Spallation Damage Research Articles

Enhanced spall strength of an Al0.1CoCrFeNi high-entropy alloy by the pre-strain method

The effects of pre-strain on spallation damage of an Al0.1CoCrFeNi high-entropy alloy are investigated with plate impact loading. During the pre-strain process, Kink bands and dislocations are generated in the microstructures. Pre-strain smears elastic–plastic transition upon shock loading and improves spall strength via dislocation strengthening. Although the level of pre-strain does not affect the ductile damage mode, it has a pronounced effect on damage evolution.

Effects of microstructures on dynamic deformation and spallation damage of high-entropy alloy Al0.3CoCrFeNi under plate impact loading

Plate impact experiments are conducted on high-entropy alloy Al0.3CoCrFeNi with different microstructures to investigate the effects of grain size, precipitates and heterogeneous structure on dynamic response. Free surface velocity histories are obtained. All initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Nanoscale L12 precipitates result in a considerable increase in dynamic yield stress. Spall strength is higher for the heterogeneous structure, the large grain size (200 μm) and small grain size (63 μm) with L12 precipitates by ∼4%, 19% and 10% than for the small grain size (77 μm) without precipitates. After shock compression, dislocation slip, stacking faults, the Lomer-Cottrell locks and nanotwins are found in the samples without the L12 precipitates. L12 precipitates increase the energy barrier and inhibit deformation twinning. Spall damage is ductile in Al0.3CoCrFeNi. For the homogeneous structures, intragranular voids are predominant. Voids are preferentially distributed in the fine-grain domains or at the fine-grain–coarse-grain domain boundaries of the heterogeneous structure.

Thermo-mechanical analysis and creep lifetime prediction of coated turbine vanes considering thermal barrier coating spallation characteristics

Quantitative analysis of the influence of thermal barrier coating spallation on the thermo-mechanical behaviors and creep lifetime of turbine vanes is crucial for their maintenance and reliability improvement. The temperature and stress distribution of vanes with preset coating spallation damage are investigated in this study, using the thermal-fluid-solid coupling method. Additionally, a further computational analysis is conducted to predict the hazardous regions and creep lifetime of the vanes. The results indicate that coating spallation leads to significant changes of the temperature and stress distribution at the spalled regions of vanes. The remaining coating on the unspalled regions continues to provide effective protection. Stress concentration primarily occurs at the upstream and downstream of the leading edge film holes, while high-stress regions are observed between adjacent rows of film holes, forming a serrated shape. The creep lifetime of the vanes decreases significantly at the region with coating spallation. When the same spallation area is considered, the coating spallation at the leading edge has a more serious influence on creep lifetime, which is more likely to cause the vanes failure. This study reveals the influence of coating spallation characteristics on the thermo-mechanical behaviors and creep lifetime of vanes, providing valuable insights for durability assessment of coated high-temperature components.

Dynamic response of additively manufactured Ti–5Al–5V–5Mo–3Cr as a function of heat treatment

Both shock and shockless compression experiments were performed on laser powder bed fusion (LPBF) Ti–5Al–5V–5Mo–3Cr (Ti-5553) to peak compressive stresses near 15 GPa. Experiments were performed on the as-built material, containing a purely β (body centered cubic) microstructure, and two differing heat treatments resulting in a dual phase α (hexagonal close packed) and β microstructure. The Hugoniot, Hugoniot elastic limit (HEL), and spallation strength were measured and compared to wrought Ti-6Al-4V (Ti-64). The results indicate the LPBF Ti-5553 Hugoniot response is similar between heat treatments and to Ti-64. The HEL stress observed in the LPBF Ti-5553 was considerably higher than Ti-64, with the as-built, fully β alloy exhibiting the largest values. The spallation strength of the LPBF Ti-5553 was also similar to Ti-64. Clear evidence of initial porosity serving as initiation sites for spallation damage was observed when comparing computed tomography measurements before and after loading. Post-mortem scanning electron microscopy images of the recovered spallation samples showed no evidence of retained phase changes near the spall plane. The spall plane was found to have kinks aligned with the loading direction near areas with large concentrations of twin-like, crystallographic defects in the as-built condition. For the heat-treated samples, the concentrations of twin-like, crystallographic defects were absent, and no preference for failure at the interface between the α and β phases was observed.

