Tip Extension Research Articles

<p>Extension Mechanism of a Flexible End for Colonoscopy</p>

IntroductionThe significance of the distal tip extension mechanism (DTEM) arises when the insertion tube of the colonoscope is no longer able to move further inside the colon, and when a longer insertion tube is needed. The main focus of this research is to investigate the development of a novel distal tip extension mechanism (DTEM).MethodsTo characterize the performance of the DTEM, the ability of the DTEM to extend the distal tip of the colonoscope 16 cm is investigated. To determine the maximum number of turns that the extension knob needs to make in order to move the distal tip 16 cm, the DTEM is used to displace the distal tip in different colon configurations using the silicon tube of a colonoscopy training model (CTM). The experimentally collected data was presented and discussed to characterize the performance of the DTEM.ResultsThe results showed that the DTEM is able to extend the distal tip 16 cm while the colon is in various configurations. Additionally, the impact of implanting the DTEM on the flexibility of the insertion tube was determined.DiscussionThe results of this research suggest that the DTEM will be an effective tool to help colonoscopists performing better colonoscopies.

Subcellular structure and behaviour in fungal hyphae.

This work briefly surveys the diversity of selected subcellular characteristics in hyphal tip cells of the fungal kingdom (Mycota). Hyphae are filamentous cells that grow by tip extension. It is a highly polarised mechanism that requires a robust secretory system for the delivery of materials (e.g. membrane, proteins, cell wall materials) to sites of cell growth. These events result it the self-assembly of a Spitzenkörper (Spk), found most often in the Basidiomycota, Ascomycota, and Blastocladiomycota, or an apical vesicle crescent (AVC), present in the most Mucoromycota and Zoopagomycota. The Spk is a complex apical body composed of secretory vesicles, cytoskeletal elements, and signaling proteins. The AVC appears less complex, though little is known of its composition other than secretory vesicles. Both bodies influence hyphal growth and morphogenesis. Other factors such as cytoskeletal functions, endocytosis, cytoplasmic flow, and turgor pressure are also important in sustaining hyphal growth. Clarifying subcellular structures, functions, and behaviours through bioimagining analysis are providing a better understanding of the cell biology and phylogenetic relationships of fungi. LAY DESCRIPTION: Fungi are most familiar to the public as yeast, molds, and mushrooms. They are eukaryotic organisms that inhabit diverse ecological niches around the world and are critical to the health of ecosystems performing roles in decomposition of organic matter and nutrient recycling (Heath, 1990). Fungi are heterotrophs, unlike plants, and comprise the most successful and diverse phyla of eukaryotic microbes, interacting with all other forms of life in associations that range from beneficial (e.g., mycorrhizae) to antagonistic (e.g., pathogens). Some fungi can be parasitic or pathogenic on plants (e.g., Cryphonectria parasitica, Magnaporthe grisea), insects (e.g., Beauveria bassiana, Cordyceps sp.), invertebrates (e.g., Drechslerella anchonia), vertebrates (e.g., Coccidioides immitis, Candia albicans) and other fungi (e.g., Trichoderma viride, Ampelomyces quisqualis). The majority of fungi, however, are saprophytes, obtaining nutrition through the brake down of non-living organic matter.

Fracture Surface Area Estimation from Hydraulic-Fracture Treatment Pressure Falloff Data

