Contact Detonation Research Articles

Modeling masonry walls under far-field and contact detonations

Masonry walls represent one of the most common applied constructions in civil engineering and architecture. The inhomogeneous nature of masonry imposes a challenge in the development of robust modeling techniques especially under high dynamic loads. In this contribution some modeling strategies are discussed which are suitable for the simulation of unreinforced masonry walls under explosion loads. The damage formation of the wall under loads resulting from far-field as well as contact explosions is investigated. Firstly the issue of an appropriate material model for bricks under high strain rates is discussed. Through a proper adaptation of a material model initially developed for concrete under blast loads all the necessary parameters for the bricks are derived. Subsequently the focus lies in capturing the local damage formation and the resulting debris due to a contact detonation. Small parts of the wall in the proximity of the explosion separate from the main body of the wall and travel at high velocities. They can establish an additional danger for persons and infrastructure which theoretically lie in a secure distance from the target of the explosion. The numerical results which will be presented in this paper are validated with appropriate experiments.

Experimental investigation of p-section concrete beams under contact explosion and close-in explosion conditions

Abstract This paper presents the results of an experimental investigation on explosive breaching of p-section concrete beams. Twenty three p-section concrete beams with a 100 cm length were tested. TNT charges were placed at three positions: contact detonation in the center, contact detonation above the web and close-in detonation in the center. The external and internal breach parameters of the panels were evaluated by measuring the diameter of the ejection crater, spalling crater and breach hole created by the charge detonation. The experimental results were compared to predict values obtained by the analytical models proposed by McVay, Morishita and Remennikov. A modified breach with crater limit line and breach without crater limit line were put forward based on the experimental results. The maximum cross-sectional destruction area ratio (MCDAR) values were used to evaluate the damage degree. The maximum value of MCDAR reached 0.331 corresponding to the C5 experimental condition, of which explosion occurred above the web.

Investigation of Spalling Damage in Ultra-High Performance Concrete Through X-ray Computed Tomography

Ultra-high performance concretes (UHPC) are increasingly used to build protective structures such as headquarters, nuclear power plants or critical civil engineering structures. However, under impact or contact detonation, concrete is exposed to high-rate tensile loadings that can lead to intense damage modes. Such complex damage modes need to be correctly characterised. When a UHPC sample is subjected to a dynamic tensile loading by means of the spalling technique the post-mortem pattern shows a large number of fractures that cannot be seen with a classical observation of the external face (inner crack network). In the framework of the Brittle’s CODEX chair project, the fracturing process in spalled samples of UHPC is investigated with X-ray computed tomography. The tensile loading is applied thanks to a spalling technique that is based on the reflection of a compressive wave on a free boundary. The concrete samples are entirely scanned using X-ray tomography prior spalling test to identify the initial microstructure, and post spalling test to analyse the damage pattern. Image analysis tools are used in both steps. The main fracturing properties are related to the microstructure of the tested concrete.

「児童・生徒の衣食住に関する意思決定」調査報告書への家庭科リーダーの意見(第2報) : 「触発意見」の分析

「児童・生徒の衣食住に関する意思決定」調査報告書への家庭科リーダーの意見(第2報) : 「触発意見」の分析

Impact and close-in blast response of auxetic honeycomb-cored sandwich panels: Experimental tests and numerical simulations

Protecting building, critical infrastructure and military vehicles from Improvised Explosive Devices (IEDs) has become a critical task. This study aims to examine the performance of a new protective system utilizing auxetic honeycomb-cored sandwich panels for mitigation of shock loads from close-in and contact detonations of high explosives. Both field blast tests and drop weight tests were performed using the proposed sandwiches asa shield for concrete panels in combination with conventional steel protective plates. The combined shield was found to be effective in protecting reinforced concrete structures against severe impact and close-in blast loadings. The honeycomb core with re-entrant hexagonal cells shows evident auxetic characteristics which contribute substantially to outstanding force mitigation and blast-resistance performances of such sandwich panels. Numerical simulations showed good agreement with the experimental results. The proposed auxetic panels were found to perform better than conventional honeycomb panels of the same size, areal density and material. Both were found to boost the energy absorption of the monolithic steel plate by a factor of 2.5 by changing its deformation pattern under close-in blast loading. In addition, a combination of the steel plate and an auxetic sandwich panel has aerial specific energy absorption (ASEA) higher than either of them, showing great potential for the development of lightweight blast protection of civil, mining, military, nuclear infrastructure and vehicles.

