Nonlinear feedforward and feedback control design for autonomous underwater glider

Underwater gliders are highly efficient, buoyancy-driven, and winged autonomous underwater vehicles. Their dynamics are multivariable nonlinear systems with unstable internal dynamics and thus their motion control is a significant challenge. To improve the inherent efficiency and enhance the behavior of the underwater glider over a wide operating regime, a nonlinear feedforward and feedback controller was developed. The nonlinear feedforward control design is based on a new stable inversion technique which determines a causal and bounded solution for the unstable internal dynamics. The feedback control law was designed by a quadratic optimal control method. Simulation results show that the derived control system is able to deal with nonminimum phase system and successfully achieves the tracking of planned output trajectories from initial to final conditions. Furthermore, the control effort is very low, which means the glider with limited power storage has longer range and higher endurance.     

Mechanical behavior of tubular T-joint after fire

This paper presents experimental results of the post-fire behavior of tubular T-joints. The research aims at the failure modes and the residual strengths of the T-joints after fire. Three tests of full-scale tubular T-joints are conducted. The first one is carried out to study the mechanical behavior of T-joints under ambient temperature. The other two tests are performed to study the influence of pre-load, heating and cooling phase on residual load-bearing capacity of the tubular T-joints. The test results show that the sustained axial load on the brace has remarkable influence on the residual deflection of the T-joints which is cooled down to room temperature. The results of the experiments also indicate that the axial load level and heating and cooling history have more significant effects on the compressive stiffness of the T-joints than the residual strength. In the numerical study, the result of finite element model agrees well with the test result. The work in this paper provides a basis for further parametric analysis and theoretical study on the structural evaluation after fire.     

Simulation of large forging flat-anvil stretching process and its optimization

In this paper, a kind of mathematic method for optimizing stretching process of large forgings is proposed. Distributions of effective strain within forged ingots is described by a Gauss function, which is obtained from the simulation of flat-anvil stretching process. Successive stretching is expressed by the superimposing Gauss functions. Optimized stretching process, with both homogeneous and certain strain in the center of forgings, is presented by derivation of this function. The relationship between effective strain and the values of feed is obtained during the successive stretching with a rotation angle of 90° and a feed displacement of 1/2 anvil width. The optimization result is verified by finite element simulation. Optimized value of feed obtained using this method can ensure both uniformity and forging penetration. It provides mathematic model and theoretic basis of optimizing large forging stretching process.     

Research on mechanical properties of 22MnB5 steel quenched in a steel die

22MnB5 is one of the most typical hot stamping boron steels. In order to study the effect of austenitizing temperature and holding time on the mechanical properties of hot stamping boron steel 22MnB5, a series of quenching process are done. The tensile strength and hardness of 22MnB5 samples are measured. The results show that 22MnB5 has a good quenching performance, and the tensile strength and hardness of samples quenched in a water-cooled steel die are similar to those of water quenching. The higher tensile strength and HRC hardness are achieved when austenitizing at temperatures of 880–910 °C. Holding at 910 °C for 5 min and then quenching gives rise to a better combination of tensile strength, hardness and ductility.     

Numerical study on bank effects for a ship sailing in shallow channel

A numerical study on bank effects in shallow channels is carried out by using a first-order Rankine source panel method. A container ship sailing along a vertical bank and a sloping bank at different forward speeds, different water depths and different distances between the bank and the ship hull is taken as example. The sway force and yaw moment acting on the hull are calculated and the influences of the speed, water depth and distance between the bank and ship hull on the hydrodynamic force and moment are analyzed. This study can provide insight into the bank effects, as well as to give guidance on ship manoeuvring and control in restricted waterways, which is helpful to the navigation safety.     

Effect of remelting current on molten pool profile of titanium alloy ingot during vacuum arc remelting process

The performance of vacuum arc remelting (VAR) ingot depends largely on ingot structure and chemical uniformity, which are strongly influenced by molten pool profile that is influenced by VAR process. To better understand the effect of remelting current on molten pool profile of titanium alloy ingot during VAR process, a 3D finite element model is developed by the ANSYS software. The results show that there are three remelting stages during VAR process when the remelting current is 2.0 kA. The molten pool depth increases gradually from 30 to 320 s, then the change of molten pool depth is very small during the steady state stage from 320 to 386 s, and lastly the molten pool depth becomes shallow after 386 s. The melting rate and temperature of superheat increase with the remelting current increasing, which leads to the augment of molten pool volume. In the end, the total remelting time and steady state molten pool time decrease with the melting current from 1.6 to 2.8 kA.     

Integrated fluid-thermal-structural numerical analysis for the quenching of metallic components

The quenching of a metal component with a channel section in a water tank is numerically simulated. Computational fluid dynamics (CFD) is used to model the multiphase flow and the heat transfer in film boiling, nucleate boiling and convective cooling processes to calculate the difference in heat transfer rate around the component and then combining with the thermal simulation and structure analysis of the component to study the effect of heat transfer rate on the distortion of the U-channel component. A model is also established to calculate the residual stress produced by quenching. The coupling fluid-thermal-structural simulation provides an insight into the deformation of the component and can be used to perform parameter analysis to reduce the distortion of the component.     

Numerical analysis of material properties in deformation of near hemispherical shells

In this paper, the effects of hardening exponent, yield strength and elastic modulus on the deformability of near hemispherical shells are investigated by means of finite element method and orthogonal experiment design. The largest eccentric angle during the deformation process and thickness reduction after the deformation are introduced to estimate the deformability quantitatively according to the deformation characteristics of near hemispherical shells. The results indicate that the hardening exponent is the most influential parameter, followed by elastic modulus and yield strength. The shell exhibits good deformability when the hardening exponent and elastic modulus are in the range of 0.1–0.125 and 70–108 GPa, respectively.     

Dynamic investigation on floating raft with elastic limiters

In this paper, we consider a floating raft isolation system with elastic limiters. The limiters might be “contact” or “no contact”, thus forming a unilateral system generating possible contacts. In order to avoid the large displacement of the floating raft under shock, limiters might be adopted but good limiters design requires the values of the gaps. Based on the contact dynamics, the multi-degree-freedom dynamic model of floating raft isolation system with elastic limiters is established. The artificial neural network has been developed to dingtinguish the contact state of elastic limiters at each step. The example analysis shows that the algorithm of neural network for contact can shorten the time of caclaution. From the example analysis, we get some interesting results that may be useful to the ship engineering.     

Upper limb musculo-skeletal model for biomechanical investigation of elbow flexion movement

A biomechanical musculo-skeletal model of upper limb is presented in this paper, which can provide accurate representations of muscles and joints, and capture important interactions between joints. The upper limb model is made up of seven segments: ribs, sternum, clavicle, scapular, humerus, radius and ulna, considered as a single rigid body respectively and includes 22 muscles. The individual muscle forces can be calculated by using an electromyography (EMG) assisted method, which is verified by comparing the simulation results with other researches of an elbow flexion motion. These comparisons show that the muscle forces and the estimated joint moment match well with previous literatures.