Adiabatic Shear Localization Research Articles

Adiabatic shear in annealed and shock-hardened iron and in quenched and tempered 4340 steel

Adiabatic shear localization is a catastrophic failure mechanism which can occur in ductile metals under high strain rate loading. This mechanism is driven by a thermal instability process in which rapid temperature rise due to plastic work couples with thermal softening to cause uniform deformation to collapse into narrow bands of intense shear within which material ductility is exhausted. Adiabatic shear localization is studied in three ferrous metals: annealed Armco and as-received Remco iron, both of which are high purity alpha iron; shock-hardened Remco iron; and 4340 steel quenched and tempered to a range of hardness levels. Using a compressive split-Hopkinson bar, punching-shear experiments were performed at room and elevated initial temperatures at shear strain rates of up to 45 000 s−1. Optical and scanning electron microscopy was performed on the deformed shear specimens to determine the extent of shear localization and mode of failure. Experimental evidence showed that the tempered 4340 steels were susceptible to localization through adiabatic shear banding; however, as-received and shock-hardened Remco iron and annealed Armco iron were not. Finite element simulations of the experiments were performed utilizing a user material subroutine developed as part of this research. This constitutive routine incorporates two adiabatic shear failure criteria, namely (i) maximum shear stress with a minimum critical shear strain rate and (ii) flow localization. These criteria proved to be capable of predicting the onset of an instability; however, the deformation which follows the instability was not predicted well.

Response of thermites to dynamic high pressure and shear

The paper begins with a review of the literature on high pressure and shear properties of materials. This is an area in which further interest was recently stimulated by published Russian work on s...

Influence of microstructure on adiabatic shear localization of pre-twisted tungsten heavy alloys

The influence of microstructure of pre-twisted tungsten heavy alloy (WHA) on the adiabatic shear localization under dynamic loading was investigated experimentally. A combined compression and shear load was applied to a cylindrical specimen with its axis inclined to the loading axis by use of a split Hopkinson pressure bar (SHPB) apparatus which generates a strain rate as high as 10 3 s −1. A relatively narrow shear band was formed approximately along the maximum shear stress direction, followed immediately by catastrophic fracture. Electronic microscopy and optical microscopy observations were made on the fracture surfaces and the microstructures of un-fractured specimens. It is found that the initial microstructures such as the aspect ratio and orientation of the W-grains have significant influence on the tendency to the shear band formation. The adiabatic shear band has general trends to propagate along the Ni–Fe matrix and more energy is needed to shear the tungsten grains. It is suggested that further studies on the optimization of microstructure and control of the fabricating process to promote the shear band formation be conducted in order to improve the penetration performance of the tungsten heavy alloy.

Strain Gradient Effect on Initiation of Adiabatic Shear Localization in Metal Matrix Composites

In this paper, the strain gradient effect on the initiation of adiabatic shear localization in particle reinforced metal matrix composites (MMCp) is investigated. By incorporating our newly developed strain gradient law into the classical scheme of the stability analysis, the analytical form of the critical strain is determined. The results demonstrate that high strain gradient in the matrix provides a strong driving force for the initiation of adiabatic shear localization in MMCp.

Dynamic deformation behavior of tungsten-fiber/metallic–glass matrix composites

Ballistic tests were performed firing 85% fiber volume fraction tungsten/Zr 41.25Ti 13.75Cu 12.5Ni 10Be 22.5 metallic-glass matrix composite rods into 4130 steel and 6061 T651 aluminum targets. The composite rods failed by localized adiabatic shear banding and exhibited self-sharpening behavior. Penetrator performance was 10–20% better than tungsten heavy alloy penetrators of comparable aspect ratio.

Microstructures and Properties of Metal Reinforced Zr57Nb5Al10Cu15.4Ni12.6Bulk Metallic Glass Composites

AbstractTo increase toughness of a metallic glass with the nominal composition Zr57Nb5Al10Cu15.4Ni12.6, it was used as the matrix in the composites reinforced with W fiber, W particles, and W/Re particles. Microstructures of the composites are studied by scanning electron microscopy and electron microprobe. Compression results are presented. Dynamic deformation behavior of the composites is studied in ballistic tests. The tests were performed firing composite rods into 6061 T651 aluminum target and 4130 steel targets. The composite rods failed by localized adiabatic shear banding and exhibited self-sharpening behavior.

