A phenomenological model of damage in articular cartilage under impact loading

Damage progression in high-strain rate and impact tests on articular cartilage is considered. A new type of kinetic damage evolution law is proposed and used to draw implications about the accumulated damage and the coefficient of restitution. Based on the developed damage model, a new fracture criterion is introduced.     

On the determination of particle impact breakage in selection function

This paper presents a thorough study of particle impact breakage in selection function with a unified breakage criterion. The impact mode and breakage pattern for particulate materials are classified based on a significant review of well-established impact testers. It was found that the lack of a unified breakage criterion to determine the breakage probability disables a direct comparison of particle breakage propensity from different impact loading testers. The literature breakage models to describe the breakage probability are reviewed where the advantage and drawback of these models are scrutinized. The sourced literature breakage models are compared with the zeolite breakage datasets in a unified breakage criterion to evaluate the model performance. A novel computational modelling workflow for a milling process is proposed to provide a guidance in implementing the digital twin in milling process prediction. The breakage probability models, i.e. the selection functions are comprehensively assessed in population balance model to examine the model serviceability. The model simplicity and fidelity in the model assessment are specifically discussed and the value of digital twin in substantially reducing the experimental trials is highlighted.     

Static Finite Element Validation of a Flexible Micro Air Vehicle

The flexible-wing approach has proven to be a successful method for designing micro air vehicles. The wing’s passive deformation under wind loads can allow for gust rejection, delayed stall, or improved longitudinal stability. As such, an accurate structural model of the flexible wing can provide greater understanding of the aforementioned phenomena. This paper seeks to formulate a static finite element wing model, with a particular emphasis on accuracy. The wing is broken into three different types of elements: beams, plates, and membranes. Individual element types are characterized and validated by constructing simple structures from the appropriate material, and then comparing experimental and numerical deformation fields. Experimental results are found through a visual image correlation system. The elements are then combined to form the complete wing model, which is also validated through experiments. The resulting finite element model is found to be very accurate, able to predict the complicated structural response of a composite wing. Due to observations made during standard wind tunnel testing, the structural response of a typical membrane MAV wing in steady level pre-stall flight is thought to be quasi-static. As such, the finite element model formulated in this work will be indispensable towards future numerical static aeroelastic optimization research efforts aimed at improving the efficiency, agility, and sensitivity of practical micro air vehicles.     

Parameter identifications for a rotor system based on its finite element model and with varying speeds

In order to achieve prediction for vibration of rotating machinery, an accurate finite element （FE） model and an efficient parameter identification method of the rotor system are required. In this research, a test rig is used as a prototype of a rotor system to validate a novel parameter identification technique based on an FE model. Rotor shaft vibration at varying operating speeds is measured and correlated with the FE results. Firstly, the theories of the FE modelling and identification technique are introduced. Then disk unbalance parameter, stiffness and damping coefficients of the bearing supports on the test rig are identified. The calculated responses of the FE model with identified parameters are studied in comparison with the experimental results.     

Damage accumulation model for monotonic and dynamic shear fracture of asphalt-stone adhesion

A damage accumulation model for shear fatigue of asphalt-stone adhesion was developed with the aid of experimental data obtained from monotonic and dynamic force-controlled tests. Constant static- and stress-rate monotonic tests and cyclic fatigue tests were performed on the special designed specimens of double-layered-sandwich using dynamic thermal mechanics analyzer. Experimental results showed that it was feasible to describe monotonic and cyclic tests of stress-controlled mode using one single damage accumulation model based on stress–time path, which makes it possible to reduce the number of specimens tested and time-consuming fatigue tests are no longer necessary, since the same information can be obtained from monotonic test data without loss of accuracy. It also found that the bonding fracture in shear mainly attributes to cohesive failure through binder interlayer due to extensive creep deformation.     

近壁声空泡溃灭微射流冲击流固耦合模型及蚀坑反演分析

超声场下液体环境中近壁空泡溃灭会产生强烈的微射流,为探究微射流冲击壁面流固耦合效应,利用流体力学及冲击动力学,考虑了率相关的J-C材料本构模型,建立并分析了微射流冲击壁面流固耦合三维模型,并通过超声空化试验和基于球形压痕试验理论的反演分析进行了验证。结果表明:微射流冲击下材料表面出现微型凹坑,凹坑深度由微射流速度和微射流直径共同决定且随其增大而增大,凹坑直径主要与微射流直径正相关,而凹坑径深比则主要与微射流速度负相关;壁面压强基本呈对称分布且最大压强出现在微射流冲击边缘;超声空化试验验证了微射流冲击下材料表面出现的微型凹坑,反演分析方法表明,在16~18的径深比下,微射流冲击强度为420~500MPa,对应的微射流速度为310~370m/s。试验及反演分析结果与理论分析结果相符,验证了流固耦合模型及反演分析方法的合理性及准确性,为后续工程应用中空化强度、微射流速度等的控制提供了理论参考。     

Analysis of a multiaxial test on a C/C composite by using digital image correlation and a damage model

The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.     

