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

http://ece.uprm.edu/~msuarez/4001/doc/read_assign3_2ndSem0910.pdf

Nanoindentation studies of materials

Instrumented indentation, also known as depth-sensing indentation or nanoindentation, is increasingly being used to probe the mechanical response of materials from metals and ceramics to polymeric and biological materials. The additional levels of control, sensitivity, and data acquisition offered by instrumented indentation systems have resulted in numerous advances in materials science, particularly regarding fundamental mechanisms of mechanical behavior at micrometer and even sub-micrometer length scales. Continued improvements of instrumented indentation testing towards absolute quantification of a wide range of material properties and behavior will require advances in instrument calibration, measurement protocols, and analysis tools and techniques. In this paper, an overview of instrumented indentation is given with regard to current instrument technology and analysis methods. Research efforts at the National Institute of Standards and Technology (NIST) aimed at improving the related measurement science are discussed.

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Review of Instrumented Indentation

http://www.123seminarsonly.com/Seminar-Reports/2013-04/80827267-Coatings-tribology-contact.pdf

Coatings tribology—contact mechanisms and surface design

Piezoresponse force microscopy (PFM) has emerged as a powerful and versatile tool for probing nanoscale phenomena in ferroelectric materials on the nanometer and micrometer scales. In this review, we summarize the fundamentals and recent advances in PFM, and describe the nanoscale electromechanical properties of several important ferroelectric ceramic materials widely used in memory and microelectromechanical systems applications. Probing static and dynamic polarization behavior of individual grains in PZT films and ceramics is discussed. Switching spectroscopy PFM is introduced as a useful tool for studying defects and interfaces in ceramic materials. The results on local switching and domain pinning behavior, as well as nanoscale fatigue and imprint mapping are presented. Probing domain structures and polarization dynamics in polycrystalline relaxors (PMN-PT, PLZT, doped BaTiO 3 ) are briefly outlined. Finally, applications of PFM to dimensionally confined ferro-electrics are demonstrated. The potential of PFM for studying local electromechanical phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of their macroscopic properties is illustrated.

Electromechanical Imaging and Spectroscopy of Ferroelectric and Piezoelectric Materials: State of the Art and Prospects for the Future

We discuss atomic force acoustic microscopy (AFAM) methods to determine quantitative values for the elastic properties of thin films. The AFAM approach measures the frequencies of an AFM cantilever’s first two flexural resonances while in contact with a material. The indentation modulus M of an unknown or test material can be obtained by comparing the resonant spectrum of the test material to that of a reference material. We examined a niobium film (d=280±30 nm) with AFAM using two separate reference materials and two different cantilever geometries. Data were analyzed by two methods: an analytical model based on conventional beam dynamics, and a finite element method that accommodated variable cantilever cross section and viscous damping. AFAM values of M varied significantly depending on the specific experimental configuration and analysis technique. By averaging values obtained with both reference materials, very good agreement (5–10 % difference) with values determined by other methods was achieved. T...

/pdf/atomic-force-acoustic-microscopy-methods-to-determine-thin-1n7zbv03lr.pdf

Atomic force acoustic microscopy methods to determine thin-film elastic properties

Progress in determination of the area function of indenters used for nanoindentation

We review current practice for describing force–displacement curves from pointed indenters, highlighting the consequences of the simplifications normally adopted. We derive two corrections, the 'variable epsilon factor' and the 'radial displacement correction.' These are especially important for highly elastic materials such as fused silica where the combined corrections can amount to 13% in the contact area, significantly increasing the accuracy of hardness and modulus results. In contrast, the so-called beta factor has minor importance. We compare our analytical results with finite element (FE) calculations and experimental results. Indenter area functions, obtained using the corrections, agree well with independent direct measurements by a traceably calibrated metrological atomic force microscope (AFM). Further formulae are derived to calculate the complete force–displacement curve of conical indenters and the indentation elastic and total energy. These formulae immediately identify a physical material limit above which a cone cannot generate plastic deformation; for a Berkovich indenter this is a hardness-to-modulus ratio of 0.18.

https://iopscience.iop.org/article/10.1088/0022-3727/41/21/215407/pdf

Higher accuracy analysis of instrumented indentation data obtained with pointed indenters

Methods to obtain tensile stress–strain properties of materials from a practically non-destructive indentation test are of great industrial interest. Nanoindentation is a good candidate. However, to do this successfully, indentation size effects must be accounted for. An indentation size effect with spherical indenters has been shown for a range of fcc metals with relatively large grain size (Spary et al 2006 Phil. Mag. 86 5581–93); the increase in yield stress being proportional to the inverse cube root of indenter radius. Here, we investigate these differences further and present results for the indentation size effect with spherical indenters on Cu samples with a range of different grain sizes from 1 µm to single crystal. The important experimental control parameter, of the relative size of the indentation compared with the grain size, is also explored by using indenters of different radii on the different grain sized samples. When the grain size, d, is less than 6 times the radius of the projected contact area, a, a Hall–Petch-like behaviour is observed superimposed on the indentation size effect. For d > 6a the indentation size effect dominates. The two effects may be combined by addition in quadrature. This new parametric function is able to predict the indentation pressure in annealed copper given input values of indenter radius and grain size.

https://iopscience.iop.org/article/10.1088/0022-3727/41/7/074006/pdf

Study of the interaction between the indentation size effect and Hall–Petch effect with spherical indenters on annealed polycrystalline copper

Size effects in plasticity at small length scales have been widely reported in a variety of loading situations. The hardness size effect has often been cited as an archetypal example. The indentation size effect was clearly demonstrated by Lim and Chaudhri [Phil. Mag. A 79 2979 (1999)] in spherical indentation experiments on copper and, subsequently, in experiments on iridium by Swadener et al. [J. Mech. Phys. Solids 50 681 (2002)]. They showed that large radius indenters produced an indentation stress–strain curve independent of indenter radius with a hardening coefficient equivalent to that in uniaxial tests on the same materials. For smaller indenters, the indentation stress–strain curves appeared at progressively higher pressures for smaller radius indenters. Here, we present similar experimental data for nickel. We simulate these experiments by inputting the uniaxial stress–strain data into a finite element model, which has no intrinsic length scale dependence (essentially using the von Mises criteri...

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Nigel M. Jennett

Papers

Atomic force acoustic microscopy methods to determine thin-film elastic properties

Progress in determination of the area function of indenters used for nanoindentation

Higher accuracy analysis of instrumented indentation data obtained with pointed indenters

Study of the interaction between the indentation size effect and Hall–Petch effect with spherical indenters on annealed polycrystalline copper

On the indentation size effect in spherical indentation