Being a multifaceted phenomenon, hydrogen embrittlement can be influenced by the kinetics of hydrogen diffusion and its distribution within components, making diffusion modeling a valuable tool for understanding and managing embrittlement risk. The presence of hydrogen trapping complicates diffusion and therefore the interpretation of diffusion-related experiments such as permeation tests. Accordingly, numerous methods have been proposed to estimate trapping parameters from permeation data. Here, a general estimation formula is presented based on the work of McNabb and Foster. When applied to published permeation transient datasets, the resulting trapping parameters improve the curve reconstruction, reducing the modeling error from 11.8% to 1.7% in the best case and from 4.65% to 4.54% in the worst case. In addition, the formulation is generalized to include multiple trap types.

In this work, we propose a Multi-task Bayesian Optimization framework for identifying constitutive parameters of material models whose calibration typically requires significant computational effort. While the objective function is formally expressed as a residual between experimental and simulated curves of the specimen, we allow a Multi-task Gaussian Process to directly learn the entire force-displacement response. This richer representation captures more structural information than the scalar objective alone, improving predictive accuracy and efficiency. The surrogate is used iteratively to infer optimal parameter values and to adaptively select new simulation points, balancing exploration of the parameter space with exploitation of current optima. A key advantage of the approach is that the trained surrogate can be reused for new specimens, progressively reducing the number of simulations required for parameter identification. The methodology is demonstrated through a case study on the Gurson-Tvergaard-Needleman damage model, applied to MS1500 steel specimens exposed to hydrogen charging at different concentrations. To ensure that the Multi-task Gaussian Process effectively represents the simulated curves, the force-displacement data were pre-processed to exclude regions minimally influenced by hydrogen for the concentrations investigated, such as the elastic regime. The proposed approach demonstrates its effectiveness by allowing the identification of parameter values through a limited number of iterations. The results confirm the suitability of the GTN model for describing hydrogen embrittlement and suggest that the proposed methodology can be potentially extended not only to different materials, but also to other constitutive models whose parameter identification entails significant computational cost.

The study investigates the evolution of elastic behavior in lattice structures subjected to cyclical loading after pre-straining to various levels of plastic deformation. Triply Periodic Minimal Surface (TPMS) gyroid lattice specimens were fabricated using the Laser Powder Bed Fusion (L-PBF) technique and subjected to controlled steps of compressive pre-straining, inducing plastic deformations. Subsequently, the specimens underwent cyclic loading-unloading tests to characterize their elastic behavior. Stress-strain curves were monitored throughout the testing to determine the apparent elastic modulus (E*) at each cycle. The results demonstrate that E* of pre-strained lattices are not static. The initial cycles after pre-straining exhibit a change in stiffness, with the E* initially increasing depending on the pre-strain level. This behavior is attributed to the morphology of the lattice itself, which is more sensible to local hardening due to an evident bending-dominated mechanical response. Over slight plastic strains, the elastic modulus stabilizes, reaching a new stiffening-to-plastic strain evolution. The magnitude of this shift and the experimental response’s dispersion are found to not be dependent on the pre-strain level.

This study employs laser interferometry to investigate the effects of key L-PBF parameters on the material dynamic response. The microstructural characteristics of AMed materials are directly influenced by process -induced thermal gradients, which impact solidification behavior and defect formation. High frequency laser interferometry enables precise characterization of the material's dynamic properties. By correlating these properties with processing conditions, we establish quantitative relationships between energy input, melt pool dynamics, and the resultant material structure. Additionally, laser -based dynamic modulus measurements offer a non-destructive approach to evaluating elastic properties and mechanical anisotropy, which are crucial for performance predictions. By integrating laser interferometry with conventional characterization techniques (microstructural investigations, tensile tests and fractographic analysis), we demonstrate a robust methodology for assessing the intrinsic properties of AM materials.

In the field of energy production, creep–fatigue interaction is a typical failure mode that might compromise the structural integrity of both rotating equipment and pressure vessels. Common design practices approach the problem in a conservative way by using high safety factors, which typically results in additional costs for manufacturing companies. The aim of this article, in the framework of continuum damage mechanics approaches, is to present a novel fatigue damage-based constitutive law. The presented law is directly inspired by well-assessed creep-based rules, suggesting a similarity in the behavior. On the other hand, creep deformation and damage are calculated with a more recent approach. The identification of the model parameters was carried out by interpreting experimental results obtained from low-cycle fatigue and creep relaxation tests performed on a commonly used ferritic–martensitic steel for power generation rotor forgings. To validate the proposed models, they were used to estimate material life consumption when the material was subjected to fully reversed axial loading conditions with hold time under tensile load. Different loading conditions at different total strain ranges and hold times were simulated, and good agreement was found between the predicted and experimental life, thus confirming the validity of the proposed models.

