Ultra-high Strain Rate Research Articles

Performance of polyimide film under hypervelocity impact of micro flyer: Experiments and simulations

The characteristics of polyimide film hypervelocity impacted (HVI) by micro flyer are investigated though experimental and numerical methods. Using the laser driven flyer (LDF) method combined with the technique of scanning electron microscopy (SEM) and explicit dynamics numerical algorithm, the fracture morphology and detailed mechanical responses of polyimide film, both in the impact and non–impact zones, are captured; and a dynamic fracture strain criteria, combing temperature and strain rate, is presented. A parametric study on the performance of polyimide film is implemented and the influence of the factors, including the impact velocity, film thickness, strain rate constant and pre–stress, are assessed. The research reveals that polyimide film exits two failure modes, i.e. ductile punching and brittle cracks. Polyimide materials in the impact zone have experienced a high temperature (>Tg = 685 K), ultrahigh strain rate (108∼109) and plastic state; while in the non–impact zone, it is normal temperature (293–295 K), high strain rate (105∼106) and elastic state.

A Study on the Measurement Method of Transient Temperature in Materials at Ultra-High Strain Rate by Using the PIR Fiber

A Study on the Measurement Method of Transient Temperature in Materials at Ultra-High Strain Rate by Using the PIR Fiber

Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions

The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.

Shockless spalling characterization of ceramics in a 1D-stress state

Ceramics are particularly interesting as protective materials due to their high compressive strength. However the maximum tensile stress withstood by these materials is usually lower by one order of magnitude. To study the dynamic strength in tension, spalling tests are commonly performed. In this work, a new spalling configuration is presented to characterize brittle materials such as ceramics at ultra-high strain-rates of about 103–104 s−1 in a one-dimensional stress state. To do so, a specific test is designed in which the ceramic specimen is a bar with a 3 mm radius. This geometry is chosen to ensure a one-dimensional stress state in the cylindrical specimen avoiding wave dispersion during the propagation of the loading pulse. Finally, the first experimental validation tests conducted at the CEA-Gramat with the pulsed power generator called GEPI are reported.

Size distribution of pores in metal melts at non-equilibrium cavitation and further stretching, and similarity with the spall fracture of solids

Non-equilibrium cavitation in liquids is similar in many aspects to the spall fracture of solids. The dynamics of cavitation and further evolution of melt with cavities are determined by the processes of nucleation and size variation in the ensemble of pores. Exploration of the statistical distribution of the pore sizes and its evolution in the course of the melt stretching is important for in-deep understanding of the spall fracture of melts and formulation of adequate models. Size distributions of pores in pure Al, Ni, Cu, Ti and Fe melts under stretching with ultra-high strain rates are investigated with the help of molecular dynamics simulations and analysis of the obtained atomic configurations by an algorithm of pore searching. The size distribution of pores fits well to exponential one at the stage of active homogeneous nucleation of pores in all considered cases. This form of distribution is explained by the exponential growth of the nucleation rate together with the decrease in the size of critical pores at the decrease in pressure. A transition from the exponential distribution to normal one in the course of stretching is found out for melts of most metals except Fe at the strain rates above 30/ns. The transition to the normal distribution is explained by depopulation in the area of small pores due to their collapse in conditions of the increase in pressure. The transition occurs near the pore number maximum, because the reaching the maximum indicates that the collapse becomes predominant over the nucleation. The collapse of small pores is accompanied by enlargement of the largest pores and rapid widening of the size distribution with increase in the mean pore size. This is a mediated mechanism of the volume redistribution from the small pores to the largest ones; this mechanism is the most pronounced in Al melt and the most suppressed in Fe melt. In most cases, it is accompanied by direct merging of pores that is, vice versa, the most active in Fe and the most delayed in Al. The merging knocks out pores from the distribution depending on both the pore size and the mutual position of pores; therefore, predominant depopulation of the area of small pores does not happens at merging. The prevalence of merging over the collapse in Fe melt at the strain rates above 30/ns prevents the transition to the normal distribution in this melt, and the size distribution remains exponential after the stopping of nucleation. The merging also leads to corruption of the size distribution at the late stages of stretching for all melts. Time evolution of the distribution parameters reflects the distribution widening and shows the beginning of the distribution corruption. It is shown for the case of Al melt that the general behavior of the size distribution of pores remains the same within the range of strain rates from 100/ns down to 1/ns. In the case of Fe melt, the transition to the normal size distribution arises for the strain rates below 10/ns because of the delaying of merging. Additional simulations for solid Ni and Al at room temperature show that the statistical behavior of the pore ensemble in solid metals at high-rate stretching is very similar to that is observed in the case of metal melts.