Adiabatic shear localization in the dynamic punch test, part I: experimental investigation

Abstract The behavior of 1018 steel, 6061-T6 aluminum, and titanium 6%Al–4%V alloy during a dynamic punch test is investigated. Specifically, the possibility and affects of adiabatic shear localization are examined. The three materials are chosen to encompass a wide range of physical properties. Punch tests are conducted at average shear strain rates from 10 −3 to 10 4 s −1 on a servo-hydraulic compression machine, a mechanical press, and a Hopkinson bar apparatus. Experimental load displacement curves are obtained and fractographic and metallographic analysis is performed. Finite element simulations of punching operations are performed in the second part of this two part investigation to examine internal deformation not visible during experimental tests. More specifically, the role of adiabatic shear localization in burr formation is determined. Adiabatic shear localization occurs in the titanium alloy for the mechanical press and Hopkinson bar tests, while localization is not present for the 6061-T6 aluminum in any test. The 1018 steel begins to exhibit some transitional behavior toward shear localization in the Hopkinson bar experiments. It is seen that in the materials tested here, a combination of high strength and low strain hardening make a material more susceptible to adiabatic shear localization in punching operations.

Recovery Experiments for Adiabatic Shear Localization: A Novel Experimental Technique

A novel experimental technique has been developed that allows the recovery of specimens within which an adiabatic shear band has been grown as the result of a single torsional pulse. The technique uses a new device called the compression-torsion Kolsky bar; this is an adaptation of the classical torsional Kolsky bar in which both compression and torsion are stored in the clamped length. Recovery of the specimen is obtained by causing the transmitted compression pulse to reflect from the free end of the output bar as a tensile pulse which then causes failure of the specimen/output-bar interface. All further pulses then cause no more than rigid motions of the specimen. Thus the specimen is recovered after it has been loaded by a single torsional pulse; microscopic examination of the recovered specimen now allows the rigorous determination of the evolution of microstructure and the identification of deformation mechanisms that are active under the dynamic shear loading. As an example of the utility of this recovery technique, its application to the study of adiabatic shear localization in alpha-titanium is presented. The technique allows a systematic study of the microscopic mechanisms that drive the initiation and growth of adiabatic shear bands by recovering samples within which shear bands have been grown under controlled conditions; further characterization through sectioning, followed by optical, scanning, and transmission electron microscopy is then possible. The technique also allows one to study the influence of the shape and size of geometric defects (for example, notch tip radius and notch depth) on the initiation of adiabatic shearing failures. The effect of normal stresses on the susceptibility of a material to shear localization can also be investigated.

Adiabatic shear localization in the dynamic punch test, part II: numerical simulations

Abstract The behavior of 1018 steel, 6061-T6 aluminum, and titanium 6%Al–4%V alloy during the dynamic punch test is investigated using the finite element method. Specifically, the possibility and effects of adiabatic shear localization and its role in burr formation are examined, and comparisons to experimental tests in the first part of this two part study are made. A maximum stress criterion involving strain and strain rate hardening and thermal softening is used to determine the occurrence of shear localization in the simulations. It is observed that adiabatic shear localization occurs in the simulations of the titanium alloy. This material exhibits narrow regions of concentrated shear strain during the deformation, and the shear localization criterion is satisfied in these regions. The strain is more widely distributed in the other two metals, and the same criterion is not satisfied. In the calculations of the shear localization criterion it is seen that strain rate hardening has a significant effect when compared to strain hardening and thermal softening. Also, contact between specimen and punch is lost around the center of the punch during operation. This loss of contact is important as it leads to higher stress concentrations at the punch corner and dishing of the blank.

