Amplitude Loading Research Articles

Cumulative deformation behavior of GRS bridge abutments under cyclic traffic loading

This paper presents an experimental study on reduced scale geosynthetic reinforced soil (GRS) abutment models subjected to cyclic traffic loading, aimed at investigating the influences of cyclic load amplitude, self-weight of bridge superstructure, and reinforcement vertical spacing on the cumulative deformations. The GRS abutment models were constructed using sand backfill and geogrid reinforcement. A static load was first applied to account for the self-weight of bridge superstructure, and then the cyclic loads were applied in several phases with increasing amplitude. The results indicate that significant cumulative footing settlement under cyclic loading mainly occurs within the first few hundred loading cycles, and the settlement increases with increasing cyclic load amplitude. The cyclic load amplitude and reinforcement vertical spacing have significant impacts on the cumulative deformations of GRS abutments under cyclic loading. The maximum facing displacement under cyclic loading occurs near the top of the wall. The cyclic load has a greater impact on the reinforcement strains near the upper middle reinforcement layers, while it has a smaller impact on the lower reinforcement layers.

Cyclic performance of geosynthetic-encased granular pile with tire chips and aggregates mixture in soft soil – A model study

The performance of encased granular piles subjected to heavy cyclic loading presents a significant concern in the current context. Meanwhile, global waste tire management poses a major challenge because it has a detrimental effect on the environment. To address both difficulties, this research utilizes recycled tire chips derived from end-of-life tires (ELTs) and substituting traditional aggregates in granular pile construction. This study summarizes laboratory model tests investigating the performance of geosynthetic encased granular piles designed for soft soil improvement under vertical cyclic loading. The composition of the granular pile comprised (25 % tire chips + 75 % aggregates). Various cyclic loading parameters were scrutinized, including the selection of encasement material and the best configuration for granular piles, cyclic loading frequency (f), cyclic loading amplitude (qca), length-to-diameter (L/D) ratios, granular pile end conditions, and strength of surrounding soft soil. The novel feature of this research is the evaluation of the cyclic induced settlement (Sc) – excess pore water pressure (Pexc) coupled performance for all considered factors and its effects on the encased granular piles improved soft ground under vertical cyclic loading. Key findings include ordinary granular piles (OGP) illustrated optimal performance when subjected to lower frequency and amplitude loading, smaller L/D ratios, and end bearing conditions. The provision of Combi-grid encasement notably improved the cyclic performance of granular piles by substantially reducing the cyclic induced settlement (Sc) on improved soft beds across all examined factors. This research also discusses the increased cyclic stress on the surrounding soft soil initiated excess pore water pressure (Pexc) development and is reduced to a greater extent with the help of Combi-grid encasement across all test cases.

A Non-Global Traversal Method for Dynamic Load Rapid Localization and Identification

Dynamic load localization and identification technology is very important in the structural design and optimization of aircraft. This paper proposes a non-global traversal method (NTM) for the fast positioning and recognition of dynamic loads on continuous beams. This method separates the load’s position and amplitude information in the modal space. Then, it constructs an interpolation function about position information, and converts load positioning to solving the zero point of the interpolation function. After determining the position of the dynamic load, the amplitude of the dynamic load is recognized. This method does not need to traverse all the position points globally, thereby greatly improving the efficiency of load positioning. Numerical simulations and experiments show that compared with the original variable separation fast positioning method (VSRPM), this method improves the calculation efficiency by more than 80% while maintaining the same recognition accuracy. NTM is a new method of great application value.

Low-high-low cyclic performance of screw connections for fibre-polymer composites

This study examines the cyclic performance of screw connections for joining fibre polymer composite members. Cyclic tensile experiments were conducted on connection specimens consisting of a flat panel (FP) and a square hollow section (SHS) at a right angle. A low-high-low (LHL) cyclic loading program was employed, consisting of seven varying-amplitude loading sequences spanning over 10,361 cycles, in order to simulate wind actions according to relevant standards. Four types of specimens, connected using screws or/and adhesive bonding, were examined, under three load amplitude levels corresponding to 50%, 80%, or 100% of the monotonic ultimate load. All specimens loaded under the LHL amplitude levels of 50% and 80% of the monotonic ultimate load survived after all LHL cycles. These survived specimens were subsequently loaded to determine their retention capacity. The results revealed that no less than 89% of the monotonic strength was preserved. In the LHL loading process, the specimens with adhesive bonding showed more severe degradation of stiffness than their screw-only counterparts, due to the initiation of delamination near the bonded side edges on the FP in the early stage of cycles, with further propagation as the number of cycles increased. The screw-only specimens also demonstrated comparable retention capacity in comparison to the adhesively-bonded specimens.

