Showing papers in "International Journal of Protective Structures in 2022"

Evaluation of critical damage location of contact blast on conventionally reinforced one-way square concrete slab applying CEL-FEM blast modeling technique

Abstract: Nowadays, accidental explosions in residential and factory buildings are common owing to poor maintenance and mishandling of fuel gas and chemical explosive appliances leading to grievous injuries and infrastructure damages. Contact blast on slabs using explosives is noticed as a simpler act of subversion as compared to other components of the building and is more damaging than a close-in blast. In general, damage caused by contact blast is localized in the form of concrete cratering, scabbing, and rupture of the reinforcement. A recently published state-of-the-art review on the performance of reinforced concrete (RC) slabs under contact and close-in explosion loading scenario by the authors (Anas et al., 2021b) reveals the common perception for the location of contact blast to cause maximum damage is the centroid of the slab. It develops a curiosity with sufficient interest to investigate the effect of the location of contact explosive charge on the damage response of the slab. Several numerical techniques such as empirical, ConWEP (semi-empirical), Smooth Particle Hydrodynamics (mesh-free method), and Coupled-Eulerian-Lagrangian (CEL) are in use for simulation of blast loading on structures. Current literature reveals that the CEL is the most advanced and realistic blast modeling technique. This study applies Coupled-Eulerian–Lagrangian (CEL) formulation with finite element method (FEM) using the dynamic computer code ABAQUS/Explicit-v.6.15 to investigate the performance of singly reinforced one-way spanning concrete slab subjected to concentric contact blast loading. The numerical model is validated with the experiment results in the open literature. The validated model is then employed to investigate whether or not the maximum damage is really caused by the central location of the contact blast. For this purpose, one-quarter of the slab with nine symmetrical points (or locations) of contact blast of explosive charge, which reflect the coverage of the entire slab, in contact with the top face of the slab is considered in the study. Two constitutive material models, Concrete Damage Plasticity and Johnson–Cook, with strain rate effects are used to simulate the non-linear behavior of the concrete and steel, respectively. The results reveal that the most critical location of maximum damage to the slab is along the line of symmetry parallel to the supports at an eccentricity of B/4 from the centroid of the slab, where “B” is the width of the one-way slab.

Performance of (1) concrete-filled double-skin steel tube with and without core concrete, and (2) concrete-filled steel tubular axially loaded composite columns under close-in blast

Abstract: Composite structural members such as concrete-filled double-skin steel tube (CFDSST) and concrete-filled double steel tubular (CFDST) columns are increasingly being utilized in modern structures owing to their capability to integrate the beneficial properties of constituent materials to carry heavy loads as compared to conventional reinforced concrete columns. Axial compression performance of such composite columns has been extensively investigated and available in the open literature. However, their response under impulsive loadings such as those induced by explosions is not very well studied because not many investigations have been conducted on these columns. Performance of composite compression members under short-duration/high-magnitude blast loading is of considerable interest under the prevailing environment of hi-tech wars, subversive activities, and accidental explosions. The recent devastating accidental Ammonium Nitrate explosion at Beirut port (Lebanon), and the ongoing invasion of Ukraine by Russia raise the concern of researchers and engineers for the safety of structural elements/components. In this study, a 3-D finite element model of axially loaded 2500 mm long CFDSST column of ultra-high-strength concrete (170 MPa) is developed in ABAQUS/Explicit-v.6.15 computer code equipped with Concrete Damage Plasticity (CDP) model, and investigation has been carried out for its blast performance under the 50kg-TNT explosive load at a standoff distance of 1.50 m in free-air. The effects of strain rate on the compressive strength of the concrete are considered as per fib Model Code 2010 (R2010) and UFC-3-340-02 (2008). The non-linear behavior of the steel is also taken into account. Damages in the form of (1) a - concrete crushing on the explosion side of the column and b - concrete cracking on the tension side and their spread over the column length, and (2) yielding of tubes are observed. Computational results are validated with the available experimental observations. To improve the column response, the analysis has been extended to investigate the blast performance of axially loaded CFDSST columns with and without core concrete having an inner steel tube of circular/square cross-section and their response have been compared with the equivalent single skin concrete-filled steel tubular circular/square columns of same axial load capacity.

Physics-informed regularisation procedure in neural networks: An application in blast protection engineering

Abstract: Machine learning offers the potential to enable probabilistic-based approaches to engineering design and risk mitigation. Application of such approaches in the field of blast protection engineering would allow for holistic and efficient strategies to protect people and structures subjected to the effects of an explosion. To achieve this, fast-running engineering models that provide accurate predictions of blast loading are required. This paper presents a novel application of a physics-guided regularisation procedure that enhances the generalisation ability of a neural network (PGNN) by implementing monotonic loss constraints to the objective function due to specialist prior knowledge of the problem domain. The PGNN is developed for prediction of specific impulse loading distributions on a rigid target following close-in detonation of a spherical mass of high explosive. The results are compared to those from a traditional neural network (NN) architecture and stress-tested through various data holdout approaches to evaluate its generalisation ability. In total the results show five statistically significant performance premiums, with four of these being achieved by the PGNN. This indicates that the proposed methodology can be used to improve the accuracy and physical consistency of machine learning approaches for blast load prediction.

