The Potential Scope of the Ultrasonic Surface Reflection Method Towards Mechanical Characterisation of Isotropic Materials. Part 1. A Theoretical Analysis

Background

This paper deals with the possible field of application of ultrasonic Surface Reflection Method (SRM) to achieve the mechanical characteristics of isotropic materials. This method is based on the measurement of the amplitude of the reflected wave at the interface between reference material and the material to be characterised. Objective: The purpose of Part 1 of this paper is to establish the theoretical conditions for the applicability of SRM.

Methods

First, the theoretical formulas necessary to obtain the mechanical properties of the material to be tested will be established. Then, on the basis of these analytical formulas, the validity of the results for the material to be studied will be discussed according to the choice of the mechanical properties of the reference material through uncertainty calculations. The measurand error of SRM is then compared to that of traditional methods (transmission, transmission in water bath, pulse-echo).

Results

The analytical solution to the inverse problem (the mechanical characteristics of the tested medium based on those of the reference medium and the waves’ amplitude) will be given. From this analytical solution, an analysis of the measurand error will be performed and a method for choosing the reference material will be proposed.

Conclusions

It appears that SRM is better suited than traditional methods in two specific cases: measurement of small deviations of mechanical properties from a reference material or characterisation of high damping materials. In Part 2 of this paper, the practical conditions of applicability of the method are described and then applied to different kinds of materials.

     

Identification of the Elastic Properties of Isotropic and Orthotropic Thin-Plate Materials with the Pulsed Ultrasonic Polar Scan

Already in the early 1980’s, it has been conjectured that the pulsed ultrasonic polar scan (P-UPS) provides a unique fingerprint of the underlying mechanical elasticity tensor at the insonified material spot. Until now, that premise has not been thoroughly investigated, nor validated, despite the opportunities this would create for NDT and materials science in general. In this paper, we report on the first-ever implementation of an inverse modeling technique on the basis of a genetic optimization scheme in order to extract quantitative information from a P-UPS. We validate the optimization approach for synthetic data, and apply it to experimentally obtained polar scans for annealed aluminum, cold rolled DC-06 steel as well as for carbon fiber reinforced plastics. The investigated samples are plate-like and do not require specific preparation. The inverted material characteristics show good agreement with literature, micro-mechanical models as well as with results obtained through conventional testing procedures.     

类岩石材料力学特性的试验及数值模拟研究

在进行多组不同配比类岩石材料单轴压缩试验和巴西试验的基础上,详细分析了石膏水泥比和石英砂含量对类岩石材料的单轴抗压强度、抗拉强度及弹性模量等力学参数的影响规律,力图找到适合模拟现场砂质泥岩的类岩石材料及配合比。利用颗粒流程序(PFC)模拟,进一步研究了高径比和围压对类岩石材料力学特性的影响。结果表明:随着石膏水泥比的增大,抗压强度和弹性模量均逐渐减小,而抗拉强度逐渐增大;随着石英粉含量的增大,抗压强度和弹性模量均先增大后减小,而抗拉强度则为先减小后增大。结合单轴压缩过程的声发射特征,揭示了裂纹扩展与声发射有密切的关系。PFC2D模拟获得的力学参数与室内试验相近,破裂模式也与实际情况相似。通过尺寸效应的研究可知试样的高径比在2.0~2.5较合理。随着围压的增大,试样的峰值强度、残余强度、峰值应变及弹性模量等力学参数均增大,且围压会改变试样的破裂模式。     

类岩石材料中表面裂纹扩展模式的实验研究

采用粘贴式轴拉方法实现了类岩石材料的直接拉伸实验,通过在类岩石材料试件中预制表面裂纹研究了类岩石材料中表面裂纹的扩展模式.研究发现,粘贴式轴拉方法可以满足一般类岩石材料的直接拉伸实验;类岩石材料中表面裂纹首先从试件正面开始扩展;表面裂纹的存在极大地影响着材料的破坏模式,类岩石材料在试件的正面上会表现出明显的二维穿透性裂纹的扩展形态,但是受其影响裂纹在厚度方向扩展会发生偏转,从而使得试件背面的最终贯通方向没有与加载方向垂直,而是呈一倾角,并且和表面裂纹的倾角和深度有关.     

