Free Surface Velocity Measurements Research Articles

Dynamic response of equiatomic and non-equiatomic CrMnFeCoNi high-entropy alloys under plate impact

A typical Fe-rich non-equiatomic CrMnFeCoNi high-entropy alloy (HEA), Cr10Mn10Fe60Co10Ni10, and the equiatomic Cr20Mn20Fe20Co20Ni20 HEA, are investigated via plate impact experiments along with in situ free surface velocity measurements. Both the initial and postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. These two HEAs contain a single face-centered cubic phase, are texture-free, and have similar grain sizes. Dynamic yield stress and spall strength of the non-equiatomic CrMnFeCoNi are lower by ∼55 % and 15 % than those of its equiatomic counterpart, respectively. After shock compression, dislocation slips, stacking faults, Lomer-Cottrell locks and nanotwins are found in the postmortem samples of both HEAs. Due to the reduced stacking fault energy in the non-equiatomic HEA, more deformation twins and newly formed hexagonal close-packed phases are found. For spallation, intergranular voids are larger in size but smaller in quantity than intragranular voids for both HEAs. Both intragranular or intergranular voids show strong dependence on grain boundary misorientation.

A spall and diffraction study of nanosecond pressure release across the iron ε-α phase boundary

The extreme response of polycrystalline iron at high pressures and high strain rates is revealed by means of high-power laser pulses. The compression portion of the pulse coupled with x-ray diffraction identifies the expected body-centered cubic (α) to hexagonal close packed (ε) displacive transformation. Upon release, observation shows that the complete reverse transformation takes approximately 8 ns and that the structure returns to its initial microstructural configuration, in a reversible transformation path. This is in good agreement with molecular dynamics (MD) simulations which predict an inverse dependence between transformation time and strain rate. The grain size is reduced from μm to nm range during compression and begins increasing back to the original grain size on decompression. The kinetics of the transition is dictated by heterogenous nucleation as it follows the Johnson-Mehl-Avrami-Kolmogorov equation with the appropriate time exponent of ∼1. This is confirmed by MD simulations which also identify profuse twinning and dislocation generation. The tensile pulse generated upon reflection at the free surface is captured by time-resolved free surface velocity measurements from which a peak tensile stress of 7 GPa is obtained, in stark contrast with its quasi-static value of ∼200 MPa. At these strain rates, the strength of grain interiors, which is determined by twinning and slip exceeds the strength of the boundaries, and failure initiates preferentially in the latter.

Spall damage of solution-treated hot-rolled Inconel 718 superalloy under plate impact

Spallation of a solution-treated hot-rolled Inconel 718 superalloy is investigated via plate impact loading along with ultrafast free-surface velocity measurements. Microstructures of the initial and postmortem samples are characterized with scanning electron microscopy and electron backscatter diffraction. Incipient spallation occurs near a peak shock stress of 5.3 GPa at a tensile strain rate of 105 s−1. Spall strength increases slightly (3.2–3.9 GPa) with increasing peak stress in the explored peak stress range (5.3–17.2 GPa) and tensile strain rate range (1.1 × 105–1.7 × 105 s−1). Ductile fracture is the main damage mode, and void nucleation occurs mostly at grain boundaries and triple junctions. Both intergranular and intragranular voids tend to nucleate preferentially at grain boundaries with a misoriention of 45°–55°. Molecular dynamics simulations are conducted to explain the mechanisms of intergranular and intragranular damage as observed in the experiments.

Dynamic compression and fracture of poly(ether-ether-ketone) under plate impact

Plate impact experiments are conducted to investigate shock compression and spallation properties of poly(ether-ether-ketone) (PEEK) up to a peak shock stress of 2.5 GPa with high-speed velocimetry and postmortem X-ray computed tomography. The Hugoniot equation of state and the shock-state Lagrangian sound velocity of PEEK are obtained via reverse impact. The free-surface velocity measurements indicate two-stage spallation. Spall strength and corresponding tensile strain rates are determined via symmetric impact. Spall strength varies in a narrow range of 0.18–0.22 GPa; it increases with increasing shock pressure as a result of increasing tensile strain rate, reaches a plateau and then drops due to shock heating. With the increasing impact velocity, the fracture toughness decreases monotonically. The compression and fracture features at different impact velocities are likely associated with the rearrangement and breakage of molecular chains.

