Statistical evaluation of the critical distance in the finite life fatigue regime

The procedure to evaluate the critical distance with an optimized V-notched specimen is initially reviewed in the paper. This procedure was devised by the authors, and another numerical methodology was recently proposed to evaluate the uncertainty of the critical distance assessment. The input of the analysis is the combination of the statistical distribution of the fatigue properties from which the critical distance is deduced. After assuming the specimen fatigue strengths as Gaussian (normal) distributions, the critical distance turns out to be well represented by a Skew-normal distribution. This statistical assessment is extended to the finite fatigue life, in the present paper, showing experimental results for the aluminium alloy 7075-T6 at two load ratios. The fatigue strength of other specimens are finally evaluated, reconsidering the critical distance deviation, thus providing a complete uncertainty analysis of the critical distance assessment, and a successful comparison with the experimental scatter is obtained

Statistical significance of notch fatigue prognoses based on the strain-energy-density method: Application to conventionally and additively manufactured materials

The inverse search determination of the strain-energy–density (SED) control radius R1 devised in Benedetti et al. Int J Fatigue 2019;126:306–318 and based on the knowledge of the notch fatigue factor estimated using an optimal V-notch specimen geometry is here reformulated to take into account the statistical properties of the input fatigue properties. It was found that R1 exhibits a non-symmetric probability density function that is well represented by a skew-normal distribution. The uncertainty in R1 can be attributed to the uncertainty in the inverse search procedure and to the material variability in notch sensitivity. By applying the devised procedure to real experimental data, it was found that the former contribution is preponderant in the assessment of very sharp notches, while the latter dictates the fatigue strength of blunt notches, especially in the case of intrinsically flawed materials, such as those additively manufactured

Inverse determination of the fatigue Strain Energy Density control radius for conventionally and additively manufactured rounded V-notches

The Strain Energy Density (SED) fatigue criterion is based on a material control radius. The value of this length is therefore required for an accurate assessment of the fatigue strength of any, especially severely, notched components. The singularity-based control radius is initially obtained by considering the hypothetical perfectly sharp V-notched specimen. The effect of the notch radius on the inverse search is then investigated with numerical simulations, and a new analytical procedure is introduced for the determination of the (actual) control radius. This procedure is applied to the experimental data of three metal alloys with different load ratios and manufacturing conditions

sensibility analysis of the fatigue critical distance values assessed by combining plain and notched cylindrical specimens

Abstract The material critical distance is often deduced from plain and notched specimens, instead of experimentally measuring the (long) crack threshold, which is a challenging task and not adequate in some cases. A dedicated V-notched specimen was proposed along with a dimensionless numerical procedure to derive the critical distance from the fatigue stress concentration factor, by implementing both the line and the point methods. An experimental validation activity is provided here on 42CrMo4+QT steel, focusing on how the critical distance result is sensitive to the actual local radius, the specimen sharpness, and the choice between the line or the point method. The determination of the critical distance with the point method systematically provides higher values than the line method. However, these length discrepancies do not produce large effects in terms of the component strength assessment if the same method for the fatigue limit evaluation is used. By alternatively considering the specimen not involved in the critical distance determination, as a potential design component, the prediction accuracy was evaluated. This analysis confirmed that a small notch radius is recommended for the fatigue strength assessment of larger radius notches or even of a crack, whereas by deducing the critical distance from a blunt notch, a noticeable inaccuracy can be found on smaller radius and crack threshold

Determination of the fatigue critical distance according to the Line and the Point Methods with rounded V-notched specimen

The critical distance length should in principle be deduced from the plain specimen fatigue limit and the threshold stress intensity factor range. However, the threshold range is difficult to measure experimentally, hence this length is usually obtained by means of a notched specimen. The critical distance inverse search, both according to the Line and the Point Methods, is presented in this paper referring to a relatively sharp V-notched specimen. Precise indications about the geometry parameters are given along with a complete analytical procedure to easily obtain the critical distance. A sensitivity analysis is discussed, providing evidence of a critical distance range for a well-posed inversion problem

optimal notched specimen parameters for accurate fatigue critical distance determination

Abstract The critical distance value should theoretically be determined from the plain specimen fatigue limit and the threshold stress intensity factor, though usually ordinary notch geometries are considered. In this paper, we proposed an optimized sharp notch with the aims of simple and reliable manufacture and, more importantly, a local strong stress gradient able to minimize the sensitivity on the deduced critical distance value. A numerical procedure is proposed to find the critical distance from the fatigue strength of the notched specimen, by implementing the line method with simple formulas based on dimensionless equations and specific coefficients derived from accurate FE analyses. A definition of the boundaries for a valid critical distance evaluation is also introduced and discussed. Finally, an application example is provided on a quenched and tempered steel also comparing the obtained critical distances with the threshold derived values

Surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes

The rolling contact fatigue is distinguished into subsurface initiated (spalling and case crushing) and surface initiated (pitting and micropitting). A characteristic depth was identified for each of these mechanism. The characteristic depth of the case crushing is the hardening depth, while for the spalling it is the maximum cyclic shear stress depth. The pitting depth is the size of the crack for which the mode I stress intensity factor range, due to the fluid pressurization, is higher than the threshold. This depth can be similar to the micropitting depth, in the order of 10 µm, for heavily loaded small radius contacts. Rolling contact fatigue cyclic shear stress indexes are then defined on the basis of the characteristic depths, and they identify the load intensity of each rolling contact fatigue mechanism. The characteristic depths and the stress index approach can be used to relate specific tests to component design, without any size effect misinterpretation

A fretting fatigue setup for testing shrink-fit connections and experimental evidence of the strength enhancement induced by deep rolling

Fretting tests are usually performed on flat specimens with lateral contacting pads. The shrink-fitted connection, which experiences fretting at the edge of the contact, prompted the alternative use of a round-shaped specimen. This simplified the equipment and provided an accurate alignment between the fretting specimen and the external hub which plays the role of the pad. The deep rolling treatment can also be efficiently applied to a round shape, which would otherwise be difficult on the flat specimen geometry. After introducing this solution for fretting testing, the paper shows an experimental campaign on three shrink-fitted connections with different sizes and material combinations. There was a significant improvement in fretting fatigue strength, induced by the deep rolling, for all three specimen types. Finally, scanning electron microscopic analyses provided insights into the fretting fatigue nucleation mechanisms both for untreated and deep-rolled specimens

Early consensus management for non-ICU acute respiratory failure SARS-CoV-2 emergency in Italy : from ward to trenches

no

Integral method coefficients for the ring-core technique to evaluate non-uniform residual stresses

The ring-core technique allows for the determination of non-uniform residual stresses from the surface up to relatively higher depths as compared to the hole-drilling technique. The integral method, which is usually applied to hole-drilling, can also be used for elaborating the results of the ring-core test since these two experimental techniques share the axisymmetric geometry and the 0°–45°–90° layout of the strain gage rosette. The aim of this article is to provide accurate coefficients which can be used for evaluating the residual stress distribution by the ring-core integral method. The coefficients have been obtained by elaborating the results of a very refined plane harmonic axisymmetric finite element model and verified with an independent three-dimensional model. The coefficients for small depth steps were initially provided, and then the values for multiple integer step depths were also derived by manipulating the high-resolution coefficient matrices, thus showing how the present results can be practically used for obtaining the residual stresses according to different depth sequences, even non-uniform. This analysis also allowed the evaluation of the eccentricity effect which turned out to be negligible due to the symmetry of the problem. An applicative example was reported in which the input of the experimentally measured relaxed strains was elaborated with different depth resolutions, and the obtained residual stress distributions were compared