Brittle State Research Articles

Forced-rupture of cell-adhesion complexes reveals abrupt switch between two brittle states.

Cell adhesion complexes (CACs), which are activated by ligand binding, play key roles in many cellular functions ranging from cell cycle regulation to mediation of cell extracellular matrix adhesion. Inspired by single molecule pulling experiments using atomic force spectroscopy on leukocyte function-associated antigen-1 (LFA-1), expressed in T-cells, bound to intercellular adhesion molecules (ICAM), we performed constant loading rate (rf) and constant force (F) simulations using the self-organized polymer model to describe the mechanism of ligand rupture from CACs. The simulations reproduce the major experimental finding on the kinetics of the rupture process, namely, the dependence of the most probable rupture forces (f*s) on ln rf (rf is the loading rate) exhibits two distinct linear regimes. The first, at low rf, has a shallow slope, whereas the slope at high rf is much larger, especially for a LFA-1/ICAM-1 complex with the transition between the two occurring over a narrow rf range. Locations of the two transition states (TSs) extracted from the simulations show an abrupt change from a high value at low rf or constant force, F, to a low value at high rf or F. This unusual behavior in which the CACs switch from one brittle (TS position is a constant over a range of forces) state to another brittle state is not found in forced-rupture in other protein complexes. We explain this novel behavior by constructing the free energy profiles, F(Λ)s, as a function of a collective reaction coordinate (Λ), involving many key charged residues and a critical metal ion (Mg2+). The TS positions in F(Λ), which quantitatively agree with the parameters extracted using the Bell-Evans model, change abruptly at a critical force, demonstrating that it, rather than the molecular extension, is a good reaction coordinate. Our combined analyses using simulations performed in both the pulling modes (constant rf and F) reveal a new mechanism for the two loading regimes observed in the rupture kinetics in CACs.

Breaking ties that bind biomolecules brings a surprising jump

Investigating how cell adhesion complexes rupture, researchers discover a new mechanism involving an abrupt shift between two brittle states.

Phase-field modeling of stress generation in polycrystalline LiCoO2

The lithium (Li) intercalation process in layered transition-metal oxides is treated as a fundamental phenomenon that affects the battery performance. To gain a quantitative assessment of the influence of the microstructure on the stress generated during the charging process of a polycrystalline secondary particle of LixCoO2, the phase-field model considers the crystallographic anisotropies of the Li self-diffusion coefficient and elastic property. Furthermore, the fracture surface ratio of the grain boundary (GB) is evaluated based on the estimated stress distribution over the microstructure. The criteria for crack initiation are obtained such that the maximum principal stress at the GB exceeds the instability criterion, σf, of the theory in the linear elastic fracture mechanics. The simulation results show that the stress distribution within polycrystalline LixCoO2 affects the Li concentration distribution and the phase separation. During the first charging process in the range of 0.51 ≤ x ≤ 1.0 (LixCoO2), the total fraction of the fracture surface of the GB is predicted to be <30% when in the most brittle state and σf is 0.5 GPa. Thus, the first charging process results in a nontrivial change to the internal microstructure.

Durability of metal structures under quasi-static load

Failure of materials and structures is one of unresolved problems of mechanics. This paper offers an approximate approach to assessing durability of products on the basis of a mechanical experiment. The experiment represents the fatigue process as a transition of a plastic material into its brittle state. A simplified physical model – which could be used to build a mathematical model of fatigue process – hangs on a local transition of a plastic material into its brittle state. The calculation methodology includes both an original part on cyclic degradation of material strength and correlations based on experiments and checked by design routines. Two approaches to calculating the durability of a randomly-loaded object are compared: using the equation of cyclic degradation of strength and the rule of linear summation of fatigue damages. The results obtained are useful for improving methodologies of calculating service life or durability of structures.

Is It not Time to Go back to Tamman’s Tg?

