Increasing Shock Stress Research Articles

Effects of shock stress and microstructure on shock response of Al-Hf reactive materials

Reactive materials are functional structural materials that balance structural strength and reactivity, and their shock response is greatly influenced by the characteristics of reactive components. This article aims at understanding the shock compression and reaction of Al-Hf reactive materials with two values of the Al-Hf particle size ratio α (1 µm:10 µm and 10 µm:10 µm), which were prepared through hot pressing with a compactness (the ratio of density to theoretical maximum density) greater than 96.46%. The free surface velocity of the materials was measured in the impact velocity range of 279–865 m/s. The Hugoniot elastic limit (HEL), spall strength, and Hugoniot relationships under shock stresses of 1.83–14.46 GPa were analyzed on the basis of the microstructure and phase change. Continuous Al phase was formed in the sample with α = 0.1, while more Hf–Hf contact and crushed Hf appeared in the sample with α = 1. The velocity profiles presented clear signs of the linear growth to the HEL, the slow increase to the peak, and the subsequent pullback, indicating the transition from elastic to plastic and spall behavior. The HEL fluctuated significantly with increasing shock stress, while the spall strength first increased and then decreased. The sample with a continuous Al phase exhibited a lower HEL (0.52–0.97 GPa) and higher spall strength (0.31–0.46 GPa). The microstructure had a significant effect on the shock induced chemical reaction of the materials. Shock velocity was linearly related to particle velocity for the α = 0.1 sample, while a notable quadratic growth was observed for the α = 1 sample owing to self-reactions under the lower shock stress (5.32 GPa). The intermetallic Hf4Al3 observed in the recovered product by X-ray diffraction further verified the phenomenon.

Experimental Study on the Dynamic Behavior of a Cr-Ni-Mo-V Steel under Different Shock Stresses

The present study aimed to provide new insights into the behavior of high-strength low-alloy steel under dynamic compression and to promote its use in high-stress applications. The dynamic compression response of a Cr-Ni-Mo-V steel under shock stresses ranging from 3.54 GPa to 19.76 GPa was investigated using loading technology. The free surface velocity of the specimen was measured using a displacement interferometer system with the range of 166–945 m/s. The Hugoniot elastic limit (HEL), spalling fracture, and microstructure evolution of specimens under different shock stresses were determined. The results showed that an α→ε phase transition occurred in the material at an impact stress of 15.63 GPa, leading to a change in particle velocity. The relationship between the shock wave velocity and particle velocity was found to be linear. The HEL of the steel was found to be consistent at 2.28 GPa, while the spall strength showed a more complex relationship with the increasing shock stress. Initially, the spall strength increased and then decreased with increasing shock stress before increasing again after the phase transformation. The fracture mode of the steel shifted from brittle fracture to ductile fracture with the increasing impact stresses, which is related to the previous plastic deformation under different impact loads.

Constraining the release of Sn to the ambient melting point following shock loading using time-resolved x-ray diffraction

The formation of liquid following release from a shocked state governs the transition from spall to cavitation and the formation of ejecta in metals. In order to build physics-based models of these processes, it is necessary to critically evaluate the relative importance of kinetics and entropy generation during the release along with the accuracy of multiphase equations of state. Tin (Sn) has served as a testbed for a variety of experiments examining strength and ejecta due to its accessible melt boundary and solid–solid phase transitions. This work presents experiments examining the phase evolution of high purity Sn following the shock and release to ambient stress near the melting point. Sn is found to release to states between its ambient solidus and liquidus from approximately 19 to 33 GPa under uniaxial loading, with the two-phase region being characterized by a reduction in the intensity of the (220), (211) β-Sn doublet. Jetting experiments performed at 27–28 GPa exhibit comparable diffraction patterns with what is observed following the uniaxial release. The solid fractions of β-Sn in the ambient mixed phase region are found to decrease linearly with increasing shock stress as increasing liquid Sn is formed. The results provide much needed information for interpreting measurements of dynamic strength at a high strain rate and experiments examining cavitation and shallow bubble collapse in Sn.

Multi-field coupling mechanical failure of Pb(Zr0.95Ti0.05)O3 ferroelectric ceramics under shock compression

PZT 95/5 ferroelectric ceramics have been used in shock-driven pulsed-power supplies for many years; their mechanical failure under shock compression plays an important role in their applications. Multi-field coupling shock experiments have been conducted to understand such failure by measuring the interface particle velocity of the quartz buffer/window interface, and the wave evolution was analyzed with or without the existence of failure zone inner PZT 95/5. Results confirm that the delay time of the failure wave decreased with increasing the electric field, and when the electric field reaches the dielectric strength, the electrical breakdown happens, which makes the threshold stress of mechanical failure shift to the low stress (2.0 GPa) and the delay time of the failure wave decreases dramatically at high stress. The phenomena that the delay time decreases with increasing shock stress are also observed. At the same time, ferroelectric → antiferroelectric phase transformation makes it so that the delay time of the poled state is longer than that of the unpoled state, so it has the effect of phase transformation toughening.

