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The application of Bayesian methods to the problem of fatigue crack growth prediction has been growing in recent years. In particular, sequential Monte-Carlo sampling is often presented as an efficient model-based technique to filter the sequential measures of the damage evolution provided as an input to the algorithm. However, a lot of measures are required to reliably identify the system state condition and the underlying model parameters. Many studies rely on the availability of a relatively dense sequence of crack length measures during damage evolution, made in most cases impractical by the consequent maintenance costs. Thus, real-time damage diagnosis is a requirement to enable prognostic health management. This work focuses on the application of sequential Monte-Carlo sampling to estimate the probabilistic residual life of a structural component subjected to fatigue crack propagation, while real-time estimation of crack length is provided through a committee of artificial neural networks, trained with finite element simulated strain patterns. Multiple crack length observations are available at each discrete time and are provided as the input to the prognostic system, based on a sequential importance resampling algorithm. Each time a new set of measures is available, the algorithm evaluates the posterior distribution of the augmented state vector, including the crack length and a material parameter governing damage evolution. This filtered information is used to numerically update the probability density functions of the residual life of the component. The methodology is applied first to a simulated crack and then to a metallic stiffened panel specimen subject to fatigue crack growth.

Modelling ductile damage is one of the most important applications of continuum damage mechanics (CDM) models. Scientists have previously tried to identify the damage parameters and have calibrated the failure behaviour of different materials using CDM models. However, more comprehensive studies are still needed to investigate the capabilities of CDM models for different materials. It is possible to extend the methodology to titanium alloys that are, currently, an optimal solution for highly stressed mechanical components. Therefore Lemaitre's CDM model parameters for ti-6Al-4V titanium alloy have been identified and reported in this work. A series of experimental tests have been carried out to identify the CDM model's parameters by the method of degradation of elastic modulus. A subroutine has been written in ABAQUS to numerically simulate the behaviour of the simple specimen using Lemaitre's model. Die Modellierung duktiler Schadigung ist eine der wichtigsten Anwendungen der Kontinuums-Schadigungsmechanik (CDM). Forscher haben versucht die Schadigungsparameter verschiedener Materialien durch CDM zu identifizieren und somit die Versagensverhalten zu kalibrieren. Um die Fahigkeit von CDM in verschiedenen Materialien zu untersuchen, sind jedoch umfassende Studien erforderlich. Es ist moglich, diese Methode auch auf Titanlegierungen zu erstrecken, die sich heutzutage als eine optimale Losung fur anspruchsvolle mechanische Komponenten anbieten. Deshalb werden in dieser Arbeit auch die Lemaitre CDM-Parameter identifiziert und prasentiert. Um die CDM-Parameter (durch die Bestimmung der Abnahme des Elastizitatmodul) zu identifizieren, wurden eine Reihe von experimentellen Untersuchungen durchgefuhrt. Um das Verhalten durch ein einfaches Modell (Lemaitre) zu simulieren, wurde eine Unterprogramm in Abaqus geschrieben.

An experimental–numerical methodology for the investigation of three point bending test (TPBT) on sandwich panels with Al skins and Nomex™ Honeycomb core is presented. A dedicated focus on numerical models is presented. Using data obtained from flatwise compressive tests, a highly detailed FE model (micromechanical level), has been built. It has, therefore, been possible to reproduce, with an elevated grade of similarity, the real behavior of sandwich panels in TPBT. Particular attention has been paid to verify the accurate modeling of the crushing behavior of the core during the last part of TPBT. Numerical results and experimental data have been compared. This comparison has not only been based on a load–displacement curve, but has been further exemplified by detailed photographical images, throughout the whole loading process, of the local behavior of the cells crushing. The influence of various parameters (friction and relative position of the puncher) has been also evaluated from a numerical point of view.

Sandwich structures are increasingly used in the aerospace field, also for primary parts. However, due to the extensive manufacturing process, the evaluation of their precise mechanical behaviour is not straightforward. This evaluation is, at present, a key task to enable future exploitation of these structures. Numerical simulations are powerful and flexible tools to study and identify the mechanical behaviour of such components in detail. Thus, in the present work, an extensive and highly accurate identification/validation strategy based on experimental tests has been developed. Finite element simulations (virtual tests) have been carried out in order to investigate, in detail, the effect that several manufacturer’s parameters have on the crushing mechanics of sandwich panels containing a Nomex™ honeycomb core. Numerical scale modelling has also been evaluated with the aim to increase the understanding of the crushing phenomena, particularly the reciprocal effect of the cells.