Spallation damage of tungsten heavy alloy under shock loading

In this study, plate-impact experiments are conducted using tungsten heavy alloy (WHA) to obtain the free-surface velocity profiles under shock stress of 6–23 GPa. By analyzing the velocity wave profile, it is demonstrated that the spall strength is weakly dependent on the peak stress. The microstructure of the recovered WHA is well characterized and it’s observed that intergranular fracture and cleavage are the primary fracture mechanisms. Deformation twins are sparsely distributed near the cleavage crack. The existence of twin boundary not only affects crack propagation but also regulates the crack path and reinitiates the cleavage propagation direction on the twin boundary. The macroscopic cleavage facet planes are detected to be {100}, {110} and {111} at all shock stress.

Impact Velocity-Dependent Patterns and Mechanisms of Spalling Behavior in Single Crystal Nickel

To reveal the impact velocity (U<sub>p</sub>) effect on the spalling and fracture behavior of single crystal nickel, a non-equilibrium molecular dynamics approach is performed to investigate the free surface velocity curve, radial distribution function, atomic crystal structures, dislocations, and void evolution process. The results show that the critical U<sub>p</sub> for spalling behavior in single crystal nickel is 1.5 km/s, the spallation mechanism is classical spallation damage (U<sub>p</sub>≤1.5 km/s) and micro-spallation damage (U<sub>p</sub>＞1.5 km/s). The number and distribution area, and stress distribution area under micro-spallation damage much higher than those under classical spallation damage. Analyzed the influence of impact velocity on the classical spalling damage behavior (U<sub>p</sub> ≤ 1.5 km/s) and obtained the corresponding spalling strength, an accident of spalling strength occurs at the U<sub>p</sub> of 1.3 km/s. The spalling strength of single crystal nickel is influenced by the combined effects of stacking faults, phase transformation, and dislocation mechanisms. The nucleation and emission of dislocations increase lead to a decrease in the spalling strength. When U<sub>p</sub> <1.3 km/s, spalling damage is primarily influenced by stacking faults. When U<sub>p</sub> =1.3 km/s, spalling strength is mainly affected by the competition between stacking faults and phase transformation. When U<sub>p</sub> >1.3 km/s, spalling strength is predominantly influenced by the body-centered cubic (BCC) phase transformation mechanism (transformation path: FCC → BCT → BCC). This study reveals the impact velocitydependent patterns, mechanisms, and effects on spalling damage and fracture, providing a theoretical basis for the protective application of nickel-based materials under extreme impact conditions.

Parameter inversion method for target plate constitutive model based on BP neural network

In this paper, a neural network-based inversion method that can be used to streamline the acquisition of polymethyl methacrylate (PMMA) material constitutive model parameters for explosive cutting simulations is presented. This approach mitigates the need for multiple tests, a requirement of traditional methods. Initially, three categories of the damage, penetration depth, impact fracture thickness and spallation damage thickness, were distinguished and quantified by cutting the14 mm PMMA target plate with a 2.5 mm wide linear shaped charge, and the adjustment interval for the constitutive model parameters could be determined by combining the explosive cutting experiment data and the empirical parameters of the Johnson Holmquist Ceramics (JH-2) constitutive model. Subsequently, the numerical simulations of the cutting process of the performed target plate tests were carried out using LS-DYNA, and a dataset of target plate damage incorporating the three types of damage data was amassed. Ultimately, a neural network model correlating the parameters of the PMMA target plate constitutive model with the damage data was developed and trained using the plate damage dataset, and several supplementary experiments as well as finite element numerical simulations involving a 4.2 mm wide linear shaped charge cutting a 19 mm PMMA target plate were conducted. The numerical results displayed minimal variance from the experimental results in terms of fracture characteristics and damage data, suggesting that the inverted JH-2 constitutive model parameters can be effectively applied to PMMA target plate explosive cutting simulations. The parameter inversion method allows for the acquisition of more accurate material constitutive model parameters with fewer experiments and will be beneficial for further theoretical studies and practical applications of target plates in the future.

