Effects Of Unloading Research Articles

ASSESSING THE POTENTIAL INFLUENCE OF DIFFERENT WALKING STRATEGIES ON PLANTAR PRESSURE DISTRIBUTION TRIGGERED BY A PORTABLE BIOFEEDBACK-BASED GAIT TRAINING DEVICE

Plantar pressure refers to the interfacial contact pressure between the foot and the supporting surface during daily locomotor activities. Information derived from plantar pressure measures is essential in gait and posture research for diagnosing patho-mechanics associated with the musculoskeletal diseases. In particular, it is compulsory to reduce the abnormally high plantar pressure in people with diabetes for the prevention and treatment of foot ulcerations in this population. In this study, a portable biofeedback-based gait training device is developed to advocate able-bodied subjects to adopt different movement patterns in walking to manipulate the plantar pressure distribution under the foot. Through the simultaneous detection of the plantar pressure pattern and the kinematics of the lower extremity joints, it was revealed that the unloading effects for the plantar site in particular to the lateral forefoot subareas were more abundant through biofeedback-assisted gait alterations than the self-awareness control for gait adjustment. In addition, the corresponding relationship between joint coordination and pressure redistribution pattern was obtained, which could potentially be used in gait retraining interventions to correct abnormal plantar pressure patterns in people with diabetes.

Mouse Digit Tip Regeneration Is Mechanical Load Dependent.

Amputation of the mouse digit tip results in blastema-mediated regeneration. In this model, new bone regenerates de novo to lengthen the amputated stump bone, resulting in a functional replacement of the terminal phalangeal element along with associated non-skeletal tissues. Physiological examples of bone repair, such as distraction osteogenesis and fracture repair, are well known to require mechanical loading. However, the role of mechanical loading during mammalian digit tip regeneration is unknown. In this study, we demonstrate that reducing mechanical loading inhibits blastema formation by attenuating bone resorption and wound closure, resulting in the complete inhibition of digit regeneration. Mechanical unloading effects on wound healing and regeneration are completely reversible when mechanical loading is restored. Mechanical unloading after blastema formation results in a reduced rate of de novo bone formation, demonstrating mechanical load dependence of the bone regenerative response. Moreover, enhancing the wound-healing response of mechanically unloaded digits with the cyanoacrylate tissue adhesive Dermabond improves wound closure and partially rescues digit tip regeneration. Taken together, these results demonstrate that mammalian digit tip regeneration is mechanical load-dependent. Given that human fingertip regeneration shares many characteristics with the mouse digit tip, these results identify mechanical load as a previously unappreciated requirement for de novo bone regeneration in humans. © 2021 American Society for Bone and Mineral Research (ASBMR).

Study on the Triaxial Unloading Creep Mechanical Properties and Damage Constitutive Model of Red Sandstone Containing a Single Ice-Filled Flaw

The freezing method is an effective approach for constructing coal mine shafts through water-rich soft rock strata. The frozen wall produced by this technique stops the movement of water and offers temporary support, ensuring the safety and stability of the shaft working face. However, most frozen rock masses generally comprise ice-filled flaws and frozen intact rock. A frozen fissured rock mass undergoing the long-term effects of excavation unloading under in situ stress gradually accumulates damage or creep deformation over time; this damage is mainly responsible for the significant deformation, cracking and even instability of the frozen rock wall. To study the creep characteristics of ice-filled fissured rock masses under unloading conditions, a series of conventional triaxial compression tests and triaxial unloading creep tests were performed on ice-filled flawed red sandstone specimens from the Shilawusu mine in Northwest China using a self-developed DRTS-500 subzero rock triaxial testing system. In conjunction with the experimental data, the instantaneous strength and deformation characteristics of the rock specimens were analyzed, and their creep deformation characteristics and damage evolution characteristics were discussed. In this paper, based on the nonlinear creep characteristics of frozen rock, fractional calculus theory and damage theory, a new nonlinear creep damage model of frozen fissured red sandstone was defined in series with the improved Burgers model and elastoplastic damage model. The proposed creep model can reasonably describe the creep deformation of frozen fissured red sandstone. Classical elastoplastic mechanics and creep theory were used to derive the three-dimensional creep damage constitutive equation, and the results of creep experiments and simulation results of the creep damage constitutive model were very consistent in this study. The new creep model not only reflects the whole creep deformation process of frozen fissured red sandstone but also exhibits better performance than the classical Burger model and improved Nishihara model in describing the primary creep stage and accelerating creep stage. Therefore, the proposed nonlinear creep damage model is suitable for studying the mechanical creep properties of frozen fissured rock under unloading conditions. The results can provide an important reference for the long-term stability assessment of frozen rock walls of coal mine shafts.

