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It is extremely important to predict the growth, aggregation, and coalescence failure of voids during the dynamic tensile fracture of ductile metals. In the present work, we used the finite element—smoothed particle hydrodynamics (FE-SPH) adaptive method to simulate the plate impact of tantalum simultaneously from macro- and meso-scales. For macro simulation results, the spallation phenomena and free-surface velocity were in good agreement with the experimental results, verifying the correctness of the simulation method and material model. Moreover, the free surface velocity profiles simulated by the FE-SPH adaptive method is closer to the experiment than those by the finite element method. According to the magnified details of the damage, we envisaged that the deleted elements are converted to SPH particles to represent the formation of voids. By comparing the porosity, we demonstrated the rationality of this envisagement and determined the fine mesh size to simulate growth, aggregation, and coalescence of actual meso-voids. On this basis, we proposed a void-position tracking method to accurately track the temporal and spatial information of voids. Such information would provide a detailed range of damage and describe the features and macro factors affecting void evolution. In general, the fine mesh FE-SPH method can well predict the damage distribution of spallation simultaneously in macro- and meso-scales, and this simple method has important applications. [ABSTRACT FROM AUTHOR]

When the severity of high-temperature environments increases, the oxidation mode changes from a normal oxidation mode, when a formed oxide layer protects the surface of austenitic heat-resistant cast steels, to an extreme mode, when scale spallation and partial vaporization intensify surface degradation. These processes can also influence each other. On the example of two oxidized austenitic heat-resistant steels with different alloying levels, we show how the described simulation/experimental methodology may be applied to analyze and quantify the synergetic kinetics of the extreme surface degradation processes. The surface degradation processes were formulated using diffusion-controlled parabolic oxidation, time-dependent vaporization, and spallation with an intensity cross-linked to the instantaneous thickness of the oxide layer. The 400 h tests were performed in air and water vapor-containing combustion gases at upper working temperatures. Three experimentally obtained surface degradation quantities included changing the weight of the specimen with adherent oxide, the weight of the spalled scale, and the thickness of the formed adherent oxide. These experimental quantities were used to compute the values of oxidation, vaporization, and spallation kinetic constants for each uninterrupted oxidation test. An additional interrupt every 100-h test was also performed to verify the model-predicted trends. It was shown that the alloying level in austenitic heat-resistant cast steel determines its ability to withstand surface degradation at extreme oxidation mode by the synergetic effects of oxidation, vaporization, and spallation. The described approach can be used for the metallic component lifetime prediction in severe gaseous environments. [ABSTRACT FROM AUTHOR]

In this study, dynamic mechanical response and the corresponding atomic mechanisms of single-crystal tungsten under extreme strain rates (109 s−1) are investigated using molecular dynamics simulations. The results show that crystal orientation plays an important role in the stress–strain relationship. The critical stresses for the beginning of plastic deformation are 59.4, 48.0, and 25.2 GPa for quasi-isentropic tensile loading along [111], [110], and [100] crystal orientations, respectively. The atomic behavior during plastic deformation suggests that [100] and [110] experience stress relaxation through phase transitions, while not in the [111] tensile direction. During spallation, sub-grain boundaries formed at twin junction in the [100] and [110] directions serve as nucleation sites for voids. The void grows in a planar way (along the twin) after generation, while stops growing at another misoriented twin junction. In the [111] tensile test, spallation occurs in the stress-concentration area, and finishes in a very short time interval with huge void coalescence. Our findings not only provide atomic insights into the anisotropic mechanical behaviors during spallation of tungsten under high strain rates, but also shed lights on the colorful plastic deformation behaviors from laser-shock experimental observation. [ABSTRACT FROM AUTHOR]

Interferometry allows to measure microscopic displacements in the nanometer scale. This work introduces an imaging pump-probe interferometer that enables to detect the spatially resolved phase shift of the probe radiation being reflected on a sample surface after ultrashort pulsed laser excitation with a temporal resolution of 40 fs and a maximum temporal delay of 3 ns. The capability of the pump-probe interferometer is demonstrated on the spallation of a thin gold film upon femtosecond laser irradiation. The pump-probe interferometer enables to measure a minimum phase change of Δ φ < π / 10 which corresponds to a displacement of Δ h < 12.5 nm for the applied probe wavelength of 500 nm. Upon irradiation, two distinct phase changes are observable: First, an abrupt minor phase shift has been measured in the femtosecond range, which is attributed to the changed optical properties of the sample surface after excitation. Second, a more pronounced continuing phase shift increase is detectable after a few tens of picoseconds resulting from the onset of spallation. Based on the measured spatially resolved phase shift, the transient surface topography during the spallation is reconstructed. The determined velocity of the ablated material reaches a maximum of a few 1000 m/s at ten picosecond after irradiation and decreases to 250 m/s afterwards. Consequently, the introduced imaging pump-probe interferometer provides important insights into the physical processes during laser excitation as well as the subsequent laser-induced ablation, and will enable to validate theoretical models quantitatively in following studies. [ABSTRACT FROM AUTHOR]

Previously, we estimated the neutron flux from the synchrotron of the Konstantinov St. Petersburg Institute of Nuclear Physics, at various moderator configurations near the internal target used to create a neutron flux by the spallation reaction. In this work, we made a successful attempt to transmute Am-241. [ABSTRACT FROM AUTHOR]

