Unloading History Research Articles

Quantized impact of rod on plate

In this work, we attacked the problem of elastic impact of a rod on a plate. By considering wave propagation on the plate and along the rod, we established a mathematic model with delay differential equation that governs such impact. The proposed model could consolidate classical impact problems from bead-on-wall, bead-on-plate, to rod-on-wall impact. Solving the governing equation, we obtained a universal loading and unloading history of such impact: the unloading history exhibits quantized plateaus, the values of which are quantitatively provided in the present study. Also, the contact duration also shows a band structure while tuning the compliance of plate. In addition, we analyzed the overshoots leading the plateaus during unloading, which cause the final detachment between the rod and the plate.

Reduced osseointegration in disuse and denervation rat models results from impaired cellular responses to multiscale microstructured titanium surfaces.

Immobilization-induced skeletal unloading results in muscle atrophy and rapid bone loss, thereby increasing the risk of falling and the need for implant therapy in patients with extended bed rest or neuromuscular injuries. Skeletal unloading causes bone loss by altering bone growth and resorption, suggesting that implant performance might be affected. To test this, we focused on early events in implant osseointegration. We used the rat sciatic neurectomy-induced disuse model under two different settings. In Study 1, 16 Sprague Dawley rats (SD) were separated into control, sham operated+cast immobilization, and sciatic neurectomy+casting groups; titanium implants with multiscale microtextured topography and hydrophilic chemistry (modSLA) were inserted in the distal femoral metaphysis. Neurectomy surgeries and casting were performed at the same surgical setting as implant placement; rats were euthanized 4 weeks post-implantation. In Study 2, we established the unloaded condition before implantation. A total of 12 SD rats were divided into control and sciatic+femoral neurectomy groups. A total of 24 days after sciatic and femoral neurectomy surgery, rats received implants. Study 2 rats were euthanized at 4 weeks post-implantation. MicroCT and histomorphometry showed that trabecular bone and osseointegration were reduced when disuse was established before implantation. Osteoblasts isolated from Study 1 sciatic neurectomy tibial bones exhibited impaired differentiation on modSLA culture disks, revealing a possible mechanism responsible for the decreased osseointegration observed in the Study 2 rats. This study addressed the importance of considering the mechanical unloading and muscle function history before implant insertion and suggests that implant performance was reduced due to poor cellular ability to regenerate.

Study of creep test and creep model of hydraulic concrete subjected to cyclic loading

Although the structure of hydraulic concrete depends on the history of cyclic loading and unloading, most studies focus on a single loading and unloading creep test, rendering dubious the assumptions of elastic creep theory. To clarify this situation, we designed uniaxial tensile and compressive cyclic loading and unloading creep tests and implemented them under uniform conditions. The tests were done with two-grade concrete with a water/binder ratio of 0.5, which is typical of a concrete dam project. The results elucidate how creep in sealed dam concrete depends on tensile and compressive cyclic loading and unloading. The Boltzmann superposition model was then used with the Grey Wolf Optimization algorithm to determine the parameters of the eight-parameter tensile and compressive creep model. The results show that the tensile specific creep is about 1.5–2.5 times greater than the compressive specific creep at the same loading age. When the compressive specific creep is superimposed during unloading for either tensile or compressive specific creep, the goodness of fit between the specific creep curve calculated by Boltzmann superposition and the measured specific creep curve exceeds 0.85. The results of this analysis indicate that when the compressive specific creep is superimposed during unloading, the specific creep calculated based on Boltzmann superposition is consistent with the measured specific creep.

Creep properties and constitutive model of salt rocks under a slow cyclic loading path

The construction of compressed-air energy storage (CAES) plants using the huge space created by salt rock mining is a key means of optimizing China's energy structure. The creep properties of salt rock are vital parameters for ensuring the long-term, stable operation of CAES plants. According to the operation mode of CAES plants, creep tests were conducted on salt rock under the influence of slow loading and unloading. The results indicated that the slow loading and unloading history locally changed the creep development process but did not affect the overall creep process. Additionally, the steady state creep rate of the salt rock depended mainly on the magnitude of external stress. Moreover, the creep test was monitored simultaneously using acoustic emission (AE) technology, and the distribution of the AE absolute energy during the initial loading phase and steady state creep phase satisfied the power law. To more accurately describe the creep characteristics of the salt rock under the influence of slow loading and unloading, a full-process creep constitutive model of salt rock was constructed using state variable, fracture expansion factor, and unloading factor.

Reambulation following hindlimb unloading attenuates disuse-induced changes in murine fracture healing.