Time of arrival as a diagnostic for far-field high explosive blast waves

Abstract: The ability to accurately determine blast loading parameters will enable more fundamental studies on the sources of blast parameter variability and their influence on the magnitude and form of the loading itself. This will ultimately lead to a better fundamental understanding of blast wave behaviour, and will result in more efficient and effective protective systems and enhanced resilience of critical infrastructure. This article presents a study on time of arrival as a diagnostic for far-field high explosive blasts, and makes use of the results from a large number of historic tests and newly performed experiments where the propagating shock front was filmed using a high-speed video (HSV) camera. A new method for optical shock tracking of far-field blast tests is developed and validated, and full-field arrival time results are compared against those determined from the historic data recorded using traditional pressure gauges. Arrival time variability is shown to be considerably lower than peak pressure and peak specific impulse, and is shown to decrease exponentially with increasing scaled distance. Further, the method presented in this article using HSV cameras to determine arrival time yields further reductions in variability. Finally, it is demonstrated that the method can be used to accurately determine far-field TNT equivalence of high explosives.

Blast wave interaction with structures – An overview

Abstract: Blast–obstacle interaction is a complex, multi-faceted problem. Whilst engineering-level tools exist for predicting blast parameters (e.g. peak pressure, impulse and loading duration) in geometrically simple settings, a blast wave is fundamentally altered upon interaction with an object in its path, and hence, the loading parameters are themselves affected. This article presents a comprehensive review of key research in this area. The review is formed of five main parts, each describing: the direct loading of a blast wave on the surface of a finite-sized structure; the modified pressure of the blast wave in the wake region of three main obstacle types – blast walls, obstacles, wall/obstacle hybrids; and finally, a brief description of some methods for predicting loading parameters in such blast–obstacle interaction settings. Key findings relate to the mechanisms governing blast attenuation, for example, diffraction, reflection (diverting away from the target structure), expansion/volume increase, vortex creation/growth, as well as obstacle properties influencing these, such as porosity (blockage ratio), obstacle shape, number of obstacles/rows, arrangement and surface roughness.

Dynamic mechanical properties of hybrid fiber reinforced concrete

Abstract: In this study, the dynamic mechanical properties of hybrid fiber reinforced concrete (HFRC) are analyzed with respect to failure mode, dynamic increase factor (DIF), and peak strain by means of a SHPB testing apparatus. The factors that influence the dynamic mechanical properties include fiber type and fiber content. It is concluded that the best dynamic mechanical properties of fibers are CS-PHFRC at medium and low strain rates and AS-PHFRC at a high strain rate. Within a certain range, the higher the fiber content is, the larger the DIF of the corresponding HFRC and the more obvious the increase in dynamic compressive strength. AS-CSHFRC improves the dynamic compressive deformability of the HFRC. The polypropylene fiber causes plasticity, as shown in the failure mode of concrete. The Ottosen nonlinear elastic model, modified by introducing the damage factor, can better describe the dynamic mechanical properties of HFRC.

Application of transfer learning for the prediction of blast impulse

Abstract: Transfer learning offers the potential to increase the utility of obtained data and improve predictive model performance in a new domain, particularly useful in an environment where data is expensive to obtain such as in a blast engineering context. A successful application in this respect will improve existing surrogate modelling approaches to allow for holistic and efficient strategies to protect people and structures subjected to the effects of an explosion. This paper presents a novel application of transfer learning for the prediction of peak specific impulse where we demonstrate that previous knowledge learned when modelling spherical charges can be transferred to provide a performance benefit when modelling cylindrical charges. To evaluate the influence of transfer learning, two artificial neural network architectures were stress tested for three levels of random data removal: the first model (NN) did not implement transfer learning whilst the second model (TNN) did by including a bolt-on network to a previously published NN model trained on the spherical dataset. It is shown the TNN consistently outperforms the NN, with this out-performance increasing as the proportion of data removed increases and showing statistically significant results for the low and high threshold with less variability in all cases. This paper indicates transfer learning applications can be used successfully with considerable benefit with respect to surrogate modelling in a blast engineering context.