The elevated temperature fatigue behavior and notch sensitivity of the Inconel 718 produced via Laser-Powder Bed Fusion (L-PBF) were investigated. A smooth specimen and three geometries of 90° V-notched specimens were employed. The material was printed by employing a set of process parameters aimed at producing a full-dense material and was post-processed adopting a conventional solution and aging heat treatment. Three V-notch geometries were defined through FEM analyses to obtain theoretical stress concentration factors ranging from 1.5 to 3.5. Isothermal HCF tests were carried out at a temperature of 650°C and a load ratio R of 0.1. Fractographic investigations pointed out the features of the crack onset site for the different geometries. Metallographic analyses were employed to assess the effective notch geometry produced by the L-PBF process, as well as the material microstructure.

The aim of this study is to model the impact of surface roughness and pores on the fatigue strength of plain and V-notched specimens made of Inconel 718 under as-built and machined conditions and produced by laser powder bed fusion (LPBF). Combining fractographic analyses with the Gumbel and the exponential distribution functions, the statistical analyses of the diameters of the pores and of their distances from the external surfaces were implemented. Surface roughness scans were performed with the optical profilometer. The finite element (FE) method was used to simulate a sample of pores generated by the identified probability distributions and the surface profiles obtained with the scans. The theory of critical distances (TCD) was implemented combining the blunt and sharp V-notched specimens in the machined condition, and it was combined with the Gumbel or the generalized extreme values distributions to calculate the fatigue strength concentration factors provided by the pores and the surface roughness at 99% of probability. Finally, the proposed model was used to predict the fatigue strength of the blunt V-notched specimens in the as-built conditions and of the plain specimens in the as-built and machined conditions resulting appreciably similar to the experimental data.

In this research, we investigate the dynamic behavior of Inconel 718 fabricated through laser powder bed fusion (L-PBF), addressing a notable knowledge gap regarding the correlation between process parameters and dynamic properties. The process parameters adopted are deducted from an extension of the Rosenthal solution, formulated to increase the process productivity while avoiding the typical production process defects. The dynamic Young modulus and the structural damping of the material are estimated as a function of the process parameters through ping tests reproducing the flexural vibrations of the specimens in as-built, solutioned, and aged conditions. The microstructure and porosity are investigated through metallographic analyses. The results show a substantial influence of the L-PBF process parameters on the dynamic Young modulus, which markedly increases as the energy density is reduced (23%) and progressively becomes more similar to the conventionally produced material. This influence stands in stark contrast to the relatively modest impact of heat treatments, which underlines a negligible effect of the process-induced residual stress. The structural damping remained approximately constant across all test conditions. The elastic response of the material is found to be primarily influenced by the different microstructures produced as the L-PBF process parameters varied, particularly in terms of the dimensions and shape of the solidification structures. The unexpected relationship between the dynamic Young modulus, energy density, and microstructure unveils the potential to fine-tune the material's dynamic behavior by manipulating the process parameters, thereby carrying substantial implications for all the applications of additively manufactured components susceptible to significant vibratory phenomena.

The aim of this study was to investigate the impact of surface roughness and size effect on the uniaxial fatigue strength of aged specimens made of Inconel 718, obtained by laser-powder bed fusion (L-PBF) both under as-built and machined surface conditions. The surface profiles were scanned with a 3D optical profilometer, and then modelled and filtered by the fast Fourier transform. This latter allowed to obtain the relevant parameters to calculate the term area in the Murakami model to account for the effect of surface roughness on fatigue strength prediction. Two additional terms were added to the standard Murakami formula: one to account for the size effect and the other to consider the nonzero value of the average stress. A cylindrical plain and a miniaturized plain specimens, in the as-built condition, were employed to calibrate the proposed model, while a machined plain cylindrical and a miniaturized notched specimens were used as validators with low prediction errors.

Gas turbines play a critical role in industries such as power generation and aviation. Additive manufacturing has emerged as a game-changing technology for gas turbine components, offering superior design flexibility and performance enhancements. The present work provides an overview of a multistep approach for integrating lattice structures into a specific gas turbine component, the Nozzle Guide Vane (NGV), using additive manufacturing technology. The first step involves a comprehensive assessment of lattice structures’ influence on the mechanical and thermal properties of the exposed part of NGV. Through computational simulations and experiments, an ideal lattice geometry is determined, optimizing structural integrity and heat transfer properties while minimizing volume usage. The second step sets the baseline performances of the current NGV system components, which were investigated and selected for additive manufacturing analysis. The third step focuses on the overall effect of additive manufacturing capabilities in the NGV system. The fourth and final step optimizes the additive manufacturing process for fabricating gas turbine components with lattice structures. Laser Powder Bed Fusion (L-PBF) technology, united with advanced Topological Optimization analyses, and high-temperature alloys were selected to withstand the demanding gas turbine operating conditions. This multistep approach represents a significant step forward in gas turbine technology, capitalizing the advanced mechanical applications as lattice designs and additive manufacturing, aiming in enhanced performance, reduced weight, and improved efficiency. These developments hold the potential to achieve more sustainable and cost-effective energy generation and transportation systems.

