Bending Zone Research Articles

Experimental studies of the strength of reinforced concrete beams with a restored protective layer

The article presents the results of experimental studies to assess the strength of reinforced concrete beams with varying degrees of destruction of the protective layer of the working longitudinal reinforcement, followed by its restoration with a repair mixture. The test results of three series of prototypes, three samples in each series, are presented. Within the series, prototypes were made with the same longitudinal reinforcement, and the samples in each series differed in the degree of destruction of the protective layer of the longitudinal working reinforcement in the transverse bending zone. The samples were tested as freely supported beams loaded in the span by two identical concentrated forces. The results obtained made it possible to evaluate the role of longitudinal reinforcement, depending on the degree of destruction of its protective layer, in ensuring strength, forming a stress-strain state and the nature of destruction of reinforced concrete beams of rectangular cross-section in transverse bending zones.

A COMPARATIVE STUDY OF THE SURFACE OF FACTORY AND HAND-MADE LOOPS ON TITANIUM-MOLYBDENUM ORTHODONTIC ARCHWIRE

The most important part of the brackets, used in orthodontic treatment, is the archwire, which, when activated, generate light and long-lasting biomechanical forces that make the teeth move. Different manufacturers offer ready-made expensive solutions for orthodontists - archwire with loops along the length of the wire of different shape, which increases the cost of treatment. However, orthodontic practice is strictly individual, depending on the patient’s orthodontic problems, the stages and methods of treatment.Orthodontists often have to choose between expensive archwires with pre-made loops or using an archwire that they can manually bend in the right places depending on the patient’s individual characteristics and specific needs. In this case, the orthodontist’s decision depends on preserving the quality of the archwire surface. The surface roughness of the orthodontic archwire plays an important role in the bracket-archwire complex and is an essential factor that determines the effectiveness of the guided movement of the teeth along the arch. Surface roughness not only affects the efficiency of sliding mechanics, due to frictional effects, but also corrosion behaviour and aesthetics. A comparison of the surface characteristics of Ti-Mo archwire - with factory and hand-made loops is made. The surface of the archwire in the bending zones of the loops was investigated by SEM analysis and Surface roughness analysis. The analysis of the results shows that manual bending practically does not change the surface of the archwire, does not introduce additional defects that increase the contact friction, as well as create conditions for faster deterioration of properties and the undesirable breakage of the archwire. Research results give orthodontists confidence that the manual bending process does not degrade the quality of the archwire surface. The hand-bent archwire preserves the possibility of trouble-free movement in the braces when tightening or loosening during the different stages of treatment.

Flexural behavior and design of aluminum alloy portal frame eaves joints

Aluminum alloy portal frames (AAPFs) are suitable for both long-term and temporary buildings owing to their light weight, ease of installation and disassembly, and excellent corrosion resistance. Due to the critical importance of joint mechanical behavior for structural safety, this paper conducted experimental and numerical studies on the flexural behavior of AAPF eaves joints. Initially, bending tests on four AAPF eaves joints revealed three failure patterns: connection plate, column, and hole wall failure. The failure patterns are determined by the weakest of these three components. Adding haunches increased the bending zone area, thereby increasing the ultimate bearing capacity by 110 % compared to joints without haunches. Furthermore, strain curves indicated that the load transfer zone was subjected to a bending-shear force state, consistent with the principle that the bolt group shear forms a force couple to transfer the moment. Subsequently, a finite element (FE) model was developed and validated to provide a foundation for subsequent parametric studies and theoretical analysis. The parametric analysis considers bolt diameter, connection plate thickness, aluminum alloy web thickness, and bolt end distance. The bearing capacity corresponding to three failure patterns was discussed, and a design method for the flexural bearing capacity was proposed. The design method for stiffness model was also proposed based on the component-method and parametric analysis. Finally, this paper presented and validated the design steps for the flexural behavior of the eaves joints throughout the whole process, providing an important reference for engineering design.

