This special issue of the International Journal of Computational Methods and Experimental Measurements contains a selected number of edited papers presented at the Conference on Structures under Shock and Impact held in Crete in 2016.

The papers cover a variety of topics, including impact and blast loading, response of buildings and other structures to large dynamic loads and their material behaviour at high rates of strain. These are all areas of active research and special interest, focused on the survivability of physical facilities and the protection of people.

The issue comprises a series of research contributions, essential to deepen the knowledge of how structures and materials behave under a wide variety of dynamic load actions. The contributors are from different centres throughout the world in which advanced impact and blast studies are carried out.

This issue is of primary importance to scientists and engineers working in a variety of academic disciplines and industrial organisations who need to be aware of the latest developments in the impact response of materials and structures and the vulnerability of our infrastructure and environment to accidental explosions and terrorist attacks.

In a joint endeavor conducted for the US Department of State (DoS) Bureau of Diplomatic Security, K&C has developed curtain-wall technology for US government infrastructure overseas capable of withstanding the threats anticipated from large explosive events such as VBIEDs at close proximity. The analysis utilized high fidelity physics-based (HFPB) calculations based on a combination of com- putational fluid dynamics (CFD) and computational structural dynamics (CSD) modeling methods. Unlike many similar analysis and simulation efforts, this work was validated by a full-scale explosive test. This provided an opportunity to compare the calculation outputs with test data to determine the efficacy and accuracy of the calculation methods as well as providing indicators for further calibra- tion of the analysis model. This paper will provide description and commentary of the calculation approach as adopted to analyze the structure using both CFD and CSD methods, as well as planning and conduct of the test including positioning of instrumentation and the purpose and nature of data collec- tion. Comparison of the simulation and test data is accompanied by discussion of the most significant discrepancies and areas in which the calculations closely matched the observed calculation results. Finally, conclusions are presented regarding the efficacy of the calculational approach adopted and recommendations presented for future calculations, and testing of conventional structural systems that are to be subject to blast loading of this magnitude.

Many terrorist attacks in the last decade around the world have exposed the vulnerability of citizens in public places. Public trash receptacles can be easily abused as well-covered places in which Improvised Explosive Devices (IED) can be simply left and then remotely activated. Therefore, blast resistance and possibility of blast loads redirection are very important characteristics of trash receptacles placed in crowded public areas. This paper presents the results of three different trash receptacles: non-blast resistant, blast resistant and blast resistant trash receptacle with blast load redirection. The results have shown that a considerable effect can be achieved by using blast resistant receptacles, thus reducing the possibility of deaths and injuries. A thickness optimization study was additionally performed, based on the size and geometry of the opening by using a finite element model. Based on the results of the study, some valuable recommendations for design of trash receptacles are also given.

The purpose of the paper is to investigate the influence of geometry of charges on the propagation of blast waves. Various shape charges (cylinder, sphere, irregular shape) were used in the field tests. The main type of explosive, homemade ANFO (Ammonium nitrate + fuel oil), was used as the most common used explosives in improvised explosive devices used in terrorist attacks. Characteristics of homemade and industrially made ANFO explosives are different. There were comparing charges of various types of industrially produced types of explosives and homemade explosive in the field tests. The blast wave propagation were investigated and compared.

Understanding the bond behaviour between reinforcing steel and concrete under high-loading rates is becoming more and more important with increasing frequency of natural disasters, impact loadings and a threat of terrorism. This paper aims to obtain a better understanding of the material interactions between the steel rebar and the concrete in the bond zone under different loading rates. During the experimental program push-in tests were conducted under quasi-static and dynamic loading conditions. Both a servo-hydraulic machine as well as an instrumented drop tower were used during the investigation. Samples with short-bond zone in the middle of a cylindrical specimen were used and only a small reinforcement bar diameter (10 mm) was investigated. This approach was chosen to ensure constant bond stress distribution and that the failure occurs during the first pass of the stress wave through the bond zone. Throughout the experimental programme the loading rate was varied from 0.01 mm/s to 8.3 m/s. Bond stress–slip relationships in dependence on the bond stress rate are presented in this paper. The results indicate a bond stress dependence on the loading rate although the scattering of the results is quite high. The experimentally determined dynamic increase factor (DIF) for concrete-steel bond stress is around 1.5 which is a value comparable to other authors.

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.

A blast retrofit technique for concrete structures using carbon fiber-reinforced polymer (CFRP) layers was investigated for use in large infrastructure systems with the overarching goal of preventing against major loss of life and considerable damage that would require extensive repair. Large-scale experiments were conducted and the retrofit behavior was investigated for application on relatively large reinforced concrete walls subjected to blast-like loadings. The experimental program utilized the University of California San Diego (UCSD) Blast Simulator. The Blast Simulator is able to induce various blast-like shock waves to the test specimen in a controlled laboratory environment. The performance of this blast retrofit was tested and then analyzed using SDOF and finite element modeling methods. A finite element model was created using LS-DYNA and utilized contact algorithms for the CFRP-concrete interface. Results and comparisons between the two analysis methods are given.

