Laser Shock Experiments Research Articles

Αn efficient numerical model for the simulation of debonding of adhesively bonded titanium/CFRP samples induced by repeated symmetric laser shocks

ABSTRACT Novel engine fan blades are made of 3D woven composite materials and incorporate a protective metallic layer at the leading edge. The end-of-life of such structures involves complicated disassembly and recycling processes. In this paper, laser-shock is being investigated as an environmentally friendly disassembly method. In this context, symmetric laser-shock experiments that have been conducted in a previous work using a time delay between the shots have been proven successful for debonding initiation and propagation as they manage to effectively focus the tensile stress field developed by the shock waves at the bondline. In this paper, a numerical model simulating the symmetric laser-shock disassembly of titanium/CFRP specimens has been developed using the LS-Dyna explicit FE code. The aim is to give a deeper insight into the physical mechanisms and to optimize the experimental process. To simulate the behavior of the titanium part, the Johnson-Cook material model in combination with Grüneisen equation of state has been employed, while for the composite part a progressive damage material model incorporating high-strain rate effects has been used. The bondline between the two parts has been modeled using cohesive zone elements. To obtain tensile and shear elastic and strength properties at high strain rates for the composite damage model, tension and punch shear Split-Hopkinson Bar tests have been conducted. The numerical results correlate well with back-face velocity profiles obtained from single sided laser shock experiments and with debonding patterns in the adhesive, observed in electron microscopy images, induced by symmetrical laser shock experiments. After the validation, the numerical model has been used successfully to simulate a larger disassembly process composed of 16 consecutive shots.

Quantifying the partial ionization effect of gold in the transition region between condensed matter and warm dense matter

It is now well known that solids under ultra-high-pressure shock compression will enter the warm dense matter (WDM) regime which connects condensed matter and hot plasma. How condensed matter turns into the WDM, however, remains largely unexplored due to the lack of data in the transition pressure range. In this letter, by employing the unique high-Z three-stage gas gun launcher technique developed recently, we compress gold into TPa shock pressure to fill the gap inaccessible by the two-stage gas gun and laser shock experiments. With the aid of high-precision Hugoniot data obtained experimentally, we observe a clear softening behavior beyond ~560 GPa. The state-of-the-art ab-initio molecular dynamics calculations reveal that the softening is caused by the ionization of 5d electrons in gold. This work quantifies the partial ionization effect of electrons under extreme conditions, which is critical to model the transition region between condensed matter and WDM.

Laser shock experiments prove insightful in high strain rate deformation

Simulated and experimental laser treatment of aluminum foils produces practical, comprehensive material characterization.

Pressure amplification effect of using resorcinol/formaldehyde foam ablators in laser-shock experiments

Resorcinol/formaldehyde (RF) foam-aluminum-quartz-layered targets were shock compressed up to 0.9 TPa in quartz to quantitatively evaluate the pressure-amplification effect of using a low-density RF foam as an ablator. The velocimetry and pyrometry were used to obtain the shock pressure and temperature in the quartz. The results show the use of an RF foam ablator with a density of 100 mg/cm3 increases the peak pressure of quartz by 76 (±11)% compared to the case with a polypropylene ablator. Our results also confirm that preheating of the quartz ahead of the shock wave due to the x rays produced in the ablated foam is negligible, indicating that a low-density RF foam is an ideal ablator material for laser-shock experiments.

Surface Forming Criteria of Ti-6AL-4V Titanium Alloy under Laser Loading

In order to study the spallation phenomenon of titanium alloy under the shock of nanosecond laser, the Neodymium-Yttrium-Aluminum Garnet laser was used to carry out laser shock experiments on the surface of titanium alloy. By observing and measuring the surface morphology of the target material, the forming factors and the changes of the surface morphology under different parameter settings, the forming criteria of the titanium alloy were obtained. The results show that under the single variable method, the change of laser energy can affect the target shape variable, and there is a positive correlation between them. When the thickness was greater than or equal to 0.08 mm, no obvious cracks were found in the targets. Moreover, the number of impact times was the key factor for the target deformation; with the growth of impact times, the target deformation gradually became larger until the crack appeared. The larger the diameter of the spot, the more likely the target was to undergo plastic deformation. The surface of titanium alloy with a thickness of 0.08 mm appeared to rebound under specific laser shock condition. The changes in the back of the target material were observed in real time through a high-speed camera, and the plasma induced by the laser was observed in the process. This study is based on the results of previous studies to obtain the titanium alloy forming criteria, which provides a basis for the setting of laser parameters and the thickness of the target when the nanosecond laser impacts the Ti-6AL-4V target.