Patients with bone and muscle loss from prolonged disuse have higher risk of falls and subsequent fragility fractures. In addition, fracture patients with continued disuse and/or delayed physical rehabilitation have worse clinical outcomes compared to individuals with immediate weight-bearing activity following diaphyseal fracture. However, the effects of prior disuse followed by physical reambulation on fracture healing cellular processes and adjacent bone and skeletal muscle recovery post-injury remains poorly defined. To bridge this knowledge gap and inform future treatment and rehabilitation strategies for fractures, a preclinical model of fracture healing with a history of prior unloading with and without reambulation was employed. First, skeletally mature male and female C57BL/6J mice (18 weeks) underwent hindlimb unloading by tail suspension (HLU) for 3 weeks to induce significant bone and muscle loss modeling enhanced bone fragility. Next, mice had their right femur fractured by open surgical dissection (stabilized with 24-gauge pin). The, mice were randomly assigned to continued HLU or allowed normal weight-bearing reambulation (HLU + R). Mice given normal cage activity throughout the experiment served as healthy age-matched controls. All mice were sacrificed 4-days (DPF4) or 14-days (DPF14) following fracture to assess healing and uninjured hindlimb musculoskeletal properties (6-10 mice per treatment/biological sex). We found that continued disuse following fracture lead to severely diminished uninjured hindlimb skeletal muscle mass (gastrocnemius and soleus) and femoral bone volume adjacent to the fracture site compared to healthy age-matched controls across mouse sexes. Furthermore, HLU led to significantly decreased periosteal expansion (DPF4) and osteochondral tissue formation by DPF14, and trends in increased osteoclastogenesis (DPF14) and decreased woven bone vascular area (DPF14). In contrast, immediate reambulation for 2 weeks after fracture, even following a period of prolonged disuse, was able to increase hindlimb skeletal tissue mass and increase osteochondral tissue formation, albeit not to healthy control levels, in both mouse sexes. Furthermore, reambulation attenuated osteoclast formation seen in woven bone tissue undergoing disuse. Our results suggest that weight-bearing skeletal loading in both sexes immediately following fracture may improve callus healing and prevent further fall risk by stimulating skeletal muscle anabolism and decreasing callus resorption compared to minimal or delayed rehabilitation regimens.

Framework architecture of a secure big data lake

This paper presents the architecture of a secure big data lake framework, the foundation of which is the Data Lake Architecture Framework (SDLAF). Security is entrusted to 2 components: Global Monitoring Tool (GMT), Check Sum Controller (CSC). The first one monitors data lake logs, automatically detecting anomalous data, the second one tracks the history of loading, processing, and unloading of file objects in the data lake. The conducted instrumental audit demonstrates both the effectiveness of the architecture and its prospects.

Modeling finite-strain plasticity using physics-informed neural network and assessment of the network performance

Physics-informed neural networks (PINN) can solve partial differential equations (PDEs) by encoding the mathematical information explicitly into the loss functions. In the context of plasticity, discussions of PINN have only focused on small-strain formulations. We present a framework of finite-strain elasto-plasticity for PINN, considering rate-independent isotropic hardening in this work. Details of the model architecture are discussed, including inputs and outputs of the neural network and the construction of the loss function that incorporates equilibrium equations, boundary conditions and constitutive relations. In addition, the overall architecture can learn the constitutive relations from discrete measurements on a stress–strain curve, hence eliminating the need for modeling hardening law in the formulation. We demonstrate the performance of PINN through a numerical example that includes a multi-step loading and unloading history. Moreover, we assess the performance of PINN in terms of its accuracy and robustness under mesh refinement and as a function of the network architecture design. Displacement, Cauchy stress and accumulated plastic strain fields are compared to the finite element results for the same problem for the purposes of this assessment, which is intended to provide insight and guidance for the future designs of PINN for plasticity and related problems in solid mechanics.

Numerical Method for the Deformation Calculation of the Shallow Buried Tunnel Caused by the Excavation of Overlying Soil Based on Soil Rebounding Characteristics

In view of strict deformation control standards, it is important to calculate the shallow tunnel deformation caused by the unloading of overburdened soil for analyzing tunnel structure safety. Due to the obvious nonlinear characteristics and the significantly smaller rebound deformation of soil under unloading conditions, previous numerical calculation methods need to be improved and adapted to the soil deformation characteristics under unloading. Based on the above background, cyclic loading and unloading tests were carried out to study the deformation characteristics of soil under different loading levels, stress history, and unloading levels. Secondly, a unified modulus calculation model was established, especially for the unloading modulus, which considers the unloading stress history and current unloading stress level, and can provide the value of soil modulus in different states. The relationship between the unloading rebound modulus Esc and the rebound modulus Ec0 used in the numerical simulation was studied. Finally, a set of numerical calculation and analysis methods for shallow tunnel deformation caused by overlying soil excavation based on its rebounding characteristics is proposed. The rationality of the method is verified by two examples, and is applied to a practical project.