Summary It has often been reported that the peak production of a well drilled in tight formations is highly dependent on the fracture-contact area. However, at present, there is no efficient approach to estimate the fracture surface area for each fracture stage. In this paper, we propose a method to calculate the fracture surface area on the basis of the falloff data after each stage of the main hydraulic-fracture treatment. The created hydraulic fracture closes freely before its surfaces hit the proppant pack, and this process can be recognized in the pressure falloff data and its diagnostic plots. The pressure-decline rate during fracture closure is mainly caused by the fluid leakoff from the fracture system into the formation matrix. For a horizontal well drilled in the same formation, with the known leakoff coefficient(s) and fracture-closure stress(es), the total-fracture surface area can be calculated for all stages to meet the requirement of the fluid-leakoff rate. The wellbore-storage effect, friction dissipation, and tip extension dominate the early pressure falloff data. Whereas the transient pressure dominated by friction losses typically lasts approximately 1 minute, the tip extension might end after approximately 15 minutes. Therefore, falloff data should be acquired for at least 30 minutes to observe a fracture-closure trend. The fracture-closure behavior can be identified on the G-function plot as an extrapolated straight line or on the Bourdet derivative in log-log plot as a late-time unit slope. The behavior of the late unit slope depends on the pressure-decline rate, or correspondingly, to the fluid-leakoff rate. Therefore, the total-fracture surface area can be estimated using hydraulic-fracture design input values for the formation-leakoff coefficient and fracture-closure stress. The calculated fracture surface area represents the combined area of primary and secondary fractures—effectively all fracture surfaces contributing to the fluid leakoff. We applied the approach to all stages in a horizontal well that exhibit the fracture-closure behavior. The approach shows some promise as a potential way to estimate fracture surface areas that could allow an early estimate of the expected well performance.

Noise reduction of a Counter Rotating Open Rotor through a locked blade row

Counter Rotating Open Rotors (CROR) have the potential to significantly reduce aircraft emissions. However, there are a number of concerns surrounding their noise levels. In this contribution, we propose a number of novel configurations to reduce the noise of CROR for general aviation aircraft. In particular, we lock either fore or aft blade row on departure and approach, and keep the CROR trimmed at the same thrust as in design configuration (both rotors running). The CROR is switched back to design configuration at cruise to regain the high propulsive efficiency. To further reduce noise, two additional cases are investigated: 1) folded aft rotor; 2) increased diameter fore rotor (with locked-aft blade row). A low-order CROR trim code that has been developed to compute the aerodynamic and aeroacoustic performance of new design configurations is used in the numerical analysis. The results demonstrate significant noise gains for a number of blade count combinations during a constant altitude flyover. The analysis found that using a locked-fore and locked-aft configuration resulted in a maximum noise reduction of around 3 dB(A) (or 3 dB (EPNL)) and 3 dB(A) (or 6 dB (EPNL)) respectively. Folding the aft rotor backward could result in a maximum noise gain of around 12 dB(A). Increasing the fore rotor diameter by 30% was shown to reduce noise by a maximum of 8 dB(A) against an operative CROR of the same tip extension.

Analysis of fracture toughness properties of wire + arc additive manufactured high strength low alloy structural steel components

The uncertainty surrounding the fracture behaviour of CMT-WAAM deposited steel, in terms of crack tip condition (J and CTOD) needed to cause crack tip extension, has made this manufacturing technique unpopular to date. Fracture toughness parameters are crucial in the structural integrity assessment of components and structures in various industries for assessing the suitability of a manufacturing process and material. In the offshore wind industry, the EN-GJS-400-18-LT ductile cast grade for the mainframe and hub has lower fracture toughness resistance for its high strength grade. Its high weight level affects the Eigen frequency of the tower and imposes high installation cost incurred from heavy lifting equipment usage. Poor fracture toughness is currently a challenge for wind turbine manufacturers in the quest for a cleaner and cheaper energy in the form of offshore wind. In this study, CMT-WAAM is used in depositing steel components with an oscillatory and single pass deposition strategy. The effects of microstructural variation, as a result of layer by layer deposition and the layer band spacing, on the fracture resistance in the build and welding direction was shown here. The fracture mechanics and failure mode of the WAAM deposited parts were investigated. The microstructural variation, again as a result of the layer by layer deposition and the layer band spacing, are the key parameters that control the fracture toughness of WAAM steel. Anisotropic behaviour in the Jq values was observed between both fracture orientations. The constructive transformation mechanism of the WAAM oscillatory process made way for intragranular nucleation of acicular ferrite on the Ti containing inclusion, thereby improving the toughness of the ER70S-6 deposit with a unique microstructure and Jq value of 640kJ/m2 .