The detailed prediction of the actual life expenditure of expanders operating in power plants has become a common topic of investigation. In the current energy market axial expanders are subjected to frequent load change and to the exposure to higher temperatures. The proper prediction of their life expenditure determines operability and availability improvements through faster start-ups and longer maintenance intervals. A common strategy to achieve this objective has led in the last years towards the development of progressively more and more detailed “service-like” tests aimed at recreating the actual loading conditions in terms of strain and temperature profiles. Recently the need to face the challenge of energy transition through the introduction of advanced cycle employing supercritical carbon dioxide is requiring to further improve this kind of tests by including the effect of environmental attack through corrosion and carburization. This work addresses this topic by presenting the development of customized specimens and of a testing campaign able to evaluate the degradation of creep and fatigue capabilities when high temperature carbon dioxide exposure occurs. After a short introduction on the state of the art, the specimen design will be detailed with FE simulation evaluating stress and temperature distribution in specimens and key insights will be given regarding the design of a proper test matrix and the essential theoretical framework for the interpretation of the results of a testing campaign.

Hydrogen embrittlement (HE) is among the limiting factors for the employment of advanced high-strength steels in the automotive industry. One of the most relevant manifestations of HE for those components is delayed fracture. To perform HE risk assessment against delayed fracture in structural components wherein hydrogen accumulates near notches or bends after manufacturing or assembly, continuum models can be used to predict hydrogen diffusion and accumulation. To this aim, it is crucial to identify both the correct model parameters and boundary conditions (BCs). This study provides the required modelling and experimental framework to estimate the constitutive diffusion, trapping and outgassing parameters for a commercial martensitic advanced high-strength steel grade 1300. Permeation testing and simple room outgassing tests showed that a natural BC with one-parameter effectively reproduces the experimental results. The proposed model can be used to predict a reasonable hydrogen distribution, both inside of the component and near to the surface, which is valuable for HE risk assessments. Finally, the same BC can be employed with the interpretation of ex-situ hydrogen charged tensile tests.

Thanks to the increasingly widespread additive manufacturing technology and promising properties, the use of Lattice Structures (LS) is becoming increasingly frequent. LS allows the components to be designed with tunable stiffness, which can unlock the control of natural frequencies. However, crucial challenges must be faced to integrate LS into the typical design process. In the present work, an experimental and numerical study of LS-enabled tuning of natural frequencies in mechanical components is proposed. In a first step, the difficulties arising with the large amount of FEM nodes, that are required to predict LS complex shapes in detail, are overcome by modeling LS with an elastic metamaterial whose stiffness properties are determined through ad hoc finite element analyses. After that, a simplified investigation can be conducted on the modal properties of components with fixed external shape and variable internal LS filling, based on Triply Periodic Minimal Surfaces (TPMS) lattices. In those conditions, the parameters of the LS core can be tuned to control and optimize the global modal frequencies of the entire geometry. In addition, the admissible range of frequencies can be estimated. Optimized plates results are validated through an experimental test campaign on additively manufactured specimens made with Selective Laser Melting (SLM) technology. The samples are hammer-tested with various boundary conditions while laser sensors measure the oscillation data of selected points. Finally, estimated and identified natural frequencies were compared. The described model is suitable to be implemented in an automated tool for designers.

The study of materials for space exploration is one of the most interesting targets of international space agencies. An essential tool for realizing light junctions is epoxy adhesive (EA), which provides an elastic and robust material with a complex mesh of polymeric chains and crosslinks. In this work, a study of the structural and chemical modification of a commercial two-part flexible EA (3M™ Scotch-Weld™ EC-2216 B/A Gray), induced by 60Co gamma radiation, is presented. Combining different spectroscopic techniques, such as the spectroscopic Fourier transform infrared spectroscopy (FTIR), the THz time-domain spectroscopy (TDS), and the electron paramagnetic resonance (EPR), a characterization of the EA response in different regions of the electromagnetic spectrum is performed, providing valuable information about the structural and chemical properties of the polymers before and after irradiation. A simultaneous dissociation of polymeric chain and crosslinking formation is observed.The polymer is not subject to structural modification at an absorbed dose of 10 kGy, in which only transient free radicals are observed. Differently, between 100 and 500 kGy, a gradual chemical degradation of the samples is observed together with a broad and long-living EPR signal appearance. This study also provides a microscopic characterization of the material useful for the mechanism evaluation of system degradation.