Shock wave loading of heterogeneous materials was numerically investigated using three models: a homogeneous alloy model with experimental parameters, an additive approximation model with parameters calculated from the constants and concentrations of the components, and a discrete numerical model constructed based on a random concentration distribution of components over the sample volume. The verification of the computational schemes was done by calculating the shock wave loading of homogeneous materials. Hugoniot curves were plotted and compared with experimental data to show a less than 5% deviation of the numerical results. A series of numerical simulations of spall fracture in homogeneous plates revealed that the free surface velocity profile resulting from spall fracture corresponds to the experimental profile. A relationship was derived to determine the ultimate spall strength for a heterogeneous medium based on the fracture parameters of its homogeneous components. The found homogeneous material parameters were used to simulate the shock wave loading of plates made of nickel titanium and tungsten carbide/cobalt cermet constructed with heterogeneous models. It was shown that the heterogeneous models can be effectively applied to problems of shock wave loading with spall fracture, and the deviation between the calculated free surface velocity of a heterogeneous plate and the experimental data does not exceed 10%. [ABSTRACT FROM AUTHOR]

In this study, effect of Fe concentration on the high temperature oxidation and microstructural stability of Fex(CrAlNi)100−x alloys (x = 25, 35, 45, 55, 65) at 1100 °C up to 168 h was investigated in air. Increasing Fe concentration decreased the molar fraction of B2 phase in as-cast alloys. However, microhardness values experienced only a 10% reduction (Fe25: 517.7 ± 19 HV, Fe65: 470.6 ± 22 HV) due to well-distributed B2 precipitates. After the exposures, coarsening of B2 precipitates was observed in all alloys, leading to a microhardness reduction of 20–25% after 168 h. Single-phase α-Al2O3 scales were formed on Fe25–Fe55 alloys. However, increasing Fe concentration resulted in deeper depletion zones due to reduced molar fraction and Al concentration of B2 phase. Moreover, Fe65 alloy failed to develop a protective α-Al2O3 scale due to decreased molar phase fraction and Al concentration of B2 precipitates, along with the low Cr concentration of the A2 phase. Additionally, α-Al2O3 scales were highly wrinkled due to the absence of reactive elements. Absence of reactive elements also resulted in oxide spallation and seemed to intensify with the increasing Fe concentration. Possible reasons for the increased oxide spallation with the increasing Fe concentration are discussed. Nevertheless, Fe25–Fe55 alloys displayed oxidation properties comparable to those of lean FeCrAl alloys while also possessing enhanced mechanical properties due to B2 reinforcement. [ABSTRACT FROM AUTHOR]

Thermal spallation is the erosion of brittle materials subjected to high heat fluxes, such as a hot fire in a concrete structure. While experimental investigations provide valuable data on the spallation process, they lack generality to be extrapolated to other materials or loading conditions. This paper presents a numerical method for predicting bulk quantities of the spallation process, such as the average recession rate, for crystalline materials. The method is based on direct numerical simulations of the microstructure. The crystal grains and their boundaries are modeled with finite elements and cohesive zones. These elements have anisotropic and temperature-dependent physical properties, a novel level of fidelity and one that accurately captures known crystal phenomena including the α – β quartz transition. These models of the crystalline material and the thermal spallation process are validated against experiment results for Barre granite, and then applied to predict the thermal spallation of Martian basalt. Highlights: This paper presents a high-fidelity model of a process called thermal spallation. Each individual grain within a rock is simulated directly, with material properties that are both temperature- and direction-dependent. This computational model reproduces experimental results for Barre granite. The model is then used to predict thermal spallation of Martian basalt, which is relevant to landing large robot and human missions on Mars. [ABSTRACT FROM AUTHOR]

Thin-walled castings made from Cr–Ni austenitic steels offer a combination of light weight of a near-net component with significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can result in intensive surface degradation due to scale spallation. Scale spallation can decrease the wall thickness which could be detrimental to the in-service life of thin-walled castings. Experiments and stochastic simulations of spallation assisted oxidation were performed with three cast austenitic heat-resistant steels having different Cr–Ni concentrations at temperatures between 900 and 1000 ℃ on air and water vapor containing combustion atmosphere. The recorded specimen and spalled scale weight together with SEM and TEM analysis were used to predict the oxidation constant to form adherent layer and spallation intensity. Three oxidation modes, including oxidation controlled by diffusion with forming a strongly adherent to steel surface multi-layered scale, spallation assisted oxidation, and oxidation with additional partial vaporization of scale components in the water vapor environment were distinguished. It was revealed that the Cr and Ni concentrations moved temperature boundaries between these surface degradation mechanisms depending on the exposed oxidation environment. Our approach is aimed to alleviate an appropriate alloy selection for service conditions. [ABSTRACT FROM AUTHOR]

Mo(Si,Al)2 with different yttrium (Y) additions (up to 2 at.%) was synthesised by dry powder mixing followed by compaction and sintering. In as-sintered materials, Y was present as yttrium aluminium garnet. The materials were exposed in air at 1500 °C for up to 250 h to study the effect of Y on oxidation behaviour. The oxides formed were investigated using scanning electron microscopy (SEM)-based techniques and X-ray diffraction. While the Y-free Mo(Si,Al)2 formed a scale consisting of Al2O3 and a small amount of mullite, the Y-containing samples formed oxides containing both yttrium silicate and larger fractions of mullite, in addition to Al2O3. Oxidation rate, scale spallation, as well as the evaporation of Mo, all increased with Y addition. [ABSTRACT FROM AUTHOR]

