The multiple reciprocity method applied to thermal stress problems

In this paper, the multiple reciprocity boundary element method is applied to transform domain integrals due to thermal loadings into equivalent boundary integrals. The resulting formulation is restricted to two-dimensional elastic and isotropic materials with any steady-state temperature distribution, including internal heat generation. Several numerical examples are presented to validate the formulation and demonstrate the accuracy of the method.     

Acousto-optic method for nondestructive testing

An alternative method for nondestructive testing based on light diffraction by ultrasonic waves is presented. It enables us to make more accurate measurements of intensity and phase of reflected waves, and therefore detailed information about the reflecting system can be obtained. Applications are numerous but special attention is payed to examination of the quality of coupling systems.     

Eddy current nondestructive testing by a perturbation method

This paper presents an analytical solution for the change in impedance due to a flaw of rectangular cross-section in the case when the conductivity of the flaw is sufficiently close to the conductivity of the surrounding medium. The solution, which is found by the small perturbation method, is used to compute the impedance change as a function of the parameters of the flaw. An eddy current nondestructive method based on this consideration can detect the presence of such flaws and, in some cases, determine their size.     

Variational approach of displacement discontinuity method and application to crack problems

This paper deals with a variational approach of the displacement discontinuity method. This method is an indirect boundary element technique which uses the double layer potential representation of displacements and stresses. The variational approach presented here is based upon the theorem of minimum potential energy in elasticity. In the numerical procedure, the global shape function used to approximate the displacement discontinuity distribution is the continuous piecewise linear function. Regular displacements and resultant force expressions are obtained from these shape functions and these expressions are used to build the system of linear equations. The method is applied to crack problems in both infinite and finite bodies. The stress intensity factors are then calculated and high accurate numerical results are obtained.     

基于声压的无损检测方法研究

为实现工业生产流水线产品的快速无损检测,结合结构动力学理论和声辐射模态理论,论述声辐射模态频率作为无损检测依据的可行性,提出一套基于声压的无损检测方案。该方案结合使用Matlab和高精度麦克风、工业声卡等硬件设备实现声音信号采集、FFT变换和模态识别等功能。以长安CS55发动机连杆为例进行试验,结果与Ansys仿真结果对比验证此试验方案的准确性。最后对上述连杆进行无损检测,通过对大量连杆进行重复试验的结果对比发现:利用声压的无损检测方案可以成功检测缺陷连杆,而且检测结果较为稳定。     

Meshfree boundary particle method applied to Helmholtz problems

This paper is concerned with the boundary particle method (BPM), a new boundary-only radial basis function collocation schemes. The method is developed based on the multiple reciprocity principle and applying either high-order nonsingular general solutions or singular fundamental solutions as the radial basis function. Like the multiple reciprocity BEM (MR-BEM), the BPM does not require any inner nodes for inhomogeneous problems and therefore is a truly boundary-only technique. On the other hand, unlike the MR-BEM, the BPM is meshfree, integration-free, symmetric, and mathematically simple technique. In particular, the method requires much less computational effort for the discretization than the MR-BEM. In this study, the accuracy and efficiency of the BPM are numerically demonstrated in some 2D inhomogeneous Helmholtz problems under complicated geometries.     

Trefftz indirect method applied to nonlinear potential problems

Boundary type solution methodologies for linear elliptic boundary value problems (BVPs) are now well established. Unfortunately, this class of problems represents very few realistic practical situations due to the presence of nonlinearities in either boundary conditions or material properties. In this paper, we demonstrate an indirect formulation of the Trefftz method for solution of nonlinear BVPs arising in potential problems. While the concept of Trefftz method itself is fairly established, its application to nonlinear BVPs has been quite limited. Past experience indicates that Trefftz complete functions often give rise to polynomial type of nonlinearities and the resulting equations that govern the unknowns are almost always poorly conditioned, for which numerical solutions are difficult to obtain from standard procedures. For the solution of such nonlinear equations, we adopt a highly robust stabilized continuation method with inherent numerical error attenuation properties. Our numerical examples demonstrate this methodology when applied to 2D heat conduction problems with material and boundary condition nonlinearities, and the results indicate good potential for the use of indirect Trefftz method for nonlinear problems.     

Electronic speckle interferometry, phase-mapping, and nondestructive testing techniques applied to real-time, thermal loading

A digital phase-mapping method has been developed for application in real-time electronic speckle interferometry studies. Its principles and application to a continuously deforming object are described. An efficient digital image-processing algorithm has been developed that permits quantitative interpretation of the resulting phase maps.     

Neural network approach to holographic nondestructive testing

A neural network approach for the automatic detection of defects by evaluation of holographic interference patterns of the loaded technical components is described. Translation- as well as rotation-invariant features are defined based on the maximal local slope of the intensity and a partition of the interference pattern into nonoverlapping areas. The training sample set is generated by computer simulation of interferograms directed by a few typical experimentally measured samples. Practical results show the feasibility of the method. A strategy for application of neural networks to any holographic nondestructive testing task is outlined.     

The sensitivity of the mechanical impedance method of nondestructive testing

The mechanical impedance method has been used to produce ‘C-scan’ presentations of the defect locations in sets of solid aluminium plates and carbon fibre reinforced plastic honeycomb panels with deliberately introduced defects. This has enabled the sensitivity of the method to be defined, it being shown that a defect which produces a 1.5 dB difference in measured impedance compared with a good area of structure is at the margin of detectability. This means that a skin-core disbond 10 mm in diameter can be found under a honeycomb structure with 0.5 mm thick CFRP skins, but that with 1 mm thick skins a defect 20 mm in diameter is close to the margin of detectability. These values apply only to stiff structures: the reliability of the test is greatly reduced in flexible testpieces. It has also been shown that an impedance head can be used as an alternative to the vibrating-rod transducer employed in the commercially available instruments.     