Consistent Model for Injection and Falloff Pressure Match of Diagnostic Fracture Injection Tests (DFITs)

Summary The diagnostic fracture injection test (DFIT), a fracture–injection/falloff test, is a reliable tool for quantifying the formation of closure stress, leakoff coefficient, formation permeability, and pressure. The current analytical DFIT model (used before and after closure) enables one to match the pressure falloff of abnormal leakoff behaviors, and quantifies more formation parameters than a traditional DFIT model. However, this model design addresses only the falloff data after shut–in; thus, analysts have expressed concerns that the net pressure implied by the falloff is inconsistent with the injection pressure behavior. Therefore, this paper provides a model capable of matching both injection and falloff pressure behaviors. The pressure–falloff model is capable of quantifying essential pressure values including, in order of occurrence, instantaneous shut–in pressure (ISIP), minimum fracture–propagation pressure, one or more closure–stress values, formation minimum principal stress, and pore pressure. The early pressure response represents the dissipation of three kinds of friction—wellbore, perforation, and near–wellbore friction. Each of them is quantified, and, together, they comprise the difference between the pressure at the end of injection and the ISIP. Presence of tip extension enables the quantification of the minimum fracture–propagation pressure. The minimum principal stress is consistent with the final closure stress. Subtracting the closure stress and friction pressure losses from the recorded or calculated bottomhole pressure (BHP) provides the fracture net pressure. The model match for injection pressure behavior incorporates the same pressures and consistent values for 2D fracture geometry and leakoff coefficient. The global match confirms not only the estimation of formation and fracture properties from the pressure–falloff analysis, but also the friction losses along the wellbore, through the perforations, and in the near–wellbore tortuosity during and after injection. In particular, by matching both injection and falloff, the model incorporates friction pressure losses that can explain apparent excessive net pressure. The match with both injection and falloff pressure variation addresses concerns that the net pressure implied by the falloff model match cannot be consistent with the observed injection behavior. When the pressure difference between the final DFIT pick for closure stress and the pressure at the end of injection is large, the reason might be tip extension and/or large friction pressure losses, the latter of which can be addressed by the main treatment design.

Apical assemblies of intermediate filament-like protein FilP are highly dynamic and affect polar growth determinant DivIVA in Streptomyces venezuelae.

Streptomyces spp. grow as branching hyphae, building the cell wall in restricted zones at hyphal tips. The organization of this mode of polar growth involves three coiled-coil proteins: DivIVA and Scy, which form apical protein complexes referred to as polarisomes; and the intermediate filament-like protein FilP, which influences cell shape and interacts with both Scy and DivIVA. Here, we use live cell imaging of Streptomyces venezuelae to clarify the subcellular localization and dynamics of FilP and its effect on hyphal morphology. By monitoring a FilP-mCherry fusion protein, we show that FilP accumulates in gradient-like zones behind the hyphal tips. The apical gradient pattern of FilP localization is dependent on hyphal tip extension and immediately dissipates upon growth arrest. Fluorescence recovery after photobleaching experiments show that FilP gradients are dynamic and subject to subunit exchange during vegetative growth. Further, the localization of FilP at hyphal tips is not directly dependent on scy, even though the strongly perturbed morphology of most scy mutant hyphae is associated with mislocalization of FilP. Finally, we find that filP has an effect on the size and position of the foci of key polar growth determinant DivIVA. This effect likely contributes to the phenotype of filP mutants.