A study of RC bridge columns under contact explosion

This paper presents a study on reinforced concrete (RC) bridge columns under contact detonation. Two 1/3 scale RC bridge columns with circular and square cross-sections are studied both experimentally and numerically. Field tests were performed on two types of columns under 1 kg TNT contact explosion, and acceleration data at different heights of the columns were collected by accelerometers. Investigation of the damage evolution and structural local response is conducted by high-fidelity physics-based numerical models that are developed in the commercial program LS-DYNA through the Arbitrary Lagrangian–Eulerian (ALE) algorithm. The results from the numerical simulation are compared with the experimental results. Field blast tests showed that for both columns the cover concrete on the proximal and side surfaces close to the explosive charge suffered serious damage, while the cover concrete on the distal face remained almost intact. Due to the column geometry, the contact explosion caused larger blast loads on the square column leading to more severe damage. The damage mechanism of the two columns is discussed based on numerical simulations. The results from the numerical model match well with those from the tests except for the back-surface damage in both cases. Field data obtained from accelerometers also show reasonable agreement with results from the numerical modelling and confirm the localized structural response of the columns under contact blast loads. It is shown that the numerical models established in this study provide reliable predictions for the structural response of bridge columns under contact explosion.

A detailed comparison of two material models for concrete in the dynamic loading regime, RHT and HPG

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.

Protective components made of steel fiber reinforced concrete under contact detonation

As a part of a research project at the University of the German Armed Forces we investigate steel fiber concrete for use in protective structures.At the chair for concrete construction of the University of German Armed Forces a literature study on protective concrete components under gunfire and contact detonation was conducted. In a first short experimental study tests were performed in order to obtain data for numerical modeling and for supplementary test setups.In the main research project different mixtures of steel fiber concrete were produced and investigated under gunfire and contact detonation. For the concrete local raw material from the area of mission was used as aggregates and cement. Plates for the contact detonation with different thicknesses, varying fiber geometries and reinforcement systems were produced. The plates for contact detonation were loaded with 850 g PETN explosive at the test facility of the German Armed Forces Technical Center for Protective and Special Technologies.During the contact detonation a pressure wave runs through the plate and is reflected as a tensile wave at the free surface, opposite to the loaded surface. This tensile wave generates a damage crater on the rear side of the plate. The volume as well as the surface of the crater is determined by Impact-Echo tests, by 3D-scans and by measurements of the crater geometry.One target of this project is to work out a semi empirical formula for the determination of the crater volume and its surface in dependence on the main input parameters in order to provide a mobile application for the soldiers in charge of the infrastructure during a mission. The input parameters for this formula include amongst others the compressive strength fck, the bending tensile strength fct, the fiber geometry and the fiber content.This publication focuses on the conduction of the contact detonation tests. Furthermore, a mathematical approach for numerical analysis is presented.

Experimental and numerical study on steel wire mesh reinforced concrete slab under contact explosion

With the rising of terrorism and rapid urbanization around the world, increasingly more structures are exposed to the threats from accidental and hostile explosion loads. To provide adequate structural protection against blast load, novel materials and strengthening techniques are under fast development. In the present study, a composite slab design aiming at high level blast resistance is studied. In the matrix of high strength self-compacting concrete, besides conventional rebars serving as primary reinforcement, steel wire meshes are embedded and served as secondary reinforcements. Moreover, on the concrete cover layer where the tensile cracks locate, steel fibres are added to provide micro crack-bridging effect. Preliminary numerical simulations adopting coupled Finite Element (FE) and Smoothed Particle Hydrodynamics (SPH) are carried out in hydro-code and the results are used as guide for field blast test. Composite slab with optimal design is field tested under 1kg TNT contact detonation, and the results are compared with slabs made of conventional and ultra-high performance concrete without steel wire meshes. The results demonstrate that slab with steel wire mesh reinforcement develops localized membrane effect when subjected to blast loads and shows better blast resistant capability as compared to the slabs without steel wire meshes.

