Research program
Methods for fatigue resistance evaluation of notched structural components under multiaxial loading
University Co-ordinator
Università degli Studi di PADOVA -
TECNICA E GESTIONE DEI SISTEMI INDUSTRIALI - PADOVA(PD)
Research Unit Leader
Paolo LAZZARIN
Description
The main aim of the research program is the definition of methodologies and computational procedures for the fatigue design of mechanical components subjected to multiaxial fatigue at constant amplitude loading, both in-phase and out-of-phase, in presence of notches and stress concentration.Although the experimental program considers necessarily specimens and not real components, the methodologies developed during the project will be explicitly extended also to components of industrial interest like transmission shafts for electrical devices and submergible pumps as well as welded joints.The expected activity could be summarised in the following phases:A. Bibliographic investigation on the fatigue strength of components subjected to multiaxial fatigue. The extension of a fatigue strength database already available by the Unit and its diffusion to the other partners is the main objective of this phase. The improved database will represent an essential tool for the validation of the models and computational procedures. B. With reference to blunt notches, high cycle fatigue life assessments will be performed by using the Carpinteri and the Susmel-Lazzarin methods, clarifying validity range and degree of accuracy.C. Extension of the criteria analysed in the previous point from high cycle fatigue (closed to or coincident with the endurance fatigue limit) to finite life prediction, in co-operations with other two Units.D. Extension of the expressions for stress fields due to rounded V-.shaped notches from plates (Filippi, Lazzarin e Tovo, 2002) to axis-symmetric geometries. E. Extension of the energetic approach already developed for plane problems (Lazzarin, Zambardi, Int. J. Fracture, 2001) to 3-dimensional components. The method will be validated on the basis of a large number of experimental data, related to the fatigue strength of steel and light alloy tubular welded joints, already reported in the literature.F. Comparison among the energetic approach, based on a combination of the stress intensity factors of Mode I, II and III, developed in the previous point, a criterion based on an extension of bi-parametric Woehler curve by Susmel-Lazzarin (2002), developed by the Ferrara Unit and the Point and Line criteria reported in the literature (Taylor, 1999) for uniaxial fatigue.G. Experimental evaluation of the fatigue strength under constant amplitude loading of axis-symmetric specimens made with two types of steel for transmission shafts (C40 steel, AISI 416 steel). The experimental activity will concern both plain and notched specimens, the latter containing circumferential V-shaped notches with very small root radius (less than 0.5 mm). The tests will be carried out under uniaxial tensile loading, torsional loading and their combination both in-phase and out-of-phase. The test will be carried our under two nominal load ratios (R=-1 and R=0), keeping constant the biaxiality ratio. It is therefore expected to obtain 16 series of fatigue tests. The influence of the biaxiality ratio will be investigated by means of two additional series of specimens, under in-phase conditions and R=-1. Particular attention will be paid to the analysis of nucleation and propagation phases of the so called "technical crack" in the different testing conditions. The crack initiation sites and modes as well as the fracture surface features will be documented by optic and scanning electron microscopy.H. The new experimental data will be made available to the other research Units to allow the comparative evaluation of the predictions given by the different methods developed during the research program.The research activities will developed according to the following planning:1. Bibliographic investigation (1-3 months);2. Comparison of the high cycle life prediction given by the criteria already presented in the literature by some researchers of the project, in respect of smooth blunted notched specimens (2-4 months);3. Final choice of the materials and geometries of the specimens for the experimental program, specimen manufacturing (1-4 months);4. Multiaxial fatigue characterisation of the specimens (5-18 months);5. Development of an energetic criterion for fatigue strength prediction of components with sharp notches and subjected to multiaxial fatigue (5-12 months);6. Validation and comparison of the different models developed by the Units and extension to real components (19-24 months).
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