Amplitude Loading Research Articles

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.

A technique for estimation of cyclic stress-strain curves using an optical strain measurement and its application to additively manufactured AlSi10Mg

The article describes a new accelerated technique for estimation of cyclic stress-strain curves of cyclically stable materials based on a full-field measurement of strain during low-cycle testing on a large amplitude of loading. Any optical method suitable for capturing the strain contours on the curved part of the specimen during cyclic loading is applicable in the approach developed. In this study, the digital image correlation method is used to measure the deformation on the curved part of the specimen. The technique is presented on chosen results of low-cycle fatigue tests performed on AlSi10Mg printed with selective laser melting technology. Proportional loading cases were performed in tension-compression and pure torsion. The case of non-proportional loading was realized in the form of 90 degree out-of-phase tests. The stress-strain behavior is evaluated using a standard procedure and an accelerated technique with a good correlation for the three loading cases.

Experimental Study on Fatigue Performance of Composite Box Girder With Steel Bottom Plate and Corrugated Steel Webs

In order to study the fatigue performance and fatigue failure mode of the composite box girder with steel bottom plate and corrugated webs (SBCSWs), the model beam was made and the constant and variable amplitude fatigue experiment were performed. Combined with ABAQUS finite element software, the strain, deflection, and fatigue failure mode of the composite box girder with SBCSWs were analyzed. The results showed that the slope of the load–deflection curve, the slope of the load–strain curve, and the neutral axis height of the model beam did not change during the constant amplitude fatigue test. In the variable amplitude fatigue test, as the fatigue loading time and fatigue load amplitude increased, the slope of the load–deflection curve and the slope of the load–strain curve of the model beam decreased significantly, while the neutral axis height increased significantly. The section strain of the composite box girder with SBCSWs was consistent with the quasi‐plane section assumption. The fatigue failure of the composite box girder with SBCSWs began at the stress concentration at the junction of the steel bottom plate and the corrugated webs. The fatigue fracture section has typical fatigue fracture characteristics.

A Hybrid ANN Multiaxial Fatigue Model for the Assessment of Fretting Fatigue under Variable Amplitude Shear Loading.

To foresee the life of fretting fatigue with variable shear amplitude loading, a hybrid model incorporating non-local multiaxial fatigue parameters and artificial neural networks (ANN) in conjunction with cumulative damage models has been proposed. The model has been developed based on equivalent stress parameters associated with crack nucleation and Miner's cumulative damage rule-inspired models, enabling the assessment of the impact of variable amplitude loading. To validate the models, fretting fatigue tests conducted under variable shear loading amplitude are employed to compare the predictions of four different models. The results demonstrate that while all models provide satisfactory estimates falling within the two-band limit, the hybrid model combining two ANN models outperforms the others and emerges as the most effective in predicting fretting fatigue life under these conditions. The proposed methodology effectively addresses the inherent complexities of fretting fatigue under variable amplitude stress, while also examining the influence of load block order on material behavior. Additionally, these types of fatigue ANN models have been demonstrating a strong capacity for generalization, making them highly promising for future industrial applications.

Effects of cyclic loading on soil-geogrid interaction characteristics

The benefits of geosynthetic-reinforced soil systems over conventional earth-retaining structures are now well established. These reinforced systems are often subjected not only to static loads, but also to seismic and/or traffic loads, in which case the effects of repeated loading on soil-geosynthetic interaction characteristics should be properly considered. This study investigates the behaviour of a geogrid typically used for soil reinforcement under cyclic pullout loading through load-controlled laboratory pullout tests. To examine the influence of cyclic loading amplitude, number of cycles and static pullout force acting on the geogrid at the onset of cyclic loading, distinct loading patterns are considered. A well-graded residual soil from granite is used as backfill material. A comparison between the cyclic and monotonic pullout response of the reinforcement is then established in order to identify any potential strength loss attributed to cyclic loading. The experimental results show that the ultimate pullout resistance of the geogrid embedded in medium dense residual soil from granite may be adversely affected by cyclic loading. The cumulative cyclic displacements of the reinforcement are more pronounced during the initial loading cycles, but tend to stabilize with the increasing number of cycles when the soil is densely compacted. In the presence of dense soil, the cyclic strains of the geogrid specimen are particularly significant at the front section and almost negligible towards the back end.