Rotorcraft airframes are complex structures designed to satisfy different goals; fatigue life represents one of the most critical issues, which must be guaranteed throughout the design and the construction phase. In this scenario riveted joints play an important structural role especially because of the variable loads they are subjected to. Their fatigue behaviour is strictly influenced by the local stress fields left very near to the holes during riveting manufacturing operations. Therefore, with the aim to improve design awareness, the effects of the different parameters involved during the riveting process are herein investigated. The effects of the squeeze force, the clearance, the rivet length and the clamping angle in the stress field of the joints are considered by means of numerical models. Detailed finite elements models (including rivet forming formation) are validated through experimental tests. These models are aimed at obtaining an accurate stress–strain field in the most stressed zone including the residual stress in the holes. Finally, using the literature fatigue data of Al8090-T81 and the Crossland multiaxial fatigue criterion, the influence of the riveting parameters on the fatigue strength is evaluated for optimisation purposes.

The main rotor hub is one of the most critical components in helicopter structures. Thus, to assess the behaviour of the component, a fatigue test until failure has been carried out on a real Ti–6Al–4V hub. A fracture mechanics analysis is used to estimate the critical crack dimensions. Subsequently, the slant fracture due to a propagating fatigue crack has also been simulated numerically with an explicit finite element model, using the Bao–Wierzbicki ductile damage criterion. Fracture toughness and failure calibration of Ti alloy have been carried out to obtain reliable results. The numerical fracture surface obtained has been compared with the experimental one.

Actual strategies and rules in peace keeping mission have led to an intensive use of helicopters exposing the aircraft and the crew to significant risks. Typical missions in fact involve low altitude flights in hostile environment where many threats can cause severe damages, leading eventually to the loss of the machine and the crew. According to this scenario, the tail rotor power transmission is one of the most critical components for its fundamental role to ensure flight stability and for its vulnerability, being very exposed during flight manoeuvre. In addition light weapons are wide spread, due to their cheapness and manoeuvrability. So the impact of 7.62x61 NATO ball 9.5 g bullet is an event anything but remote. This projectile is a full metal jacket bullet, with a brass jacket and a lead alloy core. Due to its mechanical characteristics, the soft lead core undergoes to high deformations and failures (mushroom and debris) during the impact, causing a large and extensive damaged area. Several researches have been developed to investigate the ballistic impact of conventional bullet against typical thin and lightweight aeronautical structure. As usual in this field, a complete methodology with experimental tests and numerical approaches has been carried on. In particular Finite Element analyses, although require complicate calibrations and validation which can be only made through indispensable experimental tests, represent a key resource. Very detailed numerical models are an extremely powerful tool to investigate the damage generated during an impact and allow simulating complex and extreme cases. With this premises direct impact between a 7.62x51 NATO ball 9.5 g bullet with a tube simulating an Helicopter drive shaft has been investigated by the authors in a previous work both with experimental and numerical activities with good agreement. However, considering the huge effect of bullet deformation verified during this activity, the modification of the bullet due to a preliminary impact with the surrounding frame (around the shaft in the real helicopter) could influence in a remarkable way the damage shape and extension in the shaft. This is an issue that is worth to further investigation and this is the aim of this paper. Basing only on a numerical procedure, previously assessed, an investigation of the impact of a NATO 7.62x51 mm ball 9.5 g bullet into an Al-6061-T6 pipe and its protection is presented. In particular the work will focus on the influence of the frame panel, which covers the transmission shaft, on the impact conditions. Analysis are carried out using the Finite Element commercial code ABAQUS/Explicit. Advanced materials’ descriptions, constitutive law and fracture criterion are introduced within the numerical model of the shaft and protection; projectile has been modelled as deformable body. Different impact conditions have also been tested in order to identify the worst impact condition.

An experimental impact test of a 7.62 NATO projectile perforating a helicopter tail rotor shaft is briefly presented. With the aim to reproduce the operational worst case, the damage occurs during the flight, the impacted specimen is subsequently tested with a multiaxial testing machine. This procedure allows the evaluation of the capability of the shaft to withstand a recovery mission, which is simulated by applying a torsional fatigue spectrum to the shaft. The nucleation and propagation of fatigue cracks have been monitored during the test. Finite element analyses have been carried out to simulate both impact and crack propagation. The state of stress is multiaxial: it is induced by torque, residual stress field and shape of the damage. Mixed mode propagation has hence been considered by means of an equivalent SIF. Numerical results are in agreement with experimental data.