Study on Shear Slip Characteristics of Sandstone Plane Joints under Normal Dynamic Load Disturbance

Abstract Rock joints are susceptible to slip instability due to dynamic load disturbances such as blasting, earthquakes, and fracturation. A series of direct shear tests under the dynamic load were conducted on sandstone plane joints using the RDS-200xl. The work investigated the effects of normal static loads and normal dynamic-load frequencies and amplitudes on plane joints. Besides, the following items were proposed, that is, the peak-to-valley response rate, shear velocity vibration dominant frequency, shear-stress reduction coefficient, and discrete element numerical simulation method for plane-joint direct shear tests. The results were as follows: (1) The normal dynamic load frequency played a role in attenuating the shear stress amplitude with a threshold value of 0.5 Hz. (2) The shear velocity of the plane joint was completely controlled by the high normal dynamic load frequency. Their vibrational dominant frequencies were identical. (3) The amplitude of shear stress increased, and the median stress decreased with the increased normal dynamic load amplitude. The reduction-coefficient equation for sandstone plane joints was proposed to evaluate the shear stress under the normal dynamic load disturbance. (4) The shear-stress hysteresis phenomenon existed in the plane joints under the normal dynamic load, which required excessive shear displacements to reach peak shear strength. The peak shear displacement increased with the increased normal static load. Numerical simulations and indoor tests showed that high- and low-shear-velocity regions were the main reason for shear-stress hysteresis. The findings are conducive to revealing the shear destabilization mechanism of rock joints under dynamic load disturbance.

3D analytical modeling of moving‐loading induced pile‐group behavior in stratified cross‐anisotropic poroelastic water‐saturated soils based on two stage theory

AbstractThe aim of this paper is to propose a two‐stage theory‐based analytical method for the dynamic performance of pile groups in layered poroelastic saturated cross‐anisotropic soils induced by moving loadings. Among them, the free‐field vibrational analysis of saturated soils is performed by the analytical element‐layer approach (ALEA) and Fourier transformation. Based on the free‐field response, the boundary element (BE) solution for the soil resistance at the soil‐pile interface is derived utilizing the two‐stage theory. Simultaneously, the finite element (FE) solutions for the pile shaft resistance and deformation of pile groups are derived based on the Timoshenko beam theory. Finally, the FE‐BE coupled dynamic equation for deformations and internal loadings of the soil‐pile system is obtained. Thereafter, the reliability of the proposed method is validated by comparing with existing solutions and FE data from ABAQUS. Based on the derived solutions, a comprehensive parametric study is performed to examine the effects of loading amplitude, force speed, soft soil‐layer stiffness, soil anisotropy, and pile length on the dynamic responses of pile groups.