Numerical investigations on static and dynamic responses of reinforced concrete sub-assemblages under progressive collapse

To study the effect of blast pressure on structural resistance against progressive collapse under column removal scenario induced by contact detonation, and to investigate the development of catenary action within ultra-fast dynamic regime, a physics-based finite element model is developed in this paper. The model is first validated by a quasi-static test series on reinforced concrete sub-assemblages under middle column loss assumption and a blast test series using the same structural configurations. The sub-assemblage included a two-span beam, a middle column stub and two column stubs at both sides. Besides validations with sub-structure tests, some pull-out tests are also performed to verify the numerical models. After the verifications, parametric studies are conducted to investigate the influence of important dynamic and structural factors such as the boundary stiffness, damping ratio, and charge weight attached to the middle column. The study shows that under actual blast conditions, catenary action in sub-assemblages can be mobilised to prevent a structure from collapse even when the bottom longitudinal reinforcement in the bridging beam has already fractured. Moreover, stiffness of horizontal restraints plays an important role to mitigate disproportionate collapse in both static and blast conditions. A comparison is also made between nonlinear dynamic procedure and nonlinear static analysis incorporating simplified energy method for dynamic assessment. It is concluded that the simplified static approach in lieu of dynamic analysis can be considered as a conservative method for practical design purpose. Nonetheless, this method may over-estimate structural resistance if the localised damage is induced by a contact-detonation event.

A comparison of numerical modelling strategies in contact detonation scenarios with concrete targets

With continuous advancements in computational capacity, it has become possible and feasible to numerically model very complex physical phenomena, for instance, high dynamic loads. Hydrocodes or, in other words, “wave propagation codes” were conceived to model such scenarios. Several numerical discretisations are available in these programs, which require the problem at hand to be modelled in distinct ways and which yield different results. In the present contribution, three different numerical strategies are compared. These employ a coupling of the Euler and the Lagrange scheme, the Euler scheme by itself as well as the Smooth Particle Hydrodynamics (SPH) scheme. Their application in the hydrocode ANSYS Autodyn to a contact detonation scenario with a concrete target and with a breakthrough is described as an example of a high dynamic load. This scenario is of special interest since it is a possible threat to critical infrastructure. The numerical results are compared and contrasted; individual strengths and weaknesses of the three numerical modelling strategies are identified also by validating their numerical results with an experimental one. To the authors’ knowledge, such comparison has not yet been done for contact detonation. It is concluded that the SPH method is the preferred strategy to model the considered scenario.

Response of reinforced concrete structural elements to near-field and contact explosions

The integrity of buildings and the safety of their occupants can be catastrophically affected by the action of contact or near-field explosions. Numerical simulation studies should be carried out in order to better understand the response of reinforced concrete (RC) structures to such dynamic impulsive actions. When concrete structural elements are subjected to near-field blast loads, large deformations, rate effects and overloading render the modelling of concrete extremely complex. Significant research has been done since September 11, 2001, in order to improve the computational abilities, to understand the complex non-linear behaviour of concrete under extreme loading conditions and to obtain reliable numerical results. Simulations can be done using ‘user-developed’ codes or commercial hydrocodes. One of the most common hydrocode is LS-DYNA, where concrete can be treated as a homogeneous material. In this particular project LS-DYNA is used along with the Winfrith Concrete Model to simulate the response of a two-layer RC slab to several contact detonations (using C4, Composition-4). The aim is to achieve a contact explosion between the target (slab) and the explosive since literature can provide us with only limited information regarding this. The slab and the high explosive are explicitly modelled using the multi-material arbitrary Lagrangian–Eulerian approach (MM-ALE). The obtained numerical results are validated using experimental data. The aforementioned numerical results of the damage are in favourable agreement with the experimental data, with average errors lying under 10%. In addition, it was observed that the increase in the explosive mass led to a change of the damage mode from penetration to perforation. Concrete’s crack pattern was also investigated.

“Sandwich-cladding” – Protection Measure against Contact Detonations

As part of the German national research project “Protection of critical bridges and tunnels – SKRIBTPlus” HOCHTIEF Engineering has conducted the development and testing of a protection measure against contact detonations in tunnels. The measure consists of a two layer cladding – therefore called “Sandwich-cladding” – which is installed at the inner side of the tunnel lining. It minimizes the effects of contact detonations to the tunnel lining significantly. An unprotected tunnel lining might be seriously damaged by a contact detonation, especially if the lining thickness is relatively small. If in addition the hydrological situation comprises flowable sands and high ground water pressure, the result might be a tunnel collapse or at least a water and soil swallet.The two layered “Sandwich-cladding” is build up as a precast concrete segment. The outer layer consists of micro-reinforced high-strength concrete which is known as DUCON (DUctile CONcrete). Between the outer layer and the proper tunnel lining the inner layer is placed, consisting of extremely compressible concrete. Both layers are poured as precast concrete segments, connected by reinforcement bars and then fixed to the tunnel lining using dowels and screwsThe effectiveness of the measure has been assessed in a series of blasting tests. Different materials as well as various combinations of layer thickness were tested. In order to get quantifiable results, the damaged specimens also have been subject to a load bearing test. Based on the results of these tests, in particular the remaining load bearing capacity after damage, the effectiveness of the “Sandwich-cladding” could be demonstrated and verified. In addition this was underlined by the results of numerical simulations of the test procedure.The “Sandwich-cladding” also functions as a fire protection measure, as it protects the tunnel lining from overheating and spalling. This has been assessed and verified in real scale fire-tests.