Numerical Simulation of Creep Behavior for Catheter obtained under Different Unloading Conditions after Proportional Loading of Tension and Torsion

This paper describes the effect of the braids on the creep deformation behavior of a catheter made of soft nylon resin and stainless steel wires. In our previous studies, the creep deformation behavior generated during the loading process has been mainly examined under two or three stages step stress for tension and torsion with different loading history. However, the creep deformation behavior that occurs during unloading process after loading process has not been examined yet. Therefore, in the present study, the creep deformation behavior is investigated focusing on the unloading history for tension and torsion. Especially, in order to reveal the effect of loading history for tension or torsion on the creep deformation behavior generated under step unloading, experiments are performed under different starting times of step unloading and different values of step unloading. Moreover, the numerical simulations of creep deformations when tension or torsion is unloaded after proportional combined loading for tension and torsion are carried out based on the three-element mechanical model that consists of a spring element and a Voight model.

The effects of unloading on drained cyclic behaviour of Sydney sand

New excavation or tunnelling affects the stress state of soils in ground. The change of stress state due to excavation may affect the cyclic behaviour of soils. Cyclic loading, such as traffic and earthquake loading, induced ground deformation may be greater than expected if such effect is not considered. A series of cyclic triaxial tests were performed on Sydney sand with different relative densities. The effect of unloading sequence on deformation of the sand under cyclic loading was simulated by reducing lateral stress in steps between loading cycles. The dependence of strain accumulation on the magnitude of confining pressure reduction and on unloading stress paths was studied. The results indicate that the sand has a memory of stress history and the stress history of such unloading enlarges the strain accumulation during the subsequent cycles, and the greater the reduction of lateral stress, the greater the accumulated strain. Under cyclic loading, the accumulated axial strain could increase nonlinearly or linearly with the ratio of unloading magnitude to initial mean effective stress, depending on the stress state before cyclic loading. The unloading stress paths have limited effects on the final accumulated strain if the initial and final stress states are the same. The variation of strain accumulation direction attributes to the change of average stress ratio resulting from lateral stress reduction, but hardly depends on relative density and unloading stress paths. The strain accumulation direction after unloading roughly agrees with the modified Cam Clay flow rule.

The effects of unloading on undrained deformation of a kaolin clay under cyclic loading

Excavation induced unloading may induce additional shear stress in soils. Such additional shear stress or unloading history may affect the long-term deformation of soft clay under subsequent cyclic loadings. A series of undrained cyclic triaxial tests were performed on a kaolin clay to study unloading effect on cyclic behaviour of the soil. The effect of unloading sequence on deformation of the clay under cyclic loading was simulated by reducing cell pressure in between loading cycles. The dependence of axial strain accumulation on the amplitude of cell pressure reduction and unloading schemes was investigated under different initial confining pressures and cyclic loading frequencies. The results indicate that reducing confining pressure accelerates the strain accumulation during subsequent loading cycles and the greater the reduction in cell pressure, the greater the accumulated strain and excess pore water pressure. Such acceleration of axial strain accumulation after unloading would be attributed to the variation of stress state and the soil's ability in memorizing the unloading history. The direction of strain accumulation is controlled by the shear stress ratio rather than the amplitude of unloading. The final accumulated strain is not much affected by unloading schemes if the samples are the same in the initial confining pressure and the amplitude of unloading. An empirical model was developed to fit the axial strain accumulation both before and after unloading. The effects of cell pressure reduction on the parameters of the empirical model were also analysed.

Leveraging the Rapid Retreat of the Amundsen Gulf Ice Stream 13,000 Years Ago to Reveal Insight Into North American Deglaciation

AbstractWe examine the influence of different deglacial histories of the North American ice saddle, which connected the Cordilleran ice sheet and western Laurentide ice sheet, on the stability of the fast‐flowing Amundsen Gulf Ice Stream, located in the northwestern Laurentide ice sheet. We use a simplified marine‐terminating ice stream model to simulate grounding line retreat and compare our predictions to geologic evidence for the rapid collapse of the Amundsen Gulf Ice Stream at 13 ka. We show that observations of ice stream retreat can be used to distinguish between past ice unloading histories, and that the rapid retreat of the Amundsen Gulf Ice Stream near 13 ka is most dynamically consistent with substantial deglaciation of the North American ice saddle from 13 to 11.5 ka, during the Younger Dryas cooling episode. These results suggest that short‐lived changes in ice stream behavior may be used to reveal larger‐scale and longer‐duration ice sheet dynamics.