Fibrin clot fracture under cyclic fatigue and variable rate loading

Fibrin clot is a vital class of fibrous materials, governing the mechanical response of blood clots. Fracture behavior of fibrin clots under complex physiological load is relevant for hemostasis and thrombosis. But how they fracture under cyclic and variable rate loading has not been reported. Here we conduct cyclic fatigue and monotonic variable rate loading tests on fibrin clots to characterize their fracture properties in terms of fatigue threshold and rate-dependent fracture toughness. We demonstrate that the fracture behavior of fibrin clots is sensitive to the amplitude of cyclic load and the loading rate. The cyclic fatigue tests show the fatigue threshold of fibrin clots at 1.66 J/m2, compared to the overall fracture toughness 15.8 J/m2. Furthermore, we rationalize the fatigue threshold using a semi-empirical model parameterized by 3D morphometric quantification to account for the hierarchical molecular structure of fibrin fibers. The variable loading tests reveal rate dependence of the overall fracture toughness of fibrin clots. Our analysis with a viscoelastic fracture model suggests the viscoelastic origin of the rate-dependent fracture toughness. The toughening mechanism of fibrin clots is further compared with biological tissues and hydrogels. This study advances the understanding and modeling of fatigue and fracture of blood clots and would motivate further investigation on the mechanics of fibrous materials. Statement of significanceFibrin clot is a soft fibrous gel, exhibiting nonlinear mechanical responses under complex physiological loads. It is the main load-bearing constituent of blood clots where red blood cells, platelets and other cells are trapped. How the fibrin clot fractures under complex mechanical loads is critical for hemostasis and thrombosis. We study the fracture behavior of fibrin clots under cyclic fatigue and monotonic variable rate loads. We characterize the fatigue-threshold and viscous energy dissipation of fibrin clots. We compare the toughness enhancement of fibrin clots with hydrogels. The findings offer new insights into the fatigue and fracture of blood clots and fibrous materials, which could improve design guidelines for bioengineered materials.

Dynamic soil–structure interaction of a three-span railway bridge subject to high-speed train passage

In this study, the influence of Soil-Structure Interaction (SSI) on the dynamic behavior of a three-span concrete slab railway bridge with integrated retaining walls is investigated. The bridge is subjected to controlled excitations using a hydraulic actuator with different frequencies and load amplitudes. A 3D model of the railway bridge-soil system is implemented and calibrated using the experimental frequency response functions at each sensor location. A soil-free model is also created to compare with the calibrated model. It is observed that the dynamic behavior of the railway bridge is substantially altered by the presence of the surrounding soils, and neglecting SSI can lead to underestimation and inaccurate results. Additionally, the calibrated model is used for further train-passage analyses. For the studied bridge, neglecting SSI increases the maximum acceleration response of the bridge during high-speed train passages from 5.5 m/s2 up to 14.5 m/s2. It is also shown that the response of the bridge during train passage is predominantly influenced by its first bending mode, with higher modes inducing no discernible effect. Finally, parametric studies are performed in order to study the uncertainties related to the soil properties.

Experimental study on low cycle fatigue behavior of TWIP steel under cyclic shear loading condition

This study investigated the low-cycle fatigue (LCF) behavior of Twinning induced plasticity (TWIP) steel based on the twin bridge shear specimen and cyclic shear loading condition. The cyclic shear experiments were conducted with different strain amplitudes, and we compared their softening and hardening behaviors, cyclic stress-strain curves, and fatigue life. Through studying the crack morphology and the damage mechanism of shear specimens the role of cyclic shear stress on the damage evolution of TWIP under cyclic shear conditions was shown. The results indicate that the cyclic hardening and softening capacities of TWIP steel are highly affected by the strain amplitude. With increasing the load amplitude, the hardening rate increases, and its fatigue life is negatively correlated with the strain amplitude. Meanwhile, the hardening rate is positively correlated with the strain amplitude, and the softening rate is negatively correlated. The magnitude of strain amplitude affects the ratio of cyclic softening and cyclic hardening periods. Through analyzing the obtained microstructure characteristics, it is found that there is a high density of dislocations around the crack zone which causes the final failure of TWIP.

Damage evolution and failure characteristics of a cemented gangue backfill considering direct shear and dynamic tests

