Low Frequency Cyclic Loading Research Articles

Comparative study of marine steel and welding joint in artificial seawater based on stress corrosion cracking and crack growth

In this work, the environmental fracture behavior of marine steel and its welding joint were compared in artificial seawater. The electrochemical measurement, slow strain rate tensile test, and low-frequency cyclic loading test were conducted. The results showed that the corrosion current density and stress corrosion cracking susceptibility of welding joints were higher than those of base metal, caused by high dislocation density and residual stress in the heat-affected zone. The acidification and ion concentration occurred at the crack tip of the welding joint and basic metal. pH value and Cl− concentration at the crack tip reached 4.1 and 3.8 mol/L, respectively. The crack growth rate of the welding joint was higher than that of base metal. The crack growth behavior was controlled by the mechano-electrochemical effect at the crack tip, and hydrogen embrittlement was the main factor.

Acoustic emission characteristics and fracture mechanism of sandstone in open-pit mines under different types of cyclic loads

Rock mass is one of the most important load-bearing media in geotechnical engineering. It has been continually vulnerable to geological tectonic movements, natural calamities, and human excavation activities. Its inherent weak surfaces such as primary pores, joints, and fissures have resulted in varying damage degrees. In mining operations, the damaged rock mass has a variety of negative impacts on the stability of its overlying structures and is frequently disturbed by the load. To study the damage law of rock mass under cyclic loading, in this paper, an acoustic emission (AE) device was employed to monitor the rock under the action of two types of cyclic loads: the variable upper and lower pre-loads, and the fixed upper and lower pre-loads. The damage of the loaded rock was split into three stages in this research, based on the features of the AE signals of the rock under uniaxial load, and the damage evolution of the loaded rock was analyzed in distinct stages. The AE signals of the rock under cyclic loading were mainly emitted in the first loading stage. When the stress did not exceed the maximum stress value in the stress history of the loaded rock, few new AE event was generated in the loaded rock. After the low-frequency cyclic static load, the AE signals varied with the load-bearing stress of the rock during the whole process from initial loading to failure, which was consistent with the characteristics of the AE signals of the loaded rock. The research results can be adapted to rock mass in open-pit mines stability analysis and risk prediction while providing some references for the early warning and danger relief of rock masses in engineering.

Seismic behavior of double-cell CFST columns with table tennis racket-shaped cross section

Multicell concrete-filled steel tube (CFST) columns with excellent mechanical performance have been used in super high-rise buildings in recent years. According to the requirements of a 265-m tall building, the double-cell CFST columns with an innovative cross section of table tennis racket were applied. Hence, six columns were designed according to the prototype columns and subjected to low-frequency cyclic loading. The impacts of different shear-span ratios (SSRs), axial load ratios (ALRs), and steel ratios on the seismic behavior were elaborated and clarified. The test results indicated that the seismic performance varied in different loading directions owing to the asymmetric cross section, and the columns performed better in the positive loading direction resulted from earlier and more sufficient confinement of the circular steel tube, which was also reflected by the measured strain development. With SSR deceasing from 2.5 to 1.7, earlier and more severe damage was recorded. The resisting capacity and stiffness were significantly improved at a cost of ductility loss. Increasing ALR from 0.2 to 0.4 brought about more damage and worse deformability. However, a proper increase in axial load was beneficial to the resisting capacity and stiffness, and its influence on the restoration capacity was associated with SSR. Increasing the steel ratio (by 33.36%) could effectively postpone and alleviate the local buckling of steel tube, thereby comprehensively improving the seismic performance. A finite element (FE) analysis of such columns was performed to carry out the damage evolution and parametric study, and provide references for their application in practical engineering.

