Jon Isaacs

Hopkinson techniques for dynamic recovery experiments

Novel techniques are introduced to render the classical split Hopkinson bar apparatus suitable for dynamic recovery experiments, where samples can be subjected to a single pulse of pre-assigned shape and duration, and then recovered without any additional loading, for post-test characterization; i. e., techniques for fully controlled unloading in Hopkinson bar experiments. For compression dynamic recovery tests, the new design generates a compressive pulse trailed by a tensile pulse (stress reversal), travelling toward the sample. Furthermore, all subsequent pulses which reflect off the free ends of the two bars (incident and transmission) are rendered tensile, so that the sample is subjected to a single compressive pulse whose shape and duration can also be controlled. For tension recovery experiments, the new design provides for trapping the compression pulse reflected off the sample, and the tensile pulse transmitted through the sample. In addition, a sample can be subjected to compression followed by tension, and then recovered, allowing the study of, e. g. the dynamic Bauschinger effect in materials.

Direct measurement of isothermal flow stress of metals at elevated temperatures and high strain rates with application to Ta and TaW alloys

A technique is developed for measuring the flow stress of metals over a broad range of strains, strain rates, and temperatures, in uniaxial compression. It utilizes a recent, enhanced version of the classical (Kolsky) compression split Hopkinson bar (l), in which a sample is subjected to a single stress pulse of a predefined profile, and then recovered without being subjected to any other additional loading. For the present application, the UCSDs split Hopkinson bar is further enhanced by the addition of a new mechanism by means of which the incident and transmission bars of the split Hopkinson construction are moved into a constant-temperature furnace containing the sample, and gently brought into contact with the sample, as the elastic stress pulse reaches and loads the sample. The samples temperature is measured by thermocouples which also hold the sample in the furnace. Since straining at high strain rates increases the samples temperature, the sample is allowed to attain the furnace temperature after each controlled incremental loading, and then is reloaded. using the same stress pulse and strain rate. The technique also allows for checking any recoveries that may occur during unloading and reloading. Using several samples of the same material and testing them at the same strain rate and temperature, but different incremental strains, an accurate estimate of the materials isothermal flow stress can be obtained. Additionally, the modified Hopkinson technique allows the direct measurement of the change in the (high strain-rate) flow stress with a change of the strain rate, while the strain and temperature are kept constant, i.e., the strain rate can be increased or decreased during the high strain-rate test. The technique is applied to obtain both quasi-isothermal and adiabatic flow stresses of tantalum (Ta) and a tantalum-tungsten (Ta-W) alloy at elevated temperatures. These experimental results show the flow stress of these materials to be controlled by a simple long-range plastic-strain-dependent barrier, and a short-range thermally activated Peierls mechanism. For tantalum, a model which fits the experimental data over strains from a few to over loo%, strain rates from quasi-static to 40 000/s, and temperatures from -200 to 1000°C. is presented and discussed. Copyright 0 1997 Acta Metallurgica Inc.

An experimentally-based viscoelastic constitutive model for polyurea, including pressure and temperature effects

Presented here are the results of a systematic study of the viscoelastic properties of polyurea over broad ranges of strain rates and temperatures, including the high-pressure effects on the material response. Based on a set of experiments and a master curve developed by Knauss (W.G. Knauss, Viscoelastic Material Characterization relative to Constitutive and Failure Response of an Elastomer, Interim Report to the Office of Naval Research (GALCIT, Pasadena, CA, 2003.) for time–temperature equivalence, we have produced a model for the large deformation viscoelastic response of this elastomer. Higher strain-rate data are obtained using Hopkinson bar experiments. The data suggest that the response of this class of polymers is strongly pressure dependent. We show that the inclusion of linear pressure sensitivity successfully reproduces the results of the Hopkinson bar experiments. In addition, we also present an equivalent but approximate model that involves only a finite number of internal state variables and is specifically tailored for implementation into explicit finite-element codes. The model incorporates the classical Williams–Landel–Ferry (WLF) time–temperature transformation and pressure sensitivity (M.L. Williams, R.F. Landel, and J.D. Ferry, J. Am. Chem. Soc., 77 3701 (1955)), in addition to a thermodynamically sound dissipation mechanism. Finally, we show that using this model for the shear behaviour of polyurea along with the elastic bulk response, one can successfully reproduce the very high strain rate pressure–shear experimental results recently reported by Jiao et al. (T. Jiao, R.J. Clifton and S.E. Grunschel, Shock Compression of Condensed Matter 2005 (American Institute of Physics, New York, 2005.).