Bending of Sheet Metal (st37) Using 90 Degree to Estimate Blank Dimensions

Several factors must be considered whendesigning parts that are being made by bending. Theprimary factor is minimum radius and angle of bending thatcan be bent successfully without metal cracking. Lowcarbon steel metal (st37), manufactured by Libyan Iron andsteel Company, was used in this study. Various bendingprocesses by Vee die for different radii and calculatethickness were conducted and bending zones were tested tofind the mean radius and calculate the bending allowance.These calculations depend upon the inner bend radius andthickness of the sheet metal. The thickness was reduced andthe neutral axis were moved up in the direction ofcompression side. Form the values of reduction in thicknessand different radii of bending a chart was drawn. Anotherchart was drawn to minimums the process of trial and errorfor estimating the blank length of the bending sheet metal.Form these charts, the initial blank sizes can be determined,so that products can bend without loss and crack in thematerial. As a result time and material can be saved. Theresults of the present study can have practical applicationsfor this material (st37) which is in common use by theLibyan industry.

Biophysical features of using a recombination sensor to detect lactate dehydrogenase: sensitivity mechanisms analysis

Background. Most pathologies of the human body (in particular, malignant neoplasms, myocardial hypoxia, liver diseases, etc.) are accompanied by a violation of the integrity of cells in target tissues and the release of intracellular macromolecules into the extracellular environment. Thus, an important diagnostic and prognostic indicator is the level of activity of certain enzymes in blood serum, which are normally intracellular. One of the most promising areas of modern medical electronics and biophysics is the development and optimization of enzyme screening methods in biological fluids. In this study, we aimed to investigate the biophysical characteristics of using a recombination sensor for determining LDH activity in biological fluids. Materials and Methods. Experiments were performed on preparations of standard human blood serum. The reference determination of lactate dehydrogenase activity was carried out photometrically based on the change (decrease) in the concentration of the reduced form of the NADH coenzyme. The passage of the lactate dehydrogenase reaction was experimentally recorded by measuring the photocurrent of a silicon structure with a buried barrier under light irradiation from the region of strong absorption (λ = 532 nm). Results. The biophysical features of the device were studied. The detection of lactate dehydrogenase becomes possible due to the transfer of a hydrogen ion from nicotinamide adenine dinucleotide (NADH) to pyruvate, as a result of which lactate and NAD+ are formed. The effect is explained by the local electrostatic influence on the parameters of the recombination centers in the near-surface bending zone near the silicon surface, which leads to a change in the surface recombination rate. Conclusions. Our approach can be considered as a promising way to develop a highly sensitive method for the detection of lactate dehydrogenase. It has been experi­mentally shown that effective detection is possible in two changes at the surface ben­ding of the deep barrier silicon substrate zone.

Recycling harmful plastic waste to produce a fiber equivalent to carbon fiber reinforced polymer for reinforcement and rehabilitation of structural members

Abstract The main idea of this research is to manufacture plastic fibers (PFs) by recycling harmful plastic waste after washing it with water and drying it, then cutting it into strips and immersing them in liquid nitrogen, using a rotating iron basin mixer containing iron balls that touch these strips, producing fibers that are woven later to be similar to carbon-fiber-reinforced polymer (CFRP). This process is the first in the world, and the cost of manufacturing these fibers is very low compared to the cost of producing CFRP, as the cost of producing these fibers is 25 times less than the cost of producing CFRP. All the physical, chemical, and mechanical properties of the manufactured PFs were investigated and compared with the properties of CFRP. In this research, it was found that the manufactured PFs have properties similar to the properties of CFRP. The experimental work consists of casting five reinforced concrete beams with a cross-section of 125 mm × 200 mm and a length of 1,200 mm. They were divided into two groups, each group containing two beams, one of which was reinforced by CFRP, and the other was repaired by CFRP. The second group also contained two beams, one of which was reinforced by PF, and the other was repaired by PF, while the fifth beam was a control beam. From the experimental results, a significant increase in the ultimate load of the beams reinforced by PF or CFRP ranged between 45.45 and 51%, respectively. This is due to the role of manufactured PFs and CFRP in restricting the width of cracks in the bending zone of reinforced or repaired concrete beams. Also, there was a change in the type of failure when changing the type of fiber, where the failure was ductile and gradual in the beams reinforced or repaired with manufactured PFs, unlike what was in the beams reinforced or repaired by CFRP, where the failure was brittle and sudden.