The structure of aldehyde reductase (ALR1) in ternary complex with the coenzyme NADPH and [5-(3-carboxymethoxy-4-methoxybenzylidene)-2,4-dioxothiazolidin-3-yl]acetic acid (CMD), a potent inhibitor of aldose reductase (ALR2), was determined at 1.99A resolution. The partially disordered inhibitor formed a tight network of hydrogen bonds with the active site residues (Tyr50 and His113) and coenzyme. Molecular modelling calculations and inhibitory activity measurements of CMD and [5-(3-hydroxy-4-methoxybenzylidene)-2,4-dioxothiazolidin-3-yl]acetic acid (HMD) indicated that pi-stacking interactions with several conserved active site tryptophan residues and hydrogen-bonding interactions with the non-conserved C-terminal residue Leu300 in ALR2 (Pro301 in ALR1) contributed to inhibitor selectivity. In particular for the potent inhibitor CMD, the rotameric state of the conserved residue Trp219 (Trp220 in ALR1) is important in forming a pi-stacking interaction with the inhibitor in ALR2 and contributes to the difference in the binding of the inhibitor to the enzymes.

An advantageous ligand-free protocol for Suzuki couplings is described. The synthetic procedure entails microwave irradiation for the reduction of the reaction times and the use of silica cartridges for the purification. Dihalo-pyrimidine structures, interesting scaffolds in medicinal chemistry, were chosen as test compounds.

The impact of a 7.62 NATO projectile on the Al 6061-T4 shaft of the tail rotor transmission of a helicopter is studied in detail in order to identify the effect on the residual strength of the component. The investigation is carried out numerically by means of the finite element method, adopting classical von Mises plasticity in conjunction with a detailed experimental characterization of the mechanical behaviour of the material at different strain rates. In order to validate the method, the results of further analyses simulating the impact of projectiles on moving flat targets are compared with previously published experimental data.

The aim of this work is to develop a methodology for the evaluation of the ballistic requirements of a helicopter component. The situation studied in the work refers to the possibility of a damaged helicopter performing an emergency manoeuvre and a 30-min return flight after the impact of a 7.62 NATO projectile on a shaft of the tail rotor transmission. The paper describes the finite element methodology adopted to simulate the impact of the projectile on the shaft, and outlines the hypotheses on which this methodology is based. The numerical simulation of the propagation of the crack originating from the damage caused by the impact is also described. It is thus possible to determine the time required to reach failure of the shaft. The effect of the residual stresses on the residual life of the shaft is studied in detail.

A probabilistic method to monitor and predict fatigue crack propagation is presented in this work. The technique makes use of sequential Monte Carlo sampling combined with the probability density functions of the model parameters. The technique leads to an adaptive dynamic state-space model for crack evolution able to identify the most probable parameters, enhancing the estimation of the residual life of the system. The lifetime predictor presented here could be implemented in advanced maintenance strategies for critical structures employed in civil, industrial, and aerospace fields. The algorithm is first applied to a simulated crack growth, and then to some experimental crack propagations from laboratory tests on helicopter panels. The applicability within on-line continuous monitoring systems is discussed at the end of the paper.

The main objective of the paper is to exploit a dedicated material calibration to predict an extreme loading condition: the damage of a helicopter tail rotor transmission shaft due to a small arm ballistic impact. A typical material point in the damage area undergoes to complex stress and strain history, starting from a large compressive stresses through shear deformation during crack extension all the way to the tensile fracture of the remaining ligament. The plasticity and the fracture characterization require to perform material tests covering the wide range of stress state. Therefore, a part of the paper is devoted to the description and analysis of plasticity and fracture test of different type of specimens. This includes test on round smooth and notched specimens under combined torsion/tension-compression, flat dog-bone and a curved tensile specimen cut from the wall of the shaft. The test data have been used to calibrate the phenomenological Modified Mohr-Coulomb model and the empirical Bao–Wierzbicki fracture models. The result of the numerical simulation of the projectile impact was then compared with the experiments on the ballistic impact of 7.62 ball projectile against the helicopter shaft. A very good agreement was found for both damage shape and residual velocity.

This research investigates a Lamb-wave based structural health monitoring approach matching an out-of-phase actuation of a pair of piezoceramic transducers at low frequency. The target is a typical quasi-isotropic carbon fibre reinforced polymer aeronautical laminate subjected to artificial, via Teflon patches, and natural, via suitable low velocity drop weight impact tests, delaminations. The performance and main influencing factors of such an approach are studied through a Design of Experiment statistical method, considering both Pulse Echo and Pitch Catch configurations of PZT sensors. Results show that some factors and their interactions can effectively influence the detection of a delamination-like damage.

