Local Stress Research Articles

Preconditioning Strategies for Improving the Outcome of Fat Grafting.

Autologous fat grafting is a common procedure in plastic, reconstructive, and aesthetic surgery. However, it is frequently associated with an unpredictable resorption rate of the graft depending on the engraftment kinetics. This, in turn, is determined by the interaction of the grafted adipose tissue with the tissue at the recipient site. Accordingly, preconditioning strategies have been developed following the principle of exposing these tissues in the pretransplantation phase to stimuli inducing endogenous protective and regenerative cellular adaptations, such as the upregulation of stress-response genes or the release of cytokines and growth factors. As summarized in the present review, these stimuli include hypoxia, dietary restriction, local mechanical stress, heat, and exposure to fractional carbon dioxide laser. Preclinical studies show that they promote cell viability, adipogenesis, and angiogenesis, while reducing inflammation, fibrosis, and cyst formation, resulting in a higher survival rate and quality of fat grafts in different experimental settings. Hence, preconditioning represents a promising approach to improve the outcome of fat grafting in future clinical practice. For this purpose, it is necessary to establish standardized preconditioning protocols for specific clinical applications that are efficient, safe, and easy to implement into routine procedures.

Photo-triggered Phase Transition of Crystals and Photoactuation.

Photo-triggered phase transition is a new type of phase transition in which a photochromic crystal with a thermal phase transition transforms into an identical high-temperature phase in a temperature region lower than the thermal phase transition temperature upon light irradiation. Here, we report a second crystal that exhibits a photo-triggered phase transition, thereby demonstrating that the photo-triggered phase transition is a general phenomenon that occurs in crystals. When the chiral salicylidenephenylethylamine crystal was irradiated with ultraviolet (UV) light, the photo-triggered phase transition occurred in the temperature range -30 to -10 °C. The photo-triggered phase transition is induced by local stress due to trans-keto molecules produced by photoisomerization near the irradiated surface. Crystal cantilevers exhibited stepwise bending by the combination of the photo-triggered phase transition and photoisomerization. Alternate irradiation with UV and visible light achieved locomotion of single crystals driven by repeated stepwise bending. Finally, a detailed comparison of photo-triggered and non-photo-triggered phase transition crystals revealed that a sufficient molecular conformation change in affordable crystal voids, smooth photoisomerization, and most likely a chiral molecular arrangement are required for inducing the photo-triggered phase transition.

Shallow ocean deoxygenation drove trilobite turnover during the late Cambrian SPICE event

The spread of marine anoxia is believed to have played a key role in the development of the SPICE (Steptoean Positive Carbon Isotope Excursion) event and the end-Marjuman extinction in the late Cambrian (∼497.5 m.y. ago), but their cause-and-effect relationship is poorly constrained. Here we present an integrated analysis of carbonate δ13C, cerium anomalies (Ce/Ce*), and genus-level diversity data of trilobites from the North China Platform. Our results show tightly coupled changes between the SPICE, an increase in Ce/Ce*, and a trilobite turnover event, which we interpret as indicating enhanced productivity and organic remineralization, leading to the development of low-oxygen conditions in shallow-water settings. This study therefore establishes a direct link between local ecological stress and trilobite turnover during the global SPICE event. Furthermore, the presence of low-oxygen rather than fully anoxic conditions during the peak of the SPICE event could explain the nature of the end-Marjuman crisis, which was characterized by the replacement of shallow-water fauna by deeper-water counterparts that were potentially more tolerant of hypoxia.

New Insight into Bulk Structural Degradation of High-Voltage LiCoO2 at 4.55 V.

The aggravated mechanical and structural degradation of layered oxide cathode materials upon high-voltage charging invariably causes fast capacity fading, but the underlying degradation mechanisms remain elusive. Here we report a new type of mechanical degradation through the formation of a kink band in a Mg and Ti co-doped LiCoO2 cathode charged to 4.55 V (vs Li/Li+). The local stress accommodated by the kink band can impede crack propagation, improving the structural integrity in a highly delithiated state. Additionally, machine-learning-aided atomic-resolution imaging reveals that the formation of kink bands is often accompanied by the transformation from the O3 to O1 phase, which is energetically favorable as demonstrated by first-principles calculations. Our results provide new insights into the mechanical degradation mechanism of high-voltage LiCoO2 and the coupling between electrochemically triggered mechanical failures and structural transition, which may provide valuable guidance for enhancing the electrochemical performance of high-voltage layered oxide cathode materials for lithium-ion batteries.

