Effect Of Stress Transfer Research Articles

Unloading Technology and Application Research of Variable Diameter Drilling in Dynamic Pressure Roadway

Theoretical analysis and numerical simulation are used to study the influence of different parameters of variable diameter borehole pressure relief technology on the surrounding rock and support. A strain-softening model was established to analyze the intrinsic connection between the parameters of variable diameter boreholes and the evolution of surrounding rock stress, deformation law, and support strength. The results show that: (1) With the increase in shallow borehole diameter, it is easy to cause anchor de-anchoring phenomenon. (2) After the deep borehole diameter is more than 250 mm, it transfers the peak of the shallow vertical stress to the deep surrounding rock (about 16 m away from the coal wall). (3) If the position of the variable borehole aperture is set between the anchorage zone and the stress peak of the roadway, the stress transfer effect is better, and the influence and effective binding force on the surrounding rock is smaller. (4) When the spacing is 1.0 m~2.0 m, the vertical stress starts to transfer to the deep surrounding rock, the deformation of the surrounding rock is smaller, and the reduction in the effective binding force of the anchors is smaller. The result can provide a reference for similar production conditions.

Large-scale model test and numerical analysis of load-bearing arch characteristics of large cross-section tunnel under high geostress

Understanding the failure process and the load-bearing arch effect of the surrounding rock is crucial for the stability control and support design of tunnels under high geostresses. In this study, the progressive collapse behavior and the formation mechanism of the load-bearing arch of a soft rock tunnel under high geostress were investigated by a model test and numerical simulations. First, a large-scale model test was conducted to study the stress redistribution rules and the failure process of the soft rock under different overloads. According to the model test results, the formation of the final load-bearing arch was divided into four stages including initial local load-bearing arch formation, rock failure inside the local arch, overall load-bearing arch formation and load-bearing arch outward transition. Both the progressive failure and arch mechanism were related to the stress transfer effect. Then, numerical simulations were carried out to verify the stress response results from the model test. The determination method for the inner and outer boundaries of the load-bearing arch was put forward based on the circumferential stress distribution, which was validated by the model test results. Furthermore, the relationship between the radial displacement of the tunnel boundary and the arch position was illustrated. It was found that the load-bearing arch showed an oval shape with a big top and a small bottom. The arch position was linearly correlated with the radial displacement at the tunnel wall. This study provides a comprehensive understanding of the load-bearing arch characteristics of large cross-section tunnels in soft rocks under high geostress conditions, which can be referred to in construction control and support design during tunnel construction.

Numerical study on the induced seismicity through hydraulic fracturing in the deep heat mining project in Basel, Switzerland using a pseudo dynamic model and considering fully hydro-mechanical coupling

The induced seismicity associated with fluid injection (hydraulic fracturing) or forced circulation has become a key issue in the safe exploitation of hot dry rock geothermal energy. In this paper, the induced seismicity in the Deep Heat Mining Project in Basel, Switzerland was numerically studied by using a developed 3D numerical code with considering the fully hydro-mechanical coupling and the dynamic stress transfer effects. The numerical study was based on history matching of the injection pressure and the statistic characteristics of the induced seismicity. Further, sensitivity analysis of some parameters relating to injection operation, slip behavior and strength-stress ratio was conducted. According to this study, the following conclusions were obtained: 1) the temporal and spatial distribution of the seismic events can be divided into an indirectly induced zone, a directly induced zone, and a quiet zone; 2) large seismic events can be found within the indirectly induced zone; 3) the released energy is not exactly proportional to the injection volume; 4) when the geological condition is well-known, the maximum induced seismicity by fluid injection can be approximately determined; 5) the slip behavior and the strength-stress state are dominate factors for inducing large seismic events.

Theoretical Study of the Stress Transfer Effect on the Output of a Composite Piezoelectric Nanogenerator

Combining ceramic ferroelectrics with high piezoelectric constant and polymers with high flexibility as the piezoelectric active material is one promising way to obtain flexible piezoelectric nanog...

