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The peak stress method (PSM) allows a rapid application of the notch stress intensity factor (NSIF) approach to the fatigue life assessment of welded structures, by employing the linear elastic peak stresses evaluated by FE analyses with coarse meshes. Because of the widespread adoption of 3D modeling of large and complex structures in the industry, the PSM has recently been boosted by including four-node and ten-node tetrahedral elements of Ansys FE software, which allows to discretize complex geometries. In this paper, a Round Robin among eleven Italian Universities has been performed to calibrate the PSM with seven different commercial FE software packages. Several 3D mode I, II and III problems have been considered to investigate the influence of (i) FE code, (ii) element type, (iii) mesh pattern, and (iv) procedure to extrapolate stresses at FE nodes. The majority of the adopted FE software packages present similar values of the PSM parameters, the main source of discrepancy being the stress extrapolation method at FE nodes.

The present paper investigates the variability in the static and cyclic properties of two nominally identical supplies of the aeronautical Al grade 7075-T6. Samples were extracted from extruded bars of 15 mm and 60 mm diameter and with slightly different chemical composition. Noticeable differences were found in tensile strength, total elongation, low- and high-cycle fatigue strength, despite the nearly identical hardness value. The diverse mechanical behavior has been imputed to different extrusion ratio and therefore work hardening along with a more or less fine distribution of precipitates and dispersoids. The high-cycle fatigue strength was found to be in direct correlation with the monotonic yield strength and the size of the largest intermetallic precipitate. A simple equation based on Murakami sqrt(area) parameter is proposed to predict the fatigue endurance. Tensile tests and microstructural analyses are recommended instead of conventional hardness tests to have a tighter quality control on the mechanical properties of semifinished products.

The main aim of the current study is to evaluate the compressive quasi-static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via Laser Powder Bed Fusion (LPBF) process. The topologies herein considered are lattice based regular and irregular configurations of cubic, star and cross shaped unit cell along with trabecular based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3 – 20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending dominated behavior affecting more the topologies with vertical struts. The S – N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching dominated cubic and purely bending dominated cross based structures. Trabecular structures have shown desirable quasi-static and fatigue properties despite the random distribution of struts.

Additive manufacturing is an evolving technology for fabricating porous structures used in a broad array of applications, ranging from the aerospace industry to biomedical engineering. Porous titanium alloy (Ti6Al4V) structures play a major role in biomedical implants and are preferred over conventional solid implants because their properties can be tailored to obtain the stiffness required to avoid stress shielding and improve osteointegration. The mechanical properties of these structures are dependent on unit cell topology and overall porosity. In the present work, three open cellular configurations were studied, namely regular (square), irregular (skewed square) and fully random structures, at three different porosity levels. The samples were manufactured using the selective laser melting of spherical Ti6Al4V powder. The deviations of manufactured samples from as designed were assessed using morphological characterisations and porosity analyses. The mechanical characterisations of the samples included monotonic and cyclic tensile tests, along with conventional compression tests under monotonic and cyclic conditions. The results from the study indicate a clear deviation of thickness values from as-designed values. The effect of inclination of the strut with respect to the loading axis has been studied in compression samples. The off-axis loading in compression led to the asymmetry in the Young's modulus in compression and tension. These led to finite element modelling of structures in the elastic regime and its validation using Gibson–Ashby model for cellular structures.

The procedure to evaluate the critical distance with an optimized V-notched specimen is initially reviewed in the paper. This procedure was devised by the authors, and another numerical methodology was recently proposed to evaluate the uncertainty of the critical distance assessment. The input of the analysis is the combination of the statistical distribution of the fatigue properties from which the critical distance is deduced. After assuming the specimen fatigue strengths as Gaussian (normal) distributions, the critical distance turns out to be well represented by a Skew-normal distribution. This statistical assessment is extended to the finite fatigue life, in the present paper, showing experimental results for the aluminium alloy 7075-T6 at two load ratios. The fatigue strength of other specimens are finally evaluated, reconsidering the critical distance deviation, thus providing a complete uncertainty analysis of the critical distance assessment, and a successful comparison with the experimental scatter is obtained.