Potential Method in the Linear Theory of Viscoelastic Materials with Voids

In the present paper the linear theory of viscoelasticity for Kelvin–Voigt materials with voids is considered and some basic results of the classical theory of elasticity are generalized. Indeed, the basic properties of plane harmonic waves are established. The explicit expression of fundamental solution of the system of equations of steady vibrations is constructed by means of elementary functions. The Green’s formulas in the considered theory are obtained. The uniqueness theorems of the internal and external basic boundary value problems (BVPs) are proved. The representation of Galerkin type solution is obtained and the completeness of this solution is established. The formulas of integral representations of Somigliana type of regular vector and regular (classical) solution are obtained. The Sommerfeld-Kupradze type radiation conditions are established. The basic properties of elastopotentials and singular integral operators are given. Finally, the existence theorems for classical solutions of the internal and external basic BVPs of steady vibrations are proved by using of the potential method (boundary integral method) and the theory of singular integral equations.     

Intergranular and intragranular behavior of polycrystalline aggregates.Part 2: Results

The global, intergranular and intragranular responses of a polycrystalline aggregate are investigated. It is shown that the heterogeneity of stress and strain dramatically increases from the global to the local level. Plastic deformation structures develop on a scale larger than the grain. Several types of boundary conditions are applied to polycrystalline aggregates, in order to estimate the importance of the surface effect. The results obtained are presented as contour plots on the cube, and a detailed study is performed to relate the variation obtained with the orientation of the grain and the position in the aggregate.     

Method of Solving the Problem of Thermoradiation Plasticity for Layered Axisymmetric Bodies Made of Isotropic and Orthotropic Materials*

International Applied Mechanics - A method for the numerical analysis of the elastic and plastic stress–strain state of layered bodies of revolution made of isotropic and orthotropic...     

化学腐蚀对混凝土材料力学特性影响的试验研究

对不同水化学环境腐蚀过程中砂浆试样物理力学性质进行了试验研究,分析了溶液pH值、浓度及水化学成分对砂浆的腐蚀程度,探讨了砂浆试样的化学腐蚀机制。试验结果表明:在相对封闭的环境条件下,无论是哪种水化学溶液,随着化学侵蚀时间的加长,溶液的pH值均趋于碱性;同时,溶出的Ca2+、Mg2+离子具有累积性特点。在其他条件相同的情况下,水化学溶液的离子成分对砂浆试样的腐蚀程度各不相同。酸性条件下,Na2SO4溶液砂浆试样物理化学性质的影响要比NaHCO3溶液大;中性条件下,与纯净的蒸馏水相比,Na2SO4溶液对砂浆试样的腐蚀作用要强一些。     

利用数字散斑相关法测定聚酰亚胺/SiO2合成薄膜的力学性能

聚酰亚胺 /SiO2 合成薄膜是一种具有优良力热光电性能的薄膜材料,在MEMS工艺中具有广阔的应用前景,其力学性能测量的研究具有非常重要的意义。本文将数字散斑相关技术和微拉伸试验相结合,对厚度为 37μm的聚酰亚胺(polyimide) /二氧化硅(SiO2 )合成薄膜进行了力学性能测量,获得了比较满意的结果。文章给出了测得的弹性模量和泊松比。为了解决数字散斑相关法不能直接测量较大变形的缺陷,本文提出了多级相关算法,并利用亚像素搜索和双线性插值进行了数据处理。     

Methods and Results of an Experimental Determination of the Catalytic Activity of Materials at High Temperatures

Methods of experimental determination of the rate constants of heterogeneous catalytic recombination of nitrogen and oxygen atoms in dissociated subsonic nitrogen and air flows generated by a high-frequency inductive plasmatron are discussed. Together with the values of the probability of heterogeneous recombination used in the literature, the rate constant of the elementary process of nitrogen atomic recombination is determined in accordance with the Langmuir-Hinshelwood mechanism.     

制动盘材料表面织构化处理对摩擦噪声影响的试验分析

在列车制动盘材料表面加工出径向均匀分布的具有不同宽度的沟槽,采用列车制动片为配副材料,在面-面接触模式下对其进行了销-盘式摩擦噪声试验研究,探讨织构表面对摩擦尖叫噪声特性的影响及其作用机理.结果表明:具有特定合理尺寸参数的织构表面能有效地抑制摩擦尖叫噪声.表面沟槽的存在,一方面能提高界面的排屑效率,抑制尖叫噪声的产生,另一方面沟槽棱角与摩擦片平面的碰击能有效地抑制界面摩擦自激振动的高频成分,并达到降低噪声的效果.沟槽的棱角在整个试验过程中保持完好,其降噪作用寿命时间较长.     