Crustal Stress Build-Up/Relaxation and Pore Pressure in Preparation and Sequential Brittle-Shear Fault Rupture – Implications for a General Theory of Earthquake Nucleation

Compelling geoscience evidence has heightened appreciation of abnormal (supra-hydrostatic, sub-/quasi-/supra-lithostatic) pore pressure and multi-mode (brittle-shear) fault rupture leading to seismicity. However, preparation and rupture nucleation are yet to be adequately constrained and quantitatively modelled. This challenges crucial schemes, notably physics-based earthquake forecasting/prediction. In this multidisciplinary study, transitions and associated critical points, linking pre-exisiting fluid overpressure in fault patches, temporal crustal stress and sequential brittle-shear fault failure, were considered. In preparation, tectonic loading was accompanied by processes such as off-fault yielding, permeability enhancement and foreshocks that facilitate local/regional stress relaxation. Subsequent equalization of stress and pre-exisiting local overpressure triggers hydraulic fracturing that destabilizes major asperities. Almost instant shear failure follows with spatially varying rupture velocity/intensity of frictional slip because of localized asperity stress and syn-slip fluid-/melt-driven fracturing/dilation and lubrication. Based on aforementioned critical points, quantitative modelling associated stress drop with evolution of stress and pore pressure. Equations for time to onset of stress relaxation and time to rupture were derived using fluid flow and viscoelastic models. Seismic moment was estimated with classical seismological relations after modifications accounting for less surface area during frictional slip. For retrospective testing, two cases of induced seismicity (2016 Fairview, Oklahoma USA and 2017 Pohang, South Korea) and multiple cases of natural seismicity (including the 2024 Noto Peninsula Earthquake, Japan), were considered. Replication of triggering mechanisms, source properties and time to rupture suggested that stress temporal relaxation and triggered anomalies (STRATA) encompass fundamental hydromechanical processes in seismogenesis. Setting and scale invariance of STRATA suggest it might be a general theory of earthquake nucleation. Based on identified preparatory processes/retrospective validations, a physics-based earthquake forecasting/prediction scheme was proposed. Nurseries/hypocenters of impending earthquakes are identified through simultaneous consideration of locally pre-existing fluid overpressure and spatiotemporal analysis of stress relaxation. Event size and rupture timing are estimated with derived relations herein.

Reduced internal stress of quasi-single crystalline Na4Fe3(PO4)2P2O7 electrode enhancing sodium-ion kinetics

A NASICON-type quasi-single crystalline Na4Fe3(PO4)2P2O7 (SC) cathode material was successfully synthesized via a freeze-drying method. The as-optimized SC sample displays both outstanding rate performance (107.9 and 88.5 mAhg−1 at 0.1C and 5C, respectively) and excellent cycling stability (88.1 % of capacity retention after 1000 cycles at 1C). Reduced sodium ion migration path of as-prepared SC cathode material effectively enhanced the kinetics of Na+ upon cycling. More importantly, the SC structure could suppress nearly 50 % of internal strain, prodigiously reducing the local stress accumulation accompanied by the inhomogeneous sodium concentration gradient during the charging/discharging processes. The smaller the stress accumulation, the more stable the crystal structure would be obtained, which maintained a more superb mechanical stability of the SC cathode material. At the same time, a high-spin state of Fe in the SC sample was confirmed by both of electron paramagnetic resonance and temperature-dependent magnetization susceptibility, indicating that the eg orbital occupation of Fe2+ could be regulated to optimize the bond strength between the reduction and oxidation processes. As a consequence, the band gap of SC material has decreased from 1.66 to 1.45 eV, and the electronic conductivity has increased from 6.0 to 32.4 μS/cm. It is believed that the SC cathode material could be a competitive candidate material for sodium ion batteries.

