Sudden Unloading Research Articles

IDENTIFICATION OF HIDDEN DAMAGE IN CONSTRUCTIVE ELEMENTS OF BUILDINGS AND STRUCTURES TO REDUCE THE RISKS OF THEIR DESTRUCTION

Problem statement. In emergency situations, which are activated by dynamic impacts from explosions, fires and earthquakes, it is necessary to monitor the stability of objects to assess their suitability for further operation. Purpose of the study. ensuring the safe operation of buildings and structures using methods, software and technical means for vibroacoustic monitoring of hidden damage in building structures. Methods. Analysis and generalization of data, computer modeling, experimental studies of the structures stability using the vibroacoustic method. Research results. Considered: loss of concrete stability, reinforced concrete, bricks and other elements of load-bearing structures; risk factors manifested in the gradual weakening of load-bearing building structures due to the accumulation of internal damage. Sudden loss of stability of structures occurs after the accumulation of a critical amount of damage in structural elements or the rapid growth of cracks, which are activated by the sudden unloading of extremely stressed structures as a result of any dynamic influences. By modeling the destruction process using the finite element method, it has been established that long before cracks on the surfaces of structures can be visually identified, zones of hidden damage can actively spread inside the wall structures of buildings. The main parameters for monitoring the condition of buildings and structures using visual and vibroacoustic methods have been determined. Due to the fact that the object of research is the partial destruction of buildings and structures (that is, not the root cause, but the consequences of the influence of a complex of negative main factors), two main groups of risk factors for assess the risks of stability loss have been proposed. The first group is associated with the detection of external structural damage identified by visual inspection methods. The second group is associated with the detection of hidden damage, the parameters of which are determined by the vibroacoustic method. The integral risk of stability loss is determined by comparing and selecting the maximum risk in the group. Scientific novelty. The methodology for identifying hidden damage in structural elements of buildings and structures has been further developed, which is distinguished by taking into account the parameters of vibroacoustic control of the destruction consequences and assessing the risks of the structures stability loss. Practical significance. The research results make it possible to improve the method for assessing changes in the structure of the medium as a result of the occurrence of observed and hidden systems of cracks using vibroacoustic monitoring.

A Power-Aware Control Strategy for an Elbow Effort-Compensation Device

This work presents a reactive control strategy for loading and sudden unloading of an elbow effort-compensation device controlled in force. Through this control strategy, in addition to an individual's forearm weight, an external load can be detected and adaptively compensated via a feed-forward force reference, facilitating the execution of arbitrary movements by the wearer. In case of a sudden contact/load loss, a power-aware strategy is implemented to immediately eliminate the portion of external loading in the force reference. The adaptive compensation of the external loads is achieved through an electromyography interface. Instead, to react to sudden load releases, we set a power limit on the tendon, and continuously measure it through an encoder and a load cell connected with the cable. Two sets of experiments are designed to test the proposed load-releasing method on a bench-top setup with 2 kg, and 3.9 kg, and a human subject with 0.5 and 1 kg. Next, the overall scenario including load-compensation and load-releasing are carried out on eight human subjects with 0.5 and 1 kg loads to evaluate the release and compensation time, and the effort reduction with respect to non-powered exoskeleton case. Results show that the average compensation/release time (payload) among subjects is measured as 0.98/0.91 seconds (0.5 kg), and 1/0.86 seconds (1 kg). The average effort reduction among the subjects are also reported as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$66.4\%$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$67.11\%$</tex-math></inline-formula> for 0.5 kg, and 1 kg, respectively.

Effect of pre-confining pressure on unloading-induced coalburst: Insights from distinct element modelling

Coalburst is a violent dynamic failure of coal during underground mining. It is of significance to study failure pattern as well as energy evolution and transition during coalbursts and how they are influenced by pre-confinement. This paper presents unloading-induced coalburst simulations using the distinct element method via a combined static–dynamic loading–unloading strategy. The numerical model is calibrated and validated by comparison with the failure process observed in laboratory tests. The influence of pre-confining pressure on unloading-induced coalburst was numerically investigated from the perspective of crack propagation, fracturing process, failure pattern, and energy evolution. In addition, failure mechanism and energy conversion during coalbursts under different pre-confining pressures are discussed. The results show that the stress change caused by sudden unloading of the pre-confining results in the initiation and development of cracks. Crack density and crack propagation velocity increase with increasing pre-confining pressure. As the pre-confining pressure increases, the coalburst becomes more intense and takes less time to be completed, and the main failure pattern in coalburst transforms from a tensile type to a shear type. In addition, the conversion ratio of kinetic energy and frictional energy during coalbursts increase non-linearly with increasing pre-confining pressure.

