Spall Strength Research Articles

Dynamic strength, reinforcing mechanism and damage of ceramic metal composites

• We fully study the shock responses of ceramic metal composites via NEMD . • The interaction of shock-wave with the interface affects dislocation strengthening. • An abnormal softening effect is captured under shock loading . • The interface possesses negative effect on damage for composites. Shock tolerance is desirable for ceramic particles-reinforced metal matrix composites in many applications, where the dislocation dynamics evolution under the extreme load is the key but still elusive. Herein, we have investigated the dislocation motion and interaction under shock loading of SiC/Al nanocomposites using molecular dynamics simulations. We have demonstrated that the plastic deformation occurs at an impact velocity (0.5 km/s) lower than the Hugoniot elastic limit of aluminum due to the reflected shear wave effect. The Al/SiC interfaces act as a dislocation emitter to control dislocation multiplication density and slip direction, opening a new pathway to achieve ultrahigh-strength via shock loading. When the impact velocity (1.0 or 1.5 km/s) exceeds the Hugoniot elastic limit, the effect of nanoparticles on dislocation structure has changed from multiplying to retarding dislocations. The spall strength of composites improves due to dislocations pile-up at interface. Instead, the damage in the matrix is exacerbated, owing to the enhanced residual peak stress and interface reflection waves. In addition, the effect of abnormal shock softening determined by atomic velocity is revealed, which could be suppressed by increasing impact energy dissipation. Meanwhile, dynamic compressive strength depends on pressure and dislocation structures evolution. Our atomistic insights might be helpful in designing advanced shock-tolerant materials.

Effect of Grain Boundary Misorientation on Spall Strength in Ta via Shock-Free Simulations with Relatively Few Atoms

A suite of 37 molecular dynamics simulations is conducted at two system sizes to systematically characterize the role of grain boundary (GB) misorientation on spall strength in pure BCC tantalum (Ta). The systems studied consist of bicrystals with a single [110] symmetric tilt grain boundary. Two loading conditions are compared: (i) homogeneous extension under uniaxial strain simulated in this study and (ii) piston/flyer impact of sample, which induces heterogeneous deformation via shockwave propagation along the length of the sample. The piston/flyer impact is taken from the literature and run on the same set of GB misorientation angles using LAMMPS. The major finding here is that both methods result in similar spall strength predictions, but the homogeneous extension method generally requires two to three orders of magnitude fewer atoms and similar reductions in computational costs. Spall strength results systematically overpredict using this method, by about 10% for the dataset three orders of magnitude smaller than piston/flyer simulations, and 5% for the dataset two orders of magnitude smaller. Lastly, the effect of system size and pre-compression magnitude on spall strength is systematically characterized.

High-Strain Rate Spall Strength Measurement for CoCrFeMnNi High-Entropy Alloy

In this study, we experimentally investigate the high stain rate and spall behavior of Cantor high-entropy alloy (HEA), CoCrFeMnNi. First, the Hugoniot equations of state (EOS) for the samples are determined using laser-driven CoCrFeMnNi flyers launched into known Lithium Fluoride (LiF) windows. Photon Doppler Velocimetry (PDV) recordings of the velocity profiles find the EOS coefficients using an impedance mismatch technique. Following this set of measurements, laser-driven aluminum flyer plates are accelerated to velocities of 0.5–1.0 km/s using a high-energy pulse laser. Upon impact with CoCrFeMnNi samples, the shock response is found through PDV measurements of the free surface velocities. From this second set of measurements, the spall strength of the alloy is found for pressures up to 5 GPa and strain rates in excess of 106 s−1. Further analysis of the failure mechanisms behind the spallation is conducted using fractography revealing the occurrence of ductile fracture at voids presumed to be caused by chromium oxide deposits created during the manufacturing process.

