J.N. Florando

Exploring specimen size effects in plastic deformation of Ni 3 (Al, Ta)

In this paper we present a mechanical test methodology to explore specimen size effects in Ni 3 Al, where the overall test sample dimensions artificially limit the volume for substructure evolution and hence the availability of jogs/kinks along individual dislocation lines. The test methodology consists of using Focused Ion Beam milling to micromachine cylindrical compression samples that have diameters ranging from 5 to 20 microns into the surface of a bulk sample, which is followed by nanoindentation using a flat-ended tip to measure the mechanical properties of the microsamples in uniaxial compression. The initial test results show that there is a strong increase in the flow stress with decreasing sample size, although misfit between the flat indenter tip and the top surface of the compression samples complicates complete interpretation of the mechanical test results at this time.

Analysis of deformation twinning in tantalum single crystals under shock loading conditions

The competition between dislocation slip and twinning in tantalum single crystals has been investigated utilizing a crystal level twinning model and the results from gas gun recovery experiments conducted at peak normal stresses of 25 and 55 GPa. The recovered samples were characterized using electron back scattered diffraction, and the observed twinning fractions were compared with the model. The experimental results show very low twin fractions in all orientations at 25 GPa, and that among (100), (110), (111), and (123) crystals, the (110) crystals had the largest amount of twinning at 55 GPa. The analysis shows that the general trends observed in the experimental data can be reproduced by the model when an orientation dependent dislocation evolution is used. This analysis gives insight into the possible influence of the dislocation density and its evolution on the observed twinning behavior.

Effect of strain rate and dislocation density on the twinning behavior in tantalum

The conditions which affect twinning in tantalum have been investigated across a range of strain rates and initial dislocation densities. Tantalum samples were subjected to a range of strain rates, from 10−4/s to 103/s under uniaxial stress conditions, and under laser-induced shock-loading conditions. In this study, twinning was observed at 77K at strain rates from 1/s to 103/s, and during laser-induced shock experiments. The effect of the initial dislocation density, which was imparted by deforming the material to different amounts of pre-strain, was also studied, and it was shown that twinning is suppressed after a given amount of pre-strain, even as the global stress continues to increase. These results indicate that the conditions for twinning cannot be represented solely by a critical global stress value, but are also dependent on the evolution of the dislocation density. In addition, the analysis shows that if twinning is initiated, the nucleated twins may continue to grow as a function of strain, even as the dislocation density continues to increase.

Calculation of the Slip System Activity in Deformed Zinc Single Crystals Using Digital 3-D Image Correlation Data

A 3-D image correlation system, which measures the full-field displacements in three dimensions, has been used to experimentally determine the full deformation gradient matrix for two zinc single crystals. Based on the image correlation data, slip system activity for the two crystals has been calculated. The results of the calculation show that, for one crystal, only the primary slip system is active, which is consistent with traditional theory. The other crystal, however, shows appreciable deformation on slip systems other than the primary. An analysis was conducted verifying the experimental observation that the net result from slip on the secondary slip systems is approximately one third the magnitude and directly orthogonal to the primary system.

Simulation Of The Main Physical Processes In Remote Laser Penetration With Large Laser Spot Size

A 3D model is developed to simulate remote laser penetration of a 1mm Aluminum metal sheet with large laser spot size (∼ 3x3cm2), using the ALE3D multi-physics code. The model deals with the laser-induced melting of the plate and the mechanical interaction between the solid and the melted part through plate elastic-plastic response. The effect of plate oscillations and other forces on plate rupture, the droplet formation mechanism and the influence of gravity and high laser power in further breaking the single melt droplet into many more fragments are analyzed. In the limit of low laser power, the numerical results match the available experiments. The numerical approach couples mechanical and thermal diffusion to hydrodynamics melt flow and accounts for temperature dependent material properties, surface tension, gravity and vapor recoil pressure.

