3,606 research outputs found
Soret driven convection in a colloidal solution heated from above at very large solutal Rayleigh number
Convection in a colloidal suspension with a large negative separation ratio
is studied experimentally by heating from above. Shadowgraph observation at
very large solutal Rayleigh numbers are reported as a function of time. Fast
relaxation oscillations are reported for the root mean square value of the
shadowgraph intensity. While pure fluids exhibit a transition to turbulent
convection for Rayleigh number of about 10^6, stable spoke-pattern planform
with up and down columnar flows are observed up to solutal Rayleigh numbers of
the order of 10^9. It is suggested that the surprising stability of the
planform against turbulence is due to nonlinear focusing arising from the
concentration dependence of the diffusion coefficient.Comment: 10 pages, 4 figure
Application of the continuum damage mechanics model in the three point bending test of Ti-6Al-4V titanium alloy specimens
One of the most important and challenging activities in the simulation of the mechanical behaviour of materials is the prediction of the failure phenomena. If well calibrated, damage models can simulate and predict the failure of materials in a generalized way allowing the replication of not only the calibration tests themselves but also of different loading cases. Generally damage models can be categorized into three different groups including phenomenological models, porosity models and continuum damage mechanics (CDM) models. Different CDM models have been proposed by researchers and these models have been applied in diverse loading conditions, geometries and materials. However the limitations and advantages of the CDM models are still not completely explored in the application areas. In this paper, a CDM model, (previously calibrated with round smooth specimen) has been applied in a three-point bending test model in order to simulate the correlated experiment. Specifically, the CDM framework has been applied in a finite element model and the obtained results have been compared with the experimental data. The tested material is Ti-6Al-4V titanium alloy, which is a widely used material in the aerospace industry because of its high strength and low density. Load-displacement data in the experiments and numerical simulations are the main results, which have been compared. Therefore, the ability of the CDM model to simulate the three point bending test has been investigated and the results are discussed
A schlieren method for ultra-low angle light scattering measurements
We describe a self calibrating optical technique that allows to perform
absolute measurements of scattering cross sections for the light scattered at
extremely small angles. Very good performances are obtained by using a very
simple optical layout similar to that used for the schlieren method, a
technique traditionally used for mapping local refraction index changes. The
scattered intensity distribution is recovered by a statistical analysis of the
random interference of the light scattered in a half-plane of the scattering
wave vectors and the main transmitted beam. High quality data can be obtained
by proper statistical accumulation of scattered intensity frames, and the
static stray light contributions can be eliminated rigorously. The
potentialities of the method are tested in a scattering experiment from non
equilibrium fluctuations during a free diffusion experiment. Contributions of
light scattered from length scales as long as Lambda=1 mm can be accurately
determined.Comment: 7 pages, 3 figure
A preliminary study of multi-parameter POD curves for a guided waves based SHM approach to lightweight materials
In view of an extensive literature about guided waves propagation, interaction and numerical simulation in platelike
structures made of metallic and composite materials, a lack of information is pointed out regarding their reliability in
structural health monitoring approaches. Typically, because of uncertainties in the inspection process, the capability of nondestructive
testing systems is expressed by means of suitable probability of detection curves. Based on Berens’ model, a linear
relationship is established between probability of detection and flaw size. Although the uncertain factors differ from a nondestructive
inspection technique and a structural health monitoring approach, the same mathematical framework can be
assumed. Hence, the authors investigated the application of a recently developed non-destructive testing Multi-Parameter POD
approach to a guided waves based SHM one: numerical simulations as well as experimental data from flawed plates were
combined to bring about a “master” POD curve. Once established, it can be used to build the POD curves of the single key
factors as flaw size, orientation, structural attenuation and so on
Valutazione della capacitĂ di rientro alla base di un elicottero in presenza di danno balistico ad un albero di trasmissione della linea rotore di coda
Nella progettazione di un elicottero militare, destinato ad operare a bassa quota e in ambiente ostile, il danneggiamento di componenti critici, conseguente ad impatto balistico, riveste un ruolo primario nella valutazione delle possibilità di sopravvivenza dell’intera macchina. In questo articolo è quindi proposto uno studio sperimentale, suddiviso in diverse fasi, riguardante la verifica della capacità di un elicottero di portare a termine una missione di rientro alla base a potenza ridotta e in presenza di danneggiamento balistico ad un albero di trasmissione della linea rotore di coda. Il lavoro ha richiesto dapprima l’esecuzione, su esemplari del componente in esame, di prove sperimentali di impatto balistico, condotte utilizzando un proiettile calibro 7.62 NATO. Successivamente su ciascun albero danneggiato sono state eseguite prove torsionali statiche ed a fatica, il cui scopo è stato verificare la resistenza residua del componente all’applicazione di opportuni carichi rappresentativi delle sollecitazioni riscontrate durante la missione di rientro