This paper describes a fast and reliable algorithm for automatic simulations of crack propagation in bi-dimensional and non-planar shell FE models. Thanks to its simplicity, the algorithm can be coded with little effort and in a relatively short time. Moreover, it can be interfaced with the existing FE analysis environment. An automatic iterative process makes it possible to achieve the highest degree of accuracy in the results for the FE model used. Good accuracy can also be achieved for quite coarse meshes.

Analysis of the state of stress and strain of the wire ropes is fundamental in assessing mechanical strength; a comparison between different analytical formulations utilized to estimate the state of stress in the inner and external wires of a rope is presented in this paper. In particular, different approaches are used to design a model for axial force and bending moment. For this purpose, a rope used in a helicopter rescue hoist, subjected to swinging of the recovery hook, present in anomalous working conditions, is considered. The analytical results have been compared with experimental data obtained on the rope (modulus of elasticity, fatigue tests of the wire, rope bending fatigue tests) with the aim of making a critical comparison of the methods analysed and to allow a reliability fatigue life prediction for the component.

In continuing the search for more effective 5-arylidene-4-thiazolidinones as aldose reductase inhibitors, a new set of suitably substituted compounds (4, 5 and 8) was explored. Acetic acids 5, particularly 5a and 5h, proved to be interesting inhibitors of the enzyme as well as excellent antioxidant agents that are potentially able to counteract the oxidative stress associated with both diabetic complications as well as other pathologies. Molecular docking experiments supported SAR studies. © 2011 Elsevier Masson SAS. All rights reserved.

This research aims at comparing two different methods for the construction of pressure vessel nozzles, designed with the same safety coefficient, according to ASME and VSR 1995 standards. It defines numerical and experimental analysis of behaviour under low-cycle fatigue for pulsating pressure. In particular, a nozzle with integral reinforcement, designed according to ASME standards, is compared to a nozzle with external reinforcement (applied reinforcement plate) designed according to VSR 1995 standards with the same safety coefficient. Strain gauge tests have been carried out on the plastic behaviour of the two structures in order to evaluate the expected fatigue life based on common criteria, using both the local strain and energetic approaches. At the same time, a FEM model of the nozzle with plate has been used to calculate numerically the expected fatigue life based on the same criteria. Finally, in order to identify the best system to exploit for design, comparisons are made of the fatigue life predictions, which are numerically and experimentally obtained and which are determined according to the standards of the two nozzle types to identify the better system.

Summary The early diagnosis of cracks in aeronautical structures is a fundamental task for the safe system operation and the optimization of maintenance policies, in view of the increasing interest in life extension programs of several high-investment industries. In principle, these tasks could be fulfilled within a condition-based framework, where direct or indirect observations of the degradation evolution are processed, possibly in real time, by proper diagnostic computational tools. In the past, several attempts have been made to build real-time monitoring systems collecting strain signals acquired from sensor networks. However, in real applications, some issues remain unresolved, for example, the large number of observations available to be handled within a unique diagnostic framework, their relationship with the underlying crack size, and their typical large uncertainties. In this paper, we provide a practical solution by innovatively combining a particle filtering-based model identification algorithm with a measurement system exploiting real-time observations of the crack length reconstructed by a committee of artificial neural networks. The artificial neural networks are trained by simulated strain fields generated by a finite element model. The resulting tool allows to perform an automatic, simultaneous, and real-time (a) selection of the model more properly describing the system state evolution, so as to detect the crack propagation onset time, (b) estimation of the model parameters, and (c) estimation of the crack length, within a unique probabilistic framework based on particle filtering. The methodology is demonstrated with reference to a real helicopter panel subject to fatigue and equipped with a fiber Bragg grating sensor network.

Severe and extreme loads, that introduce large strains and failure, are a present challenge in the design of critical mechanical components. Even though full scale testing is a fundamental approach for reliable structural integrity evaluation, numerical simulation is an alternative economical method that is now increasingly chosen especially because of the development of computing performances. In particular, the numerical assessment of the ductile fracture in metallic components represents an innovative and challenging field in the structural integrity scenario. Aerospace, automotive and manufacturing industries have recently boosted their interest in these kinds of simulations with the aim to make these approaches, little by little, reliable also for certifications. Taking the requirement to characterize material for further impact simulations as a starting point, the work described in this chapter contains a complete characterization of the mechanical properties of Al 6061-T6 aluminium alloy as far as material hardening and fracture locus are concerned. The calibration has been carried out through a series of experimental tests on simple specimens. These specimens have similar geometry, but are subjected to different stress triaxiality, thanks to the use of a multiaxial hydraulic test machine. All the experimental tests have been numerically simulated and a complete material constitutive model has been calibrated, which based on the results of these experimental analyses.

