High Strain Rate Data Research Articles

Genetic Programming for Multiscale Modeling

We propose the use of genetic programming (GP)—a genetic algorithm that evolves computer programs—for bridging simulation methods across multiple scales of time and/or length. The effectiveness of genetic programming in multiscale simulation is demonstrated using two illustrative, non-trivial case studies in science and engineering. The first case is multi-timescale materials kinetics modeling, where genetic programming is used to symbolically regress a mapping of all diffusion barriers from only a few calculated ones, thereby avoiding explicit calculation of all the barriers. The GP-regressed barrier function enables use of kinetic Monte Carlo for realistic potentials and simulation of realistic experimental times (seconds). Specifically, a GP regression is applied to vacancy-assisted migration on a surface of a binary alloy and predict the diffusion barriers within 0.1–1% error using 3% (or less) of the barriers. The second case is the development of constitutive relation between macroscopic variables using measured data, where GP is used to evolve both the function form of the constitutive equation as well as the coefficient values. Specifically, GP regression is used for developing a constitutive relation between flow stress and temperature-compensated strain rate based on microstructural characterization for an aluminum alloy AA7055. We not only reproduce a constitutive relation proposed in literature, but also develop a new constitutive equation that fits both low-strain-rate and high-strainrate data. We hope these disparate example applications exemplify the power of GP for multiscaling at the price, of course, of not knowing physical details at the intermediate scales.

High temperature high strain rate testing with a compressive SHPB

Both temperature and strain rate have a significant effect on the mechanical behavior of most materials. At or close to room temperature, well-designed and well-built SHPB devices can be used to produce high quality high strain rate data relatively easily, but at elevated temperatures many practical difficulties make such tests much more complicated. There is, however, an increasing interest and practical need to study and model the high strain rate behavior of materials also at high temperatures, and therefore development of sophisticated high strain rate high temperature testing devices is needed. In this paper, a fully computer controlled Split Hopkinson Pressure Bar (SHPB) testing system with a high temperature capability up to 1000 °C is described. The high temperature (HT) system is based on fast pneumatic specimen and pressure bar manipulation devices which enable heating up of the specimen only and limit the contact time of the hot specimen with cold pressure bars to 50...100 ms. Some results of the tests made with the new HT-SHPB device are also presenter in this paper.

The influence of crystal structure on the dynamic behavior of materials at high temperatures

The rate dependent thermomechanical behaviors of OFHC copper, vanadium, and α-titanium have been examined using a recently developed experimental technique for the performance of high temperature, high-strain-rate experiments in the compression Kolsky bar. These three materials represent the three most common lattice structures for metals: FCC, BCC and HCP (respectively). Stress–strain curves are obtained for each material at strain rates of 4×10 3 s −1 and at temperatures ranging from 300 to 1100 K. Quasistatic thermal softening behavior is extracted from the literature for these specific materials and is compared with the new high strain rate data. It is observed that the rate of thermal softening is a function of the strain rate, with the strongest effects in BCC vanadium and HCP α-titanium. In addition, the differences in the high rate thermal softening behavior of the three different crystal structures are studied. Finally, the experimental data for copper and vanadium are compared to the FCC and BCC formulations of the Zerilli–Armstrong model respectively.

Steady-state creep of a particulate SiC/6061 Al composite

The creep behavior of a 10 vol.% silicon carbide particulate reinforced 6061 Al composite produced by powder metallurgy (PM) has been examined by creep tests in both tension and compression at 400°C. The tensile creep data covering minimum creep rates of the orders 10 −9 to 10 −4/s show an apparent stress exponent n app≈13, but a comparison with compressive creep data reveals that some high strain-rate data in tension are due to the transition to the tertiary stage. Analysis of the data is made only for the steady-state creep rate, together with that for an unreinforced PM 6061 Al alloy, by incorporating a threshold stress. This gives a stress exponent n=3 for the matrix alloy, whereas the composite data show such a trend that the n value gradually changes from 3 to 1 as the effective stress increases. A new method of steady-state creep data analysis is formulated by taking account of the interface-confined diffusional flow and thereby the finding above is reasonably assessed.

Modeling temperature and strain rate history effects in OFHC Cu

Abstract In this paper, we report the modeling of a series of large strain deformation experiments on initially annealed OFHC Cu involving sequences of temperature, and strain rate (varying from quasi-static to dynamic). It is shown that equations-of-state with parameters determined using constant strain rate data from monotonic loading paths, although they employ instantaneous temperature and strain rate dependence, are insufficient to describe mechanical behavior. Macroscale internal state variable (ISV) viscoplasticity models, such as the Mechanical Threshold Stress (Follansbee, P.S., Kocks, U.F., 1988. A constitutive description of the deformation of copper based on the use of the mechanical threshold as an internal state variable. Acta Metall. 36, 81.) and BCJ–SNL (Bammann, D., Chiesa, M.L., Johnson, G.C., 1990a. A strain rate dependent flow surface model of plasticity. Sandia National Laboratory Report, SAND90-8227, Livermore, CA.) models, representative of a broad class of such models, are fit to the quasistatic isothermal and high strain rate (approximately adiabatic) data for compression and then predictions are compared to the experimental results for path sequence experiments. Implications for the representation of path history effects through the hardening, static and dynamic recovery functions are considered. A mesoscale polycrystal plasticity model with one and two slip system hardening variables is also examined in the context of sequence experiments.

High strain rate properties of an SiC w/2124-T6 aluminum composite at elevated temperatures

Metal matrix composites, (MMC`s) provide several important advantages over unreinforced metals and alloys. Among these, higher moduli and yield stresses and enhanced thermo-mechanical properties are normally considered important in structural applications of MMC`s. It is also possible that MMC`s may be exposed to loading conditions involving high strain rates during service, for example, components of a car in collision with another or turbine blades hit by ingestion of foreign objects. In such situations of rapidly increasing loading conditions, the material property response may be considerably different from that which applies during slow loading of normal quasi-static testing and, consequently, dynamic mechanical properties are of increasing interest and importance. Several studies involving dynamic deformation of a SiC{sub W}/2124-T6 Al MMC have already been conducted using tension, shear, compression and projectile impact testing. The purpose of the present study was to extend the high strain rate data to include high temperature effects. Mechanical test results and microstructural observations of features of dynamic deformation have been compared with data from the unreinforced alloy to obtain more complete information on the dynamic response of the composite.

Thickness effects in split Hopkinson pressure bar tests

Results are presented for the high strain rate behaviour of a medium and high density polyethylene, polycarbonate and poly(ether ether ketone) in compression for strain rates up to 10 4s −1 obtained using a split Hopkinson pressure bar apparatus. It is shown that the choice of specimen thickness in these tests significantly affects the measured flow stresses at high strain rates, with the thicker specimens showing an apparent enhancement of flow stress at high rates. Comparison of the present results with published work suggests that some high strain rate data in the literature could be erroneous due to an inappropriate choice of specimen dimensions.