PSI - Issue 50

Svetlana Barannikova et al. / Procedia Structural Integrity 50 (2023) 33–39 S. Barannikova, M. Nadezhkin, P. Iskhakova / Structural Integrity Procedia 00 (2023) 000 – 000

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microcracks) shift toward the accumulation of defects until the limit values (Pelleg (2013)). The damage evolution is also a multi-stage process, depending on both the initial structural state and its further changes. Under the action of the applied stress, plasticity and failure occur almost simultaneously and proceed in an interconnected manner. At the onset of deformation, plastic strain forms a critical structure, determining the location and mechanism of the origin of microcracks. At the final stage, the emergence of a macrocrack is the leading process of plastic deformation. In turn, the intermediate stages in plastic deformation are determined by the evolution of microcracks. Technical diagnostics and assessment of accumulated plastic deformation are based on the well-known acoustic methods that consist in measuring elastic constants or elastic moduli along with elastic wave propagation velocities by pulsed and resonant tools, as well as determining relative values of the velocities of one longitudinal and two transverse waves by electromagnetic acoustic techniques (EMA, acoustic strain gauge method, acoustic emission testing, etc.). All these approaches allow one to control the state of the material, product or the whole structure (Badidi Bouda et al. (2000), Kumar et al. (2003), Torello et al. (2015), Murav’ev et al. (2017), Gorkunov et al. (2021)). In particular, the ultrasound velocity measurement under the direct impact of external and internal loads on the material or structure is one of such promising methods. The effectiveness of this technique follows from the fact that the acoustic waves "reflect" the structure of the matter under consideration, and their parameters change when certain defects (e.g., dislocations or interfaces) occur in the material. In addition, stresses of the I-th (macro-) and II th (micro-) kinds arising in the structures alter the velocity of ultrasound propagation depending on the applied loads. One of the most popular topics in the field of ultrasound research is the investigation of changes in the structure of materials as a result of plastic deformation, fatigue loading, and heat treatment. Numerous works have been dedicated to the influence of heat treatment or alloying elements on the propagation speed of ultrasound as well as its attenuation and dispersion (Hakan et al (2005), Hsu et al. (2004)). However, the relationship between the ultrasound velocity measured during loading and the acting deformations and stresses is still not sufficiently justified by physical arguments. This is because the issues that would definitely show how the change in the metal structure is related to the variation in the acoustic parameters of materials are barely worked out in this direction. Most studies are limited to measuring physical quantities proportional to the load, i.e., only in the elasticity range, whereas the elastic stress state of the material remains beyond the consideration (Palanichamy et al. (1995), Murthy et al. (2009)). In view of the above, monitoring the patterns of changes in the velocity of ultrasonic waves during mechanical tests with the aim of developing methods pre-determining the mechanical properties of a material and its structural evolution in the course of operation or mechanical testing is of decisive importance (Vasudevan et al. ( 2002)). In this work, the stages of plastic deformation and failure of a structural steel are investigated via non-destructive control methods over a wide temperature range. Special attention is paid to establishing the influence of temperature and load on the acoustic characteristics of the steel.

Nomenclature V

speed of ultrasonic Rayleigh waves

the attenuation coefficient

α

D V

damage parameter

strain stress



σ

2. Materials and methods Austenitic stainless steels are widely used in the chemical and petroleum industry, food engineering, and medical equipment. In this study, the experiments were performed on a polycrystalline Fe-18 wt.% Cr -10 wt.% Ni alloy with a grain size of ~ 12.5  m. Samples with dimensions of 40  5  2 mm were tensile on an “ Instron-1185: testing machine with a speed of 3.3  10 -4 s -1 at temperatures of 318, 297, 270, 254, 227, 211, and 180 K. The test temperature in the working chamber with the sample was set by the purge rate of nitrogen vapor from the Dewar