Free-surface velocity measurements of opaque materials in laser-driven shock-wave experiments using photonic Doppler velocimetry

We present a novel photonic Doppler velocimetry (PDV) design for laser-driven shock-wave experiments. This PDV design is intended to provide the capability of measuring the free-surface velocity of shocked opaque materials in the terapascal range. We present measurements of the free-surface velocity of gold for as long as ∼2 ns from the shock breakout, at pressures of up to ∼7 Mbar and a free-surface velocity of 7.3 km/s with an error of ∼1.5%. Such laboratory pressure conditions are achieved predominantly at high-intensity laser facilities where the only velocity diagnostic is usually line-imaging velocity interferometry for any reflector. However, that diagnostic is limited by the lower dynamic range of the streak camera (at a temporal resolution relevant to laser shock experiments) to measure the free-surface velocity of opaque materials up to pressures of only ∼1 Mbar. We expect the proposed PDV design to allow the free-surface velocity of opaque materials to be measured at much higher pressures.

Probing the lattice structure of dynamically compressed and released single crystal iron through the alpha to epsilon phase transition

Experiments using broadband Laue x-ray diffraction (XRD) were used to examine the lattice structure of dynamically compressed [100]-oriented single crystal iron samples at the Dynamic Compression Sector at the Advanced Photon Source. These experiments used 1 μm thick iron single crystal samples sandwiched between a polyimide ablator and a polycarbonate window. A 100 J, 10 ns duration laser pulse incident on the polyimide ablator was used to shock compress the iron samples to initial stresses greater than 25 GPa, exceeding the ∼13 GPa alpha (body-centered-cubic or bcc structure) to epsilon (hexagonal-close-packed or hcp structure) phase transition stress. XRD measurements were performed at various times relative to the shock wave entering the iron sample: early times, <∼150 ps while the initial shock waves propagated through the iron; intermediate times, after the iron equilibrated with the ablator and window reaching a plateau stress state (12 or 17 GPa) lasting several nanoseconds; and late times, during uniaxial strain release. The early time measurements show that in <∼150 ps, the high-pressure hcp phase is relaxed with a c/a ratio of 1.61, contrary to previous laser shock experiments where a c/a ratio of 1.7 was inferred. In the plateau stress state and partially released states, XRD measurements showed that the hcp structure retained a c/a ratio of 1.61 with no observable changes in the microstructure. Upon stress release at ∼1 GPa/ns release rate, the reverse phase transition (hcp to bcc) to the original single crystal orientation (implying a transformation memory effect) was observed to reach completion somewhere between 13 and 11 GPa, indicating little stress hysteresis under rapid uniaxial strain release. A similar memory effect for the reverse hcp to bcc transformation has been previously observed under hydrostatic compression. However, the bcc/hcp orientation relationships differ somewhat between dynamic and static compression experiments, implying that the transformation pathway under uniaxial dynamic strain differs from the Burgers mechanism.

Shock-induced polymorphic transitions of PbF2 up to 1 TPa and their implications for the universal behavior of shocked AX2 compounds

Hugoniot data for $\mathrm{Pb}{\mathrm{F}}_{2}$ single crystals were measured for pressures up to 1 TPa through gas gun and laser shock experiments. Experimental results show that $\mathrm{Pb}{\mathrm{F}}_{2}$ transforms to a less compressible phase at 29 GPa and possibly melts at 57 GPa. Quantum molecular-dynamics simulations were also performed to calculated the liquid Hugoniot for $\mathrm{Pb}{\mathrm{F}}_{2}$. The simulated results are in good agreement with the experimental results, supporting the high-pressure phase is liquid. The current study, combined with previous studies on $\mathrm{Ti}{\mathrm{O}}_{2}, \mathrm{Si}{\mathrm{O}}_{2}$, and $\mathrm{Ca}{\mathrm{F}}_{2}$, suggests that for $A{X}_{2}$ compounds, decreasing of Hugoniot slope can be taken as an indication of melting, and the Hugoniot slope approaches 1.2 at ultrahigh pressure.

