Armor Surface Research Articles

Surface roughness measurement for the assessment of damage dynamics of existing and upgraded cube-armored breakwaters

Behavior of existing or upgraded rubble-mound breakwaters with non-conventional geometry, layering and surface porosity may be quite different from what predicted by state-of-art design approaches, and traditional techniques for damage estimation may be not sufficient. In the present work, a novel SfM-based technique for the analysis of damage mechanisms of cube-armored breakwaters is proposed, which is based on the evaluation of descriptors of the macro and micro-roughness of the armor layer. The first one represents the waviness of the armor slope, whereas the latter one is a proxy of the armor layer surface porosity. A systematic experimental investigation was performed, considering different upgrading options for a harbor rubble-mound breakwater with cube-shaped armor units, which involve, besides the rise of the wave wall, the addition of extra armor blocks similar or smaller than the existing ones. Damage measurements acquired through traditional laboratory techniques were combined with the armor surface roughness descriptors. The results of the novel analysis of the cube armor layer macro-roughness revealed that the processes which cause the displacement of the outer blocks are influenced by the level of waviness of the armor slope. Moreover, the quantitative analysis of the micro-roughness enabled the automated identification of four damage mechanisms experienced by the outer armor layer: (i) static equilibrium; (ii) incipient instability; (iii) bulk displacement; (iv) individual displacements. The use of the proposed roughness parameters makes simple and immediate the quantitative comparison between the performances of different upgrading solutions. • A novel SfM-based damage analysis technique for cube-armored breakwaters is proposed. • The armor layer surface is described through non-dimensional roughness descriptors. • Experimental data on upgraded structures are compared to the predictions of state-of-art. • The armor slope waviness significantly influences the stability of the upper layers. • Four damage mechanisms are identified as a function of a new roughness parameter.

High-heat-flux performance limit of tungsten monoblock targets: Impact on the armor materials and implications for power exhaust capacity

• Results of the extensive high-heat-flux tests of tungsten monoblock-type divertor target mock-ups are presented. • Impacts of various slow/short transient overloads relevant to European DEMO divertor on the armor materials and the joining interfaces are addressed. • Comprehensive high-quality data comprising in-situ IR thermography, ultrasonic reflectometry and electron microscopy (EBSD) are presented. • Implications of the novel test results for the power exhaust requirements are discussed. Development of a reliable high-heat-flux (HHF) technology is one of the crucial requirements of power exhaust strategy for a fusion reactor. The baseline HHF technology of the EU-DEMO, which inherited mostly the ITER technology, is based on the tungsten monoblock design and hot radial pressing joining technique. Thermal resilience and structural integrity under all off-normal transient events foreseen are essential prerequisites for validation of the technology towards the demonstration of full-scale prototype manufacture. Hence, the HHF performance of the baseline technology needs to be evaluated also in the transient overload regime. To this end, we conducted an extensive HHF testing campaign using small-scale test mock-ups of the tungsten monoblock target for two slow (10s) overloads at 20 MW/m 2 (up to 2000 pulses) and 25 MW/m 2 (1000 pulses), and for a short (0.4s) overload at 40 MW/m 2 (5000 pulses). Furthermore, excessive heat loads (32-37 MW/m 2 ) were applied beyond the armor melting event to test the structural stability at limit loads. IR thermography, ultrasonic inspection and electron microscopy (EBSD) delivered direct information and insight on the structural integrity and the impact on the armor microstructure. It was found that the tungsten monoblock target remained fully intact at least up to 1000 heating cycles at 20 MW/m 2 (10s) and survived 2000 cycles without any major failure. At 25 MW/m 2 (10s), the mock-ups remained nearly intact at least up to 500 heating cycles and survived 1000 cycles without critical failure. However, the armor surface showed substantial deformation roughening with the height of 1000 µm after 1000 cycles. At 40 MW/m 2 with short pulses (0.4s), the mock-ups remained fully intact without any serious damage at least up to 5000 cycles. The mock-ups even withstood the limit heat load of 32 MW/m 2 at least for a few pulses. Under excessive overloads (33-37 MW/m 2 ) above the critical incident heat flux, armor melting preceded any other potential failures (e.g. pipe rupture by coolant boiling).

On the Planar Geometric Patterns of the Exoskeletal Relief Formation in Early Vertebrates (Osteostraci, Agnatha).

