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• Innovative methodology to assess mechanical properties of brittle materials. • Adaptation of ultrasonic fatigue equipment to measure quasi-static tensile strength. • Elimination of mechanical fixtures and testing machine alignment requirements. • Experimental data to calibrate finite element model and obtain tensile properties. • Successful application of described methodology to graphite R4550. This paper proposes an innovative ultrasonic tensile test methodology for the assessment of quasi-static uniaxial mechanical properties and tensile strength of brittle materials. An ultrasonic testing machine, commonly employed for very high cycle fatigue tests, had its control and data acquisition systems adapted to induce specimen failure in about 100 cycles, avoiding cyclic load damage. The mechanical properties are thereafter estimated with a finite element model which simulates the experimental test. The proposed method has been validated by assessing the tensile strength of graphite R4550, characterized by brittle behaviour. The mechanical properties for graphite R4550 estimated with this experimental-numerical approach were found to be close to literature values obtained in different quasi-static testing configurations, confirming the effectiveness of the proposed method and the feasibility of employing the ultrasonic fatigue testing machine for studying the behaviour of brittle materials. Furthermore, the proposed methodology eliminated issues caused by mechanical fixtures and tensile testing machine alignment, while also allowing a considerable increase of the material loaded volume when compared to that of traditional test methods. The experimental and numerical approaches are successfully validated on graphite R4550, also highlighting its capability of characterizing the material nonlinear behaviour, including viscoelasticity and asymmetrical response in tension and compression. [Display omitted] [ABSTRACT FROM AUTHOR]

In the last few years, the use of adhesive joints for structural applications has rapidly increased and adhesives are more often subject to fatigue loads during their in‐service life. In presence of a rapidly varying load, such as a high‐frequency vibration, adhesively bonded joints may undergo fatigue lives in the Very High Cycle Fatigue (VHCF) region that are significantly larger than those investigated in usual high‐cycle fatigue tests. The present paper proposes an innovative testing technique for performing accelerated fully reversed tension‐compression VHCF tests on adhesive butt‐joints. The procedure for the design of the adherends is described and then experimentally validated. Ultrasonic VHCF tests are finally carried out on a cyanoacrylate butt‐joint up to 109 cycles: experimental results show that the proposed testing equipment permits an effective assessment of the VHCF response of the adhesive in a limited testing time. [ABSTRACT FROM AUTHOR]

• The mechanical behavior of AlSi10Mg lattice structure is characterized by quasi-static and impact tests. • Struts cross section is varied in Finite Element analysis to model the weakening influence of defects. • The influence of defect location within the structure is investigated. • The lower bound design curve is identified through statistical considerations. • A defect-free Finite Element model is identified for the safe design of lattice structures against crash. It is widely known that defects in lattice structures weaken their mechanical response, resulting in premature failure and affecting their energy absorption capability. In this work an experimental–numerical methodology for the identification of the lower bound design curve of lattice structures subjected to quasi-static compressive and impact tests is proposed and validated on an octet truss lattice structure made of an additively manufactured AlSi10Mg alloy. The defect size distribution is assessed through micro computed tomography and retained in a Finite Element Model (FEM) simulating the mechanical tests by a local reduction of the truss diameter. As the location of critical defects affects the mechanical response, simulations are repeated for different random distributions of the defects within the specimen. Results show that the FEM can correctly predict the mechanical behavior of the lattice structure, with the experimental curves comprised within the band of numerical curves. The lower bound design curve to use for the safe design of components can be finally identified through statistical considerations. [ABSTRACT FROM AUTHOR]

Experimental results have shown that high-strength steels can fail in the Very-High-Cycle Fatigue (VHCF) regime with cracks originating from internal defects. Material cleanliness thus plays a major role in the VHCF response of high-strength steels and refinement processes (eg, electroslag remelting [ESR] and vacuum arc remelting) could significantly enhance their performance. The present paper aims at investigating the effect of the ESR process on the VHCF behavior of an AISI H13 steel by carrying out fully reversed ultrasonic tension-compression tests on hourglass specimens manufactured with and without the ESR process. Size effect is also taken into account in the paper: the effectiveness in the prediction of the VHCF response of specimens with large risk-volumes from experimental data on specimens with small risk-volumes is discussed and experimentally validated. [ABSTRACT FROM AUTHOR]

