Experimental benchmark of a free plunging wing with imposed flap oscillations

The validation of fluid–structure interaction solvers is difficult since there is a lack of experimental data. Therefore, in this work an aeroelastic experiment is presented. The focus is on the temporal coupling between fluid and structure dynamics. Issues in the spatial coupling are eliminated by using a rigid wing. The wing, with a harmonically actuated 0.2c trailing edge flap, has a degree of freedom in the plunge (vertical) direction. The wing has a chord of 0.5 m and is suspended with springs. The wing motion is constrained by a vertical rail system.For simplicity attached flow is desired and therefore the set angle of attack is α=0°. The Reynolds number is approximately Re=700 000 and the flap deflects over a range of about ±2°. The damped natural frequency of the structure expressed as a reduced frequency is about k=0.194 and measurements are performed for reduced flap frequencies ranging from k=0.1 to k=0.3. Displacements and time dependent aerodynamic forces are measured and for k=0.198 2-D PIV measurements are performed. The planar PIV measurements are used to intrinsically determine the unsteady loads using Noca׳s method.As expected the aeroelastic problem shows similarities with a viscously damped mass–damper–spring, meaning the maximum excursion of the wing is found near the system eigenfrequency. The lift is dominated by the flap motion and the effective angle of attack due to the motion introduces phase shifts of the lift signal with respect to the flap phase angle.The experiment has been set up and executed with the necessary precision, but small ambiguities are found in the lift and drag disqualifying the data for validation. Nevertheless the data set provides a clear insight into typical loads and motions and can be used for comparative studies. It can also be used to (re)design future experiments to improve the quality of the data to the desired level of accuracy for validation.     

Effect of adhesive thickness,adhesive type and scarf angle on the mechanical properties of scarf adhesive joints

The effects of adhesive thickness, adhesive type and scarf angle, which are determined as the main control parameters by the dimensional analysis, on the mechanical properties of a scarf adhesive joint (SJ) subjected to uniaxial tensile loading are examined using a mixed-mode cohesive zone model (CZM) with a bilinear shape to govern the interface separation. Particularly, the adhesive-dependence of the vital cohesive parameters of CZM, which mainly include initial stiffness, total fracture energy and separation strength, is introduced emphatically. The numerical results demonstrate that the ultimate tensile loading increases as the adhesive thickness decreases. Cross the ultimate tension, the joint loses the load-bearing capacity when adopting the brittle adhesive but sustains partial load-bearing capacity while selecting the ductile adhesive. In addition, for the joint with the ductile adhesive, the maximum applied displacement until the complete failure of it is directly proportional to the adhesive thickness, which is different from the case using the brittle adhesive. Taking the combination of the ultimate loading and applied displacement into account, failure energy is employed to evaluate the joint performances. The results show that the failure energy of the joint with the brittle adhesive increases as the adhesive thickness decreases. Conversely, the situation of the joint using the ductile adhesive is vice versa. Moreover, the effect of the adhesive thickness becomes more noticeable with decreasing the scarf angle owing to the variation of the proportion of each component of the mixed-mode. Furthermore, all the characteristic parameters (the ultimate tensile loading, the maximum applied displacement and the failure energy) that adopted to describe the performances of SJ increase as the scarf angle decreases. Finally, the numerical method employed in this study is validated by comparing with existing experimental results.     

Relating geologic units and mobility system kinematics contributing to Curiosity wheel damage at Gale Crater,Mars

Curiosity landed on plains to the north of Mount Sharp in August 2012. By June 2016 the rover had traversed 12.9 km to the southwest, encountering extensive strata that were deposited in a fluvial-deltaic-lacustrine system. Initial drives across sharp sandstone outcrops initiated an unacceptably high rate of punctures and cracks in the thin aluminum wheel skin structures. Initial damage was found to be related to the drive control mode of the six wheel drive actuators and the kinematics of the rocker-bogie suspension. Wheels leading a suspension pivot were forced onto sharp, immobile surfaces by the other wheels as they maintained their commanded angular velocities. Wheel damage mechanisms such as geometry-induced stress concentration cracking and low-cycle fatigue were then exacerbated. A geomorphic map was generated to assist in planning traverses that would minimize further wheel damage. A steady increase in punctures and cracks between landing and June 2016 was due in part because of drives across the sharp sandstone outcrops that could not be avoided. Wheel lifetime estimates show that with careful path planning the wheels will be operational for an additional ten kilometers or more, allowing the rover to reach key strata exposed on the slopes of Mount Sharp.     