Adiabatic shear localization in the impact of edge-notched specimens

Impact experiments are performed on edgenotched specimens in the two-dimensional punch geometry. Materials tested include 18Ni(350) maraging steel; S7 tool steel; 4340, 300M, HP 9-4-20 and D-6ac ultra high-strength steels; and Ti6Al4V alloy. These materials have shown a high susceptibility to dynamic shear failure in previous studies. Impact velocity ranged from 25 m/s to 45 m/s, and shear bands were found to form at the notch tip and at the die corner on the back side of the specimen for all materials tested. Metallurgical analysis confirms the existence of adiabatic shear bands followed by a crack propagating through the fully developed shear band. High-speed photography was used to observe the initiation of adiabatic shear bands shortly after impact. Laser-etched lines on the specimen surfaces allowed the determination of the time of impact and the initiation time of shear failure. The elapsed time between the two was used to estimate the stress intensity factor at the time of shear band initiation. Comparisons of shear band initiation stress intensity factors at the notch tip and die corner are made. It is seen that the shear bands initiate at approximately the same stress intensity factor at both the notch tip and die corner. Finite element simulations support the use of a square root singularity for the stress in the plate near the corners of a deformable punch or die.

Adiabatic Shear Localization in a Liquid Lubricant Under Pressure

Experimental tests with a high-pressure flow visualization cell clearly reveal that the shear deformation in a lubricant film at elevated pressures can localize by a mechanism vastly different from previously reported mechanically-induced shear bands. Verification by numerical solution of the energy equation shows that for a stress-controlled experiment, provided that the Brinkman number is sufficiently large, the local temperature experiences a rapid rise. The rapid rise of temperature along with the local rate of shear favors an unstable temperature profile. The research provides valuable insight into the behavior of lubricants under extreme conditions since this phenomenon may be operating in the thermally dominated regime of EHD traction. Furthermore, the fact that the hot shear band is isolated from the metal boundaries by a cooler liquid, may confound recent attempts to infer the shear stress distribution in a concentrated contact from IR temperature measurement of the metal temperature distribution.

On the localization of shearing deformations in tungsten heavy alloys

This paper presents the results of an investigation into the dynamic shearing deformations of a WNiFe heavy alloy containing 91% W and with the Ni and Fe in a 7:3 ratio. The technique used to develop the dynamic shearing deformations is the torsional Kolsky bar, which generates shear rates as high as 10 3 s −1 for times of several hundred μs. Adiabatic shear localization has been observed in the high-rate shearing tests; relatively narrow shear bands are formed, followed immediately by catastrophic fracture. The deformed microstructures are examined using scanning electron microscopy, optical microscopy, and quantitative image analysis of the micrographs. Examination of the deformed microstructures supplies new insight into the mechanisms through which large dynamic shearing deformations occur in this tungsten heavy alloy.

Formation of adiabatic shear bands in eutectoid steels in high strain rate compression

A detailed study has been made of the localized adiabatic shear band formation in a plain carbon and a low alloy eutectoid rail steel subjected to high strain rate compression at initial deformation temperature of 298, 453 and 623 K. Localized adiabatic shearing due to impact is found to be favored with alloy steels and decreasing temperatures of deformation. It is shown that the deformed and transformed shear bands rather than being two separate phenomena are only an outcome of the extent of adiabatic strain localization occuring during deformation; the deformed bands forming with lesser localized flow and the transformed bands forming with extensive localized flow. This study explains convincingly the formation of the white phase on the surface of the rail heads during wheel-rail contact as due to the coalescence of the numerous adiabatic shear bands.

Microvoid formation during shear deformation of ultrahigh strength steels

Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamic conditions, aimed at elucidating the fundamental mechanisms of shear localization underlying both adiabatic shear localization and fracture processes. Experiments were also devised to study the effect of hydrostatic pressure and austenitizing temperature on the critical strain to localization. Experimental evidence strongly suggests that strain localization in the steels investigated is driven by microvoid softening controlled by nucleation at 100 nm scale particles. This is supported by the observed pressure dependence of the instability strain, enhanced resistance to shear instability with particle dissolution, and direct observation of microvoids at these particles in deformed material. For the steels investigated with approximately equivalent strength levels, a direct correlation between the crack extension force and shear instability is demonstrated. Consequently, both fracture toughness and shear localization are dependent on the size, type, and distribution of second phase particles.