Sensor network optimization for damage detection on aluminium stiffened helicopter panels
Health and Usage Monitoring Systems (HUMS) has received considerable
attention from the helicopter community in recent years with the declared aim to increase
flight safety, increase mission reliability, extend duration of life limited components and of
course reduce the maintenance costs. The latter is about 25 per cent of the direct operating
cost of the helicopter, thus playing an important role especially in the case of the ageing
aircrafts. In particular, with respect to helicopter fuselages, only some attempts were carried
out to monitor directly on-line the damage accumulation and propagation during life. In this
field, and in particular in the military applications, an integrated and reliable system for
monitoring the damage in the fuselage and for evaluating the time inspections and remaining
life (prognosis) is missing. However, because of the presence of many vibratory loads, the
diagnosis of helicopter structures is very critical. From one hand, a very large number of
sensors would be needed for a robust appreciation of the structural health, from the other hand
the industrialization of the product brings the need for a low impact over the existing
structures, or toward a reduction in the allowed amount of sensors. As a result, comes the
importance for an optimization of the sensor network, with the aim to find out the regions
inside the structure which are the most sensible to a damage and at the same time robust to
noise. The aim of the present work is to define a methodology for optimising the sensors
position inside an helicopter fuselage panel in order to obtain the best compromise between
the simplicity and the robustness of a sensor network. In particular, a Finite Element (FE)
model will be used to create a database of various damages inside the structure, thus
consequently optimising the network sensitivity to any damage. The evaluation of the
network performances is provided when some realistic noise [1,2] is added to the FE
calculation
fatigue crack propagation in a helicopter component subjected to impact damage
Abstract Damage tolerant methodology is increasingly used in aeronautical components, especially due the fact that the Aviation Regulation requires such an assessment in case an accidental damage occurs. At present, there is a strong and actual interest in applying such procedures to helicopter components that are subjected to high frequency cyclic loads. In this paper, an investigation on a damaged transmission shaft for a tail rotor transmission of an actual helicopter has been carried out focusing on the fatigue crack propagation. A complete sequence of experimental tests was performed in order to create an actual ballistic damage and to subsequently check the damage tolerant behaviour. The shaft was damaged by oblique ballistic impact and was subsequently subjected to torsional fatigue loading. During the fatigue cycles several cracks propagated from the ballistic damages. Both of these steps (impact and fatigue loading) were also simulated by a complex modelling approach based on Finite Element Models and fracture mechanics theory. The comparison between the experimental and numerical results shows a good agreement but it underlines the need for a very refined modelling technique capable to replicate all the features associated with the damage in order to reliably simulate the subsequent propagation phase
the numerical modelling of a middle strength rock material under flexural test by finite element method coupled to sph
Abstract: Proper fracture assessment of the geological materials, which are highly exposed to hydrostatic loading, is a persistent challenge, in particular when aiming to develop an adequate numerical modelling technique. The mechanical response of a middle strength rock, namely Pietra Serena sandstone, under a Flexural (Four-Point Bending) test is investigated numerically in this study. The FEM-coupled to-SPH numerical technique has been approached in conjunction with an advanced material model implemented in LS-DYNA, namely the Karagozian and Case Concrete (KCC) model. The state of stress is investigated in different parts of the specimen in order to determine the strength of the material and the crack initiation area. The numerical results are finally validated by experimental data to show the reliability of the model
evaluation of the effects of the numerical modelling choices on the simulation of a tensile test on cfrp composite
Abstract The goal of the present work is to define a method to build a FE model which is able to reproduce an experimental tensile test on CFRP specimen with different stacking sequences (UD and balanced). The defined method assesses the material numerical parameters by means of a simulation that replicates, as a virtual test, the experimental tensile one, and in the future, it will be possible to exploit the data obtained to create a reliable model for the simulation of low velocity impacts. Analyses have been performed using the non-linear solver ABAQUS Explicit. The current work further studies how to model damage and the effect of modifications of the numerical parameters on the results. Indeed, the numerical simulation of composite materials is very sensitive to the numerical choices made. Moreover, from the literature and experiments, the mechanical properties of composites are very variable and hence the evaluation of the model response to such modifications is of particular interest
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