Multi-scale microstructural construction in ultralight graphene aerogels enables super elasticity and unprecedented durability for impact protection materials

Ultralight graphene aerogels have gained extensive recognition in the impact protection field. However, attaining both elasticity and durability at low material density is challenging due to their intrinsic conflicts. Inspired by the mantis ootheca, we present a simultaneous improvement in the elasticity, durability, and density restrictions of ultralight graphene aerogels via constructing a multiscale honeycomb microstructure (MHM) within the graphene skeleton. This approach enables resulting graphene aerogel to achieve a strength per unit volume of 284.6 cm3 mg−1, the ability to recover its shape within 10 ms after an impact at 3.569 m/s, and maintain 97.2 % of its sample height after 20,000 cycles at 90 % strain. The operand analyses and calculation results reveal that the MHM structure facilitates this aerogel’s dual-stage stress transfer pathway. Initially, the macroscale honeycomb structure (millimeter-scale) of the graphene aerogels bear and transmit stress to the surrounding regions, followed by the microscale honeycomb structure (micron-scale) deformation to convert stress kinetic energy into elastic potential energy. This two-stage stress transition mechanism of the MHM structure can effectively mitigate excessive local stress and suppress strain localization, thus providing remarkable elasticity and durability. Ultimately, the obtained graphene aerogel demonstrates promising applications as a fall height detection device and impact protective material.

Inward motion of diamond nanoparticles inside an iron crystal

In the absence of externally applied mechanical loading, it would seem counterintuitive that a solid particle sitting on the surface of another solid could not only sink into the latter, but also continue its rigid-body motion towards the interior, reaching a depth as distant as thousands of times the particle diameter. Here, we demonstrate such a case using in situ microscopic as well as bulk experiments, in which diamond nanoparticles ~100 nm in size move into iron up to millimeter depth, at a temperature about half of the melting point of iron. Each diamond nanoparticle is nudged as a whole, in a displacive motion towards the iron interior, due to a local stress induced by the accumulation of iron atoms diffusing around the particle via a short and easy interfacial channel. Our discovery underscores an unusual mass transport mode in solids, in addition to the familiar diffusion of individual atoms.

Exploring the regulation of circulating factors involved in aortic valve sclerosis onset

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): Ministero della Salute - Ricerca Finalizzata Background Aortic valve sclerosis (AVSc), the first stage of calcific aortic valve stenosis (CAVS), has a prevalence of 25-30% in subjects older than 65 years. Being a clinically silent phase, it is characterized only by non-uniform leaflet thickening due to fibrosis and microcalcifications, without the obstruction of the blood flow with an increased local oxidative stress, inflammation, and endothelial-to-mesenchymal transition (EndMT). However, to date, AVSc is not considered a pathology but 10% of cases evolve a full-blown CAVS. Aim Our aim is to investigate which systemic process(es) are activated in AVSc, in order to better define these subjects. These evaluations will allow us to identify potential druggable targets. Methods We consecutively enrolled 157 healthy subjects and detected AVSc presence by echocardiography. Plasma and serum samples were collected to evaluate circulating microRNA (RNA-sequencing) and cytokines (Luminex), respectively. Functional analysis was performed to reveal systemic pathway regulation. Healthy human valve endothelial cells were employed to evaluate viability (MTT assay) and EndMT (morphological analysis and quantitative PCR) Results AVSc was present in 77 patients (49%) and the differential expression analysis revealed that 59 microRNAs were down-regulated, while 93 were up-regulated in these subjects compared to healthy ones (adjusted-p<0.05). The functional analysis revealed that the major enriched pathways in AVSc subjects were related to inflammation. Indeed, serum GROα and IL1β levels were higher in AVSc vs healthy (p<0.05). In vitro, chronic GROα treatment resulted in lower cell viability (100.0±17.4 vs 61.8±16.5% of vital cells; p<0.0001), while IL1β treatment induced a decrease in cell roundness and an increase in total area (0.40±0.18 vs 0.47±0.19AU, p<0.0001; and 0.75±0.73 vs 0.45±0.48AU, p<0.0001; respectively). Quantitative PCR revealed EndMT activation: SNAI1 (1.87±0.57 log2(foldchange), p<0.0001) and SNAI2 (2.19±0.46 log2(foldchange), p<0.0001) only after IL1β treatment. Conclusions Inflammation is the most regulated systemic pathway in AVSc subjects and its master regulator, IL1β, seems to be involved in the EndMT progression of valve endothelial cells, while GROα contributes to their death. These effects could be related to the early triggers of AVSc onset, leading to fibrosis and eventually calcification of aortic leaflets.