We performed tomographic investigation of the most heavily perturbed (thus highly radioactive) rod and its pristine/unirradiated replicate from the target No. 12 of the Swiss neutron spallation source (SINQ). The tomographic dataset reveals the 3D re-distribution of the lead filling inside the irradiated Zircaloy tube. The change in the linear attenuation coefficient of both the lead filling and the Zircaloy tube of the irradiated rod (due to the presence of the entrapped spallation products) in comparison with the pristine/unirradiated material is quantified. The dataset provides valuable input for the enhancement of safety and efficiency of future spallation targets at SINQ. [ABSTRACT FROM AUTHOR]

Cr–Ni austenitic steels offer significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can experience intensive degradation because of scale spallation, which could be detrimental to the in-service life. To predict the effect of scale spallation on oxidation kinetics, a simulation was implemented using a stochastic model. The model considers topological parameters and intensity of spallation which can occur, while delivering a true oxidation constant. The experimental procedure identified the amount of formed spalled scale and topology of spallation based on the use of element mapping of the surface. This information was used to determine a true kinetic constant for a corresponding spallation intensity in oxidized Cr–Ni austenitic steel. To illustrate the capability of the stochastic model, a parametric analysis was performed. The model verified how the spallation parameters could change the oxidation processes from parabolic growth of an adhered oxide layer without spallation to a mixed linear-parabolic, or with a constant thickness of residual scale at high spallation intensity. The spallation model will be used in a separate article to characterize high temperature surface degradation of several Cr–Ni austenitic steels during harsh oxidation environments. [ABSTRACT FROM AUTHOR]

Shock-induced consolidations of Ni/Al nanoparticles, including the consolidation behavior, exothermic reaction, intermetallic compound formation, and spallation, were systematically investigated through molecular dynamics simulation. The simulation indicates that shock consolidation of Ni/Al particles consists of two steps. Firstly, after the spread of shock wave, the cavity is densely filled by the flow deformation of the Al particles. Then, the Ni and Al particles are tightly joined together at the atomic scale by the produced high pressure and temperature. The exothermic reaction in the compressed particles is because the Ni atoms diffuse into the Al phase. Depending on whether melting takes place or not, shock consolidation can be divided into solid-state consolidation and liquid-state consolidation. In the solid-state consolidation, Ni/Al composite can be acquired with a little exothermic reaction. However, in the liquid-state consolidation, much more heat is released and intermetallic phase is formed. The spallation in the compressed particles is mainly because of the generated high-strength tensile wave but can be avoided by reducing the initial density of the Ni/Al particles or the particle velocity. [ABSTRACT FROM AUTHOR]

The fact that standard specimens made of high-strength bearing steel undergo spall fracture during quasi-static tension is experimentally proved. The uniqueness of this fracture consists in the fact that it proceeds due to the elastic energy of the specimen, in contrast to the classical spallation during high-speed collision or a high-energy pulsed external action. High-speed video filming is used to demonstrate that the mode I fracture of a specimen and its subsequent spall fracture are spaced in time. [ABSTRACT FROM AUTHOR]

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The dual-phase steel (martensite (M)/ferrite (F)) samples with different phase content were dynamic loaded simultaneously by one-stage light gas gun. The velocity of free surface particles was measured by Doppler pin system (DPS) during the loading experiment. The soft recovered samples were investigated with optical microscopy, nanoindentation, and EBSD techniques to study the effect of the phase content on dynamic damage evolution in dual-phase steel. The results show that the sample two (1000°C/60 min + 780°C/30 min+quenching) has a higher M area percentage (77.2%), larger M size, and smaller number of M and less M/F interfaces compared with the sample one (1000°C/60 min + 740°C/30 min+quenching, with M area percentage of 45.51%). Due to the reflection and transmission of shock wave at M/F interface, tensile stress will be generated inside M with higher shock impedance. Under the same dynamic loading conditions, the more M/F interface means the greater the probability of void nucleation inside M. Thus the sample two with less M/F interfaces has lower nucleation density and lower spallation strength. The microcrack propagation resistance increases with the increase in the area percentage of M and the size of M, which results in the lower damage evolution rate of the sample two. Meanwhile, the size of M will affect the direction of microcracks propagation. Each M with larger size in the sample two is composed of several prior austenite (A) grains, and the orientation of the martensite packets in prior A grains is very different. Therefore, the microcrack propagation in the sample two is limited to different regions, the direction of microcracks propagation is easy to be deflected. [ABSTRACT FROM AUTHOR]

Spallation, rupture under impulsive tensile loading, is a dynamic failure process involving the collective evolution and accumulation of enormous microdamage in solids. In contrast to traditional alloys, the spallation mechanism in medium entropy alloys, the recently emerged multiprinciple and chemically disordered alloys, is poorly understood. Here we conduct molecular dynamics simulations and first principle calculations to investigate the effects of impact velocities and the local chemical order on spallation microvoid nucleation in a CrCoNi medium entropy alloy under shock wave loading. As the impact velocity increases, the microvoid nucleation site exhibits a transition from the grain boundaries to the grains to release redundant imposed energy. During the intragranular nucleation process, microvoids nucleate in the poor-Cr region with a large local nonaffine deformation, which is attributed to the weak metallic bonds in this position with sparse free electrons. For intergranular nucleation, a Franke-like dislocation source forms through the dislocation reaction, leading to enormous dislocations piling up in a narrow twin stripe, which markedly increases the local stored energy and promotes microvoid nucleation. These results shed light on the mechanism of spallation in chemically complexed medium entropy alloys. [ABSTRACT FROM AUTHOR]

