Mechanical Fatigue Measurement via a Vibrating Cantilever Beam for Self-Supported Thin Solid Films

In this paper, we develop a novel experimental apparatus, referred to as the resonant frequency device, and establish methodology to measure the fatigue properties of thin solid films. Arranging thin-film strips of our specimens into the mechanical setting of a cantilever beam and using state-of-the-art piezo actuators to generate oscillation at the clamp of the cantilever, we create a system suitable for studying the material properties of the cantilever, such as Young's modulus, fatigue and possibly, loss tangent. Deformation of the cantilever is our controlled variable in the present study, and measured with fiber-optic probes pointed at the specimen and at the piezo driver. Stress is calculated from relative deformation of the cantilever specimen with respect to the piezo actuator via a photograph of the cantilever under vibration with a curve fitting method. A LabView computer program is developed for the fatigue tests to accurately count number of cycles applied on the specimens, and a feedback mechanism is adopted to maintain displacement during the tests. Here, we present our experimental setup, procedure and theoretical models for material-property extraction. For small displacement, the two-dimensional Euler–Bernoulli beam theory is adopted. With large displacement, the system behaves as the Duffing oscillator due to geometrical nonlinearity. In addition, some experimental observations of the piezo actuators and fiber optics are reported. The method is applied to evaluate the fatigue properties of nanolayered copper-niobium composites and significant increase in the fatigue endurance limit compared to the constituent materials in the bulk form is noted.     

Transition Modelling for General Purpose CFD Codes

The paper addresses modelling concepts based on the RANS equations for laminar-turbulent transition prediction in general-purpose CFD codes. Available models are reviewed, with emphasis on their compatibility with modern CFD methods. Requirements for engineering transition models suitable for industrial CFD codes are specified. A new concept for transition modeling is introduced. It is based on the combination of experimental correlations with locally formulated transport equations. The concept is termed LCTM – Local Correlation-based Transition Model. An LCTM model, which satisfies most of the specified requirements is described, including results for a variety of different complex applications. An incremental approach was used to validate the model, first on 2D flat plates and airfoils and then on to progressively more complicated test cases such as a three-element flap, a 3D transonic wing and a full helicopter configuration. In all cases good agreement with the available experimental data was observed. The authors believe that the current formulation is a significant step forward in engineering transition modeling, as it allows the combination of transition correlations with general purpose CFD codes. There is a strong potential that the model will allow the 1st order effects of transition to be included in everyday industrial CFD simulations.     

湍流转捩模式研究进展

以湍流模式理论为基础发展起来的湍流转捩模式是一种十分重要的转捩预测方法. 本文对转捩模式的研究进行了回顾. 首先, 探讨了以低雷诺数湍流模式理论研究转捩问题的局限性,此类模式中, 用来模拟黏性层次的阻尼函数经过修正之后, 具有一定的转捩预测能力. 但也有观点认为这只是一种数值上的巧合而已. 其次, 指出了考虑间歇性的模式所存在的问题. 此类模式通过各种方式将间歇因子与湍流模式进行耦合, 在一定程度上考虑了转捩的物理机制, 可较好地模拟简单流动中的转捩过程. 但其中所包含的非局部变量使其与现代CFD方法并不协调一致. 接着, 分析了完全由局部变量构造的新型转捩模式, 其中涉及了关于非湍流脉动动能等转捩特征变量的新型输运方程. 最后, 对此领域的发展方向进行了预测.      

Contribution of Kinematical and Thermal Full-field Measurements for Mechanical Properties Identification: Application to High Cycle Fatigue of Steels

The purpose of this work is to extend the use of unconventional tests and full field measurements (kinematical and thermal) to the identification of the effect of a wide plastic pre-strain range on the high cycle fatigue properties of a dual-phase steel. An unconventional specimen is designed. The geometry of this specimen permits a constant gradient of pre-strain to be obtained after a monotonic tensile test. Then, a self-heating test under cyclic loading is carried out on the pre-strained specimen. During this cyclic test, the thermal field is measured using an infrared camera. Finally, a suitable numerical strategy is proposed to identify a given thermal source model taking into account the influence of plastic pre-strain. The results show that, with the unconventional test and the procedure developed in this work, the influence of a plastic pre-strain range on fatigue properties can be identified by using only one specimen when, for a classical fatigue campaign, a great number of specimens is required.     

