Research program
Methods for fatigue resistance evaluation of notched structural components under multiaxial loading
University Co-ordinator
Università degli Studi di TRENTO -
INGEGNERIA DEI MATERIALI E TECNOLOGIE INDUSTRIALI - TRENTO(TN)
Research Unit Leader
Vigilio FONTANARI
Description
The program of the RU (Research Unit) will be focused on the definition as weel as on the validation of computational methods for studying the first propagation phases of cracks emanating from sharp notches. The activity is complementary with respect to the principal objectives of the other research groups.The approach that is intended to be used is based on the application of the weight function (WF) method for determining the fracture mechanics parameters necessary for the analysis of the fatigue crack subjected to completely general loading histories. Starting point for the activity will be the results previously obtained by the development of a matrix like WF for oblique edge cracks in unnotched components by which the conditions of mixed crack propagation mode in dependence of the principal geometrical parameters of the problem were studied. The main objective is to apply this computational technique for facing the problem of a semiplane with a sharp notch having different depth and angular aperture, carrying at his root a crack with variable length and inclination. For the definition of the WF an accurate numerical modelling will be carried out by means of finite element modelling (FEM) of the problem under reference loading conditions. In this way, a data base of SIFs values will be built up by varying the geometrical parameters of the problem and used for the numerical evaluation of the WF. The WF will be validated by comparison with the results of the FEM for loading conditions independent of the reference ones used for the WF set up. The WF will be then applied for studying the fatigue crack behaviour, by considering the results of experiments found in the literature or derived from the experimentation carried out by the other research unities. A specific experimentation will be also performed on notched specimens differently loaded to induce conditions of mixed mode of propagation. The research program can be subdivided into four principal phases:1) In the preliminary phase, the analysis of the state of the art will be completed, the sequence of the research steps defined, the experimental analysis for the WF validation will discussed with the others research unities. 2) The second operative phase will deal with the set up of the WF.3) The research will go on with the validation of the accuracy and of the validity of the proposed WF by means of comparison with the results of both FE analyses and experimental tests4) The final report of the obtained results will conclude the proposed research activity.PROGRAM AND TASKSI phase (months 1-3): preliminary activitya) The current knowledge of the state of the art will be completed not only as regard the condition of crack initiation and of the first propagation of fatigue cracks at sharp notches, but principally for a critical analysis of the applicability of the main numerical and theoretical methods, proposed for the study of crack emanating from notches, that are based on the fracture mechanics approach .b) the results of this analysis should allow to focus the attention on the research topics that will be faced and in particular will enable to identify the mathematical models to be used for the WF set up and to select the experimental conditions for the subsequent validation of the WF.II phase (months 4-12): numerical modelling and definition of the WFa) Set up of the parametric FE model of the crack in a sharp notched body, taking account of the geometrical features controlling the problem. The model will be validated by means of an accurate convergence analysis performed on problems with a known solution. (e.g. a symmetrical configuration with a crack loaded to have mode I of opening)b) the model will then be used with the reference loading conditions for building up the data base of SIF necessary for the set up of the WF. In this way, the SIF values will be represented as a function of the principal geometrical parameters of the problem. In particular the effect of the notch angle, of its depth with respect to the crack length and inclination will be studied.c) the WF will be formulated on the basis of suitable mathematical functions able to efficiently represent the results. These functions will be calibrated, by means of the numerical results, trough a regression algorithm.III phase (months 13-20): validation of the WF and application to the analysis of experimental resultsa) the WF will be firstly verified by numerical analysis, considering the FEM results for loading conditions chosen independently of the ones used to define the WF. Completely general loading conditions will be considered in order to define the limits of applicability for the WF, with particular attention to its suitability for studying multiaxial fatigue problems.b) the WF will be then adopted for the interpretation of the experimental results. First of all results obtained from the literature will be considered. At the same time the results of an experimentation performed by this research unit on notched specimens under appropriate loading conditions, inducing different mixed modes of fracture will be used. The metallic materials for the experimental testing have to be selected in cooperation with the other units. For this activity, the devices set up for the in situ observation of the fatigue cracks will be used, which have been yet efficiently adopted for studying the initiation and propagation of fatigue cracks from notches in titanium alloys (cfr. Publication of the research proposers) It is also intended to apply the WF for the analysis of the results obtained by the other units from multiaxial fatigue testing.IV phase (months 20-24): final report of the activitya) in this phase the verification of the WF will be completed and the software built up for the analysis will be documented, for an user friendly application of the WF .b) a final report of the whole activities will be produced, including the bibliographic analysis, the numerical modelling, the computational algorithms adopted and the experimental validation of the WF.
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