Vine Robots

A new class of continuum robots has recently been explored, characterized by tip extension, significant length change, and directional control. Here, we call this class of robots "vine robots," due to their similar behavior to plants with the growth habit of trailing. Due to their growth-based movement, vine robots are well suited for navigation and exploration in cluttered environments, but until now, they have not been deployed outside the lab. Portability of these robots and steerability at length scales relevant for navigation are key to field applications. In addition, intuitive human-in-the-loop teleoperation enables movement in unknown and dynamic environments. We present a vine robot system that is teleoperated using a custom designed flexible joystick and camera system, long enough for use in navigation tasks, and portable for use in the field. We report on deployment of this system in two scenarios: a soft robot navigation competition and exploration of an archaeological site. The competition course required movement over uneven terrain, past unstable obstacles, and through a small aperture. The archaeological site required movement over rocks and through horizontal and vertical turns. The robot tip successfully moved past the obstacles and through the tunnels, demonstrating the capability of vine robots to achieve navigation and exploration tasks in the field.

Numerical studies of active flow control on wing tip extension

PurposeIn the European project AFLoNext, active flow control (AFC) measures were adopted in the wing tip extension leading edge to suppress flow separation. It is expected that the designed wing tip extension may improve aerodynamic efficiency by about 2 per cent in terms of fuel consumption and emissions. As the leading edge of the wing tip is not protected with high-lift device, flow separation occurs earlier than over the inboard wing in the take-off/landing configuration. The aim of this study is the adoption of AFC to delay wing tip stall and to improve lift-to-drag ratio.Design/methodology/approachSeveral actuator locations and AFC strategies were tested with computational fluid dynamics. The first approach was “standard” one with physical modeling of the actuators, and the second one was focused on the volume forcing method. The actuators location and the forcing plane close to separation line of the reference configuration were chose to enhance the flow with steady and pulsed jet blowing. Dependence of the lift-to-drag benefit with respect to injected mass flow is investigated.FindingsThe mechanism of flow separation onset is identified as the interaction of slat-end and wing tip vortices. These vortices moving toward each other with increasing angle of attack (AoA) interact and cause the flow separation. AFC is applied to control the slat-end vortex and the inboard movement of the wing tip vortex to suppress their interaction. The separation onset has been postponed by about 2° of AoA; the value of ift-to-drag (L/D) was improved up to 22 per cent for the most beneficial cases.Practical implicationsThe AFC using the steady or pulsed blowing (PB) was proved to be an effective tool for delaying the flow separation. Although better values of L/D have been reached using steady blowing, it is also shown that PB case with a duty cycle of 0.5 needs only one half of the mass flow.Originality/valueTwo approaches of different levels of complexity are studied and compared. The first is based on physical modeling of actuator cavities, while the second relies on volume forcing method which does not require detailed actuator modeling. Both approaches give consistent results.

A Soft, Steerable Continuum Robot That Grows via Tip Extension.

Soft continuum robots exhibit access and manipulation capabilities in constrained and cluttered environments not achievable by traditional robots. However, navigation of these robots can be difficult due to the kinematics of these devices. Here we describe the design, modeling, and control of a soft continuum robot with a tip extension degree of freedom. This design enables extremely simple navigation of the robot through decoupled steering and forward movement. To navigate to a destination, the robot is steered to point at the destination and the extension degree of freedom is used to reach it. Movement of the tip is always in the direction tangent to the end of the robot's backbone, independent of the shape of the rest of the backbone. Steering occurs by inflating multiple series pneumatic artificial muscles arranged radially around the backbone and extending along the robot's whole length, while extension is implemented using pneumatically driven tip eversion. We present models and experimentally verify the growing robot kinematics. Control of the growing robot is demonstrated using an eye-in-hand visual servo control law that enables growth and steering of the robot to designated locations.