Static Topology Optimization and Dynamic Characteristics Analysis of 6000T Compression-Shear Test Machine Frame

This work aims to achieve the 6000t compression-shear test machine frame design with the lightweight. The force condition of the compression-shear test machine frame under limited working conditions is first analyzed, and the static analysis of the compression-shear test machine frame is performed using ABAQUS. Then, taking the volume of the frame of the compression-shear testing machine as the constraint condition, the topology optimization of the compression-shear testing machine frame is performed using the variable density method of topology optimization, and the model is reconstructed accordingly. Finally, not only the static characteristics of the frame before and after optimization but also the modal characteristics of the frame before and after optimization and the dynamic characteristics after sudden unloading are compared and analyzed. The results show that the weight of the frame decreases by 14.5% after optimization, and the maximum static stress of the frame is still less than the yield strength of the material; the maximum displacement is still less than the allowable maximum displacement, which meets the requirements of static strength and stiffness. The natural frequency of each mode is much greater than the working frequency, which meets the requirements of dynamic stiffness. After sudden unloading, the maximum dynamic stress of beams, columns, and base of the frame are less than the yield strength of materials, which meets the requirements of dynamic strength.

Failure Analysis and Prevention for Tower Crane as Sudden Unloading

Failure Analysis and Prevention for Tower Crane as Sudden Unloading

Crosstalk opposing view: Independent fusimotor control of muscle spindles in humans: there is little to gain.

Crosstalk opposing view: Independent fusimotor control of muscle spindles in humans: there is little to gain.

Multi-objective Optimization of Three-shaft Gas Turbine for Sudden Unloading

Considering the requirements of economy, emission and dynamic performance of three-shaft gas turbine for sudden unloading, the multi-objective optimization of three-shaft gas turbine performance was carried out. Firstly, the simulation model of three-shaft gas turbine was established, and the accuracy of the model was verified by comparing with the design point parameters. Then, the research of multi-objective optimization of the economy, emission and dynamic performance for sudden unloading is carried out based on the three-shaft gas turbine model, and the TOPSIS method was used to select the optimal point in the Pareto solution set. This paper provides a theoretical basis for the performance improvement and optimization design of the three-shaft gas turbine.

A new experimental apparatus for sudden unloading of gas-bearing coal

To study the characteristics of the deformation and failure of coal during a coal and gas outburst, we developed an experimental apparatus that subjected coal samples to sudden unloading under various gas conditions. The apparatus consisted of a coal sample cell, a fast pressure unloading system, a monitoring and control system, and a gas filling system. The gas filling system was equipped with safe, explosion-proof solenoid valves for gas pressure unloading and an accurate strain chip for measuring sample strain. The apparatus was designed to probe the influence of the gas on coal and gas outbursts. The study focused on the mechanism by which high-pressure gas influences coal and gas outbursts, leading to the deformation characteristics of the sudden unloading process. The experimental results showed that the plastic deformation of coal samples in the presence of methane gas can be divided into three stages: compression induced by increased pressure, expansion induced by adsorption, and deformation or failure induced by sudden unloading. The results of tests performed in the presence of helium revealed that the deformation could be divided into just two stages: a compression stage induced by increased pressure and a recovery stage induced by sudden pressure unloading. Coal samples were found to deform much more rapidly and to a greater degree in methane than in helium, and the expansive deformation and failure of the coal sample become increasingly apparent with increasing initial gas pressure. Overall, our results demonstrate that the experimental apparatus developed here provides a feasible means to test the characteristics of the coal damage and gas migration that occur during a coal and gas outburst.