Backfill mining is an effective technique that can prevent and control the impact of dynamic disasters which can occur in deep underground coal mines. This technique allows to reduce the surface subsidence and achieve a greener mining. The backfill shear mechanical properties under mining disturbances, are of great significance for ensuring the surrounding rock stability in deep mining. Therefore, studying the shear mechanical properties of the cemented gangue backfill body (CGBB) under coupled dynamic and static loads actions in deep coal mining is of great importance. In this paper, shear mechanical tests and the acoustic emission (AE) technology were used to study the shear mechanical response characteristics of CGBB under low-frequency dynamic normal load conditions. The results show that a dynamic normal load with an amplitude of 5 kN improves the shear CGBB strength, while a dynamic load with an amplitude greater than 10 kN will cause a shear strength deterioration, and the degree of deterioration is positively correlated with the load amplitude. Under low-frequency dynamic normal load conditions, the shear CGBB stress-strain curve exhibits classical characteristics. AE indicate that the main CGBB damage changes from a nonlinear deformation to an elastic stage under low-frequency dynamic normal load conditions. During the shear failure process, the cumulative AE counts are positively correlated with the load amplitude. The crack type identification based on RA-AF shows that the low-frequency dynamic normal load affects the CGBB crack type. Under constant normal load (CNL) conditions, tensile or mixed cracks account for 63.95% and are the main fracture mode. Conversely, under low-frequency dynamic normal load conditions, shear cracks account for more than 60% and are the main failure mode. This research provides a reference for the CGBB stability analysis under complex stress conditions for the case of a deep underground coal mining.

Characterization of crack-bridging and size effect on ultra-high performance fibre reinforced concrete under fatigue loading

The use of ultra-high performance fibre-reinforced concrete (UHPFRC) has become popular owing to its high strength, durability and toughness characteristics compared to conventional concrete. In this work, experimental investigations have been carried out to examine the fracture behaviour of UHPFRC beam specimens under static and fatigue loading with an emphasis on understanding the size effect. Experimental results of both monotonic and fatigue have been utilized to develop a mathematical formulation for predicting crack bridging degradation considering the fatigue loading history. Subsequently, the size effect behaviour on the crack-bridging strength of UHPFRC beams under fatigue loading conditions has been explored. An algorithm has been developed to predict crack propagation as a function of load cycles and to estimate the fatigue life corresponding to any load amplitude, accounting for the size effect. The crack-bridging strength has been observed to decrease with an increase in specimen size, lowering the fatigue life.

Study on shear performance of discontinuous PBL connectors with double holes

Discontinuous perfobond rib shear (PBL) connectors can be applied to fabricated steel-concrete composite beams. In this paper, six push-out specimens were designed to explore the shear performance of PBL under static and repeated loads. Four specimens were used for static push-out test. The remaining two were used for fatigue push-out test. The static load test mainly considers the influence of concrete at steel plate end and post-cast strip on the shear bearing capacity, initial shear stiffness and load-slip curve of discontinuous PBL shear connectors. On the basis of the test results, the calculating formulas of the shear capacity and shear stiffness of discontinuous PBL connectors considering the influence of end bearing, bond group effect and post-cast strip are proposed. The predicted results of the formula are close to the experiment. From the fatigue test results, it is found that the fatigue performance of above specimens with C50 and UHPC is better than that with studs, but the fatigue resistance of discontinuous PBL connectors with UHPC is not increased in proportion to the shear capacity. In addition, the residual slip and shear stiffness of PBL specimens after cyclic loading are related to fatigue load amplitude.

Testing Structural Elements under Multiaxial Loading: A Numerical Model of the Bench to Understand and Predict Complex Boundary Conditions

Airworthiness certification requires proof of structure strength, which is performed generally through a building block approach. To achieve this, representative intermediate-scale experiments generated by test benches are, in general, needed, in addition to material characterization on a coupons scale and structure testing on a large scale. The VERTEX test bench can generate the combined loading of tension/compression-shear-pressure on structural elements and was modelled with Finite Elements to perform virtual testing, representative of its intermediate-scale specificity. The numerous bolted joints of the bench were modelled and their behavior was identified in previous tests, so the model could quantitatively estimate the transfer function of the bench, which is the relationship between the displacements imposed by the jacks and the resulting loads on a given sample. The VERTEX model was identified to represent load shapes and amplitudes based on a training set and was later confronted by a validation set of tests of tension and shear. A model with ideal boundary conditions was also developed for a comparison, but it failed to predict some load shape specificities and did not give any indication of the loading amplitude. Application cases of the developed model are shown to assess a range of virtual testing possibilities.

