Enrico Maggiolini

Crack initiation and propagation paths in small diameter FSW 6082-T6 aluminium tubes under fatigue loading

This paper reports results of fatigue tests of friction stir welded (FSW) aluminium tubes. Relatively small 38 mm diameter tubes were used and hence an automated FSW process using a retracting tool was designed for this project, as the wall thickness of the aluminium tube was similar to the diameter of the FSW tool. This is a more complex joint geometry to weld than the more usual larger diameter tube reported in the literature. S-N fatigue testing was performed using load ratios of R = 0.1 and R = -1. Crack path analysis was performed using both low magnification stereo microscopy and scanning electron microscopy, in order to identify crack initiation sites and to determine the direction of crack propagation. Work is still in progress to follow the crack path through the various microstructural zones associated with the weld. A simple statistical analysis was used to characterize the most typical crack initiation site. This work forms part of a wider project directed at determining multiaxial fatigue design rules for small diameter 6082-T6 aluminium tubes that could be of use in the ground vehicle industry.

Designing aluminium friction stir welded joints against multiaxial fatigue

The present paper investigates the accuracy of the Modified Wohler Curve Method (MWCM) in estimating multiaxial fatigue strength of aluminium friction stir (FS) welded joints. Having developed a bespoke joining technology, circumferentially FS welded tubular specimens of Al 6082-T6 were tested under proportional and non-proportional tension and torsion, the effect of non-zero mean stresses being also investigated. The validation exercise carried out using the experimental results have demonstrated that the MWCM applied in terms of nominal stresses, notch stresses, and also the Point Method is accurate in predicting the fatigue lifetime of the tested FS welded joints, with its use resulting in life estimates that fall within the uniaxial and torsional calibration scatter bands.

On numerical integration for effective stress assessment at notches

This paper presents a numerical method for non-local stress assessment by means of a general FE tool and the local stress field. Unlike usual calculations by means of a numerical PDE solver, a more general numerical integration is used. Different solutions are compared theoretically and numerically by evaluating the results obtained by two different FEM commercial software. The application of the non-local tension field is applied to the strength assessment of notches, welded joints and cracks.

Effective stress assessment at rectangular rounded lateral notches

Rectangular lateral notches are not common engineering components, thus little research attention has been directed towards the investigation of their stress field properties. Indeed, no in-depth investigations have been conducted to date to assess their effective stress distributions according to the effective stress definitions provided by more recent non-local approaches (i.e. critical distance, average values, implicit gradient values, etc.). In fact, the potential applications of this kind of investigation are not even particularly relevant. However, rectangular notches could provide an interesting theoretical and experimental benchmark or reference case in order to validate the effective stress definitions. The aim of this paper is to investigate the linear elastic stress field at edges, corners and in the surrounding material of rectangular, sharp or rounded lateral notches. The consequent effective values of these notches are evaluated in relation to brittle fracture or their predicted fatigue strength values. The main goal of this paper is to investigate the relationship between geometrical proportions and the location of critical failure points according to the definitions of effective stress proposed in the literature.

On the notch sensitivity of cast iron under multi-axial fatigue loading

This work deals with the notch sensitivity of sharp notches under multi-axial fatigue loading. The main discussion concerns the differences in notch sensitivity at high cycle fatigue regime, between tensile, torsional and combined loading. For this comparison, this paper considers a large set of fatigue experimental tests and several computing simulations analyzed with several notch theories for predicting fatigue life of a component. The considered experimental data, taken from literature, deal with the fatigue behavior of cast iron circumferentially V-notched specimens under tension, torsion, and combined loading mode. This paper tries to apply several techniques for theoretical strength assessment and to compare different procedures. The examined procedures need the computation of many parameters, focusing on the importance of using the tensile resistance to set these parameters or using both tensile and torsion resistances. However, the improvements obtained by means of the more complex procedures are not noteworthy, compared to the overall scatter. In authors opinion, the differences in notch sensitivity under tensile and torsional loading remain questionable.