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Abstract: It is well known in the literature that fatigue is particularly critical for Additive Manufacturing (AM) parts, because internal defects originating during the AM process represent a critical site for crack initiation. In the literature, the High‐Cycle‐Fatigue (HCF) response of AM parts has been extensively investigated; however, there are few results on the Very‐High‐Cycle‐Fatigue (VHCF) behavior of AM parts, even if the number of machinery components that may sustain VHCF is rapidly increasing in the last years. The present paper investigates the VHCF response of an AlSi10Mg alloy produced through Selective Laser Melting. Ultrasonic tests are carried out on Gaussian specimens with a large loaded volume and show that fatigue failures in VHCF originate from surface and sub‐surface defects with the same mechanism of HCF failures. P‐S‐N curves and fatigue strength at 109 cycles are finally estimated to show the effect of defect size on the VHCF strength. Research highlights: Ultrasonic VHCF tests on AlSi10Mg Gaussian specimens produced through SLM. Identification and analysis of critical AM defects inducing VHCF failure. Statistical estimation of models for the prediction of the VHCF response of AM parts. Statistical prediction of effect of defect size on the VHCF response of AM parts. [ABSTRACT FROM AUTHOR]

Abstract: Extensive experimental investigations show that internal defects play a key role in the very‐high‐cycle fatigue (VHCF) response of metallic materials and that crack growth from internal defects can take place even if the stress intensity factor associated to the initial defect is below the threshold for crack growth. By introducing a reduction term in the typical formulation of the threshold for crack growth, the authors recently proposed a general phenomenological model, which can effectively describe crack growth from internal defects in VHCF. The model is able to consider the different crack growth scenarios that may arise in VHCF and is enough general to embrace the various weakening mechanisms proposed in the literature for explaining why crack can grow below the threshold. In the present paper, the model is generalized in a statistical framework. The statistical distributions of the crack growth threshold and of the initial defect size are put into the model. The procedure for the estimation of the Probabilistic‐S‐N curves and of the fatigue limit distribution is illustrated and numerically applied to an experimental dataset. [ABSTRACT FROM AUTHOR]

The continuous increment of durability and reliability requirements for many machinery components is significantly enhancing the research activity in the Very-High-Cycle Fatigue (VHCF) characterization of metallic materials, in particular of high-strength steels for critical structural applications. According to the [ABSTRACT FROM AUTHOR]

Abstract: Experimental tests investigating the gigacycle fatigue properties of materials are commonly performed with ultrasonic testing procedures, which allow for a significant reduction of testing time but induce a significant temperature increment in specimens. In order to evaluate the significance of size effects on the fatigue strength of materials, the Authors recently proposed to adopt Gaussian specimens for gigacycle tests. Fatigue specimens were designed without taking into account the hysteretic damping and its effects both on the stress distribution and on the heat power dissipation. However, in order to evaluate the temperature increment and the feasibility of ultrasonic fatigue tests with Gaussian specimens, the total dissipated heat power as well as the distribution of the dissipated heat power density along the specimen must be taken into account. The present paper proposes an analytical model validated through a finite element analysis, which permits to evaluate the effects of the hysteretic damping on the stress distribution, on the dissipated heat power density distribution and on the total dissipated heat power in Gaussian specimens. [Copyright &y& Elsevier]

ABSTRACT Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk-volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk-volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk-volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurements. [ABSTRACT FROM AUTHOR]

ABSTRACT In recent years, experimental tests exploring the gigacycle fatigue properties of materials suggest the introduction of modifications in well-known statistical fatigue life models. Usual fatigue life models, characterized by a single failure mechanism and by the presence of the fatigue limit, have been integrated by models that can take into account the occurrence of two failure mechanisms and do not consider the presence of the fatigue limit. The general case, in which more than two failure mechanisms coexist with the fatigue limit, has not been proposed yet. The paper presents a unified statistical model which can take into account any number of failure mechanisms and the possible presence of the fatigue limit. The case of S-N curves with different fatigue life distributions coexisting for the entire stress range covered by fatigue tests is also considered. The adaptability of the statistical model to the S-N curves proposed in the open literature is demonstrated by qualitative numerical examples. [ABSTRACT FROM AUTHOR]

Abstract: The point and line methods proposed by Taylor have been used to predict the fatigue strength of notched specimens with different stress ratios R by using literature experimental results. Predictions have been carried out by taking advantage of several equations describing the material fatigue strength dependence on the mean stress and the results obtained by using the Goodman and the Smith–Watson–Topper (SWT) equations are shown. Predicted and experimental fatigue strengths have been compared showing that the effectiveness of the point and line methods can be increased by introducing equivalent mean and alternating stresses and by introducing the dependence of the El Haddad parameter a 0 on the stress ratio R. Also with these corrections, the gap between predicted and experimental fatigue strengths for high R stress ratios still keeps being large. It is shown that the gap between predicted and experimental fatigue strengths for high R stress ratios can be reduced by introducing the experimentally evaluated mechanical properties of the material at different stress ratios R and/or by introducing the SWT fatigue strength approximating equation. [Copyright &y& Elsevier]

In recent years, Very-High-Cycle Fatigue (VHCF) behavior of metallic materials has become a major point of interest for researchers and industries. The needs of specific industrial fields (aerospace, mechanical and energy industry) for structural components with increasingly large fatigue lives, up to 10 10 cycles (gigacycle fatigue), requested for a more detailed investigation on the experimental properties of materials in the VHCF regime. Gigacycle fatigue tests are commonly performed using resonance fatigue testing machines with a loading frequency of 20 kHz (ultrasonic tests). Experimental results showed that failure is due to cracks which nucleate at the specimen surface if the stress amplitude is above the conventional fatigue limit (surface nucleation) and that failure is generally due to cracks which nucleate from inclusions or internal defects (internal nucleation) when specimens are subjected to stress amplitudes below the conventional fatigue limit. Following the experimental evidence, the Authors recently proposed a new probabilistic model for the complete description of S–N curves both in the High-Cycle Fatigue (HCF) and in the VHCF fatigue regions (duplex S–N curves). The model differentiates between the two failure modes (surface and internal nucleation), according to the estimated distribution of the random transition stress (corresponding to the conventional fatigue limit). No assumption is made about the statistical distribution of the number of cycles at which the transition between surface and internal nucleation occurs (i.e., the Transition Fatigue Life TFL). In the present paper, the TFL distribution is obtained. The resulting distribution depends on the distance between the HCF and the VHCF regions and on the distribution of the random transition stress. It is also shown that the statistical distribution of the fatigue strength at the median TFL (i.e., the Transition Fatigue Strength TFS) has median which corresponds to the mean transition stress. Finally, a procedure for computing Likelihood Ratio Confidence Intervals (LRCIs) for both the median TFL and the median TFS is given in the paper. The estimated TFL and TFS distributions can be effectively used for properly choosing the duration of HCF tests in terms of number of cycles and the stress amplitude below which VHCF failures more probably occur. LRCIs for the median TFL and TFS can be usefully computed for assessing uncertainty in the estimation procedure when a limited number of experimental data is available. A numerical example based on an experimental dataset taken from the literature is provided. [ABSTRACT FROM AUTHOR]