Reducing the anisotropy of a Brazilian disc generated in a bonded-particle model

The Brazilian test is a widely used method for determining the tensile strength of rocks and for calibrating parameters in bonded-particle models (BPMs). In previous studies, the Brazilian disc has typically been trimmed from a compacted rectangular specimen. The present study shows that different tensile strength values are obtained depending on the compressive loading direction. Several measures are proposed to reduce the anisotropy of the disc. The results reveal that the anisotropy of the disc is significantly influenced by the compactibility of the specimen from which it is trimmed. A new method is proposed in which the Brazilian disc is directly generated with a particle boundary, effectively reducing the anisotropy. The stiffness (particle and bond) and strength (bond) of the boundary are set at less than and greater than those of the disc assembly, respectively, which significantly decreases the stress concentration at the boundary contacts and prevents breakage of the boundary particle bonds. This leads to a significant reduction in the anisotropy of the disc and the discreteness of the tensile strength. This method is more suitable for carrying out a realistic Brazilian test for homogeneous rock-like material in the BPM.     

基于厚度法确定小冲杆试验能量转变温度

以A106管材钢为研究对象,进行了小冲杆断裂试验,获得断后试样的最小厚度;在对试样断裂后的变形分析中,建立了试样厚度减薄率与温度的关系,规避了低温试验中载荷和位移测量不准确导致小冲杆能量转变温度不确定的问题。结果表明:基于厚度法可以准确确定小冲杆断裂试验的能量转变温度;小冲杆断裂试验后,试样最薄厚度的减小率与温度曲线关系呈现S型,有上平台、转变区、下平台三个典型区域;根据厚度减薄率与温度曲线关系可以得到小冲杆的能量转变温度。采用厚度法得到的小冲杆断裂试验的转变温度与能量法获得的结果基本一致。     

On calibration of adjustable strain transducers

Biaxial strain transducers are suitable for simultaneous measurements of axial and transverse strains in uniaxial tensile tests. This paper describes a specific device where adjustment to different specimen widths is done by means of adjustable contacting screws. It is pointed out that the sensitivity of the axial sensors depends heavily on the screw position. This dependency is unfortunate as it complicates the calibration procedure and implies a potential risk of severe calibration errors. The performance of the transducer is modeled by simple beam theory combined with finite element calculations of two essential flexibilities. A calibration procedure based on the theoretical model is described, and experimental results for two different contacting screws are presented. A suggestion for a transducer design eliminating the problem is outlined.     

Vibration reduction in a flexible-link mechanism through synthesis of an optimal controller

In this paper, reduction of vibration of a flexible planar mechanism is achieved through synthesis of an optimal controller. A finite element model, based on the equivalent rigid-link system theory, is used to accurately describe the dynamic behavior of the system. The model, which accounts for geometric and inertial nonlinearities of the mechanism, has been fully validated through experimental tests. In order to be able to employ the classical optimal control theory, a suitable linear model has been derived from the original one by means of a suitable linearization procedure. Vibration reduction can then be obtained by first defining an adequate performance index, which accounts for vibration amplitude, then by solving Riccati’s equation in order to find the controller that minimizes the performance index, i.e. the optimal controller. The results of several tests that have been carried out are also reported, to show the effectiveness of the synthesized control system.     

Identification of Material Damage Model Parameters: an Inverse Approach Using Digital Image Processing

A reliable prediction of ductile failure in metals is still a wide-open matter of research. Several models are available in the literature, ranging from empirical criteria, porosity-based models and continuum damage mechanics (CDM). One major issue is the accurate identification of parameters which describe material behavior. For some damage models, parameter identification is more or less straightforward, being possible to perform experiments for their evaluation. For the others, direct calibration from laboratory tests is not possible, so that the approach of inverse methods is required for a proper identification. In material model calibration, the inverse approach consists in a non-linear iterative fitting of a parameter-dependent load–displacement curve (coming from a FEM simulation) on the experimental specimen response. The test is usually a tensile test on a round-notched cylindrical bar. The present paper shows a novel inverse procedure aimed to estimate the material parameters of the Gurson–Tvergaard–Needleman (GTN) porosity-based plastic damage model by means of experimental data collected using image analysis. The use of digital image processing allows to substitute the load–displacement curve with other global quantities resulting from the measuring of specimen profile during loading. The advantage of this analysis is that more data are available for calibration thus allowing a greater level of confidence and accuracy in model parameter evaluation.     

