RICERCA ITALIANA     

Research program

Methods for fatigue resistance evaluation of notched structural components under multiaxial loading

University Co-ordinator

Università degli Studi di FERRARA - INGEGNERIA - FERRARA(FE)

Research Unit Leader

Paolo LIVIERI

Description

The state of the art, as discussed at item 2.4, suggests that the problem of multiaxial notch fatigue has to be studied by starting from experimental analyses in order to correctly understand the real crack initiation and growth mechanisms in the presence of notches subjected to multiaxial loading.The methods based on the elastic-plastic stress and strain estimation at the notch tip seem not to be suitable for facing correctly the considered problem and, in particular, this holds true in the high cycle fatigue regime. By starting from this preliminary considerations, two different approaches can be employed to perform correctly the multiaxial fatigue assessment: methods based on the use of nominal stresses and the development of methodologies founded on the N-SIFs calculation.The planed program can be summarised as follows:A. The activity of the Research Unit will be initially focused on performing experimental analyses. Tests will conduce by using sharply notched specimens subjected to loads capable of generating multiaxial stress states at the notch tip. In particular, tests will be performed on V-notched specimens having a notch root radius value negligible if compared to the other notch dimensions. Specimens will be made of conventional low-carbon steels. In order to properly characterise the material from the static and fatigue point of view, tests will be done by following the standard procedures. Subsequently, the experimental activities will be performed on welded connections of different geometry and made both of steel and aluminium. Welded joints will be subjected to both in-phase and out-of-phase multiaxial loadings.B. The initial theoretical activities will be focused on the employment of the modified Wöhler curve method in the fatigue assessment of sharply notched components. The objective of this activity will be the extension of the classical fatigue notch factor Kf to the multiaxial fatigue field, together with the use of nominal stresses. The accuracy of this approach in the multiaxial fatigue life estimation will be evaluated by using data taken both from the literature and generated by the Research Unit.C. After having validated this approach, the theoretical study will be focused on the extension of this method to the fatigue assessment of welded joints. The objective of this activity will be the validation of this method in the presence of welds, when it is calibrated by using the fatigue curves suggested by standard codes and it is applied in terms of nominal stresses.D. In order to increase the precision in the multiaxial fatigue life estimation of sharply notched components subjected to multiaxial loading, the research activities will be focused on the N-SIFs calculation in the presence of mixed mode loadings. This study will be performed by means of FE analyses. The final aim will be the definition of formulations capable of giving N-SIFs values for standard geometries as the joint dimensions change. In particular, the study of N-SIF applicability will be focused on those situations where components are subjected to pure Mode III loadings. The validation of this procedure will be performed by using data taken both from the literature and generated by the Research Unit.E. By observing the precision obtained by employing N-SIFS based methods in the uniaxial fatigue assessment, the last part of the research activity will be focused on the development of numerical methods capable of estimating N-SIFs values in structures having a complex geometry. Finally, the applicability of these procedures to welds subjected to multiaxial loadings will be evaluated. Aim of this research activity will be the simplification of N-SIF approaches in order to easily extend their use to an industrial ambit by giving numerical algorithms capable of estimating automatically the N-SIFs values by employing directly linear-elastic FE results.In order to obtain the planed results, the research program will be organised as follows:1. Study of state of the art (1-3 months);2. Material choice, specimen geometries definition and specimen manufacture (2-5 months);3. Multiaxial fatigue tests carrying out (6-20 months);4. Modified Wöhler curve method extension to notched components in order to define the multiaxial fatigue notch factor (6-12 months);5. Modified Wöhler curve method extension to welded connections by using nominal stresses (6-12 months);6. N-Sifs calculation of welded connections subjected to multiaxial loadings (9-15 months);7. Evaluation of N-SIF applicability in the presence of Mode III loadings for the fatigue life estimation under torsional loadings (12-20 months);8. Development of methods for N-SIF estimation in real components subjected to multiaxial loadings (16-24 months).