Asymptotic Analyses of the Start-Up Stage of Couette Flow Subjected to Different Boundary Conditions

In this article, the process for reaching “developed” stage was investigated under both imposed shear stress and specified velocity boundary conditions. Four specific situations are investigated. These are (1) constant shear stress, (2) linearly increasing shear stress from zero shear, (3) constant velocity and (4) linearly increasing velocity from stationary. Analytical solutions of velocity distributions under these four situations were obtained. A dimensionless viscosity, defined as the ratio of the measured viscosity calculated based on the measuring principle of Couette-type viscometer to the true viscosity of fluid was proposed to describe the initial transient period. We define the “developed” stage when the dimensionless viscosity is 1% away from its final value or when it reaches 1.01. By analyzing Stokes' first problem, compact models of the dimensionless viscosity were expressed and exact quantitative relations among the initial values of dimensionless viscosity under these four specific situations were found. Time periods for Couette flow to reach the “developed” stage was calculated. The development time is the shortest under the constant velocity boundary and is the longest under the linearly increasing shear stress boundary

Non-contact magnetic driving bioinspired Venus flytrap robot based on bistable anti-symmetric CFRP structure

The Venus flytrap takes advantage of its bistability to generate rapid closure motion for capturing its prey. A bioinspired Venus flytrap robot with bistable artificial leaves is presented in this paper. Non-contact electromagnetic driving method is proposed to actuate the Venus flytrap robot's artificial leaves, which are made of anti-symmetric carbon fiber reinforced prepreg (CFRP) cylindrical shells. Magnetic force is generated by using the electromagnet and applied on the shell's curve edge to unbend the shell, and then the bending process transmits from one edge to the whole surface. The required magnetic force for the snap-through process of the bistable CFRP structure is determined from experimental test and compared with the result of finite element simulation. The test of the snap-through process of the Venus flytrap robot show that the Venus flytrap robot can generate a rapid snapping motion by the electromagnet actuation

Thermal effect and active control on bistable behaviour of anti-symmetric composite shells with temperature-dependent properties

Anti-symmetric cylindrical shells with two stable configurations have been proved to offer novel morphing structures in advanced engineering fields. The bistable behaviour of anti-symmetric composite shells under thermomechanical loading is analysed herein theoretically combined with a finite element modelling. The properties of the composite material in current study are considered to be functions of temperature. The shell is subjected to two different thermal load, i.e. the uniform temperature field and through-thickness thermal gradient. The influence of this two temperature field on the shell’s stable shapes was predicted analytically, which thereafter is determined by finite element results. This provides a feasible approach of controlling the deformation of the bistable shell through adjusting the applied temperature field. For this purpose, a superposition of uniform temperature field and through-thickness thermal gradient is imposed and its influence on the bistable shapes of bistable shells is therefore investigated, which is of great importance to the design and application of morphing structures manufactured from bistable composite shells

A novel thermo-mechanical anti-icing/de-icing system using bi-stable laminate composite structures with superhydrophobic surface

A novel anti-icing/de-icing system composed of bi-stable laminate composite structures with superhydrophobic surface and soft electrothermal patch is investigated in this paper. In this system, the superhydrophobic surface has superior performance in anti-icing and de-icing by reducing the adhesion of the ice-skin interface; meanwhile, a thermo-mechanical way to remove ice is conducted by deforming the bi-stable structures using heating actuation method. The superhydrophobic layer is fabricated by decreasing the free energy of copper oxide on the copper surface. The water contact angle of the superhydrophobic surface is tested by an optical contact angle measuring device, which reaches above 155° and the sliding angle is less than 10°. In addition, the microstructure of superhydrophobic layer is characterized by using a scanning electron microscope (SEM) to illustrate the superhydrophobic mechanism. Moreover, outstanding self-cleaning properties and UV-durability are obtained on the prepared surface. Experimental results indicate that the system has good performances in both anti-icing and de-icing processes when working at the subzero temperature. Meanwhile, there is no liquid water left on the surface after the snap-through process of bi-stable structures. Besides, the factors that affect the anti-icing and de-icing performance of system are discussed, including the superhydrophobic property, morphing characteristic of bi-stable laminate composite structures and actuating method. Finally, the finite element method is used to simulate the factors that affect the deformation of bi-stable structures independently, including the single layer thickness, stacking sequence of the laminate and the embedment of the electrothermal alloy

