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Adobe is a traditional masonry made of sundried earthen bricks and mud mortar. Despite a millennial history of buildings of architectural value, adobe still connotes a so called ‘not engineered’ construction type. Namely, the material and structural properties of adobe are still not entirely addressed, resulting in an equally uncertain normative framework for adobe buildings design. However, over the last ten years, a large research program has been conducted in the Netherlands to qualify the material and structural properties of this sustainable building technology. In this paper, a critical analysis of the current normative body for the material characterization of adobe is addressed. Guidelines, prescriptions and requirements related to test methods, materials selection and properties contained in the available building codes for adobe around the world are assessed. A critical normative review is performed using the most recent literature produced on adobe, with particular regards to the results of experimental tests and numerical simulations performed by the authors. On the basis of these findings, some issues have been identified in relation to the knowledge currently condensed in the norms for adobe. A series of programmatic guidelines is aimed at orienting future research on adobe as well as fostering the process of updating its current normative body.

A local damage model is proposed for the numerical assessment of the static performance of Adobe masonry components. The model was applied to simulate the experimental behaviour of sundried soil bricks and mud mortar tested in uniaxial compression and bending. Numerical simulations of the model are made mesh objective by means of a rate dependent regularization algorithm in statics. This is achieved using a generalization of the damage delay concept based on a decomposition of the Dirichlet boundary condition. It allows non-dimensionality of model parameters mathematically needed to prevent loss of ellipticity of the equilibrium equations of the model. The entire regularization algorithm is integrated within an implicit Newton-Raphson solver. © 2018 Elsevier Ltd

In this study, an impact strength measurement method based on high speed photography, that has been developed for testing porous concrete, was introduced. In the experiments, a drop weight impact test set-up instrumented with a high speed camera was used. The impact strength analyses were conducted using impedance mismatch method, where the wave reverberations were investigated in detail. The measurement configuration carries the advantage of being a fast, non-contact and accurate experimental method where only the known material properties and particle velocity data of the drop weight are required. The target cementitious material itself is not actually involved in the measurements or the subsequent analyses. The method also facilitates the observation of the crack patterns throughout the experiment. The measurement method was verified to be accurate and consistent by testing different types of porous concretes and by comparing the results with those from other dynamic monitoring techniques, such as laser Doppler velocimetry and direct stress gauge measurements.

The deformation-to-fracture evolution of a flexible polymer material under high-strain-rate compressive loading conducted by a split Hopkinson pressure bar (SHPB) setup was investigated. Representative tests were carried out at different strain rate levels, followed by the characterization of dynamic damage after each test. Craze and crack patterns on the end surface of the specimen were carefully analyzed. The failure patterns appear along the radial and circumferential directions. The sequence of their formation with increasing strain/stress level was revealed. The mechanisms resulting in the craze and crack patterns were analyzed. The heterogeneous stress distribution in the specimen and the resultant damage morphologies were demonstrated. This research not only shows the deformation-to-fracture evolution of a flexible polymer material under SHPB loading, but also provides a better clarification of the localized stress distribution in the tested material via SHPB technique.

Adobe is an ancient building technology made of sun dried bricks joined together by mud mortar. This paper deals with the physical and mechanical characterization of three different typologies of adobe bricks and one typology of mud mortar produced in Europe. They differed in terms of internal soil element proportions and amount of organic content. Physical tests consisted of granulometry, moisture content and density tests. The mechanical characterization consisted of uniaxial compressive tests and three point bending tests. Tests were performed according to modern material standards. The main mechanical properties both in tension and compression were determined at different curing conditions. The outcome provided in this study offers a general overview on the assessment of the mechanical performance of adobe in relation to the properties and interactions of its soil constituents. In fact, the comparison between components with the same soil mineralogical family and production process made it possible to assess both at a qualitative and quantitative level the effect of the physical properties of the mixture (such as fiber and clay percentages or moisture content) on the mechanical parameters of the resulting bricks and mortar. This paper proposes new predictive formulations of the most relevant material parameters in strength and deformation, such as compressive strength, deformation at peak stress and ultimate displacement for both adobe bricks and mortar. They quantify the influence that water content, clay percentage and fiber reinforcement produce on the mechanical performance of the tested adobe components. This was made possible by means of multivariate statistical analyses on the mechanical parameters derived from all the tested samples.

Although many aspects of the fracturing process of concrete are now well understood and successfully simulated with various models, it is still very difficult to properly simulate the different failure mechanisms observed in a concrete structure induced by ballistic impact. In this paper, an enhanced version of the effective-rate-dependent nonlocal damage model [Eng. Fracture Mechanics, 176 (2017)] is proposed to simulate the response of concrete in such events. Hydrostatic damage has been added to the formulation in order to take the damage of the material matrix observed while porosity reduces during compaction into account. Besides controlling the evolution of the nonlinear volumetric response of the material, this new damage variable contributes to the deterioration of the material stiffness upon confinement. It is demonstrated that the description of the nonlinear volumetric response of concrete by an equation of state (EOS) as a plasticity phenomenon, as it is commonly done in hydrodynamic constitutive modeling, is unrealistic for concrete. Such formulations fail to represent the effect of the loss of cohesion observed during compaction on the deviatoric response of the material. By taking this phenomenon into consideration, the proposed model systematically predicts the relevant failure modes (cratering, tunneling, radial cracking and spalling) observed during ballistic impact on a concrete plate as a function of the projectile velocity and plate thickness. © 2017 Elsevier Ltd

