Brittle-layer-tuned Microcrack Propagation for High-performance Stretchable Strain Sensors

High linearity is important for stretchable strain sensors. Herein, we propose a new strategy of brittle-layer-tuned microcrack propagation to achieve high-linearity resistive-type stretchable strain sensors by using a highly facile...

FEATURES OF DESTRUCTION OF NITRIDED SAMPLES OF 38H2MYuA AND 30H3MA STEELS

The article deals with the features of deformation and destruction of samples that have undergone chemical-thermal treatment, using the example of testing nitrided samples of steels 38H2MYuA and 30H3MA. It is established that the mechanical properties determined during the tensile testing of samples with a highly rigid brittle layer cannot serve as design characteristics when evaluating the bearing capacity of parts that have undergone chemical-thermal treatment. Testing of nitrided samples of 38H2MYuA and 30H3MA steels demonstrated a complex process of their deformation and destruction, and the test result showed that this steel treated to the «hardened + high tempering» mode sensitivity to the most dangerous concentration the crack is absent.

Accurate Estimation of Brittle Fracture Toughness Deterioration in Steel Structures Subjected to Large Complicated Prestrains

Studies have suggested that brittle fractures occur in steel because microcracks in the brittle layer at grain boundaries propagate as a result of the increase in piled-up dislocations. Therefore, prestraining can approach the limits of a material, which could lead to a decrease in fracture toughness. However, strains are tensors comprising multiple components, so the effect of prestrain on fracture toughness is not simple. Additionally, the mechanism of change in critical stress due to prestrain has not been thoroughly investigated. For the lifetime evaluation of steel structures with a complicated load history, it is important to generalize the effect of complicated prestrain on the decrease in fracture toughness. In this paper, a single prestrain was applied in a direction different from the crack opening direction. A general three-point bending test was employed for fracture evaluation. Numerical analyses using the strain gradient plasticity (SGP) theory, which is a method based on the finite element method (FEM) are carried out; conventional macroscopic material damage rules are considered as well. Using these FEM analyses, the critical stress is calculated. Finally, the change in critical stress can be expressed by the yield point increase and dislocation density and formulated based on the identified micromechanisms.

Analogue models of progressive arcs: strain partitioning and localization in fold-and-thrust belts developed over ductile layer of different geometry

<p>Although analogue models have successfully simulated many different types of arcuate fold-and-thrust belts, we were able to design a backstop whose curvature ratio diminished and its protrusion grade increased during experiments reproducing several kinematic features of progressive arcs never seen before 2016. General models were made up of an homogeneous silicone layer, where detachments tend to localize, overlain by a sand layer. They accomplished to simulate the overall structure and kinematics of fold-and-thrust belts of Mediterranean Arcs, especially that of the Gibraltar arc: (1) highly divergent thrust transport directions, (2) arc-perpendicular normal and strike-slip faults accommodating arc-lengthening, (3) transpressive and transtensional bands oblique to the main trend located in the lateral zones, (4) vertical axis-rotations up to 70º and (5) block individualization that rotated independently clockwise and counterclockwise in the left and right arc limbs, respectively.</p><p>However, the ductile layer is neither continuous nor homogeneous in natural cases, such that pinch-outs and diapirs previous to deformation are frequently found across and along strike. Thus, we have modified our original set-up including silicone pinch-outs and different sizes of silicone diapirs. Where silicone pinch-outs were subparallel to the apex movement, differences in the structural style along the foreland thrust-belt occurred. A forward thrust system over frictional detachments (no silicone), or wide, double verging thrust-systems over ductile detachments (with silicone) developed. Differential displacement between both types of thrust-belts was accommodated by transfer zones. Where silicone pinch-outs were perpendicular to the apex movement, the deformation front propagated up to the pinch-out, where it stopped and the thrust-system thickened up to its subsequent collapse. In models with pre-existing diapirs, first thrust and strike-slip faults nucleated close to diapirs and linked them. When deformation proceeded, all diapirs were added and deformed within the fold-and-thrust belts.</p><p>We also made experiments to analyze the ductile deformation and the influence of the brittle layer (sand) thickness. In only silicone models, a homogeneous deformation was observed at the grid scale, where each square was deformed by mostly simple shear in the lateral parts whilst by mostly pure shear in its most frontal part of the models. When a sand layer was sieved on top of the silicone layer, discrete structures developed. Although all models showed strain partitioning between arc-perpendicular shortening and arc-parallel stretching, as the brittle layer thickness increased, fold wavelength increased.</p><p>All these models show the high complexity derived from the different strain partitioning modes and the strain localization along and across-strike fold-and-thrust belts in progressive arcs. They can be extremely helpful to better understand this kind of arcuate orogens that are also the most frequent in nature. Even though these models were previously carried out to simulate the evolution of fold-and-thrust belts of Mediterranean arcs, they can also shed lights for the evolution of many others progressive arcs.</p>

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

The depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ∼8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate.