Physical, mechanical, thermal and sustainable properties of UHPC with converter steel slag aggregates

This paper aims to utilize converter steel slag aggregates (SSA) to develop sustainable ultra-high performance concrete (UHPC) for multifunctional applications. UHPC mixtures with different particle sizes and contents of SSA are developed by applying a modified packing model. The physical, mechanical, thermal and sustainable properties of the designed UHPC have been explored by analysing bulk density, ultrasound pulse velocity (UPV), water-permeable porosity, mechanical strength, spalling and residual strength after elevated temperature, binder efficiency and heavy metal ions leaching. The results show that the optimized particle distribution of converter SSA up to 5.6 mm could be introduced in sustainable UHPC with high 28-days compressive strength up to 177.8 MPa and low binder content as low as 567.2 kg/m3. The designed UHPC mixtures attain bulk density and UPV up to 3070 kg/m3 and 5236 m/s, respectively, which makes them suitable to be utilized as heavy-weight concrete in the protective shield against nuclear radiation. No obvious explosive spalling and relatively high residual strengths reveal the potential application in heat or fire-resistant concrete. The converter steel slag in form of aggregates in UHPC does not cause negative environmental impacts on CO2 emission and heavy metals pollution, considering the relatively high binder efficiency (0.178–0.313 MPa/(kg/m3)) and non-detectable leaching of B, Cr and V.

Constrained model calibration of grain structure dependent spall dynamics in shock-loaded tantalum

We perform a gas gun experiment by shock loading tantalum samples of varying grain structures to assess the suitability of a numerical model for simulating spall behavior. The observed differences in spall strength, as well spallation and re-compression history, are not captured in uncalibrated hydrodynamic simulations. An optimization is performed on the Johnson spall model to determine the best parameters that fit the observed trends. Linear stability analysis is employed to motivate bounds on those parameters. Herein, optimized simulations agree well with the experimental results, reproducing pullback depth and recompression timescales across the different samples tested. Further, the observed pullback time of the single crystal sample was found to imply, via the stability analysis, a percolation threshold in good agreement with the theoretical value for a body centered cubic lattice. Therefore, the combined linear stability and percolation analysis shows promise and may be applied to other materials with diverse microstructures. Collectively, the findings demonstrate that the model is suitable for reproducing spall-induced free surface behavior across various microstructures, but also points to caution in using model coefficients for uncalibrated microstructures.

Shock wave characteristics and spalling behavior of non-coherent Cu/Nb multilayers

We investigate shock wave characteristics and spalling failure of Cu/Nb nanolaminates under shock loading via molecular dynamics simulations. The features of stress wave propagation in the Cu/Nb multilayers, including the reflection-loading and reflection-unloading of wave-front stress, and the strong interfacial discontinuities of transverse normal stress components, maximum shear stress and average kinetic energy are revealed by atomistic simulations. Continuum models are proposed to explain these shock wave evolution characteristics. In addition, the simulations well reproduce experimentally observed abnormal ductile damage distribution, i.e., voids nucleate in the Cu phase rather than in the Nb phase or interfaces. Such abnormality is intrinsic dynamic characteristics of nanolaminates, independent of layer thickness and shock intensity. Differences in theoretical spall strengths of Cu, Nb and the interfaces are calculated using the Cauchy-Born kinematic constraint. Moreover, we bring novel insight into the cause of the abnormality from a dynamic perspective that the maximum tensile stress in the stronger Nb phase is restricted by the spall strength of the weaker Cu phase, sound velocity, strain rate and layer thickness. The simulations also indicate that dynamic ductility can be enhanced by reducing the layer thickness, due to that more spall fracture surfaces are generated in nanolaminates with finer layer thickness.

