Many problems in linear elastodynamics, or dynamic fracture mechanics, can be reduced to Wiener–Hopf functional equations defined in a strip in a complex transform plane. Apart from a few special cases, the inherent coupling between shear and compressional body motions gives rise to coupled systems of equations, and so the resulting Wiener–Hopf kernels are of matrix form. The key step in the solution of a Wiener–Hopf equation, which is to decompose the kernel into a product of two factors with particular analyticity properties, can be accomplished explicitly for scalar kernels. However, apart from special matrices which yield commutative factorizations, no procedure has yet been devised to factorize exactly general matrix kernels.
This paper shall demonstrate, by way of example, that the Wiener–Hopf approximant matrix (WHAM) procedure for obtaining approximate factors of matrix kernels (recently introduced by the author in [SIAM J. Appl. Math. 57 (2) (1997) 541]) is applicable to the class of matrix kernels found in elasticity, and in particular to problems in QNDE. First, as a motivating example, the kernel arising in the model of diffraction of skew incident elastic waves on a semi-infinite crack in an isotropic elastic space is studied. This was first examined in a seminal work by Achenbach and Gautesen [J. Acoust. Soc. Am. 61 (2) (1977) 413] and here three methods are offered for deriving distinct non-commutative factorizations of the kernel. Second, the WHAM method is employed to factorize the matrix kernel arising in the problem of radiation into an elastic half-space with mixed boundary conditions on its face. Third, brief mention is made of kernel factorization related to the problems of flexural wave diffraction by a crack in a thin (Mindlin) plate, and body wave scattering by an interfacial crack. 相似文献
In this article a fibre-reinforced composite material is modelled via an approach employing a representative volume element with periodic boundary conditions. The effective elastic moduli of the material are thus derived. In particular, the method of asymptotic homogenization is used where a finite number of fibres are randomly distributed within the representative periodic cell. The study focuses on the efficacy of such an approach in representing a macroscopically random (hence transversely isotropic) material. Of particular importance is the sensitivity of the method to cell shape, and how this choice affects the resulting (configurationally averaged) elastic moduli. The averaging method is shown to yield results that lie within the Hashin–Shtrikman variational bounds for fibre-reinforced media and compares well with the multiple scattering and (classical) self-consistent approximations with a deviation from the latter in the larger volume fraction cases. Results also compare favourably with well-known experimental data from the literature. 相似文献
In situ variable temperature XRD (VT-XRD) measurements on the transformation of nano-precursors to LaNiO phases are presented. Experimental results showed that LaNiO3 and La2NiO4 phases were formed at ca. 700 °C via the reaction of La2O3 and NiO (from the initial nano-precursors), where a relatively low temperature of 700 °C was found for the synthesis of La2NiO4. The formation of La3Ni2O7 at higher temperature (up to 1150 °C) appeared to proceed through a further reaction of La2NiO4 with unreacted NiO, whilst the formation of La4Ni3O10 (at 1075 °C) proceeded via a further decomposition of LaNiO3. Although phase pure La3Ni2O7 and La4Ni3O10 were not directly obtained under the processing conditions herein, the results of this study allow for a better understanding of formation pathways, particularly for the higher order La-Ni-O phases. 相似文献
Theγ-radiation from the10B(n,γ) reaction is studied using an unpolarized target. More accurate values for energies of transitions in11B could be determined. No new levels have been found. TheQ value of this reaction: 11,454.1 (2)keV, is in agreement with earlier experiments. Also a new value for the cross section could be derived: 0.29 (4) barn, which is a factor 5 more accurate than earlier experiments. The10B(n, α)7 Li reaction, leading to the 478 keV state in7Li, is studied by means of polarized10B nuclei and polarized neutrons. The resulting anisotropy in the directional distribution of the7Li particles manifests itself in the Doppler broadening of the 478 keV line. Analysis of the line shape directly yields the conclusion, that the reaction proceeds for more than 96% through theJ=7/2 channel of11B in case of destructive channel interference of theJ=5/2 channel. Constructive channel interference is only possible if the reaction proceeds for more than 99.5% through theJ=7/2 channel. It appeared that the outcomingα and7Li particles are emitted predominantly in directions perpendicular to the nuclear orientation axis. 相似文献
The formation of cluster orbitals in CsSn2Br5 is discussed and related more generally to tetragonal compounds of the type AB2X5 (A=monovalent cation; B=Sn, Pb; X=Cl, Br, I). The crystal structures of CsSn2Cl5 and CsSn2Br5 have been solved by single-crystal X-ray diffraction. These compounds are isostructural with each other and a range of AB2X5 structural analogues. In many AB2X5 compounds where B is a subvalent main group metal a tetragonal cell is observed with space group I4/mcm. The structures of CsSn2Br5 and CsSn2Cl5 are layered with polymeric sheets of [Sn2X5]n−n separated by the Cs+ cations. Stereochemical considerations suggest that stabilization of this structural form, rather than the more ionic NH4Pb2Cl5 or NaSn2Cl5 structures, is through interaction of the “nonbonding” valence electron pairs on tin with low-lying empty d-orbitals on neighboring X atoms. Electronic structure calculations based on the structural data confirm the likelihood of cluster orbital formation. Crystal data: CsSn2Cl5, tetragonal, I4/mcm, a=8.153(1) Å, c=14.882(4) Å, Z=4, R1=0.0215, wR2=0.0503 [I>2σ(I)], R1=0.0393, wR2=0.0536 (all data); CsSn2Br5, tetragonal, I4/mcm, a=8.483(6) Å, c=15.28(2) Å, Z=4, R1=0.0607, wR2=0.1411 [(I>2σ(I)], R1=0.1579, wR2=0.1677 (all data). 相似文献
Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K2[HCr2AsO10], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch‐Pouchot, Durif & Guitel [Acta Cryst. (1978), B 34 , 3725–3727], led to an independent redetermination of the structure using two separate crystals. The resulting improved accuracy allows the inference that the H atom is located in the hydrogen bonds of length 2.555 (5) Å which form between the terminal O atoms of shared AsO3OH tetrahedra in adjacent HCr2AsO102− ions. The largest atomic displacement of 0.586 Å between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius ∼0.60 Å about the 31 or 32 axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates. 相似文献