Sloshing and energy dissipation in an egg: SPH simulations and experiments

Tall structures, such as towers and bridges, can oscillate at excessive magnitudes when subjected to wind and earthquake loads. Liquid sloshing absorbers can be used to suppress these excessive oscillations by tuning the frequency of the sloshing to the critical frequency of the structure. Sloshing absorbers are simple structures consisting of a partially full container of liquid with a free surface. Tuning ensures that significant amounts of harmful energy can be extracted from the structure to the sloshing liquid. However, there needs to be a rapid means of dissipating this energy to avoid its returning back to the structure (then back to the liquid periodically).A hen׳s egg seems to have evolved to efficiently dissipate energy to protect its embryo using sloshing of its liquid content. Hence, the potential to implement the egg׳s unique properties as a sloshing absorber for structural control, is the main focus of this study. Numerical simulations, using Smoothed Particle Hydrodynamics (SPH), and experimental comparisons are presented in this paper. One objective is to demonstrate the ability of SPH to simulate complex free surface behaviour in three dimensions. Such a tool is then useful to identify different dissipation modes. Effects of fill volume and viscosity on the rate of dissipation, are also investigated.     

生物力学与基因-献给周培源教授诞辰100周年

生物界包罗万象,其中有力的作用,所以有生物力学.自Galileo,Harvey, Boreli, Hooke, Euler,Young等创始以来,生物力学阐明了鸟飞鱼游,人体运动,血液循环,人工脏器等,对人世社会,有所贡献.生物力学的基础是质点力学,传统地用连续体力学的概念来简化.但近年做生物组织在应力的作用下改造的问题,引起了必须更改传统连续体力学的几个公理的问题.我们将仔细讨论这些公理,然后指出新公理存在的理由,是由于基因在细胞里的日常工作.基因不单主宰遗传,变异;并且忙着控制日常生活.不过,现在仅见其端倪.详细的情形,要等将来来阐发了.      

Use of a modified torsional Kolsky bar to study frictional slip resistance in rock-analog materials at coseismic slip rates

The knowledge of shear resistance in earth faults is of fundamental importance to our understanding of the magnitude of stress drop and the associated energy release during typical seismic rupture events. In the present study a modified torsional Kolsky bar is employed to investigate frictional slip resistance in rock-analog materials (i.e., quartz and soda lime glass) at normal stresses of relevance to earthquake physics (30–80 MPa) and co-seismic slip rates. The results indicate the coefficient of kinetic friction to be in the range of 0.2 to 0.3. These values of the coefficient of friction are much lower when compared to those obtained in rocks at quasi-static slip rates. In all experiments slip weakening is observed and is preceded by slip strengthening. The slip weakening is understood to be due to thermal weakening induced by flash-heating at asperity contacts and requires a few mm of slip to be effective; the slip strengthening is understood to be due to an increase in the real area of contact at the asperity junctions due to localized plastic flow and subsequent coalescence and solidification of local softened/melt patches at the slip interface.     

The effect of variations in the creep exponent on the buckling of circular cylindrical shells

The senior author solved the problem of axially symmetrical creep buckling of thin circular cylindrical shells subjected to uniform axial compression. In that analysis the constitutive equation was a power law, and the exponent was taken to be equal to three. The purpose of this work was to extend the solution to a range of values of the creep exponent, n. To cope with the increasing algebraic complexity, a digital computer was employed in two ways: to generate the set of equations symbolically, and then to solve these equations. The computer programs were used to generate numerical solutions for the cases in which n was equal to 3, 5, 7 and 9. Two simple extrapolation techniques were then employed to obtain approximate solutions to the critical time problem for values of n up to 29.     

An icing physics study by using lifetime-based molecular tagging thermometry technique

