Recent experiments of Rosakis et al. have clearly shown that the crack-tip velocity can exceed the shear wave speed for a crack tip propagating between two weakly bonded, identical and isotropic solids under shear-dominated loading. This has motivated recent theoretical and numerical studies on intersonic crack propagation. We have obtained analytically the fundamental solution for mode-II intersonic crack propagation in this paper. This fundamental solution can provide the general solutions for intersonic crack propagation under arbitrarily initial equilibrium fields. We have also developed a cohesive zone model to determine the crack-tip energy release for an intersonic shear crack. © 2001 ASME.

Huang Y.;Gao H.

Journal of Applied Mechanics Transactions ASME

2001

We have prepared polyethylene thin films with the ionized cluster beam (ICB) deposition technique. There are perfect polyethylene crystal lamellas in the thin films. We have observed the polyethylene lamellar surface topography with scanning tunneling microscopy (STM), and found the fold chain structures on the (001) surface of polyethylene crystal lamellas first. We have analyzed polyethylene thin films with transmission electron microscopy (TEM) and found the (001) plane parallel to the surface of the substrate. There are strict periodic structures in the (110) plane, but no good periodic structures in the [110] direction. © 1992.

Xue Z. Q.;Gao H. J.;Liu W. M.;Zhu C.;Ma Z.;Pang S.

Applied Surface Science

1992

Mechanisms of strain relaxation and defect formation during surface roughening in Si 1-x Ge x films grown epitaxially on (100)Si substrates have been investigated by controlled annealing experiments. Epitaxial films 10 nm in thickness and containing 18% Ge, which are subcritical with respect to the formation of misfit dislocations, show strain relaxation through surface roughening on annealing at 850 °C, where surface grooves are aligned along 〈100〉 directions. Other films with 22% Ge and supercritical thicknesses have also been studied, where surface grooves are aligned along 〈110〉 directions. © 1997 American Institute of Physics.

Ozkan Cengiz S.;Nix William D.;Gao Huajian

Applied Physics Letters

1997

This paper has two goals. First, it is aimed at providing a fundamental understanding of the oscillatory behavior of an interface crack between two dissimilar materials from the viewpoint of the interface mismatch that results from the cracking. Second, we extend the Bueckner-Rice weight function method to facilitate the interface crack analysis. Using properties of the surface Green’s functions of a homogeneous solid and solutions obtained from weight function formulae, a mismatch analysis is carried out which indicates that the local mismatch near the crack tip results in the oscillatory near-tip field while the mismatch on the global scale leads to the corresponding stress intensity factors. For an oscillatory interface crack field, it is shown that, other than a few extra material constants, the interface weight function analysis is completely parallel to the well-developed homogeneous theory so that knowledge of one crack solution for a given bimaterial geometry is sufficient for determination of solutions under any other loading conditions. © 1991 by ASME.

Gao Huajian

Journal of Applied Mechanics Transactions ASME

1991

Complex-variable representations in the Stroh formalism are used to analyse a number of problems involving holes (voids) in a stressed anisotropic solid. One novel feature of our study is to consider morphological perturbations along the hole boundary. It is shown that the problem of a hole subjected to an applied stress field or an arbitrary boundary perturbation is equivalent to one with some prescribed traction along the hole surface. The boundary-peturbation analysis may be used in conjunction with a conformal-mapping scheme for elliptical hole shapes to study more complicated geometries. First-order perturbation solutions are given explicitly for smooth polygonal holes that are viewed as being perturbed from a reference circular hole. The perturbation solutions are used to estimate stress concentrations near a perturbed hole boundary, and to examine the morphological stability of a circular void in a stressed isotropic solid; in the latter study a bifurcation in the void shape, from a circle to less symmetrical geometries, is found to occur when either the stress level or the hole size reaches a critical value. © 1992 Oxford University Press.

Gao Huajian

Quarterly Journal of Mechanics and Applied Mathematics

1992

Quasi-static elastic dislocations in a homogeneous elastic half-space are commonly used to model earthquake faulting processes. Recent studies of the 1989 Kalapana, Hawaii, and Loma Prieta, California, earthquakes suggest that spatial variations in elastic properties are necessary to reconcile geodetic and seismic results (Arnadottir et al., 1991; Ebernart-Phillips and Stuart, 1992). In this paper, we use a moduli perturbation approach to investigate the effect of lateral and vertical variations in elastic properties on the elastic fields produced by dislocations. The method is simple, efficient, and in some cases leads to closed form solutions. The zeroorder solution is simply the solution for a homogeneous body. The first-order correction for elastic heterogeneity is given by a volume integral involving the spatial variations in moduli, the displacements due to a dislocation in a homogeneous half-space, and the half-space Green's function. The same representation can be also used to obtain higher-order solutions. If there are only piecewise constant variations in shear modulus, the volume integral can be reduced to a surface integral (or line integral in two-dimensions). Comparisons with the analytical solutions for a screw dislocation in a layered medium suggest that the perturbation solutions are valid for nearly an order of magnitude variation in modulus. It is shown that a simple two-dimensional model with both vertical and lateral variations in the elastic properties may explain a large part of the discrepancy between seismic and geodetically inferred fault depths for the 1989 Kalapana, Hawaii, earthquake Copyright 1994b y the AmericanG eophysicaUl nion..

