Stretch Zone Research Articles

Winter Wheat Aboveground-Biomass Estimation and Its Dynamic Variation during Coal Mining—Assessing by Unmanned Aerial Vehicle-Based Remote Sensing

Underground coal mining in coal-grain overlapped areas leads to land subsidence and deformation above the goaf, damaging cultivated land. Understanding the influencing process of coal mining on cultivated land and crops is important for carrying out timely land reclamation and stabilizing crop yield. Research has been carried out by using crop growth parameters to evaluate the damaging degree of cultivated land when the mining subsidence is stable, but few studies focus on the influence of land damage on crop growth when the subsidence is unstable during coal mining. Therefore, this study tracked the three growth stages of winter wheat by using UAV multispectral imagery to explore the dynamic influence of underground mining on winter wheat aboveground biomass (AGB). Firstly, a winter-wheat-AGB estimation model (R2 = 0.89, RMSE = 2.18 t/ha) was developed by using vegetation indexes (VIs), textures, and terrain data extracted from UAV imagery. Secondly, based on the winter-wheat-AGB estimation model, the winter wheat AGB was successfully estimated and mapped at different growth stages. The AGB of winter wheat in the coal mining-affected area was approximately 5.59 t/ha at the reviving stage, 8.2 t/ha at the jointing stage, and 15.6 t/ha at the flowering stage. Finally, combined with the progress of coal mining, the dynamic changing process of crops during underground mining can be inferred by analyzing the spatiotemporal variation in winter wheat AGB. Results showed that, in the dip direction, winter wheat AGB at the flowering stage was the highest at the compression zone, followed by the inner stretch zone, outer stretch zone, and neutral zone. The distance from the waterlogged area and the existence of cracks were found to be the important moderating variables affecting the crop growth status in the mining subsidence area. In the strike direction, there were significant differences in the wheat AGB-affected area as the mining proceeded. Even areas where AGB had previously significantly increased gradually transitioned to significant decreases with the end of mining. The research explores the dynamic changes in winter wheat AGB and land damage status during coal mining. It provides a rapid and non-destructive land-damage-monitoring method to protect cultivated land in mining areas.

Multiple aspects of simulation facilitators’ role in higher education: protecting and challenging the learners

ABSTRACT Simulation-based learning (SBL) has been the focus of higher-education literature, especially in teacher education and training. Led by a professional facilitator, the SBL process requires learners to step out of their comfort zone. This multiple case-study examines the roles of SBL facilitators to better understand the conditions for creating an optimal learning setting. Triangulation was accomplished using three data-collection methods: 218 facilitators’ written reflections were collected; a focus group was held with eight SBL facilitators; and interviews were held with 21 SBL participants. Data analysis revealed six themes describing the facilitator’s multiple roles: removing barriers to promote participants’ openness to learn; reducing stress; counterbalancing harsh criticism; gauging the intensity of the scenario; pacing the learning process; linking the learned lesson to practice. Conceptualizing the SBL facilitator’s role as the combination of these opposing poles suggests that the facilitator’s goal is to create a ‘stretch zone’ that promotes effective learning.

Determination of fracture toughness of 2.25Cr1Mo0.25V steel based on acoustic emission technique

The correct determination of fracture toughness is dependent on the accurate measurement of the point of crack initiation. In this research work, the AE technique is used to accurately detect the point of crack initiation and determine the initiation fracture toughness value JAE of 2.25Cr1Mo0.25V steel. Moreover, conventional standard procedures such as the J-R curve and stretch zone methods are also used to determine the fracture toughness JQ and JSZW values, respectively. Multiple AE parameters involving the entropy, peak amplitude, count, and energy are extracted from AE signals to identify the damage progression during fracture toughness tests. The results show several critical points associated with the point of plastic yielding, the onset of crack initiation, and the rapid crack growth and final fracture can be accurately identified by AE characteristics. Furthermore, the point of crack initiation recognized by AE is used to determine the JAE value. The fracture toughness JAE values of different specimens are also compared with the corresponding JQ and JSZW values. The JAE values are found to be about 20% lower than the JQ values but are significantly larger than the JSZW(2D) and JSZW(3D) values. It is believed that the fracture toughness parameter JAE determined by the AE technique can provide an important reference for evaluating the fracture toughness of engineering materials.