Aim of the present paper is the evaluation of the capability of a ductile damage criterion to numerically predict (finite element model) the behaviour of Al6061-T6 compact specimen C(T) involved in fracture toughness tests. The parameters of the damage model have been previously calibrated with mono and multiaxial tests; the current work is an exploitation of such calibration in a different framework, in order to check its geometrical transferability even in a scenario dominated by a high strain gradient. Finite element analysis has been carried out using the commercial finite element software ABAQUS 6.12 and a quasi-static explicit framework. The advantages and the possible drawbacks of the application of this kind of macroscopic failure criteria in a scenario dominated by very localized phenomena, thus involving material at grain size, are herein discussed. Due to the localized nature of the problem, mesh size potentially plays a relevant role in the failure, hence various mesh sizes have been analyzed and their performance is discussed on the basis of metallographic analyses.

Al6061-T6 aluminium single layer plates, of different thickness, impacted by two different 7.62 mm Armour Piercing projectiles are studied from a metallographic point of view. The material behaviour after ballistic impact is investigated by optical and scanning electron microscopy as well as by micro-hardness tests. The study allows one to highlight the physical phenomena that happen during the impact: the formation of adiabatic shear bands (ASB), the evolution of softening and hardening phenomena, the creation of petalling, plugging and the formation of a melted film between the target and the penetrator as well as the localised failure mechanism. The thermo-mechanical effects on the target and the penetrator material have been examined in order to investigate in the physical phenomena ruling the ballistic damage; the differences between the two types of impactors are also highlighted. Among interesting findings, it has been noticed that a complete penetration is less detrimental and tungsten core projectile seems more damaging for the structural integrity of Al6061-T6 plates. A complete penetration allows the material to better dissipate the plastic deformation energy induced by bullet whereas the arrest increases localised material softening effect resulting in the formation of ASB. Although the creation of numerical models is not the aim of the present paper, the findings could be useful for the building of a modelling approach capable of replicating all the involved phenomena in details.

Experimental, analytical and numerical simulations were performed to study the ballistic resistance of 6061-T6 aluminium plates subjected to a normal impact of small calibre armour piercing bullets. Two types of bullets were used, steel and tungsten carbide core bullets. Three plate thicknesses, 101.6 mm, 76.2 mm and 25 mm, were considered. Bullets were impacted at ordnance velocity thus results range from deep penetration to complete perforation with residual velocity. Ballistic results, deformations of the plates and of the bullets are discussed and reproduced through numerical simulations using both LS-DYNA and ABAQUS solvers; analytical models by means of a cavity expansion approach are also used. Exploiting analytical and numerical models the ballistic behaviour of the two bullets on the three plates is discussed with emphasis on the capability of the models to reproduce the physical features of the phenomena. X-ray diffraction measurements for the residual stress patterns of the plate's surface were also performed on all of the plates both before and after the tests and the experimental measurements are compared with the numerical model results. A specific discussion about tungsten carbide core bullet behaviour is reported highlighting the importance of the sabot in the impact phenomena. Although the work focuses on monolithic plates, the results and discussion can be of interest for the design of optimised multilayer armour shields.

Different damage mechanics models have been proposed by researchers to calibrate the failure behavior of materials. Continuum damage mechanics (CDM) models are one of the main categories of damage models that can be exploited in numerical simulations. In this paper Lemaitres damage model, has been applied to finite element models of flat specimens. These models allow assessing the geometry transferability of the previously calibrated CDM model investigating in different geometry and loading conditions. Four different types of plane stress specimens have been designed to get different stress triaxialities which cover shear dominant and high triaxiality failure. Experimental tests were also done and the obtained data were critically compared with the results from numerical models. The tested material is Ti-6Al-4V titanium alloy which is a widely used material in aerospace industry because of its high strength and low density.

An indentation test is a non-destructive and easy to implement test studied in this paper to estimate the fracture toughness of ductile materials, specifically the Al6061-T6 aluminium alloy. The Berkovich indenter is adopted to generate a crack-like indent; this established technique is based on the principal concept of continuum damage mechanics (CDM) and the model of indentation energy to fracture. It is found that the relation between the effective elastic modulus and the indentation plastic depth hp during the indentation process can be exponentially fitted. Thus the critical plastic indentation depth h p * is evaluated based on the elastic modulus degradation technique and the effective elastic modulus behaviour. Consequently, the critical indentation energy per unit area γ* is determined by using the power law behaviour between the applied load and the plastic indentation depth hp. Considering the stress intensity approach and the Griffith theory, this energy per unit area value is used to calculate the fracture toughness KIC of the ductile material. The fracture toughness of Al6061-T6 is evaluated by indentation tests. Traditional compact tension (CT) tests are performed to attest the accuracy of the indentation energy approach and a numerical analysis is adopted to investigate the stress state in the indentation test. The fracture toughness obtained by the indentation test stands in good agreement with the experimental CT tests.

