Implicit gradient approach for numerical analysis of laser welded joints

Abstract This paper analyzes the fatigue strength of laser welded steel joints by applying the implied gradient method. This method is adopted using the same procedure proposed for studying arc welded joints. The fatigue scatter band of laser joints, obtained from numerical analysis of experimental data taken from the literature, is different from the relative curve of arc welded joints. However, at high cycles fatigue the two bands maintain exactly the same average values and the same scatter. The fatigue strength of lap joints with different weld patterns confirms the use of the proposed generalized fatigue scatter band for laser joints

vibration fatigue tests by tri axis shaker design of an innovative system for uncoupled bending torsion loading

Abstract An innovative system for bending-torsion fatigue tests by tri-axis shaker is designed and presented. The system mounts a cylindrical specimen with eccentric tip mass, excited by horizontal and vertical base accelerations. A lateral thin plate prevents specimen horizontal displacement and allows torsional and bending deformations to be controlled independently. A lumped-mass model is first used to verify if input accelerations and resultant dynamic forces, required in testing, comply with shaker specifications. A finite element model is then used to perform both modal and harmonic analyses, necessary to determine the system natural frequencies and the dynamic response under horizontal and vertical accelerations. Experimental measures on a prototype are finally used to gather preliminary information for validating the numerical model and to verify that the proposed testing system can control bending and torsion loadings independently

a simplified multiscale model of degenerate graphite clusters in grey cast iron

Abstract To take into account the weakening effect of defects clusters in real microstructures, we propose a multiscale model in a two-dimensional setting. At a small scale, single defects are described as elliptical voids randomly distributed and randomly oriented in an isotropic matrix. Using an effective field method proposed by Tandon and Weng [5,6], the effective properties of a porous equivalent material can be estimated in a simple closed form. At a larger scale, defects clusters are modelled as single large elliptical inclusions characterized by the weakened effective properties calculated in the first step and embedded in an infinite, elastic, isotropic matrix under remote loading. The method allows to examine the dependence of defects interaction on some basic microstructure parameters (porosity density, voids aspect ratio, inclusion aspect ratio)

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 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

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

Numerical predictions of the fatigue life of aluminium welded joints

The fatigue life of arc welded joints made of aluminium alloy has been investigated by means of the implicit gradient method. This approach is suitable for estimating the fatigue life of welded joints characterised by different opening angles of the fillet weld and different thicknesses. The main fatigue data are related to cruciform joints, rectangular hollow section T-joints and bead removed joints. Using the principal stress as the local equivalent stress and a notch tip of the weld toe radius equal to zero, the paper analyses about two hundred experimental data taken from the literature and proposes a universal scatter band in the range of 104 to 5·106 cycles. This scatter band has an inverse slope around 4 and, at high cycle fatigue, the strength is about half that of steel joints. However, the scatter band is similar to welded joints made of steel previously analyse

An innovative system for uncoupled bending/torsion tests by tri-axis shaker: Numerical simulations and experimental results

This work describes a new testing system for applying a coupled/uncoupled bendingtorsion loading in vibratory tests by a tri-axis shaker. The system is composed of a cylindrical specimen with eccentric tip masses, excited by horizontal and/or vertical base accelerations. The specimen tip is constrained by a lateral thin and flexible plate which impedes any bending when the specimen is excited horizontally, but which permits the specimen torsional rotation. This layout then allows torsional and bending deformations to be produced and controlled independently, when vertical and horizontal base accelerations are applied simultaneously. A finite element model is first used to estimate the system dynamic response and the stresses in the notched specimen section. The model is then validated through experimental tests under harmonic base accelerations. The strains at clamping system are also monitored to indirectly estimate the bending and torsion moment in the specimen. Comparison of numerical and experimental results showed a close correlation and proved that bending-torsion loading are truly uncoupled. Preliminary fatigue tests with harmonic bending loading (vertical base excitation) are finally compared to the constant amplitude S-N curve, showing a quite satisfactory agreement