An experimental investigation was conducted to quantify the unsteady heat transfer and phase changing process within small icing water droplets in order to elucidate underlying physics to improve our understanding of the important micro-physical process of icing phenomena. A novel, lifetime-based molecular tagging thermometry (MTT) technique was developed and implemented to achieve temporally-and-spatially resolved temperature distribution measurements to reveal the time evolution of the unsteady heat transfer and dynamic phase changing process within micro-sized water droplets in the course of icing process. It was found that, after a water droplet impinged onto a frozen cold surface, the liquid water at the bottom of the droplet would be frozen and turned to solid ice rapidly, while the upper portion of the droplet was still in liquid state. As the time goes by, the interface between the liquid phase water and solid phase ice was found to move upward continuously with more and more liquid water within the droplet turned to solid ice. Interestingly, the averaged temperature of the remaining liquid water within the small icing droplet was found to increase, rather than decrease, continuously in the course of icing process. The temperature increase of the remaining liquid water is believed to be due to the heat release of the latent heat during solidification process. The volume expansion of the water droplet during the icing process was found to be mainly upward to cause droplet height growth rather than radial to enlarge the contact area of the droplet on the test plate. As a result, the spherical-cap-shaped water droplet was found to turn to a prolate-spheroid-shaped ice crystal with cusp-like top at the end of the icing process. The required freezing time for the water droplets to turn to ice crystals completely was found to depend on the surface temperature of the test plate strongly, which would decrease exponentially as the surface temperature of the frozen cold test plate decreases.     

New accurate efficient modeling techniques for the vibration analysis of T-joint thin-walled box structures

We propose new accurate efficient modeling techniques for the vibration analysis of T-joint thin-walled box structures. The essence of the present techniques is to use beam elements to model thin-walled members of the joint, but the elements are based on an eight-degree-of-freedom (8-DOF) beam theory capable of handling warping and distortion. Two approaches are considered to model the interfacing joint region connected to three adjacent thin-walled box structures: the first one is to model the joint region with plate elements and the second one is to use a joint element derived to be consistent with nearby 8-DOF beam elements. The efficiency of the present techniques comes from the use of beam elements to model the box structures while the accuracy comes from the use of the higher-order beam theory accounting for warping and distortion. The procedures to match the dissimilar elements and to develop the joint element are also presented in this work. The effectiveness of the present approaches is demonstrated by numerical examples.     

A study on the fragmentation of space debris particles at high-speed collision

A mathematical model was previously proposed by the first author to evaluate the number of fragments and their mass distribution at high-speed collision of two space debris particles and to determine their velocities after collision. The constants used in this model are assumed to be temperature-independent. In this paper it is shown that the consideration of temperature effects due to high-speed collision of particles leads to the formation of smaller fragments and to a significant increase of their total number.     

基于振动监测的静载荷滑动轴承接触摩擦故障诊断实验研究

采用静载荷滑动轴承试验台模拟轴颈-轴承从液体润滑状态逐步向干摩擦状态过渡时的接触摩擦故障的发生和发展过程,进而利用振动信号诊断滑动轴承的接触摩擦故障;通过对特征参数进行归一化处理得到无量纲特征参数,进而通过无量纲特征参数的适当数学组合得到无量纲诊断准则,并探讨了无量纲诊断准则的适用性.结果表明:利用所得到的无量纲诊断准则进行轴颈-轴承接触摩擦故障诊断时无须建立用于模式识别的标准模,可实现快捷方便的故障诊断;无量纲诊断准则对工况不敏感,而对故障更敏感,适用于可变工况下滑动轴承的故障诊断,且诊断成本较低.     

Resonant Capture and Separatrix Crossing in Dual-Spin Spacecraft

We consider the rotational motion of a spacecraft composed of two bodies which are free to rotate relative to one another about a common shaft S. A motor on one of the bodies provides a small constant internal torque which influences the relative motion of the two bodies, and which may influence the orientation of their common shaft S. Resonant capture refers to the phenomenon that the spacecraft may end up in one of several possible orientations, including a nearly flat spin (transverse to S), in addition to the expected simple rotation about S.The method of averaging is used to treat the original equations of motion, and it is shown that the essential mathematical problem involves separatrix crossing in a problem with slowly moving separatrices. Energy changes represented by Melnikov integrals are used to supplement the averaged equations in the neighborhood of the heteroclinic motions. The method is used to predict which initial conditions lead to capture into each of three distinct capture regions. The asymptotic results are compared to those obtained by direct numerical integration of the equations of motion.     

Non-nulling seven-hole probes for high angle flow measurement

This paper illustrates a method for calibrating seven-hole probes to measure local total and static pressures and relative flow angles of up to 70 degrees in subsonic compressible flows. The method of Latin Squares was used to statistically sample a large and otherwise unmanageable data set, thereby reducing to a minimum the number of data points required to construct a polynomial curve fit to the data. Calibration produces three-variable third order polynomials which permit all of the desired flow properties to be found explicitly from probe measured pressures. This method determines the flow angles to within 2 degrees and Mach number to within 0.04 with 95 percent certainty.     