Based on the analysis of numerous experimental data, it is shown that the currently generally accepted glass transition temperature Tg is not correct. It was the result of borrowing from G. Tamman the symbol Tg, which corresponds to the temperature at which a liquid in a viscous flow is transformed into a solid brittle glassy state, and using it instead of the symbol Tw. The latter corresponded to the bending temperature (for Tamman) on the property–temperature curve of glass-forming substances lying above Tamman’s Tg. The disclosed physico-chemical nature of the temperature curve Tw (currently, generally taken to be Tg), which is the temperature of the reverse direction of the interconversion of nanofragment patterns (of polymorphs) of the high- and low-temperature polymorphs that coexist in a vitreous substance, is based on the application of polymer-polymorphing representations of the structure of glass-forming substances. In the past few decades, the discovery of the preendothermal effect located, just as Tamman’s Tg, below the standard Tg, confirms the correctness of Tamman’s Tg characterized by the increased specific heat of the heated glass.

Elastic-brittle-plastic analysis of the radial subgrade modulus for a circular cavity based on the generalized nonlinear unified strength criterion

When using the bedded-beam model based on Winkler springs for cavity lining design, it is always difficult for engineers to select an appropriate value for the radial subgrade modulus (k) due to the lack of related theoretical research. The solutions of k for a circular tunnel proposed by Wood (1975) and Zhang et al. (2014) have been found to be applicable only when a tunnel is in elastic ground. However, a rock mass will exhibit elastic, brittle and plastic states under different stress conditions. Considering that the plastic state can effectively improve the bearing capacity and reduce the engineering cost, to investigate the radial subgrade modulus in an elastic-brittle-plastic rock mass, modified analytical solutions are derived that are compatible with a generalized nonlinear unified strength criterion. Using the maximum shear stress modulus, these solutions consider the comprehensive influences of the intermediate principal stress, the dilation characteristic and brittle plastic. Then, a comparative verification of the new solutions using a nonlinear Hoek-Brown yield criterion is conducted. The effects of the parameters are discussed in detail. The calculation results show that k is a comprehensive evaluation index for multiple factors, is significantly influenced by the geo-stress and support pressure, and increases with the geological strength index, elastic strain and Young’s modulus in the plastic zone. During cavity contraction, the intermediate principal stress improves the magnitude of k more significantly than dilation, whereas dilation has a greater effect during cavity expansion. Using the perfectly plastic model without considering brittle-plastic behaviour will lead to an overestimation of k, and using different strength criteria directly affects the timing of the plastic deformation of a rock mass. The influence of various factors should be comprehensively considered to select a reasonable radial subgrade modulus to take full advantage of the latent potentialities of a rock mass. The deep-buried diversion tunnel at the Jinping Ⅱ hydropower station is studied as an example to demonstrate the practical application of the proposed model.

Categorization of States Beyond Strong and Weak

The discourse on poor state performers has suffered from widely varying definitions on what distinguishes certain weak states from others. Indices that rank states from strong to weak conceal important distinctions that can adversely affect intervention policy. This deficiency is addressed by grouping states according to their performance on three dimensions of statehood: authority, legitimacy, and capacity. The resultant categorization identifies brittle states that are susceptible to regime change, impoverished states often considered as aid darlings, and fragile states that experience disproportionately high levels of violent internal conflict. It also provides a quantifiable means to analyze transitions from one state type to another for more insightful intervention policy.

Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites

Spark plasma sintering (SPS) was employed to fabricate monolithic titanium and in-situ formed TiB whisker (TiBw) reinforced titanium matrix composites (TMCs) by adding different amounts of TiB2 as boron source. The sintering process was completed at 1050°C for 5min under 50MPa. The influences of TiB2 content (0.6–9.6wt. %) on microstructural evolution and mechanical properties of TMCs were investigated. Thermodynamics, XRD analysis and microstructural investigations confirmed the in-situ formation of TiBw in the composite samples. However, some semi-reacted TiB2 phases, surrounded by TiB coronas, were remained in the microstructure due to the unfinished chemical reaction between the components during a short-time sintering process. The results showed that all samples were appropriately densified by SPS process into the almost dense parts with relative density no less than 97.5%. While bending strength decreased and hardness increased with increasing TiB2 content, the sample with 4.8wt. % TiB2 had the maximum tensile strength. Fractographical assessments showed that the addition of TiB2 hindered the grain growth of titanium matrix. With increasing TiB2 content, fracture mode changed from a multiple pattern to a predominantly transgranular and brittle state.

