Plate Impact Technique Research Articles

Plate-Impact Technique for Measuring Dynamic Friction at High Temperatures

A new pressure-shear plate impact experiment is described for measuring the friction between two sliding surfaces at high temperature. Additionally, the techniques developed for this experiment extend the capability of the general pressure-shear experiment so that the shear strength of materials can be measured at high temperatures.

The Behavior of an Elastohydrodynamic Lubricant at Moderate Pressures and High Shear Rates

Abstract Knowledge of the behavior of lubricants over a wide range of pressures and shear rates is fundamental to an understanding of elastohydrodynamic (EHD) lubrication. The mechanical properties of elastohydrodynamic lubricants have been measured by a number of researchers under the conditions of low pressures and low shear rates, as well as under high pressures and high shear rates. This paper presents experimental results for the synthetic lubricant 5P4E subjected to moderate pressures (60 MPa to 700 MPa) and high shear rates (105 s−1) using the technique of pressure-shear plate impact. Thin layers (25 μm thick) of the lubricant are confined between two hard elastic plates; the assembly is subjected to impact by a parallel plate in a manner designed to induce both compression and shear loading. For approximately 1 μs the compressed lubricant is subjected to a simple shearing motion; during that time, continuous records of the shear stress and shear rate are obtained using laser interferometry. Three test configurations were used in order to cover the pressure range, and special techniques were developed for preparing the specimen sandwich. The pressure range covered includes both the liquid and glassy states of the lubricant. The results show that a limiting shear stress model is an appropriate model for lubricant behavior under these conditions. The experimental results also show little change in the lubricant shearing behavior across the glass transition.

チタニウム粉体の衝撃圧縮における斜め衝撃波の影響

Titanium powders were shock-compressed in capsules having four kinds of cylindrical sample cavities by the flyer plate impact technique. In this study, effects due to shape changing of sample cavities in shock-compressed titanium powders were investigated. The following four types of sample cavity were utilized: type (A); 35 mm length and 6 mm diameter, type (B); 35 mm and 12 mm, type (C); 5 mm and 12 mm and type (D); 2 mm and 12 mm. In the recovered samples shock-compressed in type (A) and type (B) capsules, there observed a conical, cylindrical and spherical region involving small spherical boids. In these areas, lamellar structures due to melting were recognized with a scanning electron microscope and unknown peaks were observed with X-ray diffraction patterns. These areas are thought to be formed by effects of oblique shock waves. On the occasion of shock-compression in capsules having long length cavity, the oblique shock waves from the side wall interact in the powder samples and may generate a high temperature region, which can not be achieved only by plane shock waves. The forms of interaction vary significantly with the shapes of sample cavities.

Various microstructures suggesting possible shock compaction mechanisms

In order to reveal mechanisms of the shock compaction process for metallic powders, a shock recovery experiment for equiatomic NiTi alloy powder was performed using the flyer plate impact technique, and the microstructure at the central region of the sample after the shock treatment was observed under a metallurgical microscope. It is clearly confirmed from the microstructures that the powder is compacted by three mechanisms: (1) generation of a molten metal jet and its trapping, (2) dynamic friction between powder particles, and (3) plastic deformation around a void. The microstructure of the shock-compacted powder depends on the range of particle dimensions, which implies that the compaction process based on the above mechanisms is influenced by the powder particle size and its distribution. At the periphery of the sample, two structural features are observed. One, which is characterized by the large melt pool and the marked deformation and breakage of the powder particle, is located near the sample region to which a plane shock wave propagates through the sample capsule. The other shows the long melt band along the bottom and the side of the sample. These features are attributable to a shock wave with a nonuniform shock front and a radial shock wave, respectively.

Study of spall and recompaction of ceramics using a double-impact technique

A new plate impact technique for studying the dynamic properties of shock-damaged materials has been developed. The technique is based on impacting the specimen with two flyer plates which are separated by a small gap. Impact of the first plate causes spall in the target. Impact of the second plate closes the spall. The transmitted shock wave is monitored with a VISAR and/or manganin stress gauge. Analysis of the waves gives the properties of the damaged region. Preliminary results for copper and polycrystalline aluminum oxide have been obtained.