Numerical modeling of close-in detonations of high explosives

The effects of close-in detonations of high explosives are characterized in terms of incident and reflected overpressures and impulses. Studies are performed using a verified (1D) and validated (2D) CFD code. Calculations are performed to verify the code in 1D, estimate blast effects using 1D models, predict incident overpressures and impulses in the near field, provide guidance on the use of reflecting and transmitting boundaries in 2D and 3D models, provide recommendations on cell size, and predict reflected overpressures on a rigid reflecting surface for close-in detonations. Contact detonations and their effects are not modeled. The complex wave field in the Mach stem region is studied.

Dynamic Progressive Collapse of an RC Assemblage Induced by Contact Detonation

The nature of progressive collapse is a dynamic event caused by accidental or intentional extraordinary loading. Most published experimental programs are conducted statically, without any consideration of the accidental loading and treating progressive collapse as threat independent. This paper demonstrates the more realistic process of progressive collapse in an experimental program on reinforced concrete subassemblages collapsed by a combination of dead weight loading and contact detonation. The dynamic results are represented systematically at different aspects and compared with previous published quasi-static experiments in terms of structural mechanisms, crack patterns and local failure modes. Moreover, the dynamic increase factor (DIF) of reinforcing bars and the dynamic load amplification factor (DLAF) are investigated and discussed. Following the above comparisons and the findings in the dynamic tests, previous quasi-static test results can be linked to actual progressive collapse behavior more convincingly. Finally, the dynamic tests also highlight the effect of contact detonation on structures, which are often not considered in quasi-static tests and design guidelines. The test results indicate that contact detonation causes uplift and out-of-plane actions to the subassemblage before their downward movement under gravity load, in which the strain rate of reinforcement is between 10⁻² and 10⁻¹/s. Moreover, the structural mechanisms are similar in both quasi-static and dynamic tests.

補強量の違いが耐爆補強効果に及ぼす影響

When designing blast-resistant reinforced concrete (RC) structures, it is important to reduce spall damage due to reflected tensile stress waves. In this study, to devise a blast-resistant strengthening method for existing RC slabs, contact detonation tests using 200 g of explosive charges were conducted on RC slabs with rear surfaces strengthened with continuous fiber sheets/meshes. High-strength carbon fiber (HSCF) and high-modulus carbon fiber (HMCF) sheets were employed to strengthen 75-mm-thick RC slabs, and the number of layers used for strengthening them was varied between 2, 4, and 6 layers. Two types of carbon fiber reinforced plastic (CFRP) meshes, in which the weight per unit area of carbon fibers was varied between 215 and 430 g/m2, were also used to strengthen 100-mm-thick RC slabs. The results showed that the spall failure of the 75-mm-thick RC slabs could be prevented by bonding more than four HSCF sheets onto the rear side of the slab; however, the HMCF sheets had no effect on the blast resistance of the slabs because of fiber rupture. Further, the spall failure of the 100-mm-thick RC slabs could be prevented by bonding the CFRP meshes with fibers 430 g/m2 in volume. On the basis of the test results, the criteria for fiber rupture in CF sheets due to detonation loading was discussed in terms of the fracture energy of the sheets and the modified-scaled concrete thickness of the slab. Furthermore, high-speed photography was employed to investigate the spall failure process of each strengthened slab.

Three-dimensional modelling of steel fiber reinforced concrete material under intense dynamic loading

This paper presents a new three-dimensional numerical model to study the dynamic response and failure mode of steel fiber reinforced concrete (SFRC) material under impact and blast loading. In the 3D numerical model, SFRC is assumed to be composed of two components, that is, the homogeneous concrete matrix with lower strength and the steel fibers with higher strength. The straight round steel fibers are assumed to be dispersed with random locations and orientations in the matrix and a 3D grid algorithm is introduced to generate the finite element analysis model. The interaction between fibers and matrix is modeled by the bonding and sliding contact algorithm. The mechanical behavior of SFRC material is simulated and a good agreement with the test data is observed. The verified numerical model is then employed to perform a series simulation of split Hopkinson pressure bar (SHPB) and split Hopkinson tensile bar (SHTB) in order to study the dynamic increase factor (DIF) of SFRC material with different fiber volume percentage (Vf) under high strain rate loading. And the influence of Vf on the dynamic properties of SFRC is analyzed. Further numerical studies of the SFRC specimens under contact detonation are carried out and compared with the test data. The result shows that the developed numerical model for SFRC could precisely predict the failure mode.