A numerical investigation of blast-structure interaction effects on primary blast injury risk and the suitability of existing injury prediction methods

Abstract: Explosions increasingly occur in densely populated, urban locations. Primary blast injuries (PBIs), caused by exposure to blast wave overpressure, can be predicted using injury criteria, although many are based on idealised loading scenarios that do not necessarily reflect real life situations. At present, there is limited understanding of how, and to what extent, blast-structure interaction influences injury risk, and the suitability of injury criteria that assume idealised loading. This work employed computational fluid dynamics to investigate the influence of blast interaction effects such as shielding and channelling on blast load characteristics and predicted PBIs. The validated modelling showed that blast interaction with common urban features like walls and corners resulted in complex waveforms featuring multiple peaks and less clearly defined durations, and that these alter potential injury risk maps. For example, blast shielding due to corners reduced peak overpressures by 43%–60% at locations behind the corner. However, when the urban layout included a corner and a wall structure, higher pressures and impulse due to channelling were observed. The channelling significantly increased the injury risk at the exposed location and reduced the shielding effects behind the corner. In these cases, the application and interpretation of existing injury criteria had several limitations and reduced reliability. This demonstrates that structural-blast interaction from common urban layouts has a significant effect on PBI risk. Specific challenges and further work to develop understanding and reliability of injury prediction for urban blast scenarios are discussed.

Blast retrofit of one-way reinforced concrete members using externally bonded FRP and FRP anchorage

Abstract: This paper presents the results of nine as-built and carbon fiber reinforced polymer (CFRP) retrofitted reinforced concrete panels subjected to simulated blast loading using a pneumatically operated shock tube. The objective of the study was to characterize the blast response of CFRP retrofitted reinforced concrete panels, with and without supplemental mechanical anchorage applied to the CFRP. The results indicate that retrofitting can significantly increase the strength and stiffness of reinforced concrete flexure members and greatly enhance the displacement time-history response over non-retrofitted members. Debonding of the externally bonded CFRP was the failure mode for all retrofitted members. FRP anchors, designed to prevent or delay debonding failures through mechanical end-anchorage, were found to substantially enhance the performance of panels experiencing critical diagonal crack debonding. However, the FRP anchors were found to have no substantial effect on retrofit performance for the case plate-end interfacial debonding failures. In addition, the displacement time-histories for as-built and FRP retrofitted panel obtained through detail single degree of freedom analysis were found correlate well with those obtained experimentally. Finally, a discussion on the practical considerations of using externally bonded FRP retrofits to resist blast loads and recommendations for protective design are presented.

An experimental and simulated investigation into the validity of unrestricted blast wave scaling models when applied to transonic flow in complex tunnel environments

Abstract: Since the inception of high explosives as an industrial tool, significant efforts have been made to understand the flow of energy from an explosive into its surroundings to maximize work produced while minimizing damaging effects. Many tools have been developed over the past century, such as the Hopkinson–Cranz (H-C) Scaling Formula, to define blast wave behavior in open air. Despite these efforts, the complexity of wave dynamics has rendered blast wave prediction difficult under confinement, where the wave interacts with reflective surfaces producing complex time-pressure waveforms. This paper implements two methods to better understand blast overpressure propagation in a confined tunnel environment and establish whether scaled tests can be performed comparatively to costly full-scale experiments. Time–pressure waveforms were predicted using both a 1:10 scaled model and three-dimensional air blast simulations conducted in Ansys Autodyn. A comparison of the reduced scale model simulation with a full-scale blast simulation resulted in self-similar overpressure waveforms when employing the H-C scaling model. Experimental overpressure waveforms showed a high level of correlation between the reduced scale model and simulations. Additionally, peak overpressure, duration, and impulse values were found to match within tolerances that are highly promising for applying this methodology in future applications. Using this validated relationship, the simulated model and reduced scale tests were used to predict an overpressure waveform in a full-scale underground mine opening to within 2.12%, 2.91%, and 7.84% for peak overpressure, time of arrival, and impulse, respectively. This paper demonstrates the effectiveness of scaled, blast models when predicting blast wave parameters in a confined environment.

Experimental investigation of 3D-printed auxetic core sandwich structures under quasi-static and dynamic compression and bending loads

Abstract: Auxiliary metamaterials designed according to the Negative Poisson’s Ratio (NPR) property are exciting structures due to their high impact strength, impact energy absorption abilities, and different damage mechanisms. These good mechanical features are suitable for aviation, automotive, and protective construction applications. These structures, whose most significant disadvantages are production difficulties, have become easier to produce with the development of 3D production technology and have been the subject of many studies in recent years. In this presented study, two conventional core geometries and three different auxetic geometries, commonly used in sandwich structures, were designed and produced with 3D printer technology. The strength and energy absorption capabilities of prototype sandwich structures investigated experimentally under bending loads with static and dynamic compression. Except for the re-entrant (RE) type core, the auxetic core foam sandwich structures demonstrate higher rigidity and load-carrying capacity than classical sinusoidal corrugated (SC) core and honeycomb (HC) core sandwich structures under both quasi-static and impact-loaded compression and three-point bending experiments. Double arrowhead (DAH) and tetrachiral (TC) auxetic cores outperformed honeycomb core in terms of specific quasi-static and impact load-bearing performance under compression by 1.5 ± 0.25 times. In three-point bending experiments under both quasi-static and impact loading conditions, the load-carrying capacity of the double arrowhead and tetrachiral auxetic cores was found to be more than 1,86 ± 0.38 times that of the honeycomb core sandwich panels.

