13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- FRACTURE TOUGHNESS CRITERIA OF SMALL-SIZE SPECIMENS WITH ULTRAFINE-GRAINED STRUCTURE Yevgeny Deryugin Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences Russia, 634021, Tomsk, 2/4, Academichesky prospect, dee@ispms.tsc.ru Abstract A new method for determining crack toughness of materials is described based on test data of small-size chevron-notched specimens in terms of commercial titanium VT1-0 and titanium alloy VT6 with ultrafine-grained (UFG) structure, obtained by methods of severe plastic deformation (SPD). A problem of separating a part, connected with variations in specimen ductility under crack propagation, of the total displacement of load application point, is solved. Equations to calculate specific fracture energy are obtained. The calculated values of stress intensity factor KIc are in good agreement with known test data of standard specimens. Keywords ultrafine-grained (UFG) structure, strain localization, fracture, specific fracture energy 1. Introduction Standard crack toughness tests of materials are generally conducted using the bulk specimens not less than 10 mm in thickness. Although in many cases it is more convenient to use specimens of essentially smaller thickness for this purpose. These specimens do not require a large amount of material and high-power testing machines. Due to this, there is an issue in assessing crack toughness of ultrafine-grained (UFG) and nanostructured materials. The production of these materials in bulk specimens is connected with a series of technical problems. When testing the fracture toughness (crack toughness) of small-size specimens, the chevron-notched specimens are generally used [1-5]. Moreover, specimens with configuration of this type are not required to be fatigue pre-cracked. In the given paper, a new method for determining crack toughness of materials is described based on test data of small-size chevron-notched specimens in terms of commercial titanium VT1-0 and titanium alloy VT6 with ultrafine-grained (UFG) structure, obtained by methods of severe plastic deformation (SPD). In course of this study, important computational works, connected with using of chevron-notched specimens, were performed: • Calculation of the Young 's modulus of the material Е; • Definition of specific fracture energy (crack-driving force [6]) under crack propagation Gs. 2. Determination of Young’s modulus when testing chevron-notched specimens The data on Young’s modulus value for the materials with UFG structure is limited. It is known that SPD strongly affects the Е value [7, 8]. A method of Young’s modulus determination by test data of the chevron-notched specimens is described below. A scheme of the chevron-notched specimen is presented in Fig. 1. This configuration of the specimen can be considered as a double-cantilever design.
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