Long-term Loading Tests Research Articles

Experimental study on monopile-supported offshore wind turbine subjected to long-term wind and wave cyclic loading

Offshore wind energy has recently gained much attention. During its service life, a monopile-supported offshore wind turbine (OWT) is subjected to long-term wind and wave lateral cyclic loads with different cycle characteristics, inducing accumulated deformation of the OWT system to exceed the serviceability limit state and causing safety accidents. In this study, a 1g scaled model test with a similarity ratio of 1:100 was conducted to investigate the lateral response of a monopile-supported OWT in sand under long-term wind and wave cyclic loads. Two sets of centrifugal gear cyclic loading devices applied stable long-term wind and wave cyclic loads and different cyclic load amplitudes and directions were achieved by adjusting the input voltage and counterweight masses. Four groups of long-term cyclic loading tests were conducted for the monopile-supported OWT, considering different wind load simplification methods and various wind-wave load contribution ratios. The lateral displacement and accumulated tilt of the OWT were monitored using two laser displacement transducers installed at different heights. The results show that a simplistic treatment of wind loads as static loads results in an overestimation of the cumulative rotational deformation, and an increase in the wind-wave load contribution ratio decreases the cumulative tilts of the OWT structures.

Electrical conductivities and sensing mechanisms of low-temperature 3D printing conductive hydrogels with good sensitivity

Flexible strain sensors are core components of wearable strain sensors, and they have attracted much attention because of their applications for electronic skin and monitors of human activity. However, hydrogel-based strain sensors are greatly limited by the trade-offs between mechanical performance, electrical sensing properties and diverse structures. In this paper, we present a facile approach for preparing conductive hydrogels with good mechanical properties, outstanding electrical sensing properties and complex structures. First, a low-temperature 3D printer was designed and fabricated in this laboratory to prepare lignosulfonate/poly(vinyl alcohol) hydrogels with complex structures. The hydrogels with a 30° angle between adjacent layers showed the best optimal tensile properties. Then, extremely thin and dense silver layers were prepared by reducing silver on their surfaces using a two-step immersion process to obtain electrical sensing functionality. The process of preparing layers of reduced silver was carefully analyzed according to the Lamer model, and the resulting silver layers showed relatively high conductivity of 8571 S/cm. By changing the print path angles of the upper surface, hydrogels were prepared with different sensitivities and strain ranges. The maximum sensitivity and widest strain range were 177.65 and 0–55%, respectively. The corresponding hydrogels showed durable sensing performances in various tensile strain and long-term cyclic loading and relaxation tests. The sensing mechanisms of the hydrogels were the opening and closing of microcracks in the silver layers located in the print paths and the gaps between adjacent print paths. The sensors composed of conductive hydrogels successfully detected subtle vibrations in strains and various human activities, indicating their great potential for applications in monitors for human health and soft robotics.

Experimental Study on Dynamic Parameters of Calcareous Sand Subgrade under Long-Term Cyclic Loading

A long-term cyclic loading test of calcareous sand from an island reef in the South China Sea was conducted under conditions of unequal consolidation and drainage. The axial cumulative strain and dynamic characteristics of coral sand samples with different physical and mechanical properties were studied under different stress levels, yielding a model prediction formula. The results show that (1) the axial cumulative strain and dynamic elastic modulus increase with increasing vibration times of the cyclic load, whereas the dynamic damping ratio decreases with increasing cyclic vibration time; (2) the axial cumulative strain, elastic modulus, and damping ratio are affected by dynamic stress amplitude, load frequency, confining pressure, consolidation stress, compactness and moisture content; and (3) based on dynamic triaxial test results, the axial cumulative strain model of calcareous sand under cyclic loading and its permanent deformation prediction formula are established by introducing the cyclic stress ratio. Two parameters, the elastic modulus evolution parameter and the damping ratio evolution parameter, are introduced, and the axial cumulative strain is normalized. The results of this research have significance for understanding the long-term deformation and dynamic response of coral sand subgrade soil under cyclic vibration loads.