Violent paroxysmal activity drives self-feeding magma replenishment at Mt. Etna

A new sequence of eruptions occurred at Mt. Etna volcano during the first half of 2017, after almost 8 months of quiescence. These episodes had low-to-mild intensity and markedly differ from the violent paroxysms occurred at the Voragine Crater (VOR) during December 2015 and May 2016. Despite the general weak explosive nature of the eruptions, the activity during 2017 revealed unusually complex dynamics of magma ascent and interaction. Detection and investigation of such dynamics required a multidisciplinary approach in which bulk rock compositions, crystal chemical zoning, diffusion chronometry and ground deformation data have been combined. Bulk rock major and trace elements suggest that the 2017 magmas followed a differentiation path similar to that experienced by magmas erupted at Mt. Etna during the 2015–16 eruptions at VOR. Olivine core compositions and zoning patterns indicate the presence of multiple magmatic environments at depth that strictly interacted each other through some episodes of intrusion and mixing before and during the 2017 eruptive events. Timescales retrieved from diffusion chronometry on olivine normal and reverse zoning correlate well with the ground deformation stages detected through geodetic data and associated models, thus allowing to track the evolution through time of the 2017 volcanic activity. Combination of all petrological and geodetic observations supports the idea that dynamics of magma transfer driving the eruptive episodes of 2017 have been a direct consequence of the violent eruptions occurred at VOR on May 2016, which boosted the ascent of new magma from depth and improved the efficiency of the plumbing system to transfer it upward to the surface. We propose a mechanism of self-feeding replenishment of the volcano plumbing system during 2017, where magma recharge from depth is triggered by sudden unloading of the magma column consequential to the violent paroxysmal activity occurred on May 2016 at VOR.

Mesoscopic structure PFC∼2D model of soil rock mixture based on digital image

Soil-rock mixture (S/RM) is a very complex discontinuous medium material, which is a multiphase system consisting of high strength rock blocks (Rocks), relatively soft filling components (Soils) and corresponding pores. Because the mechanical properties of various components of soil-rock mixtures under external loads are very different, and there are extremely complex interactions between them. Therefore, the mechanical properties of this geotechnical material (such as stress transfer, failure mode, crack propagation, bearing capacity, etc.) are quite different from those of homogeneous geotechnical materials, and largely depend on the internal structure characteristics of soil-rock mixtures (such as particle size composition, particle shape, particle distribution and arrangement). Due to the complexity of the model, the simulation of its meso-mechanical properties is mostly confined to the random simulation of regular blocks. In this paper, an automatic generation method of PFC∼2D numerical model of soil-rock mixture microstructure based on digital image processing is proposed, and the experimental simulation is carried out with matlab. Thus, the rapid, real and automatic modeling of heterogeneous material microstructure by PFC∼2D software is realized. The PFC∼2D numerical calculation model of soil-rock mixtures is established. The results show that when the stone content is 80%, the analysis should be caused by the large amount of rock, which leads to the large internal voids, and the sudden unloading between the rock and the rock during compaction and then the structural reorganization.

Feedforward neural control of toe walking in humans.

Activation of ankle muscles at ground contact during toe walking is unaltered when sensory feedback is blocked or the ground is suddenly dropped. Responses in the soleus muscle to transcranial magnetic stimulation, but not peripheral nerve stimulation, are facilitated at ground contact during toe walking. We argue that toe walking is supported by feedforward control at ground contact. Toe walking requires careful control of the ankle muscles in order to absorb the impact of ground contact and maintain a stable position of the joint. The present study aimed to clarify the peripheral and central neural mechanisms involved. Fifteen healthy adults walked on a treadmill (3.0kmh-1 ). Tibialis anterior (TA) and soleus (Sol) EMG, knee and ankle joint angles, and gastrocnemius-soleus muscle fascicle lengths were recorded. Peripheral and central contributions to the EMG activity were assessed by afferent blockade, H-reflex testing, transcranial magnetic brain stimulation (TMS) and sudden unloading of the planter flexor muscle-tendon complex. Sol EMG activity started prior to ground contact and remained high throughout stance. TA EMG activity, which is normally seen around ground contact during heel strike walking, was absent. Although stretch of the Achilles tendon-muscle complex was observed after ground contact, this was not associated with lengthening of the ankle plantar flexor muscle fascicles. Sol EMG around ground contact was not affected by ischaemic blockade of large-diameter sensory afferents, or the sudden removal of ground support shortly after toe contact. Soleus motor-evoked potentials elicited by TMS were facilitated immediately after ground contact, whereas Sol H-reflexes were not. These findings indicate that at the crucial time of ankle stabilization following ground contact, toe walking is governed by centrally mediated motor drive rather than sensory driven reflex mechanisms. These findings have implications for our understanding of the control of human gait during voluntary toe walking.