The phenomenon of spall fracture of high-strength steel specimens with the formation of two fracture surfaces during standard tensile tests is discussed. The fracture along two planes is shown to occur not simultaneously: first, fracture induced by a tensile load is observed, and then a second fracture plane forms, presumably, according to the spallation mechanism. The sequence proposed for the two fractures is confirmed by the results of fractographic analysis. The spallation during tension of standard specimens is found to differ from the classical spallation under an intense external action in the scheme of deformation wave formation and the source of spallation energy. [ABSTRACT FROM AUTHOR]

Stresses in the steam-side oxide of boiler tubes are evaluated based on analytical derivations and numerical simulations. With only thermal strain considered, analytical solutions of stress distribution are obtained for a cylindrical geometry representative of boiler tubes and a flat-plate geometry—a typical simplification assumed in the literature to represent boiler tubes with extremely thin oxide scale. In these analytical derivations, the substrate metal has finite dimensions or is assumed to be very thick. The solutions under the approximations of flat-plate geometry and very thick substrate are employed to examine the accuracy of various approximations adopted in the literature to analyze the stress distribution in boiler tubes. For more complicated situations where the contributions from thermal strain, interface roughness, creep of the oxide and metal, and oxide growth are considered, numerical simulations are performed for a cylindrical geometry to evaluate the stress distribution in boiler tubes. The simulation results reveal that: (1) the local radial stress at curved oxide–metal interfaces is enhanced by interface roughness with implications about interfacial crack growth; (2) contrary to the general belief that creep relieves the oxide stresses, creep may actually increase the stresses in the oxide due to different creep rates of the oxide and substrate metal; and (3) the geometrically induced oxide growth strain substantially increases the magnitudes of the hoop and axial stresses in the oxide. Based on the assumption that the failure of the oxide scales is caused by crack growth which is dominated by the stress intensity factor, damage maps are plotted directly using the hoop, axial, and radial stresses as the critical variables. Our work provides a quantitative understanding of the interactions between different thermomechanochemical processes and oxide scale failure in boiler tubes. [ABSTRACT FROM AUTHOR]

Spallation analysis is one of important research directions in impact dynamics. By combining the newly developing Peridynamics (PD) theory, the spallation phenomenon of concrete is numerically simulated using C language and MATLAB programming. The factors that may affect the spalling are verified: the type of pulse load, the geometric size of the model and the action time of pulse load. The dynamic response of spallation of three-dimensional concrete columns under different pulse loading forms (rectangular pulse, triangular pulse and exponential pulse) is analyzed. (1) Under the same impulse effect, only one spalling occurs in the rectangular pulse, and no multiple spallation occurs when the pulse amplitude increase. Exponential and triangular pulses can produce multiple spallation phenomena, and the time for the first spallation phenomenon is rectangular pulse < triangle pulse < exponential pulse. (2) The effect of the same linear triangle pulse on spalling of concrete columns with different lengths (100 mm, 200 mm and 300 mm) is analyzed. The triangle pulse can cause single or multiple spallation, which is related to the length and size of the model. (3) Finally, by changing the number of time steps of the pulse load, the different spalling phenomena of triangular pulses are analyzed. The thickness of the first layer increases significantly with the increase of the action time. [ABSTRACT FROM AUTHOR]

Spallation induced by rapid hydrothermal heating was investigated as a possible method of drilling rock. In this study, an electrically-heated hydrothermal jet was impinged on the surface of cylindrical Barre Granite specimens (basement rock) contained in an autoclave reactor to induce localized thermal stress. Comminution of the rock surfaces was achieved at supercritical water conditions, temperatures from 535 to 580 °C and pressures of 22.5–27 MPa. These conditions simulate those encountered in drilling deep, water-filled wells at depths greater than about 2300 m. Preferential removal of quartz grains from the rock matrix was observed. This comminution cannot be attributed to erosion by either the jet's momentum, or by differential pressure forces. Additionally, silica removal (primarily from quartz grains) was observed at rates greater than those that could be attributed to dissolution of quartz alone. This implied a secondary comminution mechanism associated with spallation caused by the local thermal stresses from the impinging hydrothermal jet. The experimentally determined heat flux and surface temperature measurements indicated that hydrothermal drilling occurred below the empirically determined minima for the onset of continuous thermal spallation reported in earlier studies for low-density, high-velocity and high-temperature flame jet drilling at temperatures in excess of 1000 °C jets impinging on rock surfaces at near-atmospheric pressures. [ABSTRACT FROM AUTHOR]

Gas turbine hot-section parts can be very expensive components with a finite lifetime. Their durability is strongly dependent on the operating service conditions which control the maintenance intervals and associated expenses. Blade damage is the most frequent reason for failures in gas turbine engines and also oxidation is one of the most critical degradation mechanisms when the power system operates in partially loading condition. This paper describes the methodology of oxidation life assessment for uncoated and coated blades with various metallic coatings in the first stage of a typical gas turbine engine. The resultant weight-change curves were validated by comparing them with the experimental and numerical data from the various references. Also a computer code was developed for real-time monitoring of remaining oxidation life and damage on the maximum blade temperature where the maximum damage accumulation occurs. Results indicated that the oxidation life consumption increases if the duration of cycle decreases or the temperature of the metal is increased during real-time operational condition. Also, considering a duplex metallic coating on the first-stage blade, the rate of oxidation damage reduces about 10 times compared to an uncoated blade. [ABSTRACT FROM AUTHOR]