There is limited knowledge on deriving the strength of an explosion based on the observed explosion effects. Existing methods are based primarily on the effects of blast overpressure on surrounding structures. Window breakage and structural damage are well studied and are widely used to derive most probable explosion strengths. Other explosive effects such as debris throw are usually left out of such forensic analyses. In this paper, a quantitative method is presented that can be used to obtain the explosion strength based on debris throw. When the method is applied together with analysis of other evidence at a post blast scene, more accurate predictions of the original amount of explosive can be obtained. An overview of the required information that has to be collected from a post-blast scene is outlined, which will aid in the backward calculations that will be carried out in the next stage. This calculation method is applied to two case studies, which illustrate that it is applicable to events involving both low and high order accidental explosion events. Besides accidents, the method is also applicable to intentional explosion scenarios such as terrorist attacks. It is shown that the added value of such a method to forensic investigation depends on the precision of the data collected. Therefore, it is recommended that first responders not only register the impact locations but also attempt to identify the part of the donor installation from which the debris originated. Furthermore, estimates should be made of the launch height and ballistic properties of the debris and information should be recorded on obstructions and terrain properties in the vicinity. In this way, the quality of the backward calculations can be optimized. © 2017 Elsevier Ltd

Boşluklu beton, agrega tanelerinin birbirine ince bir çimento hamuru tabakası ile bağlanması sonucu oluşan, yüksek oranda mezo-boyutta boşluk içeren özel bir tip betondur. Güvenlik uygulamalarında kullanılmak üzere dayanımı arttırılmış boşluklu betonlar geliştirilmesi amacıyla gerçekleştirilen bir projede, boşluklu betonların dinamik davranışları sonlu eleman yöntemiyle analiz edilmiştir. Analizlerde, ABAQUS/Explicit programında tanımlı bulunan açık direct entegrasyon metodu kullanılarak dairesel agregalı boşluklu betonlar incelenmiştir. Boşluklu betonlar ve bir yalın betonda basınç gerilmesi kontürlerinin gelişiminden yola çıkarak dalga ilerlemesi hızı tahmin edilmiştir. Hesaplanan değerlerin literatürdeki değerlere ve deneysel ultrases dalga hızı sonuçlarına çok yakın olduğu belirlenmiştir. Bunun yanında iki farklı boyutta agrega içeren boşluklu betonun dayanımlarının birbirine neredeyse eşit olduğu tespit edilmiştir. Boşluklu betonlarda oluşan hasar dağılımı ve gerilme konsantrasyonları incelendiğinde, deneylerde de tespit edildiği gibi dinamik yükleme altında çoklu çatlaklar ve yaklaşık olarak agrega boyutunda fragmanlar oluştuğu görülmektedir. Bu nedenle, fragman boyutunun agrega boyutu tarafından belirlendiği tespit edilmiştir., Porous concreteis a special type of concrete that includes a high amount of meso-size airpores and is formed by the aggregate particles assembled by a thin layer ofcement paste. In the scope of a research project, having an objective ofdesigning enhanced strength porous concretes to be used in safety applications,dynamic properties of porous concretes were analyzed with finite elementmethod. In the analyses, porous concretes with circular aggregates wereanalyzed by using the explicit direct integration technique implemented inABAQUS/Explicit. Based on the analysis results, stress wave propagation speedsof porous concretes and a plain concrete were estimated based on stresscontours. The numerically estimated values were found to be very close to thereference values in literature and the experimental results. On the other hand,the impact strengths obtained for two model porous concretes having differentaggregate sizes were found to be nearly equal. When the computeddamage distributions and stress concentrations were examined, it was seen thatunder dynamic loading, the fragments formed were approximately at the size ofaggregates. Therefore, it is concluded that the fragment size in porousconcretes is mainly determined by the size of the aggregates incorporated inthe mixture.

This contribution presents a numerical study towards the propagation and branching of cracks in quasi-brittle materials, using a new effective rate-dependent damage model, enhanced by a stress-based nonlocal (SBNL) regularization scheme. This phenomenological model is mesh objective and reproduces the major phenomena associated with crack propagation and branching in quasi-brittle materials. It is discussed and demonstrated that the branching phenomenon is not controlled by a specific, material dependent, crack speed. Instead, it is governed by the evolution of the principal stresses at the crack tip, which are controlled by the evolution of damage. It is demonstrated that, with increasing crack speeds, the principal stresses at the crack tip tend to evolve from a mode-I to a mixed-mode state. Beyond a certain (critical) crack speed, the stress distribution around the crack tip reaches a critical state at which a single crack is no longer stable. When this condition is met, crack branching occurs whenever the stress field at the crack tip is destabilized by either a physical discontinuity or an interfering stress wave reflected at the specimen boundaries.

