Spall Fracture Research Articles

Microstructure evolution and wear behavior of titanium alloy under cryogenic dry sliding wear condition

Microstructure evolution and wear behavior of Ti6Al4V alloy under cryogenic dry sliding wear condition was investigated. An external cryogenic device was designed to produce a steady cryogenic environment (−172 °C) during the wear process. We found that the near surface microstructure of Ti6Al4V alloy exhibited obvious accumulation of dislocations in the grain boundaries of α phase, as well as the refinement of α and β phase grains during cryogenic temperature wear. On the contrary, the plastic deformation was predominated by twins during room temperature wear. Under cryogenic temperature wear, little plastic deformation of microstructure was observed in the near surface and subsurface of specimens, while severe deformation was found to be prevalent under room temperature wear. Worn surfaces of specimens after cryogenic wear tests were smoother with lower block spalling and friction coefficient. The cryogenic wear behavior was considered to be fracture spalling of surface induced by the initiation and extension of fatigue cracks, unlike the severe oxidative and delamination wear under room temperature. Meanwhile, as compared to room temperature wear, wear rate of Ti6Al4V alloy was reduced by 49.7%, 57.2% and 36.6% under the loading force of 100 N, 150 N and 200 N respectively after cryogenic temperature wear.

Strain rate dependence of spall strength for solid and molten lead and tin

Dynamic tensile (spall) fracture of pure Pb and Sn in solid and molten states is investigated by MD simulations. The influence of unwettable inclusions on the spall strength is revealed. Mechanical model of fracture is fitted to MD data at the strain rate $$10^{\mathrm {9}}$$$$\hbox {s}^{\mathrm {-1}}$$ and used for calculation of the rate dependencies of spall strength in the range from $$10^{\mathrm {4}}$$$$\hbox {s}^{\mathrm {-1}}$$ to $$10^{\mathrm {9}}$$$$\hbox {s}^{\mathrm {-1}}$$ in comparison with the experimental data. The model takes into account homogeneous nucleation of pores, activation of pores on unwettable inclusions or other heterogeneities and change in pore size, which is viscous for melt and elastic-plastic for solid. In the case of pure uniform material, the homogeneous nucleation gives a slow decrease in spall strength with decreasing strain rate; the calculated values significantly exceed experimental results for moderate strain rates of $$10^{\mathrm {4}}$$–$$10^{\mathrm {5}}$$$$\hbox {s}^{\mathrm {-1}}$$. Accounting of unwettable inclusions removes this contradiction and provides correspondence to experimental data. A power-law size distribution of inclusions gives in the case of melt the power-law dependence of spall strength on strain rate that coincides with the experimental data for molten Sn. In the case of solid metal, the spall strength at moderate strain rates is determined by the yield strength. Therefore, the initial power law decrease in the spall strength is replaced by almost constant level at moderate strain rates. This behavior corresponds to the existing experimental data for solid Pb. Transfer to the homogeneous nucleation mode takes place for solid and molten metals at ultra-high strain rates, when the concentration of pores activated on the existing heterogeneities is not enough for the stress relaxation.

The Mechanism of Liquid Dispersing from a Cylinder Driven by Central Dynamic Shock Loading

A systematic investigation on the mechanism of dynamic liquid dispersing process via theoretical and experimental approach is presented. Based on the experimental results, dynamic analysis, one-dimensional characteristics analysis and some numerical simulation are carried out. The stretching and shearing actions, spall fracture mechanism and characters of cavitation layered are explored. Then the thermodynamic non-equilibrium process, metastable liquid dispersing, cavitation generation and some multiscale mechanisms are revealed.

Damage characteristics of aluminum nanorod under hypervelocity impact

Understanding the dynamic behavior of nanomaterials under hypervelocity impact is of great importance to develop advanced materials or structures for protective applications. The present work gives insight into the damage characteristics of aluminum nanorod under hypervelocity impact based on atomistic simulations. First of all, the propagation of impact wave is found to experience a rapid decaying because of its release from the side surface, which leads to a complex three-dimensional stress wave and two tension regions inside the nanorod. The damage mode under this tension state is found to be very different from the classical spallation. Due to the interaction of two release waves from the side and end surfaces, a temporary spall damage is observed and its initial tensile strength is close to that of bulk material. However, that early spall damage does not develop into a complete spall fracture. More importantly, all generated voids are found to be closed eventually after their coalescence. Furthermore, the mass continues expanding outward from the impact plane and finally causes a radial annular fragmentation. The annular fragmentation shows a clear crystalline direction dependence for low impact velocities. The number and the size of final fragments are found to follow a power law relationship for all impact velocities.