3D Finite Element Pseudodynamic Analysis of Deficient RC Rectangular Columns Confined with Fiber Reinforced Polymers under Axial Compression.

This paper utilizes the advanced potential of pseudodynamic three-dimensional finite-element modeling to study the axial mechanical behavior of square and rectangular reinforced concrete columns, confined with fiber reinforced polymer (FRP) jackets and continuous composite ropes in seismic applications. The rigorous and versatile Riedel-Hiermaier-Thoma (RHT) material model for concrete is suitably calibrated/modified to reproduce the variable behavior of characteristic retrofitted columns with deficient internal steel reinforcement detailing, suffering nonuniform local concrete cracking and crushing or bulging and bar buckling. Similarly, the 3D FRP jacket or rope confinement models may account for damage distribution, local fracture initiation and different interfacial bonding conditions. The satisfactory accuracy of the reproduced experimental stress-strain envelope behavior enables the analytical investigation of several critical design parameters that are difficult to measure reliably during experiments. Additional parametric analyses are conducted to assess the effects of steel quality. The significant variation of the field of developed strains on the FRP jacket at the ultimate and of the developed strains and deformations on steel cages among different columns are thoroughly investigated. This advanced analytical insight may be directly utilized to address missing critical parameters and allow for more reliable FRP retrofit design of seismic resistant reinforced concrete (RC) columns. Further, it allows for arbitrary 3D seismic analysis of columns (loading, unloading, cyclic or loading rate effects or preloading) or addresses predamages.

Large-scale shaking table test on seismic behaviour of anti-slide pile-reinforced bridge foundation and gravel landslide: a case study

This study aimed to illustrate the seismic response and instability process of a double-row anti-slide pile-reinforced bridge foundation and gravel landslide. Selecting the gravel slope of Jiuzhai Valley along the under-construction Chengdu-Lanzhou high-speed railway as the site prototype, large-scale shaking table tests were first conducted at a similitude ratio of 1:70, with sine waves and El Centro waves as the seismic wave inputs. The amplitudes of the input seismic waves were increased, while the acceleration, dynamic earth pressure, and strain distribution were monitored in the gravel landslide. The dynamic response patterns of anti-slide pile-reinforced bridge foundations in gravel landslides were illustrated. The front-row and back-row anti-slide piles should be a reasonable distance from the bridge foundation. The response acceleration manifested an elevation amplification effect with an increasing elevation in the slope behind the anti-slide piles. Back-row anti-slide piles reinforcing the bridge foundation can reduce the effect of landslide thrust on the bridge foundation and maintain a uniform distribution of earth pressure behind the bridge foundation, mitigating seismic effects. The dynamic earth pressure peaked at the top of the bridge foundation and then decreased along the depth. The back-row anti-slide piles displayed greater resonance coupling and unloading effects before and after reaching the load-carrying limit, respectively. In seismic strengthening design involving bridge foundations and gravel landslides, when the earthquake-induced resonance coupling effect on inclined, loosely packed land masses is fully considered, pre-reinforcement measures (e.g., high-pressure grouting and anchor spraying) should be carried out on the gravel slopes.

Shear behaviour of sand-concrete interface with side post-grouting considering the unloading effect

Side post-grouting bored-pile foundations are widely used in geotechnical engineering. However, few studies have focused on analyzing the shear behaviour of soil and concrete while considering the grouting process. A total of 48 groups of self-developed, large-scale direct shear experiments were conducted to investigate the shear behaviour of sand-concrete interface with side-grouting, focusing on the effect of soil stress unloading and the on-site roughness measurements of bored pile. The influence of the grouting volume V, unloading rate α and interface roughness R on the shear behaviour are discussed, and the grouting modes and shear band deformation are investigated. The experimental results indicated that the grouting volume has a limited effect on improving the shear strength, and an optimal grout quantity will exist in post-grouting engineering. In addition, the unloading has a non-negligible influence on the grouting modes.