Prediction of blast loading on protruded structures using machine learning methods

Abstract: Current empirical and semi-empirical based design manuals are restricted to the analysis of simple building configurations against blast loading. Prediction of blast loads for complex geometries is typically carried out with computational fluid dynamics solvers, which are known for their high computational cost. The combination of high-fidelity simulations with machine learning tools may significantly accelerate processing time, but the efficacy of such tools must be investigated. The present study evaluates various machine learning algorithms to predict peak overpressure and impulse on a protruded structure exposed to blast loading. A dataset with over 250,000 data points extracted from ProSAir simulations is used to train, validate, and test the models. Among the machine learning algorithms, gradient boosting models outperformed neural networks, demonstrating high predictive power. These models required significantly less time for hyperparameter optimization, and the randomized search approach achieved relatively similar results to that of grid search. Based on permutation feature importance studies, the protrusion length was considered a significantly more influential parameter in the construction of decision trees than building height.

Fragment response of unreinforced concrete masonry walls subjected to blast loading

Abstract: Very few methods are available for predicting the secondary fragment velocity and secondary fragment production of ungrouted and nonreinforced concrete masonry unit (UGNR CMU) walls when subjected to blast loading. Existing approaches often require large computational resources or fail to predict secondary fragment velocity accurately. This paper presents an engineering-level analysis methodology that can be rapidly implemented to predict the velocity of fragments from UGNR CMU walls subjected to blast loading. The theoretical background and need for the methodology are discussed. Results from the analytical methodology are compared to results from two full-scale experiments and four quarter-scale experiments. A strong correlation is demonstrated between the experimental results and the analytical methodology.

From impact of RC flat slabs in a building to its progressive collapse

Abstract: This paper presents a realistic model for the simulation of a progressive collapse scenario in a typical low-rise building that is constructed from RC flat slabs and supported by columns. The progressive collapse scenario starts after failure of the top slab connections, where the slab is falling downward and impacts with the slab below. This impact event is analyzed, and the dynamic failure of the impacted slab’s connections starts the progressive collapse event. Two different scenarios are identified, depending on the first slab damage condition prior to impact. The first scenario refers to an undamaged impacting slab where an elastic collision occurs with the slab below; in the second scenario, the first slab is damaged, and its collision with the slab below is plastic. In the first scenario, the impacting slab velocity drops to zero while its velocity is fully imparted to the impacted slab. In the second scenario, both slabs continue their motion jointly at a common velocity. In the subsequent impacts, the impacting slabs are a-priori damaged, hence plastic collisions occur. These impact occurrences are analyzed separately, depending on the number of impacting slabs involved, damage characteristics, and impact velocity. Due to the nature of the first impact, the first scenario is characterized by separate motion of the first impacting slab which is falling behind the other slabs. This slab gains speed until it meets the other falling slabs below at a certain altitude, and an intermediate collision occurs, not necessarily at a floor level. In the analyzed five-story building, the intermediate impact occurs after the third impact event, where the slabs are located slightly above the first story level. The intermediate impact elevates the velocity of the impacted slabs such that their impact with the first level slab is more severe and its motion toward hitting the ground level is faster.

A numerical investigation of blast-structure interaction effects on primary blast injury risk and the suitability of existing injury prediction methods

Abstract: Explosions increasingly occur in densely populated, urban locations. Primary blast injuries (PBIs), caused by exposure to blast wave overpressure, can be predicted using injury criteria, although many are based on idealised loading scenarios that do not necessarily reflect real life situations. At present, there is limited understanding of how, and to what extent, blast-structure interaction influences injury risk, and the suitability of injury criteria that assume idealised loading. This work employed computational fluid dynamics to investigate the influence of blast interaction effects such as shielding and channelling on blast load characteristics and predicted PBIs. The validated modelling showed that blast interaction with common urban features like walls and corners resulted in complex waveforms featuring multiple peaks and less clearly defined durations, and that these alter potential injury risk maps. For example, blast shielding due to corners reduced peak overpressures by 43%–60% at locations behind the corner. However, when the urban layout included a corner and a wall structure, higher pressures and impulse due to channelling were observed. The channelling significantly increased the injury risk at the exposed location and reduced the shielding effects behind the corner. In these cases, the application and interpretation of existing injury criteria had several limitations and reduced reliability. This demonstrates that structural-blast interaction from common urban layouts has a significant effect on PBI risk. Specific challenges and further work to develop understanding and reliability of injury prediction for urban blast scenarios are discussed.