Long-Term Loading Experimental Research of Prestressed Glulam Beams Based on Creep Influence

In this paper, long-term loading tests of ten prestressed glulam beams were carried out for 45 days, using the loading method of three-point hanging heavy objects. We analyzed the influence of prestressed steel wire numbers and total preloading force on parameters such as failure mode of beams, variation of steel wire stress, and long-term deflection of mid-span. When the total prestress was constant and the number of steel wires was 2, 4, and 6, the total stress of steel wires decreased by 33.07%, 37.26%, and 50.04%, respectively, after long-term testing for 45 days. The ratio of mid-span deflection to total deflection was 61.5%, 54.8%, and 40.9% respectively. When the number of steel wires was constant and the values of total prestress were 0, 3.079 kN and 6.158 kN, the total stress decreased by 28.90%, 36.59%, and 36.63%, respectively, after long-term loading. The ratio of long-term deflection of mid-span to total deflection was 60.7%, 54.8%, and 39.3%, respectively. The more steel wires, the greater the value of total prestress and the loss of total stress, the smaller the mid-span long-term deflection. After the test, we analyzed and fitted the test data to obtain the specific numerical of the creep coefficient θ under normal conditions, and the formula for calculating the long-term stiffness of the glulam beams based on the creep effects is proposed.

Studies on the validity of strain sensors for pavement monitoring: A case study for a fiber Bragg grating sensor and resistive sensor

To address the problems associated with the validity of pavement sensor measurement, a method of combining indoor experiments with finite element (FE) simulations for strain measurement in asphalt pavement is developed in this paper to analyze the validity of strain sensors for practical measurements. First, correlation analysis and one-way analysis of variance (ANOVA) between the simulated strain and the measured strain of the resistive (R) sensor and the fiber Bragg grating (FBG) sensor are developed, and the results show that the strain simulation of asphalt mixtures with sensors by means of FE simulation is feasible for short-term loading tests; therefore, the simulated strain without sensors is considered the true strain. The FBG sensor is more appropriate than the R sensor for use in the measurement of horizontal strain based on the stability of the regression model. Furthermore, creep experiments and FE simulations with a modified Burgers model are developed, and the results demonstrate that the FE simulation is also effective for long-term dynamic loading tests. Finally, the effect of the ratio of the modulus of the FBG sensor to that of the asphalt mixture on the stability of the regression model is analyzed, and the results suggest that the stability worsens as the modulus ratio increases at the same temperature, which could guide the selection of encapsulating materials. Moreover, the research method could also provide resources for studying the validity of sensors under more complex loading modes.

Long-Term Loading Test of Reinforced Glulam Beam

Due to creep characteristics of wood, long-term loading can cause a significant stress loss of steel bars in reinforced glulam beams and high long-term deflection of the beam midspan. In this study, 15 glulam beams were subjected to a 90-day long-term loading test, and the effects of long-term loading value, reinforcement ratio and prestress level on the stress of steel bars, midspan long-term deflection, and other parameters were compared and analyzed. The main conclusions drawn from this study were that the long-term deflection of the reinforced glulam beams accounted for 22.5%, 20.6%, and 18.2% of the total deflection respectively when the loading value was 20%, 30%, and 40% of the estimated ultimate load under the long-term loading. The higher the loading level was, the smaller the proportion of the long-term deflection in the total deflection was. Compared with ordinary glulam beams, the long-term deflection of the reinforced glulam beam was even smaller. Under the condition of the constant loading level, the total stress value of the steel bars decreased by 17.5%, 13.6%, and 9.1%, and the proportion of the long-term deflection of the beam midspan in the total deflection was 26.9%, 24.2%, and 20.6% respectively when the reinforcement ratio was 2.05%, 2.68 %, and 3.39%. With the increase of the reinforcement ratio, the stress loss of the steel bars decreased, and the proportion of the long-term deflection decreased as well. When other conditions remained constant and the prestress level of the steel bars was 0 MPa, 30 MPa, and 60 MPa, the total stress value of the steel bars decreased by 9.1%, 9.4%, and 10.2%, respectively, and the proportion of the long-term deflection in the total deflection was 20.6%, 26.1%, and 64.9%, respectively. With the increase of the prestress value, the stress loss of the steel bars increased, and the proportion of the long-term deflection increased as well.