Dynamic Yield Strength and Spall Strength of Polycrystalline Nickel Aluminide

Dynamic Yield Strength and Spall Strength of Polycrystalline Nickel Aluminide

Deformation and damage characteristics of copper/honeycomb-graphene under shock loading

Three-dimensional graphene networks have been promising structures for improving the mechanical properties of metallic composites. This work investigated the deformation and damage characteristics of a single-crystalline and polycrystalline copper matrix with a honeycomb graphene network based on atomistic simulations. The graphene interface exerts a minor influence on the shock Hugoniot relation compared with that of the crystalline orientation of the copper matrix. Shock-induced dislocation nucleation occurs at the junction of the graphene interface and then propagates along the (111) plane of the copper matrix, and the graphene interface can be penetrated by copper atoms under strong shock. The graphene interface will dramatically reduce wave transmission and convert some shock energy into heat at the grain boundary, forming local hot spots and disordered regions. With shock wave releasing to the tensile stage, the graphene interface undergoes wrinkling, folding, and even fracture. Differing from the classical spall, the composites easily spall at the graphene interface under the release wave due to the weak Cu-C interfacial interaction, which significantly reduces the damage degree inside grains and leads to a special failure mode of grain flying. In addition, this fracture dominated by interface separation will produce a complex fracture interface and lead to a nonuniform distribution of spall strength on the fracture surface, with obvious local spallation waves, which makes the estimation of spall strength based on the acoustic approximation method difficult. The average strength of the graphene/single-crystalline copper composite is close to that of polycrystalline copper but higher than that of the graphene/polycrystalline copper composite.

The grain boundary effect on shock induced spallation of polycrystalline uranium

The shock-induced spallation mechanism of polycrystalline uranium is explored by molecular dynamics, and the role of grain boundaries in it is analysed. Present work reveals that the growth and fusion of voids are the main mechanism of uranium during spallation, and the grain boundaries have significant effects on nucleation and growth of voids. The spallation strength of polycrystalline uranium is lower than that of single crystal uranium, and the relative instability of the structure at the grain boundaries causes the conjunction point of multiple grains is the first nucleation position of voids. Besides, a mutual promotion relationship between the growth of voids and local thermal dissipation is verified, which greatly promotes the development of spallation. Furthermore, the grain boundary effect on the spallation is influenced by the shock intensity. Under lower shock intensity (0.3 km/s, 0.4 km/s, 0.5 km/s), the mechanical properties of the grain boundary are the major influence factor of the spalling strength of polycrystalline uranium. The lower the speed, the more obvious the grain boundary effect, and the larger the gap between the spallation strength of single crystal uranium and polycrystalline uranium. As the shock intensity gradually increases (0.6 km/s, 0.8 km/s), the grain boundary effect is gradually weakened. When the shock intensity approaches the melting intensity, the spallation behavior of single-crystalline and polycrystalline uranium tends to be consistent.

Effect of Heat Treatment and Test Temperature on the Strength Properties of Cast Heat-Resistant Nickel Base Inconel 718 Superalloy under Shock-Wave Loading

The influence of the heat treatment regime and the initial temperature on the strength characteristics of the cast heat-resistant superalloy Inconel 718 under shock loading has been studied. For samples of four types: in the as-received state, in the as-received state with subsequent heat treatment, in the as-received state after annealing and in the as-received state after annealing and subsequent heat treatment, measurements of the Hugoniot elastic limit and spall strength were carried out, based on the registration and subsequent analysis of the wave profiles in the samples under study. Shock-wave load pulses with an amplitude of ~6.5 GPa were generated using a light-gas gun. Measurement of the evolution of the shock-wave during loading—registration of the velocity profiles of the free surface of all types of samples of different thicknesses was carried out using a laser Doppler velocimeter VISAR. The measurements were carried out at a temperature of 20 °C and 650 °C. The analysis of the results revealed a noticeable effect of heat treatment and temperature on the characteristics of the elastic-plastic transition and the resistance to spalling of the Inconel 718 superalloy.