Method of determining parameters of void growth damage model and its application to simulation of spall test

Spallation of ductile metal is of great importance in many scientific and engineering fields, which is due to the interaction between the incident shock waves and the reflected waves. Physically, the spallation is caused by nucleation, growth and coalescence of microvoids for ductile material. Therefore, numerical simulation of spall process usually involves theoretical model of void growth. However, due to the limited knowledge of microvoid properties, many empirical parameters are included in the void growth model, which are usually determined by comparing numerical results with experimental data. Therefore, a key problem arises in the numerical modeling of damage and spall fracture, that how the parameters of the void growth damage model can be determined. In this work, we present a theoretical method to determine the parameters based on the free surface velocity (FSV) profile. Firstly, the critical state of damage is discussed based on the relationship between characteristics of FSV and change of physical quantity in spall plane. Then, the propagation and interaction of shock waves during the evolution of spall damage are considered. Lastly, the physical meanings of the parameters of the void growth damage model are further discussed. So, based on the relation among spall strength, damage and pull-back of FSV, a physics-based method to determine the parameters of the model is given. The applicability of this method is verified by the simulation of the spall experimental data on typical ductile metals OFHC copper and tantalum. The parameter-determining method given in this paper can not only expand the scope of application of the damage model and effectively improve the reliability of the calculation results, but also provide a good reference for the determination of parameters of other spall damage model.

Calculation of fracture location in multiple spalling

The paper proposes a model that allows us to determine the fracture location in case of multiple spalling. The incubation time criterion was used to predict fracture in material. Modeling was performed for spall fracture performed on concrete rods. Comparison of the calculation results with experimental data was carried out which showed the coincidence of the predicted fracture zone with spall sections in the samples.

Effects of dynamic deformation and fracture in the Klein – Gordon stress field

The interaction of a constant cross-section rod with an elastic environment under dynamic loading is considered. The directly proportional to the displacement resistance forces impacted on the rod. Elastic and viscoelastic one-dimensional models describing longitudinal vibrations occurring in a rod located in the environment are developed. Numerical analysis of deformations and stresses in the rod in an elastic environment is performed, and deformation profiles in specific sections of the rod are calculated. The wave propagation and reflection from the free boundary of a finite-length rod are considered. The obtained results are compared with the experimental data and classical models of longitudinal vibrations of rods. A good correspondence of the calculation results with experimental data was revealed. Calculations describing the fracture of the rod are performed, and the possibility of spall fracture is shown. Fracture phenomenon is predicted by the incubation time criterion. In several special cases, the dependence of the threshold value of the load amplitude on the duration of the load is found.

Effect of NiCr content on the solid particle erosion behavior of NiCr-Cr3C2 coatings deposited by atmospheric plasma spraying

Homogeneous coatings cannot meet the overall erosion-resistance requirements for an entire blade since materials present different erosion resistances at various impact angles. In this work, NiCr-Cr3C2 coatings with different NiCr contents were prepared on FV520B steels through atmospheric plasma spraying (APS) by using a double-channel powder feeder. The composition and microstructure of the as-sprayed coatings were examined by SEM, EDS, and XRD analyses, and microhardness was determined by microhardness tests. The erosion performance of NiCr-Cr3C2 coatings under different testing conditions (impact angle, solid particle size, solid particle velocity, and NiCr contents) were further evaluated with a gas–solid two-phase testing machine. Furthermore, the erosion wear mechanism of NiCr-Cr3C2 coatings with different NiCr contents at various impact angles was investigated. The results showed that the phase distribution inside the NiCr-Cr3C2 coatings became uneven with the increased NiCr content, leading to the fluctuation of microhardness. The erosion rate of 25 wt% NiCr-Cr3C2 coating initially increased and then decreased with increasing impact angle and solid particle size. The erosion rate was also improved rapidly with the increased erosion velocity for all impact angles. It was found that at low impact angles, the erosion resistance of NiCr-Cr3C2 coatings was increased with higher ceramic content because of the micro-cutting failure. By contrast, at medium and high impact angles, the erosion resistance of the coating could be improved by increasing metal content since coatings are eroded through fatigue spalling and rigid fracture.