Mechanical Properties Test and Simulation Analysis of Glass Coated Amorphous Filaments

Taking the Co68.7Fe4Ni1B13Si11Mo2.3 glass-coated amorphous microwire as the research object, the mechanical properties of the amorphous wire were tested through biaxial tensile experiments, and the fracture deformation process of the amorphous wire was analyzed. Based on the experimental data, the finite element method was used The simulation software ABAQUS numerically simulates the tensile deformation of the amorphous wire, and the fracture relationship between the glass cladding layer and the core wire, and further reveals the tensile deformation fracture behavior of the amorphous wire was analyzed. The experimental results indicate that the fabricated microwires exhibit good tensile properties, and the maximum tensile breaking strength can reach 2918MPa; the fracture of the amorphous wire is a typical brittle fracture, and there is no obvious yield phenomenon. The tensile mechanics simulation results of the amorphous wire are consistent with the experimental results. The glass cladding layer breaks before the core wire, and the stress transfer effect in the amorphous wire is good. The model fracture is consistent with the fracture morphology of the amorphous wire. These results show that the amorphous wire exhibits good tensile properties and stress transmission capabilities, confirming the huge application potential of amorphous wire materials in sensor applications and functional composite materials.

Microstructure and mechanical properties of AlCrFeCoNi high-entropy alloy particle reinforced Mg-9Al-1Zn matrix composites

Abstract AlCrFeCoNi particles were added to Mg-9Al-1Zn alloy in a rotary blowing process. The microstructures and mechanical properties of Mg-9Al-1Zn based composites were characterized by means of X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and tensile and compression tests at room temperature. Results revealed that AlCrFeCoNi particles could effectively refine the grains, and the rotary blowing process enabled the uniform distribution of these particles. The mechanical properties of composites improved with the increase of particle volume fraction. The superior wettability of AlCrFeCoNi particles supported their reliable bonding with the Mg-9Al-1Zn matrix. The Hall–Petch strengthening and stress transfer effect played a dominant role in the improvement of compressive and tensile properties.

Quasi-static and low-velocity impact responses of polypropylene random copolymer composites with adjustable crystalline structures

Currently, the effect of cooling rates on mechanical properties of continuous glass fiber-reinforced isotactic polypropylene (CGF/iPP) composites cannot be well revealed, due to the imperfect impregnation containing voids, limited selection of crystalline structures (i.e., boundary defects and lamellae thickness) and impact energy. Moreover, compared to the polypropylene random copolymer (PPR) with toughness-rigidity balance, the brittleness of iPP and incompatibility of iPP/elastomer system also limit the further improvement of impact resistance. Furthermore, to date, the influence of cooling rates on mechanical properties of CGF/PPR composites has not been reported. Herein, we reveal the influence mechanism of multiple crystalline structures on quasi-static, and on low-velocity impact behavior of CGF/PPR composites based on perfect impregnation. Results show that the tensile strength and impact resistance under perforation threshold both increase first and then decrease with the increased cooling rate, while the impact resistance at non-perforation threshold gradually improves. Severe boundary defects mask the contribution of other crystalline structures, resulting in the lowest tensile strength. After overcoming boundary defects, the type of lamella is the most important factor for the effect of stress transfer. In the case of non-perforation damage, laminate with the smallest crystallinity and the widest size distribution only suffers from subcritical damage. The stiffness of amorphous area generated by the density of tie chains and chain entanglements becomes the decisive factor for improving impact resistance of laminate above perforation threshold. This work may pave the way for improving mechanical properties of thermoplastic composites via generating specific crystalline structures.

Single edge crack effect on mechanical property of polyimide flexible composite envelope

The existence of cracks has a great influence on the life span of the envelope material of aerostats, especially for stratospheric airships that operate in harsh environments. Understanding the relationship between fracture mechanism and composite structure of envelope materials is of great significance to the design and development of envelope materials. To explore the fracture mechanism of envelope material, a novel polyimide (PI) envelope was developed by lamination method. Then, the fracture behavior and mechanism of PI envelope with single edge crack was studied and analyzed in detail by means of experimental method and theoretical model analysis. It was found that the crack orientation has no significant effect on the fracture strength of PI envelope, while the increase of crack length will increase the strength loss rate and decrease the crack sensitivity. These phenomena suggest that the fracture strength loss of envelope is mainly determined by the number of cut-off longitudinal yarns. And the stress transfer effect of coating and transverse yarns which causes stress concentration in the crack tip region is another important factor affecting the fracture strength of PI envelope. Besides, the interfacial properties between coating and fibers as well as the size effect of coating also play an important role in the fracture property of PI envelope. Finally, stress intensity factor theory and critical stress field theory were chosen to study the fracture behavior of PI envelope. The calculation results indicated that critical stress field theory can better predict the fracture strength of PI envelope.