Demonstration of a coupled floating offshore wind turbine analysis with high-fidelity methods

This paper presents results of numerical computations for floating off-shore wind turbines using, as an example, a machine of 10-MW rated power. The aerodynamic loads on the rotor are computed using the Helicopter Multi-Block flow solver developed at the University of Liverpool. The method solves the Navier–Stokes equations in integral form using the arbitrary Lagrangian–Eulerian formulation for time-dependent domains with moving boundaries. Hydrodynamic loads on the support platform are computed using the Smoothed Particle Hydrodynamics method, which is mesh-free and represents the water and floating structures by a set of discrete elements, referred to as particles. The motion of the floating offshore wind turbine is computed using a Multi-Body Dynamic Model of rigid bodies and frictionless joints. Mooring cables are modelled as a set of springs and dampers. All solvers were validated separately before coupling, and the results are presented in this paper. The importance of coupling is assessed and the loosely coupled algorithm used is described in detail alongside the obtained results.     

A novel hybrid solid-like fluid-like (SLFL) method for the simulation of dry granular flows

This paper proposes a novel hybrid method to simulate the dry granular flow of materials over a wide range of inertial numbers that simultaneously covers the quasi-static and dense granular flow regimes. To overcome the lack of incremental objectivity whenever large deformations occur in solid-like regimes and to remove computational singularities in fluid-like regimes close to rest, the elastic–perfectly plastic theory based on the Drucker–Prager yield criterion is combined with the theory of dense granular flows. By implementing some new modifications at the boundaries and removing all ghost particles, smoothed particle hydrodynamics (SPH) is used as the framework for the method. A number of benchmark problems have been solved to show the capabilities of the new modified SPH method. Precise prediction of both location and pressure makes the modifications comparable with the previous works on SPH. Finally, the method is used to solve the classic 2D dry granular cliff collapse problem and to model dry granular material flow inside a rotary drum. The outcomes of the numerical simulation show good agreement with tabletop experiments and published results.     

Stress induced phase transitions in silicon

Silicon has a tremendous importance as an electronic, structural and optical material. Modeling the interaction of a silicon surface with a pointed asperity at room temperature is a major step towards the understanding of various phenomena related to brittle as well as ductile regime machining of this semiconductor. If subjected to pressure or contact loading, silicon undergoes a series of stress-driven phase transitions accompanied by large volume changes. In order to understand the material's response for complex non-hydrostatic loading situations, dedicated constitutive models are required. While a significant body of literature exists for the dislocation dominated high-temperature deformation regime, the constitutive laws used for the technologically relevant rapid low-temperature loading have severe limitations, as they do not account for the relevant phase transitions. We developed a novel finite deformation constitutive model set within the framework of thermodynamics with internal variables that captures the stress induced semiconductor-to-metal ($\mathrm{cd-Si}\to \mathrm{\beta }\mathrm{-Si}$), metal-to-amorphous ($\mathrm{\beta }\mathrm{-Si}\to \mathrm{a-Si}$) as well as amorphous-to-amorphous ($\mathrm{a-Si}\to \mathrm{hda-Si}$, $\mathrm{hda-Si}\to \mathrm{a-Si}$) transitions. The model parameters were identified in part directly from diamond anvil cell data and in part from instrumented indentation by the solution of an inverse problem. The constitutive model was verified by successfully predicting the transformation stress under uniaxial compression and load–displacement curves for different indenters for single loading–unloading cycles as well as repeated indentation. To the authors' knowledge this is the first constitutive model that is able to adequately describe cyclic indentation in silicon.     

