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Number of results: 11
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Abstract

In order to explore the mining failure law of deep coal seam floor and clarify the mechanical behavior and energy change in the floor strata during mining, the mechanical properties and energy evolution law of sandstone under cyclic loading with different confining pressures (20, 30, 40 MPa) were studied using the Rock Top multi-field coupling tester. The results are as follows: (1) the hysteresis phenomenon of a rock stress-strain curve under cyclic loading is evident. Moreover, the hysteresis loop migrates to the direction of strain increase, and the fatigue damage caused by cyclic loading has a certain weakening effect on the peak strength of rock; (2) both the number of cycles and the axial strain show a nonlinear change characteristic that satisfies the quadratic function relationship. Among them, the stress level of the rock is the main factor affecting the fitting effect; (3) under the same confining pressure, with an increase in cycle level, the macroscopic deformation of the rock increases, the accumulation of fatigue damage in the sample increases, and the irreversible deformation of the rock increases, which leads to an increase in energy input and dissipation; (4) in terms of elastic energy and dissipation energy, elastic energy plays a dominant role. In the initial cycle, the rock is destroyed, and the rock energy loss is great. After the second cycle, the input energy is mainly stored in the rock in the form of elastic energy, and only a small part of the input energy is released in the form of dissipation energy; (5) the confining pressure can improve the efficiency of rock absorption and energy storage, enhance the energy storage limit of rock, and limit the dissipation and release of partial energy of rock. The greater the confining pressure, the more evident the limiting effect, and the more significant the dominant position of elastic energy; and (6) the change in the energy dissipation ratio can be divided into three stages: rapid decline stage, stable development stage and rapid rise stage. The greater the increase in dissipation energy, the greater the degree of rock damage. The evolution process of the energy dissipation ratio can reflect the internal damage accumulation process of rock well, which can be used as the criterion of rock instability.
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Authors and Affiliations

Daqiang Xu
1
ORCID: ORCID
Peisen Zhang
1
ORCID: ORCID
Wei Yan
1
ORCID: ORCID
Xiaole Zhang
1
ORCID: ORCID
Yuhang Dong
1
ORCID: ORCID
Hui Niu
1
ORCID: ORCID

  1. Shandong University of Science and Technology, National Key Laboratory for Mine Disaster Prevention and Control, Qingdao, Shandong 266590, China
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Abstract

At present, many studies have been carried out on the fatigue mechanical properties of conventional rocks, but there are few studies on the mechanical properties of rocks after water rock interaction. The aim is to better study the fatigue mechanical characteristics of sandstone after dry wet cycle and the research object we take was sandstone samples after a certain number of drying and wetting cycles. The WAW-2000 electro-hydraulic servo pressure system was used to carry out uniaxial fatigue cyclic loading and unloading tests with different amplitudes and different upper limit stresses. The test found that: when the sample is damaged under fatigue load, the stress-strain curve shows a sharp downward trend and a sudden instability failure occurred in the sample.With the increase of the upper limit stress and amplitude, the life of the sample decreases gradually which also conforms to the change of power function. Then the relationship between fatigue life and stress is obtained. The fatigue stress-strain curve is characterized by sparse-dense-sparse. With the increase of cycle time, the axial strain of the sample shows an inverted “S” shape. The strain change can be divided into three stages: initial stage, constant velocity stage and acceleration stage.
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Authors and Affiliations

Jin-Song Zhang
1
ORCID: ORCID
Yu Lu
1
ORCID: ORCID
Heng-Liang Zhang
2
ORCID: ORCID
Yu-Di Yang
1
ORCID: ORCID
Yi-Shun Bu
1
ORCID: ORCID

  1. Anhui University of Science and Technology, No. 168, Taifeng street, Huainan City, Anhui Province, China
  2. First Design and Research Institute MI China Co, No. 12550, bustling Avenue, Hefei Economic and Technological Development Zone, Anhui Province, China
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Abstract