The behavior of short cracks, focusing on the study of small manufacturing flaws in a critical structure working under a high frequency load spectrum, is a complex and very current topic and its investigated in this study. The modeling and predicting of the effect of these flaws is a matter of interest also for industrial manufacturers with regards to the future design of high stressed components. Aeronautical components in general, but particularly helicopter components, are subjected to a high number of fatigue cycles that potentially cause the propagation of very small flaws and thus require careful analysis. The application of multi-axial loading conditions such as axial and torsional load complicate the assessment by provoking a multimode propagation. A high cycle multi-axial fatigue loading condition (torsional and axial) on a tube that is representative of a helicopter main transmission shaft has thus been investigated in this work. The presence of a small flaw has been artificially created on the surface of the tube and the behavior of the defect in terms of the threshold of the propagation of a crack emanating from it under high cycles fatigue due to axial and torsional stresses has been investigated by means of analytical and FE models. Experimental test have also been carried out to validate the results of the simulations.

The following work utilizes a calculation method for the design of spherical containment vessels for pressure gases likely subjected to internal detonation. A centrally initiated explosion within the vessel is taken into account and the history of pressures on the internal vessel wall is investigated by means of the fundamental compressible fluid dynamics, for which a non-viscous perfect gas is assumed. The obtained differential equations are integrated with explicit finite difference techniques, introducing artificial viscosity. The response of vessel is decoupled from the fluid and it is determined by a dynamic finite element code of explicit type; the investigated material is considered to have an elastic-plastic behaviour. Comparison with data found in references on PBX-9404 high-explosive induced explosions confirms the calculation method developed.

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 possibilita di sopravvivenza dell’intera macchina. In questo articolo e quindi proposto uno studio sperimentale, suddiviso in diverse fasi, riguardante la verifica della capacita 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 e stato verificare la resistenza residua del componente all’applicazione di opportuni carichi rappresentativi delle sollecitazioni riscontrate durante la missione di rientro.

Studies of ballistic penetration into metal plates and their numerical simulation currently present an important topic in ballistics, however, no congruent results have been presented so far, especially when it comes to impacts on multi-layered plates. Presently, as far as ballistic limits are concerned, the choice between layered and monolithic structures is not completely straightforward and unproblematic. The effect of introducing air gaps between metallic layers is not fully understood and explained either. Furthermore, these issues are more investigated for normal impacts than for oblique impacts for which only limited results are available. Therefore, the aim of this paper is to conduct a numerical analysis in order to evaluate the effect on the ballistic limit on layered targets for both normal and oblique impacts. A validated numerical methodology will be used, though validated with a limited number of experiments. The target material is an Al6061-T6 aluminium alloy the mechanical behaviour of which (hardening, strain rate, failure, etc.) is already known and described. Several configurations will be numerically tested and the results critically evaluated.

Some possible applications for MEMS technology development in structural health monitoring (SHM) are investigated in this chapter, particularly exploring the area of piezoelectric based sensors and fiber Bragg grating optical technologies. Although the latter is not included among typical MEMS, its functioning not being based upon electric sources, its small dimensions and low logistic impact make it a micro optical mechanical system (MOMS).

An experimental–numerical investigation of the ballistic impact of 7.62×51 mm soft-core ball 9.5 g projectiles on aluminium tubes is presented in this paper. The tubes are made of Al-6061-T6 and simulate actual components of a helicopter tail rotor drive shaft. Several tests have been carried out: a real gun has been used to produce ordnance velocities and a dedicated support frame has been built to perform impact tests with an angle of obliquity (in order to reproduce the most critical damage on the component that is subjected to torsional service load). Initial and residual velocity of the bullets, shape and dimensions of the damage, and the residual stresses on the components, have been measured. Numerical models of the impact have been developed with the commercial Finite Element code ABAQUS/Explicit. The Johnson–Cook (JC) constitutive model and the Bao–Wierzbicki (BW) ductile fracture criterion have been calibrated for Al-6061-T6 and have been used for the analyses. The effect of the bullet type on the damage has been investigated; therefore the ball bullet has been fully modelled (core and jacket) to highlight the effect and danger of this type of soft core bullet in case of impact against aluminium thin structures. Numerical models are in good agreement with the experimental results; more specifically they show a satisfactory capability to reproduce the residual velocity of the bullets and the failure mode, as well as the residual stress fields on the tubes near the damaged zone.

