Mechanical Heterogeneity Research Articles

Оценка анизотропии механических свойств листового проката из углеродистой стали

The purpose of the article is to establish experimentally the effect of material inhomogeneity on the characteristics of strength ( σ в, σ 0.2) and plasticity (δ) on example of a rolled steel sheet. Uniaxial tensile testing was carried out on flat samples of hot-rolled sheet made of St3 alloy cut in three directions relative to rolling: along, across and at the angle of 450. The heterogeneity of structure was established by studying the fracture surface of the destroyed samples after tensile testing. A metallographic research and micromechanical testing (measurement of microhardness) of sections parallel to the fracture surface were carried out as well. The uniaxial tensile testing of flat samples resulted in obtaining the values of the characteristics of strength ( σ в, σ 0.2) and plasticity (δ). The analysis of fracture patterns, microstructure and microhardness values of the material allowed to reveal the structural heterogeneity caused by the presence of fibrousness and a banded ferrite-pearlite structure oriented along the deformation direction. The formation reason of the latter was the presence of oriented non-metallic inclusions - elongated plastic sulfides. The study determined that the material under investigation features the anisotropy of mechanical properties and structural heterogeneity. The values of the ultimate strength ( σ в) and yield strength ( σ 0.2) decrease from the longitudinal direction to the transverse direction (relative to the rolling direction) and vice versa (from the transverse to longitudinal direction) in the first case probably due to the influence of non-metallic inclusions (plastic sulfides) and, as a result, the banded ferrite-pearlite structure; in the second case due to the influence of fiber direction. The values of the relative elongation (δ) decrease from the longitudinal direction to the direction at an angle of 450 and then increase to the transverse direction as a result of different hardening of the material during plastic deformation. This is proved by the obtained microhardness values of the investigated sections and the values of the maximum applied loads during the tensile test. The obtained values are obviously the result of the influence of fiber orientation relative to the existing maximum tensile stresses.

Acquired demyelination but not genetic developmental defects in myelination leads to brain tissue stiffness changes

Changes in axonal myelination are an important hallmark of aging and a number of neurological diseases. Demyelinated axons are impaired in their function and degenerate over time. Oligodendrocytes, the cells responsible for myelination of axons, are sensitive to mechanical properties of their environment. Growing evidence indicates that mechanical properties of demyelinating lesions are different from the healthy state and thus have the potential to affect myelinating potential of oligodendrocytes. We performed a high-resolution spatial mapping of the mechanical heterogeneity of demyelinating lesions using atomic force microscope-enabled indentation. Our results indicate that the stiffness of specific regions of mouse brain tissue is influenced by age and degree of myelination. Here we specifically demonstrate that acquired acute but not genetic demyelination leads to decreased tissue stiffness, which could influence the remyelination potential of oligodendrocytes. We also demonstrate that specific brain regions have unique ranges of stiffness in white and grey matter. Our ex vivo findings may help the design of future in vitro models to mimic the mechanical environment of the brain in healthy and diseased states. The mechanical properties of demyelinating lesions reported here may facilitate novel approaches in treating demyelinating diseases such as multiple sclerosis. Statement of SignificanceMechanical characteristics of a cell's environment can have a profound influence on its biological properties. Neuronal and glial cells are sensitive to mechanical input during development, in disease and regeneration. Sustained tensile strain can promote differentiation of oligodendrocyte progenitor cells into mature oligodendrocytes, which are responsible for the myelination of axons. Changing myelination is an important hallmark in human aging and disease, such as multiple sclerosis. Our hypothesis is that these diseases might be characterized by altered tissue stiffness and that this has an influence on remyelination potential. Here we investigate tissue stiffness profiles of healthy, aged and disease model mice. Manipulating the tissue stiffness might be another promising approach for new treatments.

Robustness-Heterogeneity-Induced Ultrathin 2D Structure in Li Plating for Highly Reversible Li-Metal Batteries.