The role of growth mindset and symbol of shared event representation in the promotion of challenge-pursuit

Challenges are rigorous opportunities to grow and learn. Yet many students underutilize these opportunities. This study investigated how the growth mindset and priming of a shared-event representation might independently affect students’ tendency to challenge-pursuit. In a school-wide intervention, a long-distance running program (POMA Run) that embodies the concept and shared experiences of “ascend beyond current self” was created. In our study, students were randomly selected, then randomly assigned to one of the two event representation conditions: (1) think about the POMA Run (prime condition) or (2) think about a neutral target (control condition). Afterwards, to measure their willingness to extend one's limit, they were presented with four school-life situations and asked to indicate their intention to engage in activities of varying levels of difficulty described in each situation. Next, they ideographically categorized the activities into those that fell in the comfort zone (within their current capacity), stretch zone (beyond current capacity, challenging but attainable), or panic zone (far beyond current capacity). Challenge-pursuit was defined as the intention to engage in the activities categorized in the stretch zone. In the control condition, students with a stronger (weaker) endorsement of growth mindset reported higher (lower) challenge-pursuit intention. The effect of event representation prime on promoting challenge-pursuit intentions was significant among students with a weaker growth mindset: their challenge-pursuit intention was stronger in the prime condition than the control condition. In contrast, students in the prime condition displayed equally strong challenge-pursuit intention regardless of their level of growth-mindset endorsement. The effectiveness of forming a shared-event representation will be discussed.

Effect of the notch depth on fracture behavior of TC4 titanium alloy sheets

To evaluate the low-stress brittle fracture of TC4 titanium alloy sheet structures, the effect of notch depth (d) on the fracture behavior of the 1.8 mm thick TC4 titanium alloy sheets was investigated combied with tensile tests and digital image correlation method. The results show that the ductile–brittle transition (DBT) occurs with increasing notch depths. Ductile fracture occurs for the notched samples with d = 0.5–1.5 mm, and a uniform plastic deformation stage is observed in the load–displacement (P-L) curves, and there are stretch zone and the stable crack propagation zone in the fracture surfaces. Brittle fracture occurs for the notched samples with d = 3.0–5.0 mm, and there is only the linear elastic stage in the P-L curves, and no stretch zone in the fracture surfaces. The samples at the middle notch depths (d = 2.0–2.5 mm) are in the transition from ductile to brittle fracture, and there exists the elastoplastic deformation stage in the P-L curves, and the stretch zone and stable crack propagation zone are both decreased. The analysis of notch fracture parameters shows that fracture of notched samples is controlled by the stress concentration and the stress field intensity of an imaginary crack with the same notch depth. The effect of crack gradually increases with increasing d, which results in increased brittle fracture tendency. Moreover, the fracture energy γf can characterize the DBT of notched TC4 titanium alloy sheets, and the γf is proportional tothe ratio of critical plastic zone size to d (rpc/d). Finally, the rpc/d is proposed to characterize the DBT of the notched TC4 titanium alloy sheets, and the criterion of the DBT for the notched TC4 titanium alloy sheets is obtained.

Несуча здатність доведених до граничного стану (ULS) пошкоджених бетонних балок з ВFRP, підсилених фіброармованими пластиками (СFRP)

The aim of this article is to familiarize with the experimental and theoretical study of the load-bearing capacity of BFRP concrete beams damaged in previous studies and brought to failure, reinforced with carbon-plastic fabric in the lower stretched zone and carbon-plastic jackets in the supporting areas under the action of low-cycle sign-repeated transverse loading of high levels with the development of initial data for the physical model of the methodology for calculating the strength of their normal and inclined sections. The article presents the results of testing concrete beams reinforced with ВFRP, strengthened with carbon fiber reinforced polymer (CFRP) strips in the lower tensile zones and carbon fiber reinforced polymer jackets at support sections, previously tested to ultimate limit state (ULS). The load-carrying capacity of the reinforced FRP support sections of beam structures, brought to the ultimate limit state (ULS), should be determined primarily under the action of bending moment through the critical inclined crack. The performed experimental and theoretical studies have established the possibility and feasibility of strengthening damaged and brought to the boundary state (ULS) concrete structures with BFRP external fiber-reinforced plastics (CFRP) while observing the established technology. The bearing capacity of CFRP-reinforced damaged concrete beams with BFRP should only be determined for the action of bending moments along normal sections in elements with large (a/d = 3) and medium (a/d = 2) shear spans and along inclined sections in beams with small (a/d=1) shear spans.