Numerical investigation of monopiles in structured clay under cyclic loading

Offshore wind is undergoing rapid growth and is expected to play a vital role in the transition to a low-carbon global economy. This paper presents the results of implicit three-dimensional finite element analyses investigating cyclic lateral loading on a large diameter monopile in structured London Clay. The cyclic behaviour of the clay was simulated using the subloading tij constitutive model. Symmetric and asymmetric two-way and one-way low frequency cyclic lateral loads were used with an amplitude representing offshore wind turbine production loading conditions. Simulations showed that one-way loads induced greater cumulative pile deformation and rotation relative to two-way loads. For all load configurations, the rate of accumulation of lateral pile displacement and rotation at mudline reduced with each subsequent cycle. In contrast, the cyclic secant stiffness progressively degraded with its rate of decay reducing with each ensuing cycle. Findings are indicative of the incidence of strain softening, reconsolidation and densification during low-frequency cyclic loading. It was established that assuming the clay to be unstructured would be conservative in the design of monopiles.

Seismic performance of reinforced self-compacting concrete-filled circular steel-tube columns

Reinforced self-compacting concrete-filled circular steel-tube (RSCCFST) columns are used globally. The aim of this paper was to analyze the seismic performance of RSCCFST columns. One self-compacting concrete-filled circular steel-tube (SCCFST) column and seven RSCCFST columns were subjected to a low-frequency cyclic load. The experimental results indicated that the hysteretic curves of the RSCCFST columns were plump, and the columns exhibited good seismic performance. Parametric studies were also conducted to reveal the effects of the axial compression ratio, steel tube thickness, and the longitudinal reinforcement ratio on the seismic behavior of the RSCCFST columns. The parameter analysis indicates that increasing the reinforcement ratio and steel tube thickness can improve the load capacity and ductility of RSCCFST columns. Based on the above analysis, the entire process analysis method for RSCCFST columns under low-frequency cyclic loads was proposed using the Fortran language. The calculated results showed high accuracy compared to the experimental results; the average value of the ratio between the calculated and experimental values was 0.942, with a mean square error of 0.034. This proves that the computing programs written in Fortran are efficient for analyzing RSCCFST columns.

Experimental Study on the New Type of Connecting Nodes of Prefabricated Stairs Using Flexible Grouting Material

In recent years, prefabricated buildings have been developing positively in China. Prefabricated concrete stairs are widely used in prefabricated concrete buildings because of their quick construction, convenient handling, reduced cost, and premium appearance. They also have promising application potential in cast-in-place concrete structures and steel buildings. The stairs are important escape routes during earthquakes and play the role of “safety islands.” Therefore, the study on the seismic performance of connecting nodes of prefabricated concrete stairs is significant. For a new flexible-hinge connecting node of the prefabricated stairs with flexible grouting material (FGM), the low-frequency cyclic loading test is carried out in this paper to study the stress performance of the end nodes of the stairs under in-plane and out-of-plane low-frequency cyclic loading. The results show that the new flexible-hinge connecting nodes of the prefabricated concrete stairs have moderate bearing capacity and stiffness, which can mitigate the effect of diagonal bracing under small and medium earthquakes and dissipate energy to a certain extent.

Low Frequency Cyclic Mechanical Loading of Till Deposits from Northern Germany under Oedometric Conditions

Glacial deposits are of significant importance to geotechnical engineers and geologists in northern Europe, North America, and Northern Asia, as vast areas of these land surfaces were historically covered with ice leading to the formation of a wide variety of till deposits. The use of these areas for various engineering purposes warrants their subjection to mechanical loads (of static and cyclic forms) from manmade structures, as well as natural hazards such as earthquakes. This paper focuses on the experimental investigation of the cyclic mechanical loading behavior of two glacial tills from northern Germany under one-dimensional loading or oedometric conditions, and in different soil wetting conditions. The experimental results show a significant dependence of the cyclic mechanical response of the glacial tills on wetting condition and number of loading cycles. The recorded values of accumulated plastic strains of the glacial tills generally increase with an increase in wetting or moisture content, with the highest measured value for the two tills being around 3.9% after 19 cycles of loading. The findings of the experimental cyclic mechanical tests of the glacial tills are discussed in view of the intrinsic soil behavior and fabric.