Anode interface modification is crucial for the stabilization of Li-metal batteries (LMBs), which have been considered as the most promising system for the electric vehicle market owing to their high energy density (500 W h kg-1). However, the biggest challenge for LMBs lies in the preservation of anode reversibility, including plated Li morphology control and dendritic Li inhibition during cycling. Here, we propose a nanostructure modulation strategy of Li grains and plating to activate the anode kinetics of LMBs without the compromise of anode stability. This modulation is triggered by the rapid deposition of ultrathin polydopamine coating on the Cu foil (PDA@Cu), which results in an unusual interlaced growth of vertical or lie-down two-dimensional Li nanoflakes on PDA. The high binding energy (>3 eV) between Li atoms and rich imino/carbonyl groups enables a superior lithiophilicity of PDA to homogenize the Li-ion flowing and Li-mass electroplating with negligible nucleation overpotential. The high Coulombic efficiency (98%) and low voltage hysteresis (∼20 mV) are stabilized for at least 300 cycles in the Li-PDA@Cu cell architecture. This PDA@Cu electrode can even tolerate much higher current densities of 5 and 10 mA cm-2 for 170 and 100 cycles, respectively. The interlaced network of Li nanosheets reinforces the electric contact and therefore charge transfer at the anode-electrolyte interface characterized by small interfacial resistance (<3 Ω cm2) and activation energy (0.28 eV). A viewpoint of robustness loss or mechanical heterogeneity in Li plating is discussed to disclose the evolution from column-like Li grains to porous Li sponges and then to compactly stacked Li nanoflakes with porosity shrinkage.

Fragmentation of long period stacking ordered (LPSO) phase and its impact on microstructure evolution of a Mg–Y–Zn alloy during multi-directional forging

In this study, multi-directional forging (MDF) experiments were conducted to a homogenized Mg-2.36Y-0.98Zn (at%) alloy containing long period stacking ordered (LPSO) phase. Microstructure evolution of the MDFed alloys with different forging passes were systematically investigated, corresponding microhardness development was also concerned. The results revealed that grain refinement, texture weakening and DRX development during MDF were closely relevant to fragmentation of LPSO phase. It was also found that development of average microhardness and mechanical heterogeneity of the MDFed alloys can be attributed to the competition and balance of work hardening, grain refinement and DRX softening. Furthermore, LPSO-related deformation and dynamic recrystallization (DRX) behaviors such as (i) kink-aided DRX (KDRX, this concept was firstly nominated in this study); (ii) particle stimulated nucleation mechanism (PSN) and (iii) pinning effect on DRX nucleus were thoroughly discussed, which should help to understand the unique microstructure evolution of LPSO-containing Mg alloys, and thus offer certain enlightenment for manufacturing LPSO-strengthened Mg alloys with excellent mechanical property.

Mechanical Heterogeneity in the Bone Microenvironment as Characterized by Atomic Force Microscopy

Bones are structurally heterogeneous organs with diverse functions that undergo mechanical stimuli across multiple length scales. Mechanical characterization of the bone microenvironment is important for understanding how bones function in health and disease. Here, we describe the mechanical architecture of cortical bone, the growth plate, metaphysis, and marrow in fresh murine bones, probed using atomic force microscopy in physiological buffer. Both elastic and viscoelastic properties are found to be highly heterogeneous with moduli ranging over three to five orders of magnitude, both within and across regions. All regions include extremely compliant areas, with moduli of a few pascal and viscosities as low as tens of Pa·s. Aging impacts the viscoelasticity of the bone marrow strongly but has a limited effect on the other regions studied. Our approach provides the opportunity to explore the mechanical properties of complex tissues at the length scale relevant to cellular processes and how these impact aging and disease.

Effect of mechanical heterogeneity on hydraulic fracture propagation in unconventional gas reservoirs

The mechanical heterogeneity of unconventional gas reservoirs is a significant factor affecting the nucleation and propagation of hydraulic fractures. The ignorance of mechanical heterogeneity in the numerical calculation has been demonstrated to result in huge deviations from actual fracture propagation. In the present study, the eXtended Finite Element Method (XFEM) is used to simulate the propagation of a hydraulic fracture in a heterogeneous reservoir. The evolution of a hydraulic fracture encountering hard blocks at different positions near the initial fracture tip is investigated. The mechanical heterogeneity of reservoir rock leads to the redistribution of strength and stress, which eventually affects the evolution of hydraulic fracture. The effects of soft-hard rock interbedding and randomly distributed irregular blocks are also investigated. Generally, the hard blocks distributed in a reservoir promote the growth of hydraulic fracture in length, but limit the fracture growth in width.