On the spray drying of uniform functional microparticles

Spray drying is a typical method to produce particles in dry powder forms at industrial scale. Most spray-dried products often show a wide range of particle properties even within the same batch. At Monash University, we utilise a microfluidic spray drying approach to generate uniform microparticles with tightly controlled characteristics and sizes in a scalable, almost waste-free process. The technique is useful to correlate the effects of formulation and spray drying conditions on the properties of spray-dried particles, and can be used to test new formulations for targeted applications such as encapsulation and release of active ingredients. The synthesis route can be applied to other self-assembling systems, including mesoporous, crystalline, and hierarchically structured microparticles. As spray drying is commonly used in commercial scales, the understanding of how functional particles are formed in relation to formulations and process conditions could assist in developing a cost effective, energy and material-efficient route to produce powders with better properties and ease of handling for more advanced applications such as selective adsorption and bio-separation.     

Retrieval of aerosol size distribution using improved quantum-behaved particle swarm optimization on spectral extinction measurements

An improved quantum-behaved particle swarm optimization (IQPSO) algorithm is employed to determine aerosol size distribution (ASD). The direct problem is solved using the anomalous diffraction approximation and Lambert–Beer's Law. Compared with the standard particle swarm optimization algorithm, the stochastic particle size optimization algorithm and the original QPSO, our IQPSO has faster convergence speed and higher accuracy within a smaller number of generations. Optimization parameters for the IQPSO were also evaluated; we recommend using four measurement wavelengths and 50 particles. Size distributions of various aerosol types were estimated using the IQPSO under dependent and independent models. Finally, experimental ASDs at different locations in Harbin were recovered using the IQPSO. All our results confirm that the IQPSO algorithm is an effective and reliable technique for estimating ASD.     

Modeling dynamic brittle behavior of materials with circular flaws or pores

Compressive failure of brittle materials is driven primarily by crack growth from pre-existing flaws in the material. These flaws, such as grain boundaries, pores, preexisting cracks, inclusions and missing grains, are randomly spaced and have a range of possible shapes and sizes. The current work proposes a micromechanics-based model for compressive dynamic failure of brittle materials with circular pore flaws, which incorporates both the number density and the size distribution of flaws. Results show that the distribution of flaw sizes is very important, particularly at moderate strain rate, since analyses based solely on the mean flaw size overpredict strength. Therefore, in order to increase dynamic strength at low to moderate strain rates, it is most effective to control the presence of large flaws. At very high strain rates, however, crack growth is activated even in small flaws and therefore controlling the total number density rather than the size of the flaws is effective for increasing dynamic strength. Finally, the model shows that neglecting very small flaws in the pore population may not have significant effects on the results in many cases, suggesting that the model is a useful tool for identifying a minimum resolution required for experimental characterization of microstructure.     

准静态条件下金属材料的临界断裂准则研究

本文针对9种金属材料完成了具有不同约束程度的10类试样的延性断裂试验, 获得了发生拉、压、扭和裂尖断裂等破坏形式构型试样的载荷-位移试验关系; 基于圆棒漏斗试样拉伸试验所得直至破坏的载荷-位移曲线, 采用有限元辅助试验(finite-element-analysis aided testing, FAT)方法得到了9种材料直至破坏的全程等效应力-应变曲线, 以此作为材料本构关系通过有限元分析获得了各类试样直至临界破坏的载荷-位移关系模拟. 载荷-位移关系模拟结果与试验结果有较好的一致性, 表明用于解决试样颈缩问题的FAT方法所获得的全程材料本构关系针对各向同性材料具有真实性和普适性. 对应9种材料、10类试样的36 个载荷-位移临界断裂点, 通过有限元分析获得了对应的材料临界断裂应力、应变与临界应力三轴度, 研究表明, 第一主应力在延性变形过程中为主控断裂的主导参量; 通过研究光滑、缺口、裂纹等构型试样的断裂状态, 提出了$-1$至3范围的应力三轴度下由第一主应力主控的统一塑性临界断裂准则.      

