Study on the Dynamic Responses of Tunnel Linings with Small Net Distance Under Bias Pressures Caused by Blasting

The mutual influence of the parallel excavation of the tunnel can not be ignored. The control of the distance between the faces of the two tunnels is also a difficult point in the construction process. Taking the Nanshan Tunnel of Tieben Expressway in Liaoning Province as the background, the blasting simulation research was carried out under the conditions of different distances between the tunnel faces. Firstly, based on the field test data, the blasting conditions under different tunnel faces were simulated by numerical simulation. The research shows that: (1) The velocity effect of the preceding hole on the subsequent hole is embodied as: the smaller the distance between the face of the tunnel, the more obvious the velocity effect of the subsequent hole; the velocity effect of the subsequent hole on the preceding hole is mainly reflected in the change of the velocity propagation process; (2) The influence of the leading hole on the displacement of the trailing hole is mainly reflected in the following: the greater the distance between the tunnel faces, the later the peak appears, and the smaller the displacement peak; the influence of the trailing hole on the displacement of the leading hole is mainly reflected in the following: the greater the distance between the tunnel faces, the smaller the peak displacement.

Influence of Aerobic Exercise After Static Stretching on Flexibility and Strength in Plantar Flexor Muscles

Aerobic exercise could improve stretch-induced strength deficits. However, mechanisms of the improvement were unclear. The purpose of the study was to examine the effects of aerobic exercise after static stretching (SS) on flexibility and isometric strength in ankle plantar-flexor muscles. Fifteen healthy males received two interventions after SS of their ankle plantar-flexor muscles for 5 min. One was aerobic exercise for 10-min on a cycling ergometer, and the other was a 10-min rest as a control. Range of motion (ROM) of ankle dorsiflexion, passive torque at terminal ROM, muscle-tendon unit (MTU) stiffness, muscle tendon junction displacement, peak torque of ankle plantarflexion, and the amplitude of electromyography (EMG) were measured. Immediately after the SS, in both interventions, ROM, passive torque, and muscle tendon junction displacement increased significantly (p < 0.05), while MTU stiffness, peak torque, and the amplitude of EMG were significantly decreased (p < 0.05). After 10-min on a cycling ergometer, the decreased peak torque and amplitude of EMG indicated higher values than those before SS (p < 0.05), while MTU stiffness was no change. In conclusion, SS increased ROM because of the decreased MTU stiffness as well as increased tolerance for stretching. Aerobic exercise could increase the muscle strength and amplitude of EMG which decreased after static stretching.

Rock and roll: The influence of bicycle lean on the mechanics of non-seated cycling

When riding off the saddle during climbing and sprinting, cyclists appear to coordinate the rhythmic, vertical oscillations of their centre of mass (CoM) with the side-to-side lean of the bicycle. Is the coordination of these two motions merely a stability requirement, or could it also be a strategy to more effectively generate crank power? Here we combined a kinematic and kinetic approach to understand how different constraints on bicycle lean influence CoM movement and limb mechanics during non-seated cycling. Ten participants cycled in a non-seated posture at a power output of $5\ W\cdot kg^{-1}$ and a cadence of 70 rpm under three bicycle lean conditions: unconstrained on rollers (Unconstrained), under instruction to self-restrict bicycle lean on rollers (Self-Restricted) and constrained in a bicycle trainer (Trainer). Bicycle lean angle in the Unconstrained condition was greater than Self-Restricted and in the Trainer. Vertical CoM displacement, peak vertical crank force, and peak instantaneous crank power in the Unconstrained condition were greater than Self-Restricted but similar to in the Trainer. The amount and rate of energy lost and gained by the rider's CoM in the Unconstrained condition was greater than Self-restricted but similar to in the Trainer. The differences in joint power contributions to total joint power (hip, knee, ankle, and upper body) between conditions were inconclusive. We interpret these results as evidence bicycle lean plays an important role in facilitating the production of high crank force and power output during non-seated cycling by allowing a greater non-muscular contribution to crank power.

Rock and roll: The influence of bicycle lean on the mechanics of non-seated cycling.