Determination of crack-speed history and tip locations for cracks moving with nonuniform velocity

This paper describes the use of Rayleigh waves as observed with the reflected-light method of caustics to obtain information pertaining to crack speed and tip location. The basic idea is to use the Doppler-shift effect as applied to elastic-wave emission from a running crack to geometrically reconstruct crack-tip locations. In addition to the usual information pertaining to the stress-intensity factor at the time when the photograph was taken, the process yields a complete history of crack-propagation velocities even along curved-crack paths.     

Interpolation-based reduced-order modelling for steady transonic flows via manifold learning

This paper presents a parametric reduced-order model (ROM) based on manifold learning (ML) for use in steady transonic aerodynamic applications. The main objective of this work is to derive an efficient ROM that exploits the low-dimensional nonlinear solution manifold to ensure an improved treatment of the nonlinearities involved in varying the inflow conditions to obtain an accurate prediction of shocks. The reduced-order representation of the data is derived using the Isomap ML method, which is applied to a set of sampled computational fluid dynamics (CFD) data. In order to develop a ROM that has the ability to predict approximate CFD solutions at untried parameter combinations, Isomap is coupled with an interpolation method to capture the variations in parameters like the angle of attack or the Mach number. Furthermore, an approximate local inverse mapping from the reduced-order representation to the full CFD solution space is introduced. The proposed ROM, called Isomap+I, is applied to the two-dimensional NACA 64A010 airfoil and to the 3D LANN wing. The results are compared to those obtained by proper orthogonal decomposition plus interpolation (POD+I) and to the full-order CFD model.     

A polynomial chaos approach to the robust analysis of the dynamic behaviour of friction systems

This paper presents a dynamic behaviour study of non-linear friction systems subject to uncertain friction laws. The main aspects are the analysis of the stability and the associated non-linear amplitude around the steady-state equilibrium. As friction systems are highly sensitive to the dispersion of friction laws, it is necessary to take into account the uncertainty of the friction coefficient to obtain stability intervals and to estimate the extreme magnitudes of oscillations. Intrusive and non-intrusive methods based on the polynomial chaos theory are proposed to tackle these problems. The efficiency of these methods is investigated in a two degree of freedom system representing a drum brake system. The proposed methods prove to be interesting alternatives to the classic methods such as parametric studies and Monte Carlo based techniques.     

Electro-osmotic pore pressures in soil due to an alternating electrical field

Pore pressure development in a soil specimen due to electro-osmosis under alternating current conditions is examined theoretically. Solutions to the governing equation are derived for one-dimensional flow with boundary conditions corresponding to an impervious (conventional no-flow boundary), a partially drained boundary, and a partially drained boundary with an intervening permeable zone between the boundary and the soil. These latter two boundary conditions can arise from details of pore pressure measuring systems at the specimen boundaries during laboratory experiments. An analysis of the solutions indicates that for a perfect no-flow boundary, excess pore pressures measured at an electrode consist of a steady state and rapidly-decaying transient response. The pore pressures exhibit a 45 degree phase shift relative to the applied electric current. The effect of the partially drained boundary is to reduce the peak to peak amplitude of the pore pressure and to increase the phase shift to as much as 90 degrees depending on the compressibility of the pore pressure measuring system. The effect of the impeded and partially drained boundary is to further reduce the amplitude of the pore pressures and to increase the phase shift to as much as 180 degrees depending on the relative permeability of the impeded boundary.     

A minimal model for studying properties of the mode-coupling type instability in friction induced oscillations

A minimal two degree of freedom model is used to clarify from an intuitive perspective the physical mechanisms underlying the mode-coupling instability of self-excited friction induced oscillations. It is shown that simultaneous out-of-phase oscillations of friction force and displacement tangential to the friction force may lead to energy transfer from the frictional system to vibrational energy. Also it is shown that the friction force acts like a cross-coupling force linking motion normal to the contact surface to motion parallel to it and that a necessary condition for the onset of instability is that these friction-induced cross-coupling forces balance the corresponding structural cross-coupling forces of the system. Finally the origin and the role of phase shifts between oscillations normal and parallel to the contact surface is clarified with respect to the mode-coupling instability. It may be expected that the intuitive picture gained will be of considerable help for practical design purposes.     

