Analisi del cedimento strutturale del braccio di una gru portuale

Nel presente lavoro vengono discusse le cause del cedimento strutturale del braccio di una gru portuale a portata variabile. L’ispezione delle sezioni di rottura ha mostrato in particolare la presenza di una grossa fessura preesistente al momento del crollo, che si estendeva per circa metà sezione di uno degli elementi tubolari principali che costituiscono il traliccio del braccio. Al fine di stabilire le modalità del crollo, nel lavoro vengono presentati un modello per l’analisi della propagazione della fessura, che fa uso del metodo delle weight functions ed un modello per l’analisi di collasso della sezione. Le analisi condotte hanno permesso di concludere che il crollo si è verificato per collasso plastico della sezione ed hanno messo in evidenza come la fessura in esame, per una parte considerevole della vita, sia propagata in condizioni stabili con velocità di avanzamento circa costante

A new formulation of the understeer coefficient to relate yaw torque and vehicle handling

The handling behaviour of vehicles is an important property for its relation to performance and safety. In 1970s, Pacejka did the groundwork for an objective analysis introducing the handling diagram and the understeer coefficient. In more recent years, the understeer concept is still mentioned but the handling is actively managed by direct yaw control (DYC). In this paper an accurate analysis of the vehicle handling is carried out, considering also the effect of drive forces. This analysis brings to a new formulation of the understeer coefficient, which is almost equivalent to the classical one, but it can be obtained by quasi-steady-state manoeuvres. In addition, it relates the vehicle yaw torque to the understeer coefficient, filling up the gap between the classical handling approach and DYC. A multibody model of a Formula SAE car is then used to perform quasi-steady-state simulations in order to verify the effectiveness of the new formulation. Some vehicle set-ups and wheel drive arrangements are simulated and the results are discussed. In particular, the handling behaviours of the rear wheel drive (RWD) and the front wheel drive (FWD) architectures are compared, finding an apparently surprising result: for the analysed vehicle the FWD is less understeering than for RWD. The relation between the yaw torque and the understeer coefficient allows to understand this behaviour and opens-up the possibility for different yaw control strategies

Analisi del cedimento strutturale del braccio di una gru portuale

Nel presente lavoro vengono discusse le cause del cedimento strutturale del braccio di una gruportuale a portata variabile. L’ispezione delle sezioni di rottura ha mostrato in particolare la presenza di unagrossa fessura preesistente al momento del crollo, che si estendeva per circa metà sezione di uno degli elementitubolari principali che costituiscono il traliccio del braccio.Al fine di stabilire le modalità del crollo, nel lavoro vengono presentati un modello per l’analisi dellapropagazione della fessura, che fa uso del metodo delle weight functions ed un modello per l’analisi di collassodella sezione. Le analisi condotte hanno permesso di concludere che il crollo si è verificato per collasso plasticodella sezione ed hanno messo in evidenza come la fessura in esame, per una parte considerevole della vita, siapropagata in condizioni stabili con velocità di avanzamento circa costante

Geometry optimization of a magnetorheological clutch operated by coils

Magnetorheological fluids are smart materials responsive to magnetic field, widely applied in dampers and shock absorbers but also in clutches and brakes. The magnetorheological fluid gap shape is a very important topic in the design of clutches, since it directly influences the transmissible torque and the power loss. In this paper, an approach to magnetorheological fluid clutch design based on optimization is proposed and tested on four different layouts. Starting from a given available volume, two magnetorheological fluid gap shapes, namely single cylinder and multi-disc, and two coils positions, i.e. internal or external, were considered. A lumped parameter model was developed to analytically compute the magnetic flux along the clutch magnetic circuit and to calculate the transmissible torque of the clutch. The optimal geometry of the clutch for maximum transmissible torque, in terms of number and dimensions of the coil sectors, was determined for each shape and coil configuration and the results were validated by finite element models

Analisi del cedimento strutturale del braccio di una gru portuale

Nel presente lavoro vengono discusse le cause del cedimento strutturale del braccio di una gruportuale a portata variabile. L’ispezione delle sezioni di rottura ha mostrato in particolare la presenza di unagrossa fessura preesistente al momento del crollo, che si estendeva per circa metà sezione di uno degli elementitubolari principali che costituiscono il traliccio del braccio.Al fine di stabilire le modalità del crollo, nel lavoro vengono presentati un modello per l’analisi dellapropagazione della fessura, che fa uso del metodo delle weight functions ed un modello per l’analisi di collassodella sezione. Le analisi condotte hanno permesso di concludere che il crollo si è verificato per collasso plasticodella sezione ed hanno messo in evidenza come la fessura in esame, per una parte considerevole della vita, siapropagata in condizioni stabili con velocità di avanzamento circa costante

Fatigue endurance of welded joints subjected to different blocks of bending and torsion loading

