A diagram valid for the analysis of the fatigue limit of cracks and notches centred in an infinite plate was recently proposed by the authors of the present work with the aim to make explicit the bridging at the fatigue limit between defect sensitivity (correlated to the length parameter a0, according to El Haddad–Topper–Smith's definition) and notch sensitivity (correlated to a*, where a* is a particular notch depth corresponding to the intersection between the ΔKth and Δσ0/Kt curves). The expression being valid, defect sensitivity and notch sensitivity were seen as two sides of the same medal. Such a diagram is now extended to finite size components by simply introducing the shape factor α commonly used in fracture mechanics. The obtained critical defect size is termed aD, which is a material and geometry dependent parameter, in order to distinguish it from a0, which is a material parameter. As a consequence the critical notch depth aN is introduced, such that . This results in the proposal of a ‘universal’ diagram able to summarize experimental data related to different materials, geometry and loading conditions. The diagram, the validity of which is checked by means of several results available in the literature, is applied both to the interpretation of the scale effect and to the surface finishing effect.

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A B S T R A C T The energy dissipated to the surroundings as heat in a unit volume of material per cycle, Q, was recently proposed as fatigue damage index, and it was successfully applied to rationalise fatigue data obtained by carrying out stress-controlled and strain-controlled fatigue tests on AISI 304 L stainless steel plain and hole specimens. In this paper, it is shown that the Q parameter is independent on thermal and mechanical boundary conditions occurring during experiments. After that, additional stress-controlled fatigue tests on plain and notched specimens characterised by smaller notch tip radii than those tested previously have been performed. Present data have been compared with previous ones, and it was found that all available results can be synthesised in terms of the energy parameter Q into a unique scatter band, independently on the testing conditions (stress-controlled or strain-controlled) and on the specimens' geometry (plain or notched). About 100 data were included in the statistical analysis to characterise the energy-based scatter band of the material. Finally, some limitations of applicability of the experimental technique adopted in the present paper are discussed.Keywords AISI 304 L; energy dissipation; fatigue scatter; notch effect; thermography; thermometric methods. N O M E N C L A T U R EA% = percent deformation after fracture c = material specific heat E p = rate of accumulation of damaging energy in fatigue per unit volume f = load test frequency f acq = sample frequency of the temperature data HB = Brinell hardness k = inverse slope of the stress-life (s an -N f ) curve and of the energy-based life (Q-N f ) curve K = specimen's axial stiffness K i = initial value of specimen's axial stiffness K tn = stress concentration factor referred to the net section N = number of fatigue cycles N A = reference fatigue life (2Á10 6 cycles) N f = number of cycles to specimen's failure N s = number of cycles corresponding to complete specimen's separation Q = energy released as heat in a unit volume of material per cycle (specific heat loss per cycle) Q A,50% = characteristic value of Q at the reference number of cycles N A with a survival probability of 50% R = notch radius R m = engineering tensile strength R p02 = engineering proof stress R e = nominal strain ratio (ratio between the minimum and the maximum applied nominal strain)

This paper summarizes an attempt to devise an engineering method suitable for predicting fatigue lifetime of metallic materials subjected to both proportional and nonproportional multiaxial cyclic loadings. The proposed approach takes as a starting point the assumption that the plane experiencing the maximum shear strain amplitude (the so-called “critical plane”) is coincident with the micro-/mesocrack initiation plane. In order to correctly account for the presence of both nonzero mean stresses and nonzero out-of-phase angles, the degree of multiaxiality/nonproportionality of the stress state damaging crack initiation sites is suggested here to be evaluated in terms of the ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio is used then to define nonconventional Manson–Coffin curves, whose calibration is done through two strain-life curves generated under fully reversed uniaxial and fully reversed torsional fatigue loadings, respectively. The accuracy and reliability of our approach were systematically checked by using approximately 350 experimental data taken from the technical literature and generated by testing 13 different materials under both in-phase and out-of-phase loadings. Moreover, the accuracy of our criterion in estimating lifetime in the presence of nonzero mean stresses was also investigated. Such an extensive validation exercise allowed us to prove that the fatigue life estimation technique formalized in the present paper is a reliable tool capable of correctly evaluating fatigue damage in engineering materials subjected to multiaxial cyclic loading paths.