Experimental tests investigating Very-High-Cycle Fatigue (VHCF) properties of materials are commonly performed with ultrasonic testing machines which allow for a significant reduction of testing time but induce a relevant temperature increment in specimens. In particular, due to the large volume of material (risk-volume) under test, Gaussian specimens, recently introduced for investigating size effects in VHCF, are extremely prone to heat dissipation and to the consequent temperature increment. They were originally designed by the Authors without considering the hysteretic damping and its effects on power dissipation: however, in order to evaluate the feasibility of ultrasonic fatigue tests with Gaussian specimens, hysteretic damping effects must be taken into account. The paper proposes an analytical model that permits to evaluate the effects of the hysteretic damping on the distribution of the power density and on the power dissipation in Gaussian specimens. The theoretical model is verified through Finite Element Models and experimentally validated. [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 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]

Abstract: The literature presents several analytical models and solutions for single- and double-lap bonded joints, whilst the joint between circular tubes is less common. For this geometry the pioneering model is that of Lubkin and Reissner, Transactions of The ASME 78 (1956) 1213–1221, in which the tubes are treated as cylindrical thin shells subjected to membrane and bending loading, whilst the adhesive transmits shear and peel stresses which are a function of the axial coordinate only. Such assumptions are consistent with those usually adopted for the flat joints. A former investigation has shown that the L–R model agrees with FE results for many geometries and gives far better results than other models appeared later in the literature. The aim of the present work is to obtain and present an explicit closed-form solution, not reported by Lubkin and Reissner, which is achieved by solving the governing equations by means of the Laplace transform. The correctness of the findings, assessed by the comparison with the tabular results of Lubkin and Reissner, and the features of this solution are commented. [Copyright &y& Elsevier]

Abstract: A design-by-reliability approach is ever more required and accurate design inputs are needed to meet reliability targets while reducing costs. Different models have been proposed in the literature to describe fatigue life variation with respect to the applied stress by assuming the applied stress as a deterministic independent variable and the fatigue life as a dependent random variable. In the paper also the applied stress is considered as a random variable with its own uncertainty and the procedure to evaluate the error on the estimation of parameters usually adopted in service reliability assessment of structural components is shown. Exact equations are proposed for some special cases and an illustrative example showing the reliability errors originated by applying the ASTM recommendations is given. [Copyright &y& Elsevier]

[Display omitted] • Fully reversed ultrasonic test for the VHCF response of carbon fiber woven laminate. • Strain gage calibration to exclude the presence of spurious modes. • Embedded optic fiber to measure the internal temperature increase. • Failures of hourglass specimens in the range of 106-109 cycles and 90–155 MPa. In the present work, the feasibility of axial ultrasonic tests for exploring the fully reversed fatigue response of composite materials even in the Very High Cycle Fatigue (VHCF) regime is proved. VHCF tests are run on hourglass specimens made of twill 2x2 carbon woven fabric impregnated with epoxy resin with stacking sequences [0] 8 and [0/90/+45/-45] s and designed through Finite Element (FE) modal analysis. The stress distribution within the specimen and the absence of buckling are first determined through an extensive strain gage campaign, which has validated the FE model. As the temperature is a main concern in ultrasonic tests, the temperature increment within the composite specimen is investigated by means of an embedded fiber optic sensor and controlled during the tests with an infrared sensor. With the proposed experimental setup, fully reversed ultrasonic tests have been carried out up to 109 cycles and the failure of the two investigated specimen types has been analyzed by comparing the failure origin location in relation to the stress distributions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Application of novel methodology to find tensile strength of Alumina 99.5%. • Finite element model optimization to calculate tensile properties. • Micro-CT scanning for characterizing internal flaws and identifying the critical defect. • Stress intensity factors formulation for correlating tensile strength with critical defect size. The ultrasonic tensile test method recently developed by the Authors is applied with the micro-computed tomography technique aiming to correlate the failure-inducing nominal stress in alumina 99.5% with the manufacturing defect size. After characterizing the defect population, the critical defect size of each specimen is defined, and an empirical formulation for the stress intensity factors on this material is obtained. The ultrasonic tensile test, with a loading frequency of 20 kHz, is simulated with finite element analyses, to estimate with an optimization process the material properties that reproduce the experimental displacements. Evidence of negligible fatigue damage is verified to further support the validity of the ultrasonic tensile test as a method capable of providing a good estimate for the quasi-static tensile strength. The analysed alumina resulted in a density of 3969 kg/m3 and an elastic modulus of 371.2 GPa. All specimens failed in 100 cycles or less, with tensile strength values ranging from 79.5 MPa to 322.6 MPa. The reason for this large experimental variability is attributed to the dispersion of imperfection sizes, with critical sizes ranging from 92 μm to 3443 μm, proving the importance of combining the information on the defect population with the assessed tensile strength. [ABSTRACT FROM AUTHOR]