Optical diagnostics and chemical kinetic analysis on the dual-fuel combustion of methanol and high reactivity fuels

Dual-fuel combustion is an effective method to utilize methanol because of its high octane number and latent heat of evaporation. Two ways to use methanol are discussed. One is port-injected high reactivity fuel and directed-injected methanol, named as active-thermal atmosphere combustion (ATAC). The other is port-injected methanol and direct-injected high reactivity fuel, which is reactivity-controlled compression ignition (RCCI). In the current work, both methanol combustion modes were investigated in an optical engine. Several optically diagnostic methods were utilized to characterize flame features, including flame natural luminosity imaging, OH* natural luminosity imaging and two-color method by wavelength integration. Meantime, the chemical kinetics simulation was also combined to investigate the effects of active atmosphere and thermal atmosphere on methanol combustion. Results show that as for methanol ATAC, when the methanol is direct-injected during the low temperature heat release (LTHR) stage of n-heptane, it causes significant effects on the combustion phasing of the following high temperature heat release (HTHR). When the methanol is direct-injected during the HTHR stage of n-heptane, the single-peak HTHR turns to the double-peak HTHR. Besides, the yellow diffusion flame of methanol can also be observed. Affected by the in-cylinder vortex, the area and intensity of the OH* signal in the vortex squeeze zone are greater than those in the vortex stretch zone. The flame temperature of methanol maintains in 1800–2500 K at CA50 (the crank angle of 50% mixtures is burned completely), while the maximum KL factor maintains in 0.04–0.05. The thermal atmosphere created by the n-heptane combustion has the dominant effects on shortening the ignition delay of methanol. The effects of active atmosphere are limited. As for methanol/high reactivity fuel RCCI, because of the existence of homogeneous low reactivity methanol, the misfire condition occurs when the direct injection (DI) timing is late. With the DI timing of high reactivity fuel advancing, the combustion stability improves. But the flame development progress is mainly low-speed flame propagation, which results in long combustion duration. The combustion characteristics can be further improved by utilizing higher reactivity fuel, such as n-dodecane, specifically, the combustion phasing advances and the number of auto-ignition spots increases.

Delivering Covid-19 research during the UK pandemic: Experiences of a local research taskforce.

During the COVID-19 pandemic in the UK, clinical research nurses had to work in new ways and under significant pressure to generate evidence for the developing health crisis. Research nurse support needs, personal and professional challenges have not been explored. This study addresses that gap, generating learning for continued support and development of the research nurse specialty and its ability to respond to public health priorities. We employed a qualitative exploratory approach through online open-ended interviews to explore research nurses' experiences of delivering research during the pandemic using principles of Grounded Theory. Fifteen research nurses in the local research taskforce were identified through purposive sampling. Qualitative interviews were conducted online between November 2020 and January 2021 and analysed using the principles of constructivist grounded theory. Three themes of adapting to uncertainty, inclusive leadership and finding validity in the stretch zone were generated in the analysis. A model of inclusive leadership and support can facilitate high-functioning performance in a research team, supporting a rapid, confident and efficient response to research needs. Research nurses, a previously invisible workforce, have proved critical to the pandemic response. This study explores the experiences of a team of redeployed research nurses and develops a theory of their experience as they were undertaking the rapid delivery of urgent public health studies during COVID-19. What was found was a process of adaptation and resilience through collaborative teamwork, a strong sense of purpose and role validation enabled by an inclusive leadership style. This work will drive future development of a model of research nursing with a focus on collaboration between research and clinical colleagues.

