Purpose - The aim of the paper is the study of the change in the mechanical properties (and in particular in ductility), with the microstructure, of a biomedical Ti-6Al-4V alloy produced by different ...variants of selective laser melting (SLM).Design methodology approach - Ti-6Al-4V alloy produced by different variants of SLM has been mechanically characterized through tensile testing. Its microstructure has been investigated by optical observation after etching and by X-ray diffraction analysis.Findings - SLM applied to Ti-6Al-4V alloy produces a material with a martensitic microstructure. Some microcracks, due the effect of incomplete homologous wetting and residual stresses produced by the large solidification undercooling of the melt pool, are observable in the matrix. Owing to the microstructure, the tensile strength of the additive manufactured parts is higher than the strength of hot worked parts, whereas the ductility is lower. A pre-heating of the powder bed is effective in assisting remelting and reducing residual stresses, but ductility does not increase significantly, since the microstructure remains martensitic. A post-building heat treatment causes the transformation of the metastable martensite in a biphasic a-b matrix, with a morphology that depends on the heat treatment. This results in an increase in ductility and a reduction in strength values.Originality value - The study evidenced how it is possible to obtain a fully dense material and make the martensite transform in Ti-6Al-4V alloy through the variation of the SLM process. The stabilization of the microstructure also results in an improvement of the ductility.
Purpose - The purpose of this paper is the microstructural and mechanical characterization of a biomedical Ti-6Al-4V alloy produced by electron beam melting, and the study of the stability of the ...as-built microstructure upon heat treatment.Design methodology approach - Ti-6Al-4V alloy produced by electron beam melting has been mechanically characterized through tensile and fatigue testing. Its microstructure has been investigated by optical observation after etching and by X-ray diffractometry analysis. The stability of the microstructure of the as-built material has been deepened carrying out suitable heat treatments, after an analysis by dilatometry test.Findings - The microstructure of a Ti-6Al-4V alloy produced by electron beam melting has a very fine and acicular morphology, because of the intrinsically high-solidification rate of the process. This microstructure is very stable, and the traditional thermal treatments cannot modify it; the microstructure changes significantly only when an amount of strain is introduced in the material. However, the mechanical properties of the alloy produced by electron beam melting are good.Originality value - The paper provides evidence of the microstructural stability of the material produced by electron beam melting. Even if the microstructure of the as-built material is not recommended by the specific ISO standard, the related mechanical properties are fully satisfactory. This is a significant indication from the point of view of the production of Ti-6Al-4V orthopaedic and dental prostheses by electron beam melting.
This paper summarizes the results of a two-day dynamic monitoring of Michelangelo's David subject to environmental loads (city traffic and pedestrian loading induced by tourists visiting the ...Accademia Gallery). The monitoring was carried out by a no-contact technique using an interferometric radar, whose effectiveness in measuring the resonant frequencies of structures and historic monuments has proved over the last years through numerous monitoring activities. Owing to the dynamic behavior of the measurement system (radar and tripod), an accelerometer has been installed on the radar head to filter out the movement component of the measuring instrument from the measurement of the David's displacement. Measurements were carried out in the presence and absence of visitors, to assess their influence on the dynamic behavior of the statue. A numerical model of the statue was employed to evaluate the experimental results.
The assessment of existing masonry towers under exceptional loads, such as earthquake loads, requires reliable, expedite and efficient methods of analysis. These approaches should take into account ...both the randomness that affects the masonry properties (in some cases also the distribution of the elastic parameters) and, of course, the nonlinear behavior of masonry. Considering the need of simplified but effective methods to assess the seismic response of such structures, the paper proposes an efficient approach for seismic assessment of masonry towers assuming the material properties as a stochastic field. As a prototype of masonry towers a cantilever beam is analyzed assuming that the first modal shape governs the structural motion. With this hypothesis a nonlinear hysteretic Bouc & Wen model is employed to reproduce the system response which is subsequently employed to evaluate the response bounds. The results of the simplified approach are compared with the results of a finite element model to show the effectiveness of the method.