A study of the diversity of sculpture and histological structure of the exoskeleton in various osteostracan taxa (Osteostraci, Agnatha) enabled the first characterization of the main elements (geometric modules) of the planar organization of the complex relief on their armor surface. The armor relief was analyzed using a circular model of the formation of exoskeletal hard structures. The model was applied to unique material, fragments of a shield of the osteostracan Oeselaspis pustulata (Patten, 1931) from the Silurian of the Severnaya Zemlya Archipelago (Russia).

Metallographic analysis of piercing armor plate by explosively formed projectiles

This work discusses the impact consequences of Explosively Formed Projectiles (EFP) on the process of armor penetration. Based on the example of piercing shields made of armor steel ARMOX 370T with anti-vehicle mine of MPB type to verify the effectiveness of EFP charge. The influence of the projectile on the armor wall was analyzed on the basis of metallographic examination of the material from the armor, its plastic deformation and the change in the structure of the material within the shock influence area. As a result of the slug projectile impact on the armor surface a crater as a hole was formed with a diameter of approx. 105–120 mm, with noticeable traces of dynamic plastic deformation which occur at high speeds and high temperatures. In addition, microstructure analysis of the mines liner was performed, as in before and after piercing based on the analysis of EDS for phase precipitates identification.

Fracture mechanical analysis of a tungsten monoblock-type plasma-facing component without macroscopic interlayer for high-heat-flux divertor target

In the framework of the European DEMO divertor project, several novel design concepts of the plasma-facing components of vertical targets are being developed. One of those concepts is the tungsten monoblock design (similar to the ITER divertor) but with a very thin interlayer (roughly 25mm thick only) at the armor/tube bond interface instead of a thick (1mm) copper interlayer as has been the case in the conventional tungsten monoblock design developed for ITER divertor. The thin interlayer serves as bonding agent, but not as structural constituent. The reasoning for this novel design concept to omit the thick soft copper interlayer, which has been used as stress-relaxing buffer between the stiff armor block and tube, is to prevent plastic fatigue damage under cyclic high heat flux loads and irradiation embrittlement which the soft copper interlayer is predicted to undergo. On the other hand, the desirable stress relaxation effect on a global scale is abandoned. In this study, such trade-off effects are computationally investigated in a comparative assessment of structural impact, which the presence (or absence) of the thick copper interlayer is expected to bring forth, in terms of the fracture and fatigue behaviour of the armour block and cooling tube in two representative cases of tungsten monoblock plasma facing component design, namely, with and without a thick copper interlayer. Quantitative results of cyclic plastic strain history and crack tip fracture energy are presented for the armour surface, bond interface and tube of the respective plasma facing component models. The positive and negative implications of these impacts on the structural integrity are discussed.

Design options to mitigate deep cracking of tungsten armor

Recent high heat flux (HHF) tests showed that the tungsten monoblock armor often suffered from deep cracking, when the applied HHF load approached 20MW/m2. The deep cracks were initiated at the armor surface and grew toward the cooling tube. The deep cracking seemed not to affect the heat removal capability of tungsten divertor, as most of the cracks were perpendicular to the loading surface. However, the inherently unstable nature of brittle cracking may likely increase the risk of structural failure. In this work, three variants (reduction in width of armor, inverse trapezoid shape in the lower part and castellation) of full-W divertor armor design based on the ITER divertor design are proposed to mitigate deep cracking at 20MW/m2. The temperature, stress and strain fields are simulated with finite element method. The possibilities of crack initiation and propagation are evaluated by calculating the low cycle fatigue lifetime and J-integrals, respectively. All three variants can mitigate deep cracking of tungsten armor.

Structural impact of armor monoblock dimensions on the failure behavior of ITER-type divertor target components: Size matters

Plenty of high-heat-flux tests conducted on tungsten monoblock type divertor target mock-ups showed that the threshold heat flux density for cracking and fracture of tungsten armor seems to be related to the dimension of the monoblocks. Thus, quantitative assessment of such size effects is of practical importance for divertor target design. In this paper, a computational study about the thermal and structural impact of monoblock size on the plastic fatigue and fracture behavior of an ITER-type tungsten divertor target is reported. As dimensional parameters, the width and thickness of monoblock, the thickness of sacrificial armor, and the inner diameter of cooling tube were varied. Plastic fatigue lifetime was estimated for the loading surface of tungsten armor and the copper interlayer by use of a cyclic-plastic constitutive model. The driving force of brittle crack growth through the tungsten armor was assessed in terms of J-integral at the crack tip. Decrease of the monoblock width effectively reduced accumulation of plastic strain at the armor surface and the driving force of brittle cracking. Decrease of sacrificial armor thickness led to decrease of plastic deformation at the loading surface due to lower surface temperature, but the thermal and mechanical response of the copper interlayer was not affected by the variation of armor thickness. Monoblock with a smaller tube diameter but with the same armor thickness and shoulder thickness experienced lower fatigue load. The predicted trends were in line with the experimental observations.

