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Environment effects in plasticity and fracture of metals, well studied for several decades, still pose many unanswered questions. A micro-mechanics explanation of how dislocation activity is influenced by the material surface state, that can answer these questions, has been elusive. We build on a recently discovered effect in metal cutting – organic monolayer embrittlement (OME) – wherein metal surfaces are rendered brittle by long-chain organic adsorbates, to explore how material state variables influence surface plasticity and fracture. In particular, cutting experiments with Al containing Self Assembled Monolayers (SAMs), show that the OME is controlled by surface stress (f) induced by the adsorbates. This is contrary to many instances of environment-assisted fracture which are usually attributed to surface energy changes, and wherein fis largely ignored. Other contributions include (a) a cantilever technique to measure surface stress, (b) demonstration of strong effect of SAM molecule chain length on f, (c) characterization of how dislocation activity at crack-tips is affected by adsorbate-induced f, and (d) large improvements in machining processes enabled by controlled environment-assisted fracture. We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC.

Abstract: Roughness, defined as unevenness of material surfaces, plays an important role in determining how engineering components or natural objects interact with other bodies and their environment. The emergence of fractal roughness on natural and engineered surfaces across a range of length scales suggests the existence of common processes and mechanisms for nucleation and evolution of roughness. In this article, we review recent advances in understanding the origins of roughness and topography evolution on natural and engineered surfaces and their connection with subsurface deformation mechanisms. Directions for future research toward understanding the origins of roughness on solid surfaces are discussed. Graphical abstract:

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Graphical abstract:

Modulation-assisted machining (MAM) superimposes a controlled, low-frequency oscillation in the feed direction of conventional machining (CM). The process transforms the otherwise continuous cutting into a series of interrupted cutting events. As a result, the tool is subject to time-varying temperature field in MAM. This paper proposes an analytical model for predicting the transient tool temperature in MAM. Chip formation during MAM is studied to investigate the time-varying undeformed chip thickness. The heat flux in MAM is determined based on a non-equal distance shear zone model. A reduction in specific energy due to modulation is also considered. Then, the tool is modeled as a semi-infinite rectangular corner, and a Green's function approach is applied for calculating the temperature. The analytical results are compared with finite element modeling (FEM). A set of cylindrical turning experiments is carried out with and without modulation, showing the discrete chip formation in MAM and related reduction in tool temperature. The analytical model provides a framework for characterizing the effect of the process parameters on applications for modulation-assisted machining.

Abstract

The application of controlled, low-frequency modulation (∼100Hz) superimposed onto the cutting process in the feed-direction – modulation-assisted machining (MAM) – is shown to be quite effective in reducing the wear of cubic boron nitride (CBN) tools when machining compacted graphite iron (CGI) at high machining speeds (500m/min). The tool life is at least 20 times greater than in conventional machining. This significant reduction in wear is a consequence of the multiple effects realized by MAM, including periodic disruption of the tool–workpiece contact, formation of discrete chips, enhanced fluid action and lower cutting temperatures. The propensity for thermochemical wear of CBN, the principal wear mode at high speeds in CGI machining, is thus reduced. The tool wear in MAM is also found to be smaller at the higher cutting speeds (730m/min) tested. The feed-direction MAM appears feasible for implementation in industrial machining applications involving high speeds.

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Abstract

Deformation microstructure and texture in Mg-AZ31B bulk strips processed through extrusion-machining were studied as a function of deformation temperature. At warm deformation temperatures (~200oC), cold-worked type microstructures with predominant tilted basal texture were observed. With increase in temperature, grain structure sharply transformed into equiaxed type with predominant in-plane basal texture. This sharp transition was found to be consistent with change in temperature dependent dynamic recrystallization mechanism from continuous to discontinuous type.

FCC materials were subjected to large strain deformation by three techniques: equal channel angular pressing (ECAP), plane-strain machining and friction stir processing (FSP). Based on the orientation imaging mapping (OIM) analysis of the deformed regions, the most likely microstructure refinement mechanisms have been identified for each of the techniques and compared among one another.

Deformation history and state of chip and machined surface in low-speed cutting have been characterized using image analysis, complemented by microstructure and hardness. Fine scale details of the deformation field of relevance to machining modelling are highlighted. The severe plastic deformation inherent to chip formation results in microstructure changes which can be controlled through appropriate process parameters selected with the aid of machining simulations. Scaling of subsurface strain distribution is observed. Similarity in deformation history of chip and near-surface suggests opportunities for engineering surfaces with controlled deformation levels and microstructures, directly, by machining. The deformation characterization offers substantial scope for improvement and validation of machining models, and enhancement of machining process capability.

