Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

Abstract

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel.

Effect of heat treatment on the microstructure and fatigue behaviour of AISI 4130 steel

“…Figure 5c shows a picture of one initiation point in higher magnification. Figure 5d shows striations typical for stable fatigue crack growth [9]. The detail from the area of the final fracture (see Fig.…”

An automatic image processing based on Hough transform algorithm for pavement crack detection and classification

“…The study (ibid) results showed that the proposed model achieves a classification accuracy of 93.86%. A method for the quantitative assessment of the spacing of FS was developed by (Maruschak et al, 2021). The method includes the detection of FS by binarization of the image based on the principle of local minima and rotation of the highlighted fragments of the image using the Hough transform.…”

Analysis of Mechanical Properties of Welded Joint Metal from TPP Steam Piping after Its Operational Degradation and Hydrogenation

“…The main factors of the degradation of heat-resistant steels operated at TPPs include high temperature and stress, supplemented by the influence of a hydrogen-inducing technological environment [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. During the long-term operation of high-and low-alloy heat-resistant steels, the carbides form and coagulate along the grain boundaries.…”