Papers by Keyword: Brittle Fracture

Abstract: K-criterion that is used to boundary line and brittle fracture is presented when the size of plastic zone around crack-tip is less than that of adaptive zone of K-criterion by comparison adaptive zone of K-criterion with plastic zone around crack-tip in this paper. Finally numerical examples are given to illustrate the applications of the present method by comparison with other methods.

Abstract: The correlation between fracture toughness parameter K_Jc and cleavage initiation distance (CID) for the three point bending (3PB) pre-crecked Charpy type specimens of VVER-1000 reactor pressure vessel base and weld metals was observed. Two types of brittle fracture origin sites were found: nonmetallic inclusions and grain or subgrain boundaries. It was shown that K_Jc values are shifted to the higher temperature area for weld metal with respect to base metal data. In case when the initiation origin is grain or subgrain boundary, the K_Jc values are higher for base metal at the same CID values. This indicates the higher crack resistance of base metal.

Abstract: Cu-matrix bearing materials were prepared by powder metallurgy method and their mechanical properties were studied in present work. The continuity of copper alloy matrix was destroyed with the increase of Bi element. Because Bi is brittle phase and distributed in the copper alloy matrix grain boundaries as thin ribbon network. The hardness, crushing strength and impact toughness of copper-bearing materials were reduced with increase of the content of Bi. The mainly of Copper-bearing materials containing bismuth fracture are brittle intergranular fracture. When the Bi content is over 4wt%, the Bi has a relatively full distribution in the rich Bi phase of copper alloy matrix grain boundary like a chip mesh belt. As the further increase of Bi, the reduced trends of the mechanical properties of Cu-matrix bearing materials containing bismuth are slow down. because the role of fragmented Copper alloy matrix will not significantly increase with further increase of Bi.

Abstract: he macro fracture of broken drill pipe friction welded joints and characterizations of microstructure were performed by SEM and XRD, and tested its hardness. The results suggested that the large area of “gray leaf spot” (composition: Fe₃O₄) which is on friction welded joints is the main reason causing the drill pipe brittle fracture. In order to avoid excessive “gray leaf spot” defects during the friction welding, put forward practical recommendations.

Abstract: In this study, Mg-7Gd-5Y-1Nd-0.5Zr alloy (EW75) was produced by melting method and then press-forged into large size plate. The properties of the Mg-7Gd-5Y-1.2Nd-0.5Zr alloy were optimized through T6 heat treatment. The microstructures of alloy were observed by means of optical microscopy (OM), scanning electron microscopy (SEM). Its mechanical properties under different heat treatment conditions were determined by tensile tests. The results indicated that increasing the solid solution temperature and prolonging the solid solution time can both lead to the dissolution of second phase in the alloy back into the matrix. The solid solution temperature affects the dissolution process more than the solid solution time. Grain growth occurred during the solid solution process. The grain size of the matrix enlarges with the increase of solid solution temperature. The tensile test result showed that the tensile strength of the alloy was significantly improved after T6 heat treatment. Its tensile strength in the same direction was nearly 40% up after T6 heat treatment. The analysis shows that T6 heat treatment can effectively eliminate the larger deformed precipitates and beneficial to the formation of hard precipitates, which leads to an improvement in the alloys tensile strength.

Abstract: In order to study the concrete compound crack and fracture extension problem such as brittle material, based on the analysis of the fracture criterion theory ,on the analysis of some measured data, and the near crack tip portion of the shape change energy, the strain energy criterion of Iso- line for Ⅰ-Ⅱ mixed mode crack was proposed, a crack initiation angle formula and the fracture criterion were established, and the variation law among ， with Poisson ratio was discussed . Using the criterion, the concrete compound crack was analyzed. Results show that the theoretical values are consistent with the measured values. So the criterion can be applied to analyze the mixed mode crack fracture of brittle material such as concrete.

Abstract: The 2Cr13 axle sleeve, used in N52 water pumps, suffered wear after being tested for a relatively short time and fractured when it was beat under the dynamic load. Optical metallographic microscopy, scanning electron microscopy (SEM) and Vickers hardness tester for the fractured 2Cr13 axle sleeve were used to determine the most probable causes of the failure. The results showed that the failure was mainly due to the inappropriate technological parameters of high frequency induction heating and a large amplitude rotor vibration. Meantime, cracks were easily found not only close to the frayed surface but also in the inner part contributed to the inappropriate parameters. Finally, the proper protections were advised to be used for preventing this type of failure for the axle sleeve from happening again in the further.

Abstract: Sapphire has high mechanical strength at high temperatures and high pressure. Thus, micro devices made of sapphire are expected to be used in extreme environments such as aerospace, defense and marine. Then, the machining technology is required for manufacturing of 3 dimensional shapes. The paper discusses the micro cutting process of sapphire to determine the applicable cutting parameters in planing and milling. First, the critical stress at which crack initiation occurs is measured in the indentation test with detecting AE signal. The critical stress of sapphire is beyond 22.6 GPa, which is much higher than those of glasses. Then, the critical depths of cut, maximum depths of cut in which crack-free surfaces were finished, were measured in cutting of r crystal plane. The critical depth of cut is maximized when the cutting is performed in a crystal axis direction. In milling of sapphire, adhesion of chip onto the surface finish should be considered as well as brittle fracture. In milling of r crystal plane, the surface finish is improved better than those of the other crystal planes. Adhesion of material can also be controlled by supplying lubrication.

Abstract: Safety valve is an important component of a boiler. The failure of its spring will bring hidden trouble to the boiler operation. The fracture failure of a drum safety valve spring of a power plant boiler was discussed based on chemical composition analysis, hardness test, metallurgical microstructure observation and SEM. Results showed that improper material selection and improper heat treatment process which led to temper brittleness contributes to the fracture. In addition, oxygen corrosion pit on the surface of the spring play an important role in promoting the cracking.

Abstract: Based on the singular stress field near the V-notch tip and under the two basic assumptions in this paper, twin-shear yield criterion under mixed loading mode I and II is established and the numerical results of the crack angles are calculated by this criterion. In order to verity twin-shear yield criterion ,tension tests are carried out under mixed mode loading by tension testing machine using polymethyl methacrylate(PMMA). The experimental results coincide with the numerical prediction results, which suggests that the twin-shear yield criterion is valid for the V-notch brittle facture.