Anisotropic dynamic response of AlSi10Mg fabricated via laser powder bed fusion under plate impact

The effects of structural anisotropy on dynamic yield and spallation damage of an AlSi10Mg alloy manufactured via laser powder bed fusion are investigated via plate impact experiments along the build direction (BD) and the transverse direction (TD). Free surface velocity histories are measured to deduce its dynamic mechanical properties. The as-received and postmortem samples are characterized with scanning electron microscopy, metallograpic microscopy, electron backscatter diffraction and synchrotron X-ray computed tomography. The Hugoniot elastic limit for loading along TD is about 40% higher than that along BD, given the initial 〈100〉∥BD texture. For spallation, voids prefer to nucleate at melt pool boundaries. Spall strength for loading along TD is about 18% higher than that along BD, as a result of higher critical stress for void nucleation and less nucleation sites due to the melt pool morphology. The damage evolution features are also correlated well with melt pool morphology and grain morphology within the pool interior, demonstrating the “ductile” interpool and “brittle” intrapool fracture modes for loading along BD and TD, respectively.

Impact response of metastable dual-phase high-entropy alloy Cr10Mn30Fe50Co10

Plate impact experiments are conducted on a metastable dual-phase high-entropy alloy (HEA) Cr10Mn30Fe50Co10 with different matrix grain sizes and phase fractions. Free surface velocity histories are obtained along with microstructure characterizations. Postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Multiple deformation mechanisms are observed, including dislocation slip and stacking fault in the face-center cubic (FCC) phase, dislocation slip and deformation twinning in the hexagonal close-packed (HCP) phase, and the FCC to HCP transformation. For spallation, damage is ductile in nature. Despite different microstructures, micro-voids prefer to nucleate at high-angle grain boundaries within the FCC phase, leading to similar spall strength. Upon shock loading, a dynamic plastic deformation partitioning behavior among the two phases is observed; at high shock stress, denser dislocations in the HCP phase result in increased fractions of voids around the FCC/HCP phase interface and within the HCP phase during spallation damage.

Investigation on the spallation properties and failure mechanism of selective laser melted Ti-6Al-4V alloy with heat treatment under shock load

In this study, the effect of heat treatment on the microstructure of selective laser-melted Ti-6Al-4V alloy and the corresponding spalling performances and damage mechanism at ultra-high-speed (UHS) of 730 m/s are studied. Results show that the acicular martensite in the as-built sample is transformed into large grain basket-weave and Widmanstatten structures with solution treatment at 950 ℃ and 1050 ℃, respectively. The spallation damage mechanism is characterized by the aspects of grain refinement ratio (GRR) and damage porosity (DP). The HT-1050 sample has the highest GRR and DP, while the HT-950 sample has the lowest values, indicating that the HT-950 sample has the highest resistance of spallation. Due to the difference in microstructure morphology and grain size of the three samples, the spalling damage mode of the original sample is characterized by brittle fracture, and then it changes to pore growth and coalescence after heat treatment, larger grains have strong guiding effects on the coalescence of pores. The coalescence of the pores is always accompanied by dynamic recrystallization, thus reducing the local dislocation density and optimizing the development path of cracks. Based on the above work, can provide theoretical support for promoting the application of additive manufacturing alloys in defense industry and aerospace fields.