Experimental investigation on the reversibility of concrete creep under repeating loads

So far, creep reversibility of concrete has been tested either under single unloading or stepwise (“staircase”) stress histories. However, these investigations do not provide a complete understanding of the creep behavior under repeating stress histories, similar to variable live load histories in usual concrete structures. Typical examples are parking garages, bridges or storage buildings with frequent but still longer-term loadings and unloadings. Therefore, the paper attempts to extend the knowledge on the creep under frequently repeated stresses by testing concrete specimens under various loading and drying conditions. The creep-recovery versus creep ratio over time, considered here as a measure of creep reversibility, was studied within two separate experiments. The first experiment aims to assess the influence of different service stress levels with 30% and 45% of the concrete compressive strength fc as well as different unloading levels of full and partial unloading. The second one focuses on the influence of different hygral conditions of the specimens, namely sealed and unsealed conditions. Regardless of the stress level, the creep-recovery versus creep ratio tends to approach unity after a sufficient number of loading and unloading cycles. Drying conditions show negligible effects on the creep recovery. On the other hand, they have remarkable influence on the proportion of the creep reversible in each loading cycle. Moreover, the basic creep component shows high reversibility under repeating stress histories. The results demonstrate that the recovery behavior under repeating stresses pronouncedly differs from the ones under just sustained stresses.

Degradation of piezoelectric device as an energy harvester under equivalent traffic stress condition

Piezoelectric materials have widely been used for applications in microelectromechanical systems, electromechanical devices, civil infrastructure, in transportation and smart devices. The performance of these devices is largely affected by their loading and unloading history when used in electromechanical applications. Lead Zirconium Titanate (PZT), a specific material of piezoelectric ceramics, which upon repetitive cyclic compressive load undergoes both electrical and mechanical degradation over a period of time and number of cycles. The degradation of piezoelectric properties with the application of cyclic compressive loading and the mechanical fatigue behavior were investigated on PZT-5H.

Creep and Long-Term Permeability of a Red Sandstone Subjected to Cyclic Loading After Thermal Treatments

Long-term experiments were performed on red sandstones after different thermal treatments (25, 300, 700 and 1000 °C) under multi-step loading and unloading cycles and a confining pressure of 25 MPa. Furthermore, to quantitatively analyse the temperature influence on the deformation behaviours of the specimens, the concept of the temperature–strain rate was proposed to describe the relationship between strain and temperature, and the experimental results were corrected to identical temperatures (i.e., 20 °C), to overcome the influence of periodic fluctuations in ambient temperature. The results show that the axial mean temperature–strain rate first increased as temperature increased from 25 to 300 °C and then decreased with increasing temperature, whereas the lateral mean temperature–strain rate decreased with increasing temperature. The total strain was divided into the instantaneous elastic strain, the instantaneous plastic strain, the visco-elastic strain and the visco-plastic strain. The total axial strain increased with increasing deviatoric stress, and the irrecoverable strain increased with increasing loading and unloading history. Furthermore, the total axial strain increased with increasing temperature; specifically, at 1000 °C, it was approximately two times that at 700 °C and three times those at 25 and 300 °C. The instantaneous elastic strain and the instantaneous plastic strain increased approximately linearly with increasing deviatoric stress, whereas the creep strain varied with deviatoric stress in complicated ways at different temperatures. However, under identical deviatoric stress, the instantaneous elastic strain and the instantaneous plastic strain increased slightly as temperature increased from 25 to 700 °C and then increased substantially as temperature reached 1000 °C, whereas the variations in the creep strain, the visco-elastic strain and the visco-plastic strain were dependent on temperature and stress level. Finally, the permeability first decreased slightly as temperature increased from 25 to 300 °C and then increased with increasing temperature.

Blast and Impact Analyses of RC Beams Considering Bond-Slip Effect and Loading History of Constituent Materials

An improved numerical model that can simulate the nonlinear behavior of reinforced concrete beams subjected to blast and impact loadings is introduced in this paper. The layered section approach is based in the formulation, and the dynamic material behaviors of concrete and steel are defined with the use of the dynamic increase factor. Unlike the classical layered section approach that usually gives conservative structural responses because of no consideration of the bond-slip effect, the introduced numerical model takes into account the bond-slip between reinforcing steel and surrounding concrete by changing the bending stiffness EI of elements placed within the plastic hinge length. Since the bond-slip developed after yielding of reinforcing steel is dominant and accompanies fixed-end rotation, the equivalent bending stiffness to be used in the critical region can be evaluated on the basis of the compatibility condition. In advance, the consideration of the unloading and reloading histories of reinforcing steel and concrete makes it possible to exactly trace the structural behavior even after reaching the maximum structural response. Finally, correlation studies between analytical results and experimental data are conducted to establish the validity of the introduced numerical model, and the obtained results show that it is important to consider the bond-slip effect and the loading history of constituent materials.

