Evaluation of the stress–strain curve of metallic materials by spherical indentation

AbstractA method for deducing the stress–strain uniaxial properties of metallic materials from instrumented spherical indentation is presented along with an experimental verification.An extensive finite element parametric analysis of the spherical indentation was performed in order to generate a database of load vs. depth of penetration curves for classes of materials selected in order to represent the metals commonly employed in structural applications. The stress–strain curves of the materials were represented with three parameters: the Young modulus for the elastic regime, the stress of proportionality limit and the strain-hardening coefficient for the elastic–plastic regime.The indentation curves simulated by the finite element analyses were fitted in order to obtain a continuous function which can produce accurate load vs. depth curves for any combination of the constitutive elastic–plastic parameters. On the basis of this continuous function, an optimization algorithm was then employed to deduce the material elastic–plastic parameters and the related stress–strain curve when the measured load vs. depth curve is available by an instrumented spherical indentation test.The proposed method was verified by comparing the predicted stress–strain curves with those directly measured for several metallic alloys having different mechanical properties.This result confirms the possibility to deduce the complete stress–strain curve of a metal alloy with good accuracy by a properly conducted instrumented spherical indentation test and a suitable interpretation technique of the measured quantities

Surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes

The rolling contact fatigue is distinguished into subsurface initiated (spalling and case crushing) and surface initiated (pitting and micropitting). A characteristic depth was identified for each of these mechanism. The characteristic depth of the case crushing is the hardening depth, while for the spalling it is the maximum cyclic shear stress depth. The pitting depth is the size of the crack for which the mode I stress intensity factor range, due to the fluid pressurization, is higher than the threshold. This depth can be similar to the micropitting depth, in the order of 10 µm, for heavily loaded small radius contacts. Rolling contact fatigue cyclic shear stress indexes are then defined on the basis of the characteristic depths, and they identify the load intensity of each rolling contact fatigue mechanism. The characteristic depths and the stress index approach can be used to relate specific tests to component design, without any size effect misinterpretation

Integral method coefficients for the ring-core technique to evaluate non-uniform residual stresses

The ring-core technique allows for the determination of non-uniform residual stresses from the surface up to relatively higher depths as compared to the hole-drilling technique. The integral method, which is usually applied to hole-drilling, can also be used for elaborating the results of the ring-core test since these two experimental techniques share the axisymmetric geometry and the 0°–45°–90° layout of the strain gage rosette. The aim of this article is to provide accurate coefficients which can be used for evaluating the residual stress distribution by the ring-core integral method. The coefficients have been obtained by elaborating the results of a very refined plane harmonic axisymmetric finite element model and verified with an independent three-dimensional model. The coefficients for small depth steps were initially provided, and then the values for multiple integer step depths were also derived by manipulating the high-resolution coefficient matrices, thus showing how the present results can be practically used for obtaining the residual stresses according to different depth sequences, even non-uniform. This analysis also allowed the evaluation of the eccentricity effect which turned out to be negligible due to the symmetry of the problem. An applicative example was reported in which the input of the experimentally measured relaxed strains was elaborated with different depth resolutions, and the obtained residual stress distributions were compared

Comportamento a fatica di strutture meccaniche in piena scala: risultati sperimentali e previsioni

Il lavoro si propone di presentare le principali attività di ricerca svolte, negli ultimi anni, presso il Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione (DIMNP) dell’Università di Pisa, anche in collaborazione con l’Università di Trento, nel campo della resistenza a fatica delle strutture meccaniche, in particolare per quanto riguarda la conduzione di “test” su componenti in piena scala e la loro interpretazione. Viene quindi condotta un’illustrazione di alcune recenti campagne sperimentali (Es.: giunzioni filettate in acciaio, elementi di sospensione in alluminio, ingranaggi ad elevate prestazioni), alla quale segue una descrizione delle attività di caratterizzazione di base e di modellazione condotte al fine di costituire una adeguata base di conoscenze per la interpretazione. Infine, vengono analizzati i risultati ottenuti, evidenziando alcuni problemi aperti, sia sul piano concettuale che su quello applicativo

Modello di tenuta della flangia bullonata, senza guarnizione, mediante l'analogia della meccanica della frattura di una fessura parzialmente aperta

