There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

https://zenodo.org/record/1258680/files/article.pdf

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

[Lu, K.; Lu, L.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. [Lu, L.; Suresh, S.] MIT, Sch Engn, Cambridge, MA 02139 USA.;Lu, K (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China;lu@imr.ac.cn ssuresh@mit.edu

/pdf/strengthening-materials-by-engineering-coherent-internal-4fkjmv3btq.pdf

Strengthening Materials by Engineering Coherent Internal Boundaries at the Nanoscale

Previous studies of quasielastic release in shock-loaded FCC metals have shown a strong influence of the defect state on the leading edge, or first observable arrival, of release wave, due to large density of pinned dislocation segments behind the shock front, their relatively large pinning separation, and a very short response time as determined by drag coefficient in shock-compressed state. This effect is entirely equivalent to problems associated with elastic moduli determination using ultrasonic methods. This is particularly true for FCC metals, which have an especially low Peierls stress, or inherent lattice resistance, that has little influence in pinning dislocation segments and inhibiting anelastic deformation. BCC metals, on the other hand, have a large Peierls stress that essentially holds dislocation segments in place at low net applied shear stresses and thus allows fully elastic deformation to occur in the complete absence of anelastic behavior. Shock-compression and release experiments have been performed on tantalum (BCC), with the observation that the leading release disturbance is indeed elastic. This conclusion is established by examination of experimental VISAR records taken at the tantalum/sapphire (window) interface in a symmetric-impact experiment which subjects the sample to a peak longitudinal stress of approximately 7.3 GPa, in comparisonmore » with characteristic code calculations.« less

/pdf/shock-compression-and-quasielastic-release-in-tantalum-4p0bia8y5n.pdf

Shock compression and quasielastic release in tantalum

https://www2.nsysu.edu.tw/MSE/papers/Paper%20in%20PDF/22%20Shock%20MCP%201993.pdf

The response of metal-matrix composites subjected to quasi-static and shock-wave deformation

Energetic loading subjects a material to a 'Taylor wave' (triangular wave) loading profile that experiences an evolving balance of hydrostatic (spherical) and deviatoric stresses. While much has been learned over the past five decades concerning the propensity of deformation twinning in samples shockloaded using 'square-topped' profiles as a function of peak stress, achieved most commonly via flyer plate loading, less is known concerning twinning propensity during non-I-dimensional sweeping detonation wave loading. Systematic small-scale energetically-driven shock loading experiments were conducted on Ta samples shock loaded with PEFN that was edge detonated. Deformation twinning was quantified in post-mortem samples as a function of detonation geometry and radial position. In the edge detonated loading geometry examined in this paper, the average volume fraction of deformation twins was observed to drastically increase with increasing shock obliquity. The results of this study are discussed in light of the formation mechanisms of deformation twins, previous literature studies of twinning in shocked materials, and modeling of the effects of shock obliquity on the evolution of the stress tensor during shock loading.

/pdf/influence-of-shockwave-obliquity-on-deformation-twin-421dhuk4d9.pdf

Influence of shockwave obliquity on deformation twin formation in Ta

Based upon a study of the independent variation of peak pressure and pulse duration upon the shock loading response of OFE copper, the following conclusions can be drawn: (1) Increasing peak pressure or pulse duration was found to decrease the observed dislocation cell size and increase the yield strength and (2) the influence of pulse duration is attributed to the influence of reorganization time on the amount of dislocation generation during the rarefaction release

/pdf/influence-of-peak-pressure-and-pulse-duration-on-3xyndqtm3h.pdf

Influence of peak pressure and pulse duration on substructure development and threshold stress measurements in shock-loaded copper

Considerable understanding of fatigue crack initiation and crack growth has been achieved in recent years, with a synthesis of concepts now available which includes crack initiation phenomena and also the growth behavior of short cracks and long cracks. This understanding has been shown to apply to rail steels in both the theoretical and experimental senses. Experimental results illustrating such extension are presented and discussed for both plain carbon and alloy rail compositions, with microstructural dependence of properties featured as a characteristic amenable to exploitation for improvement of rail performance.

George T. Gray

Papers

Shock compression and quasielastic release in tantalum

The response of metal-matrix composites subjected to quasi-static and shock-wave deformation

Influence of shockwave obliquity on deformation twin formation in Ta

Influence of peak pressure and pulse duration on substructure development and threshold stress measurements in shock-loaded copper

Fatigue Crack Growth in Rail Steels