The structure of adiabatic shear bands in metals: A critical review

A review of the structure of adiabatic shear bands in metals is presented. Shear bands are redefined as being either “transformed” or “deformed” according to how the prior shear deformation is partitioned between two discrete zones in metallographic section. Metals are then classified by their general tendency to form these two types of shear zone during adiabatic shear deformation, based on available literature. Metals of low thermal diffusivity and of low resistance to adiabatic shear localization tend more readily to form “transformed” shear bands; these metals are also capable of transforming to other phases at elevated temperature (and pressure), and forming martensite on rapid cooling to room température. Shear bands of “transformed” appearance can also form in other metals during extremely localized adiabatic shear deformation resulting from the effect of localized plastic flow and elevated temperature alone. The role of phase transformations themselves in promoting the formation of “transformed” shear bands cannot be isolated using the arguments presented in this work, and may even by incidental.

Adiabatic shear localization in titanium and Ti-6 pct Al-4 pct V alloy

Ballistic impact experiments were conducted on 12.5 mm thick commercial purity titanium and Ti-6 pct Al-4 pct V alloy plates using steel “stepped” projectiles with 10.5 mm diameter. The impact velocities varied between 578 m per second and 846 m per second, and a flash X-ray technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage mechanisms associated with impact (shear band formation, shock wave propagation, and dynamic fracture) were analyzed by optical, and scanning and transmission electron microscopy. Elliptical and spherical cavities were observed along the bands. Microindentation hardness differences between the bands and adjacent regions were slight for the targets; for the projectiles, the hardness in the band was significantly lower than that of surrounding regions. Observation of the fractured regions along the bands showed unique features indicating possible melting. Transmission-electron microscopy of a shear band in titanium revealed microcrystalline features (∼0.3 µm diameter) with poorly defined grain boundaries.

The ignition of a thin layer of explosive by impact; the effect of polymer particles

In an earlier paper Heavens and Field described the use of high speed photographic and pressure-measuring techniques to study the behaviour of thin layers of pure explosive under drop-weight impact. When ignition occurred it invariably did so at local sites after bulk plastic flow of the sample. The present paper investigates the effect of added particles to the explosive layer. The case of the hard, high-melting-point particle is reasonably well understood. As shown by Bowden and Gurton the ‘hot­-spot’ temperature is then usually controlled by the melting point of the particle. However, as this paper confirms, some relatively soft, low-melting-point solids such as certain polymers can also sensitize explosive layers. Both chemical and mechanical explanations are examined. Chemical effects were assessed by using thermogravimetric analysis. High speed photography, with the sample between transparent anvils, was used to photograph impacts on polymers, explosive samples and layers of ex­plosive with polymer added. The sensitizing effect is shown to be a mecha­nical one with the production of free radicals by the polymer only of secondary importance. Polymers that sensitize are those that fail catastrophically either by fracture or localized adiabatic shear, and that have a low specific heat, latent heat and thermal conductivity. Hot spots in these polymers, produced during rapid deformation, can greatly exceed the polymer’s softening point. This was confirmed by separate experiments with a friction apparatus with hot-spot temperatures re­corded by using infrared techniques. The experiments with the polymeric samples also have implications for studies of the fracture, friction, erosion and wear of polymers.

Solid particle erosion mechanisms in copper and two copper alloys

The amount and mechanisms of metal loss has been studied for oblique and normal impact on copper, 70/30 brass, and precipitation hardened Cu-2 pct Be, using steel ball bearings and velocities from 138 to 300 m/s. The weight losses are roughly the same for the three alloys though the mecahnisms of flow and fracture change from highly plastic for copper to pronounced adiabatic shear localization for the Cu-Be alloy. At these high velocities and ball sizes the deformation is adiabatic, thus localizing the flow and enhancing loss compared to a slow isothermal indentation. Also, inertial forces are large at the base of any extruded lip, thus playing a central role in removing metal. For normal impacts the principle weight loss occurs where the impacts partially overlap, and stems from the concentration of strain at local irregularities in the surface.

Adiabatic shear localization and erosion of strong aluminum alloys

A high strength aluminum alloy, 7075-T651, was eroded by ball-bearing impact at high velocities (about 180 m s t̄1) at normal and oblique incidence. Scanning electron microscope studies of the target and ejected chip show that the fracture surface is covered with myriad small globules (roughly 1 μm in dimension) whose form was clearly established by the effect of surface tension on a fluid metal. It is argued that for high strength age-hardened alloys like this, chips are formed by fracture across molten adiabatic shear bands which then quickly solidify after separation to give the globular surface. Such structures were not found on eroded surfaces of the much softer commercially pure aluminum.