Papers by Keyword: Microcrack

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Authors: Jing Feng Guo, Ren Gen Xu, Li Zhao, Wen Zhu Shao, Lian Jie Qin
Abstract: Initiation and propagation of micro cracks in the Fe-25Cr-35Ni based superalloy were observed and investigated through in situ tensile test in SEM using a single-edge notched specimen. The results show that the micro crack sources may easily occur with one of those, the second phase particles and matrix interface, and the stress concentration region. The micro-cracks which propagated at first for those that are perpendicular to the tensile load would be growing up and connected mutual with the increase of external stress until forming the main crack. While the length of the mail crack enlarged enough in the specimen, it become instable for the main cracks and the fracture occurred rapidly.
Authors: Bing Zhang, Masato Yoshioka
Abstract: A three dimensional finite element model for Vickers indentations on brittle materials is presented in order to analyze the stress distribution. The objective of this paper is to study when and where cracks are most likely to initiate and propagate in the indentation cycle based on the analyzed stresses. Therefore the time-dependent stresses around and below the surface of the contact area during the indentation cycle, especially at the end of loading and at the beginning of unloading phase are investigated in detail. The analytical results are shown to be in good agreement and verified with the experimental results.
Authors: Wei Bo Li, Osamu Umezawa
Abstract: The characterization of subsurface fatigue crack initiate sites of near α and α-β types titanium alloys and their cracking models proposed were reviewed. The crack initiation sites consisted of facets mostly on near basal plane of α grain, although the crystallographic orientation and surface topography of the facets presented a subtle difference. The crack initiation mechanisms were a quasi-cleavage accompanying high normal stress on the plane, a combination of basal slip and normal stress across the basal plane, and a pure slip on facet plane inclined near 45 degree to loading axis.
Authors: Osamu Umezawa, Wei Bo Li
Abstract: The subsurface fatigue crack generation processes in near α type titanium alloy were divided into four steps: (1) development of a saturated dislocation structure by cyclical micro-plastic strain accumulation, (2) generation of localized slip and/or microcracking to relax the stress concentration in the vicinity of a boundary, (3) microcrack growth and transition to main crack, and (4) crack propagation. The experimentals on transgranular facets formation in Ti-Fe-O alloy were reviewed and a subsurface fatigue crack generation model was discussed. The β platelets which were aligned between the recrystallized α grain and the recovered α grain were responsible for the microcrack generation to form (0001) tansgranular facet in the recrystallized α grains. A combination of the shear stress and tensile stress normal to the basal plane may give a trigger of the (0001) microcracking in the recrystallized α grain. The localized shear stress following slip off on the basal plane was activated at the microcrack tip in the recrystallizedαgrain, and the microcrack grew into the recrystallized α grain to form (0001) transgranular facet.
Authors: Guo Shuang Shui, Yue Sheng Wang, Jian Min Qu
Abstract: In this paper, a new theoretical model is developed to characterize the damage of the adhesive joint. Elastic modulus of adhesive joints is an important parameter to represent damage characteristics. Based on the fact that the thickness of the adhesive layer is very small, it is reasonable to believe that damage will decrease the tension modulus of the adhesive joint while the compression modulus will keep unchanged. Modeling the adhesive joint as an interface with different modulus in tension and compression, and applying integral transform method, we solve the associated nonlinear boundary problem to obtain the nonlinear ultrasonic waves transmitting through the adhesive layer. With this nonlinear ultrasonic wave, variation of elastic modulus and damage variable of the adhesive layer are thereafter characterized nondestructively by a nonlinear coefficient.
Authors: Oleg Bashkov, Roman Romashko, Sergey V. Panin, Tatiana I. Bashkova
Abstract: The paper presents the results of studies the deformation behavior of aluminium alloy D16 by acoustic emission (AE) method. The purpose of this study was to establish the deformation stages and deformation mechanisms at each stage. Studies were carried out on the samples of aluminum alloy D16 (analog 7075). This paper contains a method for the separation of AE signals. The method of AE sources identification based on the the two-parameter distribution analysis (frequency parameter Kfvs energy of AE signals). The frequency parameter Kf is based on wavelet transform of AE signals. Two-parameter distribution allows one to separate the AE signals emitted by dislocations from the signals of micro cracks. The investigation results allowed the various deformation stages to establish by the different types of AE signals. By the AE analysis shows the dislocation mechanism of hardening the aluminum alloyD16. The paper presents the results that characterize the influences of heat treatment and structural condition of aluminum alloy on the AE parameters.
Authors: Kang Zhao, Qun Yan Jia
Abstract: Based on extremely complex geological characteristics of metal mines, a mining model was built up, and from the micro - meso - macro point of view, relationship between rock AE and failure characteristics of overlying rock mass was studied and AE is divided into four stages. From the view of AE energy and distribution, the generation location, shape and size of caving of overburden rock was predicted. Through studies on the difference of goaf size, the author obtained the displacement variation and failure characteristics of metal mine overburden rock under tectonic stress, self-weight stress and dynamic mining conditions.
Authors: Jian Fu Shao, F. Bourgeois, O. Ozanam
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