Du Yijun;Segall Paul;Gao Huajian

Journal of Geophysical Research Solid Earth

1994

Toluylene 2,4-dicarbamidonitrite (TDCN) and Ag-ultrafine-particle-TDCN thin films were prepared by using an ionized-cluster-beam (ICB) deposition method. TDCN is a newly synthesized conjugated polymer with novel properties. These thin films were characterized by transmission electron microscopy (TEM). The TEM results indicate that the crystalline structure of the organic thin films can be controlled by the parameters of the ICB and the ultrathin crystalline TDCN film can be achieved by using the ICB deposition method. Results also demonstrated that amounts of Ag ultrafine particles embedded in the organic thin films may play an important role in the functional properties of the composite thin films. The electrical bistability of the Ag-ultrafine-particle-TDCN thin films was also achieved, and the possible conductive mechanism of the films was suggested qualitatively.

Gao H. J.;Xue Z. Q.;Wu Q. D.;Pang S. J.

Journal of Vacuum Science and Technology B Microelectronics and Nanometer Structures

1995

An analytic proof is developed for an energy variation theorem which states that in a homogeneous isotropic linear elastic solid under equibiaxial loading, a cusp-shaped hypocycloid cavity satisfying the Griffith condition G ≥ 2γ can be formed through mass-conserved shape evolution with no associated energy barrier. In this process, the sum of the strain energy and the surface energy continuously decreases as the hypocycloid changes from a perfectly circular hole to an array of cusp cracks. The hypocycloid evolution provides a unified model of diffusive crack formation from Griffith cracks to cycloid surfaces; the latter has been used recently to model stress driven surface evolution in heteroepitaxial thin films. -from Author

Huajian Gao

Proceedings Royal Society of London A

1995

Surface roughening associated with strain relaxation of Si 1-x Ge x films grown epitaxially on (100) Si substrates has been investigated using transmission electron microscopy, atomic force microscopy and x-ray diffraction. Epitaxial films 100 angstrom in thickness and containing 18% Ge, which are subcritical with respect to the formation of misfit dislocations, show strain relaxation through surface roughening on annealing at 700 °C. Enhanced surface grooves aligned along 〈100〉 directions are observed in films annealed at 850 °C. Strain relaxation as measured by x-ray diffraction is significantly greater at the higher temperature. Prolonged anneals at 850 °C also result in islanding. The surface roughening processes have also been studied in subcritical films with 15% Ge at 900 °C. These films also show enhanced grooving aligned along 〈100〉 directions. These observations are consistent with an anisotropic elastic analysis which indicates that grooving should occur preferentially along 〈100〉 directions. Intermixing effects in these samples have also been investigated through depth profiling using Auger Electron Spectroscopy. In addition to the above subcritical films, other films with 18% and 22% Ge and supercritical thicknesses have also been studied. For these films, surface grooving is observed along 〈110〉 directions, which suggests that these grooves are related to the formation of misfit dislocation networks. The role of these surface roughening processes in the nucleation of dislocations has also been explored.

Ozkan Cengiz S.;Nix William D.;Gao Huajian

Materials Research Society Symposium Proceedings

1996

Strained layer semiconductor structures provide possibilities for novel electronic devices. When a semiconductor layer is deposited epitaxially onto a single crystal substrate with the same structure but a slightly different lattice parameter, the semiconductor layer grows pseudomorphically with a misfit strain that can be accommodated elastically below a critical thickness. When the critical thickness is exceeded, the elastic strain energy builds up to a point where it becomes energetically favorable to form misfit dislocations. In the absence of a capping layer, surface roughening may also take place which causes strain relaxation in the form of 2D ridges or islands via surface diffusion. At sharp valley regions on the surface, amplified local stresses can cause further defect nucleation and propagation. These defects can be detrimental to the electrical performance of devices by acting as electron-hole recombination centers or current leakage channels. In this paper, we present observations and analyses of two novel defects nucleated in heteroepitaxial Si 1-x Ge x thin films through surface roughening. Heteroepitaxial films 500 angstrom thick and containing 22% Ge are deposited by LPCVD. These initially flat films are subjected to various annealing conditions in a H 2 atmosphere to induce morphological evolution and defect formation. High resolution transmission electron microscopy and atomic force microscopy have been used to study the morphology of defects at the film surface and at the film/substrate interface.

Ozkan Cengiz S.;Nix William D.;Gao Huajian

Materials Research Society Symposium Proceedings

1997