Experimental, theoretical and numerical study on the shear stress of adhesive layer in FRP-bamboo scrimber composite beams

The adhesive layer is an important factor affecting the mechanical properties of FRP- bamboo scrimber composite beams (FBSCB). However, studies on the interfacial shear stresses in the adhesive layers with both ends of FRP and bamboo scrimber beam aligned have been rarely reported. To this end, a two-parameter theoretical calculation model and a finite element model (FEM) based on cohesive zone model were hereby established to solve for the adhesive layer interface shear stresses, which was verified by four-point bending experiments. The results show that both the two-parameter theoretical model and the FEM can effectively compute the shear stress of the adhesive layer. Meanwhile, the FEM simulation results not only reflect the detailed changes of the shear stress, but also provide a better analysis of the shear stress at the adhesive layer with a small fluctuation range. There are three zones of shear stress at the adhesive layer of FBSCB under four-point bending load, i.e., the bending and shearing zone, the transition zone and the pure bending zone. In the bending and shearing zone, the shear stress of the adhesive layer interface increases 2.61 times and 2.5 times, respectively when the thickness and elastic modulus of FRP increase three times. However, the stress remains constant at zero in the pure bending zone.

Damage assessment in L-shaped bends by nonlinear feature guided waves

L-shaped bends are crucial lightweight structural components in aerospace manufacturing, renowned for their high specific strength and stiffness. However, the geometric configuration of these bends makes them susceptible to stress concentration in the bending zone. Effective technique for damage assessment of the bending region of L-shaped bends is highly demanded. This paper proposes a nondestructive testing method using nonlinear ultrasonic feature guided waves (FGWs) for early damage assessment of L-shaped bends. Semi-analytical finite element (SAFE) method and perfectly matched layer (PML) technique are employed to analyze FGW propagation in L-shaped bends. Three-dimensional finite element models are utilized to investigate the FGWs propagation in the bending region of L-shaped bends in a numerical perspective. Cumulative second harmonic generation (SHG) of FGWs is successfully observed by selecting suitable mode pairs based on synchronism and non-zero power flux conditions, which provides a promising alternative for early damage assessment in L-shaped bends. Experimental findings are consistent with the numerical ones. Subsequently, thermal damage and plastic deformation with varying gradients were introduced into the bending region of L-shaped bends. The use of SHG of selected FGW mode has effectively assessed the damage with an improved sensitivity, compared to conventional linear acoustic one. This investigation substantiates the potential of nonlinear FGWs in detecting early damage in L-shaped bends, offering promise for future industrial testing applications.

Simulated and measured piezoelectric energy harvesting of dynamic load in tires

From 2007 in US and from 2022 in EU it is mandatory to use TPMS monitoring in new cars. Sensors mounted in tires require a continuous power supply, which currently only is from batteries. Piezoelectric energy harvesting is a promising technology to harvest energy from tire movement and deformation to prolong usage of batteries and even avoid them inside tires.This study presents a simpler method to simultaneous model the tire deformation and piezoelectric harvester performance by using a new simulation approach - dynamic bending zone. For this, angular and initial velocities were used for rolling motion, while angled polarization was introduced in the model for the piezoelectric material to generate correct voltage from tire deformation. We combined this numerical simulation in COMSOL Multiphysics with real-life measurements of electrical output of a piezoelectric energy harvester that was mounted onto a tire. This modelling approach allowed for 10 times decrease in simulation time as well as simpler investigation of systems parameters influencing the output power. By using experimental data, the simulation could be fine-tuned for material properties and for easier extrapolation of tire deformation with output harvested energy from simulations done at low velocity to the high velocity experimental data.