Seismic performance of end-reinforced steel pipe dampers: Laboratory test and numerical investigation

The premature end failure of the traditional hollow steel pipe damper (HSPD) greatly affects its energy dissipation performance. This paper proposes a novel hollow steel pipe damper (i.e. end-reinforced steel pipe damper, ESPD), which consists of an energy dissipation steel pipe, two inner short steel pipes for strengthening the end of ESPD, and two connecting plates. To study the effectiveness of end-reinforced structure and investigate the mechanical performance of ESPD, one HSPD and three ESPDs specimens with different lengths of inner short pipe are fabricated and subjected to low-frequency cyclic loading. To investigate the influence of the reinforced steel pipe on the mechanical properties of ESPD, numerical analyses are conducted. Results indicate that end-reinforced with short inner pipe is a simple and feasible measure to avoid premature end failure of HSPD. Compared to HSPD, ESPDs own a superior energy dissipation ability and ductility. The reinforcement thickness ratio of 1.67 to 2, and the reinforcement length ratio larger than 0.47 are suggested in designing ESPD.

Seismic performance of L-shaped short-leg shear walls with different concealed support materials

This paper reports a study of the effects of different concealed bracing materials on the seismic performance of short-legged shear walls and presents the results of low-frequency cyclic loading tests on three L-shaped short-leg shear walls—one wall without concealed bracing and the other two with concealed bracing materials of steel bars and SMA bars, respectively. The tests were aimed mainly at analyzing the effects of different concealed bracing materials on seismic performance indicators such as crack pattern, bearing capacity, ductility, hysteresis performance, stiffness, and energy dissipation of the L-shaped short-leg shear walls. The results revealed that their seismic performance with concealed bracing was better than without concealed bracing. The short-leg shear walls with SMA bar concealed bracing had the lowest number of cracks and the smallest crack width, and the effect of delaying the development of cracks was more evident. The short-leg shear walls with steel reinforcement and SMA-bar concealed bracing were better in bearing capacity, ductility, and energy dissipation capacity, with, respectively, 11% and 26.3% increase in ultimate load, 4% and 18.6% increase in ultimate displacement, and 12% and 28% increase in energy dissipation. It can be seen that setting concealed bracing effectively enhances the seismic performance of short-leg shear walls, and that short-leg shear walls with concealed SMA bar bracing have the most significant improvement in seismic performance.

Seismic performance of wooden straight-tenon joints reinforced with lightweight steel members

A large stress is produced in the straight-tenon joints of a wooden structure during an earthquake, and this can cause the tenon to pull out and destabilize the timber frame. Therefore, four types of lightweight steel reinforcements for straight-tenon joints are herein proposed. Seven straight-tenon douglas fir joint specimens include one unreinforced and six reinforced specimens, are subject to low-frequency cyclic loading according to the Chinese specification JGJ/T101-2015. Failure modes, moment-rotation hysteretic curves, skeleton curves, stiffness degradation, strength degradation, energy dissipation capacity, and tenon pulled-out values are obtained and discussed. The results indicate that the unreinforced specimens are damaged because of the large tenon pulled-out values and obvious pinching effect of the hysteretic curve. Compared with unreinforced specimens, the final failure modes of all reinforced specimens are changed, hysteretic curves become fuller, peak load increases by more than 10%, initial stiffness increases by more than 25%, and bearing capacity degradation improves significantly; they achieve a good reinforcement effect. The strength and energy dissipation of the specimens increased with an increasing triangular-rib size. An FE model is established, and the FE-predicted results match the test results well. The parametric study results indicate that increasing the size of the tenon and the length of the tapping screw can effectively improve the seismic performance of the specimen.