Experimental Study on Stress Transfer and Fracture Law During the Hydraulic Fracturing of Coal Seams

Hydraulic fracturing is a key technology for increasing the permeability of coal seams and improving the extraction effect of coalbed methane. Exploring the parameters that represent the infiltration effect is an essential part of the optimization process of hydraulic fracturing. In this work, based on a similarity principle design, a hydraulic fracturing test was carried out on large raw coal samples using an indoor hydraulic fracturing physical simulation test system. Based on the distribution of primary cracks in the coal samples, the law of crack extension during hydraulic fracturing was studied. The stress variations before and after hydraulic fracturing of the coal seams were monitored, and the stress transfer laws during confining pressure loading and hydraulic fracturing were investigated. The experiment showed that the new fissures in the fracturing process are more likely to extend to the primary fissure development area. When the applied coal body stress exceeds the strength of the coal sample body, the internal cracks in the coal sample are completely penetrated. During this complete penetration, the energy accumulated by pressure water injection is continuously released, and the count detected by the acoustic emission increases sharply. The coal is mainly subjected to compressive stress during the confining pressure loading process, whereas the coal body is mainly subjected to a shear stress during the hydraulic fracturing process. Both the stresses can be transferred through the coal samples. When the stress increase during hydraulic fracturing is greater than the confining load, the overall effect is due to different stresses. As the cracks continue to expand and extend during hydraulic fracturing, the stress transfer effect becomes weaker. Through the exploration of the law of crack extension and stress transmission, this experiment showed that large-scale physical simulation experiments in a laboratory setting can help effectively simulate on-site hydraulic fracturing conditions. The test method and results reported herein provide a reference and basis for the design and optimization of on-site hydraulic fracturing parameters.

Biomimetically Mineralized Alginate Nanocomposite Fibers for Bone Tissue Engineering: Mechanical Properties and in Vitro Cellular Interactions.

We report herein the structural and mechanical properties and in vitro cellular response of hydroxyapatite (HAp)/alginate nanocomposite fibrous scaffolds mimicking the mineralized collagen fibrils of bone tissue. The biomimetically "engineered" nanocomposites, fabricated by electrospinning and in situ synthesis strategy, were compared with pure alginate nanofibers and micrometer-level HAp/alginate composite fibers. The tensile strength and elastic modulus of the nanocomposites increased by 79.3 and 158.4%, respectively, compared to those of alginate. The uniform nucleation and HAp nanocrystal growth on the alginate nanofibers resulted in such enhancement of the mechanical properties via a stress-transfer effect. Rat calvarial osteoblasts were stably attached and stretched more extensively on the nanocomposites' surface than on the pristine alginate. The controlled deposition of the HAp nanophase contributed to a much faster cell proliferation rate on the nanocomposites than on the others. The improved structural stability and osteoblast interactions suggest the fibrous nanocomposite scaffold's potential advantages for bone tissue regeneration.

Cost-effective fabrication of bio-inspired nacre-like composite materials with high strength and toughness

A cost-effective one-step densification process based on bi-directional freeze casting was investigated to produce nacre-like alumina/poly (methyl methacrylate) (PMMA) composites with a unique micro-layered (μL) architecture. This method has the advantage of shorter processing time, as it requires only sintering once instead of twice as in the fabrication of conventional brick-and-mortar (BM) composites via freeze casting. By tuning the processing parameters, composites with different ceramic content and layer thickness were obtained. The resultant mechanical properties of μL composites showed that ceramic content and wall thickness affected mechanical properties significantly. The μL composite with fine ceramic walls (8 μm) and relatively high ceramic fraction (72 vol%) exhibited an exceptional combination of high flexural strength (178 MPa) and fracture toughness (12.5 MPa m1/2). The μL composites were also compared with the conventional BM composites. Although the fracture behaviour of both composites exhibited similar extrinsic toughening mechanisms, the μL composites with longer ceramic walls displayed superior mechanical properties in terms of strength and fracture toughness in comparison with the BM composites comprising short ceramic walls (i.e. bricks), due to the effectiveness of stress transfer of load-bearing ceramic phase within the composites.

Preparation and loads transfer behaviour of graphene/halloysite organic–inorganic hybrid aerogel

Aerogel materials have wide research and application in the fields of thermal insulation and functional materials. Although graphene sheet has high strength in conventional graphene aerogels, the interaction between sheets is weak. The overall strength of graphene aerogels is not high enough. Aerogels, especially halloysite aerogels, have been successfully synthesised in recent years, but their electrical potential expansion is slightly insufficient. Therefore, in view of the enhancement of the mechanical properties of aerogels and the development of electrical potential, the idea of organic–inorganic hybrid (OIH) aerogel was proposed. Graphene halloysite OIH aerogel materials were prepared by graphene sheets and halloysite fibres. The preparation and microstructure of materials were characterised using Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and nitrogen adsorption–desorption. Meanwhile, the mechanical properties of aerogels were also studied, and the stress transfer behaviour of aerogels was investigated by Raman spectroscopy. The results showed that the OIH aerogel was successfully obtained. The strength of the OIH aerogels was much higher than that of the mixture of graphene and halloysite aerogel. The OIH network aerogels had more effective stress transfer effect depending on the chemical bonds between the networks.