Effects of service temperature on tensile properties and microstructural evolution of CP titanium subjected to laser shock peening

Effects of service temperature on the tensile properties and microstructural evolution of tensile specimens manufactured by commercial pure (CP) titanium subjected to laser shock peening (LSP) were investigated. Special attention was paid to the microstructural features at the top surface close to the fracture zone of the failed LSPed specimens at 20 °C, 150 °C, 250 °C, and 350 °C by means of transmission electron microscopy (TEM) observations. Results indicated that the ultimate tensile strength (UTS) of LSPed specimens gradually decreased, but the area reduction and elongation of LSPed specimens increased with the increment of service temperature. Furthermore, the influence mechanism of service temperature on the microstructural evolution of the LSPed CP titanium consisted of two modes: (a) ultra-high strain rate inhibited the dislocation motion at a lower temperature which was more prone to the occurrence of deformation twinning, and (b) ultra-high strain rate was easier to activate dislocation motion at a higher temperature, making deformation twinning disappear in the plastic deformation layer of CP titanium.

Surface gradient microstructural characteristics and evolution mechanism of 2195 aluminum lithium alloy induced by laser shock peening

The peak-aged 2195 aluminum lithium alloy was treated by laser shock peening (LSP). The surface gradient microstructural characteristics of this alloy induced by ultrahigh strain rate deformation during LSP were systemically examined with transmission electron microscope (TEM), the observed results suggested that the grains refined and precipitates partially dissolved in the surface after LSP. The original coarse grains with average size of about 16 μm in length and 5 μm in width were refined instantly to equiaxed grains with size of about 91 nm at the top surface after LSP. The quantitative calculation of recrystallization kinetics proved that the grain refinement was the result of rotation dynamic recrystallization (RDR). The adiabatic temperature increase, the generation of high-density dislocations around the precipitates, and the increase of grain boundary area caused by grain refinement provided the thermodynamics and kinetics conditions for partial dissolution of precipitates. The microhardness tested results showed gradient distribution characteristics of microhardness values after LSP, and the maximum of microhardness was at the top surface of this alloy. The refined grains and deformed substructures played important roles on the enhancement of surface microhardness.

Investigation on mechanical properties and microstructural evolution of TC6 titanium alloy subjected to laser peening at cryogenic temperature

The aim of this study was to investigate the influence of the synergistic function of cryogenic temperature and ultrahigh strain rate deformation during the process of Cryogenic Laser Peening (CLP) on the mechanical properties and microstructural evolution of TC6 titanium alloy. The measurements of tensile properties at room and elevated temperatures, as well as micro-hardness on the cross-sectional direction were carried out. Meanwhile, the microstructural evolution was characterizated by Electron Backscattered Diffraction (EBSD) analysis and Transmission Electron Microscopy (TEM) observation. The experimental results demonstrated that cryogenic laser peening could significantly improve the strength and ductility, as well as the stability at elevated temperature condition of TC6 titanium alloy. At the same time, cryogenic laser peening could provide higher surface micro-hardness than room temperature laser peening (RT-LP), the surface micro-hardness of the specimen subjected to cryogenic laser peening increased by 4.86% than that of the specimen treated by room temperature laser peening. Additionally, cryogenic laser peening could generate finer grains and higher-density of dislocation structures, while large numbers of sub-grains and mechanical twins were also observed in the surface layer. Finally, the microscopic strengthening mechanism of cryogenic laser peening for better mechanical properties on TC6 titanium alloy was also analyzed in detail.