Study on the influence of rock vertical heterogeneity on the vertical extension law of hydraulic fractures

The vertical expansion of fractures during hydraulic fracturing in low-permeability bottom-water oil and gas reservoirs is a crucial factor that significantly influences stimulation effectiveness. Fractures propagate concurrently in three dimensions; however, as operation time for fracturing and fracture length increase, there is gradual reduction in fracture height. In absence of a barrier layer or with inadequate strength and thickness, fractures may extend vertically or oscillate through it leading to an “unyielding” fractured state which impedes successful operations. This study introduces a mathematical model delineating the vertical expansion of hydraulic fractures while computing stress intensity factors at both upper and lower tips of each individual fracture. We utilize numerical methods to examine how vertical heterogeneity within reservoir rocks impacts laws governing vertical fracture propagation while conducting sensitivity analysis for making informed decisions on design parameters for hydraulic fracturing operations. The research results show that an escalation in stress disparity as well as increased toughness results into diminished height for hydraulic fractures. Hydraulic fracturing augments fracture lengths along their respective axes. Nevertheless, if gap stress disparity surpasses net fluid pressure, then there’s reduction observed in fracture lengths. With an increasing ratio between local stress gradient versus fluid gravity gradient, hydraulic fractures persistently advance vertically across cap as well as bed formations.

Researches on the continuous-wave infrared laser-assisted-load icebreaking

In this paper, a novel continuous-wave infrared laser-assisted load icebreaking method for large-sized ice blocks is put forward, which employs a laser multipoint irradiation approach. By strategically changing the mode of deployment points, interlocking cracks and melt holes are generated within the ice. This process effectively weakens the strength of ice, thus decreasing the difficulty of icebreaking. Based on the interaction mechanism between laser and ice, a mathematical model describing the evolution of the local temperature and stress of ice during continuous-wave infrared laser irradiation is established. The effects of deployment mode, laser irradiation time and laser power on the temperature and stress fields are analyzed based on numerical simulation. The impact of laser multipoint irradiation on the superposition of the stress between adjacent deployment points is revealed. On this basis, the experiments of continuous-wave infrared laser-assisted static-load icebreaking and impact-load icebreaking are carried out with natural freshwater ice as the research object. The influences of deployment mode, laser irradiation time and laser power on the external force required for subsequent icebreaking and the final icebreaking effect are analyzed. The characteristics of continuous-wave infrared laser-assisted static-load and impact-load icebreaking are comprehensively compared. Ultimately, the conclusion that impact load is more suitable for laser-assisted load icebreaking is drawn. The outcomes of this study potentially provide a new perspective on the breaking of large-sized ice blocks in an efficient way.

Brittle Damage Processes Around Equi‐Dimensional Pores or Cavities in Rocks: Implications for the Brittle‐Ductile Transition

AbstractIn the Earth's upper crust, rocks deform mostly by means of brittle fracturing processes. At the micro‐scale these processes involve the formation and growth of microcracks in the vicinity of defects such as open fissures, pores and other cavities. Large defects can produce strong enough perturbations of the stress field to activate and/or intensify brittle damage around them. Here we considered the ideal cases of smooth cylindrical and spherical pores inside an infinite solid body subjected to remote triaxial compressive stresses (i.e., the intermediate and minimum principal stresses are assumed equal). We first established the resulting local stress field around one of those large pores and then verified whether certain brittle damage processes could be activated in these conditions. We mainly considered the formation of tensile microcracks and micro shear bands. The former requires the presence of tensile stresses in some regions around the pore, while the latter needs sufficiently large shear stresses on pre‐existing optimally inclined microcracks to overcome their frictional resistance to sliding. We find that shear driven deformation remains localized in the vicinity of the pore. On the other hand, dilatational, tensile cracks can propagate large distances away from the pore but cannot form at and above some threshold ratio of the least to the largest principal stresses. Faulting associated with interacting tensile cracks is therefore suppressed with increasing depth. Our analysis leads to conclusions generally consistent with published experimental observations and provides some clues to discuss the physical cause of the brittle‐ductile transition in rocks.