Study on the damage mechanism and hysteretic performance of fork column-dougong connection in traditional Chinese multi-story wooden structures

Fork column-dougong connection is an essential part in traditional Chinese multi-story wooden structures and its hysteretic performance is critical to the seismic performance of such structures. In this paper, a full-scale model consisting of a fork column and a dougong was fabricated and subjected to the pseudo-static cyclic load. Two types of damage patterns of the fork column were found and their inherent mechanism were revealed. The deformation features and hysteretic curves were studied and discussed. Results show that the constraint-induced and embedment-induced cracks are the two major damages of the connection, which both makes the cracks of column initiate at the interface with the dougong and propagate upwards. The rotation of the fork column accounts for about 90 % of the loading deformation and the proportion grows with the increase of the loading amplitude. The hysteretic curve of the connection is almost symmetric and each hysteresis loop seems fusiform, while the damping coefficient is slightly smaller compared with dougong alone.

NSM-CFRP rods with varied embedment depths for strengthening RC T-beams in the negative moment region: Investigation on high cyclic response

Throughout its service life, structural reinforced concrete (RC) encounters cyclic loads and the load amplitude might always vary, leading to a mixed high and low-cycle failure. This research investigates the high cyclic response of RC T-beams strengthened in the negative moment region using near-surface mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) rods. The distinctive aspect lies in exploring varied embedment depths, including the introduction of half-embedded configurations as an alternative NSM technique. The study comprehensively analyzes load-carrying capacity, failure modes, energy dissipation, ductility, stiffness degradation, and strain behavior, providing insights into the effects of loading rates on structural response. Through an experimental program comparing fully-embedded (BF-D) and half-embedded (BH-D) CFRP rods with a control beam (BN-D) under high-rate cyclic loading, significant increases in ultimate load capacity (21.23% for BH-D and 30.86% for BF-D) are demonstrated despite debonding occurrences. The findings highlight a trade-off between benefits (enhanced ultimate load capacities, improved energy dissipation, and increased stiffness) and drawbacks (reduced ductility and tendencies toward brittle behavior) under high loading rates. Furthermore, a rate-dependent material formula is developed and validated, predicting the flexural strength of the negative moment region in good agreement with experimental results. Finally, this research contributes practical solutions to RC element strengthening challenges and advances understanding of NSM-CFRP-strengthened beam, emphasizing the impact of loading rates on structural response.

A novel uncertainty quantification method for determining deformations and reliabilities of stochastic laminated composite plates with geometric nonlinearity

Stochastic finite element method (SFEM) for uncertainty quantification is widely applied for analysis of structures with intrinsic randomness. For determining the geometrically nonlinear deformations of laminated composite plates with random fields, the existing intrusive SFEMs have the limitations of low applicability, insufficient accuracy, or low efficiency. To this end, this paper proposes a novel and efficient non-intrusive SFEM incorporating direct probability integral method to achieve probability density functions (PDFs) of stochastic responses and reliabilities of laminated composite plates with geometric nonlinearity. Firstly, the von Kármán strain-displacement relation based on the third-order shear deformation theory is employed to model the geometric nonlinearity of laminated plates. The random field is discretized via Karhunen–Loève expansion. Secondly, the probability density integral equation (PDIE) is derived from the new perspective of probability conservation. The proposed non-intrusive SFEM decouples the equilibrium equation and PDIE to compute the response PDFs and reliabilities of uncertain laminated composite plates in a unified way. Moreover, the criterion which can judge the applicability of geometrically nonlinear theory is suggested for performing uncertainty quantification. Finally, comparisons of the results in terms of Monte Carlo simulation and the literature demonstrate the high accuracy and efficiency of the proposed method. For stochastic laminated plates, the response statistical moments vary nonlinearly with linear increase of load amplitude due to geometric nonlinearity, the deflection variability increases and structural reliability decreases with the increase in variability and correlation length of random field, and the stacking angle significantly affects the stochastic nonlinear deflections and reliabilities.

