PSI - Issue 13

Bragov A.M. et al. / Procedia Structural Integrity 13 (2018) 1811–1816 Author name / Structural Integrity Procedia 00 (2018) 000–000

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To carry out the tests at negative temperatures a cooling by nitrogen vapor, both a specimen with a surrounding strong ring, and the adjacent ends of the measuring bars was used. The specimen temperature was controlled by a miniature thermocouple welded to the side surface of the specimen. It should be noted that due to the large ratio of the cross-sectional areas of the measuring bars and the working part of the specimen (12:1), most of the loading wave in the tension cycle is reflected back from the specimen, reaches the free (rear) end of the second measuring bar, re-loads the specimen, is reflected again, etc. Thus the specimen undergoes several cycles of tensile load with gradually decreasing amplitude (Bragov et al. 2001). Usually with insufficient energy of the loading pulse and great plasticity of the test material, the destruction of the specimen does not occur in the first (main) loading cycle, but in any subsequent one. With the existing configuration of the measuring bars undistorted registration of two loading cycles is possible. The analysis of the recorded strain pulses allows us to determine the cycle in which the destruction occurred. 2.2. Method for studying fracture toughness As an alternative to the standard Charpy impact test, a modification of the Kolsky method developed by Bragov et al. (2009) was used to obtain the dynamic fracture toughness parameters (KCV) by realizing the tension of a solid cylindrical specimen weakened by V-notch cut without a fatigue crack (Fig.1,b). The length of the working part is L =5 mm, diameters: total cross-section D =7 mm, weakened section d =4 mm. The radius of the notch is 0.25 mm. The measurement procedure and the means for recording the strain pulses in the measuring bars are the same as in the simple tensile tests. As a result of the tests the energy and fracture toughness are determined. The energy of destruction KV is defined as the area under the curve P ~Δℓ. The fracture toughness for specimens with an annular notch KCV=KV/ A 0 , here A 0 =πd 2 /4 is the cross-sectional area at the notch location. The nominal stress σn is defined as the ratio of the force in the sample P (t) to the area of the weakened cross section σ n ( t )= P ( t )/ A 0 . Accordingly, the breaking stress σ * in the specimen is defined as the maximum of the time dependence σ n ( t ). 2.3. Method for analysis of strain-rate dependences of fracture toughness It is known (Savruk, 1988) that the stress intensity factor K I can be found from the stress concentration at the top of the smooth cutout by the formula max I 0 lim 2 K      , where ρ is the curvature of the notch in its vertex, σ max = P max / A 0 is the maximum normal stress at the notch tip. This expression can be rewritten in terms of the stress concentration factor K t , assuming that the stress intensity factor reaches its critical value with a breaking load σ max : �� � � � � � � σ ��� � � � . The critical stress intensity factor (static fracture toughness) K IC is defined as follows (Kogut, 1986): �� � � ��� �√� � ���� , where the shape factor �λ� � ������√��� �√� � ��������√��� , d D /   , here d is the diameter of the sample in the notch, D is the diameter of the total cross section of the working part of the sample. The constant 0.9656 in the equation is somewhat different from the one given in [Kogut, 1986] and equal to 0.7976 because the tested cylindrical specimen has no fatigue crack and therefore, about the fracture toughness can be said only conditionally. To describe the velocity dependences of the ultimate characteristics of fracture toughness, the incubation time concept was formulated by Petrov and Utkin (1989), Petrov and Morozov (1994). � � � � �� � �� � � �� , (1) where  is the structural or incubation time - an independent material parameter, K Ic is the critical value of the stress intensity factor, K ( t ) is the value of the intensity factor for a given load.