Pressure-shear waves in 6061-T6 aluminum and alpha-titanium

T he pressure-shear plate impact technique is used to study material behavior at high rates of deformation. In this technique, plastic waves of combined pressure and shear stresses are produced by impact of parallel plates skewed relative to their direction of approach. Commercially pure alpha-titanium and 6061-T6 aluminum are tested under a variety of pressure and shear tractions by using different combinations of impact velocities and angles of inclination. A laser interferometer system is used to monitor simultaneously the normal and transverse components of motion of a point at the rear surface of the target plate. The experimental results are compared with numerical solutions based on an elastic/viscoplastic model of the material. Both isotropic and kinematic strain hardening models are used in the computations. The results indicate that unlike the normal velocity profiles, the transverse velocity profiles are sensitive to the dynamic plastic response and, thus, can be used to study material behavior at high strain rates. For the materials tested the results suggest that the flow stress required for plastic straining increases markedly with increasing strain rate at strain rates above 10 4s −1. Hydrostatic pressure of the order that exists in the tests (up to 2 GPa) does not affect the plastic flow in 6061-T6 aluminum and appears to have at most a minor effect on the deformation of the titanium.

Continuum modelling of explosive fracture in oil shale

Fracture and fragmentation studies on 80 ml/kg Anvil Points oil shale with plate impact, Hopkinson bar, capacitor discharge, and HE techniques have shown that the dynamic fracture stress, fracture energy, and fragment size depend on the rate of tensile loading. A model coupling fracture, fragmentation, and stress wave propagation and based on the activation and growth of an initial Weibull distribution of fracture-producing flaws has been found effective in describing the observed rate-dependent fracture phenomena from static to high strain-rate impulse loading. The fracture model has been incorporated into one- and two-dimensional stress-wave computer codes and is being used to evaluate blasting geometries and stress-pulse tailoring for in situ rubblization of oil shale.

Compression wave studies on calcite rock

Dynamic compression wave studies have been conducted on three calcite rocks, Solenhofen limestone, Oakhall limestone, and Vermont marble, in the stress range of 0–5 GPa. Plate impact techniques provided transient stress wave input, and diffuse laser interferometry was used to study the evolution of these pulses through various thicknesses of rock. Both loading response and release wave response were determined from the time‐resolved particle velocity profiles. The calcite I–II and II–III phase transitions dramatically influence wave propagation in this stress region. The I–II transition begins between 0.6 and 1.2 GPa depending on rock type and is found to be consistent with a reversible, displacive polymorphic phase change. A constitutive model based on stress heterogeneity describes most of the features associated with the I–II transition. The calcite II–III transition proceeds above 2.4 GPa, and peak stress values traverse a metastable Hugoniot. This wave feature and others are more characteristic of a slower, reconstructive phase change. We conclude that stress wave features reveal striking differences in the nature of the shock‐induced calcite transitions and that these transitions strongly influence the dynamic constitutive behavior of calcite rock.

An electromagnetic shear-stress gage for large-amplitude shear waves

In many materials, especially plastics, ceramics and rocks, large-amplitude shear-wave propagation studies could provide valuable information for the development of constitutive equations. A newly developed electromagnetic-gage configuration provides an output voltage which is directly related to the dynamic shear stress in the material. The electromagnetic shear-stress gage has been used to make direct measurements of shear-wave stresses in PMMA and Solenhofen limestone. Large-amplitude shear waves were obtained with a new plate-impact technique which generates shear waves by a controlled-reflection process. The configuration of the stress gage permits it to be used simultaneously with more conventional electromagnetic velocity gages, thus providing both types of data in one experiment.