The effect of the Reynolds number on the Reynolds stresses and vorticity in a turbulent far-wake

Using two orthogonal arrays of 16 X-wires, eight in the (x,y)-plane and eight in the (x,z)-plane, the effect of the Reynolds number in a turbulent plane far-wake has been investigated for two values of Re_θ (based on the free stream velocity and the momentum thickness), i.e. 1350 and 4600. It is observed that as the Reynolds number increases the magnitudes of the measured Reynolds stresses increase, as does the size of two-point vorticity correlation iso-contours. Discernible differences are also observed in probability density function, spectra and three-dimensional topologies. The Reynolds number dependence seems to vanish when Re_θ5000.     

Verification of a model to predict the influence of particle inertia and gravity on a decaying turbulent particle-laden flow

In an earlier publication some of the authors presented a theoretical model for the calculation of the influence of particle inertia and gravity on the turbulence in a stationary particle-laden flow. In the present publication the model is extended for application to a decaying suspension. Also a comparison is given between predictions made with the model and experimental data (own data and data reported in the literature) on a decaying turbulent flow with particles in a water tunnel or in a wind tunnel. For most of the experiments a prediction with reasonable accuracy and an interpretation is possible by means of the model.     

The Prandtl number effect near the onset of Bénard convection in a porous medium

This note focuses on Kladias and Prasad's claim that the critical Rayleigh number for the onset of Bénard convection in an infinite horizontal porous layer increases as the Prandtl number decreases, and argues that the critical Rayleigh number (Ra_c) depends only on the Darcy number (Da), as linear stability analysis indicates. The two-dimensional steady-convection problem is then solved numerically to document the convection heat transfer effect of the Rayleigh number, Darcy number, Prandtl number, and porosity. The note concludes with an empirical correlation for the overall Nusselt number, which shows the effect of Prandtl number at above-critical Rayleigh numbers. The correlation is consistent with the corresponding correlation known for Bénard convection in a pure fluid.     

Modal part analysis of slosh of the liquid in a cylindrical container in the case of small bond number

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A simplified method for the impact test of beams using a pseudo-dynamic (PSD) process

The impact test in structural parts for dynamic applications is an essential procedure for their certification in the presence of time dependent loads. In the case of beam elements, either built with one material or as an assembly of different material members joined with recent developed bonding techniques, an impact test is of leading importance, once the dynamic resistance of the joints involved in the beam fabrication is assessed and evaluated. The pseudo-dynamic method is an alternative to dynamic analysis of structures, here offering to the researcher the possibility of examining with detail the specimen in test for the initiation and progress of eventual damage mechanisms arising in the beam element joints whenever included in the design.     

Correction of an effect of using a faulty model for the retrieval of a bulk wave slowness of a layered half space

This study concerns the retrieval of a single, real constitutive parameter (the bulk shear wave inverse velocity) of a simple, although representative, geophysical configuration involving two homogeneous, non-dissipative media, from its simulated response to pulsed plane wave probe radiation. This nonlinear inverse problem is solved exactly, at all frequencies, by equating the simulated frequency domain response incorporating the true real inverse velocity to the assumed response incorporating a trial complex inverse velocity. Due to discordance, caused by prior (a parameter of a model that is not retrieved, but rather assumed to be known, although its value may be wrong) uncertainty, between the models associated with the assumed and trial responses (the model giving rise to the latter is therefore qualified as being ‘faulty’), the imaginary part of the retrieved inverse velocity turns out to be non-nil, and, in fact, to be all the greater, the larger is the prior uncertainty. Moreover, the retrieved inverse velocity is found to be dispersive and its real part nearly equal to the true inverse velocity at all frequencies. The reconstructed signals (obtained by inserting the retrieved complex parameter into the faulty response model) are found to coincide exactly with the true signals for three types of probe pulses, even when the prior uncertainty is large.     

The effect of nozzle layout on droplet ejection of a piezo-electrically actuated micro-atomizer

We study here effects of nozzle layout on the droplet ejection of a micro atomizer, which was fabricated with the arrayed nozzles by the MEMS technology and actuated by a piezoelectric disc. A theoretical model was first built for this piezoelectric-liquid-structure coupling system to characterize the acoustic wave propagation in the liquid chamber, which determined the droplet formation out of nozzles. The modal analysis was carried out numerically to predict resonant frequencies and simulate the corresponding pressure wave field. By comparing the amplitude contours of pressure wave on the liquid-solid interface at nozzle inlets with the designed nozzle layout, behaviors of the device under different vibration modes can be predicted. Experimentally, an impedance analyzer was used to measure the resonant frequencies of the system. Three types of atomizers with different nozzle layouts were fabricated for measuring the effect of nozzle distribution on the ejection performance. The visualization experiment of droplet generation was carried out and volume flow rates of these devices were measured. The good agreement between the experiment and the prediction proved that only the increase of nozzles may not enhance the droplet generation and a design of nozzle distribution from a viewpoint of frequency is necessary for a resonant related atomizer. The project supported by the National Natural Science Foundation of China (50405001).     