Comprehensive before-closure model and analysis for fracture calibration injection falloff test

The fracture calibration injection falloff test, also known as mini-frac and diagnostic fracture injection test (DFIT), is widely used to quantify essential input parameters for the following main fracture treatment design. Over time, data presentations used to facilitate the analysis have revealed trends in the before-closure falloff transient data that have been labeled as abnormal closure. This paper provides new models for all known before-closure leakoff behaviors and a comprehensive model that enables a match with the entire before-closure transient response.The before-closure leakoff behaviors in the comprehensive model include wellbore storage (WBS), friction dissipation, tip extension, pressure dependent leakoff area (PDLA), multiple apparent closures, transferred storage (also known as transverse storage), height recession and variable fracture compliance. Besides the parameters such as closure pressure, leakoff coefficient, and the hydraulic fracture geometry that can be quantified by commonly known analysis techniques, the new model enables quantification of several new parameters.Two diagnostic plots are used to facilitate identification and quantification of the standard and newly modeled parameters: the Bourdet log-log plot of pressure change and derivative and the composite G-function plot. Tip extension is more easily identified on the G-function plot, and the model match with this behavior quantifies the tip extension distance after shut-in and the fracture minimum propagation pressure. So-called pressure dependent leakoff and transferred storage may be related to opening of secondary fractures, and may be seen as multiple closure events easily identified on either diagnostic plot. Such effects enable quantification of closure stress and surface areas for the secondary fracture sets. Alternatively, when the behavior that looks like transferred storage is explained as fracture height recession, the closure stress of overlying or underlying layers can be estimated along with the recessed fracture area. Variable fracture compliance has similar appearance with transferred storage as well in the diagnostic plots. When it occurs, the way to determine fracture closure stress and other parameters are provided.Matches with field data reflect highly enhanced understanding of the nature of hydraulic fracturing and natural fracture flow mechanisms that have not been possible with other models and that greatly refine well design strategies.

A lattice-based system for modeling fungal mycelial growth in complex environments

Fungi are one of the most destructive agents to wood and wood-based products, resulting in the decrease of the building service life by breaking down lumber structures. This work presents a discrete lattice-based model to simulate the mycelial growth, which explicitly incorporates tip extension, extension angle, branching and anastomosis. The developed algorithm reduces greatly the geometrical restriction of the lattice to hyphal elongation; thus, it is able to generate realistic mycelial networks with low computational costs. Different growth conditions are reproduced via tropisms which influence the tip dominant direction, such as thigmotropism that enables hyphae to bend around the obstacles in structurally-heterogeneous media. The validation of this model is implemented through an experiment focusing on the morphology and the growth of Postia placenta, a species of brown rot fungus. Some model parameters are directly obtained from the experimental data, while others have to be determined by inverse procedure. This validated model is an efficient tool for predicting the mycelial growth under different conditions over a large spatial scale, thereby allowing upscaling from laboratory to macroscopic scale.

Realistic Stacking Sequence Optimisation of an Aero-Engine Fan Blade-Like Structure Subjected to Frequency, Deformation and Manufacturing Constraints

Aims:A procedure to optimise the stacking sequence of a composite fan blade-like structure is proposed in this article. The aim of the optimisation is to minimise weight when respecting deformation, frequency and strain constraints. The literature often deals with stacking sequence optimisation of airplane wings or wind turbine blades whilst less attention has been dedicated to aero-engines fan blades, the objective of the present paper. The manufacturing constraints are also implemented in the optimisation process in order to obtain a manufacturable structure.Background:Stacking sequence of composite laminates can be tailored to drive the deformation towards the desired shape (potentially exploiting unbalanced laminates and their anisotropy). When optimising the stacking sequence (including blending/tapering) of an aero-engine fan blade-like structure, manufacturing constraints must be included in order to apply the results of the optimisation procedure into a “Real World” design.Objectives:To define an engineering procedure able to provide a good design point to minimise the weight of a fan blade-like structure subjected to deformation (tip extension and untwist), frequency and strain constraints.Methods:A two-level optimisation procedure is proposed. At the first level, the stacking sequence is optimised in such a way to maximise stiffness (and therefore to minimise deformation). Less stringent limits are applied to the constraints of such a level 1 optimisation. In the second step of the optimisation, the blending/tapering of each ply of the stacking sequence is searched.Results:The fan blade-like structure is loaded only with a centrifugal load (the main load acting on this kind of components). The stacking sequence obtained to minimise the weight contains 42.3% of 0 degrees fibres, 19.25% of 45 degrees fibres, 19.25% of -45 degrees fibres and 19.2% of 90 degrees fibres. Blending in terms of width and length of each layer is given in the numerical results section.Conclusions:When the fan blade-like structure is loaded with a centrifugal force only, in order to minimise weight by respecting untwist, tip extension, frequency and integrity constraints, no unbalance in the laminate has been found necessary. An “Optimum” point has been found after a two steps optimisation. This design point is claimed as a good industrial design point rather than as “optimum” in the mathematical sense. Such a “Best Solution” design point has been verified by exploring the design space near it. All the performance of the neighbour points has been found worse. A comparison between a quasi-isotropic laminate and a zero degreed dominated laminate has been also performed.