Density contrast, deep structure, rheology and metallogeny of the crust and upper mantle of the Verkhoyano-Kolymsky region

Research subject. The Verkhoyano-Kolymsky areal of ore mineralization in the Far East of Russia.Data and methods. We used the state metallogenic map of Russia, Sc. 1: 2 500 000 (2000) and the gravity map of Russia Sc. 1: 2 500 000 (2001). Modeling was conducted by studying the deep structure of the earth’s crust and upper mantle from the anomalies of the density contrast of geological media in the intervals between the centers of density inhomogeneities and the surfaces of equivalent spheres.Results. 3D-distributions of density contrast (µz-parameter) in the crust and upper mantle of the Verkhoyano-Kolymsky region related to the rheological properties of geological media were analyzed. In the gravity models designed without attraction of external information, the structures of thrust, splitting, stretching, as well as the structures of central type (CTS) of the plume nature, were identified. In the regional stretching zone, at the border of lithospheric segments, the revealed Indigiro-Kolymsky and Verkhoyansk CTSs were described in 3D space. These structures are characterized by a mushroom-like upwelling of the asthenosphere, associated with heat flow anomalies. The identified structures differ in terms of asthenosphere depth, age and ore mineralization. The location of ore clusters and regions in the zones of CTS obeys concentric ore-magmatic zoning, typical for this type of structures. The central (trunk-like) zone of Indigiro-Kolymsky CTS features mainly high-temperature gold-quarts mineralization. On the periphery, along with gold areas, there are tin-tungsten, tin and complex ore mineralization areas. The majority of gold fields with low-temperature gold-sulfide, tin and polymetallic mineralization are attributed to the flanks of Indigiro-Kolymsky CTS. In the Verkhoyanska CTS, the majority of ore regions are characterized by multi-formation ore mineralization. In the central part of this structure, areas with mainly low-temperature tin, mercury-antimony and gold-silver ore mineralization are located. On the flanks, gold mineralization is either absent or subsidiary.Conclusions. As a result of a simple procedure, implying generalization of multiple decisions of the elementary inverse problem of gravity potential, main features of the deep structure of the Verkhyano-Kolima region were defined. In the regional stretch zone, at the boundary of lithospheric segments, the Indigiro-Kolimskaya and Verkhoyanskaya CTSs of the plume nature that control the location of ore deposits were identified and described in 3D space.

Basic parameters of crack resistance of conventional and damaged reinforced concrete beams strengthened by carbon plastic under the action of a high-level load.

The main results of experimental studies of the fracture resistance of conventional and damaged and brought to the limit state according to I group in the previous experiments of reinforced concrete beams reinforced with fiberglass carbon fiber (СFRP) in the lower stretched zone and at the supporting areas. According to the adopted methodology, a full-scale experiment was performed on a four-factor three-level Box-Bank B4 plan. The tests of the prototypes were carried out according to the scheme of a single-track free-beam, alternately loaded from above, then from below by two concentrated forces without changing its (beam) position. During testing of the test specimens-beams for the action of short- term one-time and small-cycle loads, the formation, development and width of crack opening on their surface were monitored. The width of the opening of normal cracks was determined at the level of the stretched working armature, and the inclined ones - in the middle of the height of the beam in places where it was visually greatest. Due to the adopted methodology, new experimental data were obtained to substantially refine the physical models of the inclined sections of the precast concrete structures by the action of low-cycle reloading of high levels, resulting in the first identified systemic effect on fracture toughness.
 Keywords: concrete, reinforcement, carbon fiber fabric (CFRP), reinforced concrete beam, normal and inclined cracks, deformations.

Comparative study between crack tip opening displacement techniques by statistical analysis

Standardized tests may not always achieve all their analytical objectives. Particularly to structural integrity, where a reproduction of the practical occurrences of the industry is desired, new techniques are being developed. Therefore, this work aims at evaluating fracture toughness in tensile armour through four measurement methodologies (standard, video, laser and stretch zone (SZ)) to obtain crack tip opening displacement (CTOD), and compares their responses by ANOVA and Duncan’s tests. The findings showed equivalence between the standard and other approaches, thus allowing distinct methods for fracture toughness evaluation.