Mechanism of Stratum Instability and Dynamic Deformation under Discontinuous Boundary Conditions

A fault disrupts the continuity of the rock strata in a mining area. To study the law governing the fracture of overlying strata in mining areas under discontinuous boundary conditions, the overlying strata were redefined and grouped based on the activity characteristics of each rock layer during the overall movement of the overlying strata. The activity patterns of different layers of the fault were obtained through the movement and failure forms of each group of rock layers. The relationship among the size of the coal pillar at the boundary of the fault, the dip angle of the fault, and the movement angle of the rock strata was considered. A model of the spatial relationship between the overlying rock movement zone of the quarry and the fault surface was established. The limit equilibrium equations of the key layer in the fault zone before breaking were established based on the tensile strength of the rock layer. In addition, the mechanical slip instability criterion and the deflection instability criterion of the discontinuous-boundary rock mass are given herein. Based on a field case, a double criterion was used to determine the initiating activated rock layers of the fault in the cases where the fault dip was smaller than the rock movement angle. Rock movement during excavation was simulated by similar simulation tests, and different levels of rock movement patterns in the boundary fault zone were focused on monitoring and analyzing. The stress and displacement changes in different rock layers in the fault zone were analyzed with numerical simulation results. The results show the following: if the dip angle of the fault is smaller than the movement angle of the rock layer, the delamination space of the fault surface is mainly distributed in the bending and sinking zone of the overlying rock; with an increase in the working-face advancement distance, the vertical pressure of the upper part of the fault gradually decreases, and the stress-concentration area in the middle and lower part of the fault gradually increases; the rock layer of the upper part of the fault, which is mainly composed of the key stratum, is the main area of activation of the fault.

Experimental study on stiffness deterioration of cracked RC hollow-core slab beams

To accurately assess the performance degradation of RC hollow-slab beam after service, in-situ static load tests were carried out on a RC hollow-slab beam bridge with a span diameter of 11.25 m, which is part of the Chongqing–Chengdu Highway Viaduct. Indoor single-girder static load tests were also carried out to study the deflection concrete strain and reinforcement strain at key locations on the hollow-slab beam. Crack development was recorded to investigate the stiffness degradation law of hollow-core slabs. Inverse secant stiffness–residual deflection and inverse secant stiffness–maximum crack width relationship models were established. Results show that the calibration coefficients obtained in the in-situ static load tests were greater than one, indicating potential for stiffness deterioration and insufficient structural safety reserves during service. The initial crack width was increased and extended upwards before the test beam yielded, with many new vertical cracks in the pure bending zone which extended continuously. The slope of deflection curve was larger before yielding and smaller after yielding, the secant stiffness of RC hollow-slab beam was exponentially decreased with load before yielding and changed linearly with load after yielding. The inverse secant stiffness–residual deflection and inverse secant stiffness–maximum crack width curves of the test beam were approximately bilinear, with a larger slope at the beginning of the test and a smaller slope in the plastic phase.

Flexural capacity and serviceability evaluation on seawater sea-sand concrete beams reinforced by BFRP bars and TRE composite in the dry-wet environment

To promote the durability and serviceability of offshore structures, the bearing capacity and serviceability of BFRP-reinforced seawater sea-sand concrete (SWSSC) beams with textile reinforced ECC (TRE) to retard deformation and cracking under chloride dry-wet cycles were investigated. A total of a BFRP-reinforced SWSSC beam and nine TRE composite beams were designed. The dry-wet cycles, thickness, fabrication method and shape of TRE composite layer as factors the on the bearing capacity, deflection and crack behavior, curvature and ductility of beams with TRE composite were analyzed. TRE composite layer still can visibly increase the cracking moment of BFRP-reinforced SWSSC beams even in the dry-wet conditions, but the improvement effect on the ultimate moment was noteless. The bearing moments of composite beams increased with the TRE thickness and the dry-wet cycles. The failure modes for composite beams with the same reinforced radio of FRP bars under dry-wet environment changed from failure in shear cross zone to failure in pure bending zone with the increase of the thickness of TRE composite layer. The development of deformation and cracks for beams with TRE composite was significantly distinct from that for BFRP-reinforced SWSSC beams, attributed to the bonding properties of the composite layer to concrete, which with the TRE composite subjected to service moments can be judged by the curvature of 0.00375/d, below the curvature of 0.005/d. The fine cracks in TRE composite beams developed sequentially throughout the pure bending of beams. The dry-wet environment can reduce the deflection by increasing the stiffness of beams. Under service moment, the neutral axis height of TRE composite beam was clearly superior to that of BFRP-reinforced SWSSC beam. The variation trend of ductility index on energy and deformation was different. And the ductility of TRE composite beams declined as the increasing dry-wet cycles, regardless of the energy or deformation.