(Invited) Battery Safety across Different Use-Cases

Battery Safety has been discussed at length in the literature. Recently, the use of mathematical models to describe failure mechanisms and propagation modes within battery modules and packs has become more prevalent. Detailed models that represent thermal events from ohmic heating [1] to component-wise reaction heats, [2] gradual degradation of cell components, [3] plating of lithium leading to internal short circuits,[4] failure of welded joints,[5] venting of the cell via the crimp or the can walls,[6] mechanical deformation have been developed.[7] This presentation will highlight the utility of physics-based models to discuss a few practical questions posed during the design process. We begin by attempting to present an electrochemical definition of a short circuit that is consistent across different failure modes. This section of the presentation will explore in detail the origin of failure within a cell. With our mathematical derivation, we identify parameters that govern the evolution of failure under a few key scenario, including an internal-short and mechanical crush. Next, we discuss different experimental approaches to measure these parameters and issues associated with the different measurement techniques. The second part of the presentation will cover modeling tools for propagation of failure across a module. We will discuss the influence of different trigger mechanisms and compare the amounts of heat released across the different modes. The set of case-studies presented include gradual degradation of the interface, to ultra-high strain rate deformation scenario. We will present a methodology to distinguish between gradual deformation across prolonged periods of time and severe degradation across fewer use-cycles. These results are particularly relevant in designing duty cycles for different applications and to monitor how safety evolves as a function of the aging profile of the battery. Finally we conclude with a generic map (Fig. 1) of the heat evolution versus dissipation rates that summarizes the various case-studies discussed in the presentation. This approach enables us to compare different cell designs for a given module requirement, determine packaging and/or spacing requirements as well as design of cooling loads that help mitigate the risk of failure propagation for a given design.

The research of micro pattern transferring on metallic foil via micro-energy ultraviolet pulse laser shock

A micro-energy high frequency ultraviolet pulse laser was introduced for fabricating the micro pattern on pure aluminum foil by laser shock transferring. The copper mesh mold (mesh size: 400# and 1000#) and pure aluminum foil were used to investigate the effect of laser pulse energy and overlap ratio on micro pattern transferring. A FEM model was adopted to simulate the aluminum foil deformation and plastic strain in laser shock process. To investigate the micro forming mechanism of laser shock transferring, the micro morphology of significant area on processed micro pattern was observed by atomic force microscope (AFM). The experimental results revealed that the depth of transferred micro pattern increased with the increasing of single pulse energy and overlap ratio within certain range. The forming depth of aluminum foil imprinted from 400# mold was larger than that of 1000#, and more sensitive to pulse energy and overlap ratio. The aluminum foil on mold bars was smoother than that of on mold holes. The study demonstrated that the micro-energy ultraviolet pulse laser shock can realize the precise, controllable, fast and ultrahigh strain rate fabrication of surface microstructure.

Influence of rapid heating and cooling combined with deformation at ultrahigh strain rates on the microstructure evolution of pure titanium shaped charge liner

A slug prepared from pure titanium shaped charge liners was recovered after detonation deformation at an ultrahigh strain rate over 105 s−1 and high strain over 500%. Optical microscopy, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM) were employed to investigate the microstructure of the Ti shaped charged liner before and after the detonation deformation. The microstructures of the recovered slug include lamellar α grains, equiaxed α grains within localized shear bands, and jagged α grains. EBSD results indicated that the grain orientation distribution is random and no texture was observed in the slug. TEM observation revealed that the α → β → α phase transformation occurred owing to the rapid heating and cooling combined with deformation at ultrahigh strain rates in the Ti shaped charge liner. No melting phenomenon was observed during the deformation process. The deformation process consisted of the explosive detonation stage combined with rapid heating and the subsequent penetration stage combined with rapid cooling. In the explosive detonation stage, the α → β phase transformation instantaneously occurred via a martensitic mechanism. In the penetration stage, the high-temperature slug in β state was cooled rapidly and showed different deformation behaviors in three different areas.

The dynamic plasticity and dynamic failure of a magnesium alloy under multiaxial loading

This paper examines the anisotropic and rate-dependent mechanical behavior of a rolled AZ31B magnesium alloy under multiaxial loading over a wide range of strain rates ranging from 10−4 to 105s−1. It is shown that stress state, strain rate as well as loading orientation collectively influence the plastic flow. The rate dependence of the plastic flow in the material varies with the loading orientation. The texture-induced anisotropy, however, changes very little with the strain rate. The microstructures of the as-received and deformed specimens suggest that detailed analysis of initial spread in textures is necessary, especially under multiaxial stress states. The failure mechanisms are also observed to evolve with loading rate. Our results provide useful insights into the deformation and failure mechanisms in magnesium and extend the understanding of the mechanical properties of magnesium to the ultra-high strain rate regime (105s−1).

