PSI - Issue 28

M.R. Tyutin et al. / Procedia Structural Integrity 28 (2020) 2148–2156 TyutinM.R./ Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Usually, to assess the material damage under deformation, either non-destructive testing methods or acoustic emission registration are used. For a complete understanding of the fracture process, it is necessary to study the kinetics of real damage in the material. Studies of the damage accumulation are very complex and usually, the process of their development is modeled based on the concept proposed by Lemaitre [1,2] or using the Gurson-Tvergaard-Needleman model [3]. The study of the real damage, coupled with the assessment of the physical parameters of non-destructive testing, will make it possible to obtain the relationship between these characteristics. Many works have been devoted to the study of the fracture process using non-destructive testing methods, such as acoustic emission (AE), metal magnetic memory method (MMM), eddy current method (EC), and measurement of coercive force (CF). Acoustic emission is widely used to assess the technical condition of structures in the industry. In addition to detecting cracks and other defects, this method also allows assessing the damage evolution at testing metallic specimens [4], concrete [5], rocks [6], and snow [7] under static, cyclic and impact [8] loading. One of the criteria for the onset of the pre-fracture stage is a decrease in the b-value, which is used both in seismology and in studying the fracture of structural materials by the AE method [5,9–12]. The metal magnetic memory method is based on the magnetoelastic effect of changing the intensity of the self magnetic field upon deformation of the ferromagnetic materials [13–15]. The difference between the MMM and other magnetic non-destructive methods is that it does not require the application of an external magnetic field to the object, since the intensity of the self-magnetic field of the loaded object is measured. The MMM method is used for non destructive testing of the state of technical devices in the industry [16]. In parallel, laboratory studies are being conducted on the effectiveness of this method in determining stress concentration zones [17] and damage accumulation at different stages of static loading [18,19]. Eddy current method is widely used in non-destructive evaluation of structural elements. This method makes it possible to detect defects in structures made of magnetic and non-magnetic materials in such critical industries as nuclear [20], aviation [21], and in other areas [22,23]. The work [24] shows the possibility of using this method to assess material degradation during periodic operational monitoring of pipelines. One of the most sensitive magnetic parameters to structural changes under static and cyclic loads is the coercive force ( H C ). By its physical nature, the coercive force is a measure of the resistance of a ferromagnet to demagnetization, which usually occurs by displacement of interdomain boundaries. Researches devoted to the study of changes in H C in a deformed material made it possible to establish the dependence of this parameter of non-destructive testing on the values of static [25,26] and cyclic [23,27] loads. In [28], it was found that the coercive force depends on the density of dislocations at plastic deformation of low-carbon steel specimens, but this relationship is of a qualitative nature. In our work, we applied an integrated approach to assess material damage under static loading using physical non destructive techniques and optical microscopy to establish the relationship between the real damage parameters and physical characteristics. This task is important both from a scientific and practical point of view.

Nomenclature b AE

slope coefficient of the amplitude distributions of the acoustic emission (AE) signals

Σ N AE cumulated number of AE events Ṅ AE intensity of AE signals H r

intensity of the resulting self-magnetic field

magnetic field intensity estimated by the eddy current method

H EC

coercive force

H C S *

relative area of the damaged surface