Papers by Author: Wan Lin Guo

Abstract: Experimental of two kinds of compact tension (CT) specimens’ creep crack propagation are carried out in this paper. Traditional fracture mechanics and three-dimensional fracture theory are compared and the results show that: The K-Tz two-parameter model can eliminate the thickness-effect on the crack growth rates in the relatively low K range, however when K exceed certain values the effect of thickness for crack growth rates still exists; The Ct and Ct-Tz model can describe the thickness-effect of creep crack growth rates in regions of high Ct; When the crack tip stress intensity factor K of the two kinds of thickness (B=5 mm, B=10 mm) specimens equal to 35 and 31 respectively, this material’s creep crack growth control parameter change from K to Ct.

Abstract: A kind of processing strategy of total life cycle of mechanical products was designed. A logistics model of total life cycle of mechanical products was established based on eight typical states of life cycle of mechanical products. The logistics analysis of total life cycle of a sort of aero-engine was carried out by using the model. The dynamic equivalent quantity of the aero-engines in different states of life cycle was obtained when times changed from the products were first put into production to the time when stable production capacity was reached. The model can also be used to predict logistics of other products rapidly. The results give references for production departments or enterprises which use life cycle methods to configure their production resources effectively and optimize production processes, and also provide a basis for further analysis of total life cycle analysis such as economic and environmental assessment.

Abstract: Material selection in mechanical products based on total life cycle design is a complicated work, which should be studied systematically. A material selection model of mechanical products based on total life cycle design was proposed. A set of candidate materials were screened out, and then assessed according to the technical, economic and environmental assessment index. The candidate materials were ranked by using by using Z-transformation method in each of the assessment index. Different weights were assigned to each of the three assessment indexes, and global assessment was carried out according to different strategies or requirements which pay more attention to technical, economic or environmental performance of the material product used. A case in selecting aircraft structure element material was studied. The analysis results showed that the method could rank the candidate materials and selected out the “optimized material”, and the influence of the subjectivity of designer was reduced. The method provides some practical values for preliminary material selection in the early design stage of the mechanical products based on life cycle design.

Abstract: Strain-dependent electro-optic constant r33 and nonlinear optical coefficient d33 of ZnO are investigated systematically using density-functional theory based linear-response perturbation method. Miscellaneous properties, such as dielectric constants, elastic constants, piezoelectric coefficients, nonlinear optical coefficients, and electro-optic constants of other II-VI compound semiconductors (both Wurtzite and Zinc-blende structures) are also calculated for comparison with the results of unstrained ZnO. Extensive first-principles calculations show that both r33 and d33 of ZnO decrease almost linearly with increasing strains, which indicates that appropriate compression along the [0001] direction of ZnO could enhance its electro-optic and nonlinear optical properties, while stretching may weaken the corresponding properties. Among the involved Wurtzite structures, ZnO has the highest elastic constant, piezoelectric coefficient and electro-optic constant, showing practical importance.

Abstract: A method of damage detection and fault diagnosis for gears is presented based on the theory of elastomeric dynamics according to the theory of cracked beam. It takes an advantage of accurate fault diagnosis of gear body using the change of dynamic features and has some advantages for dynamic design of gear systems.The dynamics characteristics, i.e., natural frequency, vibration shape,dynamic response and so on, due to crack of gear tooth are studied, and the gear dynamics characteristics caused by the position and size of crack are deeply investigated by comparison with FEM. The theoretical analysis results are contrasted with numerical simulation results and shows good agreement with the result by FEM. The proposed method can be used to detect damage and diagnose fault for gear structures and also can be applied to designing dynamic characteristics for gear systems.

Abstract: In the recent years three-dimensional (3D) elastic-plastic analyses have been conducted extensively for the opening mode (mode I) fracture and the constraint effects are discussed in detail. However less work is focused on other modes as sliding mode (mode II), tearing mode (mode III) or the mixed mode fracture in three-dimensional. In this paper the thickness effect on pure mode II case is discussed by the finite element method (FEM). Modified Boundary Layer (MBL) model is used, which has the ability to take into account the combined effects of the in-plane constraint (T-stress) and the out-of-plane constraint (finite thickness). The result demonstrates the weak thickness dependence on the near tip stress and strain fields under mode II loading. And the size of the 3D zone at mode II loading is determined to range from 1.0 to 1.2 times the thickness. Two fracture parameters of J integral and crack tip sliding displacement (CTSD) are discussed, which are almost same at different thickness planes except those very near the surface. It is interesting to find that the relations between J and CTSD keep linear at different thickness planes. T-stress is symmetry on stress and strain distributions along the crack plane. However its effects indicate weak thickness dependent on the CTSD and J integral fracture parameter.

Abstract: Based on the classic laminate theory, a progressive damage model has been incorporated into the composite structure analysis by using a commercial code, ABAQUS, via one of its user-defined subroutine, UGENS. The developed user subroutine can be applied to simulate fiber and matrix damage processes in the general composites structures. The responses of flat laminate subjected to static contact crush have been studied to verify the efficiency of the presented damage method. The predicted load-displacement relationships, damage distribution were obtained and compared with the corresponding experiments, and the results were found to be in good agreement.

Abstract: The delamination cracks and its effects on the fracture of pipeline steel are investigated experimentally by using of Drop-Weight Tear Test (DWTT). The delamination cracks are produced by the stress perpendicular to the weak interfaces before main crack beginning or accelerating, no new delamination crack is produced during the stabile propagation of fracture. The quantity, splay degree of delamination crack and the space between two delamination cracks are influenced by the stress state of the crack tip at beginning or accelerating point of main crack and the length of delamination crack is influenced by the stress state of the crack tip during the propagation of fracture. The surface of delamination crack is cleavage fracture appearance with large cleavage facet. There is no delamination crack on the brittle fracture surface below the brittle-to-ductile temperature or on the brittle fracture region of mix-mode fracture surface with ductile and brittle region. The part of fracture surfaces with delamination crack ought to be evaluated as the shear area because the delamination cracks are produced only on the ductile fracture surface or on the ductile part of fracture surface.

Abstract: The historical developments of the fracture mechanics from planar theory to threedimensional (3D) theory are reviewed. The two-dimensional (2D) theories of fracture mechanics have been developed perfectly in the past 80 years, and are suitable for some specific cases of engineering applications. However, in the complicated 3D world, the limitation of the 2D fracture theory has become evident with development of the structure toward complication and micromation. In the 1990’s, Guo has proposed the 3D fracture theory with a 3D constraint factor based on the deformation theory and energy theory. The proposed 3D theory can predict accurately the fracture problems for practical and complicated engineering structures with defects, by integrating the 3D theory of fatigue, which has been developed to unify fatigue and fracture. Our efforts to develop the 3D fracture mechanics and the unified theory of 3D fatigue and fracture are summarized, and perspectives for future efforts are outlined.