Search

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 f is 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 FROM AUTHOR]

The study of fracture mechanics is usually within the paradigm of a failure mode that needs to be avoided. However, both in nature and in modern technology, there exist several situations where an ability to fracture is essential. In this work, we consider the problem of machining highly ductile and strain-hardening metals, such as annealed Cu, Al and Ta. These metals are known by the moniker "gummy metals" due to the large forces and poor surface finish associated with machining them. We investigate a chemo-mechanical technique involving adsorption of organic monolayers on the metal surfaces that causes the metals to become relatively brittle. This transition from ductile to brittle results in > 50% drop in the cutting force and an order of magnitude improvement in the surface finish. Molecular dynamics simulations of the phenomenon show the organic monolayers impose a surface stress on the metal surface which results in arresting of the dislocations close to the surface. The results suggest that a deeper understanding of the underlying mechanism has implications in environment-assisted cracking, stress-corrosion cracking and hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Next‐generation, high‐efficiency energy storage and conversion systems require development of lithium metal batteries. But the high cost of production and constraints on thickness of lithium (anode) foils continue to limit adoption for integration into battery cell architectures. Here, a novel lithium anode manufacturing solution is demonstrated – single‐step production of ultrathin gauge foil formats directly from solid ingot. Hybrid cutting‐based deformation processes, involving large plastic strains and strain rates, produce foil to sub‐10 µm thickness, with surface quality even superior to present Li anode processing routes. Energy analysis shows the single‐stage processing is ≈50% more efficient than conventional processing by extrusion‐rolling. Through in situ force measurements and high‐speed imaging of the cutting it also characterize – for the first time – the flow stress of Li to strain rates of 800 sec−1, revealing a power‐law relationship. The results present a paradigm shift in manufacturing and integration of solid lithium anodes for energy applications. [ABSTRACT FROM AUTHOR]

This document is an introduction to a special issue of the International Journal of Fracture on structural integrity. It emphasizes the importance of understanding how materials behave under extreme conditions and the role of advanced experimental and computational tools in studying structural integrity. The document discusses different failure mechanisms in materials, such as ductile failure in alloys and brittle failure in amorphous materials, as well as fatigue and corrosion. It also highlights the significance of considering material behavior at lower length scales and the use of advanced computational tools in analyzing and predicting structural behavior. The document concludes by stating that a multifaceted approach is necessary for designing safe and reliable structures. The special issue focuses on fracture, fatigue, and composite damage, with contributions from various authors. [Extracted from the article]

Deformation response of areca catechu palm leaf-sheath, after hydration and aqueous sodium hydroxide treatments, is studied using Limiting Dome Height testing. The leaf material can undergo large shape changes in biaxial stretching, with forming strains as high as 0.56 reached using optimized sodium hydroxide (NaOH) treatment (5% for 2 h). These limit strains are higher than for many sheet metals. The treatment also lowers the forming force, another key attribute of formability. Using parametric studies, optimal treatment conditions are established for producing high-aspect ratio products. Implications for single-step forming of palm-leaf materials and sustainable manufacturing of eco-friendly foodware are discussed. [ABSTRACT FROM AUTHOR]

The question of how soft polymers slide against hard surfaces is of significant scientific interest, given its practical implications. Specifically, such systems commonly show interesting stick–slip dynamics, wherein the interface moves intermittently despite uniform remote loading. The year 2021 marked the 50th anniversary of the publication of a seminal paper by Adolf Schallamach (Wear, 1971), which first revealed an intimate link between stick–slip and moving detachment waves, now called Schallamach waves. We place Schallamach's results in a broader context and review subsequent investigations of stick–slip, before discussing recent observations of solitary Schallamach waves. This variant is not observable in standard contacts so that a special cylindrical contact must be used to quantify its properties. The latter configuration also reveals the occurrence of a dual wave—the so-called separation pulse—that propagates in a direction opposite to Schallamach waves. We show how the dual wave and other, more general, Schallamach-type waves can be described using continuum theory and provide pointers for future research. In the process, fundamental analogues of Schallamach-type waves emerge in nanoscale mechanics and interface fracture. The result is an ongoing application of lessons learnt from Schallamach-type waves to better understand these latter phenomena. This article is part of the theme issue 'Nanocracks in nature and industry'. [ABSTRACT FROM AUTHOR]