The impact of the depth-dependence of in-situ stresses on the effectiveness of stacked caprock reservoir systems for CO2 storage

A candidate reservoir deep enough for CO2 storage might still be unsuitable if it carries a low minimum horizontal stress (Shmin) that limits its storage capacity, due to a higher risk of fracturing. This study investigated the impact that depth variations in the magnitude of in-situ stresses of a stacked caprock reservoir system have on the geomechanical response to CO2 injection. While geological and fluid flow properties (e.g., caprock permeability) are typically considered to be the main factors preventing CO2 leakage, this work investigates the importance of considering the stress variation across formations as an impeding factor that prevents CO2 leakage. The study employed well log, core sample, and field tests data from one such system in the Appalachian basin, where the target storage formations have low Shmin, but are overlain/underlain by formations with larger Shmin. Two coupled fluid flow – geomechanical models were constructed (1: vertically heterogeneous but laterally homogeneous 2: both vertically and laterally heterogeneous) for this investigation. Modeling results suggest that variations in the in-situ Shmin along the vertical profile of the stacked system are effective in containing the risks of fracturing to only those zones with the lowest Shmin. Furthermore, results suggest that inducing fractures – typically considered undesirable for CO2 storage – may in fact enhance injectivity without compromising the integrity of the storage system. While incorporating 3-dimensional mechanical heterogeneity results in stochastic locations of induced fractures, the estimated injectivity is slightly different from the laterally homogeneous model. The variability of stresses in a stacked caprock reservoir system merits detailed investigation as an entire system (rather than an isolated zone) in order to fully characterize the geomechanical responses to injection. As demonstrated in this work, stress changes across caprock-reservoir formations in the Appalachian basin is an essential factor preventing CO2 leakage.

Study of Structural and Mechanical Heterogeneity through Steel 15Kh5M Carburized Pipe Wall Thickness

Features of the change in chemical and phase composition, structure, strength, ductility, and impact strength of steel 15Kh5M pipe material used for furnace coils of catalytic reforming units after carburizing the external and internal surfaces are studied.

Analysis of Technological Capabilities and Peculiarities of Fuse Welding of Products Made of Metal-Matrix Composite Materials Based on Aluminum Alloy Produced by Internal Oxidation

This paper describes the outcomes of practical experiments in the validation of a technology for welding of the Al-Al2O3 metal-matrix composite material produced by the internal oxidation method. Technological capabilities are herein considered for argon-arc welding (Tungsten Inert Gas/ TIG) with filler wire and arc welding in a protective inert/active gas medium using a melting electrode (Metal Inert Gas/ MIG) for joining sheets of A6 aluminum alloy-based metal-matrix composite material (MMC). Mechanical properties of welded joints are determined and the fracture macrostructure is investigated. Fracture patterns and tensile strength are shown for different modes of welding procedure for alloy plates of 5, 8 mm in thickness by the TIG method and 25 mm by the MIG method. The macrostructural and mechanical heterogeneity of welded joints is shown. Welds made under optimal conditions are free of any macrodefects. The welded joint strength is up to 96% of the base material strength.

Characterization of sub-seismic resolution structural diagenetic heterogeneities in porous sandstones: Combining ground-penetrating radar profiles with geomechanical and petrophysical in situ measurements (Northern Apennines, Italy)