The effect of constraint on fracture properties of Z3CN20.09M after accelerated thermal aging

The main coolant piping material Z3CN20.09 M was heat treated at 400 °C for 18,000 h and then the fracture behavior has been studied. To understand the effect of in-plane constraint on fracture behavior, J-R curves were determined by the pre-cracked specimens with a wide range of crack depth ratios. And the stretch zone width (SZW) values were measured by experimental method and finite element method (FEM) based on fracture energy density. The comparisons of SZW value show that good agreements have been achieved among experimental tests and FEM. Moreover, to reveal and quantify the constraint effect (a/W), three-dimensional FEM was used to obtain the second terms in the Williams’ series expansion. The J-R curves, JIC and JSZW and SZW are higher at lower crack depth ratios a/W, and gradually decrease with increasing crack depth. Then, the constraint dependent fracture properties of Z3CN20.09 M were determined and could be used to predict fracture properties under other constraint conditions.

Effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M with long-term thermal aging degradation

It is of great significance to investigate effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M cut from pump casing with long-term thermal aging degradation for nuclear power plants to consider actual operation of nuclear power plants. Both multiple heat treatments and accelerated thermal aging experiment at the same temperature of 400 °C for different time were successively carried out on centrifugal casting stainless steels Z3CN20.09M in order to examine the metallographic modification and impact properties. Finally, an additional investigation on the related fracture properties was carried out, in which the critical initial fracture toughness Ji was determined by stretch zone width and 0.2 mm offset line methods. These results indicated that the multiple heat treatments led to the dispersed distribution of ferrite phases in austenite matrix and thus microhardness increased, but impact energy exhibited a decreasing tendency significantly. After long-term aging, the metallographic structure remained almost unchanged, but the size of ferrite phases showed a slight increasing trend because of spinodal decomposition in ferrite phases and G-phase precipitation. In addition, centrifugal casting stainless steels Z3CN20.09M with multiple heat treatments exhibited the higher microhardness, Charpy impact toughness, critical initial fracture toughness JIC (J-integral determined by 0.2 mm offset line method), and JSZW (J-integral determined by stretch zone width method) than those with primary heat treatment, while the specific number of the heat treatment had a low influence on fracture toughness.

Fiber reinforcement as an alternative to the compressed zone linear reinforcement and the flexible concrete elements stretched zone prestressing

The results of a numerical experiment in the framework of a theoretical study of the strength and crack resistance of the reinforced concrete beams available or non-available in fiber reinforcement are presented. The fiber reinforcement presence influence on the strength and crack resistance of the reinforced concrete beams’ normal sections is determined by varying the following parameters: working reinforcement class; percentage of reinforcement with steel reinforcement; the presence and degree of the reinforced zone reinforcement prestressing; the presence or absence of the beam compressed zone reinforcement; the steel fiber reinforcement presence. Reinforcement of concrete with steel fiber had a significant impact on the theoretical strength and crack resistance of the reinforced concrete elements’ normal sections, both in the presence and in the absence of the working reinforcement prestressing. The calculation showed that in some cases the presence of fiber reinforcement will eliminate the prestressing of the working reinforcement or significantly reduce its level, abandon the reinforcement in the compressed zone and, under the certain conditions, replace the high-strength prestressed reinforcement A600 with the reinforcement A400 without increasing the total consumption of steel by the sample.

Experimental determination of toughness under mode I/II loading

The mixed-mode fracture toughness of a 42CrMo4 high-strength steel under mode I/II loading was investigated. The steel was processed in a steel casting simulator where it was poured through a spaghetti-type filter into a ceramic crucible. The machined compact tension shear specimens were heat treated in order to obtain the quenched and tempered state. The designs of the specimen and the loading device enabled testing at different loading angles leading to different ratios KI/KII. Strain gauges were applied in order to analyze the stress field around the crack tip. It was observed that the strain gauges can be reasonably applied to measure KI. However, large errors occurred during the measurement of KII. The specimens fractured in a brittle manner. However, no valid plane-strain fracture toughness was obtained. With increased loading angles, mode I fracture toughness decreased while mode II fracture toughness increased. Furthermore, it was found that stretch zone was flattened with increased loading angle.