Mean stress dependent nonlinear hyperelasticity coupled with damage stiffness degradation. A thermodynamical approach

Porous materials, such as geomaterials, exhibit a behaviour dependent on the confining pressure. The aim of this paper is to study the degradation of the elastic stiffness of mean stress dependent materials, due to the deterioration of the microstructure during loading.Continuum damage mechanics offers a framework to model this rigidity deterioration. In addition to the concept of effective stress, a choice has to be made between two widely used hypotheses, the principle of strain equivalence and the principle of equivalent elastic energy, in order to build a complete modelling framework.A mean stress dependent hyperelastic formulation is used to ensure a conservative behaviour, and associated to the two previous damage modelling assumptions, whose effects are compared. This allows for mean stress dependent elasticity to be reproduced, with elastic moduli increasing with mean stress while decreasing with damage.     

A Methodology for Combined Rotation-Extension Testing of Simple Steel Beam to Column Joints at High Rates of Loading

Framed building structures rely on the integrity of beam to column connections to avoid collapse in the event of damage to some part of the building. This requirement is often referred to as robustness and in UK practice may be assumed to be achieved (for certain categories of building) if the beam to column connections are designed to resist a specified tensile force. However, prior to the current work, no published data existed on the behaviour of connections subjected to rapid, non-cyclical loading to failure, and models of frame response to localised damage typically use either very simplified assumptions for connection behaviour (e.g. fully rigid against translation and rotation) or data from static connection tests. This paper describes experimental equipment which has been designed, fabricated and commissioned in order to facilitate this type of testing. The experimental test rig is capable of applying an axial tension load or a combination of moment and tension to a connection. The loading can be applied at quasi static or high-rate dynamic rates, with connections taken to failure in a few hundredths of a second. A methodology for analysis of the data, in particular in allowance for inertial effects in interpreting the load/moment data is presented, along with validation tests and examples of results from connection tests.     

Bifurcation of limit cycles in fluid film bearings

Rotors supported by journal bearings may become unstable due to self-excited vibrations when a critical rotor speed is exceeded. Linearised analysis is usually used to determine the stability boundaries. Non-linear bifurcation theory or numerical integration is required to predict stable or unstable periodic oscillations close to the critical speed. In this paper, a dynamic model of a short journal bearing is used to analyse the bifurcation of the steady state equilibrium point of the journal centre. Numerical continuation is applied to determine stable or unstable limit cycles bifurcating from the equilibrium point at the critical speed. Under certain working conditions, limit cycles themselves are shown to disappear beyond a certain rotor speed and to exhibit a fold bifurcation giving birth to unstable limit cycles surrounding the stable supercritical limit cycles. Numerical integration of the system of equations is used to support the results obtained by numerical continuation. Numerical simulation permitted a partial validation of the analytical investigation.     

Development of a tyre traction testing facility

An experimental setup comprising up of an indoor soil bin, a single wheel tester (SWT), a soil processing trolley, a drawbar pull loading device and an instrumentation unit was developed to perform traction tests in the soil bin to study the effect of soil, tyre and system parameters on the performance of tyres. The design of the single wheel tester was such that the dynamic weight reaction force is equal to that measured statically. It is a simple wheeled device, capable of testing tyres of up to 1.5 m in diameter, vertical force up to 19 kN, net pull up to 7.2 kN, torque up to 5.5 kN m, and speed up to 3.5 km/h.     

A hybrid model for computationally efficient fatigue fracture simulations at microelectronic assembly interfaces

Modern microelectronic assemblies are heterogeneous, layered structures joined by interconnects made of solder alloys with low homologous temperatures. The solder interconnections join devices to circuit boards and they fail by thermal fatigue fracture at their interfaces either to the device or to the circuit board. Predicting the fatigue fracture of the solder interconnections is a challenge due to the fact that they undergo large inelastic deformations during temperature cycling tests. In this paper we develop a hybrid approach inspired by cohesive zone fracture mechanics and the Disturbed State Concept to predict the crack trajectory and fatigue life of a solder interconnection subjected to both isothermal temperature cycling and anisothermal temperature cycling conditions (representing the two common accelerated test conditions for microelectronic products). A hybrid computational approach is used in which a first order approximation of the disturbance is used to estimate incremental cycles to criticality and thereby propagate the crack. The modeled crack fronts and the fatigue lives are validated through a comparison to results from the two types of accelerated tests. Overall, the model is shown to predict the fatigue life of the critical interconnection in the assembly to within 20% of the experimentally determined life. More importantly, the predicted crack trajectory is demonstrated to agree very well with the experimentally observed trajectory. Strikingly, the microscopically observed microstructural changes during crack propagation from that corresponding to creep fatigue to that of shear overload were found to be excellently correlated with the rate of change of the disturbance calculated in the model.