From a macroscopic point of view, the dynamic tensile response of concrete is mainly due to the viscous behavior of the bulk material and inertia effects at multi-scale levels. It has been suggested that almost all of these mechanisms have to be considered as intrinsic material properties and explicitly included in the constitutive relations of the continuum model. It is discussed and demonstrated that the use of semi-empirical dynamic strength increase factor (DIF) functions to numerically describe rate effects do not characterize true constitutive relations of the material. A strain-rate dependent formulation is used to describe the strength and fracture energy increase of concrete under dynamic tensile loading conditions. However, instead of the commonly used e (instantaneous strain-rate) to update the constitutive law, an effective rate (R) is considered. With this new concept a time scale is introduced in the constitutive law which restrains the ‘evolution of rate’, to represent the inherent dynamic properties of concrete. This has a weak regularization effect and acts as a localization limiter. Mesh objectivity is recovered with the addition of a material length scale to the constitutive relations, here accomplished by an explicit stress-based nonlocal regularization scheme. Two sets of modified split Hopkinson bar tests are simulated for validation, using respectively notched and un-notched specimens. The results are objective and in good agreement with the experiments.

Adobe is one of the most ancient forms of masonry. Adobe bricks are sundried mixtures of clay, silt, sand and natural fibres locally available joined together using mud mortar. Adobe structures are largely spread in areas of the world prone to earthquakes or involved in military conflicts. Unfortunately, almost no literature concerns the dynamic assessment of soil-based masonry components. From earlier research, it was derived that the mechanical behaviour of adobe in statics fits in the class of quasi brittle materials. Its resemblance with cementitious materials concerns the main failure modes and the constitutive models in compression. This study deals with the experimental characterization of adobe components response in dynamics. It is aimed to study and quantify the rate sensitivity of adobe material from bricks at a wide range of strain rates, from statics up to impact conditions. In particular, the influence of fiber reinforcement in the mixture on the mechanical behaviour of the material has been addressed. Adobe bricks are commonly mixed using organic content locally available in the field, from straw to chopped wood. Fibres are added to prevent shrinkage cracks during the air drying process. In modern materials such as concrete, inclusion of artificial fibres is originally meant to enhance the mechanical performance of the material, benefiting from the selective properties of reinforcement and binder. An experimental campaign was carried out in a collaboration between Delft University of Technology, Dutch Ministry of Defence, TNO and the Joint Research Centre (JRC) of the European Commission. Two types of bricks were tested. They both had the same soil composition in terms of mineralogical family and soil elements proportions but only one was mixed using straw and wood. Cylindrical samples were subjected to compression tests at different rates of loadings in compression: low ( _ 1 = 3 10􀀀4 s􀀀1), intermediate ( _ 2 = 3 s􀀀1) and high ( _ 3 = 120 s􀀀1). High strain rate tests were performed using the split Hopkinson bar of the Elsa-HopLab (JRC). For each test, high resolution videos registered the failure process and force-displacement plots were recorded. Elaboration of results revealed clear trends in the dynamic material behaviour. Adobe, as concrete, is sensitive to the loading rate. The rate effects on the main properties of the material in strength and deformation are also analytically and numerically quantified. Rate sensitivity and failure mode are significantly influenced by the inclusion of fibers in the mixture. These effects are quantified, interpreted and compared with modern SFRC. This paper presents the experimental campaign and the obtained results. Moreover, physical interpretations for the observed trends are discussed. Finally, new formulations for the assessment of the dynamic increase factor of the compressive strength of adobe are proposed.

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impact has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. In this contribution, a rate-dependent stress-based nonlocal damage model has been introduced for the simulation of dynamic tensile failure of quasi-brittle materials. The recently proposed stress-based nonlocal criterion has been updated in order to be consistently combined with a rate-dependent version of the well-known Mazars damage model. The model was implemented in LS-DYNA using a fully explicit computational scheme. Two sets of numerical examples have been presented. First, one-dimensional numerical analyses were conducted to evaluate the model capabilities, applicability and limitations. Second, the model has been validated against experimental results. It has been shown that the proposed model, in addition to correcting spurious mesh sensitivity, also provides a more realistic representation of damage initiation and growth, in particular around discontinuities (notches and free boundaries) and damaged areas.

A glass-polymer system of a polyurethane elastomeric matrix with a single 3 mm-diameter glass particle was investigated using a split Hopkinson pressure bar (SHPB) setup for revealing the dynamic compressive mechanical response. This study produced the characteristics of the dynamic stress-strain relation and the relations for the rate dependencies of yield stress, maximum stress and strain energy. A high-speed camera was applied to record crack initiation, propagation and fragmentation fracture. Scanning electron microscopy (SEM) was employed to explore the dynamic damage mechanisms. The static and dynamic compressive mechanical properties of a glass-polymer system were compared with these of monolithic polyurethane elastomeric polymer material. The results of this study provide the dynamic response at unit cell level and are used for development and evaluation of transparent, impact-resistant protection concepts of glass-polymer systems.