A branching algorithm to reduce computational time of batch models: Application for blast analyses

Abstract: Numerical analysis is increasingly used for batch modelling runs, with each individual model possessing a unique combination of input parameters sampled from a range of potential values. Whilst such an approach can help to develop a comprehensive understanding of the inherent unpredictability and variability of explosive events, or populate training/validation data sets for machine learning approaches, the associated computational expense is relatively high. Furthermore, any given model may share a number of common solution steps with other models in the batch, and simulating all models from birth to termination may result in large amounts of repetition. This paper presents a new branching algorithm that ensures calculation steps are only computed once by identifying when the parameter fields of each model in the batch becomes unique. This enables informed data mapping to take place, leading to a reduction in the required computation time. The branching algorithm is explained using a conceptual walk-through for a batch of 9 models, featuring a blast load acting on a structural panel in 2D. By eliminating repeat steps, approximately 50% of the run time can be saved. This is followed by the development and use of the algorithm in 3D for a practical application involving 20 complex containment structure models. In this instance, a ∼20% reduction in computational costs is achieved.

Application verification of blast mitigation through the use of thuja hedges

Abstract: Nowadays, large gatherings of people, such as open-air concerts, outdoor-sport events, trade fairs, etc., are often attracted by the terrorists. Recently, an interesting passive alternative way of securing such events against terrorist threats appeared in the scientific literature, in which the tree hedges mitigation potential against blast waves were studied. Despite comprehensive studies regarding selected species of hedge trees, the real application outlines were reported to be still missing for those barriers. Our study verified the mitigation potential of thuja in field tests for (i) several distances behind the hedge and for (ii) several positions along the hedge wall. The explosives of 5 kg trinitrotoluene with a rectangular shape were used in four detonations. Six pressure pencil gauges were registering the overpressure histories. A high-speed camera was recording the in-plane deformation of the hedge wall, the motion of selected points on the height of the wall was plotted. For each position, the reduction of overpressure peak and overpressure impulse were obtained in reference to their counterparts for the position without a hedge. The maximal overpressure peak reductions obtained were 14% for case (i) (differing distances from the explosive) and 22% for case (ii) (differing positions along the hedge wall). The experiments' outcomes showed the safest position behind the thuja wall and the actual benefit from using them in the public application if the terrorist acts would happen.

Mitigation of blast loading through blast–obstacle interaction

Abstract: Obstructing the passage of blast waves is an effective method of mitigating blast pressures downstream of the obstacle. To this end, the interaction between a blast wave and a simplified structural shape, such as a cylinder, has been widely investigated to understand the complex flow pattern that ensues around the obstacle. The patterns include the interference zones of the incident wave, the diffracted wave, and other secondary waves in the downstream region. Such zones are responsible for causing significant modifications to the blast wave parameters. This research aims to identify and study the factors that serve to mitigate the resulting blast loads downstream of a cylindrical obstacle – both on the ground, and on a rigid wall target that the obstacle is aiming to protect. Inputs from this numerical study are also used to develop a fast-running predictive method based on an artificial neural network (ANN) model. It was found that the size of the cylinder, the strength of the blast wave, the position of the cylindrical obstruction, and the target length, all have remarkable effects on the development of the complex flow-field downstream, and on the impulse mitigation on a reflective target. A number of key mitigation mechanisms are identified, namely shadowing and interference, and their origins and significance are discussed. An ANN model trained using scaled input parameters could successfully predict impulse values on such a reflective target. Using this model to predict the response of previously unseen configurations (for the ANN) gave excellent correlation, thereby demonstrating the high fidelity of this fast-running tool, and its ability to predict the effectiveness of various wave-cylinder interactions in mitigating blast loading.

Local failure resistance of polypropylene fiber reinforced concrete plates subjected to projectile impact

Abstract: This study investigated local failure characteristics of polypropylene fiber reinforced concrete (PPFRC) plates subjected to projectile impact. Flexural and compression tests for two types of PPFRC specimens (PPFRC1 and PPFRC2) were conducted to examine mechanical properties of the PPFRC. The average flexural strength of PPFRC1 and PPFRC2 at a strain rate of 10−1/s were 12.2 N/mm2 and 10.4 N/mm2, respectively. The average compressive strength of PPFRC1 and PPFRC2 at a strain rate of 100/s were 58 N/mm2 and 74.9 N/mm2, respectively. Projectile impact tests for 60 mm- and 80 mm-thick PPFRC plates were conducted by using a 50 g-mass projectile collided at velocities corresponding to 193–423 m/s. Experimental results exhibited that the PPFRC plate had a higher effect of suppressing local failure than a plain concrete plate. Comparison of the tests results with the modified NDRC formula revealed that the limit scabbing thickness was 15–20% smaller than that of a plain concrete plate.