Particle flow study on strength and meso-mechanism of Brazilian splitting test for jointed rock mass

A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.     

薄膜涂层材料界面纯剪破坏标准试验法的开发

界面强度是薄膜涂层材料的最重要的性能指标之一，目前尚缺乏有效的测量方法。测量界面强度的主要困难在于寻求一种便于试验的试件形状和加载方式，使得界面上能够产生不同的应力状态，即在不同的剥离应力和剪应力比的状态下发生破坏。本文采用有限元数值分析（ABAQUS），研究了几种具有简单几何形状的试件的界面应力，并基于大量的数值试验，设计了剪应力为破坏支配因素的试件形状和加载方式，并且给出了便于应用的最大界面剪应力的经验估算公式。该经验公式可以适用于各种材料组合和薄膜涂层的厚度。研究结果表明，通过对薄膜涂层材料试件的基体引入缺口以产生应力集中，进行普通的四点弯曲试验，可以进行剪应力占支配地位的界面破坏试验。利用本文提出的试件形状和相应的最大界面剪应力经验公式，可以通过破坏试验简单地得到界面的剪切强度。     

Fracture toughness for CNT specimens from numerically obtained critical CTOD values

Many approaches for estimating mode I fracture toughness (K_IC) using circumferentially notched tensile (CNT) specimen have been demonstrated in the literature. In this paper, an effective approach for estimating fracture toughness from the numerical solution of critical crack tip opening displacement (CTOD) is proposed. An elasto-plastic finite element analysis is used to estimate critical CTOD values for CNT specimens. A number of materials are analysed, and the relationship between K_IC and critical CTOD for CNT specimens is obtained. The proposed relationship is validated by comparing the fracture toughness values obtained from the relationship with those obtained experimentally using CNT specimens. The fracture toughness (K_IC) calculated according to this relationship from numerically obtained critical CTOD is found to be in close agreement with the experimentally obtained fracture toughness for the respective materials.     

脆性材料的扩展裂纹前缘形态与三维J积分

本文利用超声波断口图技术对不同类型的PMMA试件进行了测试。这些试件为承受均匀拉伸或三点弯曲变形的带单侧贯穿裂纹的PMMA板，其中拉伸试件又分为光面和带表面沟槽的两种。得到了这些试件断口上的超声波线。发现它们为平面线．其凸凹方向与试件的表面状态有关。基于有限元法，计算了试件的三维J积分，考察了杨氏模量和泊松比对其的影响．结果发现泊松比影响J积分曲线形状，而杨氏模量只影响其绝对数值，不影响其曲线形状。得到的试件在不同裂纹长度下的三维J积分与它们断口图上的超声波线的形状和凸凹方向相似。说明在脆性材料中，扩展裂纹前缘曲线形态是三维J积分的作用的结果。     

Size Effect on The Bending And Tensile Strength of Micromachined Polysilicon Films for MEMS

I. INTRODUCTION Microelectromechanical systems (MEMS) have achieved impressive progress and become a very area of research. But long-term durability of various MEMS devices requires a fundamental understaof the fatigue and fracture characteristics of su…     

Constitutive modeling and the trousers test for fracture of rubber-like materials