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

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

Experimental characterization of extreme temperature granular flows for solar thermal energy transport and storage

High-temperature, dense granular flows along an inclined plane were considered for solar thermal energy transport and storage with sintered bauxite particles. A series of experiments was performed for particle inlet temperatures of ∼ 200, 400, 600, and 800 °C to understand the mechanisms of granular flows at extreme temperatures. Mass flow rates were measured using a load cell and free-surface velocities were measured and computed using particle image velocimetry. Surface temperatures were measured using infrared cameras. A significant decrease in steady-state particle mass flow rate was observed with increasing temperature due to changing flow properties. A decrease in bulk particle free-surface velocities was observed at higher temperatures. Free-surface velocity measurement error between experiments were within 20% of the average. The particle surface temperatures decreased from inlet to outlet with larger gradients at higher temperatures observed due to increasing convection and radiative heat losses. A decrease in temperature was observed along the side walls due to a decrease in particle velocities. • Refractory granular media enable high temperature solar thermal energy storage. • Granular flow experiments on inclined plane explored at elevated temperatures. • Significant changes in flow behavior observed at elevated temperatures. • Larger heat loss to surroundings observed at side walls due to decrease in velocity.

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

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

X-ray diffraction measurements and pressure determination in nanosecond compression of solids up to 600 GPa

X-ray diffraction measurements under laser-driven dynamic compression now allow us to investigate the atomic structure of matter at TPa pressures and thousands of degree temperatures, with broad implications for condensed matter physics, planetary science, and astronomy. Pressure determination in these experiments often relies on velocimetry measurements coupled with modeling that requires accurate knowledge of the optical and thermomechanical properties of a window material, resulting in significant systematic uncertainty. Here we report on a series of x-ray diffraction experiments on five metals dynamically compressed to 600 GPa. In addition to simultaneously collecting atomic structure information for multiple compressed samples, namely Pt, Ta, Au, W, and Fe, we demonstrate a different approach for pressure determination applicable to x-ray diffraction experiments under quasi-isentropic ramp compression. The method, based on the use of in situ pressure calibrants, is similar to the techniques often adopted in static compression with diamond anvil cells. Focusing on experiments using a diamond window, we discuss challenges and mitigation strategies for the novel approach. Our study provides lattice-level information on five different metals compressed to hundreds of GPa and validation to the currently used methods for pressure determination based on time-resolved measurement of the diamond free-surface velocity, revealing that the use of in situ calibrants enables a factor of four reduction in the pressure uncertainty in these experiments.

Free-Surface Velocity Measurement Using Direct Sensor Orientation-Based STIV.

Particle image velocimetry (PIV) is a quantitative flow visualization technique, which greatly improves the ability to characterize various complex flows in laboratory and field environments. However, the deployment of reference objects or ground control points (GCPs) for velocity calibration is still a challenge for in situ free-surface velocity measurements. By combining space-time image velocimetry (STIV) with direct sensor orientation (DSO) photogrammetry, a laser distance meter (LDM)-supported photogrammetric device is designed, to realize the GCPs-free surface velocity measurement under an oblique shooting angle. The velocity calibration with DSO is based on the collinear equation, while the lens distortion, oblique shooting angle, water level variation, and water surface slope are introduced to build an imaging measurement model with explicit physical meaning for parameters. To accurately obtain the in situ position and orientations of the camera utilizing the LDM and its embedded tilt sensor, the camera’s intrinsic parameters and relative position within the LDM are previously calibrated with a planar chessboard. A flume experiment is designed to evaluate the uncertainty of optical flow estimation and velocity calibration. Results show that the proposed DSO-STIV has good transferability and operability for in situ measurements. It is superior to propeller current meters and surface velocity radars in characterizing shallow free-surface flows; this is attributed to its non-intrusive, whole-field, and high-resolution features. In addition, the combined uncertainty of free-surface velocity measurement is analyzed, which provides an alternative solution for error assessment when comparing measurement failures.

