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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]

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]

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]

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]

Abstract: Tantalum powders containing different oxygen concentrations have been vacuum hot-pressed in graphite dies to study the dissolution and precipitation of oxygen and carbon in powder metallurgy (PM) tantalum. Various types of oxide and carbide precipitates were observed using microscopy and analyzed by X-ray microdiffraction. An in situ contact gettering method using zirconium has been coupled with hot-pressing to control oxygen. This method is effective at removing oxygen from the solid solution, while the precipitation behavior is not significantly altered. Hardness profiles with distance from Zr contact agree well with those expected from oxygen concentration profiles predicted by analysis assuming a diffusion-limited rate of gettering. Corresponding lattice parameter measurements by microdiffraction indicate that oxygen prefers to stay in supersaturated solid solution, even under slow cooling, where it is much more effective in hardening than in the form of precipitates. [Copyright &y& Elsevier]

Enhanced workability, as characterized by the magnitude and heterogeneity of accommodated plastic strains during sheet processing, is demonstrated in high Si content Fe-Si alloys containing 4 and 6.5 wt% Si using two single-step, simple-shear deformation techniques – peeling and large strain extrusion machining (LSEM). The model Fe-Si material system was selected for its intrinsically poor material workability, and well-known applications potential in next-generation electric machines. In a comparative study of the deformation characteristics of the shear processes with conventional rolling, two distinct manifestations of workability are observed. For rolling, the relatively diffuse and unconfined deformation zone geometry leads to cracking at low strains, with sheet structures characterized by extensive deformation twinning and banding. Workpiece pre-heating is required to improve the workability in rolling. In contrast, peeling and LSEM produce continuous sheet at large plastic strains without cracking, the result of more confined deformation geometries that enhances the workability. Peeling, however, results in heterogeneous, shear-banded microstructures, pointing to a second type of workability issue – flow localization – that limits sheet processing. This shear banding is to a large extent facilitated by unrestricted flow at the sheet surface, unavoidable in peeling. With additional confinement of this free surface deformation and appropriately designed deformation zone geometry, LSEM is shown to suppress shear banding, resulting in continuous sheet with homogeneous microstructure. Thus LSEM is shown to produce the greatest enhancement in process workability for producing sheet. These workability findings are explained and discussed based on differences in process mechanics and deformation zone geometry. [ABSTRACT FROM AUTHOR]

Surface folding in large-strain deformation of metal polycrystals, mediated by unsteady sinuous plastic flow, was recently uncovered by direct observations. Here, we examine microstructural origins and mechanics of the folding process in polycrystalline aggregates, using computational methods and in situ , high-speed imaging experiments. Our model loading system is an indenter contact that imposes large strain deformation typical of metal forming, sliding and cutting. Folding arises primarily from intrinsic, grain-level flow stress variation in the polycrystalline ensemble. This flow stress heterogeneity is incorporated, spatially, in a continuum Lagrangian finite element framework, by partitioning the metal surface into grain-like structures. This pseudograin model captures all key aspects of the folding as observed by direct imaging, from fold nucleation via microstructure heterogeneity through various stages of fold development on the surface; surface strain fields; and deformation parameter effects such as indenter geometry and friction. The folding phenomenon is quite general, and provides a direct route for formation of surface defects and delamination wear particles. The microstructure-based simulation capability, thus validated, can be used as a virtual tool for analyzing large-strain plastic flow at surfaces and its consequences. Besides demonstrating the importance of folding in surface plasticity, the study points to a critical need to consider microstructure effects on local plasticity for sliding wear and deformation processing. [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]

We demonstrate key features of a recently uncovered mode of plastic flow – sinuous flow – with vortex-like components on the mesoscale. Based on high-resolution, in situ imaging of a hard wedge (asperity) sliding against a metal surface, we contrast this flow with the more well-known smooth homogeneous (laminar) flow in wear and large strain deformation processes. Sinuous flow is characterized by folding, and arises in both pure sliding and cutting of metals with large strain hardening capacity. The folds mediating the flow can transform into wear particles and surface defects by delamination via fold-splitting. Examples of this occurrence have been captured in situ , by high speed imaging of the sliding contact. This provides a direct mechanism for delamination wear, in just a few passes of sliding. Material heterogeneity plays an important role in the folding, as revealed by finite element simulation and experiment. This combined experiment-simulation approach reveals a number of ways in which folding can be triggered, suggesting an important role for sinuous flow in delamination wear. A close relationship between sinuous flow and mechanochemical Rehbinder effects in machining of metals is also highlighted. Technological implications of sinuous flow for sliding wear and manufacturing processes are briefly discussed. [ABSTRACT FROM AUTHOR]

• Single step severe shear deformation processing was utilized to produce high-Si content electrical steel sheet. • Magnetic properties were characterized, toward decoupling the effects of crystallographic texture and silicon content. • Texture was found to have negligible impact on properties, while higher Si-content led to softer magnetic performance. • Sheet quality produced by the single step deformation process was comparable or superior to conventional (rolled) sheet. Soft magnetic Fe-Si alloys (electrical steels) possess exceptional functional properties such as high permeability, low coercivity, and low core loss, which generally improve with increasing Si content in the alloy. However, Fe-Si alloys containing > 3.5 wt% Si are also characterized by prohibitively low workability and poor ductility that have prevented their efficient commercial production in sheet form by rolling. This has limited their use for improving efficiency of motors and transformers. In this study, hybrid cutting-extrusion (HCE) is used as a single-step thermomechanical processing method to produce continuous Fe-Si alloy sheet with high Si compositions of 4 wt% to 6.5 wt%. HCE sheet is shown to have a homogeneous annealed grain structure and simple-shear crystallographic textures. By controlling the HCE deformation path, varied crystallographic shear textures are created in the sheet. Quasi-static magnetic properties of the HCE sheet are evaluated to decouple the effects of sheet texture and Si composition on resultant permeability and coercivity properties. The results suggest that HCE, with suitable process scaling, is a viable route for production of high-Si content electrical steel sheet for next-generation motors and transformers. [ABSTRACT FROM AUTHOR]