Effects of shock-induced phase transition on spallation of a mild carbon steel

Effects of shock-induced phase transition (PT) on spallation of a mild carbon steel are investigated via plate impact experiments at impact velocities of 0.433–1.163 kms−1. The free surface velocity history is recorded to deduce spall strength and wave propagation/interaction within the sample. Postmortem samples are characterized with scanning electron microscopy and electron backscatter diffraction. Detailed wave analysis indicates that the PT affects wave propagation and interaction. Such wave propagation and interaction finally lead to the change of spall strength and spall plane position for different stress ranges. In the peak shock stress range of 13–17 GPa (slightly above the PT stress), the release fan of the PT wave accelerates the free surface but is not involved in the major spallation, giving rise to an overestimation of spall strength by the conventional acoustic method. However, at higher shock stress above PT (>17 GPa), the release fans of the PT wave interact with each other and induce secondary spallation, and the acoustic method is applicable in such cases; the major spall plane moves to the sample central region, and multiple secondary spall planes appear. In addition, the reverse PT upon release facilitates nucleation and growth of deformation twins which subsequently act as nucleation sits of voids during spallation. With increasing shock stress, the spall damage exhibits mode transitions from brittle damage (transgranular cleavage cracks), to mixed brittle and ductile damage, and to ductile damage (void nucleation and growth).

The characteristics of void distribution in spalled high purity copper cylinder under sweeping detonation

The effects of different peak compression stresses (2–5 GPa) on the spallation behaviour of high purity copper cylinder during sweeping detonation were examined by Electron Backscatter Diffraction Microscopy, Doppler Pins System and Optical Microscopy techniques. The velocity history of inner surface and the characteristics of void distributions in spalled copper cylinder were investigated. The results indicated that the spall strength of copper in these experiments was less than that revealed in previous reports concerning plate impact loading. The geometry of cylindrical copper and the obliquity of incident shock during sweeping detonation may be the main reasons. Different loading stresses seemed to be responsible for the characteristics of the resultant damage fields, and the maximum damage degree increased with increasing shock stress. Spall planes in different cross-sections of sample loaded with the same shock stress of 3.29 GPa were found, and the distance from the initiation end has little effect on the maximum damage degree (the maximum damage range from 12 to 14%), which means that the spallation behaviour was stable along the direction parallel to the detonation propagation direction under the same shock stress.

3-D characterization of incipient spallation response in cylindrical copper under sweeping detonation

Abstract

Multidimensional Study on Spall Behavior of High-Purity Copper Under Sliding Detonation

The spall behaviors of high-purity copper samples with different heat treatment histories were investigated using optical microscopy and X-ray computer tomography (XRCT). The spall samples were obtained by sliding detonation experiments at low pressures (2 to 4 GPa). It was found that the spall planes created by sliding detonation in this experiment are similar to the spall planes created by plate impact test, except for more secondary damage residual around the main spall plane. The results of damage degree, the shape, and the distributions of voids obtained by the means of metallography (2D) and XRCT (3D) statistics were consistent. For similar microstructure, the maximum damage degree and damage zone width increase with increasing shock stress. Whereas the ranges of voids distribution parallel to the shock stress direction decreases with the increasing of shock stress. For the shock stress is similar, the shape of voids in annealed samples are closed to spheres, their mean flatness is 0.51. The voids in samples with thermo-mechanical treatment histories are sheet like with mean flatness 0.16. The difference in grain size (40 and 9 μm) may be the main reason of such difference.

Electrical response of Pb(Zr0.95Ti0.05)O3 under shock compressions

PZT ((Pb(Zr0.95Ti0.05)O3)) 95/5 ferroelectric ceramics have been used in shock-driven pulsed-power supplies for many years. Their electrical responses under the high electric field strengths generated by shock compression are important in various applications. Shock experiments were conducted to understand the complicated electromechanical behavior by measuring the depoling current. An equivalent circuit was built to analyze the dielectric parameters of PZT 95/5 ceramics under shock compression. The results indicated that the internal resistivity and breakdown strength decrease with increasing shock stress and abnormal behavior, whereby the internal resistivity increases with increasing electric field strength, which is also observed at 4.3 GPa. Cracks induced by mechanical failure under high stresses were found to be related to the leakage current of the samples and to play an important role in determining the electrical response of PZT 95/5 under shock compression.

Failure behavior of Pb(Zr0.95Ti0.05)O3 ferroelectric ceramics under shock compression

PZT 95/5 ferroelectric ceramics have been used in shock-driven pulsed-power supplies for many years; their mechanical failure under shock compression plays an important role in dielectric breakdown. Shock experiments have been conducted to understand such failure by measuring the velocity of the free surface or the PZT/sapphire interface. Results confirm that delayed failure exists in PZT 95/5 before dynamic yielding at 2.4 GPa; multipoint measurements indicate that the failure layer in PZT 95/5 was not a plane but a rough front. The delay time and velocity of this layer had been determined by measuring samples of varying thicknesses at fixed pressure; results indicate that this velocity is the same as the shock-wave speed and the delay time decreases with increasing shock stress. At a shock stress of 4.9 GPa, the delay time falls to zero and a ramp wave is observed. This kind of failure is a new phenomenon in electric breakdown of PZT 95/5 under shock compression.