Simplified prediction of the creep buckling of cylinders under external pressure. Part 2: Experimental verification

In this paper, the simplified method, proposed in (Combescure, 1998), for the prediction of creep buckling is compared to experimental results. The model is applied to predict the buckling time of two sets of experiments on cylinders subjected to uniform external pressure. It is shown that the proposed model is satisfactory for this type of prediction: in all cases, the times up to failure predicted by the model are generally lower than the experimental failure times. The model is rather conservative for thicker cylinders. However, it appears that a very detailed geometrical imperfection survey would be necessary if a highly accurate assessement of the creep failure time were sought. It has been observed experimentally that creep buckling is a very dangerous failure mode: nothing seems to happen during a very long “incubation” period but, when the initial imperfection reaches some critical value, buckling then suddenly occurs. For thin cylinders, the level of creep strain at which the instability starts to develop is much lower than the strain at which the tertiary creep initiates; the instability is thus clearly generated from the interaction between the material and the geometrical nonlinearity.     

The Scale-Adaptive Simulation Method for Unsteady Turbulent Flow Predictions. Part 2: Application to Complex Flows

The paper summarises the validation activity performed with the Scale-Adaptive Simulation turbulence model (SAS model) using the two commercial CFD solvers, ANSYS-FLUENT and ANSYS-CFX. Both the KSKL-SAS and the SST-SAS model variants have been tested, although most cases have been computed with the second. The turbulence-resolving capability of the SAS method has been validated with a representative set of test cases, covering both underlying generic flows as well as practical engineering applications. In addition to the purely aerodynamic flows with massive separation and heat transfer they include also such physical phenomena as turbulent combustion and aeroacoustics. The illustrating results show the potentials of the SAS approach for industrial flow simulations. Most of the test case simulations were conducted during the recent EU project “DESider”.     

A Device for Biaxial Testing in Uniaxial Machines. Design,Manufacturing and Experimental Results Using Cruciform Specimens of Composite Materials

The present study deals with the design and manufacturing of a mechanical device to perform biaxial testing in universal (uniaxial) testing machines. A review of previous definitions of similar devices is carried out and a new device is conceived and developed. The main improvement with the present device is that it allows different types of biaxial loadings (tension-compression) to be performed with few manipulations. The device allows variable displacement ratio to be used in each loading direction, giving then rise to variable loading ratios. Biaxial tension-tension tests on cruciform specimens made of composite material were carried out using very brittle samples in which the fibre direction was perpendicular to the loading plane. Strain gages were used to monitor the percentage bending parameter so that the correct alignment of the loading could be checked. Values below 5% for the bending parameter were achieved at the moment of failure.     

On the Dynamics of Mechanical Systems with the Homogeneous Polynomial Potential V = ax 4 + cx 2 y 2

This paper is devoted to analyze certain dynamical aspects of the planar mechanical system associated to the potential V = ax ⁴ + cx ² y ² as a function of the real parameters a and c.     

The Stress State of a Transversely Isotropic Plate with a Curvilinear Hole for a Given Splitting Force on the Boundary Surface

International Applied Mechanics - The problem of the stress state of an infinite transversely isotropic plate with curved (noncircular) hole is solved by using the method of expanding the unknown...     

Experimental Investigation of Thermocapillary Convection Induced by a Local Temperature Inhomogeneity near the Liquid Surface. 2. Radiation-Induced Source of Heat

The structure and stability of a thermocapillary flow from a concentrated source of heat induced by radiation and located near the free surface of the liquid filling a deep reservoir are experimentally studied. Absolute stability of this flow for all depths and powers of the heat source is found. The profile of the liquid surface near the heat spot is considered experimentally.     

Experimental characterization of the instability of the vortex rings. Part II: Non-linear phase

The first stage of the instability of a vortex ring is linear and characterized by the growth of an azimuthal stationary wave which develops around the ring. Theoretical works predict its origin, shape, number of waves and growth rate. Apart for the growth rate, experimental and numerical results in viscous fluids fit well with the predictions based on an ideal fluid hypothesis. On the other hand, the next stages of the development of the instability (which are non-linear) are not well known. Only few phenomena are described, in an isolated way, in various partial contributions. The aim of this paper is to report on a complete experimental investigation of the non-linear phase of the instability of the vortex ring. The vortices were produced in water and their Reynolds number Re _p was varied from 2,650 to 6,100. Visualizations were performed using planar laser induced fluorescence and measurements with 2D2C and 2D3C particle image velocimetry. Based on a Fourier analysis of the results, it appears that the non-linear phase begins with the development of harmonics of the linear modes (first unstable modes). But the growth of those harmonics is rapidly stopped by the development of low order modes. Then appears an m=0 mode, which corresponds to a mean azimuthal velocity around the vortex. Simultaneously, secondary vortical structures develop all around the vortex in its peripheral zone. These vortical structures are linked with the ejection of vorticity in the wake of the ring and they appear just before the transition towards turbulence. A tentative is made here to place all these phenomena chronologically, in order to propose a scenario for the transition from the linear phase to turbulence.