Effect of design geometry of the demo first wall on the plasma heat load

In this work we analyse the effect of W armour surface shaping on the heat load on the W/EUROFER DEMO sandwich type first wall blanket module with the water coolant. The armour wetted area is varied by changing the inclination and height of the «roof» type armor surface. The deleterious effect of leading edge at the tiles corner caused by misalignment is replaced in current design by rounded corners. Analysis has been carried out by means of the MEMOS code to assess the influence of the thickness of the layers and effect of the magnetic field inclination. Calculations show the evolution of the maximum temperatures in the tungsten, EUROFER, Cu allow and the stainless-steel water tube for different level of surface inclination (chamfering) and in the case of rounded corners used in the current design. It is shown that the blanket module materials remain within a proper temperature range only at shallow incident angle if the width of EUROFER is reduced at list twice compare with the reference case.

Channel adjustments to a succession of water pulses in gravel bed rivers

AbstractGravel bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply. The evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers. However, the role of successive floods on the surface texture evolution is still poorly understood. An experimental campaign in an 18 m‐long 1 m‐wide flume has been designed to study these issues. Eight consecutive runs, each one consisting of a low‐flow period of variable duration followed by a sudden flood (water pulse) lasting 1.5 h, have been conducted. The total duration of the experiment was 46 h. The initial bed surface was created during a 280 h‐long experiment focused on the influence of episodic sediment supply on channel adjustments. Our experiments represent a realistic armored and structured beds found in mountain gravel bed rivers. The armor surface texture persists over the duration of the experiment. The experiment exhibits downstream fining of the bed‐surface texture. It was found that sorting processes were affected by the duration of low‐flow between flood pulses. Since bed load transport is influenced by sediment sorting, the evolution of bed load transport is impacted by the frequency of the water pulses: short interpulse durations reduce the time over which fine material (transported as bed load) can be winnowed. This, in turn, contributes to declining reduction of the bed load transport over time while the sediment storage increases.

Interpretation of the deep cracking phenomenon of tungsten monoblock targets observed in high-heat-flux fatigue tests at 20 MW/m2

The HHF qualification tests conducted on the ITER divertor target prototypes showed that the tungsten monoblock armor suffered from deep cracking due to fatigue, when the applied high-heat-flux load approaches 20MW/m2. In spite of the critical implication of the deep cracking of armor on the structural integrity of a whole target component, no rigorous interpretation has been given to date. In this paper, a theoretical interpretation of the observed deep cracking feature is presented. A two-stage modeling approach is employed where deep cracking is thought to be a consecutive process of crack initiation and crack growth, which is assumed to be caused by plastic fatigue and brittle facture, respectively. The fatigue lifetime to crack initiation on the armor surface and the crack tip load of brittle fracture are assessed as a function of crack length and heat flux loads. The potential mechanisms of deep cracking are discussed for a typical slow transient high-heat-flux load cycle. It is shown that the quantitative predictions delivered in this study agree well with the observed findings offering insight into the nature of tungsten armor failure.

Flow Instabilities in Non-Uniformly Heated Helium Jet Arrays Used for Divertor PFCs

Due to a lack of prototypical experimental data, little is known about the off-normal behavior of recently proposed divertor jet cooling concepts. This article describes a computational fluid dynamics (CFD) study on two jet array designs to investigate their susceptibility to parallel flow instabilities induced by non-uniform heating and large increases in the helium outlet temperature. The study compared a single 25-jet helium-cooled modular divertor (HEMJ) thimble and a micro-jet array with 116 jets. Both have pure tungsten armor and a total mass flow rate of 10 g/s at a 600 °C inlet temperature. We investigated flow perturbations caused by a 30 MW/m2 off-normal heat flux applied over a 25 mm2 area in addition to the nominal 5 MW/m2 applied over a 75 mm2 portion of the face. The micro-jet array exhibited lower temperatures and a more uniform surface temperature distribution than the HEMJ thimble. We also investigated the response of a manifolded nine-finger HEMJ assembly using the nominal heat flux and a 274 mm2 heated area.For the 30 MW/m2 case, the micro-jet array absorbed 750 W in the helium with a maximum armor surface temperature of 1280 °C and a fluid/solid interface temperature of 801 °C. The HEMJ absorbed 750 W with a maximum armor surface temperature of 1411 °C and a fluid/solid interface temperature of 844 °C. For comparison, both the single HEMJ finger and the micro-jet array used 5-mm-thick tungsten armor. The ratio of maximum to average temperature and variations in the local heat transfer coefficient were lower for the micro-jet array compared to the HEMJ device.Although high heat flux testing is required to validate the results obtained in these simulations, the results provide important guidance in jet design and manifolding to increase heat removal while providing more even temperature distribution and minimizing non-uniformity in the gas flow and thermal stresses at the armor joint.