Strain rate dependence in dolomite inferred from impact and static compression studies

Plate impact techniques and laser interferometry instrumentation were used to obtain continuous one-dimensional compression wave data through states of loading and unloading on Blair dolomite in the stress range of 0 to 6.5 GPa. These data are used to infer the dynamic stress--strain response. Both the character of the wave propagation and a comparison with quasi-static uniaxial strain data obtained by other workers suggest substantial strain rate dependence in this rock. The dynamic yield stress of 2.5 GPa determined from the wave propagation data was approximately a factor of 10 higher than that observed in the quasi-static data. Wave propagation below 2.5 GPa is suggestive of viscoelastic response, and a nonlinear Maxwell constitutive equation fitted to these data provides a characteristic relaxation time of 0.01 ..mu..s. Comparison of dynamic and quasi-static loading data suggests that the large difference in observed yield stress could be the result of a transition from failure by brittle fracture to failure by intracrystalline plastic flow.

The Hugoniot and shock initiation threshold of lead azide

The Hugoniot of unreacted dextrinated lead azide has been determined using plate impact techniques in a 2.5 in. bore gas gun. The test specimens were compacted to a density of 3.4 gm/cc. Piezoresistive gauges were used to determine both the impact stress and the stress profile propagated through the sample. Each shot provided transmitted wave data for four sample thicknesses. The Hugoniot is linear up to 10 kbar. It can be represented by the equation σ=41.7 Up (σ in kilobars and Up in mm/μsec). The initiation threshold to long duration (3.5 μsec) shocks was 6.0 kbar based on evidence of reaction in the explosive inferred from increases in stress with propagation distance. Changes of wave speed were found to be noticeable only at higher stresses. Above 6 kbar the delay in initiation decreases with increasing stress levels. The initiation sensitivity to short duration (0.1 μsec) shocks was determined by impacting the lead azide with thin flyers accelerated by exploding foils. The impact velocity was determined by observation with a high-speed framing camera and the transmitted stress profiles were measured by x-cut quartz gauges. The short-pulse initiation threshold for polyvinyl lead azide is 4.2 kbar for a density of 3.6 gm/cc, and 2.1 kbar for dextrinated lead azide for a density of 2.9 gm/cc. Comparison of initiation threshold measurements suggests that there is a minimum thickness, or a run-up distance, before detonation occurs which is independent of pulse width with stresses up to 10 kbar. For long pulse (3.5 μsec) experiments with the gas gun, no evidence of detonation was detected for 1 mm thick samples. Detonation occurs with run distances in the range of 1–2 mm for impact stresses of 8.9 kbar. For stresses greater than 6.0 kbar, evidence of detonation was noted after a 2 mm run. In the thin flyer plate experiments at stresses of 8 kbar dextrinated lead azide displayed a 2 μsec initiation delay. Close to the initiation threshold the stress profiles in the explosive show a gradual transition from an unreactive shock to a stable detonation. It is postulated that reaction occuring behind the shock front produces pressure waves which travel through the explosive interacting with the unreacted explosive ahead of the reaction front causing a nonuniform rate of growth of the shock. This behavior is similar to that observed for heterogenous secondary explosives. The wave speed of the initial wave front propagating in the explosive changes abruptly from that appropriate to unreacted material to the detonation velocity.

Shock and release behavior in porous 1100 aluminum

Plate impact techniques were used to determine the dynamic response of 1.60−g/cm3 porous aluminum at energy states approaching incipient melt. Both quartz gauge and laser interferometric techniques were used to measure the shock response of this material over the stress range 0.6−7.3 GPa (6−73 kbar), which is in the region of full compaction. In addition, a nearly complete equilibrium release curve centered at 3.07 GPa (30.7 kbar) was measured using reverberating wave techniques. This is thought to represent the first accurate measurement of a release isentrope through a fully compacted state of a distended material and allows a further test of the equation of state. It is found that both the initial compaction and the release behavior of the compacted aluminum can be accurately predicted using the Mie−Grüneisen equation of state.