Assessing the stab resistive performance of material extruded body armour specimens

Abstract: This paper investigates the effect of material extruded body armour specimen size on stab penetration depth and back-face signature (BFS) and establishes the minimum thickness required for a series of material extrusion materials to provide protection against the UK Home Office Scientific Development Branch (HOSDB) body armour KR1-E1 requirements. In stage one, material extruded planar test specimens ranging from 40 × 40 mm to 80 × 80 mm in length and width with 10 mm increments at three different thicknesses, 6, 8 and 10 mm, were stab tested under 24 joules of impact energy using a gravity driven drop test apparatus. In stage two, 50 × 50 mm specimens in six material categories, PC, ABS, PLA, TPLA, PA and TPU, were manufactured at different thicknesses via material extrusion and impacted in accordance with the UK HOSDB KR1-E1 stab impact energy level as they were the optimum size when considering overall stab and BFS performance. The study established the fundamental steps towards the use of material extrusion in future personal protection solutions. Results demonstrated that stab penetration and BFS were dependent on specimen size, thickness and material type, and there was an inverse relationship between stab penetration depth and BFS. Also, a minimum thickness of 5 mm for PC and TPLA, 6 mm for ABS, 7 mm for PLA, 11 mm for PA and 12 mm for TPU, with 100% print density, was required in order to provide protection against the HOSDB KR1-E1 level of 24 J stab impact energy.

Dynamic response characteristics of adjacent tunnel lining under blasting impact in subway connecting passage

Abstract: The connecting passage between two adjacent tunnels is conducive to the rescue and evacuation of subway tunnels when disasters occur. The blasting method is usually used in the construction of connecting passage. The vibration caused by blasting construction may endanger the safety of subway tunnel structure. As a result, the influence of blasting pressure on the stability of subway tunnel lining structure during the excavation of connecting passage is studied, and the safe blasting construction distance is proposed, which is crucial to the safety of adjacent subway tunnel lining. This study takes the connecting passage of Wuhan Metro Line 8 as an example. Using Finite element software ANSYS/LS-DYNA, an accurate numerical calculation model of construction site is established. The nonlinear elastoplastic mechanical characteristics of soil, rock, and tunnel lining are simulated by Drucker Prager, Plastic Kinematic, and Johnson Holmquist Concrete constitutive material models, respectively. The credibility of the three-dimensional numerical calculation model and material constitutive model was proved by contrasting the field measured data of the connecting passage with the numerical calculated results. Analysis of numerical results, the axial and radial PPV, frequency, and Von Mises stress of subway tunnel lining are obtained. The influence of subway tunnel lining under adjacent blasting can be obtained by analyzing the distribution law of PPV and Von Mises stress. Non-static tensile strength is needed considering the high pressure and high strain rate process of concrete during blasting. By fitting the relationship between PPV and dynamic tensile stress, and referring to DIF parameters, the safety range of PPV in subway tunnel lining blasting is determined. The critical safety distance between blasting construction and tunnel lining is obtained by Sadovsky vibration velocity attenuation formula, which is used to guide the subsequent blasting excavation of continuous tunnels.

Crashworthiness investigation of draw bead in aluminum tubes under axial loading condition

Abstract: This study aims to investigate the effects of the draw beads on the crashworthiness of the aluminum tubes under axial quasi-static loading. Based on this design philosophy, a total of 12 beading tube designs with various configurations were developed. Within each design, the effect of arrangement bead form on the crashworthiness performance was also analyzed. A finite element model, validated using experimental tests, was used to study the crashworthiness performance and progressive deformation of the tubes. Based on the results, a multi-criteria decision-making method known as Technique of Order Preference by Similarity to Ideal Solution was employed to determine the most suitable tube that features high energy absorption and low impact force. The best tube with a high score was selected to investigate the effect of bead formed direction on aluminum tubes. Consequently, the study identified a bead shape tubes configuration that exhibits superior crashworthiness and low impact force. The beading tube design methodology presented in this study allows the exploitation of variable shapes geometries for the development of high-efficiency energy-absorbing structures and their crushing behaviors.

