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The paper aims at assessing the influence of the Size-Effect (SE) parameter on the VHCF response of the AISI H13 steel. Ultrasonic tests were performed on Gaussian and hourglass specimens characterized by different risk-volumes (volume of material subjected to an almost uniform stress amplitude). Experimental results showed that a significant difference exists between the VHCF strengths of the investigated material obtained by using specimens with different risk-volumes. [ABSTRACT FROM AUTHOR]

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 1010 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 statistical model for the complete description of SN 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). In the present paper, the statistical distribution of the transition fatigue life is obtained, according to the statistical model proposed. The resulting distribution depends on the distance between the HCF and the VHCF regions and on the distribution of the random transition stress. The estimated distribution can be effectively used to predict, with a specified confidence level, the number of cycles for which an internal nucleation may probabilistically occur in a VHCF test and it is also informative for properly choosing the end of HCF tests in terms of number of cycles. A numerical example, based on experimental datasets taken from the literature, is provided. [ABSTRACT FROM AUTHOR]

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]

The paper presents a topology optimisation method based on optimality criteria for total potential energy maximisation with a volume constraint. The final volume of the optimal structural configuration has not to be specified a priori and is directly controlled by the stress, displacement or stiffness constraints defined at the design problem layout phase. The proposed method leads to the identification of well defined structures characterised by a small number of discrete elements with intermediate material properties within a limited number of iterations. The results obtained by solving several two dimensional benchmark problems are shown. [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]