Laser-based Powder Based Fusion (LPBF) being one of the most widely used additive manufacturing (AM) process has not only impacted the design of components in aeronautical and automobile industries but also in development of porous components. The properties of these lattice structures can be tailored according to the structural and compatibility requirements depending on the application. Therefore, understanding the behavior of cellular structures under static and fatigue loading becomes necessary. The factors that influence their behavior include relative density, cell topology, cell geometry and microstructure of the material. In this current study, titanium alloy Ti6Al4V cellular structures with four different geometries (Cubic, Star-shaped, X-shaped and Trabecular) manufactured using LPBF are considered. The mechanical performances of the samples, designed in order to have an overall porosity of 70-75%, have been evaluated with static tests under tensile and compressive loading, and with compression-compression fatigue test (R=0.1). The results indicate a clear decrease in the mechanical properties with increase in the bending dominated behavior of the cell types. However, Cubic structures displayed a catastrophic fracture behaviour, while the other structures were more resilient. Furthermore, despite the facts being bending dominated and with the lowest number of struts per node, the Trabecular structures showed good fatigue resistance thanks to the presence of struts in all the directions.

The problem of frailty in kidney transplantation is an increasingly discussed topic in the transplant field, partially also generated by the multiple comorbidities by which these patients are affected. The criteria currently used to establish the presence and degree of frailty can be rapidly assessed in clinical practice, even in patients with chronic kidney disease (CKD). The main objectives of this work are: (i) to describe the method of evaluation and the impact that frailty has in patients affected by CKD, (ii) to explore how frailty should be studied in the pre-transplant evaluation, (iii) how frailty changes after a transplant and (iv) the impact frailty has over the long term on the survival of renal transplant patients.

Modelling the thermal behaviour of the melt pool produced in Laser Powder-Bed Fusion (L-PBF) processes is not an easy task, as many complex non-linear thermal phenomena are involved. An effective way to make the computational cost of these analyses affordable is to model powder and molten metal as continuous media, wherein all the heat transfer modes occurring in the liquid are simulated as lumped fictitious heat conduction. The augmentation factor used to enhance the thermal conductivity of the liquid is in general calibrated through experimental estimations of the melt pool size. The present work is aimed at devising a robust method for the calibration of such thermal parameters. A specific point of novelty of the present paper is the definition of a method to correlate surface roughness and numerically predicted melting pool size. This strategy is able to predict with good accuracy the roughness of L-PBF fabricated parts and could pave the way for calibration strategies based on roughness measurements. For this purpose, a 3-factor, 3-level Design of Experiment (DoE) has been carried out to investigate melting pool size and roughness by changing the machine process parameters: laser power, hatch distance, time exposure. In this way, the calibration of the thermal properties is made less sensitive to the large uncertainty usually affecting the melt pool size measurements and the range of applicability of the thermal model is explored over a broad spectrum of L-PBF process parameters. Anisotropic and isotropic enhanced thermal conductivity approaches are applied in combination with a laser source modelled either as a 2D or 3D heat source, respectively. The latter approach proved to be more accurate and robust against experimental uncertainties.

In this paper, four material variants of an EN-GJS-400-type ferritic ductile cast iron (DCI) are produced by varying solidification time and chemical composition. In this way, experimental batches with different size of graphite nodules and spheroidal graphite fraction are obtained. The material variants are characterized through static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests. It is shown that by increasing the mean diameter of the spheroidal graphite particles fatigue strength, tensile strength and total elongation decrease, while fatigue crack growth threshold and fracture toughness increase. On the contrary, reducing the spheroidal graphite fraction owing to degeneration into chunky graphite results in lower tensile and fatigue properties as well as fracture toughness, while the near-threshold fatigue crack growth resistance is improved. This effect is discussed by the light of crack closure, fracture surface roughness and fractographic analyses.

Selective Laser Melting (SLM) is a technology that allows for the realization of metallic porous solids that cannot be produced with other methods. Nevertheless, a geometric discrepancy between the as-designed and as-built part is a well-known issue that can be critically important for biomedical metallic lattices with pore size and strut thicknesses of a few hundred microns. In practice, any geometric imperfection introduces a degree of uncertainty that can alter the mechanical properties of the as-built lattice. A quantitative relationship between the as-designed geometry and the real geometry is a very useful design tool because it makes possible to predict the final morphology of the lattice by considering the manufacturing errors: a more predictable geometry of the lattice structure means more predictable mechanical properties of the implant. In this work, we investigate the relationship between the as-designed strut thickness and the printed strut thickness for regular cubic cell lattices. The joints between the struts are filleted to reduce the stress concentration and increase fatigue resistance. The size of the unit cell is scaled to find the smallest radius which can be accurately reproduced. Moreover, we also studied the effect of the printing direction on geometrical accuracy

In this work we study the mechanical behavior of Ti6Al4V cellular structures by varying the randomness in the cell topology from regular cubic to completely random and the porosity of the structure. The porosity of the structure is altered by changing the strut thickness and the pore size to obtain a stiffness value between 0.5-12Gpa. The geometrical deviation in the structures from the as-designed values is studied by morphological characterization. The samples are subjected to compression and tensile loading to obtain the stiffness and the elastic-plastic behavior of the samples. Finite element modelling (FEM) is carried out on the as-designed structures for both tensile and compressive loading to study the effect of deviation between the as-designed and as-built structures. FEM is also carried out for as-built regular structures, by introducing the geometrical deviation to match the porosity of the as-built structures. Comparison of FEM and experimental results indicated that the effect of cell topology depends on the porosity values. Simulation results of as-built structures demonstrated the importance of defects in the structure.