Experimental/ computational evaluation of flow stress at high strain rates with application to adiabatic shear banding

Abstract Experimental techniques are described and illustrated for direct measurement of temperature, strain-rate, and strain effects on the flow stress of metals over a broad range of strains and strain rates. The approach utilizes: (1) the dynamic recovery Hopkinson bar technique recently developed at UCSD (Nemat-Nasser et al., Proc. R. Soc. London A20 , 371–391), in which samples are subjected to a single predefined stress pulse and then recovered without having been subjected to any additional loads; (2) direct measurement of sample temperature by high-speed infra-red detectors; and (3) ability to change the strain rate during the course of experiment at high strain rates. In this manner, constitutive parameters of elasto-viscoplastic flow of metals and metallic alloys are established and used in constitutive models for large-scale computational simulation of high strain-rate phenomena such as adiabatic shear banding. For application to this kind of very high strain-rate events, it becomes necessary to further tune the constitutive parameters through additional coordinated experimental and computational efforts. To this end, Taylor anvil tests are performed, accompanied by high-speed photographic recording of the deformation, and the results are compared with those obtained by finite-element simulations, leading to fine tuning of parameters in the materials flow stress. This procedure is illustrated for tantalum and tantalum/tungsten alloys, as well as for VAR 4340 steel. The simulations are performed using the PRONTO-2D finite-element code, in which an explicit constitutive algorithm, recently proposed by the first two authors and co-workers, has been implemented; Nemat-Nasser (1991, Mech. Mater. 11 , 235–249), Nemat-Nasser and Chung (1992, Comput. Meth. Appl. Mech. Eng. 95, 205–219), and Nemat-Nasser and Li (1992, Comput. Struct. 44 (5), 937–963). This algorithm is always stable and incredibly accurate, independently of the time or strain increments. The constitutive model and the constitutive algorithm are used to simulate adiabatic shear bands produced in a hat-shaped specimen under controlled conditions (Beatty et al., 1991, Proc. 12th Army Symp. on Solid Mechanics , pp. 331–345), arriving at excellent correlation with experiments.

High Strain-Rate, Small Strain Response of a NiTi Shape-Memory Alloy

The compressive response of a NiTi shape-memory alloy is investigated at various strain rates using UCSDs modified 1/2-in. Hopkinson pressure bar and a conventional Instron machine. To obtain a constant strain rate during the formation of a stress-induced martensite in a Hopkinson test, a copper tube of suitable dimensions is employed as a pulse shaper, since without a pulse shaper the strain rate of the sample varies significantly as its microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. The NiTi shape-memory alloy shows a superelastic response for small strains at all considered strain rates and at room temperature, 296 K. At this temperature and below a certain strain rate, the stress-strain curves of the NiTi shape-memory alloy display two regimes: an elastic austenite regime and a transition (stress-induced martensite) regime. The transition stress of this material and the work-hardening rate in the stress-induced martensite regime increase with increasing strain rate, the latter reaching a steady state level and then rapidly increasing.

Microstructure of high-strain, high-strain-rate deformed tantalum

Abstract Hat-shaped specimens of polycrystalline tantalum are subjected to high plastic shear strains (γ = 170–910%) at strain rates exceeding 5 × 104/s in a compression split Hopkinson bar. The dynamic shear tests are performed at room and 600 K initial temperatures, under adiabatic and quasi-isothermal conditions, using UCSDs recovery Hopkinson technique [1]. The microstructure of the post-test specimens is examined with transmission electron microscopy (TEM). The plastic deformation is highly concentrated, producing a narrow shear-localization region of approximately 200 μm in width. Slip of perfect screw dislocations, on the {110} primary planes along the 〈111〉 directions, is found to be the dominant deformation mechanism. Dynamic recovery takes place in the shear-localization regions of all adiabatically tested specimens, and evidence of dynamic recrystallization is observed in the specimen deformed to a shear strain of 910% at a 600 K initial temperature. The substructures of the adiabatically tested specimens include well-defined dislocation arrays, grouped dislocations, elongated dislocation cells, subgrains, and recrystallized micron-sized grains. The microstructure of isothermally tested specimens, on the other hand, features high dislocation density and inhomogeneous dislocation distribution. In light of the TEM observations, the relation between the microstructure and shear stress, the causes of strain inhomogeneity, the estimated adiabatic temperature within the shear-localization zone, the rapid quenching of the shearband at the end of the dynamic testing, the slip characteristics of dislocations in tantalum, and the formation mechanisms of dislocation loops, are discussed.