Surface Nanocrystallization and Amorphization of Dual-Phase TC11 Titanium Alloys under Laser Induced Ultrahigh Strain-Rate Plastic Deformation.

As an innovative surface technology for ultrahigh strain-rate plastic deformation, laser shock peening (LSP) was applied to the dual-phase TC11 titanium alloy to fabricate an amorphous and nanocrystalline surface layer at room temperature. X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy (HRTEM) were used to investigate the microstructural evolution, and the deformation mechanism was discussed. The results showed that a surface nanostructured surface layer was synthesized after LSP treatment with adequate laser parameters. Simultaneously, the behavior of dislocations was also studied for different laser parameters. The rapid slipping, accumulation, annihilation, and rearrangement of dislocations under the laser-induced shock waves contributed greatly to the surface nanocrystallization. In addition, a 10 nm-thick amorphous structure layer was found through HRTEM in the top surface and the formation mechanism was attributed to the local temperature rising to the melting point, followed by its subsequent fast cooling.

Investigation on the Strain Rate Sensitivity of Polytetrafluoroethylene

Strain rate sensitivity of polytetrafluoroethylene(PTFE) across strain rate range from 10-3 to 106 s-1 was investigated. For quasi-static state of strain rates from10-3 to 10-1 s-1, the samples were tested using universal material testing machine. The values of compression strength and Young’s Modulus were calculated from the experimental results. And exponential fitting and linear fitting were done to the experimental data of compression strength and Young’s Modulus, respectively. The quasi-static experimental results are compared with Zerillie-Armstrong model for polymers, which shows better agreement at large strains. For 102~103 s-1, the samples were tested using traditional split Hopkinson pressure bars (SHPBs). The experimental result was compared with other test of as-received sample. For 105~106 s-1, the PTFE dynamic pressure experiment was conducted using pressure-shear-plate-impact (PSPI) facility. The polymer-specific Lennard-Jones model was used to analyze the results of ultra-high strain rate.

First-principles modeling of laser-matter interaction and plasma dynamics in nanosecond pulsed laser shock processing

Nanosecond pulsed laser shock processing (LSP) techniques, including laser shock peening, laser peen forming, and laser shock imprinting, have been employed for widespread industrial applications. In these processes, the main beneficial characteristic is the laser-induced shockwave with a high pressure (in the order of GPa), which leads to the plastic deformation with an ultrahigh strain rate (105–106/s) on the surface of target materials. Although LSP processes have been extensively studied by experiments, few efforts have been put on elucidating underlying process mechanisms through developing a physics-based process model. In particular, development of a first-principles model is critical for process optimization and novel process design. This work aims at introducing such a theoretical model for a fundamental understanding of process mechanisms in LSP. Emphasis is placed on the laser-matter interaction and plasma dynamics. This model is found to offer capabilities in predicting key parameters including electron and ion temperatures, plasma state variables (temperature, density, and pressure), and the propagation of the laser shockwave. The modeling results were validated by experimental data.

Experimental characterization of dynamic deformation behaviour for SCM440 steel at high strain rates

The dynamic deformation behaviours of SCM 440 steel were characterized at the strain rates from 10-3 s-1 to 106 s-1. The uniaxial tensile tests at different temperature of 25 °C, 350 °C, and 700 °C were performed by a hydraulic universal testing machine equipped with a heating stage, and the compressive tests were conducted by using a spilt Hopkinson pressure bar (SHPB) at room temperature. Material coefficients of the Johnson-Cook constitutive model considering temperature effects were obtained based on the stressstrain relations from the experimental tests. In addition, Taylor impact tests on the SCM 440 steel were carried out to evaluate the accuracy of the determined material coefficients and characterize the dynamic behavior at the ultra-high strain rates and high temperature, by comparison with numerical simulations.

