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When laser beams propagate through media with non-vanishing absorption, the media is heated resulting in a change of the refractive index, which can lead to thermal lensing and thermal blooming. However, experimental details about both phenomena for propagations in water are lacking, especially for high-power lasers in the kilowatt range. We show that significant thermal lensing occurs only for high input powers before the onset of convective flow, while for low input powers, no strong thermal lens arises. After the onset of water flow, thermal blooming occurs at low input powers comparable to that known for propagations over kilometres in the air. However, for high input powers a thermal blooming on a qualitatively higher level is shown. By wavefront sensing, the change of refractive index distribution in water is investigated. This clearly shows the fast development of a strong thermal lens for high input powers and the onset of convection. Furthermore, a qualitatively good agreement of the accompanying simulations is observed. It is found that the absorption coefficient is linear with a value of $$\mu ={13.7}\,{\mathrm{m}^{-1}}$$ μ = 13.7 m - 1 at least up to 7.5 kW, i.e. 8 $$\mathrm{kW/cm}^2$$ kW / cm 2 . However, the directed transmission into an aperture is only constant before any thermal lensing of blooming occurs.

During laser penetration, the irradiated samples form a melt pool before perforation. Knowledge of the dynamics of this melt pool is of interest for the correct physical description of the process and leads to improved simulations. However, a direct investigation, especially at the location of high-power laser interaction with large spot diameters in the centimeter range is missing until now. Here, the applicability of 2D triangulation for surface topology observations is demonstrated. With the designed bidirectional 2D triangulation setup, the material cross-section is measured by profile detection at the front and back side. This allows a comprehensive description of the penetration process to be established, which is important for a detailed explanation of the process. Specific steps such as surface melting, indentations, protrusions during melt pool development and their dynamics, and the perforation are visualized, which were unknown until now. Furthermore, a scanning 3D triangulation setup is developed to obtain more information about the entire melt pool at the front side, and not just a single intersection line. The measurements exhibit a mirror-symmetric melt pool and the possibility to extrapolate from the central profile to the outer regions in most cases.

Commercially available carbon fiber reinforced polymer matrix composite panels (Hexply® M18-1/G939 and 8552/IM7) with different thicknesses (1-6 mm) were exposed to continuous-wave high-energy laser radiation at various laser powers up to 10 kW under static conditions. The perforation times, the size of the damaged volume and the residual compressive strengths are determined and correlated to the irradiation parameters. It is found that for the time of perforation, the damaged volume, which is approximated by a cylinder shaped model, correlates linearly with the laser energy imposed onto the sample surface. This relationship can be used for a prediction of the perforation time for various laser powers, material thicknesses and laser spot diameters. Increasing laser energy results in a decay of residual compressive strength after impact. Visual inspection as well as micro-focused computed X-ray tomography and scanning electron microscopy indicate a small area of thermal damage outside the laser spot. Additionally, infrared spectroscopy characterizes incipient heat damage most sensitively.

A recently published modeling approach for the penetration into adobe and previous approaches implicitly criticized are reviewed and discussed. This article contains a note on the paper titled “Ballistic model for the prediction of penetration depth and residual velocity in adobe: A new interpretation of the ballistic resistance of earthen masonry” (DOI: https://doi.org/10.1016/j.dt.2018.07.017). Reply to the Note from Li Piani et al is linked to this article.

Selective Laser Melting (SLM) offers significant potential for a sustainable way of production. Raw material in form of metallic powder can directly be reused and the selective nature of the process offers new potential for resource economization. We introduce a mathematical model, which allows conclusions about the influence of parameters like part volume (influenced by lightweight design) and exposure parameters onto the resource consumption in an SLM process. For this purpose, time and energy consumption are classified in process shares as a function of volume and process parameters. The introduced approach is validated by experimental methods under the consideration of part volume, exposure parameters and batch size. While the approach shall be independent of the manufactured material, the experiments are executed for the aluminum alloy AlSi10Mg. The measurements quantify the impact of the part volume and process parameters on the resource consumption and provide recommendations for improvements regarding an increased material efficiency. Additionally, the established model can be used to analyze manufacturing costs for single parts or series productions. The results illustrate the importance of lightweight design methods for an efficient and sustainable production by powder bed fusion methods like SLM.