Transient Stage Comparison of Couette Flow under Step Shear Stress and Step Velocity Boundary Conditions

Couette flow has been widely used in many industrial and research processes, such as viscosity measurement. For the study on thixotropic viscosity, step-loading, which includes (1) step shear stress and (2) step velocity conditions, is widely used. Transient stages of Couette flow under both step wall shear stress and step wall velocity conditions were investigated. The relative coefficient of viscosity was proposed to reflect the transient process. Relative coefficients of viscosity, dimensionless velocities and dimensionless development times were derived and calculated numerically. This article quantifies the relative coefficients of viscosity as functions of dimensionless time and step ratios when the boundary is subjected to step changes. As expected, in the absence of step changes, the expressions reduce to being functions of dimensionless time. When step wall shear stresses are imposed, the relative coefficients of viscosity changes from the values of the step ratios to their steady-state value of 1. but With step-increasing wall velocities, the relative coefficients of viscosity decrease from positive infinity to 1. The relative coefficients of viscosity increase from negative infinity to 1 under the step-decreasing wall velocity condition. During the very initial stage, the relative coefficients of viscosity under step wall velocity conditions is further from 1 than the one under step wall shear stress conditions but the former reaches 1 faster. Dimensionless development times grow with the step ratio under the step-rising conditions and approaches the constant value of 1.785 under the step wall shear stress condition, and 0.537 under the step wall velocity condition respectively. The development times under the imposed step wall shear stress conditions are always larger than the same under the imposed step wall velocity conditions

Effect of plasticity of the cladding with different thicknesses on the bearing capacity of the brittle base wall of RPV under PTS loads

In the research field of reactor pressure vessel (RPV) subjected to pressurized thermal shock (PTS), the traditional linear elastic analytical method generally ignores the plastic properties of cladding. In fact, the neutron irradiation in RPV is easy to cause the embrittlement of the base material rather than cladding. So the elastic-plastic parameters of cladding are introduced into the FE model of the RPV with a sub-clad crack. The stress distributions in the thermal-mechanical coupling fields related to the base wall and cladding of different thicknesses are then obtained. The XFEM is used to simulate the crack growth in the nozzle area. The allowable internal pressure in the dangerous moment of the PTS is calculated to show the ultimate bearing capacity of the structure. The numerical results are compared with those only considering the elasticity of cladding. Furthermore, the law of the effect of plasticity of cladding on the structural safety is summarized

Fracture Toughness, Breakthrough Morphology, Microstructural Analysis of the T2 Copper-45 Steel Welded Joints

The performance and flaws of welded joints are important features that characteristics of the welding material influence. There is significant research activity on the performance and characteristics of welding joint materials. However, the properties of dissimilar welding materials and the cracking problem have not been thoroughly investigated. This investigation focuses on the evaluation and analysis of fracture mechanics, including fracture toughness, microstructural analysis, and crack initiation of T2 copper-45 steel dissimilar welding materials. Standard tensile and three-point bending experiments were performed to calculate the ultimate strength, yield strength, and elastic modulus for fracture toughness. The macro/micro-fracture morphology for tensile fracture and three-point bending fracture were analysed. Based on these investigations, it was concluded that the fracture types were quasi-cleavage and an intergranular brittle fracture mixed model. The deflection of the crack path was discussed and it was determined that the crack was extended along the weld area and tilted towards the T2 copper. Finally, the crack propagation and deflecting direction after the three-point bending test could provide the basis for improvement in the performance of welded joints based on experimental testing parameters and ABAQUS finite element analysis