Acute respiratory muscle unloading improves time-to-exhaustion during moderate- and heavy-intensity cycling in obese adolescent males

Obesity significantly impairs breathing during exercise. The aim was to determine, in male obese adolescents (OB), the effects of acute respiratory muscle unloading, obtained by switching the inspired gas from ambient air (AIR) to a normoxic helium + oxygen gas mixture (HeO2) (AIR → HeO2) during moderate [below gas exchange threshold (GET)] and heavy [above GET] constant work rate cycling. Ten OB [age 16.0 ± 2.0 years (mean ± SD); body mass index (BMI) 38.9 ± 6.1 kg/m2] and ten normal-weight age-matched controls (CTRL) inspired AIR for the entire exercise task, or underwent AIR → HeO2 when they were approaching volitional exhaustion. In OB time to exhaustion (TTE) significantly increased in AIR → HeO2 vs. AIR during moderate [1524 ± 480 s vs. 1308 ± 408 (P = 0.024)] and during heavy [570 ± 306 s vs. 408 ± 150 (P = 0.0154)] exercise. During moderate exercise all CTRL completed the 40-min task. During heavy exercise no significant differences were observed in CTRL for TTE (582 ± 348 s [AIR → HeO2] vs. 588 ± 252 [AIR]). In OB, but not in CTRL, acute unloading of respiratory muscles increased TTE during both moderate- and heavy-exercise. In OB, but not in CTRL, respiratory factors limit exercise tolerance during both moderate and heavy exercise.

Prevention and treatment of pulmonary congestion in patients undergoing venoarterial extracorporeal membrane oxygenation for cardiogenic shock.

Cardiogenic shock is still a major driver of mortality on intensive care units and complicates ∼10% of acute coronary syndromes with contemporary mortality rates up to 50%. In the meantime, percutaneous circulatory support devices, in particular venoarterial extracorporeal membrane oxygenation (VA-ECMO), have emerged as an established salvage intervention for patients in cardiogenic shock. Venoarterial extracorporeal membrane oxygenation provides temporary circulatory support until other treatments are effective and enables recovery or serves as a bridge to ventricular assist devices, heart transplantation, or decision-making. In this critical care perspective, we provide a concise overview of VA-ECMO utilization in cardiogenic shock, considering rationale, critical care management, as well as weaning aspects. We supplement previous literature by focusing on therapeutic issues related to the vicious circle of retrograde aortic VA-ECMO flow, increased left ventricular (LV) afterload, insufficient LV unloading, and severe pulmonary congestion limiting prognosis in a relevant proportion of patients receiving VA-ECMO treatment. We will outline different modifications in percutaneous mechanical circulatory support to meet this challenge. Besides a strategy of running ECMO at lowest possible flow rates, novel therapeutic options including the combination of VA-ECMO with percutaneous microaxial pumps or implementation of a venoarteriovenous-ECMO configuration based on an additional venous cannula supplying towards pulmonary circulation are most promising among LV unloading and venting strategies. The latter may even combine the advantages of venovenous and venoarterial ECMO therapy, providing potent respiratory and circulatory support at the same time. However, whether VA-ECMO can reduce mortality has to be evaluated in the urgently needed, ongoing prospective randomized studies EURO-SHOCK (NCT03813134), ANCHOR (NCT04184635), and ECLS-SHOCK (NCT03637205). These studies will provide the opportunity to investigate indication, mode, and effect of LV unloading in dedicated sub-analyses. In future, the Heart Teams should aim at conducting a dedicated randomized trial comparing VA-ECMO support with vs. without LV unloading strategies in patients with cardiogenic shock.