Time-dependent creep and shrinkage analysis of curved steel–concrete composite box beams

Curved steel–concrete composite box beams are widely used in urban overpasses and ramp bridges as they are light in weight, have large spanning ability and high torsional rigidity. One-dimensional models are generally used to analyze bridge performance due to their calculation and numerical efficiencies. An incremental step-by-step method based on the Kelvin rheological model together with a discrete algorithm in finite element space is adopted in this study to analyze the mechanical behavior of curved composite box beams. A model with a beam finite element with 26 degrees of freedom (DOFs) is proposed for curved composite box beams that considers the constrained torsion, distortion, shear lag, biaxial slip at the interface and time-dependent effects. Furthermore, the experimental results of the long-term loading tests of three curved composite box beams are selected to validate the proposed beam element model. Finally, the proposed model is applied to analyze the long-term behavior of a curved composite box beam that is used in real engineering applications.

Examining mechanical behavior of steel-bamboo composite I-section column under long-term loading

The steel-bamboo composite structure is a novel composite structure, which is characterized by light weight and high bearing capacity of structural components, as well as excellent seismic performance and good thermal performance of fabricated buildings. To facilitate its engineering application and better understanding the deformation law of composite column during design reference period, mechanical properties of steel-bamboo composite column under long-term loads are investigated in this paper. 27 composite I-section columns were designed and fabricated, among which 18 specimens were first investigated in static loading test with steel ratio and slenderness ratio as main parameters, failure modes and effect of each parameter on the axial compression performance was observed and analyzed. Then, a self-balanced loading device was designed to perform long-term loading test on 6 specimens with loading level and steel ratio as main parameters, and the other 3 specimens were adopted to monitor the temperature effect on creep. The creep process of specimens was observed and the nonlinear fitting analysis of creep data was carried out using Burger model. Once the predetermined long-term loading duration was reached, the secondary static loading test were carried out to study the influencing factors of bearing capacity and deformation, as well as their variation laws. Further, calculation formula of yield bearing capacity was derived by analyzing yield conditions of composite column, as two calculation methods of ultimate bearing capacity were proposed based on superposition principle and limit equilibrium theory. Results show that the composite columns have good integrity, high deformation capacity and bearing capacity, which improves significantly with the increase of steel ratio and decreases with the increase of slenderness ratio. Creep deformation become basically stable after 90 days of loading, which can be accurately described by Burger model. After the long-term loading, ductility and ultimate strain of the specimens were obviously increased, while the bearing capacity was not significantly affected. The average error between the theoretical and experimental results of yield and ultimate bearing capacity is smaller than 7%, which indicates that the established calculation methods have high accuracy and are suitable for predicting the bearing capacity of steel-bamboo composite I-section columns. It is proved that scientifically designed steel-bamboo composite columns have reliable bearing capacities and controllable deformations, and can be safely applied to practical projects.

Pressure-compacted and spider silk-reinforced fibrin demonstrates sufficient biomechanical stability as cardiac patch in vitro.

The generation of bio-/hemocompatible cardiovascular patches with sufficient stability and regenerative potential remains an unmet goal. Thus, the aim of this study was the generation and in vitro biomechanical evaluation of a novel cardiovascular patch composed of pressure-compacted fibrin with embedded spider silk cocoons. Fibrin-based patches were cast in a customized circular mold. One cocoon of Nephila odulis spider silk was embedded per patch during the casting process. After polymerization, the fibrin clot was compacted by 2kg weight for 30min resulting in thickness reduction from up to 2cm to <1mm. Tensile strength and burst pressure was determined after 0weeks and 14weeks of storage. A sewing strength test and a long-term load test were performed using a customized device to exert physiological pulsatile stretching of a silicon surface on which the patch had been sutured. Fibrin patches resisted supraphysiological pressures of well over 2000mmHg. Embedding of spider silk increased tensile force 1.8-fold and tensile strength 1.45-fold (p < .001), resulting in a final strength of 1.07MPa and increased sewing strength. Storage for 14 weeks decreased tensile strength, but not significantly and suturing properties of the spider silk patches were satisfactory. The long-term load test indicated that the patches were stable for 4weeks although slight reduction in patch material was observed. The combination of compacted fibrin matrices and spider silk cocoons may represent a feasible concept to generate stable and biocompatible cardiovascular patches with regenerative potential.