Impact and shock wave events represent typical extreme dynamic loads of interest for accessing the response of materials and structures. Here, we focus on an intense electric current pulse as an extreme dynamic load. Experimentally, metallic samples were electromechanically loaded with currents up to 400 kA. For this purpose, a test rig containing a high-voltage pulsed power supply and high-performance switches was built. In this paper, we present an approach for modeling the response of thick metallic wire samples, with diameters on the order of millimeters, to extreme electromechanical loads sufficient for deformation and fragmentation, but not for material disintegration by wire explosion. The essential electrodynamic, thermodynamic, and mechanical aspects are considered, and a way to couple those physical regimes is suggested, which allows the use of numerical simulation based on the finite-element method to determine the material response. Simulations are employed to describe the time-dependent process of the structural and material behavior of thick wires. The focus here is on the structural mechanical behavior including bending or buckling before the onset of the wire explosion. The new approach for a simulation model is able to capture the basic experimental observations.

A special phenomenon observed in hypervelocity impacts on rock targets is the so-called momentum multiplication, i.e. the momentum transferred to the target is greater than the original momentum of the projectile. This effect is caused by ejection of debris in the direction opposite to the flight direction of the projectile. In the present study momentum multiplication was investigated as a function of target material properties and projectile velocity. Hypervelocity impact experiments on target materials with different porosities were conducted and the momentum transfer was measured using a ballistic pendulum. Low porous materials like quartzite show larger momentum multiplication than porous materials like sandstone. The smallest momentum multiplication was measured for highly porous aerated concrete. Higher projectile velocity leads to higher momentum multiplication. Furthermore, this increase is stronger for low porous materials compared with porous materials. These observations can be explained by the different ejection behavior. Low porous materials show a directional and very fast ejection whereas porous materials show a slower ejection. The highly porous material shows a diffuse ejection behavior. Furthermore, cratering efficiency is reduced in porous targets leading to a smaller amount of ejected debris. This effect is attributed to energy dissipation caused by irreversible crushing of pore space.

This paper extends a recently published investigation on the dwell capability of single ceramic tiles of limited thickness for protection against tungsten-heavy-alloy (WHA) rod penetrators. Direct impact experiments with long WHA rods against unconfined SiC tiles supported by a steel backing were performed at impact velocities ranging from 1100 m/s to 1800 m/s. The ceramic tile was either bare or had a small buffer layer attached to the front. For the bare ceramic, the area density of the penetrated material ρ increases linearly with increasing impact velocity but at a reduced slope when compared to a semi-infinite RHA target. For the buffered ceramic, dwell is possible up to around 1700 m/s. Compared to the bare ceramic ρ is smaller and the buffered ceramic performs best regarding mass efficiency at an impact velocity of about 1600-1700 m/s before the performance drops sharply towards values in the range of the bare ceramic. Part of the work presented here has recently been published in a journal article [Behner et al., Int. J. Impact Eng. 95 (2016) 54-60].

The protective capabilities of three different armor steels of the HHA and UHA class against a long rod penetrator (LRP) were examined. Due to the lack of commercial availability of thick armor plates, the specific ballistic behavior under overmatch conditions, and unavoidable experimental scatter, we were unable to confirm any performance differences among the steels in ballistic testing, even when compared to standard RHA. In a second approach, numerical simulations of the ballistic behavior of the armor steels based on material testing (tensile tests and planar-plate-impact tests) were performed. The simulations provided the possibility to examine the steels in unavailable configurations, i.e. thickness. With this combined approach it could be shown that UHA steels may give a considerable protection increase against LRP in diminishing their maximum effective range by at least several hundred meters compared to HHA steel.

In the present paper, we review and extend our results concerning predictive numerical simulations of the penetration into adobe on the basis of the RHT concrete model [C. Sauer et al., Int. J. Impact Engng., 104 (2017) 164-176]. We apply this model to the theoretical description of adobe in a hydrocode. For this purpose a set of material parameters is derived on the basis of published data, own material tests, and engineering assumptions. Available ballistic results for the penetration of spherical steel projectiles into semi-infinite and finitethickness adobe targets are used for validating our approach. As a result, we obtain a hydrocode model which is expected to predict a wide range of penetration scenarios into adobe. The predictive character is exemplarily demonstrated by the successful reproduction of the instability within the penetration process and residual velocities of differently shaped tungsten heavy alloy projectiles.