Relative stability of Cu, Ag, and Pt at high pressures and temperatures from ab initio calculations

The paper presents ab initio studies into the relative stability of the crystalline structures of copper, silver, and platinum up to high pressures at $T\ensuremath{\ge}0$ K. Our calculations in quasiharmonic approximation suggest that not the fcc structure of Cu and Ag, but the body-centered cubic one, is thermodynamically most favorable at $P\ensuremath{\gtrsim}100$ GPa and $T>3$ kK. The shock Hugoniot of Cu and Ag crosses the fcc-bcc phase boundary and the calculated transition pressures agree well with the result of recent laser shock experiments by Sharma et al. The advantage of the bcc structure comes from its softer low-frequency phonon modes and the smaller contribution of lattice vibrations to free energy at high temperatures, as compared to close-packed structures. The compression of platinum crystal also causes the $\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{bcc}$ transition but at much higher pressures, $P>1.4$ TPa.

Raman spectroscopic studies of ortho-xylene under laser driven shock and static compression

Ortho-xylene (o-xylene), a derivative of benzene and an important aromatic compound is applied as an anti-knocking agent in automobiles and jet engines. Knocking being a dynamical phenomenon that occurs at very high temperatures and high pressures, here we have reported the pump–probe technique based time-resolved Raman spectroscopy studies under laser-driven shock compression (high temperature and high pressure) along with the numerical simulation to understand the molecular level response of o-xylene under shock compression. The laser shock experiments carried out up to 4.1 GPa using confinement geometry target holder assembly show indication of three phase transitions, i.e., liquid–solid phase-I, solid phase-I–solid phase-II, and solid phase-II–solid phase-III at 1.2, 2.1, and 3.6 GPa, respectively. The shock velocities calculated for 700 mJ laser energy (corresponding pressure 2.5 GPa) using intensity ratios of Raman modes scattered from the shocked and whole region of the sample for 735 and 582 cm−1 Raman modes are 3.4 ± 0.3 and 3.5 ± 0.3 km/s, respectively, which is in close agreement with the shock velocity of 3.51 km/s determined using 1D radiation hydrodynamic numerical simulation. In addition, our high-pressure static compression studies on this compound employing diamond anvil cell up to 13 GPa show that this compound shows four possible phase transitions at 0.4, 0.9, 3.8, and 12 GPa pressures to solid phase I, II, III, and IV, respectively. On release, these phase transitions are reversible.

Modeling the damage evolution and recompression behavior during laser shock loading of aluminum microstructures at the mesoscales

Damage evolution in metals during laser-shock loading (spallation) is a complex phenomenon accompanied by extremely high temperatures, pressures, and strain rates that affect the void nucleation/growth mechanisms. The current modeling efforts at the atomic scales to investigate the evolution of microstructure undergoing the spall failure at the atomic scales are limited to a hybrid atomistic–continuum method that combines the two-temperature model (TTM) with the molecular dynamics (MD) simulations. This manuscript demonstrates this capability by investigating the mechanisms of nucleation/evolution of voids for a nanocrystalline Al system experiencing an ultrafast laser pulse. This capability, however, is unable to model the laser shock response of experimental systems (with grain sizes greater than 100 nm and thicknesses in the microns) as well as the post-spall behavior (damage growth or recompression behavior). This work combines the TTM with the quasi-coarse-grained dynamics (QCGD) method to extend MD-TTM simulations to the mesoscales. The hybrid QCGD-TTM approach retains the laser energy absorption, heat generation/transfer, and microstructure evolution (melting, defects, and damage) behavior predicted by MD-TTM simulations. The QCGD-TTM simulations allow the investigation of the wave propagation behavior, the evolution of microstructure (defects and damage), temperature, and pressure at the time and length scales of laser-shock experiments. The QCGD-TTM simulations reported here investigate the nucleation and post-spall damage evolution behavior during spall failure of sc-Al and 0.5 µm grain-sized pc-Al films with a thickness of up to 2 µm. The QCGD-TTM-predicted damage evolution behavior captures the post-spall behavior observed experimentally and retains the atomistic characteristics of void nucleation and void collapse.

Development of shock-dynamics study with synchrotron-based time-resolved X-ray diffraction using an Nd:glass laser system.