Rock bolts under cyclic loading: Mechanical performance and damage assessment by acoustic emissions

Rock bolt support is one of the key methods to maintain the stability of underground space. The durability of the rock bolt can be weakened by cyclic loading, resulting in premature failure of the underground structures. A comprehensive understanding of the performance of the bolting system under cyclic loading is essential to assess the safety of the bolting system and underground structures. In this context, this study investigated the mechanical performance of rock bolts installed in simulated soft and medium hard rock and subjected to cyclic loading. The Acoustic Emission (AE) technique was used to monitor internal damage to the bolt system under different loading conditions. A cumulative AE energy slope method has been proposed to determine the onset of the macro crack formation stage. The rock bolt demonstrated an extended fatigue life in soft rock, but its condition worsened when implanted in medium hard rock. The cyclic loading resulted in a slight improvement in the load bearing capacity of the soft rock bolt system and a significant deterioration in the medium hard rock bolt counterpart. The main fatigue failure modes of the rock bolt systems were splitting and shear failure of the surrounding rock, and debonding of the interface. Increasing the amplitude of the cyclic loading reduced the macro crack initiation cycle for both rock bolt systems, whereas increasing the frequency had different effects. The damage variable increased with frequency and amplitude for both rock bolt systems. The toughness of the soft rock bolt degraded during cyclic loading, while the high frequency cyclic loading did not modify the toughness of the medium hard rock bolt.

A simplified cyclic shear test for pore water pressure build-up of different soils

Pore water pressure (PWP) build-up essentially takes place in loose, water saturated, coarse-grained soils causing the reduction of effective stresses and soil stiffness (soil liquefaction). Considering the same internal structure (soil fabric), stress level, loading amplitude etc. the response of different soils to external disturbance is different. Therefore, the increase of PWP or tendency to soil liquefaction is dependent on the granulometric properties of soils. This paper reveals a simple cyclic shear test that enables the comparison of the sensitivity of PWP build-up to density changes for different sands. The presented test allows a fast installation of a soil specimen and a subsequent constant volume cyclic shearing within a short time period (ca. 30 minutes). The results successfully confirm the repeatability of the method as well as the dependence of the PWP build-up on the initial relative density and saturation degree. It is also shown that the soil fabric has an essential influence on the build-up of PWP. The method aims to allocate an index value to every tested sand and thus to quantify a sensitivity of different sands to density changes with respect to liquefaction.

Assessment of Ratcheting in SGDHR Pipe Bend based on Linear and Bi-linear Analysis

The possibility of ratcheting in the 200 NB pipe bends of the SGDHR system has been analytically studied. Elastic analysis with a sustained load on internal pressure at 7 bars and deformation-controlled cyclic displacement of 20 mm amplitude is carried out. The results suggest that the outer surface of the pipe bend crown region becomes the potential location for the cyclic accumulation of strain. The ratcheting possibility is also verified using the conventional design standard procedure of RCC MRx and ASME. It is observed that ASME is more conservative than RCC MRx, and predicts lower displacement to initiate ratcheting. Elasto-perfectly plastic analysis carried out at 7 bar pressure and various displacements shows an elastic shakedown at lower displacements. Initiation of ratcheting at 7 bars is predicted at 35 mm and 18 mm displacements at room temperature and 600 ℃, respectively. It is observed that the ratcheting strains stabilize at higher cycles. The magnitude of the stabilized strain and number of cycles increases with an increase in the amplitude of cyclic loading for a prototype pipe bend.

An Approach to Measure Slips in Friction Contacts Using a Self-Powered Sensor

The accumulation of small tangential slip will occur on the contact surfaces of components in mechanical structures when subjected to cyclic loadings, which will lead to system failure. However, the small tangential slip is difficult to measure directly. In this paper, a method for measuring the contact slip by using a self-powered displacement sensor based on triboelectric nanogenerator (TENG) is proposed, and the accuracy is verified by the finite element analysis (FEA). The contact slips for a flat-on-flat contact configuration are measured by this method. When the test piece is subjected to a cyclic tangential load with a frequency of 10Hz, the average obtained slip displacement of the test piece under single load cycle is less than 10μm. Then the finite element simulation of the contact configuration is carried out using software Abaqus 6.14. The analysis results are close to the test results, which verifies the effectiveness of the measurement method. Moreover, the slips of the flat-on-flat contact configuration under different frequency and amplitude of cyclic tangential load are investigated.