The work presented hereafter is about the development of a diagnostic system for crack damage detection, localization and quantification on a typical metallic aeronautical structure (skin stiffened through riveted stringers). Crack detection and characterization are based upon strain field sensitivity to damage. The structural diagnosis is carried out by a dedicated smart algorithm (Artificial Neural Network) which is trained on a database of Finite Element simulations relative to damaged and undamaged conditions, providing the system with an accurate predictor at low overall cost. The algorithm, trained on numerical damage experience, is used in a simulated environment to provide reliable preliminary information concerning the algorithm performances for damage diagnosis, thus further reducing the experimental costs and efforts associated with the development and optimization of such systems. The same algorithm has been tested on real experimental strain patterns acquired during real fatigue crack propagation, thus verifying the capability of the numerically trained algorithm for anomaly detection, damage assessment and localization on a real complex structure. The load variability, the discrepancy between the Finite Element Model and the real structure, and the uncertainty in the algorithm training process have been addressed in order to enhance the robustness of the system inference process. Some further algorithm training strategies are discussed, aimed at minimizing the risk for false alarms while maintaining a high probability of damage detection.

Simulation of the behaviour of structural components subjected to high explosive detonation is one of the current challenges in the field of numerical simulation. Along with experimental tests, numerical analysis is necessary to give an in-depth insight of this event, as well to reduce costs for some further experimental tests. High values of strain rate, temperature and pressure, together with failure phenomenon, govern the complex interaction between the explosion and the structure involved. In a scenario of this type, capabilities and performance of the numerical software used are crucial to the quality and the outcome of the simulation. Besides the simulation itself, this paper provides a comparison between different finite element programs such as ABAQUS, AUTODYN and LS-DYNA in an explosion event. In the event descibed in the paper, the behaviour of tube made of aluminium alloy Al6061-T6 and filled with explosive material is under investigation. A fully coupled Eulerian and Lagrangian formulation is used together with a complete mechanical behaviour and constitutive equations of all the materials involved in the simulation (aluminium alloy Al6061-T6, explosive C4, air). Finally, results and comparison between the mentioned numerical solvers will be reported and critically discussed.

When the material yields local stress and strain behaviour changes, especially if there is a notch, determination of the local strain value can be difficult. Therefore it is not easy to predict the life of mechanical components in the low-cycle region. In the present work pressure vessels are considered and fatigue tests carried out. The most stressed zones, which are at the connections between the nozzles and the vessel, are subjected to cyclic loads with subsequent repeated plastic strains that cause nucleation and propagation of fatigue cracks. The life of such a mechanical component under low cycle fatigue can be predicted by utilizing the life strain curves e − N f or the energy approach (plastic strain and total strain energy density). In the present work, life prediction is performed by considering several different approaches. The results are compared with the experimental results obtained by fatigue testing pressure vessel nozzles. The material considered is Fe 510 B UNI 7070-72 steel.

In this study, keyhole and smooth specimens, made from a low alloy pressure vessel steel (ASTM A-533 grade B), were subjected to monoaxial fatigue tests. The results show the influence of the stress concentration factor, K[sub t], on the number of cycles to failure, N[sub f]. Total strain energy per cycle, [Delta]W[sub t] = [Delta]W[sub p] + [Delta]W[sub e], was proved to be a good parameter for predicting the life of notched components. Elasto-plastic FEM analysis, utilizing the cyclic and monotonic curve of the material, showed close agreement with the experimental values.

The present article provides a technical survey of the effects of shot peening conditions on the fatigue performance of Ti–6Al–4V specimens representative of the material (and the surface treatment) used in helicopter rotor hubs. As the effects of shot peening on non-plain surfaces have been fairly neglected in the scientific literature, the present work attempts to define the effects of shot peening on different specimens, featuring specific cross-section geometries, namely smooth and sharp edged specimens. Experimental tests also include measurements of the residual stress field caused by shot peening and the definition of the fatigue limit (by means of the ‘staircase method’) for all the tested specimen configurations. The present study proceeds with an optical and scanning electron microscopic investigation of the dynamics and causes of the different fatigue limits associated with the geometrical features. The present study conveys a strong correlation between specimen geometry and shot peenin...