Design of piezoMEMS for high strain rate nanomechanical experiments

Nanomechanical experiments on 1-D and 2-D materials are typically conducted at quasi-static strain rates of 10−4/s, while their analysis using molecular dynamic (MD) simulations are conducted at ultra-high strain rates of 106/s and above. This large order of magnitude difference in the strain rates prevents a direct one-on-one comparison between experiments and simulations. In order to close this gap in strain rates, nanoscale actuation/sensing options were explored to increase the experimental strain rates. Using a combination of COMSOL multiphysics finite element simulations and experiments, it is shown that thermal actuation, which uses structural expansion due to Joule heating, is capable of executing uniaxial nanomechanical testing up to a strain rate of 100/s. The limitation arises from system inertia and thermal transients. In contrast, piezoelectric actuation can respond in the GHz frequency range. However, given that the piezoelectric displacement is limited in range, a sagittal displacement amplification scheme is examined in the actuator design, which imposes a lower frequency limit for operation. Through a combination of analytical calculations and COMSOL dynamic analysis, it is shown that a piezoelectric actuator along with a displacement amplifier is capable of achieving ultra-high strain rates of ∼106/s during nanomechanical testing.

Effects of Laser Shock Peening on Mechanical Properties and Surface Morphology of AA2024 Alloy

This paper presents the after effect of laser shock peening (LSP) on AA2024 alloy specimen irradiated with the L-Spiral scanning pattern at different pulse energies. The surface morphology indicates an increase in surface roughness after LSP. The X-Ray diffraction analysis of the LSP treated specimens were compared with the untreated specimen. The sin²Ψ method of the X-ray diffraction technique indicates an improvement in the magnitude of compressive residual stress distribution after LSP. The hardness profile after LSP also shows substantial increment. The results suggested an improvement in the microstructural structure owing to the ultra-high strain rate deformation and refined grains. The High Score Plus software is used to analyse the crystallite size and microstrain of the alloy treated by different pulse energies.

Mechanical response of 99.999% purity aluminum under dynamic uniaxial strain and near melting temperatures

A series of reverse geometry normal plate impact experiments are employed to investigate the incipient plastic behavior of commercial purity polycrystalline aluminum under ultra-high strain-rates (∼105 /s) and test temperatures up to melt (i.e. 643 °C). The applied stress at the aluminum/target interface, as inferred from the measured normal free-surface particle velocity record, shows progressive weakening with increasing test temperatures ranging from 23–620 °C; at higher temperatures (470–620 °C) this rate of weakening is observed to decrease, and at the highest test temperatures employed in the study, i.e. temperatures approaching the melt point of aluminum (643 °C), a net increase in the longitudinal stress is observed. Additionally, estimates of longitudinal elastic impedance of the sample material, as inferred via impedance matching from the particle velocity states at shock plateau show similar monotonic decrease with temperature, except for the highest test temperature case, in which reversal of this trend is observed. Though slight, these noticeable changes in the impedance, and stress/velocity states likely indicate variances in the strengthening/hardening response of the material, which can be linked to transition in the dominant plastic flow mechanisms as the sample approaches the melting point. Scanning electron microscopy of the impact surface of the post-test samples reveal coarsening of the sample grains due to static recrystallization during the heating phase of the experiment. Additional experiments are performed on annealed specimens to probe the effects of microstructural changes, i.e. grain size, and/or relief of residual strains in the interpretation of the measured response of aluminum. The results, which show a decrease in the longitudinal acoustic impedance, and decrease in the levels of stress/particle velocity under shock loading, indicate that the effects of annealing may play a role in the observed response by promoting softening of the dynamic strength, and/or lessening of the rate of hardening in a post-yield regime.

Phase field crystal simulation of stress induced localized solid-state amorphization in nanocrystalline materials

In this work, the phase field crystal (PFC) method is used to study the localized solid-state amorphization (SSA) and its dynamic transformation process in polycrystalline materials under the uniaxial tensile deformation with different factors. The impacts of these factors, including strain rates, temperatures and grain sizes, are analyzed. Kinetically, the ultra-high strain rate causes the lattice to be seriously distorted and the grain to gradually collapse, so the dislocation density rises remarkably. Therefore, localized SSA occurs. Thermodynamically, as high temperature increases the activation energy, the atoms are active and prefer to leave the original position, which induce atom rearrangement. Furthermore, small grain size increases the percentage of grain boundary and the interface free energy of the system. As a result, Helmholtz free energy increases. The dislocations and Helmholtz free energy act as the seed and driving force for the process of the localized SSA. Also, the critical diffusion-time step and the percentage of amorphous region areas are calculated. Through this work, the PFC method is proved to be an effective means to study localized SSA under uniaxial tensile deformation.