Critical assessment of the extreme mechanical behavior of a stable nanocrystalline alloy under shock loading

A material's spall-strength and Hugoniot elastic limit (HEL), are measures of its ability to resist failure and plastic deformation under shock loading. An ideal single-crystal, i.e., a defect-free material, offers perfect lattice rectification, and therefore, provides an upper bound, or expected limit, which has yet to be exceeded by their polycrystalline counterparts. Toward this, we used a nanocrystalline (NC) copper-tantalum alloy, a model system, to probe the HEL and the spall strength of a stable NC alloy and the pertaining microstructural features that control failure. The results reveal significant increases in the HEL to about 2.0 GPa and spall strength of 1.19–1.67 GPa compared to polycrystalline Cu along with negligible changes in the residual hardness and microstructure of the shock recovered samples. The observed spall strength is approximately 2-times that of polycrystalline Cu. Further, advanced microstructural characterization using transmission electron microscopy (TEM) found no increase in dislocation density and/or mechanical twinning between the as-received and shock recovered samples, i.e., stabilized NC-alloys exhibit an unprecedented ability to resist high defect (such as dislocation) accumulation and damage. This anomalous behavior in stable NC-alloys is attributed to the elimination/limitation of defects formed under shock conditions coupled with a divergent strain-rate-insensitive behavior of its main microstructural features. The present work highlights, if designed properly, that some critical lower length-scale features including grain and phase boundaries may not contribute to the failure process. However, more fundamental research is needed to address the role processing parameters have on the resultant material that could result in spall strengths comparable to those attained for single crystals.

Dynamic response of wrought and additively manufactured nickel-based alloys to high velocity impacts of laser-launched flyers

Although nickel-based superalloys are widely used in the industry, their response to shock loading is still rarely investigated. Here, the impacts of laser-launched flyers were used to study the dynamic behavior of a Rene 65 superalloy under shock pressures of about 10 GPa at very high strain rates of about 106 s−1. Three types of samples, wrought or additively manufactured (laser powder bed fusion) and subjected to different heat treatment conditions, were investigated. These experiments allowed the measurement of the Hugoniot elastic limit (compressive yield strength) and the spall (tensile) strength, both in upper ranges compared with the most common metals. Post-recovery analyses involving various techniques provided insight into dynamic failure with a combination of transgranular ductile fracture and intergranular cracks with preferential nucleation sites, strongly dependent on the different microstructures related to fabrication routes and thermal treatments.

Atomic insights into shock-induced spalling of polyurea by molecular dynamics simulation

Some block-copolymer elastomers are regarded as competitors to metal for lightweight impact protection design. However, the microscopic spalling mechanism of polymer is still unclear owing to the complex macromolecule structure. Molecular dynamics (MD) can overcome the drawback in the experiment and reveal the nature of dynamic damage at the microscale. In this work, the spalling of polyurea under different impact velocity is analyzed using MD method and coarse-grained molecular dynamics model. The simulation results show that spalling failure originates from the nucleation and development of void-damage. The flow stress has a plateau in the early and middle stages of void flow. The Hugoniot relation is obtained. Furthermore, the free surface velocity is calculated, which shows the typical characteristics observed in the experiment. The spalling strength is calculated by the direct and indirect methods, respectively. The non-monotonic variation of spalling strength with impact velocity is the result of the competition between the strain-rate strengthening and temperature weakening. The molecular mechanism is further analyzed, which indicates that the molecules are oriented during void flow, and the hard segments are more rigid and hysteretic. Besides, microphase-separated block copolymers have a special fibril toughening mechanism. The relationship between the flow stress and deformation of a fibril is quantitatively analyzed. Finally, the unit cell simulation, which represents one point in macroscopic, is compared with the spalling simulation.