We characterize primary (shape-change) and secondary (friction) deformation, and associated temperature fields, in metal cutting and forming processes, using in situ imaging and simulation. The experimental configurations enable access to the deformation zones and die contact interfaces, for measuring deformation, temperature, and frictional drag. Infrared thermography reveals that the plastic strain-rate field is an excellent proxy for the deformation-induced heat sources. Both spatially confined and diffuse strain-rate fields occur, depending on the initial workpiece deformation state. When the strain rate is confined, as in prehardened material, the temperature modeling is greatly simplified, as the heat source is also now localized. However complex, microstructure-driven deformation modes, such as sinuous flow in annealed metals, result in spatially diffuse strain-rate and body heat sources that are more challenging to analyze. Our unified measurements should be of value for accurately estimating the fraction of plastic dissipation that is converted into heat in large-strain deformation processes. [ABSTRACT FROM AUTHOR]

Deformation response of Areca catechu palm leaf-sheath under biaxial stretching is evaluated using Limiting Dome Height (LDH) testing. It is shown that the leaf material has high formability, with biaxial limit strains ~ 0.4, comparable to that of ductile sheet metals. Hydration of the leaf-sheath prior to the forming increases the limit strains by approximately 500% and lowers deformation forces by up to 85%. The potential of the LDH test for characterizing the stretch-forming response of plant materials is also demonstrated. Implications for single-step forming of plant leaf materials into products and manufacturing of eco-friendly foodware are discussed. [ABSTRACT FROM AUTHOR]

The problem of conduction electron scattering by inhomogeneous crystal lattice strains is addressed using a tight-binding formalism and the differential geometric treatment of deformations in solids. In this approach, the relative positions of neighboring atoms in a strained lattice are naturally taken into account, even in the presence of crystal dislocations, resulting in a fully covariant Schrcodinger equation in the continuum limit. Unlike previous work, the developed formalism is applicable to cases involving purely elastic strains as well as discrete and continuous distributions of dislocations-in the latter two cases, it clearly demarcates the effects of the dislocation strain field and core. It also differentiates between elastic and plastic strain contributions, respectively. The electrical resistivity due to the strain field of edge dislocations is then evaluated and the resulting numerical estimate for Cu shows good agreement with reported experimental values. This indicates that the electrical resistivity of edge dislocations in metals is not entirely due to the core, contrary to current models. Application to the study of strain effects in constrained quantum systems is also discussed. [ABSTRACT FROM AUTHOR]

In the article "Single-Step Deformation Processing of Ultrathin Lithium Foil and Strip," Debapriya P. Mohanty and co-workers present a new method for manufacturing thin lithium foil for use in lithium batteries. This method involves machining-based deformation processing, which can produce metal foil as thin as a few micrometers in a single step from an ingot. The process is more energy-efficient than traditional multi-stage processes like rolling and can be implemented at the point of use with minimal infrastructure. [Extracted from the article]

A formulation of dislocation-indenter interaction in two-dimensional, isotropic elasticity is presented. A significant dislocation-indenter interaction is predicted when dislocations are nucleated very close to the indenter. This interaction is expected to have an important influence on dislocation motion and multiplication. Upon nucleation close to the indenter, the dislocations are shown to modify the load, load distribution, and moment acting on the indenter. This effect is seen to vary with the indentation contact length. Further away from the indenter, the indenter-dislocation interaction is shown to be negligible. [ABSTRACT FROM AUTHOR]

Shear banding, a type of inhomogeneous plastic flow involving very large local strains, occurs in a variety of material systems. We study dynamics of evolution of single shear bands at strain rates of up to per second in three different polycrystalline metal systems, using a special shear deformation framework and a micro-marker technique calibrated to track localised deformation fields at micrometer resolution. Once a band is nucleated as a weak interface, localised plastic flow occurs via Bingham-type viscous sliding between material segments on either side of the interface. As a result, the evolution and magnitude of strains and material displacements in the band vicinity are well-described by a model based on momentum diffusion. The viscosity at the band interface is very small, only a few mPa·sec, and is comparable to those of liquid metals at their melting point. Based on analysis of various contributions to band viscosity at the microscopic level, a plausible explanation based on phonon drag on dislocation motion is presented for the small viscosity. The accuracy of predictions made by the momentum diffusion model for different materials and deformation rates suggests that once nucleated, a shear band evolves by a common mechanism that is relatively insensitive to microstructure details. [ABSTRACT FROM AUTHOR]