Tubular structures find wide application in the automotive context. In particular, rectangular cross-section tubes are used to fabricate structural frames via different techniques, such as Three-Roll-Push-Bending with the addition of twisting component (TRPBT) and the Rotary Draw Bending (RDB). However, whether the accumulated plastic strains, hardening and residual stresses influence the load capacity of the tubular component is still unclear. This paper is intended to shed light on this issue. The load capacity of a tubular mock-up obtained by sequential combination of TRPBT and RDB has been empirically assessed by a destructive compression test. A finite element (FE) model has been devised and validated to analyse the manufacturing processes. This work puts in light the need to correctly model the compliance of the tool set-up for Roll Bending in the numerical calculations. The final shape of the mock-up obtained by FE analysis is the input of the numerical simulation of the compression test. The present modelling has shown clearly that the global resistance of a tubular component is sensitive to plastic strains, hardening and residual stresses resulting from the previous forming processes. Taking into account these three factors greatly improves the capability of the FE to model the mechanical response of the structural part.

Additive manufacturing of industrially-relevant high-performance parts and products is today a reality, especially for metal additive manufacturing technologies. The design complexity that is now possible makes it particularly useful to improve product performance in a variety of applications. Metal additive manufacturing is especially well matured and is being used for production of end-use mission-critical parts. The next level of this development includes the use of intentionally designed porous metals - architected cellular or lattice structures. Cellular structures can be designed or tailored for specific mechanical or other performance characteristics and have numerous advantages due to their large surface area, low mass, regular repeated structure and open interconnected pore spaces. This is considered particularly useful for medical implants and for lightweight automotive and aerospace components, which are the main industry drivers at present. Architected cellular structures behave similar to open cell foams, which have found many other industrial applications to date, such as sandwich panels for impact absorption, radiators for thermal management, filters or catalyst materials, sound insulation, amongst others. The advantage of additively manufactured cellular structures is the precise control of the micro-architecture which becomes possible. The huge potential of these porous architected cellular materials manufactured by additive manufacturing is currently limited by concerns over their structural integrity. This is a valid concern, when considering the complexity of the manufacturing process, and the only recent maturation of metal additive manufacturing technologies. Many potential manufacturing errors can occur, which have so far resulted in a widely disparate set of results in the literature for these types of structures, with especially poor fatigue properties often found. These have improved over the years, matching the maturation and improvement of the metal additive manufacturing processes. As the causes of errors and effects of these on mechanical properties are now better understood, many of the underlying issues can be removed or mitigated. This makes additively manufactured cellular structures a highly valid option for disruptive new and improved industrial products. This review paper discusses the progress to date in the improvement of the fatigue performance of cellular structures manufactured by additive manufacturing, especially metal-based, providing insights and a glimpse to the future for fatigue-tolerant additively manufactured architected cellular materials.

The main aim of the current study is to evaluate the compressive quasi-static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via Laser Powder Bed Fusion (LPBF) process. The topologies herein considered are lattice based regular and irregular configurations of cubic, star and cross shaped unit cell along with trabecular based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3 – 20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending dominated behavior affecting more the topologies with vertical struts. The S – N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching dominated cubic and purely bending dominated cross based structures. Trabecular structures have shown desirable quasi-static and fatigue properties despite the random distribution of struts.

Our study will be focused on stainless steel AISI 304&mdash, carbon steel ASTM A105 joints obtained by rotary friction welding and on their fatigue corrosion behavior in different testing environments. As a first thing, the joints will be characterized by microscopy and electrochemical techniques. The manuscript will then describe the optimization of the experimental setup and the validation of the testing procedure. After that, the fatigue behavior of the joints will be tested in different aggressive environments. This study pointed out that it is possible to build a simple and low-cost setup for the study of fatigue corrosion behavior of dissimilar joints while exploiting in situ electrochemical measurements to follow the fatigue corrosion process.