The paper deals with the effects of electrochemical and electro-discharge deep hole drilling on the high- temperature fatigue behavior of a directionally solidified GTD-111 superalloy. High-temperature HCF tests were carried out at 800 degrees C on specimens having a coaxial hole drilled with the investigated processes. The hole diameter and length were 1.5 and 95 mm, respectively. The material's high-temperature HCF S-N curve was also obtained to provide a reference curve. Despite the scatter caused by the material microstructure in the fatigue crack nucleation and propagation regime, the fatigue strength was found to be affected by the investigated hole drilling techniques. The electro-discharge deep hole drilling was found to lower the fatigue strength by 8%. Fractographic analyses showed that it can be attributed to the different hole surface roughness and micro- structural alteration in the underlying material.

This paper proposes a definition of a Hydrogen Embrittlement Index for assessing structural components made by a martensitic advanced high-strength steel. A material model, including a damage model and a fracture criterion, was developed to reproduce the tensile behaviour until fracture nucleation under different hydrogen concentrations. The analysis of the tests with different specimen geometries demonstrated that the maximum principal strain at fracture is correlated to hydrogen concentration, and it can be used to define the embrittlement index for quantifying the hydrogen susceptibility of material in a wide range of notch severity.

The enhancement of the operational flexibility of axial expanders can be obtained through the implementation of Nozzle Guide Vane (NGV). This design architecture includes moving parts typically sustained by ball joints, whose wear behavior might be important to define the maintenance interval of the expander and define its operational availability. This is especially true when considering challenging loading and temperature conditions. Usually, tribological tests are performed by pin-on disk or similar test methods. However, these techniques neglect important factors as actual sliding zone and contact morphology, which can be fundamental for ball joints. This work deals with the design of an innovative experimental test procedure for the characterization of the tribological behavior of ball joints considering the actual operating conditions of a NGV at 500°C. The core of the methodology is a customized test bench specifically designed to investigate actual commercial ball joint in field conditions. Details on the design phase and on the commissioning of the experimental set-up are given. The results of the experimental campaign performed on this novel test bench are presented: the chosen test matrix explores a wide spectrum of load and sliding speed. Specimen pre-load and working velocity have been scaled-up to achieve viable testing duration. The test bench was employed on three different kinds of commercial ball joints for high-temperature applications in absence of lubrication. Measurement of diametral clearance and friction in worn conditions were also performed. The results discussion allowed to choose the most suitable solution for a specific design targeting 50000 working hours.

Hydrogen diffusivity is strongly affected by the presence of traps, which are defects of the metal matrix that offer a location for hydrogen atoms with a lower energy state than regular interstitial lattice sites. The study of hydrogen diffusion is relevant for multiple different reasons, according to the specific application that is considered. In the context of enamelling grade steels, hydrogen is the cause of the defect known as fishscaling, or the detachment of enamel pieces from the metal surface. Study of hydrogen trapping and diffusion is part of the process of understanding fishscaling, since for many materials, a strong link was found between high diffusivity and fishscaling susceptibility. This study is performed with the use of hydrogen permeation tests, but the interpretation of permeation data is not straightforward because of the trapping effect. In this paper, temperature programmed desorption tests were performed on enamelling grade interstitial free steels to provide an initial understanding of the type of traps expected. Then hydrogen permeation tests were performed to further characterize the trapping parameters of the materials. Common methods to estimate the apparent diffusivity were compared, and the experimental curves were reconstructed using McNabb and Foster's trapping and diffusion model. Finally, a simple method is proposed that may lead to simple empiric formulae to estimate trapping parameters from permeation tests for specific families of materials, possibly overcoming the issues inherent in employing the common and simplified apparent diffusivity approach.

This work examines the effects of gamma-irradiation (up to 500 kGy) on the mechanical behavior of aluminum in situ specimens bonded with 3M (TM) Scotch-Weld (TM) EC-2216 B/A Gray, a commercial two-part epoxy adhesive. Shear tests were performed, from which failure data and incipient axial stiffness were obtained. The results were compared to those of dedicated finite element models that had been tuned with the previously obtained bulk cohesive properties of the adhesive. Analytic models to evaluate stress distribution in the adhesive and joints axial stiffness were applied to validate the developed models and to explain the relatively small stiffness variance (observed both in experimental and numerical analyses results) compared to that of the adhesive Young's modulus. On the whole, the adhesive showed a good tolerance to radiation in the interval studied, as already observed for bulk specimens. Finite element models were found to be accurate in predicting of the stiffness for the entire deformative field, while they overestimated the failure loads and displacements also applying unconservative failure criteria. This is very likely due to the significant number of defects found on the fracture surfaces of bulk specimens.