Masonry is a construction technique which typically reacts in compression. Characterization of its material properties in compression is thus of paramount importance. This especially counts for adobe bricks because their material properties are still unknown to a large extent. This traditional masonry, made of locally available soil and fibres, is spread in areas currently involved in military conflicts, where also European forces operate. Therefore, not only its static properties in compression, but even more the dynamic strength is a relevant parameter. Laboratory characterization of material properties still pose several challenges, among which so-called size dependence is one of the most controversial topics. This entails the possible variation of material properties values from tests on specimens of different size and shape. Several factors may concur to its determination and a well-founded theory does not exist yet. This counts for statics and even more in dynamics. Addressing the properties in compression of bricks at high strain rates is rare, namely no studies of size dependence on masonry bricks in dynamic regimes are published. Lately, a series of experimental campaigns were conducted by the authors at the Joint Research Centre of the European Commission. In these campaigns, a series of compression tests were performed on several types of adobe bricks. Different soil mixtures were used to produce cylindrical samples of different sizes. Compressive tests from 2e-5 s-1 to 10 s-1 and 100 s-2 were executed using hydraulic machines as well as split Hopkinson bars. Next, the static as well as the dynamic material properties as calculated from tests on specimens of different sizes and material compositions have been qualitatively and quantitatively compared and interpreted. In this paper, the experimental program is presented, next the material properties in strength and ductility as well as the dynamic increase factors are investigated.

In this study, the impact of a steel spherical projectile on an alumina ceramic is considered. New experimental and numerical results are presented and analysed. Numerical results are obtained using the Finite Element Method and the upgraded viscosity regularized Johnson-Holmquist-2 constitutive model to describe the ceramic material behaviour. First, a short investigation is done using 2d Finite Element simulations to establish a proper numerical framework. Second, the numerical framework is extended to 3d and experimental results are used to validate the framework and the ceramic material model. This shows that all relevant ceramic failure mechanisms are captured correctly and the framework and model can be used to simulate sphere impact on ceramic material. Third, the simulations are used to analyse the failure processes in the ceramic material in more detail. Here the focus lies in obtaining information which can currently not be retrieved from experiments. Timing and interaction of propagating conical and radial cracks are investigated and corroborate with the typical failure mechanisms observed in sphere impact on ceramic material.

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. © 2019 Elsevier Ltd

A local damage model has been recently developed for the numerical simulation of the static behaviour of adobe bricks. Mesh insensitivity of the local model was obtained by generalizing the damage delay concept based on a Dirichlet boundary condition decomposition integrated in an implicit solver. The regularization properties of the model were proven before only in statics. In this study, mesh independence is demonstrated in dynamics analysing the problem of a cantilever bar uniaxially loaded at high deformation rates. Furthermore, the physical background of the delay formulation is interpreted regarding the main failure processes in compression exhibited by quasi brittle materials used in masonry. Two limitations of the model in correctly simulating the dynamic behaviour of masonry bricks have been observed. Corrections to the original damage delay formulation are proposed in this study. These enhance the capability of the model to address also distributed failure of traditional geo-materials and the inherent rate dependence also at high strain rate regimes. The improvements are demonstrated in this paper by means of numerical simulations of both theoretical tests and practical applications. These consist of experimental tests in compression recently performed by the authors at different strain rates, from statics to high velocity impact tests.

This study presents the numerical analyses conducted to investigate the impact behavior of different porous concretes, which have also been cast and tested experimentally. For a realistic representation of the real porous concretes containing arbitrary shaped air pores, a mesh generation code was developed in which the aggregates in the mixtures were directly extracted through computed tomography. In the code, mineralogically different aggregates in porous concretes with gravel could also be individually defined. In the explicit finite element analyses conducted, porous concrete was considered as a four-phase material, consisting of aggregates, interfacial transition zones (ITZ), bulk cement paste and air. The pore size distribution and the fragmentation behavior of the concretes were also numerically analyzed. Among the parameters that have been investigated both numerically and experimentally, aggregate grading, which determines the porosity and pore size distribution of the material, was found to have a dominant effect on the strength as well as the fragmentation properties of porous concretes. Although the amount of ITZ is higher in mixtures containing finer aggregates, those mixtures had higher impact strengths compared to coarser aggregate ones again owing to their much finer pore structures. © 2019 Elsevier Ltd

In this work, findings of a numerical study performed to investigate the impact behavior of porous concrete, modeled as a four phase cementitious composite consisting of aggregates, cement paste, interfacial transition zones (ITZ) and air, are presented. The numerical analyses contributed to the process of designing a special type of concrete for safety purposes i.e. as a protective building material to be used in safety walls outside important buildings or munition magazines for storing explosives. In case of an explosion, large concrete fragments that are formed, cause a very important threat. Therefore, in the scope of a research project, designing a special type of concrete having sufficient strength, but fracturing into small fragments under impact loading was aimed. In the numerical analyses, model porous concretes, in which the amounts and properties of pores and aggregates could be varied individually, were used to see the sole effect of each parameter. According to the results, it was found that at constant total porosity, the impact strength increased with decreasing pore size while multiple fragmentation was observed. On the other hand, the impact strengths of porous concretes with different size aggregates (with constant total aggregate content and porosity) were approximately the same when no ITZ was defined. However, when ITZ was present, the impact strength was found to decrease as the aggregates were finer. This trend was also valid for the respective full concretes. Representative experimental results of porous concretes were also presented in order to support the numerical results.

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.

The glass interface effect on dynamic tensile response of a soft polyurethane elastomeric polymer material has been investigated by subjecting a glass-polymer system of this polymer material matrix embedded a single 3 mm-diameter glass particle to impact loading in a split Hopkinson tension bar (SHTB) setup. The characteristics of the dynamic stress-strain response of the glass-polymer system have been determined. Yield stress and maximum stress show strain rate dependency according to a power-law function. Glass interface effects on dynamic properties were studied quantitatively at different strain rates. The debonding deformation behaviour accompanied by cavitation and necking has been carefully analysed. Damage mechanisms were explored in order to understand crack initiation and propagation at the glass-polymer interface and inside the polymer matrix. This work is of significance in order to evaluate a transparent impact-resistant device of a soft polyurethane elastomeric polymer material backed by the rigid glass, to develop a particle-polymer system for propellant and to calibrate computational models for designing an advanced rigid-particle-modified composite material system for protective applications. © 2015 Elsevier Ltd.