Shock qualification of low-cost blast resistant wheels by in field tests

Abstract: This paper presents the results obtained during an experimental campaign on blast resistant wheels designed for a low-cost demining machine, derived from an agricultural tractor. Such wheels must fulfil two requirements: first, they have to be able to retain their mechanical integrity in case of blast and still work after one or more explosions, in order to be able to drive the machine out of the minefield without human intervention; second, they must reduce as much as possible the amount of energy transferred to the vehicle, to protect the on-board equipment from the effect of the detonation of a landmine. One of the goals of the experimental activity was to compare two wheels characterized by different designs. Mechanical performance and capacity of the wheels to reduce the energy transferred to the vehicle have been assessed to verify whether the wheels were suitable for the task and to identify which wheel performs best. Physical integrity of both wheels was assessed by visual inspection after each explosion. To evaluate the energy transferred to the vehicle, a measurement of the potential energy transferred, by means of a ballistic pendulum, equipped with an encoder, was performed together with a triaxial acceleration measurement in correspondence of the wheel hub. The triaxial accelerometer measurement was then also used to assess the behaviour of the wheels mounted on the vehicle after tests on the ballistic pendulum. Wheel performances have been quantified using specific features and frequency domain functions, related to the damage induced by the vibration at the interface between the hub and the demining machine. The obtained results suggest that the heaviest wheel performs better both in terms of mechanical integrity and of shock response.

The dynamic material properties of stabilised rammed earth materials using cement and calcium carbonate residue

Abstract: The use of rammed earth (RE) as a construction material has recently received renewed interest due to its sustainability characteristics and potential for low-cost construction. Modern RE includes the addition of a binder to increase its performance. The mechanical performance of stabilised RE particularly the dynamic material properties is still not well understood. During the design life, a structure could experience dynamic loading. It is necessary to properly understand the dynamic properties of stabilised RE for safe applications. In this study, the quasi-static and dynamic material properties of cement and calcium carbonate residue (CCR) stabilised RE are experimentally investigated. The failure of the stabilised RE under different loading rates is investigated. Dynamic increase effect on cement and CCR stabilised RE are studied. The unconfined uniaxial compressive strength (UCS), Young’s modulus of the two types of stabilised RE at different strain rates are quantified. Empirical formulae of dynamic increase factor are derived for engineering application.

Flexural and impact behavior of textile reinforced concrete panel

Abstract: Steel is susceptible to corrosion and requires a significant concrete cover, which increases self-weight and cost. Therefore, an alternative to traditional reinforcements is needed. Textile reinforced concrete (TRC) is a favorable composite using textile material as reinforcement with a fine-grained concrete matrix. This study represents a comparison between different TRCs having different textile reinforcements subjected to flexural bending and impact loading. Four types of textiles—glass (GT), a square oriented galvanized iron (SGIT), diagonal pattern galvanized iron (DGIT), and carbon (CT) are used. All four types of textiles are used to prepare 400 x 50 x12 mm textile reinforced mortar (TRM) and tested for tensile strength properties. This study tests TRC panel and plate samples by three-point bending and drop-weight impact methods. The uniaxial tensile strength test of the textiles shows that CTs can take around 2.3 times higher tensile load than SGITs. However, their tensile load capacity is almost similar in the case of TRM, where SGIT textile shows about 30% higher extension. The flexural bending test of the TRC panels shows that the load-carrying ability increases nearly two times with the increase of 25 mm in thickness even when the number of reinforcement layers remains the same. With the increase in thickness, SGIT textile shows better performance. Drop-weight impact test of the TRC plates shows that the impact energy absorption in CT textile plates is up to two times higher than SGIT plates for various thicknesses. This study summarizes that CT shows overall better performance than SGIT.

The development of specialised modular protective structure on continuous miners against coal burst hazards

Abstract: As mining progresses into deep ore deposits in Australia, geo-hazards such as coal burst and outbursts are becoming a major concern for mine workers. The occurrence of geo-hazards involved the ejection of coal lumps and sometimes large volumes of hazardous gases such as methane and carbon dioxide. Whilst it is extremely important to de-stress and de-gas the seam and adjacent strata before roadway development and install competent support systems such as steel mesh and bolt, the last line of protection will be the installation of a protective canopy on the Continuous Miner (CM), which is typically used for roadway developments, to shield mine workers from these deadly dynamic impacts of coal and rock resulting from a burst or outburst. This paper aims to introduce the design, manufacture and testing of an innovative modular protective structure on the CM in underground coal mines. The developed protective system can be easily assembled in the underground mining environment and provide a high level of protection against flying debris hazards in the event of a coal burst. The extensive experimental program and numerical simulations have confirmed the high performance of the protective system against high-speed impact loading by single and multiple coal rocks and projectiles.