The cutting of metals has long been described as occurring by laminar plastic flow. Here we show that for metals with large strain-hardening capacity, laminar flow mode is unstable and cutting instead occurs by plastic buckling of a thin surface layer. High speed in situ imaging confirms that the buckling results in a small bump on the surface which then evolves into a fold of large amplitude by rotation and stretching. The repeated occurrence of buckling and folding manifests itself at the mesoscopic scale as a new flow mode with significant vortex-like components--sinuous flow. The bucklingmodel is validated by phenomenological observations of flow at the continuum level and microstructural characteristics of grain deformation and measurements of the folding. In addition to predicting the conditions for surface buckling, the model suggests various geometric flow control strategies that can be effectively implemented to promote laminar flow, and suppress sinuous flow in cutting, with implications for industrial manufacturing processes. The observations impinge on the foundations of metal cutting by pointing to the key role of stability of laminar flow in determining the mechanism of material removal, and the need to reexamine long-held notions of large strain deformation at surfaces. [ABSTRACT FROM AUTHOR]

Simple shear deformation via hybrid cutting-extrusion is used to produce continuous electrical steel sheet from a commercial high-silicon (nominal 4 wt%) iron alloy of poor workability in a single deformation step, a fundamentally different route from the multi-step processing of rolling and annealing currently in use. The shear texture created in the sheet is found to be quite different from that produced by rolling. The magnetic properties of the shear-textured Fe–Si sheet are measured using closed-circuit permeametry and compared with those from sheet produced by rolling of the same alloy and a commercial non-grain-oriented sheet of similar composition. Properties compared include maximum relative permeability, induction, coercivity, and hysteresis loss. The results are interpreted in terms of microstructure, texture, and composition. A unit cell representation of the shear texture components is introduced that relates the expected orientation of easy magnetization directions with the sheet axes. [ABSTRACT FROM AUTHOR]

Shear banding is a plastic flow instability with highly undesirable consequences for metals processing. While band characteristics have been well studied, general methods to control shear bands are presently lacking. Here, we use high-speed imaging and micro-marker analysis of flow in cutting to reveal the common fundamental mechanism underlying shear banding in metals. The flow unfolds in two distinct phases: an initiation phase followed by a viscous sliding phase in which most of the straining occurs. We show that the second sliding phase is well described by a simple model of two identical fluids being sheared across their interface. The equivalent shear band viscosity computed by fitting the model to experimental displacement profiles is very close in value to typical liquid metal viscosities. The observation of similar displacement profiles across different metals shows that specific microstructure details do not affect the second phase. This also suggests that the principal role of the initiation phase is to generate a weak interface that is susceptible to localized deformation. Importantly, by constraining the sliding phase, we demonstrate a material-agnostic method--passive geometric flow control--that effects complete band suppression in systems which otherwise fail via shear banding. [ABSTRACT FROM AUTHOR]

Sheet processing of high Si-Fe alloys (up to 6.5 wt pct Si) is demonstrated by application of highly confined shear deformation in cutting-extrusion. This alloy system, of major interest to electromagnetic applications, is characterized by poor workability. By a suitable interactive combination of simple shear, high strain rates, near-adiabatic heating, and large hydrostatic pressure in the deformation zone, flow localization, and cracking inherent to this alloy system are suppressed. This enables creation of sheet and foil forms from bulk ingots, cast or wrought, in a single deformation step, unlike rolling. The sheet is characterized by strong shear textures, described by partial {110} and 〈111〉 fibers, and fine-grained microstructures (~20 µm grain size). The orientation (inclination) of these fibers, with respect to the sheet surface, can be varied over a range of 35 deg through selection of the deformation path. In contrast to rolling textures, the current shear deformation textures are negligibly influenced by recrystallization annealing. A recovery-based continuous recrystallization mechanism is proposed to explain the texture retention. Some general implications for shear-based processing of alloys of limited workability are discussed. [ABSTRACT FROM AUTHOR]

