Papers by Leonardo Bertini
The Journal of Strain Analysis for Engineering Design, 1995
The elastic-plastic stress-strain field produced in a thin plate under equibiaxial loading during... more The elastic-plastic stress-strain field produced in a thin plate under equibiaxial loading during the operation of drilling a through-thickness hole was analysed. The plane stress condition and Tresca yield criterion were assumed for an elastic-perfectly plastic material. These hypotheses allowed the problem to be formulated by means of ordinary differential equations which were analytically solved. Within these particular conditions, the results gave indications of the effect produced by plasticity on the widely used hole-drilling method for residual stress measurement. In particular, the equality of final results with those obtained for a plate with a pre-existing hole loaded in an elastic-plastic regime gives useful indications for the experimental analysis of the effect of plasticity on residual stress measurement.
Fatigue & Fracture of Engineering Materials & Structures, 1997
ABSTRACT
Strain, 2010
Residual stresses play a fundamental role in predicting the resistance of several structural comp... more Residual stresses play a fundamental role in predicting the resistance of several structural components, in particular when operating under alternating loads or in aggressive environments. Among the measurement techniques, the hole-drilling method is unrivalled in terms of cost and flexibility. The currently available data reduction techniques do not account for some important parameters, in particular the hole eccentricity, and can be directly used only with certain rosettes or require the partial holes to be drilled at fixed depths. This paper presents a general approach for interpreting hole-drilling measurements that includes the hole eccentricity and several other geometric and material parameters. The method is based on analytical influence functions relating the measured relieved strains to the residual stress by means of integral equations. By elaborating the results of accurate finite-element simulations, continuous analytical influence functions are produced, which can be used to reduce the error in the reconstructed residual stress. We focus on the definition of the influence functions, their evaluation and assessment.
Fatigue & Fracture of Engineering Materials and Structures, 1994
The paper discusses the results of fatigue crack growth rate tests conducted in the presence of r... more The paper discusses the results of fatigue crack growth rate tests conducted in the presence of residual stresses. Three different residual stress distributions, obtained by laser welds, were employed in order to characterize the crack propagation behaviour under different conditions, producing either an increase or a reduction of the stress intensity factor due to external loads. Test results are analysed by means of a non-linear numerical model (based on the weight function method) and a knowledge of the fatigue crack growth properties of the base material, free from residual stresses. The results of the analysis are discussed with reference to experimental trends, in order to clarify the predictive capabilities of the method and aspects needing further investigation. NOMENCLATURE a = crack length in CT specimen, measured from the load line K = stress intensity factor N = elapsed fatigue cycles R = ratio between minimum and maximum stress intensity factor in the fatigue cycle AK = stress intensity factor range max, min = maximum and minimum values in the fatigue cycle, respectively
Meccanica, 2008
The effect of notch geometry on the propagation of fatigue cracks emanating from sharp V-shaped n... more The effect of notch geometry on the propagation of fatigue cracks emanating from sharp V-shaped notches is investigated by means of an experimental campaign performed on Al-7075-T651 specimens carrying notches with opening angles of 45°, 90°, and 135°. The samples were tested using a servohydraulic machine under different loading directions and at several loading levels. The crack deflection induced by the variation in loading direction was determined my measuring the kinking angle and by studying the crack propagation plane through fractographic analysis. A linear elastic fracture mechanics approach was adopted for the analysis of experimental results. Stress intensity factors were calculated using an appropriate weight function set up for studying inclined edge cracks emanating from sharp V-notches. The influence of K II on the crack propagation was discussed on the basis of theoretical and semi empirical models.
Engineering Fracture Mechanics, 2008
A procedure for evaluating the fracture mechanics parameters of a subsurface two-dimensional crac... more A procedure for evaluating the fracture mechanics parameters of a subsurface two-dimensional crack parallel to the boundary in an elastic half plane is presented. A Weight Function (WF) with a matrix structure is proposed, to account for the coupling effects between modes I and II, typical in non-symmetrical problems. In order to face any loading condition, the WF was formulated
Journal of Strain Analysis for Engineering Design, 1995
The elastic-plastic stress-strain field produced in a thin plate under equibiaxial loading during... more The elastic-plastic stress-strain field produced in a thin plate under equibiaxial loading during the operation of drilling a through-thickness hole was analysed. The plane stress condition and Tresca yield criterion were assumed for an elastic-perfectly plastic material. These hypotheses allowed the problem to be formulated by means of ordinary differential equations which were analytically solved. Within these particular conditions, the results gave indications of the effect produced by plasticity on the widely used hole-drilling method for residual stress measurement. In particular, the equality of final results with those obtained for a plate with a pre-existing hole loaded in an elastic-plastic regime gives useful indications for the experimental analysis of the effect of plasticity on residual stress measurement.
International Journal of Fracture, 1999
The matrix-like structure of the Weight Function (WF) for determining the Stress Intensity Factor... more The matrix-like structure of the Weight Function (WF) for determining the Stress Intensity Factors (SIFs) in a nonsymmetric plane body is obtained from the general properties of the elastic field. General asymptotic and symmetric properties of the WF are discussed. By extending a previously proposed methodology, an analytical approximate WF is determined for an edge crack in a semiplane within
An application of the Weight Function (WF) method for studying the problem of an edge crack incli... more An application of the Weight Function (WF) method for studying the problem of an edge crack inclined with respect to the external surface is presented. Starting form a matrix like structure of the WFs, an analytical expression of the Green functions, giving the Crack Opening Displacement (COD) components for the crack under general loading conditions was obtained. These formulations was adopted for facing the problem of a travelling load applied on the boundary of a semi-plane carrying an inclined edge crack. By means of an iterative procedure, the closed portions of the crack and the effects exerted by the normal and friction contact forces on COD and on the effective stress intensity factors (SIFs) K I and K II were evaluated. Considering different friction conditions in the sliding crack surface, the evolution of K I and K II was analysed at different position of the load.