• Additive Manufactured AlSi10Mg fatigue response estimated with Machine Learning. • A Physics Informed Neural Network (PINN) inspired to Murakami's model is proposed. • PINN performs better than simple Feed Forward Neural Networks (FFNN) • SLM AlSi10Mg fatigue response is estimated from process parameters with PINN. The fatigue response of Additive Manufacturing (AM) components is driven by manufacturing defects - whose size mainly depends on process parameters - and by the resulting microstructure - mainly affected by heat treatments and process parameters. In the paper, Machine Learning (ML) algorithms are applied to estimate the fatigue response from AM process parameters and heat treatment properties. Feed-forward neural networks (FFNN) and physics-informed neural network (PINN) algorithms are designed and validated on literature datasets of AM AlSi10Mg alloy, proving the effectiveness of physics-based ML approaches in predicting the fatigue response of AM parts. Leveraging PINN interpretability, the authors analyse the relationship between process parameters and fatigue response. [ABSTRACT FROM AUTHOR]

• High-cycle and VHCF behavior of AMed AlSi10Mg with different printing powder sizes investigated. • Specimens with 20 μm powder size has higher fatigue property than with 50 μm powder size. • Unmelted powders were detected as the main source of crack initiation. • Five different types of crack initiation modes were identified. The high-cycle and very-high-cycle fatigue (VHCF) behavior of AlSi10Mg specimens manufactured by selective laser melting with different printing powder sizes was investigated. The specimens with 20 μm powder size contained more defects than those with 50 μm powder size, and the latter have a higher fatigue property than the former. Unmelted powders were detected as the main source of crack initiation. The specimens printed with 50 μm powder size are more likely to feature subsurface crack initiation in the VHCF regime. Five different types of crack initiation modes were identified, and the fatigue strength was assessed by statistical model. [ABSTRACT FROM AUTHOR]

The paper proposes a new model for the assessment of size effects affecting the fracture strength of brittle materials. The proposed model permits to accurately estimate the relation between the specimen strength, the initial defect size and to take into account the strength variation with respect to the tested volume. The proposed methodology is analytically defined and thereafter validated with the literature data obtained through tests on different types of brittle materials, and on specimens with increasing volume. A simple procedure for parameter estimation is also defined in the paper. The literature validation proves the effectiveness of the proposed methodology, with the resulting fitting models in well agreement with the experimental dataset and characterized by high values of coefficients of correlation, similar or larger than those obtained in the literature with different approaches. Unlabelled Image • New size effect model for fracture strength of brittle materials • Estimation of fracture strength considering specimen volume variation • Statistical model based on the Largest Extreme Value Distribution • Validation through uniaxial static tests data of brittle materials from literature [ABSTRACT FROM AUTHOR]

• VHCF strength decreases with building orientation. • Fatigue crack origin is always from an internal defect. • Rough area and fisheye have been observed on fracture surfaces. The effect of building orientation on the very-high-cycle fatigue (VHCF) response of Ti-6Al-4V specimens produced through selective laser melting (SLM) process with three different building orientations (0°, 45° and 90°) has been experimentally assessed. The fatigue performance decreases with different building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue and VHCF regime independent of building orientations. Size of defects induced fatigue failures and the stress intensity factor range decrease with the number of cycles to failure. By considering the VHCF strength at 109 cycles, the median value decreases from 217 MPa (0°) to 201 MPa (45°) and finally to 155 MPa (90°), with a 40% reduction from 0° to 90°. The building orientation significantly influences both the defect size and the resulting VHCF response. [ABSTRACT FROM AUTHOR]

• Ultrasonic VHCF tests were successfully conducted on AlSi10 SLM Gaussian specimens. • Annealing for two hours at 320 °C is detrimental for the VHCF response. • The evolution of Si particles during the heat treatment affects the VHCF response. • The performed tests extend the knowledge about the VHCF response of AlSi10Mg SLM parts. The present paper investigates the effect of a stress relief heat treatment (two hours at 320 °C and air cooling down) on the VHCF response of an AlSi10Mg alloy produced through SLM. Ultrasonic tension-compression tests up to 10 9 cycles were carried out on as-built and heat treated Gaussian specimens with a large loaded volume. Experimental results showed that the investigated heat treatment has a detrimental effect on the VHCF response: VHCF strength reduction is larger than 20% at number of cycles above 10 8. The microstructural changes induced by the heat treatment (i.e., rupture and spheroidization of the original eutectic silicon network) was shown to be the main reason for the VHCF decrement, thus suggesting that an optimization of the heat treatment parameters is required. [ABSTRACT FROM AUTHOR]