Applicability of tungsten/EUROFER blanket module for the DEMO first wall

In this paper we analyse a sandwich-type blanket configuration of W/EUROFER for DEMO first wall under steady-state normal operation and off-normal conditions, such as vertical displacements and runaway electrons. The heat deposition and consequent erosion of the tungsten armour is modelled under condition of helium cooling of the first wall blanket module and by taking into account the conversion of the magnetic energy stored in the runaway electron current into heat through the ohmic dissipation of the return current induced in the metallic armour structure. It is shown that under steady-state DEMO operation the first wall sandwich type module will tolerate heat loads up to ∼14MW/m2. It will also sustain the off-normal events, apart from the hot vertical displacement events, which will melt the tungsten armour surface.

Effects of Hypervapotron Geometry on Thermalhydraulic Performance

Plasma disruptions and edge localized modes can result in transient heat fluxes as high as 5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on portions of a tokamak reactor first wall (FW). To accommodate these heat loads, the FW will likely use water-cooled hypervapotron heatsinks to enhance the heat transfer. In this article, we present the results of a computational fluid dynamics (CFD) study using 70 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C inlet water at 2.7 MPa to investigate the tooth height and backchannel depth of 50-mm-wide hypervapotrons with 6-mm-pitch and 3-mm side slots. We compare a popular design with 4-mm-high teeth and a 5-mm backchannel to a more optimal case with 2-mm-high teeth and a 3-mm backchannel under nominal heat loads (0.5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) on a 100-mm-heated length and under single-phase flow conditions. Better heat transfer in the latter case and the smaller backchannel permit a factor of two reduction in the required mass flow while maintaining the same beryllium armor surface temperatures near 130 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The shallow teeth and smaller backchannel allow the 40 fingers in a typical panel to flow in parallel and simplify the water circuit. A comparison of the two hypervapotron designs during off-normal loading (5.0 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and two-phase flow then follows. The design with 2-mm teeth has a 3.5% higher beryllium surface temperature of 648 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and reduces the critical heat flux (CHF) by ~2%. Hypervapotron width also plays a role in heat transfer and CHF. CFD results for 36 and 70 mm wide hypervapotrons compared to the 50-mm case reveal similar thermal performance at low heat flux, but a reduction in CHF with increasing width. This study highlights the necessary compromise between design margin during transient events, effective heat transfer under nominal conditions, limitations on finger width, and the simplicity needed in the water circuit design.

Simulation of residual thermostress in tungsten after repetitive ELM-like heat loads

Brittle destruction of tungsten armour under action of edge localised modes of plasma instabilities (ELMs) in ITER is an important issue determining the lifetime of the divertor. Besides, cracking of the armour produces tungsten dust with characteristic size of 1–10 μm flying from the armour surface with velocities up to 10 m/s. Influx of the tungsten dust into the ITER confinement decreases the temperature of the plasma, reduces the thermonuclear gain and even may run the confinement into disruption. This paper describes experiments in QSPA-Kh50 plasma gun and modeling, which has been performed for providing more insight into the physics of tungsten cracking under action of ELMs and for confirmation of the important result on stabilization of the crack development at the tungsten armour surface, predicted in our previous paper – the same authors, 2010. The threshold value of energy density deposition for start of tungsten cracking has been measured as 0.3 MJ/m 2 after 5–10 shots. From analytical considerations three times smaller threshold value has been predicted with increasing number of shots.

