RICERCA ITALIANA     

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

Università degli Studi di Parma

Abstract

The proposed Research Project aims at developing design and assessment methodologies for civil and mechanical structural components with severe notches under multiaxial fatigue loading. In particular, constant and variable amplitude multiaxial loading, both in-phase and out-of-phase, will be considered.

The life prediction models which will be developed by the Research Units will average either the stresses or the strain energy in a material volume located in the most highly stressed zone. These theoretical models will represent an extension of what has already been proposed by some of the researchers involved in the present Project. The theoretical predictions will be compared with experimental data available in the literature or obtained from tests performed by some of the Research Units. As is well-known, the root of a notch is a preferential site for initiation and growth of a fatigue defect. Therefore, it is essential to analyse the effect of the multiaxial fatigue loading on crack initiation, crack growth and fatigue resistance of the notched structural components being examined. Such phenomena will be analysed by employing fatigue and fracture mechanics concepts. For medium/low-cycle fatigue (finite life), the notch induces stress components that, for strain hardening materials, can overcome the yield stress. In those conditions, the values of the local loading ratio R are not much lower than the unity. As a consequence, fatigue strength could be >>>

Principal Investigator

Andrea CARPINTERI Università degli Studi di PARMA

Research Objectives

The proposed Research Project aims at developing design and assessment methodologies for civil and mechanical structural components with severe notches under multiaxial fatigue loading.

As a first step, the Research Project aims at collecting information available in the literature related to the design methodologies of notched structural components subjected to multiaxial fatigue. In such a way, it will be increased the amount of data available by the researchers involved in this Project.

Several contributions on this subject have recently been presented at the international scientific community by the above researchers. In particular, high-cycle fatigue (close to fatigue limit conditions) has been analysed. Moreover, the criterion by Susmel and Lazzarin and that by Carpinteri et al. (see Section 2.2 below) have already been systematically verified by using several experimental data available in the literature, related to smooth or blunt-notched specimens.

In the presence of stress concentration due to severe notches, the above criteria need some "ad hoc" modifications. As a matter of fact, the fatigue behaviour does not follow a point criterion, but it is rather controlled by what occurs in a small but finite volume of material surrounding the most highly stressed zone. The Research Project aims at also extending the two above criteria from high-cycle fatigue to finite life, i.e. medium/low-cycle fatigue, by highlighting their >>>

First Results

Updating of the state of the art in the field of the Research Project, improvement of the available database. The validity range and the degree of accuracy of some life prediction methods already available for high-cycle fatigue will be examined. Planning of the research activities of the Research Units involved in the present Project.Strength and life prediction models for multiaxial fatigue in the presence of notches and stress concentrations as well as for finite life. Additional multiaxial fatigue data for the different structural components and loading conditions analysed. Results obtained from the numerical analyses performed.Validity range of the developed models and degree of accuracy in life and strength predictions. Procedures for the application of the models to complex civil and mechanical structural components. Final reports of the Research Project.

Timescale

24 months

National and international background

Since civil and mechanical structural components frequently work under multiaxial fatigue loading, the problem of the multiaxial fatigue assessment has long been investigated by several researchers [1]. The analysis of the state of the art shows that the approaches vary mainly as a function of the fatigue life, and are different for low-cycle fatigue and high-cycle fatigue. The most popular low-cycle fatigue life estimation techniques are based on a strain approach (see, for example, the critical plane-based criteria proposed by Socie and co-workers [2-5], Brown and Miller [6], and Wang and Brown [7] as well as the energy criterion introduced by Ellyin [8-10]). These criteria are sometimes extended also to high-cycle fatigue, the plastic strain contribution being negligible. On the other hand, all the high-cycle multiaxial fatigue criteria are based on the stress components only. This is true, for example, for the mesoscopic approach-based criteria proposed by Dang Van [11] or Papadopoulos [12,13] and for the critical plane-based criteria due to McDiarmid [14,15], Matake [16] and Findley [17]. All these methods are well-known to the scientific community and a discussion about them is, obviously, out of the aims of the present Research Project proposal.

Other "stress-based criteria", recently presented in the literature, have mainly been developed by the researchers who are proposing this Project.

In January 2002 Susmel and Lazzarin presented a >>>