EXPERIMENTAL STUDIES ON THE STRENGTHENING OF A BEAM DAMAGED IN THE TENSILE ZONE

The paper presents the results of a study of the bearing capacity of a beam damaged during hostilities, which was reinforced with steel-reinforced concrete. Consider a beam with 40% damage in the stretched zone. Before concreting, a foam insert was installed in the zone of planned damage, the shape and dimensions of which corresponded to the planned damage. After the concrete set to 70% strength, the liner was removed, and the formed cavity was filled with a 2% fiber concrete mixture. This is one of the options for beam strengthening in a complex program of testing damaged beams, which involves the study of a series of beams with different types of damage, the damage zone (tension and compression) and its dimensions, geometry and method of strengthening. For testing, the authors developed a special stand. The plane-transverse load bending of the beam under study is created with the help of a hydraulic jack and a metal I-beam with a traverse, which transmits two equally concentrated forces to the beam. The generated load is controlled by a model dynamometer of the Tokar system and a ring dynamometer, which acts as a support. Experimental studies have established that a beam whose cross-section is 40% damaged in the middle stretched zone, and the shape of the damage is close to rectangular, has a bearing capacity of 91.0 kN, which is 92.38% of the bearing capacity of an intact beam (98.5 kN ). Cracking started at the 5th loading stage, when the load value was 32.5kN, i.e. 35.7% of the bearing capacity of the damaged beam. At the same time, 4 cracks were formed. At the sixth stage, three more cracks appeared. At stages 9-12 of loading, 9 more cracks formed. The maximum final crack opening width was 1.1 mm. At the same time, 6 cracks formed in the zone of clean bending. During the entire testing process, the fiber concrete liner worked with the beam as a single unit. It can be argued that the strengthening of a beam with 40% damage in the stretched zone in the manner considered in the paper allows to achieve its bearing capacity, which is 92.38% of the bearing capacity of an undamaged beam.

EXPERIMENTAL STUDIES OF A DAMAGED BEAM REINFORCED WITH FIBER CONCRETE

The paper presents the results of a study of the load-bearing capacity of a beam damaged during hostilities, which was reinforced with steel fiber concrete. A beam with 20% damage in the compressed zone is considered. Before concreting, a foam liner was installed in the area of the planned damage, the shape and dimensions of which corresponded to the planned damage. After the concrete gained 70% of the brand strength, the liner was removed, and the resulting cavity was filled with a 2% mixture of fiber-reinforced concrete. This is one of the options for strengthening a beam in a comprehensive testing program for damaged beams, which involves studying a series of beams with different types of damage, damage zone (tension and compression) and its size, geometry and method of strengthening. The authors have developed a special stand for testing. Plane transverse bending of the loading of the beam under test is created using a hydraulic jack and a metal I-beam with a traverse that transmits two equal concentrated forces to the beam. The created load is controlled by an exemplary dynamometer of the Tokar system and a ring dynamometer, which acts as a support. Experimental studies have shown that a beam, the cross-section of which is 20% damaged in the middle compressed zone, and the shape of the damage is close to rectangular, has a load-bearing capacity of 93.6 kN, which is 95.0% of the load-bearing capacity of an undamaged beam (98.5 kN). Crack formation began on the 5th stage of loading, when the load value was 32.5 kN, that is, 34.7% of the load-bearing capacity of the damaged beam. In this case, 4 cracks formed in the pure bending zone. At the sixth stage, three more cracks appeared. At stages 9, 10 and 14 of loading, 7 more cracks formed. The maximum final crack opening width was 0.8 mm. throughout the testing process, the fiber-reinforced concrete liner worked with the beam as a single unit. It can be argued that strengthening a beam with 20% damage in the compressed zone using the method considered in the work makes it possible to achieve its load-bearing capacity, which is 95% of the load-bearing capacity of an undamaged beam.