Transportation of flammable liquefied gas in tunnels presents a significant risk of an accidental loss of containment leading to an explosion with major consequences. Possible scenarios include a BLEVE, a non-reactive gas expansion explosion and a reactive gas explosion. Quantification of the risk and consequences associated with such events is central in the design of tunnels and routing of dangerous goods. TNO previously developed a Quantitative Risk Analysis (QRA) method, which combines a probability assessment with state-of-the-art explosion effect and consequence models. The current article extends this model to combine the dispersion of a flammable cloud with its probability of ignition and the resulting physical effects such as overpressure. The model assumes an increasing probability of ignition with both the number and the duration of vehicles present within the flammable cloud. Various case studies are considered to illustrate the effect of different ignition probability parameters. These cases deal with instantaneous and continuous LPG releases with varying release rates including the effect of ventilation. They clearly show the capability to quantify the ignition probabilities and gas explosion load. The combination of the gas dispersion, gas explosion and ignition probability models are needed to derive design loads for tunnels, to perform tunnel risk assessments, and to develop safety measures. These models form the backbone for quantitative risk assessments.

This paper presents the results of an experimental research aimed at assessing the material performance of adobe bricks in compression for a wide range of induced strain rates, from statics to high velocity impact. Adobe connotes a traditional masonry whose bricks are made of sundried soil mixtures possibly reinforced with natural fibres and joined together using mud mortar. The inclusion of fibre and the presence of water in the mixture have a dominant effect on the mechanical performance of adobe bricks and masonry. Their influence on the dynamic behaviour of this material is quantified and interpreted in this study at high strain rates also with data produced through Hopkinson bar testing. Appropriate dynamic increase factors and constitutive equations for adobe materials in dynamics are also investigated. The paper presents the experimental campaign, shows the main results and offers qualitative and quantitative interpretations for the principal damage patterns observed. © 2019 The Author(s)

Porous concrete incorporates a high percentage of meso-size air voids that makes its mechanical characteristics remarkably different from normal concrete. A research project was undertaken to design a special type of porous concrete, that fractures into small fragments when exposed to impact loading while having sufficient static strength, to be used in protective structures such as safety walls or storages for explosives. In the concretes designed, while a sufficient static strength was required, high porosity was essential to facilitate the formation of multiple cracks and the subsequent fracturing. Production of porous concretes having improved static compressive strengths was accomplished by modifying the mixture design and the compaction technique; while the design procedure was supported by macro and mesoscale mechanical testing, computed tomography, microscopy and X-ray diffraction analysis.

This paper presents a time integration scheme capable of simulating blast loading of relatively high frequency on porous media, using coarse meshes. The scheme is based on the partition of unity finite element method. The discontinuity is imposed on the velocity field, while the displacement field is kept continuous. The velocity discontinuity is postulated to occur in the time domain. The developed time integration scheme is unconditionally stable and has controllable numerical dissipation in the high frequency range. An important feature of the time scheme is that it allows for controlling the numerical damping in a consistent way. The time scheme has been implemented in combination with Biot's theory of wave propagation in saturated porous media. Numerical examples have demonstrated that the proposed time scheme is, in addition to being accurate and stable, highly effective for coarse meshes. This makes the developed scheme suitable for large scale finite element analysis. © 2011 Elsevier Ltd.

The resilience of a city confronted with a terrorist bomb attack is the background of the paper. The resilience strongly depends on vital infrastructure and the physical protection of people. The protection buildings provide in case of an external explosion is one of the important elements in safety assessment. Besides the aspect of protection, buildings facilitate and enable many functions, e.g., offices, data storage, -handling and -transfer, energy supply, banks, shopping malls etc. When a building is damaged, the loss of functions is directly related to the location, amount of damage and the damage level. At TNO Defence, Security and Safety methods are developed to quantify the resilience of city infrastructure systems (Weerheijm et al. 2007b). In this framework, the dynamic response, damage levels and residual bearing capacity of multi-storey RC buildings is studied. The current paper addresses the aspects of dynamic response and progressive collapse, as well as the proposed method to relate the structural damage to a volume-damage parameter, which can be linked to the loss of functionality. After a general introduction to the research programme and progressive collapse, the study of the dynamic response and damage due to blast loading for a single RC element is described. Shock tube experiments on plates are used as a reference to study the possibilities of engineering methods and an explicit finite element code to quantify the response and residual bearing capacity. Next the dynamic response and progressive collapse of a multi storey RC building is studied numerically, using a number of models. Conclusions are drawn on the ability to predict initial blast damage and progressive collapse. Finally the link between the structural damage of a building and its loss of functionality is described, which is essential input for the envisaged method to quantify the resilience of city infrastructure.