#957 The immunometabolic profile of human peritoneal macrophages during peritoneal dialysis and its relation to systemic cardiovascular effects

Abstract Background and Aims Patients with end-stage kidney disease (ESKD) treated with peritoneal dialysis (PD) exhibit a high risk of heart failure (HF). This largely concerns HF with a preserved ejection fraction (HFpEF). Uremic toxins, oxidative stress and systemic inflammation related to ESKD contribute to this HFpEF phenotype, which may be aggravated by the daily administration of glucose-rich PD-fluids. These induce insulin resistance and formation of advanced glycaemic end products, resulting in microvascular injury and arteriosclerosis. A hallmark of HFpEF is the presence of profibrotic cardiac macrophages that aggravate fibrosis. In the peritoneum, the non-physiological high glucose concentrations trigger local oxidative stress, fibrosis and inflammation, processes alike those seen in HFpEF. While the role of macrophages in HFpEF is established, the involvement of human peritoneal macrophages (HPMs) in PD-induced peritoneal injury is largely unknown. We hypothesize that HPMs, chronically exposed to the glucose-rich fluids, exhibit an inflammation-driven immunometabolic profile. This may contribute to cardiovascular (CV)-risk in PD-treated patients. We present preliminary results on the isolation of HPMs from PD effluent (PDE) and a detailed protocol to establish the immunometabolic profile of HPMs. These profiles will subsequently be related to cardiac function (left ventricular global longitudinal strain (LV-GLS), an echocardiographic measure of HFpEF) and parameters of systemic inflammation. Method In 15 adult PD-treated patients we will prospectively perform cardiac echography, draw a blood sample and collect PDE for HPM isolation. To obtain control HPMs a peritoneal saline flush will be collected from 5 adult patients with non-metastasized colon cancer during a routine diagnostic laparoscopy. HPMs will be isolated from the PDE and saline by centrifugation, followed by freezing for storage. We compared fresh and thawed cells of a PDE-sample to evaluate the impact of storage on HPM phenotype and functionality, respectively using flow cytometry and measurement of IL-6 production, with and without a lipopolysaccharide (LPS) challenge. In-depth HPM characterization will be performed by measuring glucose and lactate, conducting metabolic flux analysis, assessing glycolysis and mitochondrial characteristics using Seahorse®, and employing cytometry to evaluate reactive oxygen species and metabolic fuel preference. A phenotypical profile will be established using fluorescent probes and antibodies. To assess functionality, HPMs will be exposed in vitro to “M1” and “M2” inducers and we will use human monocyte-derived macrophages from healthy adult controls as a comparison. We will relate the immunometabolic, phenotypical and functional HPM characteristics to LV-GLS, blood cytokine (obtained with Bio-Plex multiplex) and blood immune profiles (obtained with cytometry) in PD-treated patients. Furthermore, we will compare the HPM characteristics of the PD-treated patients with the control group. Results Preliminary results of 6 patients show that we can isolate HPMs from the PDE with a yield of >5M cells. The phenotype and functionality of HPMs was evaluated both directly after isolation and after a freeze-thaw cycle. Flow cytometry showed high percentages of CD14+HLA-DR+CD64+ macrophages in both fresh and biobanked HPMs. IL-6 production was observed in both fresh and biobanked HPMs even without LPS-stimulation (Figure). Conclusion By successfully isolating and storing HPMs from the easily accessible PDE, we optimized the conditions to assess their immunometabolic characteristics. The proposed protocol not only aims to provide valuable insights into HPM phenotype and functionality, but also allows us to relate these to cardiac function and systemic inflammation. With this strategy, we aim to gain insight in the immunometabolic effects of PD on peritoneal immune cells and how these relate to CV-effects.