Abstract: In the last years, spallation reactions have increasing in several works and research applications. They are applying as optimum neutron sources investigations or for energy production and nuclear-waste transmutation in accelerator-driven systems. In this paper, a series of Monte Carlo calculations using the MCNP-6 code performed to calculate, identify the behavior and number of proton-produced neutrons and photons. [ABSTRACT FROM AUTHOR]

Pure chromium oxidized at 900 °C at low oxygen partial pressure (10−12 atm) gives duplex Cr2O3 scale with an internal part made of equiaxed grains and exhibiting an n-type conduction, and an external part made of columnar grains and exhibiting a p-type conduction. Spalled regions occurring during cooling have been studied with photoelectrochemical techniques at a microscale. New information in the form of a specific image (structural quality image) could be obtained and revealed a level of structural defect density in the internal chromia subscale higher than that measured in the non-spalled region. The results complement the spallation scenario proposed in part I of this work. [ABSTRACT FROM AUTHOR]

Self-assembled monolayers (SAMs) are aggregates of small molecular chains that form highly ordered assemblies at the nanoscale. They are excellent contenders of molecular-level tailoring of interfaces because of the wide choice of terminal groups. Molecular dynamics (MD) simulations and experimental observations of spallation of two SAM-enhanced gold-film/fused silica-substrate interfaces have shown that the cohesive strength of SAM-enriched transfer-printed interfaces is strongly dependent on the choice of terminal groups. Though the MD results of perfectly ordered atomistic surfaces show the same qualitative trend as the experiments, they over-predict the interfacial cohesive strengths by a factor of about 50. Previous studies have revealed that the roughness of these interfaces may significantly impact their cohesive strength. In this manuscript, we perform a multiscale study to investigate the influence of surface roughness on cohesive strength of an interface between a Si/SAM substrate and a transfer-printed gold film. We approximate the film as a 2D deformable medium while the rough SAM-enhanced substrate is modeled using 2D harmonic functions with the cohesive interaction between the SAM and the film described by a simple exponential relation. Spallation is simulated on this system to evaluate the effective traction-separation response for the rough SAM-gold interface. Beyond the idealized harmonic interface, we extend our studies to real surface profiles obtained by AFM. We demonstrate how interfacial roughness can reduce the cohesive strength of the SAM-enhanced interface by more than an order of magnitude. [ABSTRACT FROM AUTHOR]

The Deflection Test in Monofacial Oxidation (DTMO) was used to assess the parameters of a phenomenological model aimed to represent the mechanical behavior of the martensitic T91 steel during oxidation at 550 °C under wet atmosphere. The constitutive equations of the mechanical model were determined from the knowledge of the growth mechanism of the oxide scale. Some model parameters were found in the literature and complementary data were obtained by the comparison between experimental DTMO curve and simulated results. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBC) can be used to reduce the metal temperature of gas turbine blades enabling higher Cr alloys (lower strength) to be used when gas turbines are to be used in corrosive environments (where hot corrosion resistance is required). However, the TBC must also be resistant to the corrosive environment and remain attached to the blade. A 1000 h test to evaluate air plasma-sprayed (APS) TBC adhesion to a low-pressure plasma-sprayed CoNiCrAlY bond coat (with and without through thickness cracking) under hot corrosion conditions at 850 °C has been carried out. The APS TBC significantly reduced the hot corrosion rate of the CoNiCrAlY; however, delamination cracking occurred with a thinner thermally grown oxide than would be expected from isothermal and cyclic oxidation testing. [ABSTRACT FROM AUTHOR]

In this paper, the commercial finite element code LS-DYNA is employed to simulate the process of a certain kind of a linear shaped charge jet penetrating into a TC4 (Ti-6Al-4V) titanium alloy plate of moderate thickness. The fracture profiles agree well with experimental observations, which confirms the validity of the code and the Johnson-Cook material model applied to describe the TC4 plate. The fracture pattern of the plate is drawn based on this comparison. [ABSTRACT FROM AUTHOR]

The effect of peak shock stress on the incipient spallation damage in a cylindrical sample under sweeping detonation is presented. The free surface velocity curve was measured by photon Doppler velocimetry and the quantitative investigation of voids in a spalled sample was performed using X-ray computer tomography. The results revealed that the maximum volume and the mean volume of voids in the spalled sample increased with increasing shock stress. The sphericity of voids decreases with the increasing of shock stress. The rod voids were the result of the independent growth of voids along the grain boundaries in samples with lower shock stress, while the rod shaped voids in sample with higher shock stress were formed due to coalesce. The rod voids can be found in a cylindrical sample, while the voids in plate samples were in the shape of spheres or ellipsoids, and the difference of stress state induced by the curvature in the geometry of samples may be the main reason. [ABSTRACT FROM AUTHOR]

Long-term cyclic oxidation testing was conducted on oxide dispersion strengthened (ODS) and cast FeCrAlY alloys at 1100 and 1200 °C with 1 or 100 h hold time in air, O, and humid atmospheres. These data were used to optimize four different cyclic oxidation models and calculate the Al consumption rate due to oxidation. The pkp and COSIM-GSA models were able to reproduce accurately the experimental oxidation curves, and lifetime predictions based on these models were in good agreement with experiments. Microstructure characterization revealed that the degradation of the ODS FeCrAlY cyclic oxidation performance was mainly due to the incorporation of Ti carbonitrides in the alumina scale and the formation of tensile cracks. Controlling the levels of C, N, and S in one ODS alloy resulted in oxidation performance only moderately lower than the performance of cast FeCrAlY alloys. A few, very deep, tensile cracks formed at the cast alloy surface upon thermal cycling. Finally, the effect of specimen mechanical strength and the presence of HO on ODS FeCrAl durability are discussed. [ABSTRACT FROM AUTHOR]