Threshold position control of anticipation in humans: a possible role of corticospinal influences.

Sudden unloading of preloaded wrist muscles elicits motion to a new wrist position. Such motion is prevented if subjects unload muscles using the contralateral arm (self-unloading). Corticospinal influences originated from the primary motor cortex maintain tonic influences on motoneurons of wrist muscles before sudden unloading but modify these influences prior to the onset and until the end of self-unloading. Results are interpreted based on the previous finding that intentional actions are caused by central, particularly corticospinal, shifts in the spatial thresholds at which wrist motoneurons are activated, thus predetermining the attractor point at which the neuromuscular periphery achieves mechanical balance with environment forces. By maintaining or shifting the thresholds, descending systems let body segments go to the equilibrium position in the respective unloading tasks without the pre-programming of kinematics or muscle activation patterns. The study advances the understanding of how motor actions in general, and anticipation in particular, are controlled. The role of corticospinal (CS) pathways in anticipatory motor actions was evaluated using transcranial magnetic stimulation (TMS) of the primary motor cortex projecting to motoneurons (MNs) of wrist muscles. Preloaded wrist flexors were suddenly unloaded by the experimenter or by the subject using the other hand (self-unloading). After sudden unloading, the wrist joint involuntarily flexed to a new position. In contrast, during self-unloading the wrist remained almost motionless, implying that an anticipatory postural adjustment occurred. In the self-unloading task, anticipation was manifested by a decrease in descending facilitation of pre-activated flexor MNs starting ∼72ms before changes in the background EMG activity. Descending facilitation of extensor MNs began to increase ∼61ms later. Conversely, these influences remained unchanged before sudden unloading, implying the absence of anticipation. We also tested TMS responses during EMG silent periods produced by brief muscle shortening, transiently resulting in similar EMG levels before the onset and after the end of self-unloading. We found reduced descending facilitation of flexor MNs after self-unloading. To explain why the wrist excursion was minimized in self-unloading due to these changes in descending influences, we relied on previous demonstrations that descending systems pre-set the threshold positions of body segments at which muscles begin to be activated, thus predetermining the equilibrium point to which the system is attracted. Based on this notion, a more consistent explanation of the kinematic, EMG and descending patterns in the two types of unloading is proposed compared to the alternative notion of direct pre-programming of kinematic and/or EMG patterns.

Experimental investigations on the transformation-induced plasticity in a high tensile steel under varying thermo-mechanical loading

Transformation-induced plasticity (TRIP) also known as transformation plasticity (TP) occurs during solid state phase transformation in the case of applied stress and may lead to irreversible macroscopic distortions in steel components after heat treatment. Particularly, in the context of cost-efficient hot forging, where heat treatment is integrated in the process chain, various complex stress states can occur during the cooling phase due to irregular part geometry, temperature gradients and local differences in the deformation history. Varying local temperature, unsteady stress state or even sudden unloading during the transformation can have a strong impact on the resulting TRIP strain. Thus prediction of the final distortions in hot formed steel components becomes challenging. For this reason process simulation based on the finite element (FE) method offers great opportunities for the accurate virtual process design, reducing time- as well as cost-intensive trial and error cycles. However, a realistic FE-simulation requires reliable mathematical models as well as detailed thermo-mechanical material data. In order to improve the modelling of the material behavior in a hot forging and quenching process, physical simulations for particular process-related time-force-temperature profiles have been carried out on a uniaxial thermo-mechanical testing machine. The relative dilatation of the steel specimens for several applied stresses as well as for the case of sudden unloading have been recorded and evaluated for both compressive and tensile loads. It has been shown that other process parameters (e. g. heating strategy) also have a significant influence on the resulting TRIP strains.

