Impact of COVID-19 on maxillofacial surgery practice: a worldwide survey

The outbreak of coronavirus disease 2019 (COVID-19) is rapidly changing our habits. To date, April 12, 2020, the virus has reached 209 nations, affecting 1.8 million people and causing more than 110,000 deaths. Maxillofacial surgery represents an example of a specialty that has had to adapt to this outbreak, because of the subspecialties of oncology and traumatology. The aim of this study was to examine the effect of this outbreak on the specialty of maxillofacial surgery and how the current situation is being managed on a worldwide scale. To achieve this goal, the authors developed an anonymous questionnaire which was posted on the internet and also sent to maxillofacial surgeons around the globe using membership lists from various subspecialty associations. The questionnaire asked for information about the COVID-19 situation in the respondent's country and in their workplace, and what changes they were facing in their practices in light of the outbreak. The objective was not only to collect and analyse data, but also to highlight what the specialty is facing and how it is handling the situation, in the hope that this information will be useful as a reference in the future, not only for this specialty, but also for others, should COVID-19 or a similar global threat arise again

Evaluation of the stress–strain curve of metallic materials by spherical indentation

AbstractA method for deducing the stress–strain uniaxial properties of metallic materials from instrumented spherical indentation is presented along with an experimental verification.An extensive finite element parametric analysis of the spherical indentation was performed in order to generate a database of load vs. depth of penetration curves for classes of materials selected in order to represent the metals commonly employed in structural applications. The stress–strain curves of the materials were represented with three parameters: the Young modulus for the elastic regime, the stress of proportionality limit and the strain-hardening coefficient for the elastic–plastic regime.The indentation curves simulated by the finite element analyses were fitted in order to obtain a continuous function which can produce accurate load vs. depth curves for any combination of the constitutive elastic–plastic parameters. On the basis of this continuous function, an optimization algorithm was then employed to deduce the material elastic–plastic parameters and the related stress–strain curve when the measured load vs. depth curve is available by an instrumented spherical indentation test.The proposed method was verified by comparing the predicted stress–strain curves with those directly measured for several metallic alloys having different mechanical properties.This result confirms the possibility to deduce the complete stress–strain curve of a metal alloy with good accuracy by a properly conducted instrumented spherical indentation test and a suitable interpretation technique of the measured quantities

Plain fatigue resistance of shot peened high strength aluminium alloys: Effect of loading ratio

Abstract The effect of different shot-peening treatments on the reverse and pulsating bending fatigue behaviour of Al 7075 T651 was studied. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the effects of surface modifications and residual stresses. In particular, the extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements. No significant residual stress relaxation was observed in samples tested to a load level corresponding to the fatigue endurance at 5⋅10 6 cycles. Residual stress relaxation was observed only when the material plastic flow stress was achieved during the compressive part of the fatigue load cycle. Accordingly, shot peened samples with deep sub-superficial compressive residual stress peak showed a reversed fatigue endurance level corresponding to the condition of incipient plastic flow. This phenomenon was also accompanied by subsuperficial fatigue crack initiation. On the contrary, samples tested at shorter fatigue lives or under pulsating loading conditions showed crack initiation close to the surface. The initial and the stabilised residual stress profiles were considered for discussing the improvement in the fatigue behaviour due to peening. For this purpose, a multiaxial fatigue criterion was adopted to account for the biaxial residual stress field. The fatigue life was quite accurately predicted as long as fatigue initiation occurs on the surface

Critical behavior in a cross-situational lexicon learning scenario

The associationist account for early word-learning is based on the co-occurrence between objects and words. Here we examine the performance of a simple associative learning algorithm for acquiring the referents of words in a cross-situational scenario affected by noise produced by out-of-context words. We find a critical value of the noise parameter $\gamma_c$ above which learning is impossible. We use finite-size scaling to show that the sharpness of the transition persists across a region of order $\tau^{-1/2}$ about $\gamma_c$, where $\tau$ is the number of learning trials, as well as to obtain the learning error (scaling function) in the critical region. In addition, we show that the distribution of durations of periods when the learning error is zero is a power law with exponent -3/2 at the critical point

effect of porosity and cell topology on elastic plastic behavior of cellular structures

Abstract 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

Comportamento a fatica di strutture meccaniche in piena scala: risultati sperimentali e previsioni

Il lavoro si propone di presentare le principali attività di ricerca svolte, negli ultimi anni, presso il Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione (DIMNP) dell’Università di Pisa, anche in collaborazione con l’Università di Trento, nel campo della resistenza a fatica delle strutture meccaniche, in particolare per quanto riguarda la conduzione di “test” su componenti in piena scala e la loro interpretazione. Viene quindi condotta un’illustrazione di alcune recenti campagne sperimentali (Es.: giunzioni filettate in acciaio, elementi di sospensione in alluminio, ingranaggi ad elevate prestazioni), alla quale segue una descrizione delle attività di caratterizzazione di base e di modellazione condotte al fine di costituire una adeguata base di conoscenze per la interpretazione. Infine, vengono analizzati i risultati ottenuti, evidenziando alcuni problemi aperti, sia sul piano concettuale che su quello applicativo

Fatigue properties of Ti6Al4V cellular specimens fabricated via SLM: CAD vs real geometry

Abstract Fully dense titanium alloy implants have long been used for the replacement and stabilization of damaged bone tissue. Nevertheless, they can cause stress shielding which brings to a loss of bone mass. Additive manufacturing (AM) allows obtaining highly porous cellular structures with a wide range of cell morphologies to tune the mechanical properties to match that of the patient's bone. In this work, the fully reversed fatigue strength of cellular specimens produced by Selective Laser Melting (SLM) of Ti-6Al-4V alloy was measured. Their structures are determined by cubic cells packed in six different ways and their elastic modulus is roughly 3GPa to match that of trabecular bone. Part of the specimens was left as sintered and part treated by Hot Isostatic Pressing (HIP). The fatigue resistance of such AM parts can be affected by surface morphology, geometrical accuracy as well as internal defects. Micro X-ray computed tomography (CT) was used in this work to compare the geometry of the produced specimens with the CAD model and to carry out residual stress measurements using the Plasma FIB-SEM-DIC micro-hole drilling method

Tensile and compression properties of variously arranged porous Ti-6Al-4V additively manufactured structures via SLM

Abstract Additively manufactured porous structures find increasing applications in the biomedical context to produce orthopedic prosthesis and devices. In comparison with traditional bulk metallic implants, they permit to tailor the stiffness of the prosthesis to that of the surrounding bony tissues, thus limiting the onset of stress shielding and resulting implant loosening, and to favor the bone in-growth through the interconnected pores. Mechanical and biological properties of these structures are strongly influenced by the size and spatial arrangement of pores and struts. In the present work irregular and regular cellular as well as fully random porous structures are investigated through tensile and compression uniaxial tests. Specific point of novelty of this work is that, beside classical compressive tests, which are standard characterization methods for porous/ cellular materials, tensile tests are carried out. Mechanical tests are complemented with morphological analysis and porosity measurements. An attempt is made to find correlations between cell arrangements, porosity and mechanical properties