Prediction, evaluation and mechanism governing interphase strength in tensile fractured PA-6/MWCNT nanocomposites

Present investigation focuses on exploring the tensile fracture mechanism in Polyamide-6 (PA-6) reinforced with low contents of Multi-walled Carbon nanotubes (MWCNTs). Fractography revealed not only uniform dispersion of MWCNTs but also appreciable interfacial interaction between PA-6 matrix and MWCNTs. Polymer sheathing and formation of bead-like morphology on fractured MWCNTs also confirmed strong interfacial adhesion. Upon fracture, the MWCNTs bridged over broken polymer facets and contributed towards improvement in strength of the composites by stress transfer effect. The bonding of MWCNTs on favourable sites along PA-6 chain was observed from FTIR, while the formation of strong interphase was confirmed by theoretical modelling of interphase strength and the thickness of interphase layer by using modified combination model of Leidnar-Woodhams model and Pukanszky model for cylindrical nanofillers. Mechanism for the formation of a strong and dense interphase and its contribution for improved tensile properties of PA-6/MWCNT nanocomposites are also explained herein.

Microstructure and mechanical properties of the TiN particles reinforced IN718C composite

TiN particles reinforced IN718C superalloy composite (TiN/IN718C composite) was prepared by investment casting, and the microstructure and mechanical properties of TiN/IN718C composite was investigated. TiN particles were uniformly distributed in the matrix and the harmful δ phase and Laves phase were decreased. The microstructure evolution of the TiN/IN718C composite was caused by the redistribution of the Nb, Ti and C elements in the matrix and TiN particles. The significantly increment of ultimate tensile strength and yield strength of TiN/IN718C composite compared to the matrix IN718C superalloy was attributed to the stress transfer effect of TiN particles and the optimized microstructure.

Flexible and self-assembly anisotropic FeCo nanochain-polymer composite films for highly stretchable magnetic device

Magnetic field induced self-assembly is used to successfully fabricate flexible and anisotropic FeCo nanochain-PDMS composite films. The embedded nanochains are controllable in average length (<200 μm) and width (<2 μm) through optimizing the filler mass fraction and the magnetic field intensity, and an ultra-high aspect ratio of 150 can be achieved. In-situ observations on the growth of a 600 μm long nanochain verify the connection and coarsening mechanism. The composite films are anisotropic along the directions parallel or perpendicular to the alined nanochains or nanobundles. The highest tensile strength in the tension direction perpendicular to the nanochains is 3.20 MPa for the composite film fabricated at 60 mT with 1.0 wt% of filler, which is 85.0% higher than that of the pure PDMS films (1.73 MPa). Different tensile models are proposed to discuss the fracture mechanism, concerning the effectiveness of stress transfer between soft matrix and filler. VSM measurements indicate that the nanochains are easier to be magnetized in parallel geometry due to the larger dipolar coupling. The saturated magnetization (Ms) of nanocomposite films with 1.0 wt% FeCo nanocube filler is 0.45 emu g−1. Such highly flexible, magnetically responsive anisotropic composite films have potential applications especially for highly stretchable devices.

SIGNIFICANT EARTHQUAKES NEAR THE CITY OF THESSALONIKI (NORTHERN GREECE) AND PROBABILITY DISTRIBUTION ON FAULTS

Stress triggering must be incorporated into quantitative earthquake probability estimate, given that faults are interacted though their stress field. Using time dependent probability estimates this work aims at the evaluation of the occurrence probability of anticipated earthquakes near the city of Thessaloniki, an urban center of 1 million people located in northern Greece, conditional to the time elapsed since the last stronger event on each fault segment of the study area. A method that calculates the macroseismic epicenter and magnitude according to macroseismic intensities is used to improve the existing earthquake catalog (from AD 1600 - 2013 with M≥6.0) in order to compute new interevent and elapsed time values which form the basis for time-dependent probability estimates. To investigate the effects of stress transfer to seismic hazard, the probabilistic calculations presented here employ detailed models of coseismic stress associated with the 20 June 1978 M=6.5 Thessaloniki which is the latest destructive earthquake in the area in the instrumental era. The combined 2015-2045 regional Poisson probability of M≥6.0 earthquakes is ~35% the regional time-dependent probability varies from 0% to 15% and incorporation of stress transfer from 0% to 20% for each fault segment.