Dynamic mechanical properties, deformation and damage mechanisms of eutectic high-entropy alloy AlCoCrFeNi21 under plate impact

• Both L12 and B2 phases remain ordered at 12 GPa • The accurate Hugoniot equation of state of AlCoCrFeNi2.1 is obtained • Nanotwins are observed in L12 phase • Both ductile and brittle fracture modes are observed Shock compression and spallation of a eutectic high-entropy alloy (HEA) AlCoCrFeNi 2 . 1 with lamellar structure are investigated via plate impact loading with free-surface velocity measurements. The as-cast and postmortem samples are characterized with transmission electron microscopy, electron back-scatter diffraction and scanning electron microscopy. An accurate Hugoniot equation of state is determined. After shock compression to ∼ 12 GPa, both the L1 2 and B 2 phases retain their ordered structures. Dense dislocations in the {111} slip planes, stacking faults and deformation twins are found in the L1 2 phase, along with fewer dislocations in the {110} slip bands in the B 2 phase. Shock-induced deformation twinning within the L1 2 phase of this HEA is observed as a new deformation mechanism under various loading conditions. For spallation, both ductile and brittle damage modes are observed. The micro voids and cracks prefer to nucleate at the phase boundaries chiefly, then in the B 2 phase. Under similar shock stress, the spall strength of AlCoCrFeNi 2 . 1 HEA is about 40% higher than those of other reported dual-phase HEAs due to the high stability of its semi-coherent phase boundaries. Graphical Abstract .

Deformation, damage, and reaction characteristics during the collision between Ni and Al nanoparticles.

The exothermic reaction during the collision between nanoparticles is of importance for the engineering applications of energetic powder materials. This work investigates collision-induced nanoparticle deformation, damage and reaction characteristics in a reactive Ni/Al system via molecular dynamics simulations. The morphological changes and reaction process are explored thoroughly for a wide range of impact velocities v and initial particle radius R. For lower impact velocities (1 km s-1 ≤ v ≤ 1.5 km s-1), the fully melted Al gradually clad the plastic deformed Ni nanoparticles to form an Al-shell/Ni-core structure, and the morphology ultimately develop into a nearly spherical shape possessing minimal surface energy. During this period, the self-sustaining reaction driven by the diffusion of Ni atoms into molten Al leads to slow melting of Ni nanoparticles, and the reaction and melting rates increase with the decrease of the particle radius. There exists one critical radius (R = 10 nm) beyond which the reaction is severely blocked due to the occurrence of fracture behavior at v = 1.5 km s-1. For intermediate velocities (2 km s-1 ≤ v < 3 km s-1), collision-induced debris clouds are observed, which satisfies the power-law distribution in the size of debris and results in an obvious reduction of the final reaction degree. Interestingly, we found that the reactive component in generated debris is lower for the larger-radius nanoparticle, which is also responsible for the lower final reaction degree and thermal kinetic energy. For higher velocities (v ≥ 3 km s-1), the occurrence of spallation damage reduces the contact area due to the formed micro-voids within Al and Ni nanoparticles and consequently the final reaction degree further.

Spallation damage of underground openings caused by excavation disturbance of adjacent tunnels

Spallation damage in underground mines and tunnels is a common phenomenon, especially during blasting excavations, and revealing the mechanism of spallation damage is an urgent issue for the stability of underground tunnel. In this study, the circumferential and radial stress distributions around the circular tunnel under a transient cylindrical P-wave were examined by combining wave function expansion method and Duhamel’s integral technique. Theoretical results revealed that the spallation damage around the tunnel was mainly caused by high radial dynamic stress concentrations face to a short-wavelength incident waves, and the circumferential and radial stresses around the tunnel decrease as the clearance increases. Furthermore, in order to reappear the spallation damage around the tunnel under blast load and obtain the effect of initial stress and bolt support, the CSCM material model was used to simulate the damage of surrounding rock by LS-DYNA. The numerical simulation results indicated that the initial stress reduced the radial cracks, while the presence of the initial stress exacerbated the spallation damage on the incident side. Finally, the stresses in each time stage of the circular tunnel surrounding rock under the action of transient cylindrical P-waves were analysed.

