Notch and defect sensitivity in uniaxial and multiaxial fatigue of 42CrMo4 Q&T steel for marine engines

Manufacturing of metal products is energy-intensive and resource-heavy, with notable environmental impacts. Europe targets waste reduction via better design/manufacturing and the twin transition. In 2022 the ENGINE project set out to deliver a zero-defect, first-time-right system for mechanical components. Marine engine connecting-rod heads were the case study. A key contribution was the fatigue characterization of these parts, focusing on notch and defect sensitivity under cyclic loading by a multi-institutional team. This dissertation addresses notch and defect fatigue sensitivity of quenched-and-tempered 42CrMo4 steel, commonly adopted in the marine engines, under uniaxial and multiaxial loadings representative of service. An extensive program on specimens cut from full-scale big-end bearing housings is combined with analytical, numerical, and experimental methods to obtain a design-oriented description across plain, bluntly notched, severely notched, and defected geometries. Chapter 1 frames the problem, the multiaxial context with stress raisers, and the thesis role within ENGINE. The University of Padova contributed experimental multiaxial notch/defect fatigue characterization and new assessment methodologies. Chapter 2 reviews fatigue of notches/defects. Three approaches are implemented: (1) Atzori–Lazzarin–Meneghetti links endurance to an effective crack depth under fully reversed axial loading; (2) a thermographic Q-based energy–life method on plain specimens at two R-ratios; (3) an averaged strain energy density (SED) method extended to elastic–plastic behavior for fully reversed multiaxial loading, with curves for total failure and SED-consistent crack initiation calibrated on bending and combined-loading data. Chapter 3 presents the novelty: extending averaged SED from linear-elastic to elastic–plastic multiaxial conditions. Chapter 4 details specimen extraction from two rod heads. The test matrix covers tensile and fatigue tests in axial, bending, torsion, and combined bending-torsion with two biaxiality ratios and phase shifts, using plain, U- and V-notched, and U-notched with artificial defects. Chapter 5 describes equipment: an axial servo-hydraulic machine and a dual-actuator bench for bending/torsion/combined loading. Monitoring included extensometers, thermocouples, and DCPD on notched specimens. Chapter 6 reports 42CrMo4 properties and cyclic behavior from strain-controlled tests, deriving stabilized loops and Basquin–Manson–Coffin and Ramberg–Osgood curves.Chapter 7 analyzes damage across loadings: fracture morphologies, S–N curves, axial temperature data for Q-based analysis, and for notched tests DCPD signals and calibration curves. Cyclic plasticity protocols are included, with preliminary non-linear kinematic hardening parameters fitted in Ansys® Workbench. Chapter 8 synthesizes results via the three models.