Multiple-particles-polymer systems of a polyurethane elastomeric matrix embedding 25wt.% and 50wt.% 0.5 mm-diameter PMMA particles were investigated using a split Hopkinson pressure bar (SHPB) setup for revealing the dynamic compressive mechanical response. Taking the 25wt.%-particles-polymer system as a reference case, the characteristics of the dynamic stress-strain relation were quantified and analyzed. Yield stress, maximum stress and strain energy show strain rate dependent behaviour, following a power law function. Based on the power law functions, a strength-toughness relation induced by strain rate was derived. The static and dynamic compressive mechanical properties of the 25wt.%-particles-polymer system and these of the monolithic polyurethane elastomeric material were compared. A high-speed camera was applied to record crack initiation, propagation, interaction and final fragmentation. Scanning electron microscopy (SEM) was employed to explore the damage mechanisms of the multiple-particles-polymer system. By comparing the dynamic compressive data of the 25wt.%- A nd 50wt.%-particles-polymer systems, the multiple particles effect on the dynamic compressive response was analysed. The results of this study are of significance for developing a transparent particles-polymer framework with a required static stiffness for impact-resistant applications and for designing and evaluating hybrid particles-filled polymer composite material systems. © 2016 Elsevier Ltd. All rights reserved.

Traffic infrastructure in urbanised areas is increasingly projected in tunnels underground or covered over, these days. A consequence is that in case of an incident with hazardous materials the safety level for fellow road users in tunnels is considerably less than it is in surface infrastructure. To reduce the consequences of incidents for fellow tunnel users, urban tunnels are sometimes interrupted by open spaces of limited length. Open spaces allow, for instance, the release of smoke in case of a fire. In this way, possible lethal effects are limited to the tunnel section in which the incident occurred. To what extent an open space may also be effective in the mitigation of blast effects from an explosion in a tunnel system is subject of this paper. To this end, the blast effects originating from the rupture of a 50 m(sup-scr)3 LPG pressure vessel in an urban tunnel system have been computed by numerical simulation. The results show that an open space in a tunnel system has a significant mitigating effect on the blast effects indeed. However, as a consequence of the ingress of a high-velocity jet flow that follows on a primary blast wave, a second blast wave develops in the tunnel section following on an open space. The strength of this second blast wave is not very dependent on the length of the open space. It shows that an open space in a tunnel system may not always limit the lethal effects of explosion incidents in tunnels to the tube in which the incident occurred. The second blast wave in the tunnel section following on an open space may have lethal consequences for fellow tunnel users by car window failure. Language: en

A new method is presented to calculate the blast effects originating from an exploding vessel of liquefied gas. Adequate blast calculation requires full knowledge of the blast source characteristics, i.e., the release and consequent evaporation rate of the flashing liquid. As the conditions that allow explosive evaporation are not entirely clear and the evaporation rate of a flashing liquid is unknown, safe assumptions are the starting point in the modelling. The blast effects from a BLEVE are numerically computed by imposing the vapour pressure of a flashing liquid as boundary condition for the gas dynamics of expansion. The numerical modelling is quantitatively explored just for liquefied propane. In addition, it is demonstrated that often an estimate of BLEVE blast effects is possible with very simple acoustic volume source expressions. The modelling shows that the rupture of a pressure vessel containing a liquefied gas in free space only develops a blast of significant strength if the vessel nearly instantaneously disintegrates. Even if a rupture and the consequent release and evaporation of a liquefied gas extend over just a short period of time, the blast effects are minor.

The dynamic mechanical behaviour of a soft polymer material (Clear Flex 75) was studied using a split Hopkinson pressure bar (SHPB) apparatus. Mechanical properties have been determined at moderate to high strain rates. Real time deformation and fracture were recorded using a high-speed camera. Fracture micrographs were examined to explore the deformation and fracture mechanisms. Crazing and microcracking mechanisms were indicated to be decisive for the dynamic response and impact resistance of this soft polymer material. The stress-strain curves at various strain rates were derived to investigate the strain rate sensitivity. The yield stress shows a rate-dependent behaviour. Temperature rise was also measured by an infrared radiation (IR) camera to investigate the transformation of strain energy at different strain rates. It is of crucial significance to understand the deformation and fracture mechanisms, to study the rate-dependent behaviour as well as to develop a new impact-resistant framework for real engineering application. © 2015 Elsevier Ltd.

The dynamic tensile mechanical response of a soft polymer material (Clear Flex 75) is investigated using a split Hopkinson tension bar (SHTB). Stress-strain relations are derived to reveal the mechanical properties at moderate and high strain rates. These relations appear to be rate dependent. Under static loading, the polymer exhibits an elastomeric behaviour, while under dynamic loading, the response is elasto-plastic with a hardening branch. The critical strain rate for transition from a rubbery-like behaviour at low strain rates to a glassy-like behaviour at high strain rates at room temperature is determined. The axial and lateral deformation of the specimen in the SHTB test is recorded by a high-speed camera. The final fracture surface is examined by SEM to explore the physical origins of deformation and fracture behaviour: void formation, craze nucleation, craze extension, crack initiation and propagation. Meanwhile, a shielding mechanism is revealed by the observation of crazing and micro cracking in the crack tip zone, which contributes to the dynamic tensile toughness of CF 75 polymer material. © 2015 Elsevier Ltd. All rights reserved.