Investigations of transient sloshing induced impulsive hydrodynamics

Sloshing-induced impulsive pressures may be extremely high and of short rise time duration. A compressible Volume of Fluid (VoF) method was adopted to study these pressures in a partially filled three-dimensional tank with various oblique tank ceiling angles. Comparative available experimental model test measurements of free surface profiles and velocity and pressure time histories validated this numerical method. To obtain a sufficiently fine grid to yield converging predictions, grid sensitivity studies were performed. The influence of the oblique tank ceiling angle on sloshing-induced impulsive pressures was examined. Results demonstrated that the VoF method obtained accurate predictions of sloshing phenomena. The oblique tank ceiling angle dampened impulsive pressure and, therefore, this angle had to be accounted for in the design of the tank.

Effect of Porosity on Dynamic Response of Additive Manufacturing Ti-6Al-4V Alloys.

Additive manufacturing is a rapidly developing manufacturing technology of great potential for applications. One of the merits of AM is that the microstructure of manufactured materials can be actively controlled to meet engineering requirements. In this work, three types of Ti-6Al-4V (TC4) materials with different porosities are manufactured using selective laser melting using different printing parameters. Their dynamic behaviors are then studied by planar impact experiments based on the free-surface velocity measurements and shock-recovery characterizations. Experimental results indicate that the porosity significantly affects their dynamic response, including not only the yield, but also spall behaviors. With the increasing porosity, the Hugoniot elastic limit and spall strength decrease monotonically. In the case of TC4 of a large porosity, it behaves similar to energy-absorbing materials, in which the voids collapse under shock compression and then the spallation takes place.

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.

The diameter effect in Bullseye powder

Detonation velocity as a function of charge diameter is reported for Alliant Bullseye powder. Results are compared to those of mixtures of ammonium nitrate mixed with aluminum and ammonium nitrate mixed with fuel oil. Additionally, measurements of free surface velocity of flyers in contact with detonating Bullseye are presented and results are compared to those of hydrocode calculations using a Jones–Wilkins–Lee equation of state generated in a thermochemical code. Comparison to the experimental results shows that both the free surface and terminal velocities were under-predicted.

Laser Shock Peening: Toward the Use of Pliable Solid Polymers for Confinement

This paper presents the first extensive study of the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP) as opposed to the exclusively used water-confined regime up to now. The use of this new confinement approach allows the treatment of metal pieces needing fatigue behavior enhancement but located in areas which are sensitive to water. Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system for any reflector (VISAR). Pressure could reach 7.6 and 4.6 GPa for acrylate-based polymer and cross-linked polydimethylsiloxane (PDMS), respectively. At 7 and 4.7 GW/cm 2 , respectively, detrimental laser breakdown limited pressure for acrylate and PDMS. These results show that the pressures produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behavior exhibited by this type of treated materials.

Spall strength of a mild carbon steel: Effects of tensile stress history and shock-induced microstructure

The effects of loading on spall strength of a mild steel are investigated systematically under the flat-top impact loading, including the effect of tensile stress history and the effect of shock-induced microstructure. Peak stress, strain rate, pulse duration, spall strength and tensile stress history are obtained directly or indirectly from free-surface velocity measurements, and the recovered samples are characterized with electron backscatter diffraction analysis. For different tensile stress histories, spall strength depends on peak stress or strain rate, depending whether spall occurs on the plateau or rising edge of a tensile pulse. Pulse duration has an effect on whether spall occurs (incipient spallation) but not spall strength. Low shock stresses induce small microstructure change (dislocations) which has weak effects on nucleation and propagation of brittle cleavage cracks, while high shock stresses lead to deformation twins (in addition to dislocations) which act as the source of crack nucleation. A model is proposed to predict the spall strength of the mild steel under arbitrary loading conditions (peak stress below the phase transition).