Statistics of energy dissipation in the hypervelocity impact shock failure transition

In the hypervelocity impact event, shock waves subject material to failure transitions with the attendant dissipation of the imparted energy. Under shock compression, failure and dissipation entail intense inelastic shear and compaction. Through shock interactions, states of dynamic tension are achieved and further failure dissipation involves fracture and fragmentation. The nature of failure of solids in the shock environment has encouraged considerable experimental effort through the past several decades. Such efforts have yielded results that suggest a level of universality in the shock failure transition over significant spans of shock intensity and solid material types. Examples include the fourth-power relation between pressure and strain rate in solid-material compressive shock waves, and power-law relations capturing spall fracture strength and fragmentation size scale in dynamic tensile failure. Comparable power-law relations also describe the shock compaction of distended solids. The present paper explores a statistical perspective of the underlying micro failure dynamics for the purpose of achieving better understanding of the macro failure trends noted above. A statistical correlation function description of the random micro velocity field is introduced. Through the attendant kinetic dissipation, the statistical fluctuation-dissipation principle is applied to the shock failure transition. From this statistical approach, power-law relations for compressive and tensile shock failure emerge that replicate the reported experimental behaviors. Results are compared with selected experimental data.

Investigation of dynamic fragmentation of laser shock-loaded tin at different phases with the integrated diagnostic techniques

We investigated the dynamic fragmentation of laser shock-loaded tin to gain insights on the underlying mechanism of this process. In the experiments, tin samples were shock-loaded by nanosecond laser over sequential pressures ranging from 7 GPa to 43 GPa. Integrated diagnostic techniques, including four-frame optical transverse shadowgraph imaging system, soft recovery, and photonic Doppler velocimetry (PDV), were employed. They provide the shape of the ejecta in the dynamic fragmentation process, the recovered ejecta, as well as the loading parameters. These experimental results were compared with those obtained using one-dimensional Lagrangian hydrodynamics simulation. The crossed results over sequential pressures allowed us to gain better insights on the different dynamic fragmentation processes of spall fracture and micro-spalling, which can help develop reliable models for dynamic fragmentation of triangular-wave shocking metals.

Evolution of pore ensemble in solid and molten aluminum under dynamic tensile fracture: Molecular dynamics simulations and mechanical models

Construction of the mechanical model for dynamic tensile (spall) fracture is one of the cornerstone problems in the mechanical description of the material behavior under dynamic loading. The spall fracture takes place for both solid and liquid substances, typically referred as cavitations in the latter case. We perform larger-scale MD simulation of the uniform triaxial stretching of molten and solid Al with the strain rate of 3/ns at different temperatures. Formation of multiple pores under used conditions of loading allows us to obtain detailed information about the evolution of pore ensemble at dynamic tensile fracture. This information can be used for verification of the mechanical models of dynamic tensile fracture. We formulate the mechanical models for the pore ensemble evolution in both liquid and elastic–plastic mater. The common part of the model describes the random nucleation of pores in the sample bulk. The size variation part of the model for liquid uses the Rayleigh–Plesset equation. The corresponding part of the model for elastic–plastic solid takes into account the dislocation activity in the pore vicinity. The detailed comparison of the model predictions with the MD data shows their adequacy in description of both the pore ensemble evolution and the general behavior of system parameters for room and elevated temperatures. The collapse of small pores after the relaxation of pressure from strong negative to close to zero values takes place for both molten and solid Al. In the case of melt, the larger and larger pores become collapsing with time, and the number of survived pores tends to one. In the case of solid, the elastic resistance of the surrounding material suppresses the collapse of the middle-sized pores, and only small pores disappear after the pressure relaxation. This feature does not allow one to use simple viscous growth models for pores in plastic solids. MD simulations show the nucleation of secondary voids in the vicinity of the primary nucleated pores and a more ordered defect structure around the pores in the case of low temperature of 100 K. These features cannot be addressed in the frames of the present mechanical model, which needs further development for description of the fracture at low temperatures.

