Analytical and Numerical Methods for Assessing the Fatigue Life of Threaded Bores

Fatigue damage in Internal Combustion Engines (ICE) is affected by most of the physical phenomena characterizing the study of mechanical components. Mechanical energy transformation by means of deformable bodies inevitably leads to stress and strain fields. Thermal energy involvement makes the role of temperature primary since thermal expansion and thermal diffusivity effects are not negligible, especially if transient states are taken into account. Moreover, thermal cycling for certain temperature regimes may lead to creep phenomena that contribute to fatigue damage and Thermomechanical Fatigue (TMF). Additionally, the contribution of inertia in the dynamic equilibrium of the system combines with elastic forces and external loads, paving the way to the world of resonances and vibrations. Furthermore, manufacturing processes introduce residual stresses whose reliable estimation in the engine block is one of the challenges of modern automotive engineering. Finally, predicting loads requires simplifying hypotheses, given the wide working range of ICE. In this complex scenario, assessing the fatigue life of ICE threaded bores is not a simple task, even by neglecting many of the above-mentioned factors. Firstly, fatigue damage in presence of stress concentration features and its relationship with fracture mechanism is studied. Among the analysed methods, Theory of Critical Distances (TCD) is examined in depth within non-local approaches. Although critical distance arguments are already detectable in Neuber’s and Peterson’s works, only researches carried out in last decades showed TCD ability to predict failures in notched structures. TCD stands out as a general fracture theory in which microscale seems to be taken into account by introducing a new material constant. TCD is found to be a generalization of Linear Elastic Fracture Mechanics (LEFM) relatable to process zone and statistical models, Finite Fracture Mechanics (FFM) and microstructure. High Cycle Fatigue (HCF) and models for fatigue crack initiation are explored. Even though designed for uniaxial loadings, threaded joints exhibit multiaxial stress state due to notch effect. Multiaxial fatigue is analysed in the most general case of non-proportional loadings. In this sense, Critical Plane criteria are discussed by comparing the approach of commercial fatigue post-processors, inspired by the research of the lowest Safety Factor (SF), with methods based on actual fracture process. Modified Wöhler Curve Method (MWCM), Dang Van Criterion (DVC) and Maximum Variance Method (MVM) are analysed both from a local and TCD standpoint. These concepts are applied in a critical review of one of the possible methodologies for fatigue life assessment in ICE threaded bores. Engine FEM models are used to extract boundaries conditions for thermo-structural analyses of the bore sub-model. Conservative hypotheses on the engine working range are advanced and the resulting stress history is post-processed for fatigue assessment. A benchmark analysis is carried out by comparing numerical/analytical predictions with experimental evidence in aluminium specimens. On the basis of this study, strength and weakness of different methods are underlined proposing new TCD and non-TCD approaches. The versatility of DVC stands out since no cycle-counting method is needed. However, mesoscopic stress tensors should be computed. Finally, models for Safety Factor computation are compared with fatigue life approaches.