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For describing the time-dependent mechanical property of rock during the creep, a new method of building creep model based on variable-order fractional derivatives is proposed. The order of the fractional derivative is allowed to be a function of the independent variable (time), rather than a constant of arbitrary order. Through the segmentation treatment, according to different creep stages of the experimental results, it is found that the improved creep model based on variable-order fractional derivatives agrees well with the experimental data. In addition, the fact is verified that variable order of fractional derivatives can be regarded as a step function, which is reasonable and reliable. In addition, through further piecewise fitting, the parameters in the model are determined on the basis of existing experimental results. All estimated results show that the theoretical model proposed in the paper properly depicts the creep properties, providing an excellent agreement with the experimental data.

To fulfil their function, epithelial tissues need to sustain mechanical stresses and avoid rupture. Although rupture is usually undesired, it is central to some developmental processes, for example, blastocoel formation. Nonetheless, little is known about tissue rupture because it is a multiscale phenomenon that necessitates comprehension of the interplay between mechanical forces and biological processes at the molecular and cellular scales. Here we characterize rupture in epithelial monolayers using mechanical measurements, live imaging and computational modelling. We show that despite consisting of only a single layer of cells, monolayers can withstand surprisingly large deformations, often accommodating several-fold increases in their length before rupture. At large deformation, epithelia increase their stiffness multiple fold in a process controlled by a supracellular network of keratin filaments. Perturbing the keratin network organization fragilized the monolayers and prevented strain-stiffening. Although the kinetics of adhesive bond rupture ultimately control tissue strength, tissue rheology and the history of deformation set the strain and stress at the onset of fracture. Tissue monolayers avoid rupture at large tensile stresses through a strain-stiffening process governed by intermediate keratin filaments.

Strain-induced crystallisation in elastomers markedly increases their elastic moduli and rupture resistance. However, the mechanisms underlying this self-reinforcement in filled elastomers remain unclear owing to the nanoscale nature of the involved processes. Herein, isoprene rubber with/without silica nanoparticles is stretched to strains of >5 and concomitantly imaged via in situ transmission electron microscopy. Nanoscale electron diffraction mapping and in situ transmission electron microscopy results reveal that the self-reinforcement mechanism depends on the filler presence/absence. The unfilled isoprene rubber exhibits a spatially homogeneous strain-induced crystallisation behaviour resulting in drastic elastic modulus enhancement above the crystallisation onset strain. In contrast, the silica-filled isoprene rubber displays preferential crystallite formation in highly stressed regions along the silica aggregates aligned in the stretching direction. This reinforces the stress propagation pathways within the material and results in a lower crystallisation onset strain and higher rupture strength than those of the unfilled system. The insights on the role of fillers in determining strain-induced crystallisation phenomena and mechanical properties facilitate the rational design and development of elastomers. The mechanism underlying the increase in elastic moduli and rupture resistance upon strain-induced crystallization in elastomers remains unclear. Here, the authors use nanoscale electron diffraction mapping and in situ transmission electron microscopy to study the self-reinforcement mechanism of isoprene rubber with silica nanoparticles during stretching.

We performed a complete set of laboratory experiments on a rock salt specimen to study the complex evolution of gas permeability under different loading conditions. The porosity of the studied rock salt is very low (~ 1%) and the initial permeability varies over 4.5 orders of magnitude. The Klinkenberg effect is only observed for the less permeable and damaged samples. The poroelastic coupling is almost negligible in our samples. Deviatoric loading under low confining pressure (1 MPa) induces a moderate increase in gas permeability from the dilatancy threshold due to microcracking. Measurement of ultrasonic wave velocities during uniaxial compression test showed an almost irreversible closure of pre-existing micro-cracks and the opening of axial micro-cracks that are perpendicular and parallel, respectively, to the uniaxial stress direction and allowed a precise determination of the dilatancy threshold. Under higher confining pressure (5 MPa), no increase in permeability was measured because the material becomes fully plastic which practically eliminates microcracking and thus dilatancy. Under hydrostatic loading, gas permeability decreases because of cracks closure and this decrease is irreversible due to the time-dependent self-healing process. Permeability increases slightly during dynamic mechanical and thermal fatigue due to microcracking, while it reduces during static fatigue (creep) thanks to the self-recovery process. All these results give strong confidence in the underground hydrogen storage in salt caverns which remains by far the safest solution because the different mechanisms (viscoplasticity with strain hardening, microcracking and cracks healing) involved in material deformation act in a competitive way to annihilate any significant permeability evolution.