The deviatoric response of an alumina filled epoxy composite during shock loading

The deviatoric response of a particulate alumina-epoxy composite to shock loading has been investigated using manganin stress gauges sensitive to the lateral component of stress. Results show that the lateral stress and thus the shear strength are near constant behind the shock front, indicating that the presence of alumina has a diluting response of the epoxy resin. The shear strength has been observed to increase with increasing shock stress, in agreement with comparisons between the measured shock stress and the calculated hydrodynamic pressure. Finally, the Hugoniot elastic limit of this material has been estimated at ∼1.6GPa by the intersection between the elastic and inelastic shear strengths.

Measurement and Analysis of Spall Characteristics of High-PureAluminium at One-Dimensional Strain Loading

With an impact velocity varying from 196.9 m/s to 317.9 m/s and ratios offlyer/sample thickness of 2:4 and 3:6, the free-surface velocityprofiles of the shock compressed high purity aluminium (HPA 99.999%)samples are measured with a velocity interferometer system for anyreflector. Based on the vibrating features of the velocity profiles,the damage behaviour of HPA is analysed. The results indicate that thevibrating amplitude increases with increasing shock stress, and thesubsequent reverberations describing the spall become more obvious.When the shock stress in the material is below a critical or smallerthan the threshold level, the free-surface velocity profile replicatesvirtually the form of the compression pulse inside the sample. When theimpact stress exceeds a critical value (1.4 GPa), the micro damage wouldappear, and the free-surface velocity profile changes significantly,showing a series of short-duration reverberations in the profile. Whenthe impact stress exceeds the threshold of damage, a compressivedisturbance called the ``spall pulse'' appears in the free-surfacevelocity profile, and the subsequent reverberation becomes regularagain. The measured spall strength of HPA is much higher than those ofcommercially pure aluminium reported in many references. In addition,the strength of HPA is similar to that of single-crystal aluminium.

Shock compression response of nanoiron powder compact

The shock compression response of nano-Fe powder (∼25nm) pressed to ∼35% theoretical maximum density was determined based on shock stress and wave velocity measurements using piezoelectric stress gauges. The obtained data show a discontinuity in shock wave velocity plotted against particle velocity and an inflexion in specific volume from compression to expansion with increasing shock stress. It is found that the Hugoniot of 25nm Fe powder cannot be fully described using analytical models that are otherwise capable of predicting the Hugoniot of micron-sized powder or highly porous materials.

The shock properties of a La2O3 filled silicate glass

A survey of the shock properties of the silicate glass, LACA has been carried out using manganin stress gauges. The principal Hugoniot has been measured and found to have significantly higher values than for other common silicate glasses. Gauges mounted on the rear of the target (supported with a block of polymethylmethacrylate) show reloading signals superimposed on the main compressive shock pulse. This has been interpreted as evidence of dynamic compressive failure (the failure wave or front). Manganin gauges mounted so as to be sensitive to the lateral component of stress support this hypothesis. Finally, failure front velocities, measured using known lateral gauge separations increase with increasing shock stress, tending towards the shear wave speed.

The equation of state of two alumina-filled epoxy resins

The shock Hugoniots of two different alumina–epoxy particulate composites have been measured in terms of shock velocity, particle velocity and shock stress. Both shock velocities and shock stresses increase with increasing amounts of alumina loading. The shock velocity–particle velocity relationships have been shown to be linear, and that with increasing alumina, behaviour shifts from a viscous to a response more commonly seen in metals and ceramics. Comparisons of the calculated hydrodynamic pressure and shock stress suggest that these materials have a constant shear strength with increasing shock stress. Finally, comparison of the material with the highest amount of alumina, with the work of others, shows close agreement.

Lateral stress measurements and shear strength in a shock-loaded γ- TiAl alloy

Manganin stress gauges have been placed in samples of a γ-titanium aluminide alloy in such orientation that renders them sensitive to the lateral component of stress during one-dimensional shock loading. The impact stresses were in the range 1.8 to 6 GPa. The resultant stress histories show clear evidence of a transition from elastic to plastic behaviour, which, through the elastic relations correlates with the Hugoniot Elastic Limit of this material. In combination with the longitudinal stress data, it has been possible to use the lateral stresses from this series of experiments to calculate the shear strength during shock loading. The Hugoniot of the γ-titanium aluminide studied is seen to be significantly different than a standard γ-β Ti-alloy such as Ti-6Al-4V. The results show this titanium aluminide alloy displays a significant degree of hardening with increasing shock stress.