Modulation Assisted Machining (MAM), based on controlled superimposition of low-frequency modulation to conventional machining, effects discrete chip formation and disrupts the severe contact condition at the tool-chip interface. The underlying theory of discrete chip formation and its implications are briefly described and illustrated. Benefits such as improved chip management and lubrication, reduction of tool wear, enhanced material removal, particulate manufacturing and surface texturing are highlighted using case studies. MAM represents a new paradigm for machining in that it deliberately employs ‘good vibrations’ to enhance machining performance and capability.

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The deformation field associated with chip formation in plane strain (2-D) machining has been simulated using the finite element method (FEM), with the objective of developing 2-D machining as an experimental technique for studying very large strain deformation phenomena. The principal machining parameters are the tool rake angle, cutting velocity and the friction at the toolchip interface while the deformation field parameters are strain, strain rate and temperature. The relation between rake angle and the shear strain in the deformation zone is studied for the low-speed cutting of lead. This correspondence is validated by comparison with measurements of the deformation parameters made by applying a Particle Image Velocimetry (PIV) technique to highspeed photographic image sequences of the deformation. It is shown that plastic strains in the range of 1-15 can be realized in a controlled manner by appropriate choice of the rake angle. The unique capabilities offered by 2-D machining for studying micro- and nano- mechanics of large strain deformation, and the creation of ultra-fine grained materials are highlighted in the context of these results.

Diffusion of water into plant materials is known to decrease their mechanical strength and stiffness but improve formability. Here, we characterize water diffusion through areca palm leaf-sheath—a model plant material, with hierarchical structure, used in eco-friendly foodware. The diffusion process is studied using mass gain measurements and in situimaging of water transport. By treating the areca sheath as homogeneous ensemble, and incorporating effects of material swelling due­ to water absorption, a factor typically neglected in prior studies, the diffusion coefficient Dwfor water is estimated as (6.5 ± 2.2) × 10−4mm2s−1. It is shown that neglecting the swelling results in gross underestimation of Dw. Microstructural effects (e.g. fibre, matrix) on the diffusion are characterized using in situimaging of the water transport at high resolution. The observations show that the water diffuses an order of magnitude faster in the matrix (8.63 × 10−4mm2s−1) than in the fibres (7.19 × 10−5mm2s−1). This non-uniformity is also reflected in the swelling-induced strain in the leaf, mapped by image correlation. Lastly, we vary salt concentration by controlled additions of NaCl and note a non-monotonic dependence of the diffusion on concentration. Implications of the results for improving foodware manufacturing processes and product life are discussed.

In this paper we propose a simple and elegant distributed self-stabilizing algorithm to solve the all-pairs shortest path problem in graph theory. We have proved the correctness of our algorithm without using a bounded function (which is customary for proving correctness of self-stabilizing algorithms); the new approach is simple and seems to have potential in developing self-stabilizing algorithms for other graph-theoretical problems.

Bezout's theorem gives an upper bound on the degree of the intersection of properly intersecting algebraic varieties. In spaces of dimension higher than two, however, intersections between many algebraic varieties such as curves are improper. Bezout's theorem cannot be directly used to bound the number of points at which these curves intersect. In this paper an algebrogeometric technique is developed for obtaining an upper bound on the number of intersection points of two irreducible algebraic curves in k-dimensional space. The theorems obtained are applied to the specific case of intersecting algebraic space curves in three-dimensional space, and a number of examples are analyzed in this regard. The implications of the derived results for computer-aided geometric design are discussed.

Environment-assisted fracture phenomena in metals are usually associated with surface energy reduction due to an adsorbed film. Here we demonstrate a unique embrittlement effect in Al that is instead mediated by surface stress, induced by an adsorbed organic monolayer. Atomistic simulations show that the adsorbate carbon-chain length lccontrols the surface stress via van der Waals forces, being compressive for lc< 8 and tensile otherwise. For lc> 8, we demonstrate experimentally that the nanoscale film causes a ductile-to-brittle transition on the macroscale. Concomitant with this transition is a nearly 85% reduction in deformation forces. Additional simulations reveal that the microscopic mechanism for the embrittlement is via suppression of dislocation emission at incipient crack-tips. In addition to challenging long-held views on environment-assisted fracture, our findings pertaining to surface-stress induced embrittlement suggest profitable utility in manufacturing processes such as machining and comminution.