Analytical model of the deformation response of bedding slopes to excavation on the basis of Mindlin’s strain solution

To reveal the deformation law and mechanism of bedding slopes under excavation unloading, an unloading rebound model of slope deformation is established on the basis of Mindlin’s strain solution, and a bedding-slip model of slope deformation is deduced on the basis of the Kalhaway constitutive model. Combining the two models, this study presents a computational model for the deformation response of bedding slope excavation. This model can reflect the mechanism of excavation unloading and the characteristics of rock mass structure. The reliability of the model is verified by comparing the calculated results of the analytical model with the experimental results in the laboratory. On this basis, the influences of excavation angle, excavation depth, and stratum thickness are analyzed by using several calculation examples. The calculation results show that the deformation induced by excavation increases with the increase in excavation angle or depth and decreases with the increase in stratum thickness. The excavation response of bedding slopes is mainly affected by the effect of unloading and the slip of the structural plane. Moreover, the unloading effect is controlled by the amount of excavation, and the rebound deformation of slopes is approximately linearly correlated with excavation volume. Bedding slip is affected by many factors because the increase in excavation angle or the amount of rock stratum leads to the increase in slip deformation. The proposed model can provide a basis for the deformation mechanism of bedding slopes under excavation.

Failure Mechanisms and Mitigation Measures of the G1006 Electricity Pylon Landslide in the Dam Area of the Jinping I Hydropower Station

On the 29th and 30th of August 2012, more than 100 geological disasters occurred near the dam area of Jinping I Hydropower Station, Xichang, China due to local intense rainstorm. The upper part of the left valley slope, which is 500 m upstream of the dam, failed and slid, exposing the C-pile of the G1006 electricity pylon and threatening the entire power transmission lines. Therefore, ensuring the stability of the residual rock masses in the rear of the main scarp and the safety of the G1006 electricity pylon became a primary emergency task. Field geological survey, topographic mapping, and the instability mechanism study of the G1006 electricity pylon landslide were conducted. Results revealed that the G1006 electricity pylon landslide (here after represented as the G1006 EPL) occurred in the middle block cut by two faults, and the rock masses had inferior mechanical properties. Intense unloading effects and frequent microseism events during the dam construction relaxed the rock mass structural planes and reduced the geotechnical parameters. Under the influence of intense rainfall, the failure mechanisms of the slope were a combination of slope surface effect from rainfall and lateral erosion and undercutting of the slope toe by high flooding. The residual rock masses and landslide deposits around the pylon foundation were unstable under natural conditions and during the rainstorm. To prevent the continuous damage and exposure of C-pile foundation and ensure the stability of pylon foundation, the mitigation measure of “retaining piles + platform + retaining wall” was arranged. Moreover, a check dam was added to stabilize the slope toe and prevent the landslide deposits, which are the starting material source of debris flow that threatens the safety of the arch dam.

The Acute Hemodynamic Unloading Effects of Angiotensin Receptor-Neprilysin Inhibition in Patients Receiving Intravenous Vasoactive Therapy

Purpose PIONEER-HF established the safety of the angiotensin receptor-neprilysin inhibitor (ARNI), sacubitril-valsartan (S/V), in patients hospitalized with acute decompensated heart failure after achieving hemodynamic stability. We have previously shown that transitioning patients in ADHF with low cardiac output directly from intravenous (IV) vasoactive (i.e. vasodilators or inotropes) drugs to ARNI can be done safely with tolerance to one-month follow-up. Here we further characterize the acute hemodynamic impact of ARNI therapy after patients have been optimized on IV vasoactive therapy. Methods A single-center, retrospective analysis of all patients with HFrEF (EF Conclusions ICU patients can be successfully bridged from vasoactive IV therapy to oral ARNI with sustained improvement in cardiac index garnered from vasoactive agents. We also observed improvement in PAPI along with maintenance of LV/RV unloading with ARNI. These encouraging findings merit prospective validation of ARNI compared to more commonly used oral vasodilators in ICU care.

Effects of Strain Rate and Dwell Time on Fatigue Damage Accumulation in Aerosol Jet Printed Nanosilver Traces on Flex

Sintering of aerosol jet printed nano-Ag traces leads to nanoporous structures that are macroscopically brittle, but the reduction in strain localization offered by the support of a flexible polyimide substrate means that the traces are still quite flexible and can withstand much higher strains than the free-standing structures. The resulting behavior under cyclic loading conditions is very different from what we are used to for conventional microelectronics interconnects. As a part of an ongoing systematic effort, the present work is focused on the effects of strain rates and brief dwells, either between loads or at the maximum strain. Lower strain rates led to more damage per cycle, but the effect of slower unloading was greater than the effect of slower loading. Interestingly, although ongoing work shows an hour-long dwell at zero loads to allow for some degree of recovery and, thus, longer life, dwells on the order of minutes led to more damage per cycle. In contrast, short dwells at the maximum strain led to less damage in subsequent cycling. All of this can be rationalized in terms of an emerging phenomenological picture.