The assessment of existing masonry structures under seismic loading requires reliable and expedite numerical modelling approaches. These approaches should take into account both the specific ...non-linear behaviour of the material (f.i. the small tensile strength) and, in case of time-history analysis, the hereditary nature of the restoring forces that describe the masonry mechanical behaviour. The paper, considering the need of simplified but effective methods to assess the seismic response of masonry towers, proposes an expeditious approach based on an equivalent Bouc and Wen model. The model, that has been extensively employed to describe a wide range of hysteretic behaviours like degradation of stiffness and strength, has the advantage of the computational simplicity since only one auxiliary non-linear differential equation is needed to describe the hysteretic behaviour. As a prototype of masonry towers, a cantilever masonry beam is analysed and the non-linear Bouc and Wen model is employed to reproduce the system hysteretic response assuming that the first mode shape governs the dynamic behaviour. A two-steps identification procedure is proposed. Static non-linear analyses are employed to assess a few of the Bouc and Wen model parameters; remaining ones are assessed in order to minimize the error between the estimated displacement and the one obtained in a reference finite element model of the structure. Within the paper, the identification of the parameters needed to build the single-degree-of-freedom oscillator is analysed in depth analysing the effects of four different seismic natural records. The results of the identification procedure show a general trend concerning the values to be selected for the Bouc and Wen parameters.
•Modal identification based on artificial neural network (ANN).•The ANN can be trained on a structure completely different from the one of concerns.•A reduced scale three-storey steel spatial frame ...was instrumented and progressively damaged.•An effective assessment of the modal parameters is obtained.
This paper proposes the application of neural networks for output-only modal identification of structural systems. Four frequency-dependent indicators, based on specific properties of the spectral tensor of vibration measurements, are defined and employed to build a likelihood function for the presence of structural resonances. Subsequently an artificial neural network (ANN), fed with the four indexes, was built and adopted to assess structural eigenvalues and eigenmodes. The proposed technique was tested on a three-storey steel-frame. After training the trained ANN was able to assess eigenvalues and eigenmodes, as demonstrated by comparison of the obtained results with those provided by literature methods.
Background and objectives: Whether the intensity of the conditioning regimen affects febrile neutropenia (FN) and severe bacterial infections (SBIs) is not well established. We analyzed the risk ...factors (RFs) for the development of FN and SBI in the first 100 d post‐transplant in 195 consecutive adult recipients of a reduced‐intensity conditioning allogeneic hematopoietic stem cell transplantation (RIC‐allo). Materials and methods: The RIC regimens consisted of fludarabine plus melphalan (62%) or busulphan (38%) (FluMel or FluBu). SBIs include pneumonia, urinary tract infections, and bacteremia. Results: FN occurred in 141 patients (72%), always in the first 30 d post‐allo‐RIC. However, a SBI occurred in only 27 patients (14%) during this early post‐transplant period (<day +30), while 29 evaluable patients (15%) developed a SBI in the intermediate post‐transplant period (days +31 to +100). In multivariate analysis, RFs for the development of FN included onset of neutropenia before day +5 after allo‐RIC (P < 0.02) and NCI CTC grade III–IV mucosal damage in the first 10 d post‐transplantation (P = 0.03). RFs identified to SBI by multivariate analysis included corticosteroid therapy before day +100 (P < 0.01), mycophenolate mofetil‐based graft‐versus‐host disease (GVHD) prophylaxis (P < 0.01), and previous SBI before day +30 (P < 0.01). The rate of SBI from day +30 to +100 varied according to the number of RFs; thus, the rate of SBI was 1% in patients without any RF, 17% in patients with one RF, 29% with one RFs, and 53% in those with all three RFs. Conclusions: After an RIC‐allo, FN and early SBI occurred mostly in patients with severe mucositis and early‐onset neutropenia, while postengraftment high‐dose steroid therapy for acute GVHD was the major RF.
The paper, through the discussion of an experimental investigation, considers a combined approach based on artificial neural networks and genetic algorithms for structural damage identification. A ...reduced scale three-storey steel spatial frame was instrumented by a series of 12 accelerometers and progressively damaged by cutting one of its columns just above the first storey. Accelerations induced by ambient vibrations were recorded as the frame was progressively damaged, and the deepness of the cut was taken as the entity of the damage. At every damage level the modal properties (natural frequencies and modal shapes) of the steel frame were evaluated through a neural network based approach. Subsequently, two error functions that measure the differences between the experimental results and those calculated from a finite element model of the steel frame were defined and a genetic algorithm was employed for damage detection. Results of the experimentation (where damage was known as both location and extent) were compared with the results of the optimization algorithm in order to verify its ability to match the actual damage.
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