The spallation phenomenon was studied through numerical analysis using a coupled Lagrangian particle tracking code and a hypersonic aerothermodynamics computational fluid dynamics solver. The results show that carbon emission from spalled particles results in a significant modification of the gas composition of the post-shock layer. Results from a test campaign at the NASA Langley HYMETS facility are presented. Using an automated image processing of short exposure images, two-dimensional velocity vectors of the spalled particles were calculated. In a 30-s test at 100 W/cm of cold-wall heat flux, more than 722 particles were detected, with an average velocity of 110 m/s. [ABSTRACT FROM AUTHOR]

In multiple engineering fields such as rock drilling or building constructions or extreme events like earthquakes or impacts, the dynamic properties of rock play an important role. A way to model these events and define measures to minimize the damage derived from these events is created by means of numerical analysis. Hence, the knowledge of the dynamic material behavior is essential for studying the effects of such a loading scenario. Solid geological materials, from the family of the sedimentary rocks, have been analyzed under quasi-static loads. However, there is a lack of knowledge when high strain rate loadings are involved. Within this context, the paper focuses on the experimental characterization of two sedimentary rocks, sandstone and limestone, under impact loading using the Hopkinson-Bar spallation and compression tests. The analysis encompasses the determination of the tensile and compressive properties as well as the comparison between the quasi-static and dynamic behavior (dynamic increase factors). The paper fills the gap of information existing about dynamic behavior of sedimentary rocks under strain rates between 10 and 5.2 × 10 s. Furthermore, the fragmentation under different strain rates is investigated and conclusions with respect to energy absorption capacity are drawn. [ABSTRACT FROM AUTHOR]

The spallation behaviour of alumina scales grown on FeCrAlY alloy was investigated. Substrates with different thicknesses were oxidized at 1200 °C for 25 h and cooled at various cooling rates. Generally, the scale formed on a thicker substrate or with a faster cooling rate exhibits a larger compressive stress. However, the failure behaviour of alumina scales is more complicated than expected for a compressed film. Specifically, (i) the extent of the spallation is not proportional to the residual stress in the oxide; (ii) the spallation does not occur immediately after cooling, but requires a period of incubation. This indicates that the residual stress is not the sole reason for the failure of scales. It was found that the carbide forms at the oxide-metal interface after cooling, which acts in conjunction with the residual stress to control the spallation of oxides. In addition, the mechanics analysis suggests that the microscopic roughness at the interface is another important factor. [ABSTRACT FROM AUTHOR]

The spallation behavior of oxide scale on a ferritic stainless steel and a austenitic stainless steel during cooling after oxidation in air at 1273 K for 100 h were investigated by dynamic visual observation. The spallation spread as minor spallation that was generated in a scattered pattern. The spallation for the ferritic and austenitic stainless steels started at 770 K and above 983 K (at which the temperature measurement was started) respectively. The masses of spalled scale for the ferritic and austenitic stainless steels began to increase dramatically below 473 and 673 K, respectively. The spallation unit size of the ferritic stainless steel was larger than that of the austenitic stainless steel. It was considered that the scale was spalled by the thermal stress generated during cooling. The strain energy accumulated by the thermal stress were discussed with respect to the above results. [ABSTRACT FROM AUTHOR]

Thin-walled castings made from Cr–Ni austenitic steels offer a combination of light weight of a near-net component with significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can result in intensive surface degradation due to scale spallation. Scale spallation can decrease the wall thickness which could be detrimental to the in-service life of thin-walled castings. Experiments and stochastic simulations of spallation assisted oxidation were performed with three cast austenitic heat-resistant steels having different Cr–Ni concentrations at temperatures between 900 and 1000 ℃ on air and water vapor containing combustion atmosphere. The recorded specimen and spalled scale weight together with SEM and TEM analysis were used to predict the oxidation constant to form adherent layer and spallation intensity. Three oxidation modes, including oxidation controlled by diffusion with forming a strongly adherent to steel surface multi-layered scale, spallation assisted oxidation, and oxidation with additional partial vaporization of scale components in the water vapor environment were distinguished. It was revealed that the Cr and Ni concentrations moved temperature boundaries between these surface degradation mechanisms depending on the exposed oxidation environment. Our approach is aimed to alleviate an appropriate alloy selection for service conditions. [ABSTRACT FROM AUTHOR]

In this work real-time images of the initiation and arrest of a laser-induced substrate spallation event have been captured over its ~40 ns duration using ultra-high speed photography. A modified version of the laser spallation technique is presented, introducing additional optical components allowing for control of the laser loading fluence (energy/unit area) without changing the emitted loading pulse energy, spot size, and profile. Spallation experiments were conducted on (100) silicon wafer substrates, 500 μm thick, coated with a 100 nm thick Al thin film at three fluence levels, 0.124, 0.174 and 0.336 J/mm. The capabilities of the modified laser spallation technique capture spallation area evolution and spallation front velocity results over time are reported for the fluence levels examined. Following the spallation event, stress wave reflections and residual thin film stresses drive additional film delamination and substrate failure. Comparisons are made between the real-time spallation images and postmortem static images highlighting the effects of stress wave reflections. [ABSTRACT FROM AUTHOR]