Deformation bands and structurally-related diagenetic heterogeneities, here named Structural Diagenetic Heterogeneities (SDH), have been recognized to affect subsurface fluid flow on a range of scales and potentially promoting reservoir compartmentalization, influencing flow buffering, and sealing during production. Their impact on reservoir hydraulic properties depends on many factors, such as their permeability contrast with respect to the undeformed reservoir rock, their anisotropy, thickness, geometry as well as their physical connectivity and arrangement in the subsurface. We used the Ground Penetrating Radar (GPR) for detection and analysis of the assemblage of deformation bands – carbonate nodules, in high-porosity arkose sandstone in Northern Apennines of Italy. 2D GPR surveys allowed the description of the SDH spatial organization, their geometry, and their continuity in the subsurface. Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-m thick stratigraphic log to characterize the permeability and strength variations of those features. The assemblage “deformation bands – nodules” degrade porosity and permeability and produce a strengthening effect of the rock volume, imparting a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties between deformation bands, nodules, and host rock result in different GPR response (permittivity). Thus, GPR response could be used to extend outcrop data (petrophysical and geomechanical) of SDH to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogs of faulted aquifers and reservoirs in porous sandstones. • GPR effectively detects deformation bands and diagenetic heterogeneities in sandstones. • Deformation bands and cement nodules show high compressive strength and low permeabilities. • GPR response correlates with petrophysical and geomechanical rock properties. • An improved reservoir analog is obtained from integrating outcrop-based and GPR data. • Deformation bands and nodules impart anisotropy to the host rock.

Emerging concepts in ventilation-induced lung injury.

Ventilation-induced lung injury results from mechanical stress and strain that occur during tidal ventilation in the susceptible lung. Classical descriptions of ventilation-induced lung injury have focused on harm from positive pressure ventilation. However, injurious forces also can be generated by patient effort and patient–ventilator interactions. While the role of global mechanics has long been recognized, regional mechanical heterogeneity within the lungs also appears to be an important factor propagating clinically significant lung injury. The resulting clinical phenotype includes worsening lung injury and a systemic inflammatory response that drives extrapulmonary organ failures. Bedside recognition of ventilation-induced lung injury requires a high degree of clinical acuity given its indistinct presentation and lack of definitive diagnostics. Yet the clinical importance of ventilation-induced lung injury is clear. Preventing such biophysical injury remains the most effective management strategy to decrease morbidity and mortality in patients with acute respiratory distress syndrome and likely benefits others at risk.

Slow slip source characterized by lithological and geometric heterogeneity.

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

Bacterial vs. thermal degradation of algal matter: Analysis from a physicochemical perspective

Bacteria are ubiquitous in all depositional environments, especially in marine environments where anoxic/euxinic conditions prevail. In such environments, sulfate-reducing bacteria play a critical role to supply sulfur as a biogenic source for H2S through biomass degradation. In the biodegradation process, chemical and mechanical properties of the organic matter alter. In order to document these variations in-situ, selected samples from a deeply buried mudrock (Bakken Formation), were examined through microscopy analysis. Two separate but adjacent telalginite particles were selected; An unaltered telalginite and a bacterially degraded telalginite, which still contains relicts of the parent Tasmanites. A combination of AFM-based IR spectroscopy with high-resolution amplitude-frequency modulation was used to evaluate and compare the physicochemical variations across these two particles at the nanoscale. Results indicate that all aromaticity indexes increase for both particles but at a higher rate as a result of bacterial degradation. Furthermore, it was found that bacterial degradation imposes a major mechanical heterogeneity to the organic matter under study, which was detected through phase imaging and modulus mapping captured from submicron to micron-scale level, which exposed the remnants of the parent Tasmanites. This study reveals that bacterial degradation can accelerate the maturation process, thus the generation of hydrocarbons from the kerogen to happen at the earlier stages of thermal adavance.

A Pictorial View of Viscosity in Ionic Liquids and the Link to Nanostructural Heterogeneity.

Prototypical ionic liquids (ILs) are characterized by three structural motifs associated with (1) vicinal interactions, (2) the formation of positive-negative charge-alternating chains or networks, and (3) the alternation of these networks with apolar domains. In recent articles, we highlighted that the friction and mobility in these systems are nowhere close to being spatially homogeneous. This results in what one could call mechanical heterogeneity, where charge networks are intrinsically stiff and charge-depleted regions are softer, flexible, and mobile. This Letter attempts to provide a clear and visual connection between friction-associated with the dynamics of the structural motifs (in particular, the charge network)-and recent theoretical work by Yamaguchi linking the time-dependent viscosity of ILs to the decay of the charge alternation peak in the dynamic structure function. We propose that charge blurring associated with the loss of memory of where positive and negative charges are within networks is the key mechanism associated with viscosity in ILs. An IL will have low viscosity if a characteristic charge-blurring decorrelation time is low. With this in mind, engineering new low-viscosity ILs is reduced to understanding how to minimize this quantity.