Experimental and numerical study of long-term behaviour of timber-timber composite (TTC) connections

Long-term sustained load (push-out) tests under an indoor uncontrolled environment were conducted on twelve specimens (in seven groups/types) of Timber-timber composite (TTC) joints with up to two identical specimens of each TTC joint type. The TTC joints were fabricated by connecting either Laminated veneer lumber (LVL) or Glued laminated timber (GLT) joists to Cross laminated timber (CLT) slab panels. Types of joists (LVL and GLT), size of coach screw shear connectors, type of shear connection (screws with or without adhesive) between the joists and CLT slab thickness and orientation (i.e., lengthwise, or crosswise) with respect to the loading direction were the main variables in the experimental program. The laboratory tests revealed that the creep coefficient after 420 days is in the range of 1.05 to 3.90 and the CLT slab orientation has minor and CLT thickness has major influence on the creep coefficient of TTC joints (for a given service load over loading capacity ratio). A creep model was calibrated against the experimental slip-time curves and the model was used to predict the creep coefficient for the 50-year design life of the structure. It was shown that more than 50% of the creep in the TTC joints occur in the first year of loading. Additionally, a 3D coupled Finite element (FE) model implemented as user defined subroutines in ABAQUS/Standard was validated against the long-term test results and accordingly input parameters for coupled long-term FE analysis of the LVL-CLT and GLT-CLT joints (with screw shear connectors) are recommend. The validated FE model was used to further confirm effect of the CLT panel thickness and orientation on the long-term slip-time and creep coefficient of the TTC joints.

Shear modulus of ionomer interlayer: Effects of time, temperature and strain rate

The transparent ionomer material SentryGlas® or SG for short is widely used as interlayers in laminated glass and is also a potential adhesive in laminated connection. The material’s shear modulus is of practical interest before glass breakage because it allows a shear transfer between glass components. Due to the viscoelasticity behaviour of the material, its modulus is significantly dependent on the working conditions. This work investigates the effects of temperature, time, and strain rate on SG’s shear small-strain modulus from the point of practical application. The shear property at room temperature is obtained from the results of shear tests. Based on the viscoelastic theory, the time and temperature dependence within the actual application range is quantitively determined from DMA measurements. The glass transition temperature associated with modulus decline is then recalibrated. The effect of strain rate is also qualitatively measured. Finally, the time-temperature dependent equations of shear modulus are applied to numerical simulation of a long-term loading test on framed ionomer interlayer laminated glass.

Long-Term Cyclic Loading Tests for Offshore Pile Foundations Based on Hydraulic Gradient Modeling

Abstract Long-term dynamic response of pile foundations for offshore engineering has attracted more and more attention. The deflection accumulation and the natural frequency offset of the soil-pile system are two vital issues for the safety of offshore projects. However, the research on this topic is deficient because of the lack of effective test approaches. In this study, a new device was developed to investigate the long-term lateral cyclic response of a soil-pile system based on hydraulic gradient modeling. The downward seepage force was employed to create a supergravity field of high g-level. A long-term laterally cyclic loading unit was developed to simulate the cyclic loads under high g-level condition. This device was validated by comparing the results of a short-term centrifuge model test. The device was successfully applied into long-term cyclic loading tests of pile foundation. The test results showed that long-term cyclic loading induces the densification of the sand, the accumulative deflection of the piles, and the natural frequency variations of the soil-pile system. The limitations of 1 g-level model test in characterizing the long-term dynamic response of soil-pile system were clarified by comparing the results from 1 g-level and 100 g-level long-term tests.