Dynamic fracture behavior of low carbon bainitic steel after different welding thermal cycles

Dynamic fracture toughness is an important parameter to assess the fracture resistance at crack initiation although the actual crack initiation point is hard to be precisely detected. In this work, three different heat treatment microstructures (i.e., hot-rolled, welding thermal simulation coarse grained HAZ (CGHAZ) and intercritically reheated CGHAZ microstructures) were studied by instrumented Charpy impact tester. The dynamic fracture toughness was obtained based on impact load-deflection curves using various estimation methods. The dynamic fracture toughness decreased with the applied welding thermal cycles irrespective of the estimation methods. The stretch zone width proved that the fracture toughness is most acceptable when the crack initiation point is considered as the middle point between the general yield load and maximum load in the load-deflection curves.

Strength and Fracture Toughness of Hardox-400 Steel

This paper presents results of strength and fracture toughness properties of low-carbon high-strength Hardox-400 steel. Experimental tests were carried out for specimens of different thickness at wide temperature range from −100 to 20 °C. The dependences of the characteristic of material strength and fracture toughness on temperature based on experimental data are shown. Numerical calculation of the stress and strain distributions in area before crack tip using the finite element method (FEM) was done. Based on results of numerical calculation and observation of the fracture surfaces by scanning electron microscope (SEM), the critical local stress level at which brittle fracture takes place was assessed. Consideration of the levels of stress and strain in the analysis of the metal state at the tip of the crack allowed to justify the occurrence of the brittle-to-ductile fracture mechanism. On the basis of the results of stretch zone width measurements and stress components, the values of fracture toughness at the moment of crack initiation were calculated.

J-integral Fracture Toughness of High-Mn Steels at Room and Cryogenic Temperatures

The J-integral fracture toughness values of high-Mn steel (Fe-25 wt pct Mn) and 304L stainless steel were evaluated at 25 °C, 0 °C, − 50 °C, − 100 °C, − 163 °C, and − 196 °C using precracked compact tension (CT) specimens and compared to those determined using Charpy impact tests. The high-Mn steel exhibited excellent J-integral fracture toughness at both room and cryogenic temperatures, with values comparable to those of 304L stainless steel. However, the trend of the J-integral fracture toughness of high-Mn steel with the decreasing temperature differed from that of the Charpy impact test results. Electron backscattered diffraction and micrographic analyses suggest that the varying stacking fault energies of high-Mn steels at different temperatures affected the deformation behavior in the stretch zone at the crack tip of the CT specimen. The effect of this temperature-dependent deformation behavior of high-Mn steels on the fracture process in the J-integral test could differ from that in the Charpy impact test, resulting in the different trends in the fracture resistance with the decreasing temperature.

Method comparison for the determination of stretch zone parameters

The fracture behaviour of ductile failing materials is evaluated by parameters of yielding fracture mechanics. According to ISO 12135 the determination of fracture mechanical characteristic values involves, among other things, analysing the stretch zone width (SZW) in the scanning electron microscope (SEM). This procedure is very time-consuming and requires great experience in handling SEM images of fracture surfaces. An alternative method for determining the stretch zone parameters works with 3D elevation profiles of the fracture surface in the stretch zone area taken with a digital microscope. The fracture mechanical parameters determined using the above methods were finally compared. The Ji and δi values of scrap steels resulting from the digital determined SZW show a good agreement with analytically determined crack initiation values (Ji/BL, δi/BL).

A comparison between fracture toughness at different locations of SMAW and GTAW welded joints of primary coolant piping

Two primary coolant pipes were narrow-gap multipass circumferentially butt welded by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) methods separately and then subjected to micro-hardness tests to distinguish the base metal (BM), heat affected zones (HAZs), fusion zones (FZs) and weld metal (WM). Subsequently, uniaxial tensile tests were performed with a 3D DIC system to investigate the strain evolution of each area in GTAW and SMAW welded joints and to compare respective tensile properties. The fracture toughness has been investigated at the above four different locations of the SMAW and GTAW welded joints. Then the 0.2 mm offset line method and the stretch zone width method have been both employed to determine the critical initial fracture toughness Ji. The results indicate that the fusion zones (FZs) have the worst fracture toughness compared with other locations over both two types of weld joints. In addition, the GTAW welded joints have a better comprehensive performance than the SMAW welded joints.