Vadim V. Silberschmidt

Bio: Vadim V. Silberschmidt is an academic researcher from Loughborough University. The author has contributed to research in topics: Machining & Materials science. The author has an hindex of 44, co-authored 543 publications receiving 8619 citations. Previous affiliations of Vadim V. Silberschmidt include University of Rhode Island & Universities UK.

Fractal approach in modelling of earthquakes

Abstract: A model for an earthquake simulation is proposed with the use of a fractal approach. Multiple generation and coalescence of shear faults in stochastic brittle media (modelled as a 2d lattice) are considered to be a source of seismicity. Dynamics of local failure events are governed by accumulation of shear defects, described in terms of continuum damage mechanics. Fractal tree structure is used as an analogue for a stress redistribution process. Energy release, caused by the non-uniform failure, is studied for a non-conservative case. Effect of various types of rocks' properties stochasticity on energy release dynamics is analysed with a utilization of multifractal formalism. The latter is shown to be an additional method for seismicity characterization.

Numerical study of strain-rate effect in cold rolls forming of steel

Abstract: Cold roll forming (CRF) is a well-known continuous manufacturing process, in which a flat strip is deformed by successive rotating pairs of tools, without changing the material thickness. In the past decades, to lessen the process-development efforts, finite-element simulations have been increasingly employed to improve the process design and predict the manufacturing-induced defects. One of the important aspects in design of the CRF process is consideration of resulting strains in the final product as the material passes through several complex forming stands. Sufficient knowledge of longitudinal strain in the workpiece is required to set various process parameters. Increasing a process speed in a roll forming operation can bring cost advantages, but the influence of the forming speed on the strain distribution should be explored. This study is focussed on a strain-rate effect in the CRF process of steel sheets. The strain-rate dependency of a plastic behaviour observed in most metals can affect the finished product's quality as well as process parameters. This paper investigates the influence of the strain rate on longitudinal strains induced in the roll forming operation by incorporating a phenomenological Johnson-Cook constitutive model, which allows studying the impact of the process speed on the output product. Taking advantage of 3D finite element analysis, a roll forming process was simulated using MCS.Marc, comprising a complete set of forming stations. Through the changing of the process speed, the strain rate impact on longitudinal peak strains and forming length was investigated. The results highlight the effect of the strain rate on edge thinning and subsequent undesirable distortions in the product.

Modelling plastic deformation in a single-crystal nickel-based superalloy using discrete dislocation dynamics

Abstract: Nickel-based superalloys are usually exposed to high static or cyclic loads in non-ambient environment, so a reliable prediction of their mechanical properties, especially plastic deformation, at elevated temperature is essential for improved damage-tolerance assessment of components. In this paper, plastic deformation in a single-crystal nickel-based superalloy CMSX4 at elevated temperature was modelled using discrete dislocation dynamics (DDD). The DDD approach was implemented using a representative volume element with explicitly-introduced precipitate and periodic boundary condition. The DDD model was calibrated using stress–strain response predicted by a crystal plasticity model, validated against tensile and cyclic tests at 850 °C for and crystallographic orientations, at a strain rate of 1/s. The DDD model was capable to capture the global stress–strain response of the material under both monotonic and cyclic loading conditions. Considerably higher dislocation density was obtained for the orientation, indicating more plastic deformation and much lower flow stress in the material, when compared to that for orientation. Dislocation lines looped around the precipitate, and most dislocations were deposited on the surface of precipitate, forming a network of dislocation lines. Simple unloading resulted in a reduction of dislocation density. Plastic deformation in metallic materials is closely related to dynamics of dislocations, and the DDD approach can provide a more fundamental understanding of crystal plasticity and the evolution of heterogeneous dislocation networks, which is useful when considering such issues as the onset of damage in the material during plastic deformation.

Improvements of material removal in cortical bone via impact cutting method.

Abstract: Bone cutting with high efficiency as well as low levels of forces and damage has a great significance for orthopaedic surgeries. Due to the brittleness and anisotropy of cortical bone, a conventional cutting process can cause irregular crack propagation and fractured bone chip, affecting the tissue removal process and postoperative recovery. In this paper, a high-frequency impact cutting method is investigated, and its effect on fracture propagation, chip formation and cutting forces is studied for orthogonal cutting. Experimental results show that cracks are deflected by cement lines in conventional cutting, forming fractured blocks or split chips. In impact cutting, the cutting-induced fractures expand along a main shear direction, generating small pieces of triangular segmented chips. Cutting forces are significantly reduced with vibration-induced impacts; especially, the main cutting force is nearly 70% lower than that in the conventional cutting. The main reason for this is much higher strain rates in high-frequency impact cutting than in a conventional process, and direct penetration of fractures across the osteonal matrix without deflections along the cement lines. This results in a straighter path along the main shear plane and totally different chip morphology; so, a lower consumption of cutting energy in the main shear direction reduces the macroscopic cutting force. The results of this study have an important theoretical and practical value for revealing the mechanism of impact cutting, improving the efficiency of osteotomy and supporting the innovation in bone surgical instruments.

Fracture of 3D-printed micro-tensile specimens: filament-scale geometry-induced anisotropy

Abstract: The interlayer interface was widely considered as the reason for anisotropic mechanical properties in 3D-printed parts produced by material extrusion additive manufacturing (MEAM). Still, the cause has remained widely debated. Utilising a specially developed micro-tensile specimen formed by single filaments, this study examines the roles of their orientation and filament-scale geometric features on mechanical performance. The specimens were loaded in two directions: (i) longitudinal (F), coinciding with the main axis of extruded filaments, and (ii) transverse (Z), normal to the interface between layers. To replicate the geometrical groove features found at the interlayer interfaces in Z specimens, some of the F specimens were scored manually perpendicular to the load prior to tensile testing to produce similar filament-scale features. Tensile testing of all specimens with microscopic characterisation showed that both F specimens (with and without manual grooves) and Z specimens shared very similar strength characteristics, close to those of bulk polylactide (PLA). Manually grooved F specimens demonstrated significantly reduced plasticity, strain-at-fracture and load-bearing area - very close to the Z specimen’s characteristics indicating that the presence of natural grooves in Z specimens is the predominant cause of mechanical anisotropy in MEAM as opposed to commonly assumed poor interlayer molecular diffusion.

Fast parallel algorithms for short-range molecular dynamics

Abstract: 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.

The Design and Analysis of Experiments

Abstract: THE DESIGN AND ANALYSIS OF EXPERIMENTS. By Oscar Kempthorne. New York, John Wiley and Sons, Inc., 1952. 631 pp. $8.50. This book by a teacher of statistics (as well as a consultant for \"experimenters\") is a comprehensive study of the philosophical background for the statistical design of experiment. It is necessary to have some facility with algebraic notation and manipulation to be able to use the volume intelligently. The problems are presented from the theoretical point of view, without such practical examples as would be helpful for those not acquainted with mathematics. The mathematical justification for the techniques is given. As a somewhat advanced treatment of the design and analysis of experiments, this volume will be interesting and helpful for many who approach statistics theoretically as well as practically. With emphasis on the \"why,\" and with description given broadly, the author relates the subject matter to the general theory of statistics and to the general problem of experimental inference. MARGARET J. ROBERTSON

American Society for Testing and Materials

Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

Effect of Temperature

Abstract: A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.