Broadband ultrasonic backscattering applied to nondestructive characterization of materials

A simple method based on the spectrum features of the backscattered signal is proposed to the assessment of scatterers size. The method takes advantage of the band-limited response of the scattering media to an incident broadband pulse. The single to multiple scattering regime transition determines this limited frequency content, which is strongly related to scatterer size. The method is applied to the estimation of graphite particle sizes in cast irons. Good quantitative correlation is obtained between graphite sizes measured by metallography and that obtained using this new ultrasonic method, in spite of the band limitations of the ultrasonic system used in the experiment. By this method, the absolute mean value of the scatterer size can be evaluated, provided that a reasonably good estimate of the concentration of particles is known.     

A scaled boundary finite element method applied to electrostatic problems

The scaled boundary finite element method (SBFEM) is a novel semi-analytical technique, combining the advantages of the finite element and the boundary element methods with unique properties of its own. In this paper, the SBFEM is firstly extended to solve electrostatic problems. Two new SBFE coordination systems are introduced. Based on Laplace equation of electrostatic field, the derivations (based on a new variational principle formulation) and solutions of SBFEM equations for both bounded domain and unbounded domain problems are expressed in details, the solution for the inclusion of prescribed potential along the side-faces of bounded domain is also presented in details, then the total charges on the side-faces can be semi-analytically solved, and a particular solution for the potential field in unbounded domain satisfying the constant external field is solved. The accuracy and efficiency of the method are illustrated by numerical examples with complicated field domains, potential singularities, inhomogeneous media and open boundaries. In comparison with analytic solution method and other numerical methods, the results show that the present method has strong ability to resolve singularity problems analytically by choosing the scaling centre at the singular point, has the inherent advantage of solving the open boundary problems without truncation boundary condition, has efficient application to the problems with inhomogeneous media by placing the scaling centre in the bi-material interfaces, and produces more accurate solution than conventional numerical methods with far less number of degrees of freedom. The method in electromagnetic field calculation can have broad application prospects.     

Extended preconditioners for the FETI method applied to constrained problems

An efficient and simple approach for handling linear multipoint constraints in a class of substructure‐based solvers, namely the finite element tearing and interconnecting (FETI) method, is proposed. Previously, it was argued that multipoint constraints should be handled in FETI by adding a second level iteration on a coarse grid such that the FETI iterates satisfy the multipoint constraints exactly. The procedure presented here does not require an additional coarse grid but instead takes account of the multipoint constraints in the preconditioning step. The preconditioning strategy is shown to be mechanically consistent and to incur only a small additional computational cost. This strategy is simpler and computationally less expensive than the two‐level FETI procedure. Its numerical scalability even for highly heterogeneous problems is demonstrated in several test examples. Copyright © 2002 John Wiley & Sons, Ltd.     

Orthogonal meshless finite volume method applied to elastodynamic crack problems

An orthogonal meshless finite volume method has been presented to solve some elastodynamic crack problems. An orthogonal weighted basis function is used to construct shape function so there is no problem of singularity in this new form. In this work, for three-dimensional dynamic fracture problems, a new displacement function is used at the tip of the crack to give a new OMFVM. When the new OMFVM is used, the singularity of the stresses at the tip of the crack can be shown to be better than that in the primal OMFVM. High computational efficiency and precision are other benefits of the method. Solving some sample crack problems of thin-walled structures show a good performance of this method.     

An extended finite element method applied to levitated droplet problems

We consider a standard model for incompressible two‐phase flows in which a localized force at the interface describes the effect of surface tension. If a level set method is applied then the approximation of the interface is in general not aligned with the triangulation. This causes severe difficulties w.r.t. the discretization and often results in large spurious velocities. In this paper we reconsider a (modified) extended finite element method (XFEM), which in previous papers has been investigated for relatively simple two‐phase flow model problems, and apply it to a physically realistic levitated droplet problem. The results show that due to the extension of the standard FE space one obtains much better results in particular for large interface tension coefficients. Furthermore, a certain cut‐off technique results in better efficiency without sacrificing accuracy. Copyright © 2010 John Wiley & Sons, Ltd.     

Metrological support to means of comprehensive nondestructive mechanical testing

Translated from Izmeritel'naya Tekhnika, No. 7, pp. 30–31, July, 1988.     

The mixed finite element method applied to two-dimensional elastic contact problems

The application of the mixed finite element method to two-dimensional elastic contact problems is investigated. Since in the mixed method, both displacements and stresses are retained as variables, it is found that all the contact conditions—displacement as well as stress—can be approximated directly. A significant novelty is that some of the displacement variables are treated as natural boundary conditions in the contact region. In cases where the contact region is independent of the applied loading, an iterative procedure is used to establish the sliding and adhering portions of the contact region. In cases where the contact region is a function of the applied loading, for example progressive contact, an incremental formulation is employed to describe the discretized contact stages before invoking the former iterations. Several numerical examples are presented and the results are compared with those from the conventional potential energy or displacement finite element method.     

Isotherm migration method applied to fusion problems with convective boundary conditions

A numerical technique based on the Isotherm Migration Method is presented for solving Moving Boundary Problems involving convective boundary conditions. The method is illustrated by solving sample problems in one dimension as well as in two dimensions. The comparison of present results with those of earlier authors shows an extremely good agreement.