Papers by Keyword: Fracture Mechanic

Abstract: This paper solves the penny-shaped crack configuration in transversely isotropic solids with coupled magneto-electro-elastic properties. The crack plane is coincident with the plane of symmetry such that the resulting elastic, electric and magnetic fields are axially symmetric. The mechanical, electrical and magnetical loads are considered separately. Closed-form expressions for the stresses, electric displacements, and magnetic inductions near the crack frontier are given.

Abstract: The microstructure, mechanical, impact and fracture properties of Australian bamboo have been investigated. The graded composition and property has been confirmed by depth-profiles obtained by synchrotron radiation diffraction and Vickers indentation. The mechanical performance of bamboo is stronly dependent on age. Results showed that young bamboo has higher strength, elastic stiffness and fracture toughness than its old counterpart. Both crack-deflection and crackbridging are the major energy dissipative processes for imparting a high toughness in bamboo.

Abstract: A novel integral equation method is developed in this paper for the analysis of two-dimensional general piezoelectric cracked bodies. In contrast to the conventional boundary integral methods based on reciprocal work theorem, the present method is derived from Stroh’s formalism for anisotropic elasticity in conjunction with Cauchy’s integral formula. The proposed boundary integral equations contain generalized boundary displacement (displacements and electric potential) gradients and generalized tractions (tractions and electric displacement) on the non-crack boundary, and the generalized dislocations on the crack lines. The boundary integral equations can be solved using Gaussian-type integration formulas without dividing the boundary into discrete elements. The crack-tip singularity is explicitly incorporated and the generalized intensity factors can be computed directly. Numerical examples of generalized stress intensity factors are given to illustrate the effectiveness and accuracy of the present method.

Abstract: The fatigue characteristic of thermal barrier-coated Ni-base superalloy IN738LC at room temperature was studied by fatigue test using SEM-servo testing machine. In order to eliminate the influence of micro-shrinkage in IN738LC and purely investigate the effect of TBC on fatigue characteristic, HIP treated IN738LC was used in comparison with NON-HIP treated one. The results indicate that HIP treatment is effective to remove micro-shrinkage and fatigue life is greatly improved. The TBC coating layer is a strength member in the small test specimen at room temperature, and TBC coated material showed lower fatigue life rather than IN738LC without TBC.

Abstract: The crack, which occurs because of fatigue, should often consider not only the single crack but also plural cracks occurring. In this paper, the material strength when two or more cracks occurred because of fatigue etc. was examined by calculating stress intensity factor K value by using caustics method and the photoelasticity. In general, plural cracks are known KI value decreases compared with the single crack. Therefore, the influence on K value was experimentally examined from the point of the distance between the cracks, the angle, and the crack length and the mode change. As a result, KI value influences destruction in plural in exist only mode I and the mixed mode of mode I and mode Ⅱ like this research cracks. When the crack length becomes long if the distance between the cracks narrows, the decrease of KI value grows. In addition, it has been understood that the decrease of KI value is influenced by a/w on the boundary of d/a=1 (the distance between the cracks is equal to the crack length).

Abstract: LiTaO3 (5--25vol%) dispersed Al2O3 matrix composite (LTA) with high relative density were fabricated by hot pressing at 1300°C. The microstructure and mechanical properties of the composite were studied. The main results are as follows: Al2O3 was found to be phase compatible with piezoelectric LiTaO3 during high temperature sintering process. The relative density of the LTA composites was greatly improved by the addition of LiTaO3 particles. Domains are clearly discerned in LiTaO3 grains in the LTA composite, confirming that LiTaO3 particles remain ferroelectrics. 90 degree domains were observed in LiTaO3 grains constrained strongly by Al2O3 matrix, which has never been found in LiTaO3 single crystal before. Comparing with those of pure Al2O3, sintered at same temperature (1300°C), both the flexural strength and the fracture toughness of the composites are all enhanced. Domain switching is suggested as a new toughening mechanism in the ferroelectric particles dispersed structural ceramics.

Abstract: The polycrystallined LaB6-ZrB2 composites with different content of ZrB2 were fabricated by vacuum hot-pressed sintering technique in this paper. The effects of ZrB2 content on the microstructure and mechanical properties were investigated. For the eutectic LaB6-ZrB2 (21wt%) composite, the hardness, flexural strength and fracture toughness is of 93.0 (HRA), 330.0MPa and 3.70MPa·m1/2, respectively. The results showed the hardness and flexural strength of LaB6-ZrB2 polycrystalline have been enhanced with the increasing of ZrB2 content, but the fracture toughness increases first then arrives at peak value, which is corresponding to the ZrB2 content of 21wt%. The microstructure observation revealed an improved densification due to addition of ZrB2. The fracture morphology showed a tendency of the fracturing from intergranular to transgranular manner with increasing the ZrB2 content. The fracture behavior of the composites was analyzed in the paper

Abstract: Strength evaluation method for MEMS micromirror has been proposed. Pure bending and combined loading tests have been performed and torsion strength has been estimated from those results. Two-parameter Weibull distribution was used to evaluate the fractured stresses estimated from FEM model. There exists the difference in strength between pure bending and combined loading. From the load factor analysis, it is found that both geometry and stress distribution have to be considered to estimate the strength of MEMS since flaw population is non-uniformly distributed. It is also found torsional strength can be estimated on the safe side by using the result of combined loading test. From the comparison between two kinds of specimen fabricated by different etching conditions, it is found that the fracture strength is greatly affected by the amount of etching damage (notching).

Abstract: Vanadium alloys in the composition range around V-4Cr-4Ti have been proposed as candidate materials for fusion reactor applications and structures. These applications will require detailed characterization of constitutive and fracture properties. This study is aimed at understanding the basic constitutive and fracture mechanisms in vanadium alloys. Understanding of the basic constitutive and fracture mechanisms is achieved through a series of mechanical tests. These test results are combined with quantitative models of the underlying macro- and micromechanics. In addition to these experimental studies, finite element analysis (FEA) techniques are used to determine stress and strain fields to verify the constitutive law used in the fracture specimens.

Abstract: In the structural design of ceramics components especially for graphite materials, it is important to apply the realistic fracture model in the design method so as to reduce the large safety margin. In this study, we proposed the multiaxial strength model by expanding the microstructure based brittle fracture model applicable to both uniaxial tensile and compressive stress conditions. The advantage of the model is a treatment of the microstructural information such as grain size, pore size and pore size distribution. The proposed model was applied to biaxial strength prediction of near isotropic nuclear graphite using grain/pore related microstructure parameters. Predicted results were compared with biaxial strength data, and it was found that the proposed fracture model showed fairly good strength prediction.