Spatial coordinate corrected motion tracking for optical coherence elastography.

We develop a spatial coordinate corrected (SCC) motion tracking method for optical coherence elastography. SCC motion tracking refers the instantaneous velocity field extracted from optical coherence tomography (OCT) data to the laboratory coordinate system and accurately reconstructs the displacement field established during the mechanical excitation (compression) process. We acquired image data from compression OCE experiments on human breast tissue specimens, and reconstructed the displacement field through Doppler analysis of OCT data. Our results suggested that SCC tracking enables accurate reconstruction of displacement field, and enables effective identification mechanical heterogeneity that can be used as a biomarker for cancer diagnosis and tumor margin assessment.

Anisotropic stiffness gradient-regulated mechanical guidance drives directional migration of cancer cells

Interfacial interactions between cancer cells and surrounding microenvironment involve complex mechanotransduction mechanisms that are directly associated with tumor invasion and metastasis. Matrix remodeling triggers heterogeneity of stiffness in tumor microenvironment and thus generates anisotropic stiffness gradient (ASG). The migration of cancer cells mediated by ASG, however, still remains elusive. Based on a multi-layer polymerization method of microstructured hydrogels with surface topology, we develop an in vitro experimental platform for mechanical interactions of cancer cells with ASG matrix microenvironment. We show that mechanical guidance of mesenchymal cells is essentially modulated by ASG, leading to a spontaneous directional migration along the orientation parallel to the maximum stiffness although there is no stiffness gradient in the direction. The ASG-regulated mechanical guidance presents an alternative way of cancer cell directional migration. Further, our findings indicate that the mechanical guidance occurs only in mesenchymal cancer cells, but not in epithelial cancer cells, implying that cell contractility may contribute to ASG-regulated migration of cells. This work is not only helpful for elucidating the role of matrix remodeling in mediating tumor cell invasion and metastasis, but has potential implications for developing specific cancer treatments. Statement of SignificanceLocal extracellular matrix (ECM) stiffening triggers mechanical heterogeneity in tumor microenvironment, which can exert a crucial impact on interfacial interactions between tumor cells and surrounding ECM. The underlying mechanobiological mechanism that tumor cells are modulated by mechanically heterogeneous ECM, however, still remains mysterious to a great extent. Through our established in vitro platform and analysis, we have demonstrated that anisotropic stiffness gradient (ASG) has the ability to elicit directional migration of cells, essentially depending on local stiffness gradients and the corresponding absolute stiffness values. This study is not only crucial for revealing the role of matrix remodeling in regulating tumor invasion and metastasis, but also offers a valuable guidance for developing anti-tumor therapies from the biomechanical perspective.

Dynamic microscale crack propagation in shale

Hydraulic fracturing and horizontal well are the two key techniques for the development of shale oil/gas. Mechanical heterogeneity and anisotropy of shale strongly influence the effects of fracturing. While numerous studies have focused on the fracture behavior of shale, the microscale fracture mechanisms of shale remain poorly understood. In this study, three representative test areas, which were dominated by a stiff mineral, a blob of organic matter and clay layers, respectively, were selected in a terrestrial shale sample from the Yanchang Formation. Micro cantilever beams were manufactured using the micro fabrication method in each test area, and in situ fracture experiments were performed to investigate crack propagation under a scanning electron microscope. The crack propagation process was characterized at micrometer scale in shale. The crack paths and the load-displacement curves revealed various fracture mechanisms. Crack deflection and crack branching owing to mechanical contrast, toughening in organic matter, and crack bridging of clay layers were observed and discussed. This study provided a novel insight into micro crack behavior in shale and presents a new framework for the investigation of the fracture characteristics of shale. The microscale shale characterization and crack mechanisms can serve as a basis or building blocks for mesoscale modeling of fracturing in shale, and the load-displacement data and fracture behaviors can provide a valuable dataset for validating modeling approaches.