A systematic comparison of experimental set-ups for modelling extensional tectonics

Analogue modellers investigating extensional tectonics often use different machines, set-ups and model materials, implying that direct comparisons of results from different studies can be challenging. Here we present a systematic comparison of crustal-scale analogue experiments using simple set-ups simulating extensional tectonics, involving either a foam base, a rubber base, rigid basal plates or a conveyor base system to deform overlying brittle-only or brittle-viscous models. We use X-ray computed tomography (CT) techniques for a detailed 3-D analysis of internal and external model evolution. We find that our brittle-only experiments are strongly affected by their specific set-up, as the materials are directly coupled to the model base. Experiments with a foam or rubber base undergo distributed faulting, whereas experiments with a rigid plate or conveyor base experience localized deformation and the development of discrete rift basins. Pervasive boundary effects may occur due to extension-perpendicular contraction of a rubber base. Brittle-viscous experiments are less affected by the experimental set-up than their brittle-only equivalents since the viscous layer acts as a buffer that decouples the brittle layer from the base. Under reference conditions, a structural weakness at the base of the brittle layer is required to localize deformation into a rift basin. Brittle-viscous plate and conveyor base experiments better localize deformation for high brittle-to-viscous thickness ratios since the thin viscous layers in these experiments allow deformation to transfer from the experimental base to the brittle cover. Brittle-viscous-base coupling is further influenced by changes in strain rate, which affects viscous strength. We find, however, that the brittle-to-viscous strength ratios alone do not suffice to predict the type of deformation in a rift system and that the localized or distributed character of the experimental set-up needs to be taken into account as well. Our set-ups are most appropriate for investigating crustal-scale extension in continental and selected oceanic settings. Specific combinations of set-up and model materials may be used for studying various tectonic settings or lithospheric conditions. Here, natural factors such as temperature variations, extension rate, water content and lithology should be carefully considered. We hope that our experimental overview may serve as a guide for future experimental studies of extensional tectonics.

Quasi-Static and Impact Response of Graded Aluminium Matrix Syntactic Foams

The behaviour of aluminium matrix syntactic foams (AMSFs) with homogeneous and graded structures have been studied under quasi-static compression and impact. Particle size of ceramic microspheres and impact velocity had significant effects on the static and impact responses. Smaller microspheres led to higher strength but lower toughness. The compressive yield stress, plateau stress and specific energy absorption of the graded AMSF specimens were approximately the averages of the constituent layers, following the rule of mixture, although the order of the layers had some influence on the compressive behaviour. The syntactic foams were brittle under impact, no matter whether they were brittle or ductile in quasi-static compression. They had higher peak stresses and absorbed more energy in impact than in quasi-static compression. The location of the most brittle layer of the small ceramic microspheres had a significant effect on the impact failure pattern and sequence of the three-layer graded AMSFs.

A systematic comparison of experimental set-ups for modelling extensional tectonics

Analogue modellers investigating extensional tectonics often use different machines, set-ups and model materials, so that direct comparisons of results from different studies can be challenging. Here we present a systematic comparison of crustal-scale analogue experiments using simple set-ups simulating extensional tectonics, involving either a foam base, a rubber base, rigid basal plates or a conveyor base to deform overlying brittle-only or brittle-viscous models. We use X-ray computed tomography (CT) techniques for a detailed 3D analysis of internal and external model evolution. We find that our brittle-only experiments are strongly affected by the specific set-up, as the materials are directly coupled to the model base. Experiments with a foam or rubber base undergo distributed faulting, whereas experiments with a rigid plate or conveyor base experience localized deformation and the development of discrete rift basins. Pervasive boundary effects may occur due to extension-perpendicular contraction of a rubber base. Brittle-viscous experiments are less affected by the experimental setup than their brittle-only equivalents as the viscous layer acts as a buffer that decouples the brittle layer from the base. Brittle-viscous plate base and conveyor base experiments only localize deformation with high brittle-to-viscous thickness ratios that increases brittle-viscous coupling. This effect is further enhanced by higher strain rates. Our set-ups are most appropriate for investigating crustal-scale extension in continental and selected oceanic settings. Specific combinations of set-up and model materials may be used for studying young or old regions, or wide or narrow extension. Here, natural factors as temperature variations, extension rate, water content and lithology should be carefully considered. We hope that our experimental overviews may serve as a guide for future experimental studies of extensional tectonics.