J. Weerheijm

BLEVE blast by expansion-controlled evaporation

This report presents a new method to calculate the blast effects originating from an exploding vessel of liquefied gas. Adequate blast calculation requires full knowledge of the blast source characteristics, that is, the release and subsequent evaporation rate of the flashing liquid. Because the conditions that allow explosive evaporation are not entirely clear and the evaporation rate of a flashing liquid is unknown, safe assumptions have been adopted as the starting point in the modeling. The blast effects from a boiling liquid expanding vapor explosion (BLEVE) are numerically computed by imposing the vapor pressure of a flashing liquid as boundary condition for the gas dynamics of expansion. The numerical modeling is quantitatively explored just for liquefied propane. In addition, this paper demonstrates that often an estimate of BLEVE blast effects is possible with very simple acoustic volume source expressions.

How to Determine the Dynamic Fracture Energy of Concrete. Theoretical Considerations and Experimental Evidence

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.

Response mechanisms of concrete under impulsive tensile loading

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.

Research developments and experimental data on dynamic concrete behaviour

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.

Dynamic test devices for analyzing the tensile properties of concrete

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.

Drop weight impact strength measurement method for porous concrete using laser doppler velocimetry

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.

Dynamic Response of Concrete at High Loading Rates a New Hopkinson Bar Device

Abstract Explosion scenarios in tunnels, the potential hazards from storage of high energetic materials and terrorist attacks have become important safety issues. The mechanical response of concrete structures exposed to impact loading can only be predicted with proper material modelling that includes the rate effect of concrete. The influence of moisture on the rate effect of concrete is studied with a Split Hopkinson Bar. The results are presented in this paper. A new testing device for very high loading rates is developed. The development of the device and a new measurement technique is presented. The first results on concrete are promising.

Axial Dynamic Tensile Strength of Concrete under Static Lateral Compression

The rate effect on concrete tensile strength can be modeled by the description of crack extension in a fictitious fracture plane [1,2].The plane represents the initial, internal damage and the geometry of the final fracture plane. In the paper, the same approach is applied to model the failure envelope for the biaxial loading condition of static lateral compression and axial impact tensile load. The predicted failure envelope is compared with data from experimental work.

Ultimate Deformation Capacity Of Reinforced Concrete Slabs Under Blast Load

In this paper a test method to determine the deformation capacity and the resistance-deformation curve of blast-loaded slabs is described. This method was developed at TNO-PML. The method has been used to determine the ultimate deformation capacity of some simply supported reinforced concrete slabs in order to increase the knowledge of this parameter. The influence of the following parameters on the dynamic deformation capacity has been studied: the thickness of the slab, the amount of bending reinforcement and the diameter of the bending reinforcement. Preliminary results have been obtained.

DAİRESEL AGREGALI MODEL BOŞLUKLU BETONLARIN DİNAMİK DAVRANIŞININ SONLU ELEMAN YÖNTEMİ İLE ANALİZİ

Bosluklu beton, agrega tanelerinin birbirine ince bir cimento hamuru tabakasi ile baglanmasi sonucu olusan, yuksek oranda mezo-boyutta bosluk iceren ozel bir tip betondur. Guvenlik uygulamalarinda kullanilmak uzere dayanimi arttirilmis bosluklu betonlar gelistirilmesi amaciyla gerceklestirilen bir projede, bosluklu betonlarin dinamik davranislari sonlu eleman yontemiyle analiz edilmistir. Analizlerde, ABAQUS/Explicit programinda tanimli bulunan acik direct entegrasyon metodu kullanilarak dairesel agregali bosluklu betonlar incelenmistir. Bosluklu betonlar ve bir yalin betonda basinc gerilmesi konturlerinin gelisiminden yola cikarak dalga ilerlemesi hizi tahmin edilmistir. Hesaplanan degerlerin literaturdeki degerlere ve deneysel ultrases dalga hizi sonuclarina cok yakin oldugu belirlenmistir. Bunun yaninda iki farkli boyutta agrega iceren bosluklu betonun dayanimlarinin birbirine neredeyse esit oldugu tespit edilmistir. Bosluklu betonlarda olusan hasar dagilimi ve gerilme konsantrasyonlari incelendiginde, deneylerde de tespit edildigi gibi dinamik yukleme altinda coklu catlaklar ve yaklasik olarak agrega boyutunda fragmanlar olustugu gorulmektedir. Bu nedenle, fragman boyutunun agrega boyutu tarafindan belirlendigi tespit edilmistir.