Biting intentions modulate digastric reflex responses to sudden unloading of the jaw.

Reflex responses in jaw-opening muscles can be evoked when a brittle object cracks between the teeth and suddenly unloads the jaw. We hypothesized that this reflex response is flexible and, as such, is modulated according to the instructed goal of biting through an object. Study participants performed two different biting tasks when holding a peanut half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut half only (single-split task), and in the other task, they were asked to split both the peanut and the chocolate in one action (double-split task). In both tasks, the peanut split evoked a jaw-opening muscle response, quantified from electromyogram (EMG) recordings of the digastric muscle in a window 20-60 ms following peanut split. Consistent with our hypothesis, we found that the jaw-opening muscle response in the single-split trials was about twice the size of the jaw-opening muscle response in the double-split trials. A linear model that predicted the jaw-opening muscle response on a single-trial basis indicated that task settings played a significant role in this modulation but also that the presplit digastric muscle activity contributed to the modulation. These findings demonstrate that, like reflex responses to mechanical perturbations in limb muscles, reflex responses in jaw muscles not only show gain-scaling but also are modulated by subject intent.

Task-dependent control of the jaw during food splitting in humans

Although splitting of food items between the incisors often requires high bite forces, rarely do the teeth harmfully collide when the jaw quickly closes after split. Previous studies indicate that the force-velocity relationship of the jaw closing muscles principally explains the prompt dissipation of jaw closing force. Here, we asked whether people could regulate the dissipation of jaw closing force during food splitting. We hypothesized that such regulation might be implemented via differential recruitment of masseter muscle portions situated along the anteroposterior axis because these portions will experience a different shortening velocity during jaw closure. Study participants performed two different tasks when holding a peanut-half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut-half only (single-split trials) and, in the other, to split both the peanut and the chocolate in one action (double-split trials). In double-split trials following the peanut split, the intensity of the tooth impact on the chocolate piece was on average 2.5 times greater than in single-split trials, indicating a substantially greater loss of jaw closing force in the single-split trials. We conclude that control of jaw closing force dissipation following food splitting depends on task demands. Consistent with our hypothesis, converging neurophysiological and morphometric data indicated that this control involved a differential activation of the jaw closing masseter muscle along the anteroposterior axis. These latter findings suggest that the regulation of jaw closing force after sudden unloading of the jaw exploits masseter muscle compartmentalization.

The effects of general warm up, specific warm up and taping on electrical activity of lower limb's muscles in reaction to sudden unloading while walking

Background Ability of balance maintenance and postural control is important to prevent falling and injuries when balance perturbations occurs during walking. Injuries due to unloading of the base of support in walking and running such as ankle sprain has high incidence. Poor muscle function may put the joint in unstable condition. The association of the unloading and poor muscle function in the ankle joint may increase the rate and the intensity of the ankle sprain. Various techniques have been applied to maximize the joint’s stability. The objectives of this study was to compare the effects of general warming, local warming and taping of the ankle joint on the electrical activity of few muscles during a sudden unloading of the base of support during walking.

Motor control and position sense: action-perception coupling.

According to previous studies, muscles become active in response to deviations from a threshold (referent) position of body segments. To test the hypothesis that corticospinal pathways set and reset the referent position in a task-specific way, we evaluated corticospinal influences at wrist positions established before and after voluntary motion as well as before and after involuntary motion elicited by the sudden removal of a load. Although tonic electromyogram (EMG) levels at pre- and post-unloading wrist positions were different, the corticospinal influences and thus the referent wrist position remained the same. These influences and the referent position changed, however, when subjects voluntarily moved their wrists to the other position. Thus, referent control strategies underlying the two types of motor actions are fundamentally different. We also tested the hypothesis that somatosensory afferents inform the brain about the deviation (P) of body segments from the centrally set referent position, R. To identify the actual position (Q) of body segments and form the position sense (PS), the central and afferent signals are combined. The PS rule was confirmed by demonstrating that: subjects are able to successfully reproduce involuntary changes in position elicited by sudden unloading of wrist flexors by making voluntary wrist movements; successfully reproduce elbow joint positions under different constant loads; subjects may not be aware of elbow flexion elicited by tendon vibration until a certain limit (no-motion illusion). In conclusion, R and P are additive components of PS and, contrary to conventional assumptions, PS is independent of sense of effort or efference copy.