Dynamic response and safety control of civil air defense tunnel group during the whole process of underpass tunnel blasting excavation

Abstract: To ensure the safety and stability of adjacent underground structures is a key problem for the subway tunnel blasting construction. In this paper, there is a tunnel group (Sheshan civil air defense engineering) composed of several tunnel units right above a subway tunnel under blasting construction (Wuhan Metro Line 5). The vibration of the tunnel group induced by two blasting excavations of the subway tunnel was monitored. For further research, an effective 3D numerical model established by LS-DYNA, which was verified by field monitoring data, was used to analyze the dynamic response of the tunnel group in the whole process of the subway tunnel blasting. According to the numerical simulation results, the dynamic response characteristics of each tunnel unit were studied, and the most vulnerable area in each tunnel unit was determined. Then, the functional relationships between the maximum vibration velocities and the maximum tensile stresses of the vulnerable areas were established. Based on the maximum tensile stress criterion, the safety vibration velocity threshold of each vulnerable area was calculated using the relationship models. Furthermore, for convenient field monitoring during the subway construction, the safety vibration threshold at the floor of the tunnel group was also calculated. Lastly, to obtain the maximum charge per delay, five cut blasting with different charges were simulated. The maximum charge of the cut blasting in different stages of the subway tunnel blasting excavation was proposed. The research results of this paper have reference value for the blasting vibration safety control of similar tunnel excavation projects in the future.

Experimental verification of different analytical approaches for estimating underwater explosives

Abstract: The clearance of underwater ordnance is one of the most complex tasks entrusted to appropriately trained and equipped soldiers. State-of-the-art knowledge in this area is rarely published and is most often possessed by a narrow group of navy specialists. The aim of this paper was to find a link between the existing mathematical models for the peak pressure of underwater explosion with measurements of small charge detonations for long ranges to the observation point in real life scenarios. We have shown the results of the research in which the underwater explosion tests were presented for different TNT equivalents and standoff distances and thus distance ratios. The curves of pressure versus time of ignition were reported. The measurements were confronted with empirical formulas. The comparison showed large, but expected, differences, since the empirical formulas are advised for smaller distance ratios. Based on the conclusions from the study, the new methodology to identify the loading from underwater explosions based on a database collected was postulated. By creating a survey methodology for ships crew for recording explosion parameters, a large number of events can be registered without a strict setup of the test area. The database obtained can be used by military commanders to identify the explosive hazard in the Baltic Sea region.

Progressive damage in pretensioned and reinforced concrete plates against repeated impacts

Abstract: This study is planned to explore the performance of pretensioned concrete (PC) plates under multiple impacts. A detailed investigation has been carried out on pretensioned concrete plates (0.8 × 0.8 m2) against drop impact. The plates prepared using Mix-40 and Mix-60 grade concrete have been induced with two different levels of initial prestress, that is, 1/10 and 1/5 (i.e. level-1 and level-2) times the strength of the concrete. The PC plates have been impacted by a falling impactor (2382 N) dropped from 0.5 m height. The response of those plates has been obtained and compared with the reference RC plates. The post-impact performance of the damaged plates has been further discovered by subsequently dropping the impactor multiple times from the identical height. The FE simulations of the problem have been carried out using Johnson-Holmquist-2 and metal-plasticity constitutive models for concrete and steel, respectively. The models have been initially verified with the experimental results available in literature, and subsequently the simulations for drop impact have been carried out. The simulation results are also compared with the results of drop impact experimentations performed. In general, both the pretensioned and reinforced concrete have witnessed flexural cracks at the beginning, such that pretensioned concrete witnessed lesser cracks compared to reinforced concrete. As the number of drops increased, one major splitting crack developed only in pretensioned concrete, whereas the reinforced concrete exhibited additional punching cracks. For a given concrete grade, the pretensioned concrete level-2 witnessed the smallest damage, minimal cracks, and also minimal spalling followed by the pretensioned concrete level-1 and reinforced concrete. The reinforced concrete absorbed the minimal impact energy followed by the pretensioned concrete level-1 and level-2 under the multiple impacts. The FE simulations predicted the impact force and reaction within 11.9 and 9.9% variation, respectively, with the corresponding experimental results.

Experimental investigation of blast mitigation of pre-fractal obstacles

Abstract: Obstacles arranged into a pre-fractal shape (Sierpinski carpet) were tested for their blast attenuation abilities using 250 g PE4 at three different scaled distances ( Z = 1.87, 2.24, 2.99 m/kg1/3). Three pre-fractal iterations were tested, as well as free-field tests for comparative purposes. Reductions in peak overpressure up to 26% and peak specific impulse up to 19% were observed, attributed to a mechanism known as ‘trapping’. This mechanism is characterised by a reduction in the ability of a blast wave to advect downstream, with corresponding increases in pressure observed within the bounds of the pre-fractal obstacle. Attenuation magnitudes and areas of reduced pressure and impulse were found to be drastically different with each pre-fractal iteration, with a transition from shadowing to wave trapping as the obstacles more closely resembled true fractals. A linear dependence on a newly-defined obstruction factor ( OF) was found for arrival time, overpressure and impulse at the sensor locations, suggesting that the attenuation of a pre-fractal obstacle is inherently determinable. The results indicate that the mechanism of blast mitigation of pre-fractal obstacles is fundamentally different from singular or arrays of regular obstacles, and could be exploited further to develop novel protective structures with enhanced blast attenuation.