The kinematic flow pattern in slow deformation of a model dense granular medium is studied at high resolution using in situ imaging, coupled with particle tracking. The deformation configuration is indentation by a flat punch under macroscopic plane-strain conditions. Using a general analysis method, velocity gradients and deformation fields are obtained from the disordered grain arrangement, enabling flow characteristics to be quantified. The key observations are the formation of a stagnation zone, as in dilute granular flow past obstacles; occurrence of vortices in the flow immediately underneath the punch; and formation of distinct shear bands adjoining the stagnation zone. The transient and steady state stagnation zone geometry, as well as the strength of the vortices and strain rates in the shear bands, are obtained from the experimental data. All of these results are well-reproduced in exact-scale non-smooth contact dynamics simulations. Full 3D numerical particle positions from the simulations allow extraction of flow features that are extremely difficult to obtain from experiments. Three examples of these, namely material free surface evolution, deformation of a grain column below the punch and resolution of velocities inside the primary shear band, are highlighted. The variety of flow features observed in this model problem also illustrates the difficulty involved in formulating a complete micromechanical analytical description of the deformation. [ABSTRACT FROM AUTHOR]

Annealed metals are surprisingly difficult to cut, involving high forces and an unusually thick "chip." This anomaly has long been explained, based on ex situ observations, using a model of smooth plastic flow with uniform shear to describe material removal by chip formation. Here we show that this phenomenon is actually the result of a fundamentally different collective deformation mode--sinuous flow. Using in situ imaging, we find that chip formation occurs via large-amplitude folding, triggered by surface undulations of a characteristic size. The resulting fold patterns resemble those observed in geophysics and complex fluids. Our observations establish sinuous flow as another mesoscopic deformation mode, alongside mechanisms such as kinking and shear banding. Additionally, by suppressing the triggering surface undulations, sinuous flow can be eliminated, resulting in a drastic reduction of cutting forces. We demonstrate this suppression quite simply by the application of common marking ink on the free surface of the workpiece material before the cutting. Alternatively, prehardening a thin surface layer of the workpiece material shows similar results. Besides obvious implications to industrialmachining and surface generation processes, our results also help unify a number of disparate observations in the cutting of metals, including the so-called Rehbinder effect. [ABSTRACT FROM AUTHOR]

The flow dynamics, deformation fields and chipparticle formation in cutting and sliding of metals are analysed, in situ, using high-speed imaging and particle image velocimetry. The model system is a brass workpiece loaded against a wedge indenter at low speeds. At large negative rake angles, the flow is steady with a prow of material forming ahead of the indenter. There is no material removal and a uniformly strained layer develops on the workpiece surface--the pure sliding regime. When the rake angle is less negative, the flow becomes unsteady, triggered by formation of a crack on the prow free surface and material removal ensuing--the cutting regime. The strain on the prow surface at crack initiation is found to be constant. Chip morphologies, such as discrete particle, segmented chip and continuous chip with mesoscale roughness, are shown to arise from a universal mechanism involving propagation of the prow crack, but to different distances towards the indenter tip. The simple shear deformation in continuous chip formation shows small-angle oscillations also linked to the prow crack. Implications for material removal processes and ductile failure are discussed. [ABSTRACT FROM AUTHOR]

Using high-resolution, in situ imaging of a hard, wedge-shaped model asperity sliding against a metal surface, we demonstrate a new mechanism for particle formation and delamination wear. Damage to the residual surface is caused by the occurrence of folds on the free surface of the prow-shaped region ahead of the wedge. This damage manifests itself as shallow crack-like features and surface tears, which are inclined at very acute angles to the surface. The transformation of folds into cracks, tears and particles is directly captured. Notably, a single sliding pass is sufficient to damage the surface, and subsequent passes result in the generation of platelet-like wear particles. Tracking the folding process at every stage from surface bumps to folds to cracks/tears/particles ensures that there is no ambiguity in capturing the mechanism of wear. Because fold formation and consequent delamination are quite general, our findings have broad applicability beyond wear itself, including implications for design of surface generation and conditioning processes. [ABSTRACT FROM AUTHOR]

A study is made of the rotation field in wedge indentation of metals using copper as the model material system. Wedges with apical angles of 60° and 120° are used to indent annealed copper, and the deformation is mapped using image correlation. The indentation of annealed and strain-hardened copper is simulated using finite element analysis. The rotation field, derived from the deformation measurements, provides a clear way of distinguishing between cutting and compressive modes of deformation. Largely unidirectional rotation on one side of the symmetry line with small spatial rotation gradients is characteristic of compression. Bidirectional rotation with neighboring regions of opposing rotations and locally high rotation gradients characterizes cutting. In addition, the rotation demarcates such characteristic regions as the pile-up zone in indentation of a strain-hardened metal. The residual rotation field obtained after unloading is essentially the same as that at full load, indicating that it is a scalar proxy for plastic deformation as a whole. [ABSTRACT FROM PUBLISHER]