Factors implicating the validity and interpretation of mechanobiology studies in simulated microgravity environments

AbstractSimulated microgravity (s‐μg) devices provide unique conditions for elucidating the effects of gravitational unloading on biological processes and are increasingly being applied for mechanobiology studies. However, without proper characterization of the mechanical environment generated by these systems, the interpretation of results is confounded and limited. Furthermore, the cell culture approaches central to s‐μg experimentation introduces new factors that can fundamentally affect results, but these are currently not addressed. It is essential to understand the complete culture environment and how constituent conditions can individually and synergistically affect cellular responses in order to correctly interpret results, otherwise outcomes may be misattributed to the effects of microgravity alone. For the benefit of the growing space biology community, this article critically reviews a typical s‐μg cell culture environment in terms of three key conditions: fluid‐mediated mechanical stimuli, oxygen tension and biochemical. These and the implications of other experimental variables for biological analysis are categorically discussed. A new set of controls is proposed to properly evaluate the respective effects of these conditions in s‐μg culture, along with a reporting matrix and potential strategies for addressing the current limitations of simulated microgravity devices as a platform for mechanobiology research.

Problems of creating a promising biaxial bogie of a shunting diesel locomotive

Recently, in domestic and foreign practice, shunting and industrial diesel locomotives with hydraulic transmission are replaced by diesel locomotives with electric transmission and the number of axles from 2 to 4, which allows reducing the cost of fuel and repair. Accordingly, the problem arises of creating a biaxial bogie of a shunting diesel locomotive with power transmission, as much as possible unified with a triaxial non-pedestal bogie with a wheel diameter of 1050 mm. The analysis of a possible version of a biaxial shunting diesel locomotive revealed the insufficiency of the existing scientific backlog for a rational choice of design solutions. The need has been established to conduct research on irregularities of access tracks of industrial enterprises to determine the effect of dynamic unloading of diesel locomotive axles on coupling properties and to study its horizontal dynamics by modeling methods with experimental verification of the result on diesel locomotives already produced in order to determine typical options for cross-link nodes between the bogie and the body. For a traction drive with a rigid gear wheel, when assessing the load of nodes under the action of a dynamic moment in the drive, it is impossible to accept the assumption of the shockless nature of the processes in the traction drive and neglect the wheel slippage along the rail, which makes it impossible to correctly model dynamic processes in the drive, and to use previously known empirical regularities for the design of suspension devices for traction electric motors fail according to the layout of the nodes. In order to reduce possible errors in the design of a new individual traction drive for shunting diesel locomotives in conditions of insufficient scientific reserve, it is proposed to choose the option with an elastic gear wheel and suspension with articulated movable links already tested on mainline diesel locomotives that do not have special shock-absorbing elements. Variants of such a suspension are proposed, in which the traction motors are moved relative to the bogie frame with a short length between the axes of the upper and lower hinges, as well as the possibility of its unification with pendulum and spring traverse suspensions. A patent for an invention and two patents for a utility model were obtained for the proposed solutions.

Combined Hind Limb Suspension and sRANK-L on Bone Strength in Mice by Finite Element Analysis: Effects of Anatomic Model Height.