Shock compression and spallation damage of high-entropy alloy Al0.1CoCrFeNi

• The accurate Hugoniot equation of state of Al 0.1 CoCrFeNi. • Nanotwins are only observed at high shock stress. • Damage in the Al 0.1 CoCrFeNi is ductile in nature. • Void nucleation depends on GB misorientation and peak stress. Shock compression and spallation damage of a face-center cubic phase high-entropy alloy (HEA) Al 0.1 CoCrFeNi were investigated via plate impact experiments along with free surface velocity measurements. Postmortem samples were characterized with transmission electron microscopy and electron backscatter diffraction. The Hugoniot equation of state and spall strength at different impact strengths were determined. There exists a power-law relation between spall strength and strain rate. The spall strength of Al 0.1 CoCrFeNi HEA is about 50% higher than those of previously studied HEAs and comparable to those widely applied structural stainless steels at the same shock stress. Dislocation glide and stacking faults are the important deformation mechanisms in the Al 0.1 CoCrFeNi HEA. Nanotwins are only observed at high shock stress. Damage in the Al 0.1 CoCrFeNi HEA is ductile in nature. Voids are nucleated preferentially in grain interiors, and the intragranular voids show a strong dependence on grain boundary misorientation and peak stress.

Health and Durability of Protective and Thermal Barrier Coatings Monitored in Service by Visual Inspection

Protective and Thermal Barrier Coatings (TBC) applied on gas-turbine blades gradually degrade due to oxidation, aluminum depletion and impacts of environmental particles. Among various non-destructive coating testing methods (NDT), visual inspection can be undertaken regularly in service, but it provides little quantitative information, and only surface defects can be detected. This work aims at in-service monitoring of turbine blades with multilayer coatings applied by atmospheric plasma spraying (APS) in a few variants. They were validated during a series of accelerated mission tests of a retired military turbofan engine in a test cell together with five other technologies. The fifty-hour rainbow test focused on assessing coating durability. Between engine runs, 12 borescope inspections were conducted to monitor the health of the blades. Finally, the blades were disassembled and examined using computed tomography (CT) and metallographic methods. Throughout the testing, 31 newly-coated blades (66%) withstood the tests, producing results comparable to the reference blades. However, 16 blades suffered intolerable failures observed as increased roughness, gradual loss of the topcoat, spallation and minor foreign object damage. Visual inspection results were generally in agreement with subsequent laboratory tests.

Characteristics of the damage evolution and the free surface velocity profile with dynamic tensile spallation

The spalling behavior of ductile metals is a process involving void nucleation, growth, and coalescence. Limited by diagnostic techniques, spallation experiments only provide the free surface velocity profiles and the information about recovered targets, but some quantitative damage evolution information about the spalling target is lacking. In this research, the damage nucleation seeds are randomly arranged on the grain boundary in the central region of a target with grain geometry, and a two-dimensional mesoscale numerical model of a plate impact spall experiment is established. By analyzing the free surface velocity profile and the stress history, it is demonstrated that the spall strength obtained with the pull-back velocity essentially corresponds to the maximum tensile stress at the target center. The effects of the impact stress and the stress pulse duration on the dynamic characteristics of the void growth and coalescence are analyzed in-depth by using the damage evolution dissipation energy and the plastic strain contours at different times. The dynamic process of the damage evolution determines the characteristics of the oscillation after the pull-back signal. The stress history controls the damage degree and the kinetic process of the target in the spallation damage process. The impact stress has the most important effect in determining the damage evolution rate, while the stress pulse duration only affects the void coalescence process and irrelevant to the void growth. The damage degree of the void growth and the coalescence process are the result of the joint action of the impact stress and the pulse duration.

Spallation damage of 90W–Ni–Fe alloy under laser-induced plasma shock wave

Laser shock loading is a more promising technology for investigating spallation damage in materials under shock-wave loading. In this paper, shock-induced spallation in a 90W–Ni–Fe alloy at an ultrahigh tensile strain rate of 106 s−1 is investigated using a superintense ultrafast laser facility. The spallation of the 90W–Ni–Fe alloy was dominated by a transgranular fracture of tungsten(W) particles with a high spall strength of 6.46 GPa. Here, we found an interesting phenomenon that the formation of nanograins inside W particles leads to a new mode of transcrystalline fracture of W particles during the laser shock loading. Futhermore, most voids were nucleated inside the W particles rather than at the W/γ-(Ni, Fe) matrix-phase interface. This result contradicts the fracture theory under quasi-static loading, which posits that the W/γ-(Ni, Fe) matrix-phase interface is not the preferred site for the initial failure under shock loading.

