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

On the evolution of snow roughness during snow fall

The deposition and attachment mechanism of settling snow crystals during snowfall dictates the very initial structure of ice within a natural snowpack. In this letter we apply ballistic deposition as a simple model to study the structural evolution of the growing surface of a snowpack during its formation. The roughness of the snow surface is predicted from the behaviour of the time dependent height correlation function. The predictions are verified by simple measurements of the growing snow surface based on digital photography during snowfall. The measurements are in agreement with the theoretical predictions within the limitations of the model which are discussed. The application of ballistic deposition type growth models illuminates structural aspects of snow from the perspective of formation which has been ignored so far. Implications of this type of growth on the aerodynamic roughness length, density, and the density correlation function of new snow are discusse

Massless radiation from Strings: quantum spectrum average statistics and cusp-kink configurations

We derive general formulae for computing the average spectrum for Bosonic or Fermionic massless emission from generic or particular sets of closed superstring quantum states, among the many occurring at a given large value of the number operator. In particular we look for states that can produce a Bosonic spectrum resembling the classical spectrum expected for peculiar cusp-like or kink-like classical configurations, and we perform a statistical counting of their average number. The results can be relevant in the framework of possible observations of the radiation emitted by cosmic strings.Comment: 13 pages, 4 figures, improved explanations, an appendix added on rotating folded strin

Micrometeorological processes driving snow ablation in an Alpine catchment

Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micrometeorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological model (ARPS) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modelled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances except for very strong wind conditions. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is below a critical value. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.<br/

A Critical Evaluation of BS PD 7974-7 Structural Fire Response Data Based on USA Fire Statistics

Probabilistic techniques deal with the randomness of variables and reliability of safety system but their application in fire safety engineering is limited due to the lack of data related to real structures subjected to real fires. This can be overcome by analysis of national fire statistics provided by fire departments. Fire statistics databases are a collection of data from real structures subjected to real fires and provides an understanding of real effectiveness of different fire safety measures (i.e. compartmentation) which influence the spread and growth of fire, and ultimately their monetary consequence. The ability to understand the realistic responses of buildings in fire is the fundamental basis of British Standards PD 7974-7, which provides data to perform probabilistic risk assessments for fire. However, the current data presented by BS PD 7974-7:2003 (referred to as PD 7974-7 within this paper) was developed between 1966 and 1987. This research has used the USA fire statistic database of 2014 to recreate the tables present in the PD 7974-7, compare the results, and understand their evolution in time. The comparison between PD 7974-7 and the USA fire statistics introduced in this paper shows that modern fire frequency can be up to more than 10 times smaller than presented in PD 7974-7; area damage in m2 and spread of fire are linked to automatic extinguish systems effectiveness and greater in the USA fire statistics than predicted by PD 7974-7. This clearly demonstrate the need of updates to PD 7974-7 and feeds towards a better understanding of the robustness, and potentially the resilience, of real structures in fire

Microstructural effects on capacity-rate performance of vanadium oxide cathodes in lithium-ion batteries

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.Includes bibliographical references (leaf 29).Vanadium oxide thin film cathodes were analyzed to determine whether smaller average grain size and/or a narrower average grain size distribution affects the capacity-rate performance in lithium-ion batteries. Vanadium oxide thin films were prepared by sputtering onto ITO-coated glass substrates and crystallized in a refined annealing process to generate diverse microstructures. Average grain size and grain size distribution were determined in SEM analysis. No significant difference was observed in capacity rate behavior with changes in microstructure. However, it is speculated that further in situ analysis may show different relative diffusion rates into grains of differing sizes is related to different microstructures.by Robin M. Davis.S.B

Valutazione della capacità 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

Designing international public sector accounting standards: An analysis of constituents’ participation through comment letters

Harmonization of public sector accounting is attracting increasing attention from scholars and practitioners. A focal component of this phenomenon is the setting of accounting standards, whose legitimacy is paramount to their application. As participation by constituents is considered fundamental for ensuring this legitimacy, in this study, we focus on participation through comment letters in the due process. In particular, we explore the type of respondents, their geographical area, their agreement/disagreement with the documents prepared by the International Public Sector Accounting Standards Board (IPSASB) and the issues of importance to them, through an analysis of the comment letters submitted for six projects launched over the period 2017–2020 by the IPSASB. Furthermore, we analyze some factors that may affect countries’ participation in the due process. The analysis enriches our understanding of the IPSASB’s due process and provides relevant insights for the growing research into accounting standard-setting

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