With commercial space travel on the horizon, it is important to understand how the microgravity environment of space effects bone strength. The reduction in skeletal loading is known to cause a rapid loss in bone density. How this corresponds to losses of bone strength is not well known, especially when combined with the osteoporotic effects of aging. In this study, a mouse model of hind limb suspension (HLS) was used to simulate the effects of gravitational unloading. This was combined with soluble receptor activator of nuclear factor kappa beta ligand (sRANK-L), which simulates age related osteoporosis. The proximal region of the tibia in mouse legs was scanned in-vivo pre-treatment as well as at the conclusion of the study with high resolution micro computed tomography (µCT). Subject specific finite element (FE) models were constructed from these 3D images to assess bone strength by simulating mechanical loading on these bone microstructures. Parameters indicative of bone strength obtained from the FE models were bone volume, stiffness, structural efficiency, and the 10th and 90th percentile nodal Von-Mises Stresses. Additionally, a model sensitivity analysis was performed to assess how these parameters varied with changes in anatomic model height. In regards to FE stiffness, HLS resulted in a 31% decline, sRANK-L resulted in a 16.8% decline, and HLS combined with sRANK-L (HLS+sRANK-L) resulted in a 38.6% decline. One interesting finding is that HLS caused a reduction in both bone stiffness and bone structural efficiency, while sRANK-L did not cause changes in bone structural efficiency, suggesting the importance of skeletal loading for maintaining bone health. In addition, sRANK-L combined with HLS caused an additional decline in bone stiffness, but did not further alter bone structural efficiency. In conclusion, this study shows that depending on the cause of osteoporosis, bone strength changes are not necessarily proportional to bone density changes. Thus, it is important to develop new clinical bone assessments beyond the current bone density measurement.Clinical Relevance- These parameters are associated with the microstructural mechanics of bone, and understanding how strength is decreased on a structural level may lead to the development of in-vivo bone strength testing clinically.

Transformation of Mature Osteoblasts into Bone Lining Cells and RNA Sequencing-Based Transcriptome Profiling of Mouse Bone during Mechanical Unloading.

BackgroundWe investigated RNA sequencing-based transcriptome profiling and the transformation of mature osteoblasts into bone lining cells (BLCs) through a lineage tracing study to better understand the effect of mechanical unloading on bone loss.MethodsDmp1-CreERt2(+):Rosa26R mice were injected with 1 mg of 4-hydroxy-tamoxifen three times a week starting at postnatal week 7, and subjected to a combination of botulinum toxin injection with left hindlimb tenotomy starting at postnatal week 8 to 10. The animals were euthanized at postnatal weeks 8, 9, 10, and 12. We quantified the number and thickness of X-gal(+) cells on the periosteum of the right and left femoral bones at each time point.ResultsTwo weeks after unloading, a significant decrease in the number and a subtle change in the thickness of X-gal(+) cells were observed in the left hindlimbs compared with the right hindlimbs. At 4 weeks after unloading, the decrease in the thickness was accelerated in the left hindlimbs, although the number of labeled cells was comparable. RNA sequencing analysis showed downregulation of 315 genes in the left hindlimbs at 2 and 4 weeks after unloading. Of these, Xirp2, AMPD1, Mettl11b, NEXN, CYP2E1, Bche, Ppp1r3c, Tceal7, and Gadl1 were upregulated during osteoblastogenic/osteocytic and myogenic differentiation in vitro.ConclusionThese findings demonstrate that mechanical unloading can accelerate the transformation of mature osteoblasts into BLCs in the early stages of bone loss in vivo. Furthermore, some of the genes involved in this process may have a pleiotropic effect on both bone and muscle.

Mechanical behaviour and damage evolution of Beishan granite considering the transient and time-dependent effects of excavation unloading

A series of triaxial unloading tests on integrated rock, triaxial compression tests and creep tests on unloading-cracked rock were carried out on dry and saturated specimens. The spatial and temporal distribution of microcracking process were obtained with the help of 3D acoustic emission (AE) monitoring system. The experiment data revealed that the saturation had a degeneracy effect on the transient failure strength of integrated granite, and the failure strain at high confinement. The creep characteristics of unloading-cracked granite in lateral direction were found to be greater than in axial direction. At high confinement, the critical strain of creep failure, the creep deformation and the lateral creep strain rate were shown to be degraded by the saturation. When the applied stress surpassed the level of 90% of the compressive strength of unloading-cracked specimen, the variation of AE event showed a consistent tendency with the evolution of lateral strain, and the AE events were found to be mainly accumulated along the macroscopic failure surface. The AE signal was noticed to be more sensitive than the macroscopic deformation to the occurrence of creep failure. Before the appearance of large increase in lateral strain, the rapidly increase of AE activity had occurred.