Boşluklu beton, yüksekmiktarda mezo-boyutta hava boşluğu içeren özel bir tip betondur. İçerdiğiboşluklar nedeniyle boşluklu betonun mekanik özellikleri normal betonlardanoldukça farklıdır. Bu nümerik çalışmanın amacı, boşluklu betonun dinamik yükaltındaki davranışının mezoskopik olarak analiz edilmesidir. Gerçekleştirilensonlu eleman analizlerinde, açık direkt entegrasyon (explicit directintegration) yöntemi kullanılmıştır. Betonun çimento bazlı fazlarınıntanımlanmasında Beton Hasar Plastisite Modeli kullanılmıştır. Boşluklu betonundört fazlı bir malzeme olarak gerçeğe yakın bir şekilde temsil edilebilmesiiçin her bir fazın ayrı bir şekilde tanımlanabildiği bir sonlu eleman ağıgeliştirme programı oluşturulmuştur. Boşlukların etkilerinin daha iyi araştırılabilmesiiçin dairesel boşluklar içeren yalın betonlar şeklinde tanımlanmış modelboşluklu betonlar ayrıca incelenmiştir. Gerçek boşluklu betonların nümerikincelemeleri ile elde edilen sonuçlar, gerek darbe dayanımı gerekse çatlakdağılımı yönünden deneysel sonuçlarla uyum göstermektedir., Porous concrete is a special type of cementitious material incorporatinga high amount of meso-sized air pores thatmakes its mechanical characteristics markedly different from normal concrete.The objective of this numerical study ismesoscopically analyzing the behavior of porous concrete under dynamic loading.In the finite element analyses, explicit direct integration method was adopted.Concrete Damage Plasticity Model was selected to define the material propertiesof the cementitious phases. With the aim of realistically representing porousconcrete as a four-phase material, a mesh generation program was developedwhere each phase was separately defined. In order to better investigate theeffects of the properties of pores, model porous concretes were also analyzedin the form of plain concrete meshes incorporating circular pores. Thenumerical analysis results of real concrete mixtures were in good agreementwith the experimental results both in terms of quantifying the impact strengthas well as demonstrating a realistic crack pattern formation for the porousconcretes that have been analyzed.&nbsp

The handling of explosives and ammunition introduces a safety risk for personnel and third parties. Accidents related to storage, transport and transshipment may result in severe injury and material damage. Dispersion of structural debris is one of the main hazards resulting from detonations inside structures. Reliable prediction models for debris dispersion are essential for risk assessment methods. In this article we give a statistical description of the dispersion of explosion produced debris. The basis is a general expression for the projectile areal number density in the horizontal and vertical plane. Combined with engineering models for the launch conditions, predictions can be made. An analytical solution to the equations of motion may be used to allow for fast calculations. A parameter study shows consistent results. The model has been validated with internal detonation tests of bare charges in reinforced concrete structures. The validation clearly shows the prediction capabilities of the model for three loading regimes described in the literature. We give a thorough description of the validity and limitations of the model, and an outlook of current and future research. Examples are the failing debris mass distribution prediction for reinforced concrete in the shock overloading regime, and ricochet and roll and break-up at impact phenomena.

Owing to their low tensile failure strain, concrete is a difficult material to test under dynamic tensile loading. Indeed, conventional testing apparatuses such as high-speed hydraulic presses or Split Hopkinson Bar facilities rely on a mechanical balance of the specimen implying a short round-trip time in the specimen in comparison with the loading time to failure and consequently loading-rates below few hundreds of GPa/s. Above this threshold the specimen is clearly unbalanced and these methods are inadequate. Other techniques, such as spalling tests, plate-impact experiments that do rely on stress-wave analysis or edge-on impact tests that are used to visualize the tensile damage in the target, then come into play. In this chapter, different experimental methods are sorted in four sets to point out their field of use, their limitations and a number of results obtained in the literature. © 2013 Woodhead Publishing Limited All rights reserved.

The response of concrete up to complete failure in tension is represented in the load deformation relation. The characteristic parameters are the ultimate strength, stiffness in the ascending branch and the fracture energy. All these parameters depend on concrete composition and environmental conditions, as discussed in the previous chapters. The observed response of concrete at macro-level is determined by the damage initiation and damage accumulation mechanisms at meso- and micro-scale levels. The failure process is governed by (i) the stress condition, (ii) the mechanisms governing microcrack nucleation, propagation and obscuration of critical flaws, (iii) the ability to absorb energy in fracture, and (iv) the energy flow from the surrounding material into the fracture zone. In dynamics, all four conditions vary in time and depend on the loading rate. This chapter discusses the background and mechanisms of the rate-dependent behaviour of concrete, focusing on the effects of (i) inertia and limited cracking velocity at material level, (ii) concrete composition at meso-level, and (iii) moisture content and pore distribution. The available experimental data on dynamic strength and fracture energy are presented and related to response mechanisms for the different loading rate regimes. © 2013 Woodhead Publishing Limited All rights reserved.