Action–perception coupling in kinesthesia: A new approach

According to recent findings, intentional motor actions are controlled by resetting the referent position, R, at which neuromuscular elements, including reflexes, begin to act. It is suggested that somatosensory afferents inform the brain about the deviation (P) of body segments from the centrally set referent position. To perceive the actual position (Q) of body segments and form the position sense (PS), the central and afferent signals are combined: Q=R+P. In previous studies, the R has been shown to remain invariant during involuntary changes in the wrist position elicited by sudden unloading of muscles, suggesting that only the afferent component is responsible for the PS during this reflex. In contrast, the central PS component, R, is predominantly responsible for PS during intentional motion in isotonic conditions. We tested the hypothesis that the R and P are interchangeable PS components such that involuntary changes in wrist position elicited by the unloading reflex can easily be reproduced by making intentional changes in wrist position in isotonic conditions, in the absence of vision. The PS rule also suggests that PS is independent of sense of effort, which was tested by asking subjects to reproduce elbow joint angles under different constant loads. We also tested the hypothesis that tendon vibration may elicit motion that may not be perceived by subjects (no-motion illusion). These hypotheses were confirmed in three experiments. It is concluded that the R and P are additive components of PS and that, contrary to the conventional view, PS is independent of the sense of effort or efference copy. The PS rule also explains kinesthetic illusions and the phantom limb phenomenon. This study advances the understanding of action–perception coupling in kinesthesia.

Increased neck muscle activity and impaired balance among females with whiplash-related chronic neck pain: A cross-sectional study

To investigate neck muscle activity and postural control in patients with whiplash-associated disorder compared with healthy controls. Cross-sectional study with convenience sampling. Ten females with whiplash-associated disorder (age 37.7 years (21-58), neck pain > 2 years and neck disability index (NDI) > 10) and 10 healthy female controls (age 35.9 years (21-53), NDI < 6). Surface electromyography measured muscle activity of the anterior scalene, sternocleidomastoid, neck extensors and upper trapezius muscles, expressed as mean relative activity related to maximum voluntary electromyography (%MVE). On a force plate, 3 balance tasks (Romberg stance with open and closed eyes, 1-legged stance) and a perturbation task with sudden unloading, were performed. The total area, areas from slow and fast components, and range of displacements were calculated from decomposed centre of pressure anterior-posterior and medial-lateral signals. During balance tasks with closed eyes and one-legged stance, the relative mean activity of all 4 muscles was significantly increased in whiplash-associated disorder compared with healthy controls. Postural sway was also significantly increased. Increased neck muscle activity and increased postural sway during simple balance tasks indicate disturbed sensory feedback patterns in people with whiplash-associated disorder, which may have negative consequences when performing daily activities.

Handle height and expectation of cart movement affect the control of trunk motion at movement onset in cart pushing

As unexpected sudden unloading of the trunk may cause low-back injury, the objective of the present study was to investigate whether handle height and the expectation of cart movement in pushing affect trunk control at movement onset. Eleven healthy male participants pushed a 200-kg cart with handles at shoulder and hip heights. The cart would suddenly move when externally released (externally triggered condition) or when static friction was overcome (self-initiated condition). Before self-initiated cart movement, trunk stiffness and muscle activity were significantly higher than before an externally triggered onset at comparable pushing force. Lower muscle activity and trunk stiffness at shoulder height compared with the hip height before the onset resulted in higher trunk inclination after the onset. In conclusion, higher preparatory activation of trunk muscles serves to increase trunk stiffness in anticipation of cart movement and may reduce the impact of the perturbation associated with the onset of cart movement. Statement of Relevance: Sudden cart movement in pushing causes an unexpected unloading perturbation to the trunk. This perturbation was shown to cause uncontrolled trunk movement, which may explain how pushing tasks can be associated with low-back injury. Effects of handle height and awareness of the subjects of the possible cart movement suggest directions for prevention.