접촉 폭발 하중을 받는 RC 슬래브의 국부 손상 및 내폭 성능 평가

본 연구에서는 다양한 RC 슬래브의 접촉 발파 실험을 수행하여 내폭 성능을 평가하였다. RC 슬래브의 내폭 성능 향상을 위해 섬유 보강과 외부 CFRP 시트 보강을 도입하였다. 폭발하중 실험은 <TEX>$2,000{\times}1,000{\times}100mm$</TEX> RC 슬래브를 제작하였고, 일반 콘크리트와 강섬유 보강 콘크리트, 하이브리드 PVA 섬유 보강 시멘트 복합체, 초고성능 콘크리트를 적용하였다. 접촉 발파로 생긴 RC 슬래브의 손상 정도를 크레틀, 스폴과 브리치의 직경과 깊이로 평가하였다. 실험 결과를 LS-DYNA 유한요소해석 프로그램과 Morishita 등의 예측식으로 검증하고 비교분석하였다. 분석 결과, LS-DYNA 프로그램을 이용하여 크레틀, 스폴, 브리치의 직경 및 깊이에 대한 개략적인 예측이 가능하며, 폭발하중 하에서 손상부의 거시적 거동을 모사함으로써 부재의 파괴 이력을 나타낼 수 있었다. 국부 손상에 대한 세가지 예측식이 소개되어 있으나 경험식으로써의 한계가 존재하며, 이에 대한 추가 연구가 필요하다고 판단된다. In this study, the resistance of various reinforced concrete (RC) slabs subjected to contact detonation was assessed. In order to enhance the blast resistance, fibers and external FRP sheets were reinforced to RC slabs. In the experiment, the <TEX>$2,000{\times}1,000{\times}100mm$</TEX> sized RC slabs were fabricated using normal concrete (NC), steel fiber reinforced concrete (SFRC), polyvinyl alcohol fiber reinforced cementitious composite (PVA FRCC), and ultra-high performance cementitious composites (UHPCC). The damage levels of RC slabs subjected to contact detonation were evaluated by measuring the diameter and depth of crater, spall and breach. The experimental results were compared to the analyzed data using LS-DYNA program and three different prediction equations. The diameter and depth of crater, spall and breach were able to be predicted using LS-DYNA program approximately. The damage process of RC slabs under blast load was also well expressed. Three prediction equations suggested by other researchers had limitations to apply in terms of empirical approaches, therefore it needs further research to set more analytical considerations.

Blast Resistance of Double-Layered Reinforced Concrete Slabs Composed of Precast Thin Plates

When designing blast-resistant reinforced concrete (RC) structures, reducing spall damage due to reflected tensile stress waves is a major problem. Furthermore, for rapid construction of the blast-resistant structures against sudden terrorist bomb attacks, it is necessary to build it with precast concrete walls and reduce the weight of the precast elements by reducing their size for ease of transportation and construction.In this study, to propose an idea for rapid construction and better blast resistance of blast-resistant RC structures, double-layered RC slabs composed of precast thin plates, 50 mm thick, were fabricated and utilized for contact detonation tests. The tests were conducted under a condition that the amount of explosives and the dimensions of specimen were constant respectively, and two types of concrete, normal concrete and polyethylene fiber reinforced concrete (PEFRC), were employed as the slab materials. Our results showed that creating an air cavity between the two layers of PEFRC slab was effective in reducing spall damage, while the air space had no advantage in normal RC, under a condition that the thickness of the air space was fixed at 15 mm. Furthermore, the above difference between PEFRC and normal RC slabs was discussed, based on the numerical result on the fracture process of the air-sandwiched normal RC slab.

Blast Resistance of Polyethylene Fiber Reinforced Concrete to Contact Detonation

When designing blast-resistant reinforced concrete (RC) structures, reducing spall damage due to reflected tensile stress waves is a major problem. To investigate the applicability of polyethylene fiber reinforced concrete (PEFRC) for use in blast-resistant RC structures, experimental investigations were conducted to evaluate the damage to PEFRC slabs subjected to contact detonation. As a result, it was shown that PEFRC was effective in reducing the spall damage due to contact detonation as compared with normal RC. Moreover, an equation for estimating damage depth to the PEFRC slab subjected to contact detonation was derived based on the test results.