多亚层柔性节点模型及其在双材料4ENF试件界面分析中的应用

提出了多亚层柔性节点模型用于分析双材料裂纹尖端的应力和变形。该模型考虑了胶层的变形,各亚层视为独立的剪切变形梁,采用两个界面柔度系数考虑界面应力对各亚层界面变形的影响,界面变形包括双材料界面和胶层的变形。通过对FRP-混凝土末端切口四点弯试件(Four-point bending end-notched flexure specimen,简称4ENF)进行界面分析,并与其他模型和有限元分析对比表明:刚性节点模型忽略了裂纹尖端的应力和变形集中,只能粗略地估计构件的整体变形和界面应力;半刚性节点容许裂纹尖端的转动,对裂纹尖端的变形估计优于刚性节点模型,但精度依然不高;多亚层柔性节点模型反映了裂纹尖端的应力和变形集中,与数值分析结果吻合很好,该研究对进行双材料结构的工程设计具有理论指导和参考价值。     

混凝土断裂能测试方法研究

基于局部断裂能分布的双直线模型,推导出混凝土真实断裂能和受尺寸影响的断裂能的计算公式;并进一步通过分析四组不同尺寸试件的楔形劈裂试验数据,得出了不受尺寸影响的混凝土真实断裂能.本文为确定混凝土的断裂能提供了一种实践可行的测试方法.通过实验数据拟合,给出了断裂能非均匀分布的外部区域长度与试件尺寸的关系表达式.这对于在规范中规定测试断裂能的标准尺寸试件是有意义的.     

On-Chip Mechanical Characterization using an Electro-thermo-mechanical Actuator

A new micro-system for the on-chip mechanical characterization of thin polysilicon films was designed, fabricated and tested. The device contains a micro electro-thermo mechanical actuator which is able to load a specimen until rupture in purely tensile conditions. The elongation of the specimen is measured during the test through the capacitance variation of a set of parallel plate capacitors, while the force in the specimen can be computed starting from the applied voltage. Through a unique loading cycle in which the voltage is first increased until specimen rupture and then decreased, the new device allows for the determination of the specimen elastic stiffness and nominal tensile strength. The obtained experimental results were compared with values previously obtained by means of different on-chip test devices. Multi-physics FE simulations were performed for additional comparison with analytical formulae used in the data reduction procedure and with the obtained experimental results.     

Elastic characterization of orthotropic plates of any shape via static testing

The paper presents an inverse procedure for identifying elastic properties of isotropic or orthotropic materials from the full-field measurement of the surface displacements of plates under flexural loading configurations. The procedure is based on a numerical–experimental optimisation process which minimizes an error function defined by subtracting the experimental data from the outputs of the numerical analysis. In each iteration the optimisation process updates the values of the elastic constants in a finite element model of the specimen used in the experimental tests. The unknown parameters are simultaneously identified by a single test and without damaging the structural integrity of the specimen. The possibility of using the methodology for characterizing any-shaped plates was investigated. The applicability and the robustness of the procedure were carried out on aluminum and unidirectional Graphite/PEEK laminate specimens. Phase-shifting speckle interferometry was employed to detect the out-of-plane displacement field of a portion of the observed surface of the specimen.     

On two distinct types of drag-reducing fluids,diameter scaling,and turbulent profiles

Two distinct scaling procedures were found to predict the diameter effect for different types of drag-reducing fluids. The first one, which correlates the relative drag reduction (DR) with flow bulk velocity (V), appears applicable to fluids that comply with the 3-layers velocity profile model. This model has been applied to many polymer solutions; but the drag reduction versus V scaling procedure was successfully tested here for some surfactant solutions as well. This feature, together with our temperature profile measurements, suggest that these surfactant solutions may also show this type of 3-layers velocity profiles (3L-type fluids).The second scaling procedure is based on a correlation of τ_w versus V, which is found to be applicable to some surfactant solutions but appears to be applicable to some polymer solutions as well. The distinction between the two procedures is therefore not simply one between polymer and surfactants. It was also seen that the τ_w versus V correlation applies to fluids which show a stronger diameter effect than those scaling with the other procedure. Moreover, for fluids that scale according to the τ_w versus V procedure, the drag-reducing effects extend throughout the whole pipe cross section even at conditions close to the onset of drag reduction, in contrast to the behavior of 3L fluids. This was shown by our measurements of temperature profiles which exhibit a fan-type pattern for the τ_w versus V fluids (F-type), unlike the 3-layers profile for the fluids well correlated by drag reduction versus V. Finally, mechanically-degraded polymer solutions appeared to behave in a manner intermediate between the 3L and F fluids.Furthermore, we also showed that a given fluid in a given pipe may transition from a Type A drag reduction at low Reynolds number to a Type B at high Reynolds number, the two types apparently being more representative of different levels of fluid/flow interactions than of fundamentally different phenomena of drag reduction. After transition to the non-asymptotic Type B regime, our results suggest that, without degradation, the friction becomes independent of pipe diameter and that the drag reduction level becomes also approximately independent of the Reynolds number, in a strong analogy to Newtonian flow.     