Seismic behaviour of RC columns retrofitted with textile-reinforced mortar (TRM) optimized by short PVA fibres

Textile-reinforced mortar (TRM) optimized by short polyvinyl alcohol (PVA) fibres was used as a strengthening material for seismically deficient reinforced concrete (RC) columns. Seven RC columns, including a reference column, a column strengthened with the inorganic matrix added short PVA fibres, and five columns strengthened with the optimized TRM, were subjected to low-frequency cyclic loading. The influences of the reinforcing ratio of carbon textile and axial load ratio on the seismic performance of the jacketed columns were studied. The results showed that the shear capacity and deformability of RC column respectively increased by 54.3 ∼ 55.2 % and 40.34 ∼ 78.62 % after strengthening with the optimized TRM system. Increasing the reinforcing ratio of carbon textiles in the strengthening layer, the displacement ductility and energy dissipation of the jacketed column were further enhanced. As the axial load ratio increased, the shear capacity of the jacketed column increased; however, the corresponding ductility and energy-dissipation ability decreased. The relationship between the shear contribution and material configuration of the TRM was established. The shear capacity of the TRM-strengthened columns can be estimated by a modified shear model considering the reinforcement ratio of textile and the volume content of PVA fibres.

Experimental study on the seismic performance of clay brick masonry wall strengthened with stainless steel strips

Strengthening the existing masonry structures with stainless steel strips is a new strengthening approach, which combines the advantages of compatible-to-masonry, chemically stable, durable, effective retrofitting, and low impact on the existing masonry structure. The horizontal and vertical stainless strips was anchored into the masonry wall with the angle steel and steel pipe to improve its in-plane tensile strength and ductility. To study the seismic performance of brick masonry walls strengthened with stainless steel strips, one masonry wall and five stainless steel strip strengthened masonry wall specimens with different packing modes and grid spacings were tested under low frequency cyclic loading. The experimental results show that the proposed strengthening approach can efficiently improve the seismic performance of the existing masonry structures, such as the load bearing capacity, ductility, stiffness, and energy dissipation capacity. The failure modes and damage evolution law of all the specimens were compared and analyzed. The load bearing capacity and hysteretic curves of the specimen were calculated using the proposed calculation formula and restoring force model, respectively. The calculated results are consistent with the test results, which provides an appropriate theoretical support and design basis for the masonry reinforcement design with stainless steel strips.

Comprehensive study on fatigue degradation of road piezoelectric energy harvesters under thermal-mechanical coupling effect

Recent years have seen significant advancements in the field of road vibration energy harvesting using piezoelectric technology, including the watt-level road piezoelectric energy harvesters (RPEHs). However, research on the fatigue life of RPEH under thermal-mechanical coupling effect is lacking. In this study, six lead zirconate titanate (PZT) block piezoelectric transducers with distinct properties were fabricated, and five transducer structures were compared. The effects of piezoelectric material properties, pavement temperature, and transducer structure on RPEH fatigue degradation were investigated using approximately 23 million mechanical loads. The findings revealed that the output power of the soft PZT was significantly greater than that of the hard PZT under low-frequency cyclic loading. However, the Curie temperature decreased with an increasing piezoelectric constant of the soft PZT. Therefore, the stability of the output voltage worsened as the ambient temperature approached half the Curie temperature. Specifically, when the ambient temperature was 50 °C, the output failure phenomenon readily occurred during the initial stage of fatigue loading. By optimizing the transducer structure, the fatigue characteristics and high-temperature failure phenomenon of RPEH could be effectively improved. The coupling output modes of d 33 and d 31 were superior to that of the d 33 mode. Among these, the drum transducer exhibited the highest output performance and operational stability across different ambient temperatures, excitation frequencies, and displacements. After six million intermittent loads over 15 d (equivalent to two years of traffic load), the output power decreased from 6.51 to 6.02 mW with a degradation rate of merely 7.53%, indicating a promising application prospect. The results provide an crucial design foundation for the entire life cycle operation of RPEH in road engineering.

Study on the inherent magnetism and its relationship with mechanical properties of structural round steel

Inherent magnetism is an important properties of ferromagnetic materials. In this study, the internal magnetic field intensity (IMFI) and internal magneto-mechanical effect (IMME) of Q390B in a structural field were investigated to detect and verify the inherent magnetism in structural settings. In the IMFI test, the magnetic flux was used to detect the change in magnetic field to verify the existence of magnetism. In the IMME test, a novel instrument was implemented to measure the magnetic variation in the Q390B specimen without magnetic flux. Based on the low frequency cyclic (LFC) tensile loading tests, the inherent magnetism was fully described. Experimental results indicate that IMME shows great potential and more efficiency in inherent magnetism studies and can be promoted in the near future.