Quasi-Static and Dynamic Buckling of Thin Cylindrical Shape-Memory Shells

To investigate the buckling behavior of thin and relatively thick cylindrical shape-memory shells, uniaxial compression tests are performed at a 295 K initial temperature, using the CEAM/UCSDs modified split Hopkinson bar systems and an Instron hydraulic testing machine. The quasi-static buckling response of the shells is directly observed and recorded using a digital camera with a close-up lens and two back mirrors. To document the dynamic buckling modes, a high-speed Imacon 200 framing camera is used. The shape-memory shells with an austenite-finish temperature of A f =281 K, buckle gradually and gracefully in quasi-static loading, and fully recover upon unloading, showing a superelastic property, whereas when suitably annealed, the shells do not recover spontaneously upon unloading, but they do so once heated, showing a shape-memory effect. The thin shells had a common thickness of 0.125 mm a common outer radius of 2.25 mm (i.e., a common radius, R, to thickness, t, ratio, R/t, of 18). A shell with the ratio of length, L, to diameter, D (LID) of 1.5 buckled under a quasi-static load by forming a nonsymmetric chessboard pattern, while with a LID of 1.95 the buckling started with the formation of symmetrical rings which then changed into a nonsymmetric chessboard pattern. A similar buckling mode is also observed under a dynamic loading condition for a shell with LID of 2. However, thicker shells, with 0.5 mm thickness and radius 4 mm (R/t=8), buckled under a dynamic loading condition by the formation of a symmetrical ring pattern. For comparison, we have also tested shells of similar geometry but made of steel and aluminum. In the case of the steel shells with constrained end conditions, the buckling, which consists of nonsymmetric (no rings) folds (chessboard patterns), is sudden and catastrophic, and involves no recovery upon unloading. The gradual buckling of the shape-memory shells is associated with the stress-induced martensite formation and seems to have a profound effect on the unstable deformations of thin structures made from shape-memory alloys.

Experimental observation of high-rate buckling of thin cylindrical shape-memory shells

We investigate the buckling behavior of thin cylindrical shape-memory shells at room temperature, using a modified split Hopkinson bar and an Instron hydraulic testing machine. The quasi-static buckling response is directly observed using a digital camera with a close-up lens and two back mirrors. A high-speed Imacon 200 framing camera is used to record the dynamic buckling modes. The shape-memory shells with an austenite-finish temperature less than the room temperature, buckle gradually and gracefully in quasi-static loading, and fully recover upon unloading, showing a superelastic property, whereas when suitably annealed, the shells do not recover spontaneously upon unloading, but they do so once heated, showing a shape-memory effect. The gradual and graceful buckling of the shape-memory shells is associated with the stress-induced martensite formation and seems to have a profound effect on the unstable deformations of thin structures made from shape-memory alloys.

Controlling acoustic-wave propagation through material anisotropy

Acoustic-wave velocity is strongly direction dependent in an anisotropic medium. This can be used to design composites with preferred acoustic-energy transport characteristics. In a unidirectional fiber-glass composite, for example, the preferred direction corresponds to the fiber orientation which is associated with the highest stiffness and which can be used to guide the momentum and energy of the acoustic waves either away from or toward a region within the material, depending on whether one wishes to avoid or harvest the corresponding stress waves. The main focus of this work is to illustrate this phenomenon using numerical simulations and then check the results experimentally.

Structural composites with integrated electromagnetic functionality

We are studying the incorporation of electromagnetic effective media in the form of arrays of metal scattering elements, such as wires, into polymer-based or ceramic-based composites. In addition to desired structural properties, these electromagnetic effective media can provide controlled response to electromagnetic radiation such as RF communication signals, radar, and/or infrared radiation. With the addition of dynamic components, these materials may be leveraged for active tasks such as filtering. The advantages of such hybrid composites include simplicity and weight savings by the combination of electromagnetic functionality with necessary structural functionality. This integration of both electromagnetic and structural functionality throughout the volume of the composite is the distinguishing feature of our approach. As an example, we present a class of composites based on the integration of artificial plasmon media into polymer matrixes. Such composites can exhibit a broadband index of refraction substantially equal to unity at microwave frequencies and below.