Advanced damage behaviour of reinforced concrete is modelled using a mixed modelling approach, in which concrete is represented through the spherical-type discrete element model, whereas steel reinforcement is modelled by using beam-type finite elements. An original steel-concrete bond model developed and calibrated on pull-out tests is used to ensure transfer of forces between steel and concrete. The proposed approach is applied to simulate soft and hard-type impacts on RC beams within a very complete modelling framework, thus allowing validating by comparison with experimental data the overall numerical approach developed.

Precast concrete components are manufactured in a well-controlled environment. It has been proven to show good behaviour under gravity and lateral loads. However, the beam to column connections remain the critical part in the precast concrete structures under the column loss scenario in a progressive collapse scenario. In this paper, different beam to column connections, wet and dry connections, are studied and investigated numerically under the column removal scenario. A detailed model for the different connections is developed using the Applied Element Method (AEM). Different column removal locations are considered in the study to provide a comprehensive assessment. The performance of the connections is studied in terms of ultimate load capacity and rotational ductility. According to the results obtained, a connection enhancement is suggested to increase the resistance of precast concrete structures to progressive collapse.

Recent studies with numerical models regarding edge protection systems (EPS), class C according to standard EN 13374, showed that some requirements are inadequate for human safety. This problem mainly arises when a person is injured by falling directly against the EPS supports.

To analyse this subject, three series of numerical models, in accordance with EN 13374, have been produced. The paper describes these studies, which have been carried out using straight supports, different weight ballasts and also a deformable ballast.

In the first series, impacts against straight supports have been analyzed and a standard EN 13374 ballast has been used. These first studies showed too many high impact factors on the ballast. The obtained values are absolutely inadequate and dangerous to the integrity of the human body. The second series was conducted to know how different weights and shapes of ballast affect the maximum accelerations suffered by the human body. Finally, in the third series, a more deformable ballast has been used to simulate impacts of workers against straight supports, nearest to the real behaviour of a human body.

Results confirm and measure an excessive impact factor suffered by the falling person. Mainly in the first series, with direct impacts of standard ballast against straight supports, acceleration values have been generated that could seriously injure the body or even could kill workers.

The second series showed that different weights and shapes – cylinder or sphere – of ballast affect the acceleration values calculated.

Finally, in the third series, the deformable ballast has achieved results truer than previous studies with the more rigid ballast established by EN 13374.

The integrity of reactor pressure vessels (RPVs) of nuclear power plants is one of the most important topics in the field of nuclear energy production. Therefore, the integrity of RPVs has to be assessed for normal operation as well as for emergency transients. A critical transient concerning the RPV integrity is the emergency cooling of a pressurized water reactor, initiated by a leak in the hot leg. Such shock-like cooling in combination with the pressure, the so-called pressurized thermal shock (PTS), causes high thermal stresses in the RPV wall and stress intensities of pre-existing cracks which could exceed the remaining fracture toughness of the material, which is additionally embrittled due to neutron irradiation. This may result in a cleavage fracture of the most safety relevant reactor component.

We present a PTS study of a reference reactor, starting with the calculation of the thermal-hydraulic system behaviour, followed by the simulation of the cold water temperature injection and mixing by means of computational fluid dynamics (CFD) method and the subsequent structural and fracture mechanics calculation. In the safety assessment, we compare the evolution of the stress intensity factors (SIF) during an emergency cooling transient with the fracture toughness at the tip of postulated cracks. Results and open questions will be discussed in the light of a realistic estimation of safety margins.

To reduce the vulnerability of both civilian and military aircraft, it is important to take the hydrodynamic ram (HRAM) effect into account when designing their fuel tanks. HRAM is especially dangerous for liquid- filled thin walled lightweight structures that cannot be armoured due to weight penalty reasons. However, the response of the tank structure during HRAM events depends on a coupling model between fluid and structure. Water is generally used as a liquid candidate for experimental observations of HRAM, since it is a safe and affordable solution. However, its characteristics in thermal transfers are far different from the ones of hydrocarbons, and it may influence the bubble behaviour and thus its resulting loading on the tank walls. A good understanding of all these aspects is still needed to enhance the tank designs. Similarities in bubble behaviour between HRAM and underwater explosion situations were observed in recent high-speed tank penetration/water entry experiments. A confined version of the Rayleigh-Plesset equation – which is classically used for bubble dynamics analysis (including underwater explosion) – has been previously proposed to simulate a bubble created by an HRAM event. The work the presented work is a first attempt to the estimation of the influence of thermal effects in HRAM processes, by using the Rayleigh-Plesset equation in confined regime.