Study of the tensile properties of individual multicellular fibres generated by Bacillus subtilis

Multicellular fibres formed by Bacillus subtilis (B. subtilis) are attracting interest because of their potential application as degradable biomaterials. However, mechanical properties of individual fibres remain unknown because of their small dimensions. Herein, a new approach is developed to investigate the tensile properties of individual fibres with an average diameter of 0.7 μm and a length range of 25.7–254.3 μm. Variations in the tensile strengths of fibres are found to be the result of variable interactions among pairs of microbial cells known as septa. Using Weibull weakest-link model to study this mechanical variability, we predict the length effect of the sample. Moreover, the mechanical properties of fibres are found to depend highly on relative humidity (RH), with a brittle–ductile transition occurring around RH = 45%. The elastic modulus is 5.8 GPa in the brittle state, while decreases to 62.2 MPa in the ductile state. The properties of fibres are investigated by using a spring model (RH < 45%) for its elastic behaviour, and the Kelvin–Voigt model (RH > 45%) for the time-dependent response. Loading-unloading experiments and numerical calculations demonstrate that necking instability comes from structural changes (septa) and viscoelasticity dominates the deformation of fibres at high RH.

Predicting the critical stress intensity in structural steels

A method is proposed for predicting the critical stress-intensity coefficient K 1c of structural steels (their crack resistance) on the basis of uniaxial tensile tests of standard samples. The derivation of this method relies on the concepts of mechanical stability and embrittlement when nonuniform force fields act on a metal. The strain-hardening index n of the metal at the critical transition temperatures from the plastic state to the quasi-brittle state (T c or T 0) and from the quasi-brittle state to the brittle state (T k2) plays a key role here. The proposed method may prove effective in monitoring the crack resistance of steels certified in scientific and plant laboratories.

Hierarchical Supramolecular Cross-Linking of Polymers for Biomimetic Fracture Energy Dissipating Sacrificial Bonds and Defect Tolerance under Mechanical Loading.

Biological structural materials offer fascinating models how to synergistically increase the solid-state defect tolerance, toughness, and strength using nanocomposite structures by incorporating different levels of supramolecular sacrificial bonds to dissipate fracture energy. Inspired thereof, we show how to turn a commodity acrylate polymer, characteristically showing a brittle solid state fracture, to become defect tolerant manifesting noncatastrophic crack propagation by incorporation of different levels of fracture energy dissipating supramolecular interactions. Therein, poly(2-hydroxyethyl methacrylate) (pHEMA) is a feasible model polymer showing brittle solid state fracture in spite of a high maximum strain and clear yielding, where the weak hydroxyl group mediated hydrogen bonds do not suffice to dissipate fracture energy. We provide the next level stronger supramolecular interactions toward solid-state networks by postfunctionalizing a minor part of the HEMA repeat units using 2-ureido-4[1H]-pyrimidinone (UPy), capable of forming four strong parallel hydrogen bonds. Interestingly, such a polymer, denoted here as p(HEMA-co-UPyMA), shows toughening by suppressed catastrophic crack propagation, even if the strength and stiffness are synergistically increased. At the still higher hierarchical level, colloidal level cross-linking using oxidized carbon nanotubes with hydrogen bonding surface decorations, including UPy, COOH, and OH groups, leads to further increased stiffness and ultimate strength, still leading to suppressed catastrophic crack propagation. The findings suggest to incorporate a hierarchy of supramolecular groups of different interactions strengths upon pursuing toward biomimetic toughening.

Effect of Water Glass on Early Hardening of Portland Cement

In spite of extensive literature devoted to the use of water glass as an additive to Portland cement, there is no practical data concerning the influence of its small amounts on cement hardening and mechanical strength. Traditional methods for determination of the above mentioned parameters are non-effective at the early stages of hardening. Therefore in this case we offer to use a time point of the system transition from plastic to brittle state as an individual property of cement. Time position of the point can be determined by penetration, using the dependence of the hardening system cohesion strength on time.

