Blunt notch effect on the fatigue response up to 10^9 cycles of selective laser melting Ti6Al4V specimens

In this paper, the influence of a blunt notch on the VHCF response of SLM Ti6Al4V specimens is investigated. Ultrasonic fully reversed tension-compression tests up to 10(9) cycles were carried out on unnotched specimens and specimens with a blunt notch. Unnotched specimens show a slightly larger fatigue response, with limited differences. All fatigue failures originated from defects, which are bigger in unnotched specimens, mainly due to the different risk volume of the tested specimens and the related size effect. Interactions between notch, stress gradient, and defect size distribution must be considered to properly assess the influence of notch on the VHCF response

Statistical estimation of fatigue design curves from datasets involving failures from defects

In the present paper, two methodologies for the estimation of the design curves of datasets with failures originating from defects are proposed. With the first methodology, the Likelihood Ratio Confidence Bound of a specific quantile P-S-N curve is considered. The second method is based on the bootstrap approach, with a large number of datasets simulated starting from the stress life and the defect size distributions estimated from the experimental data. The two approaches have been validated on literature datasets covering also the Very High Cycle Fatigue (VHCF) life region, proving their effectiveness

Statistical models for estimating the fatigue life, the stress–life relation, and the P-S–N curves of metallic materials in Very High Cycle Fatigue: A review

The research on the Very High Cycle Fatigue (VHCF) response of materials is fundamental to guarantee a safe design of structural components. Researchers develop models for the fatigue life in VHCF, aiming at assessing the stress–life relation and, accordingly, the probabilistic S–N (P-S–N) curves. In the paper, the models for the stress–life relation in VHCF are comprehensively reviewed. The models are classified according to the approach followed for defining the stress–life dependency, that is, power law, probabilistic, fracture mechanics, or Paris law-based approach. The number of failure modes that can be modeled, the statistical distribution for the fatigue life, and the characteristics of the estimated P-S–N curves are also reviewed by analyzing the fitting capability of experimental datasets for each model. This review is supposed to highlight the strengths and weaknesses of the currently available models and guide the future research

Surrogate modeling in the design optimization of structures with discontinuous responses with respect to the design variables - A new approach for crashworthiness design

Advances to computational technology have resulted in the reduction of computational effort for crashworthiness analysis, hence enabling structural design optimization. Surrogate modeling has been shown to further reduce computational effort as well as to smooth noisy responses. Crashworthiness optimization problems are, though, ill posed as they include nonlinear, noncontinuous and noisy responses. This violates the Hadamard conditions for well-posed problems and therefore the applicability of gradient-based algorithms is limited. Here, discontinuities in the responses with respect to the design variables will be handled that result in large changes in the system functions with only small changes in the design variables using a novel surrogate modeling technique. The applicability of typical global surrogate models is limited when critical discontinuities are present. An efficient method has been developed here to identify the number of discontinuities and their position in the design domain. Previous works assume a said number of discontinuities; here though, the number of discontinuities is not given a priori. The discontinuities are identified by examining the relative difference in the response value of samples in immediate proximity of each other. Samples in the same continuous subdomain are clustered and a support vector machine for classification is exploited to locate discontinuities. Local approximations are then used for the continuous subspaces between the discontinuities. Lastly, a surrogate-based design optimization is carried out. Starting with a two-bar truss, demonstrating a snap-through discontinuity, this method is shown to account for such discontinuities. This is then integrated into an optimization framework. Further, a crash-absorbing tube is optimized that is impacted with an angle resulting in a noncontinuous design space: desired axial crushing and undesirable global buckling. After summarizing the results, advantages and possible limitations are discussed

Size-effect in Very High Cycle Fatigue: A review

It is well-known that fatigue failures in VHCF originate from defects present within the risk-volume. Moreover, experimental results have shown that the larger the specimen size and the risk-volume, the shorter is the VHCF life. This is known as size-effect and is generally due to the statistical increment of the defect size with the material volume. In the paper, size-effect in VHCF is critically revised. The experimental results obtained by testing specimens with different sizes are analyzed and the methodologies proposed to model size-effect are discussed, with the aim of guiding the future research on size-effect in VHCF

Duplex LCF-VHCF P-S-N design curves: a methodology based on the Maximum Likelihood Principle

Probabilistic-S-N (P-S-N) curves are commonly employed to model the fatigue response of specimens and components and are generally obtained by assuming the statistical distribution of the fatigue life. In industrial applications, starting from the P-S-N curves, the so-called “design curves” or “lower bound S-N curves” are defined for the design of components, in order to ensure a safety margin against fatigue failures. Due to the growing interest in the Very High Cycle Fatigue (VHCF) response of materials, methodologies for the estimation of the design curves should not be limited to the Low Cycle Fatigue (LCF)-High Cycle Fatigue (HCF) life range but should also cover the VHCF region. In this work, the Maximum Likelihood Principle is exploited for the assessment of the design curves of datasets obtained through tests in the LCF-VHCF range, with the P-S-N curves showing a duplex trend. First, the model for the P-S-N curves with duplex trend is defined. Then the proposed methodology for the design curves is described and finally validated with literature datasets

Size-Effects on the VHCF Response of Flat Metallic Specimens for Automotive Applications: Analysis of Fatigue Data with a Method Based on the Stress Gradient

The design against fatigue failures at very high number of cycles (VHCF) is fundamental to guarantee the integrity of components used in structural applications (aerospace, energy production, automotive). Experimental tests to assess the VHCF response of materials are generally carried out on small specimens with sizes that are significantly different from those of the components to be designed. Size-effect, which is widely known to affect the VHCF response, must therefore be properly modeled and accounted when components are designed to ensure their structural integrity. Size-effects in VHCF have been generally investigated by testing specimens with circular cross-sections and modeled by considering the probabilistic increment of the defect size with the loaded volume. In the present paper, ultrasonic fatigue tests have been carried out on hourglass flat specimens and larger dog-bone flat specimens to investigate size-effects. One aluminum alloy and four steels used for automotive applications have been tested. The experimental results have been analyzed with an innovative statistical model based on the weakest-link principle and on the stress gradient within the specimens, which does not require the size of the defect at the origin of the fatigue failure, generally not available if the specimen fails from the surface. Size-effects were found to significantly influence the VHCF response of the investigated materials, further confirming that it is strongly material dependent, even for flat specimens. A general rule for size-effect in VHCF of flat specimens was not found, proving that it must be properly experimentally verified and safely accounted when designing large components against VHCF failure

Blunt notch effect on the fatigue response up to 109 cycles of selective laser melting Ti6Al4V specimens

In this paper, the influence of a blunt notch on the VHCF response of SLM Ti6Al4V specimens is investigated. Ultrasonic fully reversed tension–compression tests up to 109 cycles were carried out on unnotched specimens and specimens with a blunt notch. Unnotched specimens show a slightly larger fatigue response, with limited differences. All fatigue failures originated from defects, which are bigger in unnotched specimens, mainly due to the different risk volume of the tested specimens and the related size effect. Interactions between notch, stress gradient, and defect size distribution must be considered to properly assess the influence of notch on the VHCF response