The classical constitutive modeling of incompressible hyperelastic materials such as vulcanized rubber involves strain-energy densities that depend on the first two invariants of the strain tensor. The most well-known of these is the Mooney-Rivlin model and its specialization to the neo-Hookean form. While each of these models accurately predicts the mechanical behavior of rubber at moderate stretches, they fail to reflect the severe strain-stiffening and effects of limiting chain extensibility observed in experiments at large stretch. In recent years, several constitutive models that capture the effects of limiting chain extensibility have been proposed. Here we confine attention to two such phenomenological models. The first, proposed by Gent in 1996, depends only on the first invariant and involves just two material parameters. Its mathematical simplicity has facilitated the analytic solution of a wide variety of basic boundary-value problems. A modification of this model that reflects dependence on the second invariant has been proposed recently by Horgan and Saccomandi. Here we discuss the stress response of the Gent and HS models for some homogeneous deformations and apply the results to the fracture of rubber-like materials. Attention is focused on a particular fracture test, namely the trousers test where two legs of a cut specimen are pulled horizontally apart. It is shown that the cut position plays a key role in the fracture analysis, and that the effect of the cut position depends crucially on the constitutive model employed. For stiff rubber-like or biological materials, it is shown that the influence of the cut position is diminished. In fact, for linearly elastic materials, the critical driving force for fracture is independent of the cut position. It is also shown that the limiting chain extensibility models predict finite fracture toughness as the cut position approaches the edge of the specimen whereas classical hyperelastic models predict unbounded toughness in this limit. The results are relevant to the structural integrity of rubber components such as vibration isolators, vehicle tires, earthquake bearings, seals and flexible joints.     

用试验室尺寸的试样预测混凝土及岩石结构的准脆性断裂

0Introduction Thefracturepropertiesofconcreteandrockmaterials,suchasfracturetoughnessKICand strengthfthavebeenofgreatinterestsformanyyearsbecauseoftheirimportantroleincontrolling thestructuralintegrityofvariousengineeringstructures[1-27].Varioustestingtec…     

聚氨酯泡沫塑料的强度与断裂韧性

针对3种密度的聚氨酯泡沫塑料进行了拉伸实验。通过无缺口试件确定了3种密度泡沫塑料的拉伸断裂强度,而利用有缺口试件确定了这些材料的拉伸断裂韧性。为了研究高密度泡沫塑料的拉伸断裂机制,还对破坏后试件进行了扫描电镜分析。此外,还简要讨论了泡沫塑料拉伸断裂力学性能的理论预测问题。     

线性规律功能梯度材料断裂行为实验研究

焦散线方法与高速摄影技术结合,对线性规律功能梯度材料的I型静、动态断裂特性进行实验研究,并利用有限元软件进行数值模拟. 首先推导了线性规律功能梯度材料静、动态裂尖的焦散线初始曲线方程和参数方程,提出通过迭代方法根据功能梯度材料的复杂焦散线方程求解应力强度因子. 为对比材料梯度对断裂行为的影响,利用重力沉降法制备了3种梯度变化规律的试件. 然后对功能梯度材料试件进行三点弯曲实验,通过CCD和高速摄影装置采集试件焦散线图像,提取各种静、动态断裂参数. 最后通过与有限元模拟结果进行对比,证明在功能梯度材料断裂实验中焦散线方法的有效性,并详细分析了不同梯度变化规律对材料静、动态断裂性能的影响.      

Tensile fracture behavior of heterogeneous materials based on fractal geometry

Many of heterogeneous structural materials, like concrete, have different behavior under tensile stresses in comparison to their behavior under compressive stresses. The aim of this paper is to interpret behavior of such materials subjected to tensile stresses, by using newly introduced concept of fractal geometry. In the first part of this paper, tensile behavior of granular composites has been studied by using fractal geometry. It is shown that the fractality of the cross section in this kind of composites can be used to interpret the size effect on tensile strength. In fact, this work is a modification with innovations on the previous studies on fractal based size effect.This hypothesis that the fracture surfaces of quasi-brittle materials are fractals has been verified by several investigations. Accordingly, in the other part of this paper, softening process in heterogeneous materials is studied. Resulting from presented approach, a new softening curve for quasi-brittle materials is proposed. This new softening curve is denominated “Quasi-fractal softening curve” and is consisted of two parts, a linear portion in beginning part and an exponential portion in rest of the curve. This makes it very compatible to the pre-existing softening curves.