In this work, the energy transfer from intense continuous-wave laser beams with a wavelength of 1070 nm, a power in the kilowatt range, and with diameters in the millimeter and centimeter range to metal samples is investigated. While the absorptivity of iron and steel samples is almost constant for laser intensities below 3.4 kW/cm2, a decrease in the absorptivity is observed for higher intensities which is attributed to the formation of a vapor plume in the interaction zone. The dynamics of the formation and expansion as well as the emission of light in the visible spectral range from the vapor plume are further characterized with a fixed beam diameter of 2.6 mm at a laser power of 10 kW in detail for iron and aluminum samples. The analysis of high speed video sequences yields expansion velocities of the vapor plume of 5.0 m/s for the iron and 0.29 m/s for the aluminum samples. In the spectra from the aluminum samples, emission lines from atomic aluminum as well as emission bands from molecular aluminum monoxide are identified and allow for the estimation of the basic thermodynamic parameters. A special focus is on the investigation of the effect of vapor and plasma formation on the energy transfer from the laser to the sample and on the analysis of the role of inverse bremsstrahlung in this process. The measurements indicate that the metal vapor is partially ionized and that there is a significant contribution of inverse bremsstrahlung to the absorption of laser energy in the partially ionized vapor plume.

The development of reliable techniques for the safe neutralization of improvised explosive devices (IEDs) is an active field of research. Recently, innovative approaches for the neutralization of IEDs were developed and tested within the FP7 project ENCOUNTER (“Explosive Neutralisation and Mitigation Countermeasures for IEDs in Urban/Civil Environment”) and were compared to existing, established technologies. Here, the ENCOUNTER project is presented with a special focus on the neutralization of IEDs by high-power lasers. The working principle of the application of high-power lasers for the neutralization of explosive devices is based on thermal effects. Heating of the IEDs main charge may occur either by direct irradiation of the explosive material or by heat transfer through the main charge’s confinement. The aim of the application of the laser is to achieve a low order burning reaction of the explosive charge and thus a controlled neutralization of the IED. Since laser beams allow for the directed transport of energy, this technique can be applied over long stand-off distances and has thus potential for an increase of the safety of clearing forces and population in the case of terroristic attacks in a civilian environment. Within the ENCOUNTER project, a laboratory environment has been set up which allows for the irradiation of IEDs with a laser power of up to 10 kW. Experiments have been carried out on a broad spectrum of different types of IEDs. The processes during neutralization were studied in detail with high-speed diagnostics. On the basis of these experimental data, the safety and the reliability of the application of the laser was analyzed, and recommendations to end users were given. In addition to the results of the ENCOUNTER project, approaches for the numerical simulation of the neutralization of IEDs are discussed.

The phenomena occurring during laser-metal interaction depend on a variety of parameters such as laser wavelength, intensity, and thermophysical and optical properties of the irradiated material. This work comprises a detailed study of the perforation process of thin iron and steel plates under CW laser radiation at a wavelength of 1.07 mm. Experiments were carried out over a wide range of intensities between 0.1 and 100 kW/cm2, and with beam radii in the millimeter and centimeter range. Additionally, we describe a method applying contact-free radiative temperature measurements, which allows the measurement of melt-through times without an exact knowledge of the plate's spectral surface emissivity. A detailed numerical model is presented and employed to analyze, interpret, and predict data gathered from the experiments. The model is shown to produce accurate predictions and is in good agreement with experimental data. Furthermore, our results indicate that in the investigated regime perforation is governed by gravitational and surface forces. Based on this hypothesis, a criterion for the onset of the perforation explaining the observed local minimum in the perforation time versus beam radius curve is presented.

Failure front (FF) and penetration velocity have been measured for long gold rods impacting and penetrating borosilicate (BS) glass. Data are obtained by visualizing simultaneously FF propagation with a high speed camera and rod penetration with flash X‐rays. Results for BS glass are qualitatively similar to those of DEDF (PbO) glass. FF velocity rapidly decreases from an initial value to a lower, approximately constant value. FF velocity increases with impact velocity, vp. The FF velocity remains significantly lower than the shear velocity, even at the highest impact velocity tested, about 2.5 km/s. The ratio of the FF velocity to the rod penetration velocity, vF/u, decreases with increasing vp and appears to be approaching vF/u = 1 asymptotically, as observed previously for DEDF glass. The separation of the FF and the tip of the rod decreases with increasing impact velocity. Importantly, since vF/u⩾1, the gold rod is always penetrating glass behind the FF.