Effect of waste PET strips as reinforcement in concrete under cyclic loading

Reuse of waste PET bottles in construction industry is emerging as a potential option for improving the properties of unreinforced concrete. Although earlier studies have explored the effect of PET reinforced concrete under monotonic loading, limited studies have explored its behaviour under cyclic loading. In this context, the present study quantifies the impact of PET strips on characteristics of concrete under low frequency cyclic loading at different stress levels. A comparative assessment of stress–strain response, deformation characteristics, loading/unloading modulus, Poisson’s ratio and strain energy density for PET macro-reinforced concrete has been performed with conventional concrete (CC). PET concrete exhibits higher loading capacity, lower variability, delay in damage propagation and better damage tolerance under cyclic loading. The study also proposes a simple equation for quantification of confinement effect developed due to the presence of PET strips in concrete. Overall, the utilization of waste PET in concrete presents a sustainable approach for improving the properties of unreinforced concrete (viz., pavement application, flowable fill application, brick manufacturing), while reducing the negative environmental impact of waste PET.

Seismic behavior of circular CFST columns with different internal constructions

To improve the seismic performance of circular concrete-filled steel tube (CFST) columns with large dimensions, circumferential stiffeners, vertical stiffeners, and reinforcement cages were suggested to be set in the columns. Seven circular CFST column specimens with four internal constructions and two shear-span ratios were designed and tested under low-frequency cyclic loading. The failure modes, hysteretic behavior, bearing capacity, ductility, stiffness and strength degradation, energy dissipation capacity, and effect on the seismic performance based on different internal constructions and different shear-span ratios were herein discussed and clarified. The test results illustrated that the internal constructions could effectively postpone the local buckling of steel tube and improve the ultimate strength, ductility, and stiffness of circular CFST columns. A finite element (FE) model was developed to carry out the working mechanism and parametric analysis. Finally, a theoretical model to estimate the ultimate bearing capacity of CFST columns with different internal constructions was established, the results obtained from the model were in good agreement with the test results.

Experimental and Numerical Studies on the Traditional Penetration Mortise–Tenon Connection Reinforced by Self-Tapping Screws

In this paper, the mechanical performance improvement of traditional penetration mortise–tenon (PMT) connections reinforced by self-tapping screws (STSs) were experimental and numerical studied. Four unreinforced penetration mortise–tenon connections and four STS-reinforced connections were experimentally investigated under monotonic and low-frequency cyclic loading. Besides, the numerical model of PMT connections reinforced by STSs with different diameters and yield strength were established and analyzed.Their failure modes, rotational stiffness, moment-resisting capacity, ductility, and seismic behavior were studied. The experimental results indicated that the STSs reinforcements could enhance the moment-resisting capacity of the penetration mortise–tenon connection. Compared with unreinforced connections, the initial stiffness of STS-reinforced connections increased by 75% with the moment-resisting capacity increasing by 69%, respectively. Furthermore, the STSs reinforcement method can effectively restrict the pull-out of tenon during the whole loading process. Numerical simulation results showed that the yield strength of STS has little effect on the moment-bearing behavior of the connection. The diameter of STS significantly influences the connection performance, and the diameter of 8 mm is suggested.