The simulation of ductile fracture in real components is becoming a strategic issue in numerical simulations. Numerical simulations of crashes, forming processes, impacts and fractures are reliable only if carried out with an accurate material calibration. The topics involved in this kind of simulation require a complete calibration of both the true stress–strain curve and the failure. The focus of this work is the accurate calibration of the constitutive relations of the titanium alloy Ti–6Al–4V. The approach proposed is based on different experimental tests supported by numerical simulations performed by means of detailed FE models. The Bao–Wierzbicki ductile failure criterion is calibrated using a total of 11 specimens. These specimens are tested on a multiaxial test machine to investigate the failure at different stress triaxialities. Furthermore, the sensitivity to the mesh size and the assessment of the calibration accuracy are analysed in detail on different components in order to verify the geometry transferability.

Calibration and exploitation of failure criterion is at present a challenging field in the structural integrity scenario. Calibrated failure criteria allow the simulation/reproduction of damages using virtual approach and eventually further assessment of the residual integrity of the components. Therefore the increase of awareness in failure issues makes the numerical simulation an actual, useful and reliable tool for the analysis of complex structures under extreme loads, especially in aerospace field where full scale tests are often very expensive and difficult to carry out. With this aim, the constitutive relations of an Aluminium Al 6061-T6 alloy have been calibrated with dedicated focus on failure criterion. The results obtained have been discussed considering the crystallographic measurements that permit to point out the dissipative behavior on the basis of texture formation as a function of the load type. The final aim is to confirm and explain the different failure behavior depending on the different stress triaxiality.

Numerical simulations are now an actual option in order to try to reproduce and understand the mechanical response in components subjected to extreme loading conditions, like in a ballistic impact. A correct materials calibration is therefore necessary in order to extract the materials parameters. In this work the simple and widely used Johnson-Cook model was used to analyse the experimental data obtained for the characterization of the bullet materials. The bullet under investigation is a full metal jacket ball, with a lead-antimony alloy core and a brass jacket. The experimental tests cover a wide range in strain-rate, starting from quasi-static tests up to high dynamic tests performed on a standard Split Hopkinson Pressure Bar setup. In general, there is a great lack in strain-rate sensitivity and failure data. Pure lead is very soft and ductile, so antimony is used to give greater hardness and strength. The results of this study show a significant strain-rate influence for this alloy that can be associated with the presence of the lead-antimony phases and their structures. Also in case of the brass the results showed significant strain-rate sensitivity in the material response.

In the present work a validated numerical approach has been used in order to build a robust and reliable FE model of the impact of a NATO 7.62 mm ball bullet, against an aluminium transmission shaft. The bullet is a full metal jacket type, with a lead alloy core and a brass jacket. Target shaft is made by an Al6061-T6 aluminium alloy. According to the soft core (lead alloy) of the bullet, most effort has been spent in order to evaluate the effect of bullet materials mechanical properties on the numerical results. Numerical analyses, carried out using the non-linear dynamic finite element solver Abaqus∖Explicit 6.10, have been performed focusing on core and jacket material behaviour (target material, Al6061-T6, has been previously calibrated by the authors). Thus numerical analyses have been performed considering for the mechanical behaviour of the bullet both a simplified approach (as reported in literature) and new material data (with strain rate effect) obtained by means of experimental tests on the two materials (lead and brass) with specimens cut directly from the bullet. Finally the results of the analyses have been compared with real experimental ballistic tests.

In this paper, the authors apply the ASME Boiler & Pressure Vessel Code to the structural assessment and design optimization of dissimilar welded joints subjected to transitory thermo-mechanical loads. The verification is based on the finite element stress linearization methodology. A large number of dissimilar P22/P91 steel welded joints of a power generation plant are taken into consideration. The joints vary in geometrical and operational conditions. In comparison with experimental investigations on some damaged structures, the methodology proves to be feasible, easily implemented, and reliable. Further, a second relevant result is obtained using the validated method to perform an optimization of the geometry of the joints for a safer design.

Ballistic requirements in the design of military aerospace components are critical for the structural assessment. Although experimental tests remain fundamental, numerical analyses allow us to simulate such a very complex scenario. However, in order to improve the consistency of such simulations, several aspects have to be considered in detail. The material behavior is a key one: constitutive laws, able to describe the material behavior in terms of hardening, strain rate, damage criteria, temperature, etc…, is fundamental. Moreover a detailed validation of numerical results is needed. The analysis of ballistic data (such as the residual velocity and the direction of the bullet after the impact) is only one option, but reliable simulations of damage shape and size are the real target. Therefore the paper will focus on the comparison between the experimental damage and the simulated one using a 3D acquisition of the experimental impacted area. This approach allows us not only to evaluate the damage in the macroscale field but also in the near micro-scale: shape on the border of the damage.