Lung protection in acute respiratory distress syndrome: what should we target?

Most clinical trials of lung-protective ventilation have tested one-size-fits-all strategies with mixed results. Data are lacking on how best to tailor mechanical ventilation to patient-specific risk of lung injury. Risk of ventilation-induced lung injury is determined by biological predisposition to biophysical lung injury and physical mechanical perturbations that concentrate stress and strain regionally within the lung. Recent investigations have identified molecular subphenotypes classified as hyperinflammatory and hypoinflammatory acute respiratory distress syndrome (ARDS), which may have dissimilar risk for ventilation-induced lung injury. Mechanically, gravity-dependent atelectasis has long been recognized to decrease total aerated lung volume available for tidal ventilation, a concept termed the 'ARDS baby lung'. Recent studies have demonstrated that the aerated baby lung also has nonuniform stress/strain distribution, with potentially injurious forces concentrated in zones of heterogeneity where aerated alveoli are adjacent to flooded or atelectatic alveoli. The preponderance of evidence also indicates that current standard-of-care tidal volume management is not universally protective in ARDS. When considering escalation of lung-protective interventions, potential benefits of the intervention should be weighed against tradeoffs of accompanying cointerventions required, for example, deeper sedation or neuromuscular blockade. A precision medicine approach to lung-protection would weigh. A precision medicine approach to lung-protective ventilation requires weighing four key factors in each patient: biological predisposition to biophysical lung injury, mechanical predisposition to biophysical injury accounting for spatial mechanical heterogeneity within the lung, anticipated benefits of escalating lung-protective interventions, and potential unintended adverse effects of mandatory cointerventions.

Layering and structural inheritance controls on fault zone structure in three dimensions: a case study from the northern Molasse Basin, Switzerland

Mechanical heterogeneity of a sedimentary sequence exerts a primary control on the geometry of fault zones and the proportion of offset accommodated by folding. The Wildensbuch Fault Zone in the Swiss Molasse Basin, with a maximum throw of 40 m, intersects a Mesozoic section containing a thick (120 m) clay-dominated unit (Opalinus Clay) over- and underlain by more competent limestone units. Interpretation of a 3D seismic reflection survey indicates that the fault zone formed by upward propagation of an east–west-trending basement structure, through the Mesozoic section, in response to NE–SW Miocene extension. This configuration formed an array of left-stepping normal fault segments above and below the Opalinus Clay. In cross-section a broad monoclinal fold is observed in the Opalinus Clay. Folding, however, is not ubiquitous and occurs in the Opalinus Clay where fault segments above and below are oblique to one another; where they are parallel the fault passes through the Opalinus Clay with little folding. These observations demonstrate that, even in strongly heterogeneous sequences, here a four-fold difference in both Young's modulus and cohesion between layers, the occurrence of folding may depend on the local relationship between fault geometry and applied stress field rather than rheological properties alone.

Effect of Load Ratio on the Mechanical Field around Crack Front of Dissimilar Metal Weld Joints

Stress corrosion cracking (SCC) of the dissimilar metal weld (DMW) joints in nuclear power equipment has been paid more attention in recent years. Besides the geometric size, load level, multiaxial stress state and mechanical heterogeneity, the relaxation and redistribution of residual stress make it very difficult to directly study the influence of residual stresses on the actual welded components. To study the effect of load ratio on residual stress around the crack front in the weld zone of DMW joints, the distributions of residual stress and stress triaxiality under different load ratios are simulated and discussed. The analyses show that the stress becomes larger with the increasing load ratio. The stress is tensile stress near the crack front. While the stress becomes compressive stress when it is far away from the crack front. The external load rotio has great effect on the stress around the circumferential direction at the crack front. The higher load ratio are more likely to cause the crack propagation. In the region of crack angle θ=80-120 °, the load ratio has little effect on the stress triaxiality.