The combination of high-power laser and synchrotron X-ray pulses allows us to observe material responses under shock compression and release states at the crystal structure on a nanosecond time scale. A higher-power Nd:glass laser system for laser shock experiments was installed as a shock driving source at the NW14A beamline of PF-AR, KEK, Japan. It had a maximum pulse energy of 16 J, a pulse duration of 12 ns and a flat-top intensity profile on the target position. The shock-induced deformation dynamics of polycrystalline aluminium was investigated using synchrotron-based time-resolved X-ray diffraction (XRD) under laser-induced shock. The shock pressure reached up to about 17 GPa with a strain rate of at least 4.6 × 107 s-1 and remained there for nanoseconds. The plastic deformation caused by the shock-wave loading led to crystallite fragmentation. The preferred orientation of the polycrystalline aluminium remained essentially unchanged during the shock compression and release processes in this strain rate. The newly established time-resolved XRD experimental system can provide useful information for understanding the complex dynamic compression and release behaviors.

Localized atomic segregation in the spalled area of a Zr50Cu40Al10 bulk metallic glasses induced by laser-shock experiment

Laser-shock experiments were performed on a ternary bulk metallic glass. A spalling process was studied through post-mortem analyses conducted on a recovered sample and spall. Scanning electron microscopy magnification of fracture surfaces revealed the presence of a peculiar feature known as cup-cone. Cups are found on sample fracture surface while cones are observed on spall. Two distinct regions can be observed on cups and cones: a smooth viscous-like region in the center and a flat one with large vein-pattern in the periphery. Energy dispersive spectroscopy measurements conducted on these features emphasized atomic distribution discrepancies both on the sample and spall. We propose a mechanism for the initiation and the growth of these features but also a process for atomic segregation during spallation. Cup and cones would originate from cracks arising from shear bands formation (softened paths). These shear bands result from a quadrupolar-shaped atomic disorder engendered around an initiation site by shock wave propagation. This disorder turns into a shear band when tensile front reaches spallation plane. During the separation process, temperature gain induced by shock waves and shear bands generation decreases material viscosity leading to higher atomic mobility. Once in a liquid-like form, atomic clusters migrate and segregate due to inertial effects originating from particle velocity variation (interaction of release waves). As a result, a high rate of copper is found in sample cups and high zirconium concentration is found on spall cones.

Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions

The effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades 1 . Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres 2 , are believed to undergo structural transitions 3,4 that release hydrogen from deeper layers and may lead to compact stratified cores 5–7 . Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature–pressure regime in which methane first transforms into a mixture of hydrocarbon polymers 8 , whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene (C8H8) n samples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus 9 . Our findings demonstrate the necessity of high pressures for initiating carbon–hydrogen separation 3 and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments 4,8,10 . Our results will inform mass–radius relationships of carbon-bearing exoplanets 11 , provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon–hydrogen separation could influence the convective heat transport 7 . Diamonds precipitate from methane under the intense pressures of the atmospheres of Neptune and Uranus. Here, a laser shock experiment on a hydrocarbon sample shows that diamonds may require ten times as much pressure to precipitate as was previously thought.

Spall fracture and twinning in laser shock-loaded single-crystal magnesium

As a major failure process in materials subjected to dynamic loading, spall fracture is one of the most widely studied issues in shock physics. To investigate its dependence on the microstructure, including both initial and shock-induced features, laser shock experiments were performed on single crystal magnesium. Shock loading was applied in directions parallel and perpendicular to the c-axis of the crystals. Both the spall strength and the fracture surface morphology are found to depend on the direction of the shock application with respect to crystal orientations. The results complement data obtained previously over ranges of lower strain rates. A detailed analysis of the residual microstructure and crack patterns in the recovered samples shows strong correlations between damage localization and twins, both pre-existing and shock-induced. Thus, cracks match specific twinning directions, which is discussed on the basis of deformation mechanisms reported under quasi-static loading conditions, either prismatic slip or twinning depending on local orientations.

Laser induced damage studies in borosilicate glass using nanosecond and sub nanosecond pulses

Confinement geometry target assemblies are very important in the study of laser shocked materials. The shock pressure in confinement geometry depends on incident laser intensity and has main limitation of dielectric breakdown of transparent materials used for confinement. This makes study of damage mechanism induced by laser pulses in dielectric materials necessary. We investigated the front and rear surface damage threshold of borosilicate glass (dielectric material) and their dependency on laser parameters. The induced damage width, geometries and microstructure changes are analyzed with optical microscope, scanning electron microscope and Raman spectroscopy. The results show that at low energies symmetrical damages are found and these damage width increases nonlinearly with laser intensity. The emitted optical spectrum during the process of breakdown is also investigated and is used for the characterization of emitted plasma such as plasma temperature and free electron density. Optical emission lines from Si I, Si II and Si III ions are used for temperature and density estimations. The plasma temperature and density are measured as ~0.94eV and 1016–1017cm−3 respectively. All results are found consistent with the earlier published results and are used in designing the confinement geometry targets for laser shock experiments.