In this study, the compressive deformation of crushed sandstone was tested using a crushed rock deformation-seepage test system, and the effects of various factors, including crushed rock grade, grade combination, water saturation status, and stress loading method (i.e., continuous loading or cyclic loading and unloading), on the compressive deformation of crushed sandstone was analyzed from four perspectives including stress-strain, bulking coefficient, deformation mechanism and energy dissipation. The results indicate that the stress-strain relations of crushed sandstone are closely associated with all factors considered, and are well represented by exponential functions. The strain observed for a given applied stress increased with increasing crushed rock grade throughout the loading period. Crushed sandstone grades were combined according to a grading index (n), where the proportion of large-grade rocks in the sample increased with increasing n. The bearing capacity of a water-saturated crushed sandstone sample with n = 0.2 was less than that of an equivalent dry sample for a given applied stress. The stress-strain curve of a water-saturated crushed sandstone sample with n = 0.2 under cyclic loading and unloading was similar to that obtained under continuous loading. Observation and discovery, the deformation mechanism of crushed sandstone was mainly divided into four stages, including crushing, rupture, corner detachment and corner wear. And 20% of the work done by testing machine is used for friction between the crushed sandstone with the inner wall of the test chamber, and 80% is used for the closing of the void between the crushed sandstone, friction sliding, crushing damage.

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Authors and Affiliations

Yanan Sun
Peisen Zhang
ORCID: ORCID
Wei Yan
ORCID: ORCID
Fenqian Yan
Junda Wu
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Abstract

Suspension line insulators are during their operation subject to static forces and variable loads, usually of a cyclic character. These variable loads have a significant impact on the mechanical durability of composite insulators. A method of providing durability forecast for composite line insulators based on fatigue characteristics has been proposed. The method allows providing durability forecast of insulators in a wide range of variable loadings, i.e. from quasi-static to high amplitude loadings.
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Authors and Affiliations

Jerzy Bielecki
1
ORCID: ORCID
Jacek Wańkowicz
1
ORCID: ORCID

  1. Institute of Power Engineering – Research Institute, 8 Mory Str., 01-330 Warsaw, Poland
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Abstract

Iron production’s waste materials include significant quantities of blast furnace slag (BFS) which could potentially be used as a substitute for natural aggregates in hot mix asphalt (HMA) used in highway projects. Although many of properties of slag are interesting, its porosity and absorption rate would lead to greater consumption of asphalt. For this study, a Portland cement (PC) paste was used to reduce the porosity of a BFS. This PC treated BFS (called BFS-C) was then used in an HMA to replace the coarse fraction of a natural aggregate. Marshall, Indirect Tensile Strength (ITS), resilient modulus and Cantabro tests were then carried out on different HMA mixtures that included BFS-C. Using BFS-C, HMA’s resistance under monotonic loading, stiffness under cyclic loading, and resistance to moisture damage increased remarkably. In addition, the Cantabro abrasion resistance of BFS-C improved was better than that of the HMA mixture produced with untreated BFS.

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Authors and Affiliations

H.A. Rondón-Quintana
J.C. Ruge-Cardenas
J.G. Bastidas-Martínez
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Abstract

The current fatigue evaluation procedures in Europe and North American bridge codes do not account for the degree of uncertainty in load and resistance models. However, the variability of cycling loading and material properties have a significant influence on fatigue safety verification. A fatigue verification is contingent on the accumulated load cycles and the fatigue category; which, in turn, depends on member type and its connections. Assessment of structural safety can be evaluated more completely using probabilistic methods that provide fatigue prediction in terms of the probability of crack initiation. This method provides more information about the expected performance of a structural component; therefore, the structure can be used in service for a significantly longer time. In this article, the comparison of fatigue evaluation is presented using Eurocode, North American Standard – AREMA, and the new approach using the probabilistic method. These methods are demonstrated on the riveted built-up beams of the steel deck plate girder (DPG) railway bridge using data from field monitoring.
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Authors and Affiliations

Anna Maria Rakoczy
1
ORCID: ORCID

  1. Road and Bridge Research Institute, ul. Instytutowa 1., 03-302 Warsaw, Poland
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Abstract

In this research, nonlinear analysis of composite shear walls (CSWs) with a gap between reinforced concrete wall and steel frame is investigated under cyclic loading by the use of the finite element method (FEM) software ABAQUS. For the purpose of the verification, an experimental test is modelled and comparison of its obtained result with that of the experimental test demonstrates an inconsiderable difference between them; therefore, the reasonable accuracy of the modelling is revealed. Then, effects of different parameters on the behaviour of the CSWs are examined. Gap size between reinforced concrete wall and steel frame, reinforcement percentage, steel sections of beams and columns, and existence of reinforced concrete wall are considered as parameters. It is concluded that change of the parameters affects the ultimate strength, ductility, and energy dissipation of the system. A steel shear wall (SSW) is also modelled and compared with the CSWs. Buckling of the walls is presented as well.