Quantitative assessment of corrosion-induced wall thinning in L-shaped bends using ultrasonic feature guided waves

The utilization of L-shaped bends has become widespread due to their versatility and ease of application in various industrial fields. Nevertheless, the geometric configuration of these bends makes them susceptible to stress concentration in the bending zone, which can lead to wall thinning caused by accelerated corrosion. This poses a profound challenge to the bearing capacity and reliability of L-shaped bends and highlights the urgent necessity for an effective technique to assess corrosion-induced wall thinning in the bending zone. In this study, we propose a nondestructive testing (NDT) method based on ultrasonic feature guided waves (FGWs) for the evaluation of local corrosion-induced wall thinning in L-shaped bends. Propagation characteristics of FGWs in L-shaped bends were analyzed using the semi-analytical finite element (SAFE) method and perfectly matched layer (PML) technique. Time-reversal (TR) method was employed to address the dispersion and multi-modes phenomenon of the selected guided wave modes and improve the signal-to-noise ratio (SNR) of measured FGW signals. Enhanced SNR of two types of FGW modes signals at different frequencies were verified by experimental and numerical results. Obtained results reveal that FGWs can effectively detect corrosion-induced wall thinning inside L-shaped bends, with improving the signal reflection coefficient by TR method.

Analysis of debonding mechanism and concrete cover failure of steel and steel fibre reinforced concrete composite beams

To solve the construction problems of steel reinforced concrete structures, the rebar cage was replaced by steel fibres, thus an innovative structure, steel and steel fibre reinforced concrete (SSFRC) structure, was proposed. Based on the four-point bending tests and numerical modelling of 18 SSFRC beams, the debonding mechanism and concrete cover failure were investigated. The results reflected that SSFRC beams experienced shearing failure, debonding failure and bending failure. Bonding cracks existed in most specimens, and the generation of abundant bonding cracks led to the sudden load decrease. The uncoordinated deformation between concrete cover and top flange appeared during the internal force transmission, making the bonding interface between them resist strong tensile stress and resulting in the debonding failure. After the chemical bond force at the bonding interface failed, the lateral concrete cover was firstly torn at the top flange tip, forming bonding cracks. The trapezoidal failure zone would form in the pure bending zone if bonding cracks developed sufficiently. Increasing the amount of steel fibres could prevent the trapezoidal failure zone from separation. Influenced by debonding failure, the extrusion action between bottom flange and bottom concrete cover was aggravated, generating longitudinal splitting cracks on the bottom surface of specimen.

BEARING CAPACITY OF A BEAM DAMAGED DURING COMBAT ACTIONS STRENGTHENED WITH THE USE OF FIBER CONCRETE

The paper presents the results of a study of the load-bearing capacity of a beam damaged during hostilities, which was reinforced with steel-reinforced concrete. A beam with 20 percent damage in the compression zone is considered. Before concreting, a foam insert was installed in the area of the planned damage, the shape and dimensions of which corresponded to the planned damage. After the concrete was set to 70% grade strength, the liner was pulled out, and the resulting cavity was filled with a 2% mixture of fiber concrete. This is one of the beam strengthening options in a comprehensive damaged beam testing program, which involves the study of a series of beams with different types of damage, varying damage zone (tensile and compression) and its magnitude, geometry and method of strengthening. The authors developed a special stand for testing. The load-bearing capacity of the damaged beam reinforced with fiber concrete was 80.6 kN, or 89.5% of the load-bearing capacity of the intact beam. Cracking started at the 4th load stage, when the load value was 16.1 kN, or 20% of the bearing capacity of the damaged beam. At the same time, 10 cracks formed in the zone of pure bending, the opening width of which did not exceed half the price of the microscope division, which is 0.005 mm. At the next eight stages of loading (up to 48.4kN), no new cracks were observed, and the width of the opening of all the ones that had previously formed doubled - it reached 0.01mm. At the 12th, 13th and 19th stages of loading, 4 more cracks appeared. The maximum final crack opening width was 1.2 mm. Up to a load of 68.5kN, which is 85% of the load-bearing capacity of the damaged beam, the reinforced concrete liner works with the beam almost as a unit, and then the compressed zone of concrete squeezes out the liner. It is claimed that the strengthening of a beam with 20% damage in the compressed zone in the manner considered in the work allows to achieve its bearing capacity, which is almost 90% of the bearing capacity of an undamaged beam.