Intraplate brittle deformation and states of paleostress constrained by fault kinematics in the central German platform

The structural evolution of Central Europe reflects contrasting tectonic regimes after the Variscan orogeny during Mesozoic – Cenozoic time. The brittle deformation related to each tectonic regime is localized mainly along major fault zones, creating complex fracture patterns and kinematics through time with diverging interpretations on the number and succession of the causing events. By contrast, fracture patterns in less deformed domains often provide a pristine structural inventory. We investigate the brittle deformation of a relatively stable, wide area of the central German platform using fault-slip data to identify the regional stress fields required to satisfy the data. In a non-classical approach, and in order to avoid local stress variations and misinterpretations, the fault-slip data are scaled up throughout the study area into subsets of consistent kinematics and chronology for sedimentary cover and crystalline basement rocks. Direct stress tensor inversion was performed through an iterative refining process, and the computed stress tensors were verified using field-based observations. Criteria on relative tilt geometry and indicators of kinematic change suggest a succession of events, which begins with a post-Triassic normal faulting regime with σ3 axis trending NE-SW. The deformation then follows by strike-slip and thrust faulting regimes with a change of σ1 axis from N-S to NE-SW, supposedly in the Late Cretaceous. Two younger events are characterized by Cenozoic normal and oblique thrust faulting regimes with NW-SE-trending σ3 and σ1 axes, respectively. The fracture patterns of both the cover and basement rocks appear to record the same states of stress.

Embrittlement mechanisms and magnetic properties optimization conditions in amorphous Co69Ge3.7Cr3.8Si12.5B11 alloy without ductile-brittle transition

The influence of annealing time on the ductile-brittle transition temperature (embrittlement temperature) Тf in amorphous Co-based alloy Со69Fe3.7Cr3.8Si12.5B11 with extremely low saturation magnetostriction λs (λs < 10–7) is investigated. It is revealed that the embrittlement temperature Тf dependence on annealing time ta can be described by the Arrhenius equation. Embrittlement at annealing temperatures higher and lower than 300°С can be described by different kinetic parameters owing to the different states of the amorphous phase. It is shown that, in the studied alloy, the embrittlement proceeds in a very narrow annealing temperature range, not exceeding 5°С. On the basis of experimental data on the evolution of hysteresis magnetic properties during isochronous annealing and isothermal exposure, the thermal treatment mode is investigated, providing rather high values of permeability μ5 (Н = 5 mOe, f = 1 kHz) of about 50000, without transforming studied alloy into the brittle state.

Embrittlement and conditions of the optimization of magnetic properties in the amorphous alloy Co69Fe3.7Cr3.8Si12.5B11 in the absence of a viscous–brittle transition

The influence of the holding time upon annealing on the temperature of the viscous–brittle transition (temperature of embrittlement) Tf in a cobalt-based amorphous alloy of the composition Co69Fe3.7Cr3.8Si12.5B11 with a very low saturation magnetostriction λs (<10–7) has been studied. It has been established that the dependence of the embrittlement temperature Tf on the of time of holding ta can be described by an Arrhenius equation and that the embrittlement at the annealing temperatures above and below 300°C is described by different kinetic parameters. In the alloy under study, irrespective of the holding time, embrittlement occurs in a very narrow range of annealing temperatures, which does not exceed 5 K. Based on the experimental data on the evolution of the hysteresis magnetic properties upon the isochronous annealings and upon the isothermal holding, the regime of heat treatment that ensures a very high (about 50000) magnitude of the permeability µ5 (H = 5 mOe, f = 1 kHz) without the transition of the alloy into a brittle state has been determined.

Is not it time to go back to Tg Tamman?

Based on the analysis of numerous experimental data shows that the generally accepted today, the glass transition temperatureTg is not. It was the result of borrowing from tammana symbolTg, is meant the temperature of viscous flow liquid transformation into solid brittle glassy state, and use it together symbolTw, means the temperature tammana bend on the dependence of the «property-temperature» glass-forming substances aboveTg of tammana. On the basis of the application of polymer-polymorphing representations of the structure of glass-forming substances are disclosed physico-chemical nature of the temperature curveTw (conventional Segodnya), which is the temperature of the reverse direction interconversion of nanofragments patterns (polymorphical) high and low temperature polymorphs that coexist in the vitreous substance. The discovery in recent decades prezentatsionnogo effect located as tammanaTg, is below the standardTg,Tg confirms the truth of tammana characterized by increasedspecific heat of the heated glass.