A molecular dynamics study of the shock-induced defect microstructure in single crystal Cu

Molecular dynamics simulations of shock compression are carried out over a range of pressures (from 22GPa to 75GPa) to investigate the shock-induced defect microstructure in single crystal Cu. The results show that under increasing shock pressure, we can detect three distinct phases of the defect regimes for single crystal Cu: dislocations→stacking faults→twins. We show the evolution processes of different defect microstructures. According to the analyses of these evolution processes, the formation mechanisms of various defect microstructures are studied. When the shock pressure is less than 30GPa, the motion of dislocations is a significant mechanism in single crystal Cu under the shock compression. The motion between various dislocations and loops widely exists on the (111) close-placed plane. The interesting dislocation loops are obtained at a relatively low pressure, and a new mechanism of nucleation and development of the dislocation loop is proposed. When the shock pressureis greater than 30GPa, a large number of intersecting stacking faults are formed. On the (001) surface, stacking faults align along the [220] and [2¯20] directions at exactly 90 degrees, and they are present roughly in the same proportion. The stacking fault spacing decreases with the increase of shock pressure, and shock-induced plasticity increases as a function of shock strength. The structure of the simulation, based on the topology, matches extremely well with that found in recent transmission electron microscopy studies of single crystal Cu recovered from laser shock experiments. At a relatively high pressure (61GPa), we find that the stacking faults and twins exist together in single crystal Cu. It is shown that two types of stacking faults with vertical directions are the competitive mechanism. When one of them dominates, twins will be formed from this type of stacking fault. Thus, the directions of twins are randomly distributed in the initial stage. Rotation and combination of twins are significant mechanisms, and these mechanisms lead to the final formation of twins. At 75GPa, twins elongate along [12¯1] and [1¯2¯1¯] directions on the (1¯01) surface. The configuration and direction of the twins are in agreement with experimental results.

Numerical modeling of laser-induced shock experiments for the development of the adhesion test for bonded composite materials

In this work, laser shock experiments on composite material are modeled. Focus is made on the development of a reliable numerical model to be used for the laser shock wave adhesion test of bonded composites. Technique principle is explained as well as the laser shock experiment procedure. Then, the numerical investigations are presented. A calibration method is given to set the model input parameters, and the modeling choices are detailed. Dynamic material parameters are identified thanks to experimental results, and validated through a complete campaign of laser shocks on various carbon fiber reinforced plastic (CFRP) materials (monolithic and bonded). Finally, numerical results for bonded composites are discussed. They enable to understand the stress distribution within the composite assembly during the wave propagation. This is a key step toward the development of a reliable and controlled laser shock adhesion test.

Recovery of entire shocked samples in a range of pressure from ~100 GPa to Hugoniot elastic limit

We carried out laser shock experiments and wholly recovered shocked olivine and quartz samples. We investigated the petrographic features based on optical micrographs of sliced samples and found that each recovered sample comprises three regions, I (optically dark), II (opaque) and III (transparent). Scanning electron microscopy combined with electron back-scattered diffraction shows that there are no crystal features in the region I; the materials in the region I have once melted. Moreover, numerical calculations performed with the iSALE shock physics code suggest that the boundary between regions II and III corresponds to Hugoniot Elastic Limit (HEL). Thus, we succeeded in the recovery of the entire shocked samples experienced over a wide range of pressures from HEL (~ 10 GPa) to melting pressure (~ 100 GPa) in a hierarchical order.

Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium

Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow reaching unprecedented high densities and moderately high ∼103–104 K temperatures. We describe here a complete analysis framework for the velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments. Since the precompression increases the initial density of both the sample of interest and the quartz reference for pressure-density, reflectivity, and temperature measurements, we describe analytical corrections based on available experimental data on warm dense silica and density-functional-theory based molecular dynamics computer simulations. Using our improved analysis framework, we report a re-analysis of previously published data on warm dense hydrogen and helium, compare the newly inferred pressure, density, and temperature data with most advanced equation of state models and provide updated reflectivity values.