The strength of concrete is its heterogeneous composition. It is a system that is formed by the chemical process of hydration, producing crystalline and amorphous reaction products interlocking and binding the aggregates together. The material grows in time, resulting in a resilient system that is sufficiently strong to carry loads but can also respond to environmental conditions. Crack initiation and crack growth at the various scale levels govern the mechanical tensile response of the heterogeneous concrete material. Therefore, the fracture mechanics principles of strength and energy criteria help in understanding and modelling the response mechanisms. The internal stress conditions and defect distributions are at (i) meso-level, governed by the aggregate grading, mortar and bonding (ITZ) properties, and at (ii) micro-level, defining the mortar properties (aggregates-cement matrix, ITZ and capillary pore system). The structure at micro/nano-level (cement matrix and micro-pore system) gives the sub-scale condition for the mortar. In this chapter we will describe the concrete system and the material structure from the material science point of view at the microscopic and mesoscopic levels, respectively. It provides general background information for the chapters that follow. © 2013 Woodhead Publishing Limited All rights reserved.

In this study, an experimental configuration that reveals the dynamic response of porous concretes in a drop weight impact test was introduced. Through the measurement of particle velocity at the interface, between the impactor and the concrete target, the dynamic response was obtained in an easily applicable way. Laser Doppler velocimetry (LDV) was used in monitoring the time history of the particle velocity at the interface, which was subsequently analyzed to determine the dynamic strengths of the concrete specimens tested. The velocity measurements were analyzed using a special reverberation application of the impedance mismatch method. The test results showed that the experimental configuration was sufficient to measure the dynamic strengths of porous concretes and a normal concrete with moderate strength. The method was validated by using impactors having different dynamic impedances in testing the same material and was also verified to be precise enough to distinguish between different types of porous concrete mixtures.

Data on the dynamic fracture energy of concrete are scarce and also not consistent due to different test methods, data analyses and definitions. In [1] the authors summarized and evaluated the test methods. Suggestions for the standardization of dynamic tensile testing were given. In the current paper the discussion is continued. First, definitions for the fracture energy and the relevant parameters are given. Next, theoretical considerations are given for the different rate dependency regimes of the dynamic tensile strength. Fracture and damage mechanics form the basis for the theoretical modeling. Based on the same principles, it is shown that the enhancement of the fracture energy occurs at higher loading rates than for the tensile strength. Phenomenological models to quantify the dynamic fracture energy are still lacking. To quantify the dynamic fracture energy, uniaxial test conditions are required. The Hopkinson bar technique meets this requirement. The paper presents and evaluates available data and relates these to the theoretical considerations. © (2011) Trans Tech Publications.

This paper deals with the effect of an internal explosion of cased ammunition on RC magazine structures. By means of a numerical approach, the main differences between the effect of a bare and cased charge are illustrated. A bottom-up strategy has been implemented which is based on modeling the single fragment impact, and enables to compute a complete structure. The simulations confirm the fact that the cased charge has a more destructive and localized effect than the bare charge. The study shows that the numerical erosion has a small overall effect on the response.

At the Delft University of Technology the dynamic material response of concrete is being investigated, using the split Hopkinson bar technique (SHB). To gain more insight into the influence of the initial stress conditions on the dynamic response, the SHB has been combined with a static compression loading device perpendicular to the SHB. This paper describes the test set-up, the monitoring techniques and the tests performed during the development of the biaxial loading device. Special attention is paid to the interaction of both loading devices in the biaxial set-up.

Terrorist attacks, explosion scenarios in tunnels and the potential hazards from storage of high energetic materials have become important safety issues. Knowledge about the response of concrete structures to impact and explosive loading is required for reliable safety assessment and the design of protective structures. A complicating factor is the fact that concrete is a rate-dependent material, which means that the mechanical properties of concrete depend on the applied loading rate. The mechanical response of structures exposed to explosive loading can only be predicted properly with material models that include this rate effect.

A deterministic model has been developed that describes the throw of debris or fragments from a source with an arbitrary geometry and for arbitrary initial conditions. The initial conditions are defined by the distributions of mass, launch velocity and launch direction. The item density in an exposed area, i.e. the number of impacting debris or fragments per unit of area, has been expressed analytically in terms of these initial conditions. While existing models make use of the Monte Carlo technique, the present model uses the source function theorem, an underlying mathematical relation between the debris density and the initial distributions. This gives fundamental insight in the phenomenon of throw, and dramatically reduces the required number of trajectory calculations. The model has been formulated for four basic source geometries: a point source, a vertical cylinder, a horizontal cylinder, and a vertical plane. In combination with trajectory calculations the item density can be quantified. As an illustration of the model, analytical results are presented and compared for the vertical plane and the vertical cylinder geometry under simplified assumptions. If uncertainties exist in the initial conditions, the model can be used to investigate these initial conditions based on experimental data. This has been illustrated on the basis of a trial with 5 ton of ammunition stacked in an ISO container. In this case the model has been successfully applied to determine the debris launch angle and velocity distribution, by means of backward calculations. If, on the other hand, sufficient information on the initial conditions is available, the model can be used as an effect model in risk assessment methods, or for the requirements on protective measures. The model can be used to predict safety distances based on any desired criterion. © 2007 Elsevier Ltd. All rights reserved.

The response of concrete structures exposed to explosive and impulsive loading is an important safety issue. Numerical modelling can be used to predict the dynamic response. However, a proper prediction is only possible when the material behaviour of concrete and the failure mechanisms at high loading rates are known. The importance to know and understand the rate dependency of concrete was recognised by Reinhardt. In the early eighties he initiated experimental and theoretical research in Delft on the behaviour of concrete under dynamic tensile loading.