Transient thermal response of a thermally activated pile during a long-term maintained load test at Lambeth College, London

The energy pile test at Lambeth College, London was first reported just over 10 years ago; however, the original appraisal of the test only looked at the thermal response at the end of extended cooling and heating stages, and did not examine the transient response within these stages or in the daily cyclic thermal loading stage that followed. This paper revisits the earlier results, allowing for concrete creep under sustained load and using a revised strain-dependent pile modulus. The strong influence of the latter and the possible importance of stiffness changes during load reversal is highlighted. Detailed evaluation of the long-term transient results suggests that within a given thermal loading interval the thermal effects are similar, but there is some further discrepancy in the results, possibly due to an underestimation of the concrete creep. This re-evaluation has mitigated pile head displacement and increased thermal stress in the cooling phase, with the reverse occurring in the heating phase, and appears to be in better agreement with back-analysis reported by others. The transient response within the thermal loading stages illustrates that the development of the thermal stress–displacement response is related to the time for heat to diffuse through the pile body – around 3 days in this case.

Centrifuge modelling of pipe-soil interaction in clay with crust layer

Seabed in regions, such as the Gulf of Guinea and North West Shelf of Australia, may exhibit a crust layer where the undrained shear strength can be an order of magnitude higher than that of the immediately underlying sediment. This can complicate design of steel catenary risers, where fatigue depends on the cyclic vertical stiffness of the pipe-soil interaction. Potential punch-through of the riser into the underlying soft soil may invalidate design assumptions based on the pipe-soil stiffness within the crust layer. The long-term evolution of pipe-soil stiffness within the crust layer, which exhibits similar properties to an over-consolidated soil, is also poorly understood. This paper describes centrifuge model tests undertaken in a clay sample with a crust layer, simulating the punch-through process of a pipe under load control and investigating the pipe-soil stiffness during long-term cyclic loading tests under displacement control. Results confirm that the potential for punching-through the crust layer depends strongly on the relative ratio of pipe diameter to crust layer thickness. The long-term evolution of pipe-soil stiffness showed a steady increase after an initial remoulding stage in contractile soils (normally consolidated and lightly over-consolidated), but a steady reduction in the heavily over-consolidated, more dilatant, crust. The magnitude of pipe-soil stiffness changes (during both remoulding and reconsolidation) is governed by the over-consolidation ratio of the soil and the amplitude of the cyclic displacements. This study provides insights on the relevant cyclic stiffness to consider when assessing SCR fatigue life in over-consolidated soils and soils exhibiting a superficial crust layer.

Short-term and long-term longitudinal load tests of wooden rods

The purpose of the experimental studies was to verify the reliability of the method for solving the stability problem of centrally compressed rods, taking into account the creep of wood. To achieve this goal, it is necessary to have a diagram of deformations of real wood and experimental data on the behavior of rods of the same wood under continuous load. Experimental studies include short-term axial compression tests of small specimens with recording of load-deformation diagrams, as well as short-term and long-term tests of cores of different flexibility for central compression, the description of which is presented in this article. The authors of the paper obtained the values of critical stresses under longitudinal bending for wooden rods with different lengths and flexibility. These results have a high convergence with the values obtained as a result of theoretical calculations.

Long-term performance of lightweight aggregate reinforced concrete beams

The results of long-term sustained flexural loading tests of 17 large scale reinforced concrete beams fabricated with lightweight aggregate (LWAC) having a density of 1800 kg/m3 are presented. Deflections, strains and crack widths were recorded over a period of 30 years. Subsequently, each beam was tested to failure. The observations are compared to US, European and Chinese standard provisions for long-term concrete behaviour and LWAC concrete design. The dominant conclusion drawn from this study is that, accounting for reduced material properties, the long-term behaviour of LWAC exhibits no differences from that of normal weight concrete (NWC). Existing provisions for addressing long-term behaviour of NWC may be applied to LWAC and remain suitably conservative. Experimental results confirmed the beneficial effects of the presence of compression reinforcement or a wider compression flange (tee beam). Creep effects on deflection and compression strains were more pronounced that those on maximum measured crack width. The paper presents extensive data accumulated over the 30 year long-term load study reported.

