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 specimen's 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.

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

Numerical simulations of high-strain-rate and high-temperature deformation of pure metals and alloys require realistic plastic constitutive models. Empirical models include the widely used Johnson-Cook model and the semi-empirical Steinberg-Cochran-Guinan-Lund model. Physically based models such as the Zerilli-Armstrong model, the Mechanical Threshold Stress model, and the Preston-Tonks-Wallace model are also coming into wide use. In this paper, we determine the Mechanical Threshold Stress model parameters for various tempers of AISI 4340 steel using experimental data from the open literature. We also compare stress-strain curves and Taylor impact test profiles predicted by the Mechanical Threshold Stress model with those from the Johnson-Cook model for 4340 steel. Relevant temperature- and pressure-dependent shear modulus models, melting temperature models, a specific heat model, and an equation of state for 4340 steel are discussed and their parameters are presented.

The Mechanical Threshold Stress model for various tempers of AISI 4340 steel

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.

http://www.dtic.mil/dtic/tr/fulltext/u2/a202522.pdf

Microscopic observations of adiabatic shear bands in three different steels

https://www.sci.utah.edu/publications/Ban2007a/Banerjee_IJSS2007.pdf

A new plate impact experiment is presented for studying dynamic fracture processes that occur under sub-microsecond loading. The experiment is designed to provide comparatively straightforward interpretation within the framework of fracture mechanics. A disc containing a mid-plane, pre-fatigued, edge crack that has been propagated halfway across the diameter is impacted by a thin flyer plate of the same material. A compressive pulse propagates through the specimen and reflects from the rear surface as a step, tensile pulse with a duration of 1μs. This plane wave loads the crack and causes initiation and propagation of the crack. The motion of the rear surface is monitored during this event using a laser interferometer system. The location of the crack front is mapped before and after the experiment using a focussed ultrasonic transducer.

Experiments have been conducted on a hardened 4340 VAR steel at temperatures ranging from room temperature to — 100°C. Crack advance increases monotonically with increasing impact velocity and with decreasing temperature. Critical values of the stress intensity factor K_Ic are inferred from known elastodynamic solutions and the assumption that the measured crack advance occurs at a constant energy release rate. Fracture modes are characterized by means of scanning electron microscopy of the fracture surfaces.

A finite difference method is used for numerical simulation of the experiments. The loading is modelled as that of a plane, square, tensile pulse impinging at normal incidence on a semi-infinite crack. Crack advance is assumed to initiate when the crack-tip stress intensity factor reaches the critical value K_Ic. Crack velocities are prescribed corresponding to various fracture models. The predicted motion of the rear surface is found to be in good agreement with the measured motion when the crack velocity is taken to be a constant.

Dynamic fracture under plane wave loading

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.

K. Cho

Papers

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