Damage of actively cooled plasma facing components of magnetic confinement controlled fusion machines

Plasma facing components (PFCs) of magnetic fusion machines have high manufactured residual stresses and have to withstand important stress ranges during operation. These actively cooled PFCs have a carbon fibre composite (CFC) armour and a copper alloy heat sink. Cracks mainly appear in the CFC near the composite/copper interface. In order to analyse damage mechanisms, it is important to well simulate the damage mechanisms both of the CFC and the CFC/Cu interface. This study focuses on the mechanical behaviour of the N11 material for which the scalar ONERA damage model was used. The damage parameters of this model were identified by similarity to a neighbour material, which was extensively analysed, according to the few characterization test results available for the N11. The finite elements calculations predict a high level of damage of the CFC at the interface zone explaining the encountered difficulties in the PFCs fabrication. These results suggest that the damage state of the CFC cells is correlated with a conductivity decrease to explain the temperature increase of the armour surface under fatigue heat load.

Rebounding of a shaped-charge jet

The phenomenon of rebounding of a shaped-charge jet from the armour surface with small angles between the jet axis and the target surface is considered. Rebounding angles as a function of jet velocity are obtained in experiments for a copper shaped-charge jet. An engineering calculation technique is developed. The results calculated with the use of this technique are in reasonable agreement with experimental data.

Melt damage simulation of W-macrobrush and divertor gaps after multiple transient events in ITER

Tungsten in the form of macrobrush structure is foreseen as one of two candidate materials for the ITER divertor and dome. In ITER, even for moderate and weak ELMs when a thin shielding layer does not protect the armour surface from the dumped plasma, the main mechanisms of metallic target damage remain surface melting and melt motion erosion, which determines the lifetime of the plasma facing components. The melt erosion of W-macrobrush targets with different geometry of brush surface under the heat loads caused by weak ELMs is numerically investigated using the modified code MEMOS. The optimal angle of brush surface inclination that provides a minimum of surface roughness is estimated for given inclination angles of impacting plasma stream and given parameters of the macrobrush target. For multiple disruptions the damage of the dome gaps and the gaps between divertor cassettes caused by the radiation impact is estimated.

Reply to the discussion by S. Chatterji of the paper “Mercury porosimetry—an inappropriate method for the measurement of pore size distributions in cement-based materials”

This work discusses the impact consequences of Explosively Formed Projectiles (EFP) on the process of armor penetration. Based on the example of piercing shields made of armor steel ARMOX 370T with anti-vehicle mine of MPB type to verify the effectiveness of EFP charge. The influence of the projectile on the armor wall was analyzed on the basis of metallographic examination of the material from the armor, its plastic deformation and the change in the structure of the material within the shock influence area. As a result of the slug projectile impact on the armor surface a crater as a hole was formed with a diameter of approx. 105–120 mm, with noticeable traces of dynamic plastic deformation which occur at high speeds and high temperatures. In addition, microstructure analysis of the mines liner was performed, as in before and after piercing based on the analysis of EDS for phase precipitates identification.

Thermomechanical behavior of actively cooled, brazed divertor components under cyclic high heat flux loads

Actively cooled divertor mock-ups consisting of various low- Z armor tiles brazed to refractory metal heat sinks were tested in the electron beam test facility at Jülich. Screening and thermal cycling tests were perfomed on the mock-ups to estimate the overall thermal performance under cyclic high heat flux (HHF) loadings. By detecting the temperature of the armor surface and the braze layer, it was possible to assess the heat removal capability and the accumulation of interfacial damage. Microstructures were investigated to elucidate the degradation of the joints. Finite element analyses are carried out for the simulated HHF test conditions. Temperature fields and thermal stresses are calculated for a typical divertor module. The nature of thermomechanical behavior of the divertor mock-ups under cyclic HHF loadings is discussed.

Plasma gun experiments and modeling of disruptions

Potentially high erosion due to ablation from plasma disruptions looms as a nemesis for advanced fusion devices such as ITER. Under some conditions, it is believed that the material ablated during a disruption forms a “vapor shield” that mitigates subsequent ablation. This paper presents new experimental data that identifies an absorption surface in the ablating plasma that is above the irradiated armor surface. We also present a summary of progress in modeling using a 1-D Lagrangian hydrodynamics code. Experimental erosion data will be reviewed from the PLADIS facility, a plasma gun at the University of New Mexico on several materials, including Be, tungsten, copper, graphites. Profile measurements of the crater topology showed the eroded Be surface to be much rougher than that of carbon and to demonstrate erosion rates that were almost factors of four greater than graphite. Plasma guns at the D.V. Efremov Scientific Research Institute and at TRINITI, both in the Russian Federation, are being utilized to confirm spectroscopic vapor shield data.