H. Fan

A Micro-mechanics Model for Imperfect Interface in Dielectric Materials

The interface between two dielectric bodies is considered imperfect if there are defects (micro-voids and micro cracks) present on the interface. For such interface, the perfect continuity condition across the interface is no longer valid and its use in analysis becomes questionable. To account for this imperfection, we propose a micro-mechanics model based on self-consistent scheme, leading to the establishment of a constitutive relationship between the electric displacement and potential discontinuity across the imperfect interface.

Two-dimensional contact on a piezoelectric half-space

Abstract Due to their intrinsic electro-mechanical coupling effect, piezoelectric materials have been widely used in industry. In the present paper, stress and electrical field distributions in a piezoelectric half-plane under contact load at the surface are considered. Since a piezoelectric material is intrinsically anisotropic, stress analysis has been impeded by the complexity raised by too many material constants. Hereby, Strohs formalism is applied in the present study to overcome this difficulty. The solution for a concentrated force and charge acting on the boundary of the half-space, the Green function, is obtained in a neat form. The non-slip and slip indentor contacts on the piezoelectric half-space are also formulated.

Piezoelectric waves near an imperfectly bonded interface between two half-spaces

We show the existence of certain waves propagating near an imperfectly bonded interface between two half-spaces of different piezoelectric ceramics. Some of the waves reduce to known waves when perfect bonding is assumed. There are also waves that rely on the imperfection of the interface bonding and they do not remain as interface waves when the bonding is perfect. In particular, it is shown that interface imperfection causes dispersion in general, although there does not explicitly exist a geometric characteristic length of the two-half-space structure. The theoretical results obtained can also be used to design experiments for measuring interface properties.

Screw dislocation interacting with imperfect interface

The study of dislocations interacting with interfaces is of great interest to understand the strengthening and hardening mechanisms in materials. Most of the existing results in this field dealt with perfect interfaces where the displacements are continuous across interfaces. In reality, however, due to damage of interfaces or other reasons, the displacements across interfaces could have a jump. In the present study, a linear spring model is applied to study imperfect interfaces, and the interaction of a screw dislocation with an imperfect interface is formulated. The results show that the interacting force on a screw dislocation with an imperfect interface varies between that with a free surface and that with a perfect interface. It should be pointed out that the present study is the first step of the dislocation/ imperfect-interface research. The interaction between edge dislocations and imperfect interface is a more practically important and challenging problem. 2002 Elsevier Ltd. All rights reserved.

Interaction between a screw dislocation and viscoelastic interfaces

Abstract The analytical solutions for the interaction between dislocations and interfaces are of great importance to materials scientists as well as to mechanics researchers. The interfaces are treated as perfectly bonded in the most of the existing research works, where the traction and displacement vectors are continuous across the interfaces. However, in reality, there are discontinuities of displacements across the interfaces. In the present paper, the interaction between a screw dislocation and an imperfect interface is considered. The imperfect interface is modeled by linear spring and dashpot, i.e. linearly elastic and viscoelastic behaviors are introduced to model the imperfection of the interface. Particularly, we solved the boundary value problem analytically for Kelvin and Maxwell type of interface. In terms of geometrical configurations, we obtained the solutions for two joint half-spaces and a circular inclusion embedded in an infinite matrix. The analytical results show that the force acting on the dislocation depends on the mismatch of materials and the imperfection of the interface and evolves as time elapses.

Thickness-shear vibrations of rotated Y-cut quartz plates with imperfectly bonded surface mass layers

A solution is obtained from the three-dimensional equations of linear piezoelectricity for the pure thickness-shear vibration of rotated Y-cut quartz or langasite plates with imperfectly bonded surface mass layers. The solution includes a few results in the literature as special cases. It is shown that the mass layers lower the resonant frequencies when they are relatively perfectly bonded to the crystal surface, and that loosely bonded mass layers may raise the frequencies. The results are useful in the analysis of frequency stability of quartz resonators and acoustic wave sensors.

A ZENER-STROH CRACK NEAR AN INTERFACE

Abstract A micro crack can be initiated by coalescing dislocations piled up along a slip plane. This mechanism was firstly proposed by Zener and later on analyzed by Stroh. The micro crack is thus called Zener-Stroh crack, which is a counterpart of the well-known Griffith crack in linear elastic fracture mechanics. In the present paper, we consider a Zener-Stroh crack initiated near a bi-material interface. Due to image force acted on the dislocations, a Griffith crack mechanism is introduced even where the crack is purely loaded by net dislocations. It is seen that the stress intensity factor, which consists of a Zener-Stroh component and a Griffith component, and the critical crack length are strongly affected by the presence of the interface.

Rubber-like shape memory polymeric materials with repeatable thermal-assisted healing function

A thermo-responsive shape memory polymeric material is reported, which is not only rubber-like from above to below its shape recovery temperature, but also has a repeatable thermal-assisted healing function. The mechanisms behind both features are identified. It is found that while micron-sized inclusions play a part in the shape memory effect (SME), tangled polymer chains contribute to the rubber-like phenomenon and repeatable thermal-assisted healing function atop the SME. This tangling effect enables a perfect combination of high elasticity, SME and repeatable thermal-assisted healing to be simultaneously achieved.

A finite element approach for computing edge singularities in piezoelectric materials

By using the eigenfunction expansion technique and the weak form of the governing equations for prismatic sectorial domains composed of piezoelectrics and air, an one-dimensional finite element procedure is formulated for computing the eigensolutions of the electromechanical field problem. Generalized displacement and electric potential are taken to be the nodal variables. The resulting global equation is a second order characteristic matrix equation. Validity of the formulation is verified by comparing the computed results with the existing solutions for impermeable cracks and interfacial cracks. Configurations which are of practical interest including conducting cracks, permeable and impermeable notches are studied.

Competition Between Fatigue Crack Propagation and Wear

Based on a semi-empirical derivation of the Paris fatigue law, the fatigue crack length a is related to the yield limit or flow stress, which ultimately is related to the hardness of the material. The analysis considers together the cyclic loading, which tends to increase the surface crack length, and the wear, which tends to decrease the crack length at the surface, and shows that under certain conditions a stable crack length may be developed. Experiments conducted on two test groups ((i) Rc = 58.5 and (ii) Rc = 62.7) tend to support the present analysis.