplinary approach, which integrates knowledge of risk analysis, explosion and evaporation of liquefied gases, structural dynamics and soil dynamics. Project description: The project contains two main stream research lines: 1. Loading due to BLEVE. The BLEVE research is mainly executed in a PhD project at Delft University of Technology. This part focuses on an improved understanding and modelling of the BLEVE phenomenon. TNO Defense and Safety will participate in this research line by introduction of practical mechanical modelling of the vessel behaviour and creation of a practical engineering model for BLEVE load prediction, based on the results of a PhD-study. 2. Dynamic Response of the structure-soil system under BLEVE and a reduced BLEVE loading. Here TNO Built Environment and Geosciences concentrates on the structural part of the problem, whereas Deltares and Delft University of Technology will take care of the soil response. TNO Defense and Safety will provide data on appropriate loads for realistic cases. The project is divided into the following work packages: · L1: Mechanical aspects of the initiation of a BLEVE · L2: Thermodynamic and gas dynamic aspects of a BLEVE · R1: Preliminary structural response · R2: Soil behaviour · R3: Full system response · R4: Consequences for surrounding

Simulating brittle and ductile response of alumina ceramics under dynamic loading

Alumina ceramic is often used in armour systems. This material is known to have a brittle response under tensile loading, while a ductile response is found when sufficiently high pressures are reached. During projectile impact a ceramic material experiences both tensile loading and high pressures, hence fails in both a brittle and ductile way. Properly capturing the ceramic failure in a single material model remains challenging. A viscosity regularized Johnson-Holmquist-2 model has been used to simulate dynamic loading on alumina ceramic. The simulations show that the brittle and ductile nature of the material can not be captured simultaneously in the current material model. A new failure strain formulation is proposed where the behaviour under tensile and compressive loading can be controlled independently. This allows to properly capture both the brittle and ductile response of the material in a single constitutive framework, with a single set of model parameters.Accepted Author ManuscriptApplied MechanicsMaterials- Mechanics- Management & Desig

A fully implicit plasticity model for the characterization of ceramics in ballistic protection

The Johnson-Holmquist-2 ceramic model is used for quasi-static indentation simulation. A modification is proposed to an associated plasticity formulation. This allows for a fully implicit solution scheme, where dilatation is used instead of the traditional explicit bulking formulation. Dilatation is shown to have an important influence on ring-crack formation during indentation. A mesh refinement study is performed to show the current tensile failure behaviour leads to mesh-dependent results.Applied Mechanic