Effects of the Reynolds number on a scale-similarity model of Lagrangian velocity correlations in isotropic turbulent flows

A scale-similarity model of a two-point two-time Lagrangian velocity correlation(LVC) was originally developed for the relative dispersion of tracer particles in isotropic turbulent flows(HE, G. W., JIN, G. D., and ZHAO, X. Scale-similarity model for Lagrangian velocity correlations in isotropic and stationary turbulence. Physical Review E, 80, 066313(2009)). The model can be expressed as a two-point Eulerian space correlation and the dispersion velocity V. The dispersion velocity denotes the rate at which one moving particle departs from another fixed particle. This paper numerically validates the robustness of the scale-similarity model at high Taylor micro-scale Reynolds numbers up to 373, which are much higher than the original values(R_λ = 66, 102). The effect of the Reynolds number on the dispersion velocity in the scale-similarity model is carefully investigated. The results show that the scale-similarity model is more accurate at higher Reynolds numbers because the two-point Lagrangian velocity correlations with different initial spatial separations collapse into a universal form compared with a combination of the initial separation and the temporal separation via the dispersion velocity.Moreover, the dispersion velocity V normalized by the Kolmogorov velocity V_η≡η/τ_η in which η and τ_η are the Kolmogorov space and time scales, respectively, scales with the Reynolds number R_λ as V/V_η∝ R_λ~(1.39) obtained from the numerical data.     

Adaptive nonlinear model predictive control design of a flexible-link manipulator with uncertain parameters

This paper presents a novel adaptive nonlinear model predictive control design for trajectory tracking of flexible-link manipulators consisting of feedback lineariza-tion, linear model predictive control, and unscented Kalman filtering. Reducing the nonlinear system to a linear system by feedback linearization simplifies the optimization prob-lem of the model predictive controller significantly, which, however, is no longer linear in the presence of parame-ter uncertainties and can potentially lead to an undesired dynamical behaviour. An unscented Kalman filter is used to approximate the dynamics of the prediction model by an online parameter estimation, which leads to an adaptation of the optimization problem in each time step and thus to a better prediction and an improved input action. Finally, a detailed fuzzy-arithmetic analysis is performed in order to quantify the effect of the uncertainties on the control structure and to derive robustness assessments. The control structure is applied to a serial manipulator with two flexible links containing uncertain model parameters and acting in three-dimensional space.     

Investigation of the soil thrust interference effect for tracked unmanned ground vehicles (UGVs) using model track tests

The traction force of a tracked unmanned ground vehicle (UGV) depends on the soil thrust generated by the shearing action on the soil-track interface. In the development of soil thrust, because the continuous-track system consists of a number of single-track systems connected to each other, interference occurs between the adjacent single-track systems through the surrounding soil. Thus, the total soil thrust of the continuous-track system is not equal to the sum of the soil thrust of each single-track system, and the interference effect needs to be carefully considered. In this study, model track tests were conducted on model single-, double-, and triple-track systems according to relative density of soil and shape ratio (i.e., the length of the track plate to grouser depth). The test results indicated that the interference effect reduced soil thrust due to the overlapping shear zones between adjoining single-track systems. The loss of soil thrust increased as the relative density of the soil increased and the shape ratio decreased. Based on these findings, a soil thrust multiplier that can be utilized to assess the soil thrust of a continuous-track system was developed.     

基于拟流体模型的SPH新方法及其在弹丸超高速碰撞薄板中的应用

引入颗粒动力学理论(拟流体模型)建立了适用于超高速碰撞的SPH新方法。将超高速碰撞中处于损伤状态的碎片等效为拟流体,在描述其运动过程中引入了碎片间相互作用和气体相对碎片的作用。采用该方法对球形弹丸超高速碰撞薄板形成碎片云的过程进行了数值模拟,得到了弹坑直径、外泡碎片云和内核碎片云的形状、分布,并与使用传统SPH方法、自适应光滑粒子流体动力学(ASPH)方法的模拟结果进行对比,结果显示：新方法在内核碎片云形状和分布上计算结果更加准确。同时对Whipple屏超高速碰撞问题进行了研究,分析了不同撞击速度下防护屏弹坑尺寸及舱壁损伤特性等特性,计算结果与实验吻合较好且符合Whipple防护结构的典型撞击极限曲线。