Strength Properties of the Heat-Resistant Inconel 718 Superalloy Additively Manufactured by Direct Laser Deposition Method under Shock Compression

By recording and analyzing complete wave profiles using the VISAR laser interferometer, measurements of the Hugoniot elastic limit and critical fracture stresses were carried out under the spalling conditions of the heat-resistant Inconel 718 alloy, additively manufactured by direct laser deposition, at shockwave loading up to ~6.5 GPa using a light-gas gun. For comparison, similar experiments were performed with the Inconel 718 alloy made by the traditional method of vacuum induction melting. The process of the delay of an elastic compression wave during its propagation through the sample and the dependence of the spall strength on the strain before fracture in the range 105–106 s−1 were investigated. To identify the anisotropy of the strength properties of the material under study, two series of experiments were carried out on loading additively manufactured samples along and perpendicular to the direction of the deposition. The measurements performed showed that the additively manufactured Inconel 718 alloy demonstrates weak anisotropy of strength properties for both the initial and thermal-treated samples. The thermal treatment leads to a noticeable increase in the Hugoniot elastic limit and the spall strength of the samples at low strain rates. For all types of samples, there is an increase in the spall strength with an increase in the strain rate. The spall strength measured for the cast alloy practically coincides with the strength of the as-received additive alloy and is noticeably lower than the strength of the thermal-treated additive alloy over the entire range of the strain rates. The process of the decay of the elastic precursor in the cast alloy occurs much faster than in the additive one, and the minimum values of the Hugoniot elastic limit are measured for thick samples in the cast alloy.

Effect of Symmetric Tilt and Twist Grain Boundaries on the Void Nucleation, Growth and Spall in polycrystalline Al : Multiscale modelling

Multiscale modelling using molecular dynamics (MD) and hydrodynamics (HD) in sequence is carried out to obtain the dynamic spall strength of aluminum (Al). Specifically, we simulate the effect of symmetric tilt and twist grain boundaries (GB) on the void nucleation and growth in Al using MD and transfer the information to HD simulations of plate impact. MD is used to create bi-crystals mimicking 11 tilt and 12 twist grain boundaries (GB) along <100>, with orientation angles spanning 0° to 90° using an NPT ensemble. The grain boundary energy (GBE) is validated with published results. MD simulations of the tri-axial tensile deformation of the Al bi-crystals show that the voids nucleate at the grain boundaries and then grow and coalesce. The time history of the bulk pressure and void volume fraction are recorded for calculating the material specific parameters of a void nucleation and growth (NAG) fracture model. 1-D Hydrodynamic flyer plate impact simulations are performed using these NAG parameters to obtain the free surface velocity vs time curve. The spall strength and spall thickness of Al, estimated from the free surface velocity profile, show a good match with published experimental values of the dynamic spall strength (relative error of 2.2%–6%) and spall thickness (relative error of 2.4%–4%) for three impact velocities viz. 518, 1588, and 2275 m/s respectively.

Evolution of Preset Void and Damage Characteristics in Aluminum during Shock Compression and Release.

It is well known that initial defects play an essential role in the dynamic failure of materials. In practice, dynamic tension is often realized by release of compression waves. In this work, we consider void-included single-crystal aluminum and investigate the damage characteristics under different shock compression and release based on direct atomistic simulations. Elastic deformation, limited growth and closure of voids, and the typical spall and new nucleation of voids were all observed. In the case of elastic deformation, we observed the oscillatory change of void volume under multiple compression and tension. With the increase of impact velocity, the void volume reduced oscillations to the point of disappearance with apparent strain localization and local plastic deformation. The incomplete or complete collapsed void became the priority of damage growth under tension. An increase in sample length promoted the continuous growth of preset void and the occurrence of fracture. Of course, on the release of strong shock, homogeneous nucleation of voids covered the initial void, leading to a wider range of damaged zones. Finally, the effect of the preset void on the spall strength was presented for different shock pressures and strain rates.