Computational fluid dynamics-based surrogate optimization of a wind turbine blade tip extension for maximising energy production

This article presents a design study into the redesign of a wind turbine blade tip seeking to increase the energy production subject to the loads constraints of the existing blade. The blade shape is parameterized to allow for planform changes in the tip region with respect to chord, twist and blade length extension, and additionally three parameters that allow to explore winglet-like shapes. The design strategy uses 3D computational fluid dynamics computations of the geometrically resolved rotor to create a surrogate model, after which the tip shape is numerically optimized based on the surrogate model, subject to a number of geometric and loads-based constraints. The study shows that it is possible to increase power production by 2.6% for a blade extension with a winglet, without increasing the flapwise bending moment at 90% radius, whereas for a straight blade extension it was only possible to achieve an increase of 0.76%.

A numerical lattice method to characterize a contact fatigue crack growth and its Paris coefficients using configurational forces and stress-life curves

Paris’ law material constants have computationally been determined using the implications of Configurational Mechanics and total stress-life curves to characterize fatigue crack growth rate, which is a challenge in experimental programs for measuring these parameters. A 2D plane strain lattice approach with a brittle erosion algorithm was employed to characterize total fatigue life and fracture behavior of propagating cracks of a pad-substrate system under high-cycle fatigue loadings in the context of LEFM. The capability of the lattice in predicting the direction of crack tip extension was validated by comparing with the analytical results of a center inclined cracked domain under uniaxial direct tension and also the surface crack initiation angle of the contact fatigue pad-substrate system. The lattice not only does fairly predict the Paris’ coefficients comparing to experimental measurements in literature, but also obtains total fatigue life by assuming a failure value for the crack length and demonstrates a curved fatigue crack. Having a simple constitutive formulation and a straightforward erosion algorithm, the numerical lattice can be integrated with the proposed fatigue fracture approach to obtain the Paris’ material constants, contact fatigue life, and crack propagation path.

Correction: Morphological Characterization and Quantification of the Mycelial Growth of the Brown-Rot Fungus Postia placenta for Modeling Purposes.

[This corrects the article DOI: 10.1371/journal.pone.0162469.].

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures.

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

Comparative study on carbon–nitrogen metabolism and endogenous hormone contents in normal and overgrown sweetpotato

