PSI - Issue 26

V.N. Kytopoulos et al. / Procedia Structural Integrity 26 (2020) 113–119 V. N. Kytopoulos / Structural Integrity Procedia 00 (2020) 000 – 000

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1. Introduction

There is considerable evidence in literature that atomic hydrogen may segregate by rapid diffusion at internal particle matrix interface, grain boundaries, microvoids and stress gradient sites by lowering the cohesive atomic bonding at these sites (Briant 1985). This in turn results in formation of extensive regions of voids and interfaces. In undeformed materials the introduced atomic hydrogen will occupy any interstitial site in the lattice where it appears. In body – centered cubic steel materials the location is the tetrahedral interstice. Furthermore, there is also evident that in addition to the interfacial site hydrogen resides in traps such as dislocations and point defects, debris left behind the jogs of dislocations as they move through the metal during plastic deformation (Das 1996; Hertzberg 1989). During plastic deformation, hydrogen appears to affect the mechanical properties of the metal in a more specific way by controlling the ease by which dis locations nucleate and move through the crystal lattice. Many models suggested at this problem require the redistribution of hydrogen during the test. Especially, during slow – strain – rate tests at ambient temperatures hydrogen transported by moving dislocation, may essentially contribute to this redistribution (Kikuta et al. 1978; Megn & Bayles 1987). In this aspect, it has been shown that hydrogen affects the plastic flow by promoting the onset of localized plastic instability and so the premature fracture (Hertzberg 1989). However, today, despite extensive research on these problems, a fully understanding the complexity of hydrogen-assisted microstructural damaging processes is not achieved (Ioakei midis et al. 2013; Nagumo & Takai 2019; Byju et al. 2016; Martin et al. 2019; West & Holbrook 1981). Because of this, new variant aspects and approaches in the direction of mechanical and physical characterization of mechanically loaded steels after their exposure to corrosive hydrogen environment would be of significant importance for many structural steel components of various industries. Today, micromagnetic emission (ME), also called Barkhausen noise (BN), is a well established, versatile and stress-sensitive, non-destructive testing method for such characterization of steels at micro scopic level, where by using well known related micromagnetic events certain mechanical and stress-assisted micro structural changes in the material can be relieved and analyzed (Stefanita et al. 2000; Sulliran et al. 2004). Magnetization in ferromagnetic materials is due to the nucleation and growth of domains which result from creation and motion of domain walls with increasing external excitation magnetic field. The motion of domain walls can be often pinned by microstructural inhomogeneities such as precipitates, dislocation tangles and several lattice defects existing in the material. This leads to their discontinuous and abrupt motion. As a result of this kind of magnetic domain wall motion, a pulsating induced voltage in form of counts can be measured associated with the rapid change of magnetic volume due to interrupted jumping displacement between obstacles of domain walls. This phenomenon is called the micromagnetic emission effect known in the literature also as Barkhausen noise. The change of the magnetization during one wall jump or event is related to the integral of the area under the resultant pulse. Consequently, the Barkhausen effect would be very sensitive to microstructural changes of the material caused by several environmental chemical, physical and mechanical factors (Blaow et al. 2007). When applicable the measurement of magnetic Barkhausen noise is a fast, reliable, and simple technique for non – destructive material evaluation, as compared to X-ray diffraction, ultrasound and other more sophisticated measurements. The Barkhausen effect is especially well-suited for the study of steel, one of the most important structural materials. A block diagram of the experimental set up used for the measurement of micromagnetic Barkhausen emission (MBE) is shown in Fig.1. The applied sampling analysis frequency was 100KHz and maximum magnetic induction field of the excitation was about 20 G. The MBE signal at 10 Hz magnetic excitation was acquired by a 2 mm ferrite surface probe which had 1000 turns and then amplified to 40dB using a low noise amplifier. The total number of Barkhausen counts (events) and the corresponding rms-voltage level were measured for one second or (10 cycles of magnetization frequency) using the counter processing module of the given apparatus. The specimens, made of low-carbon steel (see Table 1) had dogbone-type geometry of thickness 2 mm, width 10 mm and effective gauge length 100 mm. The samples were subjected to uniaxial tensile test at room temperature and nominal low strain rate 10 -4 /s, using a universal Instron-type testing machine. The ultimate stress was 380 MPa and the yield stress (0.2% offset) was 190 MPa. Before on line testing, the samples were exposed to a corrosive environment produced by a continuously sprayed 3.5% NaCl aqueous solution in a Salt Spray Fog (SSF) apparatus. 2. Experimental procedure