Influence of High-Temperature Annealing on the Resistance to High Strain Rate and Fracture of Tantalum at Temperatures of 20 and 500°C

Two series of shock-wave experiments have been conducted in order to measure the Hugoniot elastic limit and determine the strain rate dependence of critical fracture stress for tantalum experiencing spall fracture. Tantalum specimens have been preannealed in vacuum at 1000°C. The evolution of elastoplastic compression shock waves at room and elevated up to 500°C temperatures has been presented from complete wave profiles recorded by a VISAR laser Doppler velocimeter. The spall strength dependence on the strain rate during the expansion of the material in a rarefaction wave has been determined.

Fracture mechanism of steel plate loaded by explosive-induced shock waves

The fracture mechanism of in-plane notched steel plate were experimentally investigated under the loading of the triangle shock waves. A series of 20 mm thick steel plates were loaded by explosive charges with different weights and the steel plates were notched in the load surface and free surface. The explosives cut the central part of the plate along the notches and the recovered specimens were processed and analyzed by the scanning electron microscope (SEM). It was found that the explosive charges and the notch properties had a significant effect on the notch fracture and the spall observed in the experiments. Thick explosive charge plate generated deeper, smaller and more dimples on the spall fracture surface which means the ductile of the fracture was increased and the crack extension resistibility was strengthened. This led to a step-like morphology on the spall surface. Moreover, the fractures of two sides of the notch were different. The shear fracture emerged on the loading side and the tensile fracture appeared to the free surface side. However, when there was no notch in the loading surface, the shear fracture appeared to the free surface side and extended to the loading surface. At the same time, the spall and notch fractures interrelated near the notch. Based on the experimental results, a new damage level function was proposed considering the statistic of crack length instead of the void volume. The theoretical analyses of spalls under all six loadings were performed and compared with the experimental results. The comparison showed a good agreement and the reliability of the proposed model was verified. The fracture behavior and mechanism obtained from the experiments provided a good foundation for the further theoretical investigations and the improved damage model was applicable in engineering analysis.

Dynamic Spallation in Uranium under Laser Shock Loading**Supported by the Science Foundation of China Academy of Engineering Physics under Grant No A090504.

The spall behavior of uranium is investigated using direct laser ablation loading experiments. The uranium targets are cut and ground to 0.05 mm, 0.1 mm, and 0.15 mm in thickness. Laser energies are varied to yield a constant peak pressure. This results in different strain rates and varying degrees of damage to the uranium targets. The spall strength is calculated and analyzed from the free surface velocity histories recorded using a line velocity interferometer for any reflections system. The spall strength increases from 4.3 GPa to 9.4 GPa with strain rates ranging from 4.0 × 106 s−1 to 1.7 × 107 s−1. Post-mortem analysis is performed on the recovered samples, revealing the twin-matrix interfaces together with the inclusions to be the primary factor governing the spall fracture of uranium.

Влияние высокотемпературного отжига на сопротивление высокоскоростному деформированию и разрушению тантала при температуре 20 и 500-=SUP=-o-=/SUP=-C

AbstractTwo series of shock-wave experiments have been conducted in order to measure the Hugoniot elastic limit and determine the strain rate dependence of critical fracture stress for tantalum experiencing spall fracture. Tantalum specimens have been preannealed in vacuum at 1000°C. The evolution of elastoplastic compression shock waves at room and elevated up to 500°C temperatures has been presented from complete wave profiles recorded by a VISAR laser Doppler velocimeter. The spall strength dependence on the strain rate during the expansion of the material in a rarefaction wave has been determined.

Shock induced damage and fracture in SiC at elevated temperature and high strain rate

Large-scale molecular dynamics simulations are used to investigate shock-induced damage and fracture in 3CSiC single crystals at an elevated initial temperature of 2000 K and a high tensile strain rate of ∼1010 s−1. Three crystal orientations have been evaluated: [001], [110] and [111]. A comprehensive comparison has been made between cases at 2000 K and at 300 K to address the effects of high temperature on the mechanical performance of SiC under shock loading. Results show that for shock compression, the high temperature decreases the longitude elastic wave speeds as well as the shock stresses. The shock-induced plasticity is mainly in the form of deformation twinning at 300 K, but twinning is absent at 2000 K. The high temperature decreases the structural phase transition threshold pressure in SiC from ∼90 GPa at 300 K (for all three orientations) to ∼75 GPa in [001], ∼57 GPa in [110] and ∼64 GPa in [111] at 2000 K, with corresponding particle velocities of 2.75 km/s, 2.0 km/s, and 2.25 km/s, respectively, in agreement with trends observed in recent experiments. The spall fracture behavior reveals that high temperature reduces the spall strength with an average spall strength of ∼20.7 GPa in [001], ∼21.4 GPa in [110] and ∼22.5 GPa in [111] at 2000 K in the classical spall regime, which are about 33% lower than strengths measured at 300 K. However, in the micro-spall regime the spall strengths are very similar at both temperatures. The corresponding thresholds of particle velocity to trigger spall decrease at elevated temperature except for [001] loading, as well as the thresholds for generating overdriven phase transition waves.