Stress Pulse Attenuation in Cloth-Laminate Quartz Phenolic

Using flat plate impact techniques and quartz stress gages, a study of the propagation of short-duration stress pulses in a cloth-laminate quartz phenolic composite was made. Propagation characteristics, wave shape and peak stress attenuation, were measured for 14.5 and 7 kbar initial impact stresses and 0.1, 0.2, and 0.3 μsec initial pulse durations for propagation distances up to 52 times the impactor thickness. The experimental results showed marked dispersive spreading of the wave profiles and very strong attenuation of stress, but no evidence of elastic-plastic effects. The amount of attenuation was much greater than could be attributed to "normal" atten uation in homogeneous materials with a modulus equivalent to the com posite modulus. Experimental results were compared to calculations based on a rate dependent analogy using a generalized Maxwell model. The instantaneous and equilibrium response of the model was defined solely in terms of bulk properties of the components of the composite; while a single relaxation time of 0.023 μsec was determined from experiment. Model predictions agreed extremely well with the measured profiles, giving ± 10% agreement to the measured half-amplitude wave velocities. Attenuation was predicted to better than 20% in stress, which is quite good. Quantitative agreement was found between calculations and the measured results of other investigations, as well.

Dynamic fracture of a quartz-phenolic composite under stress-wave loading in uniaxial strain

A two-dimensionally reinforced quartz-phenolic composite material is subjected to a high-amplitude stress-wave loading condition in uniaxial strain to determine its dynamic fracture behavior. This loading condition is achieved by using exploding-foil plate-impact techniques. The direction of wave propagation through the thickness of the material is normal to the principal direction of reinforcement. Two failure levels are defined: the visual threshold, and a microscopic threshold represented as the lower limit of connected delamination. These data indicate that the macroscopic and microscopic spallation thresholds of the laminated, 0-degree shingle-angle, quartz-phenolic composite are sensitive to the tensile-pulse duration, amplitude, impulse and shape at the spall-plane location. The time-dependent aspects of the spallation threshold of two-dimensionally reinforced quartz phenolic are found to obey failure theories which are rate-process oriented, and which reflect the effects of tensile-pulse duration, peak tensile stress, tensile impulse and tensile pulse shape.

Filament Fracture and Postimpact Strength of Boron-Aluminum Composites

A preceding study of the fracture behavior of shock-loaded boron-aluminum composites indicated that these materials can have a spall resistance substantially greater than that of unreinforced alu minum. In the present investigation, filament fracture and postimpact tensile strength of a similar composite system were studied. Filament damage was produced by the thin-flyer, plate-impact technique using momentum traps to suppress spall. The specimens contained approxi mately 50 volume percent boron filaments in a plasma-sprayed 6061- aluminum matrix. Filaments were in two mutually perpendicular di rections and the specimens were impacted normal to the plane of reinforcement. Filament fracture was shown to be an important impact damage mechanism in brittle-filament reinforced composites. Filaments were fractured during the initial passage of the loading wave by interactions between adjacent or contacting filaments and by focusing of stresses in isolated fibers. The tensile strength of specimens was reduced by two-thirds after severe impact. However, the impact velocity necessary to produce a substantial decrease of postimpact tensile strength was much higher than that required to produce spall in unreinforced 6061 aluminum. The impact velocity required to cause degradation in postimpact strength was independent of the tape-layup fabrication process, i.e., brazed versus diffusion bonded.

Advances For Modelling Of Geological Materials

Geologic material models have been developed, based on static triaxial data, that improve hydrocode modelling capabilities in the assessment of Kinetic Energy warheads against soil and concrete targets. Recent work has demonstrated that the flow and failure of geological materials under confinement are strongly dependent on the load path and strain rate. This paper will describe test methods that are currently being developed by DERA to generate new models that are also predictive for shaped charge, Explosively Formed Projectile and contact explosive loading against such targets. Existing GREAC (Gauged REACtive Confinement) and Hoek cell methodologies have been used to determine the physical states that control the path dependent response of concrete. The variation of the state variable has been determined under different hydrostatic loading conditions and this behaviour will form the basis from which the computational algorithm can be constructed. Elements of the supporting test programme will utilise the newly developed Stabilised Hoek Cell and this will be extended to a high pressure testing technique. Plate impact techniques will be developed to directly determine the equation of state of geologic materials and investigate the failure mechanisms (such as failure waves). A dynamic triaxial test rig will be developed in order to investigate the high strain rate behaviour of concrete. The new geologic material models will be validated against precision instrumented trials, covering a range of warhead mechanisms. Transactions on the Built Environment vol 32, © 1998 WIT Press, www.witpress.com, ISSN 1743-3509