Inks for 3D printing based on cellulose nanofibrils (CNFs) or mixtures of CNFs and either cellulose nanocrystals (CNCs) or alginate were assessed by determining their viscoelastic properties i.e. complex viscosity and storage and loss moduli (G′ and G″). Two types of alginates were used, i.e. from Laminaria hyperborea stipe and Macrocystis pyrifera . Shape fidelity of 3D printed grids were qualitatively evaluated and compared to the viscoelastic properties of the inks. The biocomposite gels containing alginate were post stabilized by crosslinking with Ca 2+ . Mechanical properties of the crosslinked biocomposite gels were assessed. The complex viscosity, G′ and G″ of CNF suspensions increased when the solid content was increased from 3.5 to 4.0 wt%, but levelled off by further increase in CNF solid content. The complex viscosity at low angular frequency at 4 wt% was as high as 10 4 Pa·s. This seemed to be the necessary viscosity level for obtaining good shape fidelity of the printed structures for the studied systems. By replacing part of the CNFs with CNCs, the complex viscosity, G′ and G″ were reduced and so was also the shape fidelity of the printed grids. The changes in complex viscosity and moduli when CNFs was replaced with alginate depended on the relative amounts of CNFs/alginate. The type of alginate (from either L. hyp. stipe or M. pyr. ) did not play a role for the viscoelastic properties of the inks, nor for the printed grids before post stabilization. Replacing CNFs with up to 1.5 wt% alginate gave satisfactory shape fidelity. The effect of adding alginate and subsequent crosslinking with Ca 2+ , strongly affected the strength properties of the gels. By appropriate choice of relative amounts of CNFs and alginate and type of alginate, the Young’s modulus and rupture strength could be controlled within the range of 30–150 kPa and 1.5–6 kg, respectively. The deformation at rupture was around 55%. The alginate from L. hyp. stipe yields higher Young’s modulus and lower syneresis compared to M. pyr . This shows that the choice of alginate plays a significant role for the mechanical properties of the final product, although it does not influence on the viscoelastic properties of the ink. The choice of alginate should be L. hyp. stipe if high strength is desired. Graphical abstract

P92 martensitic heat-resistant steel is widely used in ultra-supercritical (USC) thermal power units due to its excellent creep resistance and high-temperature strength. However, prolonged exposure to high temperatures induces significant microstructural degradation, compromising mechanical properties and operational safety. This study investigates the evolution of microstructure and mechanical properties in P92 steel extracted from main steam pipelines after service durations of 30,000 h, 47,000 h, 56,000 h, 70,000 h, and 93,000 h. Comparative analyses of impact toughness, tensile strength, and creep strength were conducted and advanced characterization of SEM and TEM was used to investigate the microstructural evolution. The results reveal a progressive decline in mechanical properties with increasing service time. Specifically, impact toughness decreased by approximately 66.8%, room-temperature tensile strength reduced by 9.62%, and high-temperature tensile strength at 610 °C declined by 31.6%. Notably, the 105 hour creep rupture strength exhibited a 10.4% decrease compared to as-received material. This decline is attributed to microstructural changes including precipitate coarsening, martensite lath boundary degradation, dislocation reconfiguration, and severe grain coarsening. The coarsening of precipitates weakens their bonding with the matrix, while the widening of martensite laths reduces resistance to crack propagation and dislocation movement, jointly contributing to strength deterioration.