Shock compression and spallation of a medium-entropy alloy Fe40Mn20Cr20Ni20

Shock compression and spallation of a low-cost cobalt-free medium-entropy alloy (MEA), Fe40Mn20Cr20Ni20 (at%), with a face-centered-cubic structure are investigated via plate impact experiments, to reveal its dynamic mechanical properties and corresponding microscopic deformation/damage mechanisms. The Hugoniot equation of state, yield strength, spall strength and pullback rate are obtained from free-surface velocity histories. Post-deformation samples are characterized with transmission electron microscopy, scanning electron microscopy and electron backscatter diffraction. The Fe40Mn20Cr20Ni20 MEA exhibits a good balance in spall strength and ductility (low pullback rate) at a relatively low cost, compared to several other types of medium/high entropy alloys and steels. At sufficiently high impact velocity (e.g., 500 ms−1 here), nanoscale deformation twinning becomes a key deformation mechanism for Fe40Mn20Cr20Ni20 MEA in addition to dislocation slip. During spallation, voids (i.e., damage) nucleate preferentially at grain boundary triple junctions, and grow isotropically with increasing loading. Owing to fine grains and strong plastic deformation capability of the Fe40Mn20Cr20Ni20 MEA, void coalescence is accomplished by intragranular shear deformation bands and cracks, which contributes to its high ductility.

Spall damage of TiNb alloys under Taylor-wave loading

Spall damage of two cast TiNb binary alloys, Ti75Nb25 and Ti50Nb50, is investigated under Taylor-wave loading at different impact velocities. The dynamic yield stresses are similar for both alloys (∼1.2 GPa). The incipient spall strength of Ti75Nb25 (3.2 GPa) is considerably lower than that of Ti50Nb50 (4.3 GPa). The damage in Ti75Nb25 is ductile in nature, and abundant {332}〈113〉 deformation twins are activated. For Ti50Nb50, only a small amount of {112}〈111〉 deformation twins are observed, and cracks are along grain boundaries or along the {112} planes in a grain interior, leading to the typical brittle spall failure. Spall strength, and deformation and damage modes of TiNb binary alloys depend on chemical compositions.

Dynamic response of a Li2O-Al2O3-SiO2 transparent glass-ceramic under shock compression

Li2O-Al2O3-SiO2 transparent glass-ceramics (LAS-TGCs) are promising candidates for transparent armour materials due to their excellent physical and mechanical capabilities. In this work, the dynamic behaviour of a LAS-TGC were further investigated using the planar impact technique and photon Doppler velocimetry. The shock stress, shock wave velocity, failure wave velocity and spall strength were obtained. In addition, the recompression signal, as a signature of failure waves, was observed to evolve into an oscillation signal as the impact stress decreases, indicating that the failure wave is gradually formed at a threshold. It has been noted that the failure wave velocity decreases with the increase in external loading and then turns upwards. The damage parameter of the LAS-TGC was assessed to be 0.410(5) under a shock stress of ~5.5 GPa, which is smaller than that of K9 glass. It is suggested that the LAS-TGC has better shock resistance than K9 glass.

Dislocation-dominated void nucleation in shock-spalled single crystal copper: Mechanism and anisotropy

Large-scale molecular dynamics simulations are conducted on single crystal copper along eight representative orientations to investigate dislocation-dominated void nucleation during shock loading and spall failure, including mechanisms and anisotropy. Spall strength estimated from free surface velocity decreases in the order of group I ([001]), group IV ([012] and [011]), group III ([122] and [123]) and group II ([114], [112] and [111]), respectively, in good agreement with anisotropy of spall strength in previous experiments and simulations. A lower spall strength is statistically associated with a higher void nucleation rate and a higher density of stable immobile dislocations (1/6〈110〉 and 1/3〈100〉) formed before void nucleation. Stable immobile dislocation plays a key role in void nucleation. The formation of stable immobile dislocations requires three conditions: high resolved shear stress for activating mobile dislocations, two or more main slip planes for dislocation reaction, and high angle between activated slip directions for high stability of immobile dislocations. Based on crystal elastic–plastic theory, resolved shear stress on all slip systems is calculated to analyze orientation effects on the three conditions. The three conditions can be fulfilled by group II orientations, but cannot by group I, III, and IV orientations, leading to anisotropy in void nucleation and spall strength.