Sweetpotato is easily overgrown because of improved production conditions and soil fertility. In this study, normal and overgrown sweetpotato plants were obtained from the long vine cultivar J18 (J18) and the short vine cultivar X32 (X32) in a field experiment with different nitrogen (N) treatments (0, 120, and 240kgNha−1). The carbon (C)–N metabolism and endogenous hormone contents were compared between normal and overgrown sweetpotato plants. Results revealed that the total C content and C/N ratios in leaves of the overgrown plants were significantly lower than those of the normal plants at 75days after planting (DAP) and 105DAP. Also, the sucrose phosphate synthase (SPS) activities significantly decreased whereas the nitrate reductase (NR) and glutamine synthetase (GS) activities significantly increased. In the overgrown plants, the sucrose content in leaves significantly decreased at 75DAP and 105DAP, whereas the free amino acid and soluble protein contents significantly increased at 50DAP, 75DAP and 105DAP. The indole-3-acetic acid (IAA) and zeatin riboside (ZR) contents in leaves of the overgrown plants were significantly higher than those of the normal plants at 50DAP, 75DAP and 105DAP. The changing of IAA and ZR contents in the stem tips of different growth types showed similar treads to those in leaves. The abscisic acid (ABA) content in the stem tips of the overgrown plants at 75DAP and 105DAP was significantly lower than that of the normal plants. The ZR and ABA contents in the storage roots of the overgrown plants at 50DAP, 75DAP and 105DAP were significantly lower than those of the normal plants. Compared to the normal plants, the overgrown plants exhibited more vigorous N metabolism in leaves. Meanwhile, the levels of endogenous hormones that promote cell elongation and division in the leaves and stem tips increased whereas those restraining stem tip elongation and promoting storage root enlargement decreased, which could promote leaf hypertrophy and stem tip extension at above-ground parts and restrain storage root enlargement at under-ground parts. Thus, source–sink growth became incoordinate and sweetpotato underwent overgrowth.

Assessment of Six Indicators of Street Tree Establishment in Melbourne, Australia

A number of factors can influence successful street tree establishment, including inappropriate selection of stock, poorquality stock, poor planting technique, insufficient irrigation, poor weed control, and inadequate maintenance. Urban tree managers need reliable, accurate, and practical indicators of whether street trees have successfully established to inform decisions about removing supporting stakes or continuing irrigation regimes. The establishment of street trees planted as large format stock was investigated using a survey of 510 trees from 21 species managed by three municipalities in Greater Melbourne, Australia. A planted tree can only be considered established when the partial root system in the root ball or the confined root system in a container has developed into a spreading root system, utilizing moisture and available nutrients throughout a larger soil volume. Thus, an indicator of root expansion into soil of a greater volume is necessary, as are measures of canopy growth, such as shoot-tip growth or canopy dieback. The survey assessed six factors indicative of successful or poor establishment: whether trees had been planted too deeply, whether trees exhibited trunk movement at or below the soil surface, damage to the trunk, trunk sunscald injury, presence of epicormic shoots on the trunk, and whether there were co-dominant stems. The results demonstrated that trunk damage (8%) and codominant stems (5%) occurred at low rates in the trees, but the presence of epicormic shoots (12%) and sunscald damage (12.5%) was more common, and 12% of trees had been planted too deeply and 42% of trees were unstable. Three criteria—shoot tip extension, trunk movement in the ground, and percentage of canopy dieback—provided useful measures of whether street trees were assessed as being well- or poorly-established. This is the first study to record a high incidence of sunscald injury to urban trees in Australia.

The coupled criterion for description of fatigue fracture. Material embrittlement in pre-fracture zone

Step-wise extension of a crack in quasi-brittle materials under low-cycle loading conditions is considered. Both steady and unsteady loadings in pulsating loading mode are studied. It is proposed to use quasi-brittle fracture diagrams for bodies under cyclic loading conditions. When diagrams are plotted, both necessary and sufficient fracture criteria by Neuber-Novozhilov are used. A specific implementation is made on the base of the Leonov-Panasyuk-Dugdale model for the mode I cracks when the pre-fracture zone width coincides with the plasticity zone width near the crack tip. The condition of a step-wise crack tip extension has been derived. A crack extends only in the embrittled material of the pre-fracture zone. The number of cycles between jumps of the crack tip is calculated by the Coffin equation, when damage accumulation in material in the pre-fracture zone is taken into account. Critical fracture parameters under low-cyclic loading conditions have been obtained in a closed form. Estimates of the average rate of crack tip advance for a loading cycle at step-wise crack extension and S − N curves have been obtained.