A series of triaxial creep tests were conducted on warm frozen silts extracted from Qinghai–Tibet Plateau at temperature of −1.5 °C under confining pressures of 0.5, 1.0, and 2.0 MPa, respectively. The applied test stress levels were 30, 50, 60, and 70% of triaxial shear strength, respectively. The test results indicate that the creep strain increases with the increase in applied stress level and there is a stress threshold, based on which the test results can be classified into two types of creep strain curves. The creep strain curve only includes primary and secondary creep stages when the stress level is less than the threshold value. When the stress level exceeds the threshold value, the creep strain velocity gradually increases and the specimen quickly fails in tertiary creep stage. Based on the creep test results, a fractional order rheological element model is established for warm frozen silt, which is also generalized from uniaxial stress state to the three-dimensional stress state. From the analysis on the features of the stress threshold, a creep strength criterion is also proposed simultaneously. Comparing the calculated results of the warm frozen silt with the tested ones, it is found that the predicted results of the proposed model are in good agreement with the test results. In the proposed fractional order model, the relationship between the damage factor and time is established to describe the damage degree of the specimen. Compared with the existing creep constitutive model of frozen soil, the proposed fractional order model has advantages of fewer model parameters, higher simulation precision and wider applicability in analyzing the mechanical properties of warm frozen silt.

The difficulties in using the conventional Norton creep model to rationalise short-term creep data and to subsequently predict long-term creep rupture strengths for creep resistant alloys are presented and analysed. The results of this study show that these difficulties can be resolved if a new tensile creep model that integrates the tensile strength at creep temperature is applied to rationalise the short-term creep data. This is illustrated with the creep and tensile strength data measured for a grade of Ni-based superalloy. Based on this new tensile creep model, the activation energy of creep determined is independent of stress and the stress exponent is not influenced by temperature. Consequently, the model constants obtained from the short-term creep data can be applied together with the Monkman-Grant relationship to make the long-term creep rupture strength predictions at different temperatures. The factors affecting the reliability of the predictions made by this method are also analysed.

Monolayer molybdenum disulfide (MoS₂), a two-dimensional transition metal dichalcogenide (2D TMD), is at the forefront of logic device scaling efforts due to its semiconducting properties, good carrier mobility, and atomically thin structure. However, the high defect density of monolayer MoS 2 hinders its reliability for long-term, device-scale applications. Here, we show that a superacid treatment, previously shown to enhance the photoluminescence efficiency of sulfur-based 2D TMDs by two orders of magnitude, also improves the mechanical reliability and electronic uniformity of monolayer MoS₂. Treated samples exhibit a ~2× increase in static fatigue reliability, a ~10× improvement in cyclic wear reliability, and no premature failure during mechanical testing. X-ray photoelectron spectroscopy confirms reduced defect density, while ab initio molecular dynamics and density functional theory suggest that passivation delays failure propagation and reduces vacancy-induced stress. Finally, atomic-resolution conductive atomic force microscopy shows a drastically more uniform current distribution due to elimination of midgap states. Nonoxidizing organic superacid treatments of 2D transition metal dichalcogenides have been shown to drastically boost their electrical and optical characteristics while passivating and repairing defects. Here, the authors demonstrate that these treatments can also be leveraged to boost the mechanical reliability and atomic-scale electronic uniformity of MoS 2 monolayers.

This paper summarizes the results of investigations into heterogeneous P23/P91 welds after long-term creep exposure at temperatures of 500, 550 and 600 °C. Two variants of welds were studied: In Weld A, the filler material corresponded to P91 steel, while in Weld B, the chemical composition of the consumable material matched P23 steel. The creep rupture strength values of Weld A exceeded those of Weld B at all testing temperatures. Most failures in the cross-weld samples occurred in the partially decarburized zones of P23 or WM23 steel. The results of the investigations on the minor phases were in good agreement with kinetic simulations that considered a 0.1 mm fusion zone. Microstructural studies proved that carburization occurred in the P23/P91 weld fusion zones. The partial decarburization of P23 steel or WM23 was accompanied by the dissolution of M7C3 and M23C6 particles, and detailed studies revealed the precipitation of the Fe2 (W, Mo) Laves phase in decarburized areas. Thermodynamic simulations proved that the appearance of this phase in partially decarburized P23 steel or WM23 is related to a reduction in the carbon content in these areas. According to the results of creep tests, the EBSD investigations revealed a better microstructural stability of the partially decarburized P23 steel in Weld A.