Spall fracture in additive manufactured Ti-6Al-4V

We present a study on the spall strength of additive manufactured (AM) Ti-6Al-4V. Samples were obtained from two pieces of selective laser melted (SLM, a powder bed fusion technique) Ti-6Al-4V such that the response to dynamic tensile loading could be investigated as a function of the orientation between the build layers and the loading direction. A sample of wrought bar-stock Ti-6Al-4V was also tested to act as a baseline representing the traditionally manufactured material response. A single-stage light gas-gun was used to launch a thin flyer plate into the samples, generating a region of intense tensile stress on a plane normal to the impact direction. The rear free surface velocity time history of each sample was recorded with laser-based velocimetry to allow the spall strength to be calculated. The samples were also soft recovered to enable post-mortem characterization of the spall damage evolution. Results showed that when the tensile load was applied normal to the interfaces between the build layer...

Experimental and numerical investigations of beryllium strength models using the Rayleigh-Taylor instability

We present a set of high explosive driven Rayleigh-Taylor strength experiments for beryllium to produce data to distinguish predictions by various strength models. Design simulations using existing strength model parameterizations from Steinberg-Lund and Preston-Tonks-Wallace (PTW) suggested an optimal design that would delineate between not just different strength models, but different parameters sets of the PTW model. Application of the models to the post-shot results, however, suggests growth consistent with little material strength. We focus mostly on efforts to simulate the data using published strength models as well as the more recent RING relaxation model developed at VNIIEF. The results of the strength experiments indicate weak influence of strength in mitigating the growth with the RING model coming closest to predicting the material behavior. Finally, we present shock and ramp-loading recovery experiments.

The trianvil test apparatus: Measurement of shear strength under pressure

An experimental apparatus has been developed for performing shear tests on specimens held under moderately high hydrostatic pressures (up to the order of 10 GPa). This testing procedure experimentally determines the pressure dependent shear strength of thin foil specimens. This information is necessary for models of materials subjected to extreme pressures and can assist in model validation for models such as discrete dislocation dynamics simulations, among others. This paper reports the development of the experimental procedures and the results of initial experiments on thin foils of polycrystalline Ta performed under hydrostatic pressures ranging from 2 to 4 GPa. Subsequent characterization of the samples held under pressure established that the procedure described herein represents a reliable method to impose nearly uniform hydrostatic pressure on thin foil specimens. Both yielding and hardening behavior of Ta are observed to be sensitive to the imposed pressure.

HIGH RATE PLASTICITY UNDER PRESSURE USING A WINDOWED PRESSURE‐SHEAR IMPACT EXPERIMENT

An experimental technique has been developed to study the strength of materials under conditions of moderate pressures and high shear strain rates. The technique is similar to the traditional pressure‐shear plate‐impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample‐window interface. Experimental and simulation results on vanadium samples backed with a sapphire window show the utility of the technique to measure the flow strength under dynamic loading conditions. The results show that the strength of the vanadium is approximately 600 MPa at a pressure of 4.5 GPa and a plastic strain of 1.7%.

Microstructure and Mechanical Properties of 21-6-9 Stainless Steel Electron Beam Welds

Welds can either be stronger or weaker than the base metals that they join depending on the microstructures that form in the fusion and heat-affected zones of the weld. In this paper, weld strengthening in the fusion zone of annealed 21-6-9 stainless steel is investigated using cross-weld tensile samples, hardness testing, and microstructural characterization. Due to the stronger nature of the weld, the cross-weld tensile tests failed in the base metal and were not able to generate true fusion zone mechanical properties. Nanoindentation with a spherical indenter was instead used to predict the tensile behavior for the weld metal. Extrapolation of the nanoindentation results to higher strains was performed using the Steinberg–Guinan and Johnson–Cook strength models, and the results can be used for weld strength modeling purposes. The results illustrate how microstructural refinement and residual ferrite formation in the weld fusion zone can be an effective strengthener for 21-6-9 stainless steel.