PSI - Issue 30

V.P. Gulyaev et al. / Procedia Structural Integrity 30 (2020) 40–44 Gulyaev V.P., Petrov P.P., Stepanova K.V. / Structural Integrity Procedia 00 (2020) 000–000

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methods and methods of Rigid Deformed Body Mechanics, as described by Arabey (2010), Kushnarenko (2016), Klyuev (2015), R 50-54-52-88 Rekomendacii (1988), Botvina (2008), Klevtsov (2006). The detection of the mechanisms of substructure changing processes of a loaded metal, determining the characteristics of the mechanical properties of structural steels was investigated by Sih (2008), Chirkova (2009), Shmidt (1983). Today, the X-ray diffractometry is the most promising method among the experimental methods for reliability testing of operated metal products and structures. The X-ray and neutron diffractometry methods are used to study inhomogeneous elastic deformation of crystals, extents of coherent-scattering regions, and other microdistortions of the crystalline lattice of structural steels caused by external actions: mechanical, thermal, radiation, etc. Separate experimentally detectable effects of the reaction of the nanoscale structure of a solid crystalline body on the action of external mechanical loads focus the attention of researchers on the physical processes that occur inside the crystal lattice at the nano and micro levels. There are sufficiently long periods during the operation of steel structures, just as the structures do not experience the effects of external static and / or dynamic loads, except for their own gravity. So, it can be assumed that a decrease or stopping of the external action promotes relaxation of the stress-strain state of the structure, affecting the substructure characteristics of the structural material, which can be detected by X-ray diffractometry. Experimental study by X-ray diffractometry methods of fatigue accumulation of damage in the crystalline structure can help to establish reasonable margins of strength and durability of structural materials in the designing of machine parts and structural elements. At the same time, issues related to using the characteristics of X ray lines diffraction profile to diagnose possible changes in the load capacity of metal structures were studied by Botvina (2008), Sih (2008), Kile (2011), Korchevskiy (2012), Panin (2006). It should be noted that issues are insufficiently researched, in particular, in a comparative analysis of loaded and unloaded elements of multiply connected sheath structures and other technical systems that require periodic monitoring of changes in the stress strain state. The possibilities of using X-ray diffractometry methods to solve such practical problems have not been systematically studied. Therefore, the main goal of this work is to experimentally verify the relaxation of the stress-strain state in 09G2S structural steel samples by X-ray diffractometry, and to detect a change in the profile characteristic of the diffraction line — the FWHM (full width at half-maximum) broadening depending on the level of acting cyclic stresses. 2. Experimental procedures A commercial grade 09G2S steel plate samples were subjected to cyclic loading at 3 different levels of the maximum stress of the pulsating cycle: σ max = 0,5 σ 0,2 , σ max = 0,7 σ 0,2 , σ max = 0,9 σ 0,2 . The diffraction lines profile characteristics of the surfaces of samples effective part were obtained. Diffractometry of the samples was conducted twice: immediately after loading and after 10 years of storage in an unloaded condition. Samples used for cyclic load and mechanical tests were extracted and machined from sheets of blankets along cut from a gas pipe with a diameter of 530 mm and a thickness of 7 mm. The results of static tensile tests according to State Standard GOST 1497 - 84 “Metals. Tensile test methods” verified certification compliance of 09G2S steel ( σ в = 544 … 551 МПа , σ 0.2 = 436 … 439 МПа , δ = 25 … 29 %, ψ = 73,1 … 75,8 %). The general view and geometric dimensions of samples for cyclic loading and further X-ray photography are shown in Fig.1. The cyclic loading was performed on INSTRON 8802 universal servo-hydraulic testing machine with a pulsating, zero-stress cycle at 3 different levels: level 1 σ max = 0,5 σ 0,2 , σ m = 0,25 σ 0,2 = σ a , R σ = 0; level 2 σ max = 0,7 σ 0,2 , σ m = 0,35 σ 0,2 = σ a , R σ = 0; level 3 σ max = 0,9 σ 0,2 , σ m = 0,45 σ 0,2 = σ a , R σ = 0. Three samples in each batch were load, the main parameters of cyclic loading are listed in table 1. In 2009, X-ray photography of the effective part of samples with a length of 60 mm (see Fig. 1) of pre-cyclically loaded 09G2S steel samples was performed on a DRON-3M diffractometer using a chromium anode. The operating mode of the X-ray source: U=30kV; I=20mA; scanning speed is ¼ deg/min with 0.020 deg. interval, time at a point - 0.01 seconds. The X-ray line profile was plotted along the (211) plane, with a Wolf - Bragg reflection angle 2 θ = 157.040 degrees. The plotting and output of the X-ray line profile were realized by computer device equipped with software approximating the intensity discrete values of the reflected radiation by the Cauchy functions.