Effects of additive manufacturing on the dynamic response of AlSi10Mg to laser shock loading

In this study, the dynamic behaviour of light aluminum alloy AlSi10Mg obtained by additive manufacturing was investigated under laser shock loading. Two types of AlSi10Mg specimens were obtained by Selective Laser Melting (SLM) with two sets of building parameters, leading to specific architecture and microstructure compared to classical manufacturing processes. Their dynamic response to laser driven shocks was investigated on the basis of time-resolved measurements of free surface velocity, transverse visualization of shock-induced fragmentation, and post-recovery observations by means of microscopy. The results reveal a significant influence of the building parameters and SLM-inherited defects on both yield strength and spall strength values, as well as a strong dependence of high rate fracture behaviour on building direction of the material, mainly governed by melt pools shape and dissymmetry, with a combination of “interpool” and “intrapool” fracture modes.

Application of photogrammetry for spatial free surface elevation and velocity measurement in wave flumes

This article presents a method for obtaining the spatial free surface elevation and velocity field for the water surface in a wave flume over a relatively large measurement area for this type of application (approximately 1.5 m × 1.5 m). The technique employs proprietary videogrammetry software to post-process stereo images captured by multiple synchronised machine vision cameras. Dimensional resolution and other limitations are similar to that experienced for particle imaging velocimetry systems ( x, y resolution of 2 mm). Imaging of the free surface was enabled by the use of millions of bespoke slightly positively buoyant fluorescent flakes. Ultraviolet light was used as the primary light source to excite the fluorescent flakes. Reflected ultraviolet light was attenuated by a high-pass filter fitted to the cameras so that only the emitted light from the fluorescent flakes was visible. The software was validated using a simple linear translation experiment. An application is demonstrated for the radiated wave field generated from a submerged sinusoidal heaving sphere for two cases: one single and five consecutive oscillations. Results agree with linear wave theory which indicates that the floating flakes had minimal impact on the water surface particle motion at the scale tested. It is, therefore, concluded that spatial measurement of the free surface elevation and velocity using the method presented has good resolution over a large measurement field. The flakes were found to follow the free surface well, but the measurement area is constrained to where the pattern of flakes exists in the image. Hence, application of floating markers is not suitable for experiments with significant outflow/upwelling which would wash away the floating markers from the intended measurement area.

Effect of peak stress and tensile strain-rate on spall in tantalum

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. These results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements.

Ignition sensitivity study of an energetic train configuration using experiments and simulation

A full scale hydrodynamic simulation intended for the accurate description of shock-induced detonation transition was conducted as a part of an ignition sensitivity analysis of an energetic component system. The system is composed of an exploding foil initiator (EFI), a donor explosive unit, a stainless steel gap, and an acceptor explosive. A series of velocity interferometer system for any reflector measurements were used to validate the hydrodynamic simulations based on the reactive flow model that describes the initiation of energetic materials arranged in a train configuration. A numerical methodology with ignition and growth mechanisms for tracking multi-material boundary interactions as well as severely transient fluid-structure coupling between high explosive charges and metal gap is described. The free surface velocity measurement is used to evaluate the sensitivity of energetic components that are subjected to strong pressure waves. Then, the full scale hydrodynamic simulation is performed on the flyer impacted initiation of an EFI driven pyrotechnical system.

Study of laser interaction in water flow confinement at high repetition rate

This paper presents a study on the confined interaction with water flow for two successive laser pulses. The dynamic of the renewal of water films after shock produced by the laser is observed using a high speed camera. Pressure produced by the two pulses is measured from rear free surface velocity measurements using a velocimeter interferometry system for any reflector. The results show a threshold delay between the two laser pulses for which laser/target coupling of the second pulse decreases. This depends on the spot diameter, the laser intensity, and flow rate. This threshold can be calculated from the maximum jet diameter and flow rate. At an incident power density of 3 GW/cm2, a spot diameter of 1 mm, and a flow rate of 10 m/s, the maximum repetition rate ensuring target/coupling of successive laser pulses can be 1 kHz. The results open perspective for laser shock peening at high repetition rates.