Long-term performance of prestressed concrete continuous box girders in the natural environment

A long-term load test performed for 470 days on two two-span prestressed concrete (PC) continuous box girders is reported in this paper. Load types were selected as the test variates, and structural responses such as support reactions, deflections, and concrete strains were monitored. Simultaneously, affiliated experiments such as material strength, creep, and shrinkage tests were conducted to investigate the time-dependent performances of the materials. Data obtained from these tests showed that deflections, strains, and support reactions develop rapidly in the beginning and stabilize afterward; the reactions of mid- and end-supports decline and rise over time, respectively. Time-dependent patterns of deflections and support reactions were analyzed on the basis of an effective modulus method, and a practical calculation method for long-term deflections considering reaction redistributions was proposed. The effects of the service environment on the performance of PC girders were evaluated through an incremental analysis method.

Low embankment dynamic response under vehicle traffic loads in arid-oasis areas

The use of low embankments is of significant concern for ecological protection in arid-oasis areas. Based on the project of Sansha Expressway located in Kashgar City, Xinjiang, China, physical model tests were conducted in this study to investigate the dynamic response of the low embankment as per the effects of road structure, load amplitude, load frequency, load cycle, and moisture content. The dynamic stress is shown to increase with load amplitude while the dynamic elastic modulus decreases with load amplitude under short-term loading. The load frequency slightly influences the soil’s dynamic behavior; higher frequencies can improve the dynamic elastic modulus of the subgrade soil. The moisture content has greater influence on the mechanical properties of the subsoil than that of subgrade layer. The subgrade bears the majority of the traffic load as the stress dissipates to 37% of the whole value on its surface. The number of load cycles has the greatest effect on the dynamic response among the influencing factors tested. The dynamic elastic modulus with the type of long-term dynamic loading is only 40%–52% of that with static loading across the entire depth range. The dynamic stress shows significant accumulation with load cycles over the long-term dynamic loading test and becomes stable after 8 × 104 cycles of loading. An equation is established to quantify the cumulative dynamic stress in the low embankment under long-term dynamic loading conditions.

Laboratory investigations on the mechanical properties of cement treated RAP-natural aggregate blends used in base/subbase layers of pavements

The use of reclaimed asphalt pavement (RAP) matrials in road construction, maintenance, and rehabilitation projects has a number of economic and environmental benefits. This study evaluates the performance of cement-treated blends prepared by mixing the RAP content with fresh granular (G) material in different percentages and for different curing periods. For this purpose a comprehensive laboratory testing campaign was executed considering the resilient modulus tests, unconfined compressive strength tests, long-term cyclic tri-axial load tests, creep loading tests, and drying shrinkage tests on the blended samples. Analysis of the high-quality test result data demonstrates that applying even a small cement dosage, in the range of 1.5–4.5%, to the blended samples can considerably improve their resilience and strength properties in addition to limiting the accumulative strains occurring under the applied cyclic and sustained creep loading. However, drying shrinkage of the blended sample may be an adverse effect due to the addition of cement content. Additionally, statistical analysis of experimental results shows that resilience and strength properties of the cement-treated blends are significantly dependent on the blend composition and curing period, in addition to cement dosage. A regression model is presented for the estimation of compressive strength of the blended samples, which simultaneously takes into account the blend composition, curing period, and cement dosage.

Analysis of the service life of railway hydraulic dampers considering temperature and loading

Hydraulic dampers are mainly used to maintain high-speed trains in a stable state when running at high speed. In this paper, an equivalent method for the lifetime of dampers is proposed to establish the relationship between the long-term loading test and the mileage in service condition. The performance degradation of the damper at different temperatures is calculated using the long-term loading test carried out under the corresponding temperature conditions. Arrhenius model is applied to convert the degradation under different temperatures, and the sensitive parameter is selected for the conversion of the damper’s performance degradation under different temperatures. An energy method is proposed to solve the problem of the damper lifetime under different loads, so as to achieve the equivalent of the damper between the long-term load test and the mileage in service condition. Finally, taking the anti-yaw damper of a certain type of high-speed train in China as the research object, the lifetime equivalent method proposed in this paper is applied and the result showed that the anti-yaw damper had remaining service life after 2.4 million kilometers of service.