Laminar pipe flow is becoming an important experimental test case for new high efficiency heat carriers like nano- and ferrofluids. Here, a new scaling approach for mixed convection in laminar pipe flow with constant heat flux is proposed. The model relates the radial temperature gradient at the wall, represented as the local Nusselt number, with local Reynolds, Prandtl, and Grashof numbers. The proposed scaling approach is successfully employed to collapse data from different test rigs with horizontally oriented pipes and operated with water. Influences of differing strengths following from free convection are gathered with the new scaling. Moreover, the new scaling approach is successfully utilised to value experimentally obtained heat transfer data of nanofluid flow. In this regard, the impact of nanoparticles, namely the suppression of heat transfer in mixed convection, is experimentally shown and theoretically analysed. Finally, the influence of pipe orientation (vertical / horizontal) is discussed.

This work is an experimental study of the dynamic fields of a turbulent premixed methane-air flame in a Bunsen burner. The Particle Image Velocimetry (PIV) is used to determine the dynamic fields, and Laser Sheet Tomography (LST) for the flame fronts. The turbulent main jet has a Reynolds number Re = 10 000. Turbulence is generated using perforated grids having three whose provide different inlet turbulence intensities. Velocity fields are measured for various equivalence ratio (F = 0.6–1.3) and different axial flame positions. For the reactive jet, interesting results are obtained concerning the dynamic field and the flame front. It is shown that radial profiles of U and V correspond to the axial positions located before the end of the potential core in the reactive jet. The velocity increases at the jet center to 20 m/s, and is less influenced by turbulent mixing in the flame. The greatest velocity and turbulent kinetic energy are obtained using the grid with the smallest ratio (d/M). Most important values of the radial velocity correspond to the lean flames.

Sandwich panels are commonly used for aerospace structures that require a high-bending stiffness, but the thin facesheets that are bonded to the core can be susceptible to impact damage. It is necessary to be able to identify and assess the severity of the damage, but this can be difficult when dents are not visible on the surface of the skin. This can occur when the dent elastically springs back immediately after impact, and can cause the skin to return close to its original position, leaving little indication that a damaged core exists. Identifying combinations of skin thickness and core density that are more susceptible to spring back can enable better decisions to be made with respect to inspection procedures. Finite element simulations of metal-skinned honeycomb panels indicate that more spring back is expected to occur from panels composed of thicker skins and lower density core.

There are some difficulties of non-destructive test and evaluation for immersed or submerged structures such as nuclear reactor pipe line, submarine or huge ship. This paper proposes the method of damage detection of immersed metallic structure which has crack on weld zone and placed in the water with slight and random surface oscillation using ultrasonic wave propagation imaging (UWPI) system with piezoelectric transducer. A T-shape metallic structure with artificial surface crack on weld zone, which with size 2 mm by 0.3 mm and the depth 2 mm, used as the specimen. A 532 nm Q-switched continuous wave laser is used for scanning an area of 20 mm by 40 mm. A piezoelectric sensor with magnetic sensor head, which is attachable to metallic structure is used as a contact ultrasonic sensor. The tests are performed in three cases: a specimen without water, a specimen immersed in water and a specimen immersed in water with random surface oscillation. Ultrasonic wave propagation image algorithm and adjacent wave subtraction (AWS) algorithm are used for visualizing wave propagation and detecting the crack. For the case where the specimen is immersed in the water, the signal amplitude is increased compared with the specimen without water case. In AWS algorithm results of the immersed case, scattering waves which are generated by cracks were observed. In random surface oscillation case, the excitation laser beam is refracted randomly by Snell’s law resulting in the diluted wave propagation images. However, improvements in signal-to-noise ratio using repeat scan technology enable to the detection of the crack and estimation of the crack size.

This paper examines the damage detection using ultrasonic wave propagation caused by laser excitation from outside of the water even when the water surface oscillation exists and implies the possibility of application of ultrasonic propagation imaging system to submerged structures.

The preferred procedure for steel guardrails in the state of Georgia, USA for vehicle impact employs a post-installation machine to drive the posts through a layer of asphalt placed to retard vegetation growth around the system. However, in order to avoid undesirable restraint at the ground line, the AASHTO Roadside Design Guide recommends incorporating leave-outs. Using a leave-out in vegetation barriers is seen as less desirable because of issues including significantly higher expected costs, variability in the placement and spacing of posts, and the need for variable construction scheduling. In lieu of leave- outs, predetermined fracture planes, or “pre-cuts” were installed in the asphalt and evaluated in terms of ground restraint. An experimental program was carried out on an outdoor test site. Posts were installed in pre-cut asphalt and subjected to static loading to provide a better understanding of the behavior of a post restrained with an asphalt layer at the ground line. In parallel with the experimental program, a three dimensional finite element model was developed for a guardrail post installed through an asphalt layer. The model was refined using the experimental results from the test program as well as material testing. Results from the experimental program and finite element analyses indicate that certain pre-cutting configurations lead to significantly less ground restraint as desired.