Formation and chemo-mechanical characteristics of weak clay interlayers between alternative mudstone and sandstone sequence of gently inclined landslides in Nanjiang, SW China

Sedimentary rocks with alternating mudstone and sandstone sequence are susceptible to landslides even under very gentle slope inclinations. Continued occurrences of such landslides in Nanjiang, South Western part of China, bring out the necessity for more understanding of their failure mechanism. In this study, we present a series of investigations on the physical, chemical, and mechanical characteristics of weak interlayers during their formation at landslide slip surfaces, both in macro- and microscales. Our field investigations at these deep-seated landslides revealed the presence of extensive weak interlayers of clay in between alternating sandstone mudstone formations. Tectonic processes combined with loading and unloading effects and strain incompatibility might lead to the argillization of rock by easing water intake, which leads to the formation of these weak layers. We studied how changes in material microstructures, mineral composition, and ionic concentrations during the formation process (argillization) might cause a fall in mechanical strength. We found that degradation of mechanical resistance takes place even at the initial stages of argillization and shear strength of the weak layers decreases proportionally to the time of saturation and argillization.

Dose-dependent skeletal deficits due to varied reductions in mechanical loading in rats

Reduced skeletal loading leads to marked bone loss. Animal models of hindlimb suspension are widely used to assess alterations in skeleton during the course of complete unloading. More recently, the effects of partial unloading on the musculoskeletal system have been interrogated in mice and rats, revealing dose-dependent effects of partial weight bearing (PWB) on the skeleton and skeletal muscle. Here, we extended these studies to determine the structural and functional skeletal alterations in 14-week-old male Wister rats exposed to 20%, 40%, 70%, or 100% of body weight for 1, 2, or 4 weeks (n = 11–12/group). Using in vivo pQCT, we found that trabecular bone density at the proximal tibia declined in proportion to the degree of unloading and continued progressively with time, without evidence of a plateau by 4 weeks. Ex vivo measurements of trabecular microarchitecture in the distal femur by microcomputed tomography revealed deficits in bone volume fraction, 2 and 4 weeks after unloading. Histologic analyses of trabecular bone in the distal femur revealed the decreased osteoblast number and mineralizing surface in unloaded rats. Three-point bending of the femoral diaphysis indicated modest or no reductions in femoral stiffness and estimated modulus due to PWB. Our results suggest that this rat model of PWB leads to trabecular bone deterioration that is progressive and generally proportional to the degree of PWB, with minimal effects on cortical bone.

Strength-weakening effect and shear-tension failure mode transformation mechanism of rockburst for fine-grained granite under triaxial unloading compression

The occurrence of rockburst is closely related to initial high stress state and excavation unloading effects. To investigate the effects of three-dimensional (3D) stress state and unloading rate on the rockburst mechanism, triaxial unloading compression tests were performed on cubic fine-grained granite specimens (50 mm × 50 mm × 50 mm). Two unloading rates (0.08 and 40 MPa/s) were employed in the tests with seven initial 3D confining pressures (5, 10, 20, 30, 40, 50, and 60 MPa). For comparison, triaxial and biaxial compression tests were conducted at the same initial confining pressures. The results show that, under the same confining pressures, the variations in peak strength of granite specimens are in the following order: triaxial compression test > biaxial compression test > triaxial unloading compression test under low unloading rate > triaxial unloading compression test under high unloading rate, which indicating that unloading induces an obvious strength-weakening effect on fine-grained granite. In the triaxial unloading compression test with the same unloading rate, a higher confining pressure results in a greater rock bearing strength and stress drop. More energy will be stored in the rock, and the rockburst intensity will be more serious when the rock is destroyed. When the confining pressure is constant, the lower unloading rate is beneficial for improving the rock bearing strength, and more elastic energy will be accumulated and stored in the rock. Once rockburst occurs, the energy released by rock is larger and the degree of rockburst is stronger. The failure mode of fine-grained granite specimens in triaxial unloading compression tests transforms from shear-tension failure to tension failure with increasing confining pressure, which is the main reason for the weakening of rock strength. Establishing a strength criterion for 3D rock considering the unloading effect is a key for understanding the mechanism of rockburst.