Компьютерное моделирование процесса изготовления криволинейного отвода из аустенитной стали на трубогибочном станке с применением нагрева ТВЧ

The finite element method was used to evaluate residual stresses and deformations arising in the Du350 bend with a radius of 1200 mm made of the 08Х18Н10Т type austenitic steel during its manufacture with local heating by the high frequency current (HFC) and water cooling. The paper shows that compressive stresses appear on the bend inner surface, and the tensile stresses — on the outer. The bend cross-section acquires the elliptical shape. Maximum deformation in the compression zone reaches 34%, and 22% in the bend tension zone. Heat treatment and hydrotesting influence on the residual stresses magnitude was studied. Based on the tensile test data, characteristics of the metal mechanical properties in various zones of the full-scale bend were determined. It was established that in the non-heat-treated bend an increase in the yield strength (by 10...20%) compared to the original pipe properties was registered in the compressed and tensile zones. Relative elongation in the tensile zone decreased by approximately the same length, and elongation in the tensile zone increased. After heat treatment with heating of up to 900 °C (30 hours), properties in the different bend sections were leveling.

Study on Overburden Structure Characteristics and Fracture Evolution Law in Upward Mining at Kuangou Coal Mine

The development of vertical fractures in the overburden rock below after upward mining of a coal seam can influence the safe mining of both this seam and the seams above. This paper employs the method of physical similarity simulation to experimentally study the overburden structure and fracture evolution law during upward mining of the No. 4 coal seam in Kuangou Coal Mine. The study reveals that the overburden rock of the lower No. 4 coal seam after mining exhibits a typical "three-zone" characteristic, with the caved zone overburden showing a distinct cantilever beam-hinged structure, and the overburden structure of the bending and sinking zone is relatively intact. In the experiment, no significant vertical through-layer fractures were observed during the first five periodic pressures. However, during the 6th to 11th periodic pressure periods, when mining pressure became intense, a total of six vertical through-layer fractures appeared near the coal wall of the working face. These fractures, aligned with the caving angle direction, extended upwards through the No. 3 coal seam located 60m above the No. 4 seam. The overall fracture evolution showed opening during intense mining pressure and gradual closing after the pressure subsided. Two of these vertical through-layer fractures had a larger extension range, eventually reaching the surface. During the upward mining process, the appearance of vertical through-layer fractures did not lead to sliding instability or rapid sinking of the overburden roof. The working face could thus continuously advance. This research has implications for the safe upward mining in similar mines.

Impact of Internal CFRP strips on the flexural behavior of heat-damaged reinforced concrete beams

Recent theoretical developments revealed that reinforced concrete (RC) structures are susceptible to deterioration risk upon exposure to high temperatures where the mechanical properties of their constituents are affected and therefore require upgrading their overall performance. However, the overall behavior could be improved by strengthing the RC beams using the well-known carbon fiber-reinforcement polymers (CFRP) materials where its efficiency is highly limited by detachment, and debonding problems appear as a result of the weakness in the bond between the concrete surface and the strengthening material or upon the stress concentration induced by the various anchoring systems. The CFRP sheets have been integrated as internal reinforcement in the maximum bending zone within the thermally damaged beams, a new technology used in this study. The suggested method was the first of its kind and did not need an adhesive to be applied where debonding problem is eliminated. In contrast to conventionally reinforced steel, CFRP composite materials are fully compatible with flexural steel and constrained concrete. A total of 40 RC with (150 × 200) mm2 and an overall length of 1100 mm concrete beams were cast, and the studied parameters were the CFRP length, position, and exposure temperature. The internal strengthening technique has been found to ensure the full utilization of the strengthening material where the externally-strengthened beams fail preceding the CFRP strain reached, and this was confirmed using the linear weighted sum method where the internal strengthening has the highest ranking based on the mechanical characteristics comparisons. Moreover, the internal CFRP reinforcement improves RC beam performance, strength, stiffness, toughness, and serviceability more than exterior CFRP sheets. However, the enhancement percentages are twice as much for internal strengthening as the external one. It has also been found that the reinforcement's location substantially impacted the number and length of flexural cracks and its failure mode. In addition, for every 1% reduction in concrete compressive strength in heat damage, the average ultimate load was reduced by 0.8%. The CFRP profitability indexes decrease as sheet number and temperature increase; the average toughness decrease at 150 °C, 250 °C, and 500 °C is 12%, 21%, and 47%, respectively.