Yield behavior of photoplastic materials

A mixture of flexible and rigid polyester resins has been used previously by Morris and Riley¹⁶ and by Zachary and Riley⁹ to model plastic deformations. The last of these papers furnished mechanical and optical properties under uniaxial tension and compression for several different mixture ratios of the polyester resins and also presents some results under multiaxial-stress conditions from thin cylinders under internal pressure. In a recent paper, Burger, Gomide and Scott¹⁴ used the rigid polyester resin at elevated temperature to model plastic deformations in upset rings; the behavior of the rigid polyester was verified with diametrically compressed disks and uniaxial-compression specimens. A very important similitude requirement for model to prototype scaling in photoplasticity work is that the macroscopic yield behavior of model and prototype materials must be the same. Thus, not only uniaxial tension and compression properties must be examined, but also yield properties under multiaxial-stress states have to be determined. The purpose of this paper is to provide additional information on the yield behavior of polyester mixtures which appear suitable for model studies of manufacturing methods such as rolling and extruding. For these processes, mixture ratio, test temperature and strain rate can be used to control the shape of the stress-strain curve and the yield behavior. The experimental procedure used to determine the initial yield locus of the photoplastic materials employed a new specimen geometry proposed by Arcan, Hashin and Voloshin¹⁸ which produces uniform biaxial-stress fields of opposite sign in one section of the specimen. Both polycarbonate and polyester materials were evaluated using this procedure and results are compared with those available in the technical literature.     

A methodology to assess the rate and pressure sensitivity of polymers over a wide range of strain rates

This paper details a methodology to test the mechanical response of soft, pressure sensitive materials, over a wide range of strain rates. A hybrid experimental-numerical procedure is used to assess the constitutive parameters. The experimental phase involves axial compression of a cylindrical specimen which is confined by a tightly-fit sleeve that is allowed to yield plastically, thus applying a constant confining pressure. The usually neglected frictional effects between the specimen and the sleeve are fully accounted for and characterized in detail. With commercial polycarbonate as a typical example, it is shown that pressure sensitivity and rate sensitivity are not coupled, thus reducing the number of tests needed to characterize a material. The results of numerical simulations indicate that the pressure sensitivity index (angle β in the Drucker-Prager material model) has little influence on the hydrostatic and confining pressures, whereas the equivalent stress sustained by the specimen increases with β, which for commercial polycarbonate is found to be β=15°.     

Application of strain rate potentials with multiple linear transformations to the description of polycrystal plasticity

This paper reviews a class of anisotropic plastic strain-rate potentials, based on linear transformations of the plastic strain-rate tensor. A new formulation is proposed, which includes former models as particular cases and allows for an arbitrary number of linear transformations, involving an increasing number of anisotropy parameters. The formulation is convex and fully three-dimensional, thus being suitable for computer implementation in finite element codes. The parameter identification procedure uses a micromechanical model to generate evenly distributed reference points in the full space of possible loading modes. Material parameters are determined for several anisotropic, fcc and bcc sheet metals, and the gain in accuracy of the new models is demonstrated. For the considered materials, increasing the number of linear transformations leads to a systematic improvement of the accuracy, up to a number of five linear transformations. The proposed model fits very closely the predictions of the micromechanical model in the whole space of plastic strain-rate directions. The r-values, which are not directly used in the identification procedure, served for the validation of the models and to demonstrate their improved accuracy.     

A Test Method for Characterizing Clear Wood Using a Single Specimen

The constitutive models used for wood and timber have been formulated in different scales. The term clear wood corresponds to the macroscopic scale, where the orthotropic elastic model is commonly used. This model, which is typically applied to local modeling (e.g. connections), is defined by a compliance matrix composed of nine parameters, the elongations and angular distortions of which are uncoupled. The current experimental standard methods for obtaining these nine parameters are time consuming, as different specimen configurations and careful preparation for specific grain orientation are needed. As different specimens are used and the material properties of wood can vary greatly at a local level, this would have a notable influence on the outcome, as this method is not robust. Optical digital measurements (ODM) are changing and improving these traditional procedures. This work presents a new procedure for determining the compliance matrix of clear wood using just one rectangular prism as a specimen, with an orientation of the grain that does not coincide with the axes defined by the edges of the specimen. The specimen is loaded in the three mutually perpendicular directions defined by the prism, in a single axis load set up using a 3D ODM. The method was initially validated using ARAMIS® 3D, with 5 M pixels, on ash (Fraxinus excelsior L). Reasonable results were obtained, according to the resolution used; however, a higher resolution is needed to attain proper accuracy.