Is not it time to go back to Tg Tamman?

Based on the analysis of numerous experimental data shows that the generally accepted today, the glass transition temperatureTg is not. It was the result of borrowing from tammana symbolTg, is meant the temperature of viscous flow liquid transformation into solid brittle glassy state, and use it together symbolTw, means the temperature tammana bend on the dependence of the «property-temperature» glass-forming substances aboveTg of tammana. On the basis of the application of polymer-polymorphing representations of the structure of glass-forming substances are disclosed physico-chemical nature of the temperature curveTw (conventional Segodnya), which is the temperature of the reverse direction interconversion of nanofragments patterns (polymorphical) high and low temperature polymorphs that coexist in the vitreous substance. The discovery in recent decades prezentatsionnogo effect located as tammanaTg, is below the standardTg,Tg confirms the truth of tammana characterized by increasedspecific heat of the heated glass.

The influence of properties of metal and temperature on cyclical crack resistance of power-plant steels

The cyclical crack resistance of power-plant steels is studied. The influence of metal condition (properties) within a specific steel grade on the characteristics of cyclical crack resistance is analyzed. It is shown that, at a room temperature, i.e., in the range of viscous-brittle transition, the cyclical crack resistance of steel decreases with the increase in its tendency to the brittle state. With increasing temperature from room to 300–320°C, the cyclical crack resistance steel decreases, and for state-specific conditions, the degree of fatigue crack growth correlates with the level of yield strength decrease at the same temperature. Taking this into account, the approach to approximation of the kinetic cyclical crack resistance diagram is experimentally proved by a generalized dependence invariant to temperatures in the range 20–300 (320)°C.

Crunchiness Loss and Moisture Toughening in Puffed Cereals and Snacks.

Upon moisture uptake, dry cellular cereals and snacks loose their brittleness and become soggy. This familiar phenomenon is manifested in smoothing their compressive force-displacement curves. These curves' degree of jaggedness, expressed by their apparent fractal dimension, can serve as an instrumental measure of the particles' crunchiness. The relationship between the apparent fractal dimension and moisture content or water activity has a characteristic sigmoid shape. The relationship between the sensorily perceived crunchiness and moisture also has a sigmoid shape whose inflection point lies at about the same location. The transition between the brittle and soggy states, however, appears sharper in the apparent fractal dimension compared with moisture plot. Less familiar is the observation that at moderate levels of moisture content, while the particles' crunchiness is being lost, their stiffness actually rises, a phenomenon that can be dubbed "moisture toughening." We show this phenomenon in commercial Peanut Butter Crunch® (sweet starch-based cereal), Cheese Balls (salty starch-based snack), and Pork Rind also known as "Chicharon" (salty deep-fried pork skin), 3 crunchy foods that have very different chemical composition. We also show that in the first 2 foods, moisture toughening was perceived sensorily as increased "hardness." We have concluded that the partial plasticization, which caused the brittleness loss, also inhibited failure propagation, which allowed the solid matrix to sustain higher stresses. This can explain other published reports of the phenomenon in different foods and model systems.

Thermal damage pattern and thresholds of granite

High temperature may lead to the development of new microcracks or growth of pre-existing microcracks within granite, varying its physical and mechanical properties. Experiments were conducted to study the evolution of the physical and mechanical properties of granite specimens from room temperature to 800 °C. The specimens were heated in heating furnace and uniaxial compression tests were done using MTS servo-controlled testing machine. The results indicate five phases in the variation of physical and mechanical properties with temperature: from room temperature to 100, 100–300, 300–400, 400–600, and 600–800 °C. The first phase corresponds to the vaporization-escaping interval of adhered water, bound water, and structural water. Larger changes of physical and mechanical parameters in the temperature range of 300–600 °C, mostly 400–600 °C, are probably caused by the transition from the brittle state to plasticity (or ductility) of granite, and 400 °C may be a critical threshold of its thermal damage. These results confirm the important link among physical and mechanical properties in response to thermal treatment.