• P-S-N design curves in presence of defects are estimated. • The Likelihood Ratio Confidence Bound is used for P-S-N design curves. • The bootstrap approach is exploited for estimating the P-S-N design curves. • The developed methods have been validated on literature data. • The proposed methods are effective also for duplex P-S-N design curves. 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. [ABSTRACT FROM AUTHOR]

• Defect-driven topology optimization for fatigue design of AM components. • Defect size distribution accounted to limit the maximum principal stress. • Iterative procedure as the defect distribution depends on the final volume. • Additional 15% of mass saving by applying the proposed formulation to an aerospace component. In this paper, a generalized formulation of defect-driven topology optimization (TO) for fatigue design, named TopFat , is proposed, where the first principal stress, that causes the crack propagation, is limited in accordance with the defect size distribution of the additive manufacturing (AM) process. As the largest defect depends on the final volume of the component, which changes according to the topology, an iterative procedure is adopted to avoid volume constraints in the optimization problem. The procedure is applied to an actual aerospace bracket component and its strength and limitations are discussed in comparison with the TO formulations presented in the literature. Results show that considering the defect population significantly affects the final topology, while leading to a feasible optimum. Further, even though the computational time increases, an additional 15% of mass saving is achieved by adopting the proposed iterative procedure with respect to a volume constraint-based formulation of TopFat. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three methods for the design P-S-N curves have been compared. • The Likelihood Ratio Confidence Intervals are used for the design P-S-N curves. • A method based on the bootstrap approach is used for the design P-S-N curves. • The influence of the runout numerousness is investigated in the paper. • The number of runout data can be reduced with the proper testing strategy. In the present work, three methodologies for the estimation of the fatigue design P-S-N curves are compared. The approximate Owen with the staircase method, a method based on the likelihood ratio confidence interval, and an original method based on the bootstrap approach are considered. Three experimental datasets are employed for the experimental validation, showing that the three methods permit to have a reliable estimation of the design curves and that the number of data and, accordingly, the test duration can be significantly reduced with the proper testing strategy. [ABSTRACT FROM AUTHOR]

• Size-effect on the VHCF response of SLM AlSi10Mg has been investigated. • AlSi10Mg specimens with increasing risk-volume have been tested. • Size-effect strongly affects the VHCF response of SLM AlSi10Mg specimens. • Size-effect is relevant for horizontal and vertical building orientation. • Size-effect is larger for AM parts than for traditionally built parts. This paper experimentally investigates the effect of the specimen volume (size-effect) on the Very-High-Cycle-Fatigue (VHCF) response of SLM AlSi10Mg specimens. Tension-compression tests are carried out on hourglass and Gaussian specimens with different risk-volumes produced in horizontal and vertical direction. Experimental results show that size-effects significantly influence the VHCF response regardless of the building orientation. Furthermore, size-effects are more evident for SLM specimens than for traditionally built specimens, pointing out that the design of SLM components cannot rely on the results obtained by testing traditionally built specimens. [ABSTRACT FROM AUTHOR]

• Size-effect strongly affects the VHCF of materials. • A general rule for size-effect in VHCF could not be found in this research. • Size-effect depends on the material, manufacturing process and material cleanliness. • The ODA/FGA size must be taken into account when components are designed. • Tests on specimens with large risk-volumes are recommended. 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. [ABSTRACT FROM AUTHOR]

• Microstructure can have a prevalent effect on the VHCF response than defect size. • Influence of residual stresses on the VHCF response increases with the defect size. • Interactions between microstructure, defects and residual stresses are detrimental. • Building orientation affects the VHCF response of AlSi10Mg specimens. • VHCF strength of horizontal specimens is larger than that of vertical specimens. It is well-known in the literature that the fatigue response of specimens produced through Additive Manufacturing (AM) processes is mainly controlled by the defect population. Nevertheless, microstructure, residual stresses and building orientation must be considered as they may also play a significant role. In the present paper, the influence of microstructure, residual stresses and building orientation on the Very-High-Cycle-Fatigue (VHCF) response of SLM AlSi10Mg specimens has been experimentally assessed. Ultrasonic tests on AlSI10Mg specimens with different building orientations and subjected to different heat treatments showed that, together with the defect size, microstructure and residual stresses significantly affect the VHCF response. [ABSTRACT FROM AUTHOR]