When riding off the saddle during climbing and sprinting, cyclists appear to coordinate the rhythmic, vertical oscillations of their centre of mass (CoM) with the side-to-side lean of the bicycle. Is the coordination of these two motions merely a stability requirement, or could it also be a strategy to more effectively generate crank power? Here we combined a kinematic and kinetic approach to understand how different constraints on bicycle lean influence CoM movement and limb mechanics during non-seated cycling. Ten participants cycled in a non-seated posture at a power output of 5 W·kg-1 and a cadence of 70 rpm under three bicycle lean conditions: unconstrained on rollers (Unconstrained), under instruction to self-restrict bicycle lean on rollers (Self-Restricted) and constrained in a bicycle trainer (Trainer). Bicycle lean angle in the Unconstrained condition was greater than Self-Restricted and in the Trainer. Vertical CoM displacement, peak vertical crank force, and peak instantaneous crank power in the Unconstrained condition were greater than Self-Restricted but similar to in the Trainer. The amount and rate of energy lost and gained by the rider’s CoM in the Unconstrained condition was greater than Self-restricted but similar to in the Trainer. The differences in joint power contributions to total joint power (hip, knee, ankle, and upper body) between conditions were inconclusive. We interpret these results as evidence bicycle lean plays an important role in facilitating the production of high crank force and power output during non-seated cycling by allowing a greater non-muscular contribution to crank power.

A nonlinear stiffness and nonlinear inertial vibration isolator

A nonlinear stiffness and nonlinear inertial vibration isolator is proposed in this article. It consists of an inerter, a damper, and spring elements. The nonlinear stiffness characteristic is achieved through a negative stiffness structure based on the diamond-shaped structure that generates nonlinear displacement terms. The nonlinear inertial characteristic is implemented via a geometrical nonlinear inerter that produces a nonlinear acceleration term and a nonlinear quadratic velocity term. An averaging method is developed to analyze approximately the dynamic response. The isolation performance is evaluated using four performance criteria: dynamic displacement peak, force transmissibility peak, isolation frequency band, and high-frequency force transmissibility. The effects of the nonlinear inertial characteristic on the dynamic response and isolation performance are examined. The results show that the backbone curve of the nonlinear stiffness and nonlinear inertial vibration isolator has two changing trends: bending first to the right and then to the left and bending directly to the left. The corresponding frequency response curve displays linear, hardening, and softening characteristics. The isolation performance of the nonlinear stiffness and nonlinear inertial vibration isolator is compared with that of the nonlinear stiffness and nonlinear stiffness and linear inertial ones. It could achieve a smaller force transmissibility peak, lower resonant frequency, and larger isolation frequency band than the nonlinear stiffness one and could have smaller dynamic displacement peak and smaller high-frequency force transmissibility than the nonlinear stiffness and linear inertial one. The proposed nonlinear stiffness and nonlinear inertial vibration isolator achieves a better integrated isolation performance among the three vibration isolators.

Study on Mechanical Response Changes of Pavement to Aircraft Loads with Different Landing Gear Configurations

The finite element model of runway is established based on elastic layered theory and it is used for analyzing the influence on mechanical responses due to changes of main landing gear configurations. The results show that the more the total number of wheels on a main landing gear is, the smaller displacement peak, maximum tensile stress under panel bottom and solid foundation influence depth are in conditions that aircraft loads are equal. When the landing gear spacing is smaller, the displacement curves in the loading area are gentler. The changes of configurations influence little on displacements in regions far away from loading area.

The Dynamic Analysis of Portal Crane with Self-Funnel under Unloading Process

In order to get the displacement response curve of the Portal Crane under four phase of unloading process, the vibration system was built, the vibration differential equation and displacement response expression were deduced, combining with the actual case, the displacement response expression and displacement response curve were deduced in the open grab time 1.5 s and 4 s. The results show that: the unloading process is an important factor affecting the structural vibration; Displacement peak and open grab time are inversely proportional; Increase the open grab time can reduce the vibration of the structure.

Dynamics Response of Rails for Electromagnetic Launcher with Acceleration Load

One of the important problems in developing the railgun is design of the rail and the related elements. The evaluation of the loads on rails is an important task. On operation, the armature runs at an acceleration. The acceleration load can cause more complicated dynamic responses of the railgun. Here, the equations for the forced responses of the rail on acceleration load are developed. Using these equations, the dynamic displacement responses of the rail to the running electromagnetic force for various accelerations are investigated. The results show that the maximum dynamic displacement peak of the rail and its position depends on the acceleration of the running armature. The larger is the acceleration, the more obvious is the increase of the dynamic displacements under condition that the armature velocity is above the critical velocity. The results are useful for design and application of the railgun system.

Research on Earthquake Resistant Materials in Mountain Tunnels Crossing Fault

Seismic damage, such as falling, fissure and dislocation of the lining may happen at location of fault movement. The model of enhancing stiffness and that of adding seismic decrease layer are built by the finite element software MIDAS to compare the two earthquake-resistant methods. Some useful results are obtained: a)The displacement is decreased, but the peak acceleration and first principle stress of the lining are increased in the model of enhancing the lining materials stiffness. b) The displacement, peak acceleration and first principle stress are all decreased in the model of adding seismic decrease layer. c) The max-displacement when using c20 as lining material or adding seismic decrease layer is in the area of fault, but that of the model when using c40 as lining material is in the area of entrance.