An Improved Apparatus for Testing the Friction Variation of Soil-Structure Interface Induced by One-Dimensional Vibration

Abstract Forced vibration of underground structures is a common activity during the construction or service period of rail infrastructure, such as vibro piling, the vibration of piles, and tunnel lining under moving train loads, etc. To better predict the structure’s performance under vibration, a lot of attention has been paid to investigating the behavior of interfaces between soils and structures. However, limited by the capabilities of the current test apparatus, the soil-structure interface friction is mainly determined under static and low-frequency cyclic loading conditions. While for dynamic conditions, the interface friction is generally configured as a reduction of the static friction coefficient. By now, the interface friction under vibration is not fully addressed, especially under high-frequency vibration. In this study, an apparatus is developed from a direct shear device to test the interface friction variation between soil and structure under vibration. Static and vibration forces are imposed on the structure to implement coupled monotonic shear and high-frequency vibration to the interface. The apparatus allows for the controlling of soil property, surface property of structure, normal stress, shear rate, and vibration intensity and frequency. The variation of shear stress and displacement, normal stress ,and displacement are mainly monitored. To show the capability and test procedure of the developed apparatus, dry sand and a smooth-steel plate are installed and tested on the apparatus with a vibration frequency 45 Hz and an acceleration of 0–0.15 g. The results show that the apparatus can stably obtain the friction weakening of the sand-steel interface, including the reduction of shear strength, volume change, and shear-displacement slip.

The Seismic Performance of New Self-Centering Beam-Column Joints of Conventional Island Main Buildings in Nuclear Power Plants

In order to improve the deformation energy consumption and self-centering ability of reinforced concrete (RC) frame beam-column joints for main buildings of conventional islands in nuclear power plants, a new type of self-centering joint equipped with super-elastic shape memory alloy (SMA) bars and a steel plate as kernel components in the core area of the joint is proposed in this study. Four 1/5-scale frame joints were designed and manufactured, including two contrast joints (a normal reinforced concrete joint and a concrete joint that replaces steel bars with SMA bars) and two new model joints with different SMA reinforcement ratios. Subsequently, the residual deformation, energy dissipation capacity, stiffness degradation and self-centering performance of the novel frame joints were studied through a low-frequency cyclic loading test. Finally, based on the OpenSees finite element software platform, an effective numerical model of the new joint was established and verified. On this basis, varying two main parameters, the SMA reinforcement ratio and the axial compression ratio, a simulation was systematically conducted to demonstrate the effectiveness of the proposed joint in seismic performance. The results show that replacing ordinary steel bars in the beam with SMA bars not only greatly reduces the bearing capacity and stiffness of the joint, but also makes the failure mode of the joint brittle. The construction of a new type of joint with consideration of the SMA reinforcement and the steel plate can improve the bearing capacity, delay the stiffness degradation and improve the ductility and self-centering capability of the joints. Within a certain range, increasing the ratio of the SMA bars can further improve the ultimate bearing capacity and energy dissipation capacity of the new joint. Increasing or decreasing the axial compression ratio of column ends has little effect on the overall seismic performance of new joints.

Battered minipile response to low-frequency cyclic lateral loading in very dense sand

This experimental study investigates the response of vertical and battered minipiles to two-way symmetrical low-frequency (0.1 Hz) cyclic lateral loading. Laboratory (1-g) tests were performed on scaled-down minipiles in very dense cohesionless soil, for batter angles of 0°, 25° and 45°. The cyclic loading is classified into two categories: multi-amplitude and long-term single amplitude, where force-controlled load was applied at a constant frequency. The minipiles were instrumented with optic fibres, and strain profiles were obtained at each loading stage, in both compression and tension stroke. The results are presented in terms of hysteresis loops, variation of normalised stiffness, minipile strain and bending moments under cyclic loading. In the multi-amplitude loading category, backbone curves show a stiffer force–displacement response in tension stroke than in compression stroke. For the single-amplitude category, the area of the hysteresis loop is largest for 45° battered minipiles with the lowest accumulated deformation. The normalised stiffness at the end of 50 cycles is highest for 25° minipiles with a value slightly greater than one. The strain profiles along the minipiles show stabilisation of measured strain before the number of cycle reaches 50, for all three battered conditions. A multi-surface hardening constitutive model is used to explain the effect of shearing and cyclic loading, with increasing loading amplitude on 25° battered minipiles. These test results are indicative of better performance capability of 25° battered minipiles, in terms of secant stiffness, compared to the vertical and 45° battered cases.