International Journal of Fatigue, 2015
ABSTRACT
Key Engineering Materials, 2007
Composite Structures, 1991
This paper illustrates the general organization and the preliminary results of a comprehensive re... more This paper illustrates the general organization and the preliminary results of a comprehensive research program aimed at the characterization of fatigue delamination growth in laminated carboresin composites. The study included two types of test, conducted to determine both the basic material properties (under single fracture mode conditions) and the behavior of a simple structural component, i.e., a laminated plate specimen with a central hole. Results obtained in the two types of test are presented and discussed, with reference to the problem of predicting the behavior of actual laminated components on the basis of the results of commonly employed laboratory tests.
Materials Science Forum, 2000
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any fo... more All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 66.249.66.204-28/04/11,23:02:13) ... Residual Stress ECRS 5 doi:10.4028/www.scientific. ...
Surface and Coatings Technology, 2014
ABSTRACT The residual stress distribution induced by deep rolling depends on several factors: the... more ABSTRACT The residual stress distribution induced by deep rolling depends on several factors: the material elasto-plastic curve, roller shape, indentation force and rolling feed. Among these, the force and the feed are those parameters that can be easily handled without cost or layout modifications. This paper shows an experimental investigation about these two parameters and a comparative analysis on the obtained residual stress profiles. The deep rolling treatment was performed on aluminium alloy 7075-T6 samples and the used tool was a carbide roller with conical and rounded contact. The residual stresses were measured by combining the hole drilling method and the X-ray diffraction technique. A first evident result was the large difference between the two principal residual stress components. The feed direction residual stress was almost a factor of two larger than the rolling direction residual stress. Parameter trends on residual stress distributions were investigated. The depth of the compressive region increased with the rolling force and the maximum stress position also tended to be subsurface, while for lower loads the maxima were at the surface. On the other hand, the feed parameter did not produce any effect at large depth, and just the initial subsurface distribution was slightly influenced. However, the surface hardness was noticeably affected by the feed, while the rolling force had a less predominant role. Finite element simulations were also carried out and reported in the paper, mainly to obtain information on induced work hardening. The plasticity depth was only affected by the load, indeed it was very similar to the compressive residual stress depth, while the maximum accumulated plasticity was significantly increased by the feed.
In the present paper, two techniques for assembling aluminum drill pipe to steel tool joint were ... more In the present paper, two techniques for assembling aluminum drill pipe to steel tool joint were investigated: 'hot' assembling and 'cold' assembling. During hot assembling a difference of temperature between the two parts is exploited to impose contact interferences and then friction bond generates torsional strength. Hot assembling is ordinarily used. Cold assembling technique is here presented as an alternative. During cold assembling connection make up, the contact interferences are obtained just imposing an external torque. A clear advantage of cold assembling is not to expose the aluminum alloy to high temperature, that can be a risk for the aluminum alloy microstructure. However, this technique requires a proper lubricating compound between the mating surfaces in order to reduce the torque required to make up the connection. After lubricating compound curing, the coefficient of static friction rises and high torsional strength of the connection is obtained...
Surface and Coatings Technology, 1998
ABSTRACT
Sommario Nel presente lavoro vengono analizzate le problematiche di misura del metodo del foro, i... more Sommario Nel presente lavoro vengono analizzate le problematiche di misura del metodo del foro, in particolare relative alla valutazione dello stato di tensione ad una profondità inferiore a 100 μm. Il metodo del foro non può misurare correttamente lo stato di tensione oltre un valore di profondità circa pari al 50% del diametro del foro stesso, dato che lo stato di tensione rilassato ha una bassa correlazione con le deformazioni misurate in superficie. Al contrario, sarebbe lecito attendersi una misura accurata a bassa profondità. Nel presente lavoro si mostra sperimentalmente, che nonostante l'elevata sensibilità del segnale estensimetrico, intervengono altri fattori che sono causa di forti errori di misura, in particolare l'incertezza dell'effettiva profondità dei primi passi di foratura dovuta alla non planarità della superficie di fondo foro generata della superficie inferiore del tagliente della fresa, e all'eventuale non perfetta perpendicolarità dell'...
Materials Letters, 2007
The aim of the present paper is to propose an innovative hydrogenation method based on metal hydr... more The aim of the present paper is to propose an innovative hydrogenation method based on metal hydride dissociation.By heating metal hydride, high pressure of molecular hydrogen is generated, which can be used to promote diffusion through the steel, resulting in very high atomic hydrogen concentration.This new hydrogenation method was tested on a F82H steel, a Reduced Activation Ferritic/Martensitic (RAFM) steel developed for nuclear fusion application, where high temperature Hydrogen Embrittlement (HE) is a matter of serious concern.The main goal of this study is to propose a methodology to perform tensile tests on steel at high temperature and high hydrogen contents.The technique is easy and cheap. Furthermore, by using different metal hydrides and tuning temperatures, different hydrogen concentrations can be tested.
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Papers by Leonardo Bertini