Issue 77

M. V. Boniardi et alii, Fracture and Structural Integrity, 77 (2026) 405-420; DOI: 10.3221/IGF-ESIS.77.23

in rotating bending (R = -1). The article does not report the residual stress profile along the direction of maximum stress, but only the residual stress value measured at the surface (-600 MPa). The authors experimentally detect a fatigue limit of  FAb = 735 MPa (the standard deviation is not shown); fatigue initiation occurs 0.45 mm from the treated surface. The simulation obtained with the proposed model is shown in Fig. 18. The fatigue limit estimate is  FAb = 665 MPa (the error, compared to the measured value, is approximately 9%); the fatigue crack initiation point prediction is 0.4 mm from the component surface. Tab. 2 summarises the results obtained in the three real-world experiments and the related estimates of the bending fatigue limit and the trigger position: the validity of the proposed method is underlined, with errors lower than 10% compared to the experimentally measured values.

Crack Initiation experimental [mm]

Crack Initiation estimated [mm]

 FAb (R=-1) experimental [MPa]

 FAb (R=-1) estimated [MPa] 390 (-7.1%) 714 (-4.8%) 665 (-9.4%)

Steel grade

Heat treatment

Surface hardening Case Hardening

50CrV4

420 750 735

> 0.80

1.05

16MnCr5 42CrMo4

Superficial

Superficial

Nitriding

0.45 0.40 Table 2: Comparison between experimental data [15,22,49] and the estimation by the local fatigue limit method.

C ONCLUSIONS

T

his literature review systematically assessed the influence of the most common thermal and thermochemical surface hardening methods, specifically surface hardening, carburising, and nitriding, on the fatigue life of mechanical components. Given that fatigue remains the primary mechanism of structural failure in industrial applications, understanding the interaction between these treatments and cyclic loading is crucial. By synthesising the mechanical and metallurgical transformations induced by these processes, we presented a predictive model using the local fatigue limit approach. This model effectively integrates critical material variables to predict performance under bending stresses: (i) HV microhardness profiles, (ii) residual stress distributions, (iii) base material properties, and (iv) external stress states. The validity of the proposed model was rigorously tested on three different cases: notched surface hardened specimen, smooth carburised specimen and smooth nitrided specimen. The high degree of correlation between model predictions and experimental results offers significant value for both academic and industrial applications. It provides a quantitative framework for identifying which metallurgical parameters should be optimised to maximise the service life. Ultimately, the application of the local fatigue limit approach has proven to be an accurate methodology for predicting the durability of superficial layers modified by surface hardening, carburising or nitriding. This research facilitates the design strategy, allowing engineers to determine the most effective approach, such as optimizing the effective depth of the hardened layer or adjusting tempering parameters, to improve the fatigue behaviour of mechanical components even in case of high stress concentration. [1] Casaroli, A., Scabini, E., Boniardi, M. V., Gerosa, R., Rivolta, B. (2026). Optimization of austenitic and ferritic steels for deep drawing. Part 1: metallurgical and mechanical analyses., Fracture and Structural Integrity, 20(75), pp. 104–123. DOI: https://doi.org/10.3221/IGF-ESIS.75.09. [2] Casaroli, A., Scabini, E., Boniardi, M. V., Andreotti, R., Rivolta, B. (2026). Optimization of austenitic and ferritic steels for deep drawing. Part 2: FEM analyses with damage development, Fracture and Structural Integrity, 20(75), pp. 179– 199. DOI: https://doi.org/10.3221/IGF-ESIS.75.13. [3] Boniardi, M. V., Casaroli, A., Sirangelo, L., Monella, S., Mazzola, M. (2023). Failure analysis of boron steel components for automotive applications, Frattura Ed Integrita Strutturale, 17(64), pp. 137–147. DOI: https://doi.org/10.3221/IGF-ESIS.64.09. [4] Bannantine, J.A., Comer, J.J., Handrock, J.L. (1990). Fundamentals of Metal Fatigue Analysis, Prentice Hall. R EFERENCES

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