J. Duffy

An Experimental Study of the Formation Process of Adiabatic Shear Bands in a Structural Steel

Abstract A series of experiments is described in which the local temperature and local strain are measured during the formation of an adiabatic shear band in a low alloy structural steel (HY-100). The specimen employed consists of a short thin-walled tube and the required rapid deformation rates are imposed by loading the specimen in a torsional Kolsky bar (split-Hopkinson bar). The local temperature is determined by measuring the infrared radiation emanating at twelve neighboring points on the specimens surface, including the shear band area. Indium-antimonide elements are employed for this purpose to give the temperature history during deformation. In addition, high speed photographs are made of a grid pattern deposited on the specimens surface, thus providing a measure of the strain distribution at various stages during shear band formation. By testing a number of specimens, it is possible to form a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequent loss in the load carrying capacity of the steel. It appears that plastic deformation follows a three stage process which begins with a homogeneous strain state, followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. It is estimated that the shear band propagates at a speed of about 510 m/s in the material tested. Results also include data on the stress-strain behavior of HY-100 steel over the temperature range —190°C to 250°C and at quasi-static as well as dynamic strain rates.

Measurement of the Temperature Profile During Shear Band Formation in Steels Deforming at High Strain Rates

Abstract A torsional Kolsky bar (split-Hopkinson bar) was used to deform tubular specimens of AISI 1018 cold rolled steel and AISI 1020 hot rolled steel at a nominal strain rate of 103s−1. Shear bands were observed to form in both steels, and the temperature of the material in the bands was measured by determining the infrared radiation emitted at the metal surface. For this purpose, a linear array of ten indium-antimonide detectors was used to determine temperature history at ten neighboring points lying across the projected path of the shear band. Results showed that shear bands in these steels are relatively wide, that the maximum temperature rise in the band is about 450°C and that the temperature distribution across the band is consistent with results of stability analyses. The two steels have very different work hardening rates and the strain at which localization is first observed is very different for the two steels : in the cold-rolled steel it occurs at about 15% strain, while in the hot-rolled the strain is near 100%. This result also is consistent with predictions of the analyses.

Adiabatic shear bands in a TI-6Al-4V titanium alloy

Dynamic (γ≈103⧹sec) torsional experiments were performed to investigate the process of initiation and formation of adiabatic shear bands in Ti-6Al-4V alloy. In this study, thin-wall tubular specimens were deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar) . Through high-speed photography of a grid pattern previously printed on the specimens outer surface, the local strain and the local strain rate were found to be in the range of 75%–350% and 8.0×104⧹sec, respectively. The width of the shear bands ranged from 12–55 μm. In addition, an array of infrared detectors was employed to measure the local temperature rise during the deformation process. A peak temperature of 440–550°C was found in the various tests. The fracture surface of the shear band material was characterized by (1) regions of elongated dimples within which no second phase particles were observed, and (2) regions with a relatively flat and smeared appearance. There was no clear evidence based either on the appearance of the shear band in SEM or the measured temperature rise to suggest that the material within the shear band had undergone a phase transformation.

The determination of dynamic fracture toughness of AISI 4340 steel by the shadow spot method

Abstract Dynamic crack propagation experiments have been performed using wedge loaded double cantilever beam specimens of an austenitized, quenched and tempered 4340 steel. Measurements of the dynamic stress intensity factor have been made by means of the optical method of caustics. The interpretation of experimental data, obtained from the shadow spot patterns photographed with a Cranz-Schardin high speed camera, is based on an elastodynamic analysis. The instantaneous value of the dynamic stress intensity factor K d I is obtained as a function of crack tip velocity. Finally, the interaction of reflected shear and Rayleigh waves with the moving crack tip stress field is considered.