The fatigue strength of pipe-to-plate welded joints under bending, torsion and combined (in-phase and out-of-phase) bending and torsion has been already investigated in previous works by the authors. The specimen consisted of a pipe joined by seam welding to a plate. Both the pipe and the plate were made of S355JR steel. The test apparatus allows to apply any combination of proportional and non-proportional bending and torsion loads to the specimen. For the analysed specimens failure originated mainly from the weld root, where a severe notch is present, even if some failure from the weld toe was observed in case of bending loading. However, the crack propagation and fracture surface under bending and under torsion were significantly different. For this reason, in order to investigate any possible influence of the loading order on the fatigue endurance, the effect of different loading blocks was analysed in this work. This subject has not been widely investigated in the technical literature about welded joints. In a first series of tests, specimens were loaded in bending for a given fraction of the estimated endurance and then were loaded in torsion till failure. A similar series of tests was then conducted by varying the loading order: specimens were loaded in torsion for a given fraction of the estimated endurance, followed by a block of bending loading till failure. The whole test campaign was repeated for two different fractions of the estimated life, i.e. 0.3 and 0.45, respectively. The failure was intended as the presence of a through the thickness crack, whose presence was monitored by a drop in the internal pressure of the pipe. Results are discussed in terms of the Miner’s rule based on nominal stresses and to the cumulative damage suggested by Eurocode and IIW

Effects of plate stiffness on the fatigue resistance and failure location of pipe-to-plate welded joints under bending

A series of tests have been carried out using specimens made of a tube, having a thickness of t=10 mm, joined to a plate by fillet welding. Two different kinds of specimen were employed, differing in the plate geometry (stiffness). Both kinds of specimen were tested under bending (prevalent load) and shear loading in as welded conditions. Different initiation regions for the fatigue cracks were found and significantly different fatigue resistances were obtained for the two geometries in terms of the nominal stress approach (or in terms of applied load vs cycles to failure). Two local methods for the fatigue life assessment were then applied to independently analyse the experimental results: the fictitious notch rounding approach proposed by Radaj, which is also recommended by some international standards and the more recently proposed peak stress method, which is based on the NSIF concept. It is shown that the nominal stress method, which is by far the simplest method among those recommended in standards for analysing the joint under study, fails to explain the observed different endurances. On the other side, the methods based on local stresses account for the different joint stiffness and provide a reduced scatter in the results. However, even if local approaches, accounts for differences in the structural behaviour of the joint, the knowledge of the actual geometry of the weld need to be accounted for, in order to be able to identify the fatigue crack initiation region. For a design purpose, a safe prediction of the fatigue endurance of the joint can be obtained by all the analysed methods, if the corresponding recommended design curve is used

Analisi di stabilità di veicoli basculanti a tre ruote

Il Dipartimento di Ingegneria Civile e Industriale dell’Università di Pisa è partner del progetto europeo RESOLVE (Range of Electric SOlution for L-category VEhicles), avente come obiettivo lo sviluppo di motorizzazioni elettriche integrate, scalabili e modulari ad elevata efficienza energetica e costi contenuti da impiegare su una vasta gamma di veicoli di categoria L, comprese nuove formule di veicolo basculanti a tre/quattro ruote. Nei veicoli a più di due ruote sono presenti gradi di libertà aggiuntivi relativi al cinematismo di basculamento che condizionano la dinamica del veicolo, influenzando i modi propri e essi possono generare instabilità in determinate condizioni di marcia. In questo articolo viene presentata un’analisi numerica preliminare, in cui vengono studiati i modi propri di alcune architetture di veicoli a tre ruote attualmente in commercio, al fine di evidenziarne le differenze, con particolare riferimento al cinematismo anteriore. I modi propri di maggiore interesse sono quelli che caratterizzano anche i motocicli a 2 ruote, in particolare capsize, weave e wobble, le cui caratteristiche (parte reale ed parte immaginaria dell’autovalore) sono legate alla velocità di avanzamento e alla geometria del cinematismo utilizzato

Finite element parametric study of the influence of friction pad material and morphological characteristics on disc brake vibration phenomena

Since nowadays the NVH performance of vehicles has become an important priority, the noise radiating from brakes is considered a source of considerable passenger discomfort and dissatisfaction. Creep groan and squeal that show up with annoying vibrations and noise in specific frequency ranges are typical examples of self-excited brake vibrations caused by the stick-slip effect, the former, by the mode coupling of brake disc and friction pads or calliper, the latter. In both cases, the friction coefficient, which depends, among other factors, on the morphology of the mating surfaces and on the operating conditions, is a fundamental parameter but not the only one for the occurrence of the vibratory phenomena. Finite element complex eigenvalue parametric analyses were performed on a disc brake assembly to evaluate propensity to dynamic instability of brakes with multiple pads, as in railway brakes, as a function of the number of pads, pad shape and size, and material parameters

A multi-gap magnetorheological clutch with permanent magnet

This paper describes the design and testing of a novel permanent magnet clutch based on a magnetorheological fluid. It was inspired by a prototype previously developed by the authors and contains a novel gap shape conceived to reduce torque loss in the disengaged operating mode. Several geometries and material arrangements were investigated and the performance in terms of transmissible torque in different operating conditions was assessed using finite element numerical models. The prototype was manufactured and some experimental tests were performed. The new prototype was rated on the basis of performance indices and the design effectiveness was proven by a higher value of efficiency in the disengaged operating mode