We use the molecular dynamics code, large-scale atomic/molecular massively parallel simulator (LAMMPS), to simulate high strain rate triaxial deformation of crystal copper to understand void nucleation and growth (NAG) within the framework of an experimentally fitted macroscopic NAG model for polycrystals (also known as DFRACT model). It is seen that void NAG at the atomistic scales for crystal copper (Cu) has the same qualitative behaviour as the DFRACT model, albeit with a different set of parameters. The effect of material temperature on the nucleation and growth of voids is studied. As the temperature increases, there is a steady decrease in the void NAG thresholds and close to the melting point of Cu, a double-dip in the pressure-time profile is observed. Analysis of this double-dip shows disappearance of the long-range order due to the creation of stacking faults and the system no longer has a face centred cubic (fcc) structure. Molecular dynamics simulation of shock in crystal Cu at strain rates high enough to cause spallation of crystal Cu are then carried out to validate the void NAG parameters. We show that the pre-history of the material affects the void nucleation threshold of the material. We also simulate high-strain-rate triaxial deformation of crystal Cu with defects and obtain void NAG parameters. The parameters are then used in a macroscale hydrodynamic simulation to obtain spallation threshold of realistic crystal Cu. It is seen that our results match experimental results within the limit of 20 % error. [ABSTRACT FROM AUTHOR]

The isothermal oxidation behaviour of TBC systems based on a first-generation superalloy, AM1 and a fourth-generation one, MCNG, was investigated and compared. The main difference between both is the addition of Re, Ru and Hf in the MCNG composition. The systems consisted of a Pt-modified nickel aluminide (Ni,Pt)Al bond coat deposited on the superalloy and with or without a yttria-stabilized zirconia ceramic top coat. Isothermal oxidations were performed at 1,100 °C in synthetic air for 10, 50, 500 and 1,000 h. The MCNG-based system exhibited a better spallation resistance than the AM1-based one and a lower oxidation rate. Spallation always occurred at the thermally grown oxide/bond coat interface. For both systems, transformation of the β-phase into the γ′-phase was observed. In the MCNG-based samples, the β-(Ni,Pt)Al was enriched in ruthenium and the secondary reaction zone strongly extends, whereas chromium precipitation occurred in the AM1-based ones. [ABSTRACT FROM AUTHOR]

To obtain long-lasting thermal barrier coating (TBC) systems, two types of Pt-rich γ-Ni+γ′-NiAl bond-coatings (BC) were fabricated by spark plasma sintering (SPS). The former had the highest possible Pt content (Ni-30Pt-25Al in at.%) while the latter had the highest possible Al level (Ni-28Al-17Pt in at.%). Hf was added as a reactive element. TBCs were fabricated on different superalloys (AM1, René N5 and MCNG) with the aforementioned BCs and with zirconia stabilized with yttria top coats made by SPS or electron beam physical vapor deposition (EBPVD). The cyclic oxidation resistance of these systems was studied at 1,100 °C in air. Most TBCs with a Pt-rich γ-γ′ BC showed better thermal cycling resistance when compared to the reference TBCs (β-(Ni,Pt)Al diffusion BC and EBPVD ceramic top coat), with lifetimes up to 1,745 cycles instead of 700 for the reference, and despite the fabrication defects observed within the SPS BCs. Cu was tested as an addition in the BCs and proved to have a slight negative effect on the system lifetime. Moreover, the fourth generation MCNG substrate led to the best cyclic oxidation behavior. [ABSTRACT FROM AUTHOR]

Compression tests at room temperature are used to estimate the critical strain to spallation of EBPVD thermal barrier coating deposited on Ni based single crystals for blades used in aero-engines. The observation of fracture surfaces allows for investigating the location of delamination events leading to spallation, the eventual porosity at bond coat-alumina interface and kinetics of damage evolution. The effect of isothermal oxidation at 1,100 °C is presented for standard or low sulfur AM1 superalloy, and coating process variants. The degradation is shown to depend on the thermal-mechanical loading and varies from isothermal oxidation, cyclic oxidation and thermal-mechanical fatigue with hold time. These observations are consistent with the damage observed in blades in engine tests or in service. These tests are a useful complement to standard cyclic oxidation tests to identify engineering lifetime models. [ABSTRACT FROM AUTHOR]

Cast Ni-Cr-Al base alloys with two Cr ranges, 7-9.5 at.% and 3-4 at.%, respectively, and with Re + Ta refractory element contents of up to 7.5 at.% were subjected to oxidation experiments in air. The oxidation conditions were 100 h continuously at 1,000 °C and discontinuously for 500 h at temperature by cycling between room temperature and 1,000 °C. The times at temperature to the onset of scale spallation were monitored. Scale formation was investigated by SEM, EDS and XRD. The thermal expansion of the alloys and the lattice parameter of their γ-Ni phases were determined to assess their effect on the oxidation performance. Scale adhesion appears to be affected by mechanical interlocking at the alloy/scale interface. [ABSTRACT FROM AUTHOR]

Fe-Ni-Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050 °C with varied sulfur partial pressures. The oxide-scale growth on Fe-Ni-Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCrO spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe-Ni-Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed. [ABSTRACT FROM AUTHOR]

The behaviour of an oxide scale grown on the surface of a high-speed steel subjected to thermal cycling was studied. The oxide layer formed on the steel by means of the exposure of samples of this material to a mixture of dry air and water vapour at either 615 or 550 °C. The samples were held for 1 h at either temperature and were cooled to room temperature by quenching with distilled water. This procedure was repeated for up to forty cycles. The oxide layer was subjected to the action of thermal stresses generated during either the cooling or heating stages. It was found that both, the composition of the oxide layer and the amount of oxide spalled from the surface depended on the number of cycles imposed to the samples. The mechanisms that promote oxide spallation and the preferential composition of the oxides that remain on the surface of the steel after spallation are analyzed and discussed. [ABSTRACT FROM AUTHOR]

