PSI - Issue 23

I.O. Sinev et al. / Procedia Structural Integrity 23 (2019) 565–570 I.O. Sinev/ Structural Integrity Procedia 00 (2019) 000 – 000

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acoustic emission (Tyutin et al., 2018; Vettegren et al., 2004) and the method of estimating the velocity and attenuation coefficient of ultrasound (Ohtani, 2009), the study of electromagnetic radiation (Borisov, 2018), magnetic methods for estimating the coercive force (Tyutin et al., 2018), eddy current (Gonchar et al., 2016), metal magnetic memory (Yao et al., 2012) and others. The real damage is estimated by few researchers, since this requires a difficult and lengthy study. Gonchar et al. (2011) use a portable metallographic complex during cyclic tests to obtain images and estimate the change in pixel intensity at locations with defects, and, as a result, analyze the kinetics of microcrack accumulation. Beretta et al. (2015) study the residual lifetime of the material under conditions of corrosion fatigue, using for this purpose the method of optical microscopy. Tretyakova and Vildeman (2016) apply the method of digital images correlation, based on the study of the displacements and deformations fields obtained by digital cameras. It should be noted that the intensive study of the problem of small cracks was the focus of attention of researchers in the 70s and 80s of the last centuries. To describe the behavior of small fatigue cracks Miller (1987) subdivides the microcracks into 3 groups: (1) microstructurally short cracks, the size of which is comparable with the grain size of the metal of the studied sample; (2) physically small cracks, the size of which does not exceed 500 microns; (3) long cracks larger than 500 μm. A si milar classification can also be applied to small cracks developing under static loading conditions. Smith (1977) shows that cracks of various ranks appear at different stages of loading and differ in the growth kinetics. The purpose of this work is to study the development of surface defects (microcracks) under tension in samples with different geometry from three structural steels and to establish the influence of stress concentration and material structure on the studied damage characteristics.

Nomenclature σ stress ε*

relative deformation

S*

relative area of the damaged surface

k n

concentration criterion of damage accumulation

strain hardening coefficient stress concentration factor

σ α

d

index of the exponential function, approximating the k - ε* curves

2. Materials and methods

Tensile tests were carried out using samples of 5 mm thick of low- (0.2% C), medium carbon (0.45% C) and stainless (12C-18Cr-9Ni) structural steels with stress concentrators in the form of two notches with a diameter of 6 mm (type IX according to GOST 25.502) and standard flat samples (220 × 40 × 6 mm in size with the working part 80 × 20 × 6 mm) on an Instron 3382 machine with a deformation rate of 0.5 mm/min. Samples of similar geometry were used in many studies, such as studies Zhang and Fatemi (2011) and Swain (1992), but mainly to investigate the growth of small fatigue cracks using replicas, which allow studying the kinetics of microcracks from ~ 5 μm in length , as shown in work by Botvina et al. (2005). To assess the damage, the loading of samples alternated with stops, during which photographs of microcracks on the polished surface of the samples were obtained using an Olympus GX51 optical microscope equipped with a digital video cam era. This made it possible to estimate the change in the patterns of surface microcracks and their development at different stages of deformation. By processing the images using the image analysis program, the average length ( l av , µm), density ( n, 1/µm 2 ) and total area of microcracks ( S , µm 2 ) were estimated at different stages of loading. C racks with a length of at least 5 µm were considered (cracks of shorter length are difficult to distinguish from large pores or chains of pores). According to the measurement results, the relative area of the damaged surface S* , equal to the ratio of the area occupied by microcracks to the frame area and the concentration k -criterion of damage which is determined by the ratio k = 1/( l av · n 1/2 ) (Zhurkov et al.,1969; Botvina and Soldatenkov, 2017) were evaluated. Cumulative distributions of the microcracks concentration along their length at different stages of fracture were plotted and indexes in the power-law and exponential dependences approximating these curves were estimated.