Understanding transitions from homogeneous to localized flow, and mechanisms underlying flow localization, is of paramount importance for deformation processing of magnesium. In this study, a shear-based deformation method is utilized for imposing large strains ( ∼ 1 ), under controllable strain rates (10–10 5 /s) and temperatures (80–300 °C), in order to examine flow patterns in a magnesium alloy. Based on microstructure characterization, deformation twinning is suggested to contribute to the localized flow at temperatures below 200 °C and at low strain rates. The transition from the localized to homogeneous flow with increasing temperature is due to reduction in twinning activity, and enhanced strain-rate sensitivity. At constant temperature, an increase in the strain rate decreases the propensity for flow localization. A model is presented for characterizing the maximum uniform strain as a function of temperature and deformation state (simple shear, plane-strain compression). The model incorporates temperature-sensitive microstructural changes and flow properties of magnesium into a classical framework to capture the flow localization phenomena at low temperatures and strain rates. [ABSTRACT FROM AUTHOR]

We examine a, hitherto, little-studied and curious machining chip morphology, with tell-tale signs of folding at two different length scales, that is common in cutting of certain ductile and highly strain-hardening metals like soft aluminum alloys, tantalum and niobium. This chip morphology does not appear in the usual catalogues of common chip types. The mechanics of formation of the "dual-scale folded chip" is studied in model material systems of commercially pure aluminum alloys (AA 1100 and AA 8040), that prominently exhibit this chip morphology. The flow, folding and associated plastic instabilities are investigated using micro/macro structure observations of the chip in a plane-strain cutting framework, with high-speed in situ imaging and image analysis of material flow; and force measurements. The smaller-scale folding is shown to develop in the primary deformation zone while the larger-scale folding occurs as the chip traverses the rake face of the tool. The resulting chip is composed of irregularly-spaced large folds, superimposed onto which are the quasi-periodic small folds. The representative wavelengths of the two folds differ on average by an order of magnitude, 0.1 mm vs. 2 mm. The observations reveal a direct coupling between the material flow and chip morphology, and how specific attributes of the dual-scale folded chip arise from the flow mechanism. Plastic buckling is found to play a key role in the folding at both length scales. The small-scale folds are characteristic of a sinuous plastic flow mode, while the large-scale folding is characterized by buckling and stick-slip along the tool rake face, triggered by adhesive pinning of the chip to the tool. Important consequences of the dual-scale folding are very large cutting forces, and force oscillations of large amplitude, despite the alloys being very soft, only ∼25 HV. The dual-scale folding is why many of these alloys are classified as "gummy" to machine. Since the dual-scale folded chip is associated with large cutting forces and poor surface quality, there is much to be gained by disrupting this flow type in practical machining applications. Methods for controlling the folding to improve machining performance with the gummy alloys are briefly discussed. [Display omitted] • Dual-scale folding in machined chips has one-to-one correlation with flow mode and plastic instabilities. • Both fold types are triggered by Plastic buckling instability. • Small-scale folds are formed in the primary deformation zone. • Large-scale folds are formed along the rake face of the tool. [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]

Abstract: The specific energy in modulation assisted machining (MAM) – machining with superimposed low frequency (<1000Hz) modulation in the feed direction – is estimated from direct measurements of cutting forces. Reductions of up to 70% in the energy are observed relative to that in conventional machining, when cutting ductile metals such as copper and Al 6061T6. Evidence based on chip structures and strains, stored energy of cold work, recrystallization, and finite element simulation of chip formation, is presented to show that this reduction is due to smaller strain levels in chips created by MAM. A simple geometric ratio of the length to thickness of the ‘undeformed chip’, which can be estimated a priori from MAM and machining parameters, is shown to be a predictor of the transient chip formation conditions that result in the reduction in specific energy and deformation levels. [Copyright &y& Elsevier]

Abstract: An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed. [Copyright &y& Elsevier]

Abstract: The deformation history and state of machined surfaces in low-speed cutting of metals with sharp, wedge-shaped tools have been characterized using high-speed image analysis, complemented by hardness and microstructure study. Large surface and subsurface strains are observed, and have been shown to arise from the severe plastic deformation intrinsic to chip formation. The deformation history of the chip and the near-surface region during the process of surface generation are found to be equivalent. The dependence of surface and subsurface deformation on parameters such as tool rake angle and undeformed chip thickness has been explored. The subsurface strain distribution is shown to scale with undeformed chip thickness (self-similarity), and to be influenced by the rake angle. Based on the observations, and process–deformation–microstructure correlations, a framework is developed for directly engineering surfaces with controlled deformation levels and microstructures by machining. The results also offer scope for enhanced validation of machining simulations and future development of multiscale models of machining. [Copyright &y& Elsevier]

Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.

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'.

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 industrial machining 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.