Extending the intermedullary nail will not reduce the potential risk of femoral head varus in PFNA patients biomechanically: a clinical review and corresponding numerical simulation

Femoral head varus is an important complication in intertrochanteric fracture patients treated with proximal femoral nail anti-rotation (PFNA) fixation. Theoretically, extending the length of the intramedullary nail could optimize fixation stability by lengthening the force arm. However, whether extending the nail length can optimize patient prognosis is unclear. In this study, a review of imaging data from intertrochanteric fracture patients with PFNA fixation was performed, and the length of the intramedullary nail in the femoral trunk and the distance between the lesser trochanter and the distal locking screw were measured. The femoral neck varus status was judged at the 6-month follow-up. The correlation coefficients between nail length and femoral neck varus angle were computed, and linear regression analysis was used to determine whether a change in nail length was an independent risk factor for femoral neck varus. Moreover, the biomechanical effects of different nail lengths on PFNA fixation stability and local stress distribution have also been verified by numerical mechanical simulations. Clinical review revealed that changes in nail length were not significantly correlated with femoral head varus and were also not an independent risk factor for this complication. In addition, only slight biomechanical changes can be observed in the numerical simulation results. Therefore, commonly used intramedullary nails should be able to meet the needs of PFNA-fixed patients, and additional procedures for longer nail insertion may be unnecessary.

Hierarchical microstructure induced by hydrostatic pressure in a metastable β-Ti alloy

Hydrostatic pressure is a crucial tool to regulate the structure and properties of materials. However, the influence of hydrostatic pressure on the microstructure of metastable β-Ti alloys remains unclear. In this work, the microstructure and microhardness evolution of a metastable β-type Ti-30Zr-10Nb alloy under hydrostatic pressure (≤10 GPa) were investigated and compared with those under uniaxial pressure (≤1106 MPa). The results revealed that under hydrostatic pressure, Ti-30Zr-10Nb alloy formed a peculiar hierarchical microstructure composed of submicron-scaled α” plates with self-accommodating morphologies, nanoscale ω particles, and microscale shear bands containing nanoscale α” domains. The formation of a hierarchical microstructure effectively coordinates not only the isotropic macroscopic strains but also the local shear stresses under hydrostatic pressure. Compared to uniaxial pressure, the hydrostatic pressure facilitates the β to ω transformation, but hinders the β to α” transformation. The microhardness of the Ti3010 alloy reaches approximately 319 HV at a hydrostatic pressure of 10 GPa, which is 28 % higher than the highest microhardness obtained under uniaxial pressure. The current results highlight the promising potential of hydrostatic pressure in optimizing the mechanical properties of metastable β-Ti alloys.

Stress localization investigation of additively manufactured GRCop-42 thin-wall structure

A full-field crystal plasticity (CP) framework is presented for the GRCop-42 alloy to study microscopic mechanical behavior and local stress heterogeneities. The microstructures of additively manufactured (AM) materials are often unique relative to conventionally processed materials, and the local thermal histories drive these differences during the build process. These thermal histories depend on the process parameters (laser power, scan speed, and scan strategy) and the part geometry. Prior research has shown that the mechanical properties of thin-walled structures can vary significantly with wall thickness due to changes in the thermal boundary conditions during manufacturing. It is, therefore, desirable to perform CP simulations based on the phenomenological constitutive model to predict the local mechanical responses induced by microstructural heterogeneities. This work generates representative microstructures based on experimentally collected grain information (i.e., texture) for grain scale stress analysis, and the material constitutive parameters are calibrated using the experimental mechanical testing data. We specifically investigated the effect of crystallographic texture and grain morphologies on the size-dependent mechanical properties of AM GRCop-42. The selection of appropriate material properties for implementing an effective free surface boundary condition and the influence of adjacent buffer layers are also discussed. Analysis of local field results reveals a strong correlation between stress localization and the initial grain orientation. However, no significant relationship between the misorientation of the individual adjacent grains and the average misorientation is observed.

Simulation of Mechanical Properties of Injection‐Molded Thermoplastic Foam Structures

This article presents an approach regarding the interlinking of simulation programs to determine mechanical properties of foamed polymers. The software Moldex3D is used to simulate the heterogeneous foam structure, which serves as a database for material modeling. This material modeling is performed with the software Digimat and evaluated using various modeling approaches. The material models are characterized on one side by a homogeneous structure and on the other by the simulated heterogeneous foam structure. In addition, different failure indicators are used and evaluated, which were originally developed for fiber‐reinforced materials. The simulation of the tensile test is carried out with the software Marc‐Mentat. Herein, local stresses and strains are calculated, which reach a maximum value due to the influence of the failure indicators. The results are used to show the extent to which the integration of the heterogeneous foam structure proves to be beneficial. It is additionally demonstrated that the applied failure indicators cannot be used for the prediction of maximum stress and strain simultaneously. Thus, for the time of this study, it is formulated to include both indicators to calculate the maximum stress and strain or to evaluate alternative approaches regarding failure prognosis.