Dependence of spallstrength on temperature, grain size and strain rate in pure ductile metals

When a shockwave, which can be generated by high velocity impact or explosive detonation, reflects from the free surface of a metal, it usually creates tensile stress inside the metal. While the tensile stress is large enough, voids nucleation, growth and coalescence happen inside the metal, causing the metal to spall. As one of the main contents of the spallation damage research, the spallation strength, which is often characterized by features of the free surface velocity history measured in spallation experiments, represents the maximum tensile stress that the material can withstand, and is actually a complex interaction among several competing mechanisms. Optimizing the spallation strengths of metals is important for their applications in the aerospace, automotive, and defense industries, and can be achieved by using the advanced manufacturing strategies, if we can know better the meaning and present analytic model of the spallation strength of metal. A large number of experiments show that the spallation strength of ductile metal is strongly dependent on the tensile strain rate, grain size and temperature of material. Based on the analysis of early spallation evolution and influence of grain size and temperature on the material, a simple analytic model of spallation strength is presented in this paper, which takes into account the effects of strain rate, grain size and temperature in materials. The applicability of this model is verified by comparing the calculated results from the model with the experimental results of spall strength of typical ductile metals such as high purity aluminum, copper, and tantalum.

Numerical Simulations of Laser-Induced Shock Experiments on Graphite.

The development of particle accelerators with ever increasing energies is raising the standards of the structures which could interact with the particle beams. These structures could be subjected to strong shockwaves in accidental scenarios. In order to test materials in such conditions, one of the most promising techniques is the impact with high-power lasers. In view of the setting up of future experimental campaigns within the Petawatt High-Energy Laser for Heavy Ion Experiments (PHELIX), the present work aims at the development of a numerical approach for the simulation of graphite impacted by laser beams. In particular, the focus is on the spallation damage caused by shockwave reflection: a sufficiently intense laser beam could ablate the matter until plasma conditions, hence producing a shockwave which could travel inside the material and reach a free surface. A numerical model to properly describe the spall fragmentation of graphite has been calibrated on the basis of literature-available experimental data. The numerical approach is a ‘two-step’ procedure: the first step is the definition of the laser–matter interaction and the second one concerns the description of the shockwave evolution into matter. The simulations satisfactorily reproduce the dynamic response of graphite impacted by two different laser sources with various intensities, despite the difficulties of characterising a phenomenon which is extremely fast and chaotic.

Thermal-mechanical-damage coupling model for thermal spallation in rock

In deep underground engineering, thermal spallation occurs when the brittle rock is suffering from high temperature. This phenomenon severely jeopardizes safety of worker and weakens the underground structure. Based on the rock thermal-mechanical-damage coupling model and theory, a novel model is proposed to explore the rock thermal spallation mechanism, including influential factors and damage evolution. The model mainly focuses on how the rock heterogeneity and micropore distribution influence thermal spallation under rapid heating. We analyse the internal temperature field, stress field, and damage field in the varying cases. This simulation results show that heterogeneity plays a pivot role in determining thermal spallation and damage in rocks. The greater the heterogeneity (more materials with larger thermal conductivity or elastic modulus), the more likely thermal spallation occurs. Moreover, the damage behaviour is also strongly influenced by micropore distribution inside the rock. The initial micropore distribution shapes preliminary damage in the rock. With the micropores increasing, the thermal spallation damage area is mostly concentrated around the micropores, as a result of the thermal stress and the existing damage together. In addition, this study indicates that the thermal damage in the rock is mainly caused by tensile stress induced by thermal expansion. The numerical results capture main features of the thermal spallation process in brittle rock, showing good agreement with the experimental observations.