The correlation of fractal dimension to fracture surface slope for fatigue crack initiation analysis under bending-torsion loading in high-strength steels

• Published in: Measurement : journal of the International Measurement Confederation 2023-08, Vol.218, p.113169, Article 113169
• Main Authors: Macek, W., Branco, R., Podulka, P., Masoudi Nejad, R., Costa, J.D., Ferreira, J.A.M., Capela, C.
• Format: Article
• Language: English
• Subjects: Bending-torsion fatigue; Fatigue crack initiation; Fractal dimension; Fractography; Fracture; Steel
• ISSN: 0263-2241
• DOI: 10.1016/j.measurement.2023.113169

•Comparison of fractal dimension under bending-torsion loading is studied.•Effect of fatigue crack initiation life on fractal dimension of fracture surface is evaluated.•Bending moment to torsion moment ratios and maximum local von Mises equivalent stress affects fracture surface topography.•Fatigue crack initiation life is successfully predicted from fracture surface parameters and maximum local von Mises stress. In this study, the fractal dimension of fatigue fracture surfaces is investigated in order to find an alternative failure loading indicator. Some of many metrological factors are generalized by reducing the fracture surface structure to one factor and develop an entire fracture surface procedure by analyzing the impact of surface slope and calculation resolution. Three notched geometries are studied under cyclic bending-torsion: 34CrNiMo6 high-strength steel bars with transverse blind holes; (ii) 34CrNiMo6 high-strength steel bars with lateral U-shaped notches; and (iii) 18Ni300 maraging steel hollow bars with transverse holes produced by selective laser melting. The surface topographies of fatigue fractures were measured with an optical profilometer. The bending-torsion ratio, maximum local von Mises equivalent stress and the number of cycles to crack initiation are examined using the fractal dimension. Moreover, a comparison was also made for conventional surface topography parameters such as root mean square height and void volume. A fatigue crack initiation model based on surface topography and loading conditions is proposed. The model relies on the product of the maximum local von Mises equivalent stress and the fractal dimension divided by the root mean square height to void volume ratio. The results show that the fracture plane geometry, expressed by the fractal dimension Df, can facilitate the estimation of post-failure loading history. In particular, the analysis based on the enclosing boxes method (EBM) is more accurate when it is used as an extra-fine resolution without any plane leveling.