Direct observation of the chip-tool interface was made using transparent sapphire tools in combination with a CCD-based high-speed imagining system. The observations made for various nonferrous workpiece materials suggest that the contact conditions at the chip-tool interface be classified into three types depending on the nature of the zone of stagnant material - negligible zone of stagnant material (Type 1), zone of stagnant material that is stable and confined to the vicinity of the cutting edge (Type 2), and zone of stagnant material that expands upward from the cutting edge as cutting progresses (Type 3). Velocity profiles obtained using the particle image velocimetry (PIV) show that retardation of the chip underside occurs in the intimate contact region for the Type 2 materials while it is negligible for the Type 1 materials. Nanoindentation hardness profiles measured with depth into the chip from the chip underside indicate that the expansion of the zone of stagnant material observed for the Type 3 materials could be related to the work-hardenability of the chip material in the secondary deformation zone. [ABSTRACT FROM AUTHOR]

Large plastic strains between 1 and 15 can be imposed in chips formed by plane-strain (2-D) machining of metals and alloys. This approach has been used to examine microstructure changes induced by large strain deformation in model systems—copper and its alloys, precipitation-hardenable aluminum alloys, high-strength materials such as titanium, Inconel 718 and 52100 steel, and an amorphous alloy. It is shown that materials with average grain sizes in the range of 60 nm–1 μm can be created by varying the parameters of machining, which in turn affects the deformation processes. Furthermore, a switch-over from an elongated subgrain microstructure to an equi-axed nanocrystalline microstructure, with a preponderance of large-angle grain boundaries, has been demonstrated at the higher levels of strain in several of these materials. This switch-over can be readily controlled by varying the deformation conditions. Dynamic recrystallization has been demonstrated in select material systems under particular conditions of strain and temperature. This study may be seen as providing an important bridge between furthering the understanding of microstructural refinement by large strain deformation and the practical utilization of nanostructured materials in structural and mechanical applications. Conventional plane-strain machining has been shown to be a viable SPD method for examining the underlying processes of very large strain deformation. [ABSTRACT FROM AUTHOR]

Measurement of strain field in the primary deformation zone is of major interest for development of machining as an experimental technique for studying phenomena associated with large strain deformation. A study has been made of the primary deformation zone and tool-chip interface in planestrain (two-dimensional) machining of metals. The use of a high-speed, charge-coupled device (CCD) imaging system in conjunction with an optically transparent, sapphire cutting tool has enabled characteristics of the deformation field such as velocity, strain, and material flow, to be obtained at high spatial and temporal resolution. The velocity distributions in the primary deformation zone and along the tool rake face have been obtained by applying a particle image velocimetry (PIV) technique to sequences of high-speed images of the chip-tool interface taken through the transparent tool, and of the primary deformation zone recorded from a side of the workpiece. A procedure is presented and demonstrated for determining the strain and strain rate distributions in the primary deformation zone. The measurements have provided data about the variations of velocity, strain rate, and strain, in and around the cutting edge and primary deformation zone: confirmed the existence of a region of retarded sliding in the region of intimate contact between tool and chip; and highlighted the occurrence of a region of dead metal ahead of the cutting edge when cutting with a negative rake angle tool. [ABSTRACT FROM AUTHOR]

Highly strain-hardening metals such as Al, Ni, and stainless steels, although relatively soft, are well known as being difficult to cut, because of an unsteady and highly redundant mode of plastic deformation—sinuous flow—prevailing during chip formation. This difficulty in cutting is greatly ameliorated, if the workpiece surface ahead of the chip formation region is coated with certain chemical media such as glues, inks, and alcohols that are quite benign. High-speed imaging shows that the media effect a change in the local plastic deformation mode, from sinuous flow to one characterized by periodic fracture—segmented flow. This flow transition, due to a mechanochemical effect, results in significant reduction of deformation forces and energy, often > 50%, thus facilitating the cutting. The effect is mostly pronounced at smaller undeformed chip thickness, typical of finish and semi-finish machining regimes. The quality of the cut surface, as measured by defect density and surface roughness, improves by an order of magnitude, when the media are applied. Furthermore, this surface is relatively strain free in contrast to conventionally machined surfaces. The mechanochemical effect, with a strong coupling to the flow mode, is controllable, with the media showing similar efficacy across different metal systems. The results suggest opportunities for improving performance of machining processes for many difficult-to-cut gummy metals. [ABSTRACT FROM AUTHOR]