I compressori centrifughi di elevate dimensioni non permettono l'utilizzo di guarnizioni deformabili, per cui le due meta della flangia di connessione sono forzate mediante bullonatura e la tenuta e affidata al contatto completo delle due superfici. La previsione della pressione di perdita e un aspetto di progetto di notevole interesse per questa tecnologia. L'azione della pressione interna sollecita la separazione delle superfici della flangia, che invece e contrastata dall'azione di serraggio dei bulloni. Il presente lavoro propone un modello per prevedere la condizione di perdita, basato sulla meccanica della frattura. Dato che le due superfici della flangia sono semplicemente a contatto, esse costituiscono una vera e propria fessura parzialmente aperta. Come ben noto il fattore di intensificazione di una fessura parzialmente aperta e nullo. Imponendo che le due superfici siano parzialmente separate ad una distanza fino al bordo del foro del bullone (che offre un canale di fuoriuscita per il fluido in pressione), e imponendo la condizione di fattore di intensificazione nullo, e possibile determinare la pressione di perdita, analiticamente, mediante la tecnica delle "weight functions" (o "funzioni peso"). Il presente lavoro riporta una positiva validazione del modello proposto mediante sia simulazione numerica sia risultati sperimentali in piena scala e in scala ridotta. Il modello analitico proposto offre uno strumento di progetto di immediata implementazione per comparare diverse geometrie di flangia bullonata

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

Bacterial Faecal Flora in Healthy Women of Different Ages

The composition of the intestinal flora is the result of host physiology, microbial interaction and environmental influences. The possible relationship between faecal flora composition and hormonal modifications in healthy women of different ages was studied. Forty-four normal women were divided into the following groups according to age: group I, 2747 yr; group II, 50-55 yr, 5 yr after menopause. The subjects received no pharmacological treatment. Samples were collected on the 8th and 23rd day of the cycle; two samples were obtained from each postmenopausal woman. Qualitative and quantitative determination of microorganisms was carried out using slightly modified standard methods. In fertile women (group I), the microflora composition was similar for samples collected on the 8th-10th day and during the premenstrual period (23rd day). In postmenopausal women (group III), an increase in fungi, clostridia and aerobic lactobacilli mean concentrations were observed. Escherichia coli mean levels increased and Enterobacteriaceae such as Enterobacter cloacae and Citrobacter freundii were present in 80 per cent of subjects studied. The length of menopause was found to have only a slight influence on flora: the behaviour of the microflora composition in menopausal women in group II may be considered intermediate between groups I and III. This preliminary study demonstrates that there are fluctuations in the composition of the faecal flora in healthy women. The differences observed between premenopausal and postmenopausal women may be a consequence of modifications of the steroid sex hormone pattern.Keywords - Intestinal flora; Age; Premenopausal women; Postmenopausal women; Menopause

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

Funzione peso per una fessura laterale inclinata e deflessa

In questo lavoro viene presentato un metodo per determinare i fattori di intensità degli sforzi (stress intensity factors SIF) per una fessura di bordo inclinata e deflessa in un semipiano. Il problema è stato affrontato mediante un’analisi parametrica ad elementi finiti che ha permesso di studiare l’influenza dei principali parametri che governano il problema: l’angolo d’inclinazione iniziale, l’angolo di deflessione del segmento terminale della fessura ed il rapporto tra le lunghezze dei due segmenti di fessura. Sulla base dei risultati numerici è stata definita una Funzione Peso (Weight Function WF) con struttura matriciale, estendendo una tecnica mista analitico-numerica già sviluppata per lo studio di fessure di bordo inclinate. La correttezza della WF è stata verificata mediante confronto con i risultati numerici ottenuti per condizioni di carico indipendenti. La WF può essere usata per calcolare i SIF per fessure aventi inclinazione iniziale compresa tra 0° e 60 °, angolo di deflessione rispetto al segmento iniziale tra -90° e + 90° e rapporto di lunghezza tra i segmenti iniziale e terminale compreso tra 0.005 e 0.1

Bacterial faecal flora in healthy women of different ages.

The composition of the intestinal flora is the result of host physiology, microbial interaction and environmental influences. The possible relationship between faecal flora composition and hormonal modifications in healthy women of different ages was studied. Forty-four normal women were divided into the following groups according to age: group I, 27–41 yr; group II, 50–55 yr, 5 yr after menopause. The subjects received no pharmacological treatment. Samples were collected on the 8th and 23rd day of the cycle; two samples were obtained from each postmenopausal woman. Qualitative and quantitative determination of microorganisms was carried out using slightly modified standard methods. In fertile women (group I), the microflora composition was similar for samples collected on the 8th—10th day and during the premenstrual period (23rd day). In postmenopausal women (group III), an increase in fungi, clostridia and aerobic lactobacilli mean concentrations were observed. Escherichia coli mean levels increased and Enterobacteriaceae such as Enterobacter cloacae and Citrobacter freundii were present in 80 per cent of subjects studied. The length of menopause was found to have only a slight influence on flora: the behaviour of the microflora composition in menopausal women in group II may be considered intermediate between groups I and III. This preliminary study demonstrates that there are fluctuations in the composition of the faecal flora in healthy women. The differences observed between premenopausal and postmenopausal women may be a consequence of modifications of the steroid sex hormone pattern