Spall fracture in additive manufactured tantalum

We present a series of experiments on the response of additive manufactured (AM) tantalum to dynamic loading, specifically the spall strength. Rectangular plates of AM tantalum were produced, with subsequent characterization revealing a highly anisotropic microstructure. Samples were taken from these plates to investigate the effect of anisotropy on the spall strength: the resistance to high strain-rate tensile damage. A conventional, wrought tantalum sample, possessing an equiaxed microstructure, was also tested to serve as a control. Shock loading was performed via light gas-gun flyer-plate impact experiments, with laser velocimetry on the rear of the samples to record the shock wave profiles and soft-recovery techniques to allow post-mortem analysis. In general, the AM samples were found to have a higher Hugoniot elastic limit, the dynamic yield strength under shock loading, while having a reduced spall strength, when compared to the wrought tantalum samples.

Numerical simulations of pore isolation and competition in idealized micro-spall process

The ‘micro-spall’ phenomenon is a variant of fragmentation process—or spall fracture—that is traditionally discussed in context of solid materials (metals). However it concerns situations in which the medium is fully or partially melted—be it due to kinetic impact, detonation or laser loading. The phenomenon takes place at sub-micrometer and sub-microsecond scales making it inaccessible to direct experimental observation; so far, investigations have been restricted to observations of late time “post-mortem” fragments.In this context, it becomes a viable approach to apply analysis using numerical description for fluids. This work presents such an application for an idealized rapid uniaxial (one-dimensional) system expansion. Cavitation in the medium is represented by including vacuous pores or cavities with surface tension whose growth and interaction are traced in time. The simulations reveal two main regimes of pore growth regulated by a characteristic Weber number.

The Contact Mechanics of Novikov’s Surface-Hardened Gearing during Running-in Process

The article is devoted to the analysis of the state of the contact surfaces of the higher kinematic pair in the general case of relative motion, that is, in the presence of rolling, sliding, and twisting, which is characteristic of Novikov’s circular-screw gears. The purpose of the work is to assess the impact of friction forces, the state of contact surfaces after tool processing, and the localization of the instantaneous contact spot on the level of contact—fatigue durability of gears. Power contact in the presence of geometric slippage of the mating surfaces leads to a significant change in the initial geometry and the mechanical properties of surface layers. In the existing methods of calculations of contact strength, the effect of running-in is investigated insufficiently, which leads to an incorrect result, especially for gear with high hardness of the teeth. In this work, the conditions of contact interaction close to the real requirements are studied on the basis of experimental material, numerical solution of the contact problem, determination of the terms of the contact areas of slip, and adhesion within the instantaneous spot. The shape of the instant contact spot has asymmetry and can be approximated by an ellipse with the introduction of a correction factor. The running-in period is of a plastic nature with cold deformation and reduction of the roughness of surfaces. As a result of the run-in period, the area of actual contact (tooth height) is increased by 2 or more times. It is not desirable to spread the area of contact at the area of adhesion that initiates the formation of pitting. The presence of defective surface area on the level of contact strength does not have significant influence, because of the running-in period, but increases the risk of spalling and brittle fracture.

The Formation of Elastoplastic Fronts and Spall Fracture in AMg6 Alloy under Shock-Wave Loading

Full wave profiles were monitored by the laser interferometry method by means of a VISAR laser Doppler velocimeter under shock-wave loading of samples of AMg6 aluminum alloy. Analysis of these profiles was used to study the laws of elastic precursor formation and its amplitude variation during elastic–plastic transition front propagation in samples loaded by a shock wave of variable intensity. Critical stresses leading to the spall fracture of samples were determined as dependent on the strain rate under unloading.