Evaluation of a New Rotator Cuff Trainer Based on Oscillating Hydraulic Damping

In order to provide a convenient way to strengthen the rotator cuff muscles and prevent rotator cuff injury, this study designed an innovative strength trainer specifically for shoulder rotator cuff based on oscillating hydraulic damping. We carried out a myoelectric testing experiment to evaluate the shoulder rotation training effect and compared the results with traditional training equipment to verify the feasibility and validity of the new rotator cuff trainer (RCT). Then, we further explored the influence of different training postures and motion speeds on shoulder rotation training. In the experiment, subjects used three types of equipment (RCT, dumbbells and elastic bands) to perform shoulder rotation training under two movement speeds and two motion postures. The surface electromyography (sEMG) signals of targeted muscles were collected in real time and then further analyzed. The experimental results showed that when using the RCT, the muscle force generation sequence was more aligned with the biomechanical principles of shoulder rotation than using the other two training methods, and the target training muscles had the higher percentage of muscle work. During RCT training, a higher speed of movement (120°/s) led to a higher degree of muscle activation; coronal axis rotation was better for the infraspinatus training, and sagittal axis rotation was better for teres minor training. Based on these results, the RCT was proved to be more effective than traditional training methods. In order to exercise the different muscles of rotator cuff more comprehensively and extensively, different postures should be selected. Furthermore, the movement speed can be appropriately increased within the safe range to improve muscle activation

Hygroscopic influence on bistable characteristics of antisymmetric composite cylindrical shells: an experimental study

This paper investigates the effect of moisture on bistable characteristics of antisymmetric composite cylindrical by using experimental and finite element method. The bistable characteristics are characterized by the curvatures of antisymmetric composite cylindrical shells in different stable states and snap processes between the two stable states which are indicated by the load–displacement curve and snap load. The manufactured specimens after dried in the oven are immersed in distilled water to full saturation and the saturated salt solutions (MgCl) to full saturation. The specimen achieves different moisture that is immersed in distilled water at the different period until full saturation and in the saturated salt solution (MgCl) with the same period of saturation in distilled water. Specimens with different moisture are then mechanically loaded on a testing machine to transform between two stable states. Load–displacement curves are recorded in the computer, from which the snap loads can be found. After the test, the principal and twisting curvatures are captured by a digital image processing. The results are contrasted with hygroscopic influence on another kind of bistable composite structure (asymmetric cross-ply laminates) in this paper. The results show that the shapes and snap loads of antisymmetric composite cylindrical shells are influenced by the moisture increasing

Effects of adjustment devices on the fore-and-aft mode of an automobile seat system: headrest, height adjuster, recliner and track slide

The automobile seat is an important and unique device in the automobile system, which directly contacts the human body. The dynamic characteristics of the automobile seat are the key parameters for its vibration security and dynamic performance. Both experiments and finite element simulations are conducted to analyse the fore-and-aft operating modes of a new type of automobile seat system in this paper. The results and their correlation analysis between the experimental data and the computational models are given. By using finite element simulations, the computational results of the modes with the first 12 orders for the automobile seat are in the range 0-100 Hz, including the integral modes and the local modes of the automobile seat. An experimental method was designed to measure the seat frame mode using an impact hammer, a triaxial accelerometer, uniaxial accelerometers and a 24-channel LMS vibration test system. The experimental data were analysed by estimating the frequency response function and the modal parameters and validated using LMS Test.Lab software. The experimental results for five integral modal parameters of the automobile seat between 0 Hz and 100 Hz are given. The orthogonal experimental method is used to design nine different working states of the automobile seat with variations in the positions of different adjustable devices (the headrest, the height adjuster, the recliner and the track slide). Finally, the influences of the above-mentioned adjustable devices on the fore-and-aft operating mode are analysed by the single-factor analysis method using finite element simulations