Research on the Interaction Mechanism of Multi-Fracture Propagation in Hydraulic Fracturing

During the hydraulic-fracturing process, stress interference occurs among multiple wells and fractures, potentially affecting the trajectory of hydraulic fracture propagation. Previous studies have largely overlooked the influence of proppant support stresses on the trajectories of fracture propagation. This paper establishes a mathematical model, grounded in the boundary element method, designed to compute the propagation of multiple fractures, considering both proppant support on the fracture surface and dynamic perturbations within the local stress field. The findings of this research reveal that the stress field induced by hydraulic fracturing exhibits dynamic evolution characteristics, necessitating a comprehensive study of the fracture initiation and extension across the entire fracturing time domain. The effect of the residual fracture width under proppant action on the in situ stress field cannot be ignored. During simultaneous fracturing, hydraulic fractures are inclined to propagate in the direction of the maximum horizontal principal stress, particularly as the in situ differential stress escalates. Staggered fracturing between wells has been proven to be more effective than head-to-head fracturing. Simply increasing the well spacing cannot solve the problem of inter-well fracture interference. In zipper fracturing, adjusting the fracturing sequence can inhibit the fracture intersections between wells, thereby controlling the trajectory of fracture propagation. The aforementioned research has considerable significance in guiding the control of fracture morphology during hydraulic-fracturing processes.

Constitutive modelling of glassy polymers considering shear plasticity and craze yielding

The inelastic behaviour of thermoplastic polymers below the glass transition temperature can involve shear plasticity or crazing, depending on the strain rate and temperature. Shear plasticity is driven by local shear stresses and is essentially volume preserving. In contrast, crazing is a failure phenomenon occurring under tension and causes significant volume change. In some cases, crazing can also result in large inelastic deformations at macroscopic scale, referred to as craze yielding. The aim of this study is to propose a thermodynamically-consistent constitutive framework that accounts for both shear plasticity and craze yielding in glassy polymers. The main assumption is that shear plasticity and craze yielding occur exclusively from each other, depending on the local stress state. Both are modelled as thermally-activated processes with different activation stresses and flow rules. The theory is validated against experimental data for polylactic acid (PLA). Our model is capable of reproducing the stress–strain response including yielding, softening, and drawing under both shear plasticity and craze yielding, and also accurately predicts the volumetric deformation under tension. In particular, our simulations well capture the interesting phenomenon that craze yielding stabilises localised deformations and prevents necking. Our model can also simulate complex scenarios where both mechanisms occur simultaneously at different locations of the specimen.

Mechanical Properties of Folding Arch Frame Joints for Unmanned Arch Erection

The application of folding arch frames is deemed crucial for unmanned arch frame erection, with the selection of the joint form being a determining factor in the overall mechanical performance of the folding arch frame, particularly in influencing the primary support safety. In light of the geological conditions of the New Wushaoling Tunnel project, three feasible joint forms for folding arch frames were proposed: buckle, adhesive, and interference-fit joints. Numerical simulations were conducted to analyze the arch’s overall mechanical and the joints’ local mechanical performances, aiming to identify the optimal joint form. On-site construction data were collected, and the effectiveness of unmanned arch frame erection was evaluated. The design requirements for the vertical displacement results of the steel arches with different joints were met. The maximum shear stress of the buckled arch frame was found to be the lowest, whereas that of the interference-fitted arch frame was the highest. The local shear stress of the adhesive joints was the lowest, while that of the interference-fit joint was the highest. Considering the material application limitations and calculation results, buckle joints are recommended. Unmanned arch frame erection, compared with manual arch frame erection, can save 66.6% of human resources and reduce the construction time by 33.3% to 50%. Statistical analysis has confirmed that the quality of automated arch construction can be guaranteed.