Poly(p-phenylene terephthalamide)/carbon nanotube composite membrane: Preparation via polyanion solution method and mechanical property enhancement

Abstract Since the discovery of carbon nanotubes (CNTs) over twenty years ago, their potential applications have been extensively explored in nanocomposite fields owing to their excellent mechanical properties. Unfortunately, researches on CNTs reinforced polymers with liquid crystal properties are scare, especially on multi-scale interphase understanding of the newly combined system. In this work, a combination of functionalized nanometer-scale aramid structures and multi-walled carbon nanotubes (MWNTs) was prepared based on synergetic effects. After the re-proton reaction and filtration process, a nanocomposite membrane material was prepared. The mechanical properties of nanacomposite membrane were significantly enhanced and positively related to the weight fraction of carbon nanotubes below 2%. A network structure was formed by the interweaving of MWNTs which carried on applied tensile stress together with aramid matrix, reaching the highest tensile strength of 327 MPa, with a 54.2% increase compared with the matrix material. The interfacial stress transfer effect from substrate to surface of carbon nanotubes was improved through inducing crystallization of aramids around the network properly, and that was confirmed by the in-situ Raman test. The relation between Young's modulus of the nanocomposites and volume fraction of nanotubes can be accurately predicted by Halphin–Tsai equation under a certain range of CNT content. Furthermore, the thermal decomposition temperature of nanocomposites was enhanced compared to the base material, resulting in a better thermal stability.

Finite strain compressive behaviour of CNT/epoxy nanocomposites: 2D versus 3D RVE-based modelling

The macroscopic finite strain compressive behaviour of CNT/epoxy nanocomposites at quasi-static and high strain-rates was predicted and compared using 2D and 3D RVE approaches, a nonlinear and rate-dependent model for the epoxy matrix and nonlinear numerical homogenisation. Effects of CNT orientation, CNT volume fraction, CNT aspect ratio on the nanocomposite response were predicted and analysed, to compare 2D and 3D modelling approaches and obtain an insight into the nonlinear finite compressive behaviour of CNT/epoxy systems. It was found that the nanocomposite nonlinear compressive stress–strain response cannot be accurately captured by 2D RVEs (assuming the plane strain condition), when compared to 3D RVEs, primarily because of the stress transfer effect and the interparticle interaction accurately captured only in 3D. In general, 3D models predicted that the increasing CNT aspect ratio, CNT volume fraction and CNT alignment enhance the nonlinear finite strain compressive response by increasing the yield peak true stress and changing the post-yield deformation behaviour from softening to hardening. Also, the CNT alignment was identified as the major factor for enhancing the nonlinear stress–strain response at both quasi-static and impact rates of strain.

Property enhancement of polymer-based composites at cryogenic environment by using tailored carbon nanotubes

At high attitude, all polymer-based materials would suffer from degradation at very low temperature in the range between 220 and 77K (commonly called “at cryogenic environment”) and low atmospheric pressure. Within this temperature range, polymer-based composites behave very brittle and many micro-cracks are formed due to differential thermal coefficients of expansion (CTEs) between polymer matrix and high strength reinforcements. An anti-cracking mechanism in the composites is necessary and can be tailored by using nano-particles. Many studies have addressed that the use of single-walled (SWNTs) and multi-walled carbon (MWNTs) nanotubes could enhance the mechanical properties of polymer-based composites. However, interfacial bonding properties are always an issue as it would affect the efficiency and effectiveness of stress transfer in the composites. This paper addresses the viability of using coiled carbon nanotubes (CCNTs) and randomly-oriented nanoclay-supported nanotubes (NSCNTs) to enhance the mechanical properties of epoxy resin at the cryogenic environment.

Tensile property of a hot work tool steel prepared by biomimetic coupled laser remelting process with different laser input energies

Coupled with the biomimetic principle, a hot work tool steel (4Cr5MoSiV1) was manufactured using a laser with different input energies. Results of tensile tests confirmed that the biomimetic coupled laser remelting (BCLR) process had an advance effect on improving the strength and ductility of 4Cr5MoSiV1 steel simultaneously. Microstructure examinations demonstrated that a fine microstructure along with nano scale carbide was acquired in the BCLR units, which produced an accumulative contribution of grain refinement, precipitation strengthening and a mixed microstructure. Based on the well distribution of the BCLR units, the beneficial effect of stress transfer from the matrix to the units on tensile property was also analyzed.