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Authors and Affiliations

Alireza Bahrami
Mojtaba Yavari
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Abstract

Joints in cold-formed steel framing structures are usually designed as bolted lap type ones with a gusset plate. Unlike the end-plate joints in hot-rolled structures, the load in such joints is transferred through shearing of the bolts and bearing of the material. The prediction of their structural properties may be problematic in viewof unfavourable influence of the hole clearance and hole ovalization resulting from low bearing resistance of thin walls. A few experimental programmes showed that these issues lead to a different behaviour of the whole joint comparing to common end plate type. These concerns may be particularly important for joints under variable loading, which are prone to deterioration of structural properties. The testing programme conducted by the authors was focused on their behaviour under monotonic and cyclic loading with attention to a potential drop of resistance and stiffness. Monotonic tests revealed quite similar course of the joints’ response. In view of high deformability of the specimens at the intermediate stage of each monotonic test, plastic moment resistances of joints were associated with the initial part of the moment-rotation curves and were multiple times lower than maximum moments obtained in the experiments. The quantities of deterioration of structural properties were determined based on cyclic tests. Drop of resistance and stiffness was observed for several levels of loading range, but the trend of decrease varied for each property. Application of the DIC technique allowed one to identify qualitatively and quantitatively the sources of joint deformability.
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Authors and Affiliations

Rafał Budzinski
1
ORCID: ORCID
Lucjan Ślęczka
1
ORCID: ORCID

  1. RzeszowUniversity of Technology, Faculty of Civil and Environmental Engineering and Architecture, Al. Powstanców Warszawy 12, 35-959 Rzeszów, Poland
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Abstract

The subject of this paper is a new procedure applied for more precise determination of material fatigue characteristics. The proposed approach is based on a special definition of the strain energy density parameter used for material fatigue property descriptions and, in the consequence, on the new algorithm of the fatigue machine control in the feedback loop. On the basis of fatigue tests under uniaxial tension-compression with the strain energy density parameter control it was proved that the fatigue characteristics in energy approach (Wa - N1) determined directly from the tests and indirectly from the Manson-Coffin relation (E,, - Nj) with the strain control for material showing cyclic stability are similar. However, for material cyclic hardening these characteristics differ significantly in high cycle fatigue regime.
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Authors and Affiliations

Włodzimierz Będkowski
Ewald Macha
Jacek Slowik
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Abstract

This paper describes results of tensile mechanical strength testing of two types of composite suspension line insulators from two manufacturers. In order to take into account the operation of composite insulators in overhead transmission lines with high-temperature low-sag (HTLS) conductors, the testing of their static and fatigue strength was performed at both ambient and elevated temperatures. The results showed that the static mechanical strength of composite insulators decreased with an increase in the temperature of the lower end fitting of the insulator, and proved that it followed a third-degree polynomial function. Calculations performed demonstrated that a significant cause of reduction in strength was the increase in the radial stress following the temperature increase in the crimped glassepoxy resin core of the insulator. The results of the fatigue strength testing demonstrated that the increase in the temperature of the lower end fitting of the insulator up to 85°C degree had a little effect on the fatigue strength of the tested composite insulators.
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Authors and Affiliations

Jerzy Bielecki
1
ORCID: ORCID
Piotr Kotowski
2
Jacek Wańkowicz
1
ORCID: ORCID

  1. Institute of Power Engineering, ul. Mory 8, 01-330 Warsaw, Poland
  2. 2Wroclaw University of Technology, ul. Smoluchowskiego 25, 50-370 Wroclaw, Poland

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