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

development of a simplified model for the vibration analysis of lawn mowers

Abstract The vibrational behavior of vehicles is a crucial issue for the comfort, especially for the professional vehicles. This paper presents a simplified modelling approach for studying the vibrational behavior of a lawn tractor. The vibrational response of a real vehicle is analyzed by an extensive experimental modal analysis and Finite Element model (FE) simulating the modal behavior of the whole tractor. The FEM was then validated by the comparison with the experimental results and then used for identifying the components and connections effectively driving the modal response. Based on these results, a simplified Multi-Body (MB) model, able to reproduce the vibrational response of the studied lawn mower, was then setup, showing good correspondences with experimental results. General guidelines for defining effective vehicles Multi-Body modal models were also derived

experimental study of hydrogen embrittlement in maraging steels

Abstract This research activity aims at investigating the hydrogen embrittlement of Maraging steels in connection to real sudden failures of some of the suspension blades of the Virgo Project experimental apparatus. Some of them failed after 15 years of service in working conditions. Typically, in the Virgo detector, blades are loaded up to 50-60% of the material yield strength. For a deeper understanding of the failure, the relationship between hydrogen concentration and mechanical properties of the material, have been investigated with specimens prepared in order to simulate blade working conditions. A mechanical characterization of the material has been carried out by standard tensile testing in order to establish the effect of hydrogen content on the material strength. Further experimental activity was executed in order to characterize the fracture surface and to measure the hydrogen content. Finally, some of the failed blades have been analyzed in DICI-UNIPI laboratory. The experimental results show that the blades failure can be related with the hydrogen embrittlement phenomenon

High temperature fatigue testing of gas turbine blades

Abstract With the increasing use of renewable energy sources, Gas Turbines (GTs) are currently required to accomplish more flexible operations for supplying the back-up energy. As a result, thermo-mechanical fatigue issues in the GTs components are emphasized. In this paper, the design of a novel rig for assessing the fatigue behavior in the trailing edge of full scale GTs blades is presented. Based on a detailed Finite Element (FE) analysis of the blade response under thermo-mechanical loads, it is demonstrated that the stress and strain cycles arising in this area during a start-up/shut-down transient can be accurately reproduced by clamping the blade in the shank zone and applying a transversal load to the trailing edge. It is also shown that the stress/strain states can be obtained using a Test Article (TA) extracted from the actual blade. In this configuration, the load magnitude and direction, and the distance of the application point from the blade platform are the test control parameters. A FE model simulating the TA test is developed to determine the test parameters. A tooling for clamping and loading the TA is finally proposed along with a rig apparatus consisting of standard equipment used in material testing

Sviluppo di un modello per lo studio delle azioni sulle dentature di rotismi epicicloidali impiegati in campo aeronautico

Scopo di questo lavoro è lo sviluppo di un modello a parametri concentrati di un riduttore planetario per la stima delle forze agenti sulla dentatura e lo studio delle condizioni in cui si verifica la perdita di contatto e/o l’incuneamento dei denti delle ruote. Diversamente da altri lavori esistenti in letteratura, che analizzano il comportamento dei riduttori in presenza di carichi di coppia ed errori di montaggio, nel seguente lavoro è stata svolta un analisi statica che prende in considerazione il disallineamento delle strutture di supporto dei componenti del riduttore. Il modello sviluppato è stato implementato in un programma in ambiente Matlab (c). Tale programma è in grado di stimare il carico agente su ogni satellite, anche in presenza di fenomeni di separazione tra i denti. Il software valuta, inoltre, le condizioni di funzionamento per le quali si ha contatto su entrambi i fianchi del dente

Mechanical characterization of metallic materials by instrumented spherical indentation

The mechanical characterization of metallic materials by instrumented spherical indentation testing is addressed. To this purpose, a Finite Element model (FE) able to simulate spherical indentation tests was developed and indentation responses corresponding to different material models were explored. Load-indentation depth curves (L-h curves), crater profile and plastic strains evolutions in the sub-indenter region were analysed. The role of friction was also accounted for. Since friction conditions between contacting bodies cannot be a priori established, a suitable range of friction coefficients (0.0-0.5) was considered. The FE model was validated by comparison with experimental spherical indentations carried out on two different materials (Al 6082-T6, AlSI H13) and a good agreement between the experimental results and numerical predictions was found. No friction effects were observed on material L-h curves, whereas modifications were found in crater profile and plastic strains evolutions. It was established that the stress-strain curve evaluation on the basis either of the strain field in the sub-indenter region or the residual crater geometry seems particularly critical whereas it appears more reliable to deduce the constitutive properties by a proper analysis of the L-h curve

Numerical simulation of residual stress relaxation in shot peened high-strength aluminum alloys under reverse bending fatigue

The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on speci-mens subjected to three different shot peening treatments and tested under reverse bend-ing fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing com-bined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely, the residual stress field remains stable at load levels corresponding to the 5x10^6 cycles fatigue endurance