The effect of a nanosecond laser irradiation of thin (60 and 145 nm) amorphous, diamond-like carbon films deposited on Si substrate by an ion beam deposition (IBD) from pure acetylene and acetylene/hydrogen (1:2) gas mixture was analyzed in this work. The films were irradiated with the infrared (IR) and ultraviolet (UV) radiation of the nanosecond Nd:YAG lasers working at the first (1.16 eV) and the third (3.48 eV) harmonics, using a multi-shot regime. The IR laser irradiation stimulated a minor increase in the fraction of sp bonds, causing a slight decrease in the hardness of the films and initiated SiC formation. Irradiation with the UV laser caused the formation of carbides and increased hydrogenization of the Si substrate and the fraction of sp sites. Spalliation and ablation were observed at a higher energy density and with a large number of laser pulses per spot. [ABSTRACT FROM AUTHOR]

The phase constitution and associated oxidation behavior were determined for an alloy of composition (in wt%) Ni–31.5Cr–11.5Al–0.61Y. The identity, relative amount, and approximate composition of each phase were determined after equilibration at 700, 900, and 1100°C using SEM/EDS, XRD, and image analysis. The CALPHAD method was used to predict the phase equilibria in this system, and the results showed good agreement with experiment. Cyclic oxidation behavior at 900, 1050, and 1100°C in air was determined and assessment of the results was aided by CALPHAD predictions in conjunction with dilatometric measurements. [ABSTRACT FROM AUTHOR]

The purpose of this study is to quantify the influence of textural and microstructural anisotropy on spallation. This includes the influence of anisotropically-oriented MnS inclusion stringers in the HY-100 and 1080 steels on spallation, within two crystallographically-isotropic steels, and the influence of strong, anisotropic crystallographic texture in high-purity polycrystalline Ta and Zr materials to assess the role of texture on damage evolution and spallation responses. The effect of anisotropic crystallographic texture on the spallation response of Ta and Zr is shown to play a minimal role in the spallation response of each material, as seen in wave profile pull-back signals, compared to the effect of texture on the shock arrival time and the Hugoniot elastic limit that reflects strength in these two high-purity materials. In the case of both the 1080 and HY-100 steels, the influence of elongated MnS stringers, resident within the essentially crystallographically isotropic steels, was found to be dominated by the heterogeneous nucleation of damage orthogonal to the MnS stringers. Delamination between the pearlitic matrix microstructure and the MnS stringers in the 1080 steel, or inclusions and the martensitic matrix in the HY-100 steel, was seen to correlate to a lower pull-back signal during transverse loading than to that parallel to the stringer axis in each steel. [ABSTRACT FROM AUTHOR]

This paper proposes a detailed theoretical analysis of the development of dynamic damage in plate impact experiments for the case of high-purity tantalum. Our micro-mechanical model of damage is based on physical mechanisms (void nucleation and growth). The model is aimed to be general enough to be applied to a variety of ductile materials subjected to high tensile pressure loading. In this respect, the work of Czarnota et al. (J Mech Phys Solids 56:1624–1650, 2008) has been extended by introducing the concept of nucleation law and by entering a nonlinear formulation of the elastic response based on the Mie-Grüneisen equation of state. This later aspect allows us to consider high impact velocities. All model parameters are directly assessed by experimental measurements to the exception of the nucleation law which is characterized by the way of an inverse identification method using three free-surface velocity profiles (at low, intermediate and high impact velocities). It is shown that the nucleation law can be consistently determined in the range of operating pressures. The nucleation law being identified, the development of internal damage happens to be a natural outcome of the modelling. The model is applied to predict damage development and free-surface velocity profiles for various test conditions. The variety and the quality of results support the physical basis (in particular micro-inertia effects) upon which the proposed model of dynamic damage is based. [ABSTRACT FROM AUTHOR]

The spallation of the concrete slabs or walls resulting from contact detonation constitutes risk to the personnel and equipment inside the structures because of the high speed concrete fragments even though the overall structures or structural members are not destroyed completely. Correctly predicting the damage caused by any potential contact detonation can lead to better fortification design to withstand the blast loadings. It is therefore of great significance to study the mechanism involved in the spallation of concrete slabs and walls. Existing studies on this topic often employ simplified material models and 1D wave analysis, which cannot reproduce the realistic response in the spallation process. Numerical simulations are therefore carried out under different contact blast loadings in the free air using LS-DYNA. Sophisticated concrete and reinforcing bar material models are adopted, taking into account the strain rate effect on both tension and compression. The erosion technique is used to model the fracture and failure of materials under tensile stress. Full processes of the deformation and dynamic damage of reinforced concrete (RC) slabs and plain concrete slabs are thus observed realistically. It is noted that with the increase of quantity of explosive, the dimensions of damage crater increase and the slabs experience four different damage patterns, namely explosive crater, spalling, perforation, and punching. Comparison between the simulation results of plain concrete slabs and those of RC slabs show that reinforcing bars can enhance the integrity and shearing resistance of the slabs to a certain extent, and meanwhile attenuate the ejection velocity and decrease the size of the concrete fragments. Therefore, optimizing reinforcement arrangement can improve the anti-spallation capability of the slabs and walls to a certain extent. [ABSTRACT FROM AUTHOR]