A deformation-theory based model of a damaged metal matrix composite

A shear-lag and deformation-theory based model for a metal matrix composite reinforced by continuous unidirectional fibres is proposed. The model accounts for fibre and matrix cracking, matrix plasticity, and fibre-matrix interfacial sliding through seven characteristic non-dimensional parameters, which combine geometric, phase and interface properties. It allows arbitrary tensile loading and unloading history along the fibre direction, and predicts the history-dependent elastoplastic displacement, strain, and stress fields in all the fibre and matrix elements. Broken elements may be present initially, or form during the imposed loading history. Non-linear one-dimensional governing differential and algebraic equations are formulated on the basis of the model. A computationally fast solution methodology based on pseudospectral collocation is implemented. The present model is employed to predict the elastic strain profiles in a Ti/SiC composite tape near pre-existing breaks. These predictions agree well with experimental measurements reported in the literature.

Thermal contact conductance and its dependence on load cycling

Heat transfer between contacting surfaces is an important factor in the thermal behaviour of engineering components in turbomachinery and various other areas of technology. Thermal contact conductance (TCC) is a parameter that quantifies this heat flow. Any theoretical prediction of TCC should take into account the effects, if any, introduced by repeated loading and unloading. This study aims to add to the limited volume of work available on this topic in the literature. In particular, the focus of this investigation is machined surfaces that typify the mating surfaces in some turbomachinery applications. Experimental work investigating the effect of loading and unloading history for numerous cycles is presented. An instrumented split tube with in line washers, loaded and unloaded under carefully controlled conditions, was used to measure the TCC of washers made of nickel alloy PE16 and 316 stainless steel. The study also examines the load cycle effect on TCC for a variety of interface surface geometries and pressures that are relevant to turbomachinery applications. The results show that load cycling, beyond the first cycle, has a minimal effect on TCC, in disagreement with other studies in the literature. This observation is seen for variety of surface topographies and maximum contact pressures.

Early Holocene in Gästrikland, east central Sweden: shore displacement and isostatic recovery

In Gästrikland in central Sweden, deglaciation took place c. 11 000 cal. a BP. In the present study the shore displacement during the earliest ice‐free period is investigated by the 14C dating of sediment from isolated lake basins. The shore displacement in Gästrikland includes an initial phase (∼500 years) of rapid regression, followed by a slowing of the relative sea level (RSL) fall to a rate similar to that of the remaining Holocene c. 9250 cal. a BP. The Ancylus Lake stage of the Baltic Sea belongs to the analysed interval. The RSL curve and glacial unloading history are used to separate and quantify elements of isostatic uplift. The postglacial uplift is ∼260 m, of which ∼45 m forms a rapid initial rise, which can be treated as qualitatively separate from the later, slower rise. There is considerable glacial unloading just prior to the deglaciation, but calculations suggest that only a small part of this relates directly to the rapid early Holocene rebound: most unloading is transferred either to uplift immediately prior to the deglaciation or to subsequent Holocene or future uplift. The isostatic rise in Gästrikland occurring between the end of the Younger Dryas stadial and the deglaciation, c. 11 500–11 000 cal. a BP, is estimated to be 100–110 m. Observations and estimations are incompatible with a Weichselian maximum ice thickness much smaller that 3000 m. The lack of glacial unloading during the Younger Dryas has a measurable impact on the Holocene isostatic rebound in Gästrikland, reducing it by an estimated 20–25 m.

Phase transition and influence of phase transitionon spall in α phase Fe-based alloy

This paper provides an investigation of the phase transition and the spalling characteristics induced during shock loading and unloading in an α phase Fe-based alloy. Two kinds of impact set-ups are used. One is referred to as the direct impact, and the other is the reverse impact. The shock Hugoniot and the release stress-volume adiabat of the α phase Fe-based alloy are obtained. And the results show that the history of loading and unloading is determined by the relationship among the phase transition threshold and the reverse transition threshold and the shock pressure. It is found that abnormal spallations happen in the α phase Fe-based alloy sample as the pressure exceeds the phase transition threshold. Such abnormal spalling phenomena are believed to be related to the shocked α→ε phase transition, and a possible reason for the abnormal spalling is discussed also from the interaction process of stress wave.