On the measurement of local strain and temperature during the formation of adiabatic shear bands

Abstract A series of experiments was performed to study the process of adiabatic shear band initiation and formation in steels. The steels include a low carbon cold-rolled steel and three martensitic steels (HY-100 and two tempers of AISI 4340 VAR steel of varying hardness). In each case the specimens are machined as thin-walled tubes that are deformed dynamically in a torsional Kolsky bar (torsional split Hopkinson bar). Shear band initiation and formation are observed by ultrahigh-speed photography of a fine grid pattern deposited on the specimens surface. It is shown that the critical strain for shear band initiation depends on the magnitude of a preexisting defect, in accordance with the predictions of Molinari and Clifton, J. Appl. Mech., 54 (1991) 806–812. Ultrahigh-speed photographs of the grid pattern show that local strains of 100–1000% may be attained and that the local strain rates reach 10 5 s −1 . In addition, the local temperature in the shear band is measured by employing an array of small high-speed infrared detectors that provide a plot of temperature as a function of time and position. Within the shear band region, temperatures of 600 °C have been measured.

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈103/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described.

Strain rate history effects and observations of dislocation substructure in aluminum single crystals following dynamic deformation

Abstract Single crystals of aluminum are deformed in shear at a number of constant strain rates in the range 10−5–1600 s−1 and to strains of about 20%. These constant-rate tests are supplemented by a series of jump tests in which a sharp increment in strain rate is imposed during the quasi-static straining. Thin foil samples of the strained specimens are examined by transmission electron microscopy (TEM). The constant-strain-rate tests show a linear increase in flow stress with the logarithm of strain rate up to 500 s−1, followed by a more rapid increase at higher strain rates. TeM observations after deformation at quasistatic strain rates reveal a cellular dislocation arrangement that develops into well-outlined subgrains as strain increases. Cells are also visible after dynamic deformation, but these cells are about one-quarter of the size of those produced by quasi-static deformation and their walls are not as well defined. Furthermore, results indicate that, as the cell size is reduced, both the flow stress and the strain rate sensitivity increase. The jump tests show a pronounced strain rate history effect and their results permit the calculation of an activation volume applicable to quasi-static straining. Results of this calculatin, which is based on macroscopic measurements and on the theory of thermally activated deformation of metals, are compared with an activation volume based on the dislocation densities measured after deformation.

The effects of tempering and test temperature on the dynamic fracture initiation behavior of an AISI 4340 VAR steel

Abstract An investigation was conducted into the effect of tempering, test temperature and loading rate on the fracture initiation behavior of an AISI 4340 VAR steel. The fracture initiation tests were conducted by stress wave loading of a prefatigued circumferential crack in a notched round bar. The geometry provides for plane strain conditions at the fracture site, and the instrumentation gives records of average stress at the fracture site and of crack opening displacement, both as functions of time. A stress intensity rate KI of about 2 × 10 6 Mpa m 1 2 s −1 is attained by this technique, and a comparison is presented with results obtained in static loading with the same geometry. Finally, the dynamic and quasi-static stress-strain behavior in shear was determined by loading a thin-walled tubular specimen in a torsional Kolsky bar. The strain rates were γ = 103s−1 and γ = 10−4s−1. In addition, Charpy specimens of the various tempers were tested. These showed a toughness trough indicative of tempered martensite embrittlement in those specimens tempered at 350°C. However, in the fracture initiation tests with the notched round bar the embrittlement trough was observed only in tests conducted below room temperature. Extensive quantitative fractography was performed on the plane strain fracture specimens to identify the dominant mode of fracture initiation for each temper at the different rates and test temperatures.

Strain rate history effects in LiF single crystals during dynamic loading in shear

Abstract A description is presented of experiments performed with LiF single crystals loaded at high strain rates in simple shear by means of a torsional Kolsky bar. Four specimens are used in each test; they are shaped as rectangular parallelepipeds and the tractions are applied in the [1 1 0] directions on the faces parallel to the ( 1 1 0) planes. The experiments provide a measure of flow stress as a function of strain at strain rates of 400 and 800 s−1; the results are compared with those of quasi-static tests performed at strain rates of 4.8 × 10−5 and 1.6 × 10−4s−1 in the same apparatus. In addition, incremental strain rate experiments were performed. These consist in imposing first a quasi-static strain rate which is increased suddenly and with no intermediate unloading to a dynamic rate. The results showed that the flow stress of LiF is strongly sensitive to strain rate but that the effects of strain rate history, although present, are not pronounced. Microscopic observations using the etch pit technique showed that the dislocations are distributed uniformly after deformation at a constant quasi-static strain rate but that they congregate into wide bands as the result of incremental strain rate testing.