Francesco Vivio

A spot weld finite element for structural modelling

A new finite element assembly, to account for the structural behaviour of the region surrounding a spot weld, is developed. The formulation derives from the analytic solution of two circular sheets externally clamped, with a central rigid core connecting them (representing the spot weld). The effective modelling strategy of the above mentioned region makes use of equivalent beam elements; therefore, the implementation is possible in any structural code. The aim is to use this assembled element for the region around the spot: two assembled elements, connected by a link, form the spot weld connection between two metal sheets. The analytic solution is provided for any possible typology of loading acting on a spot weld (shear, bending and orthogonal loads). Moreover, the link connecting the two sub elements may be considered rigid as well as deformable, this can be very useful to evaluate fatigue damage evolution in structures jointed with many spot welds. Using these assembled elements for the spot weld region, an important increase in the accuracy is reached, as demonstrated in the example. However, the accuracy does not imply an increase in the total number of degrees of freedom of the whole model.

Enforcing of an analytical solution of spot welds into finite element analysis for fatigue-life estimation

The present paper deals with the problem of numerical estimation of fatigue life in spot welded structures. It is evidenced that the use of a particular suited analytical solution inside the finite element calculation is possible and provides gains in both accuracy and computer time saving. The main point discussed in the paper regards the approach to discretise the analytical solution of the region surrounding the spot weld to the mesh of the rest of the structure. Independence to mesh refinement is achieved very early as discussed through examples. Chance is given to account of progressive damaging of spot welds. Comparison with traditional mesh techniques is also performed.

Thermal Stresses of Rotating Hyperbolic Disks as Particular Case of Non-Linearly Variable Thickness Disks

A theoretical thermoelastic analysis of the Stodolas hyperbolic disk, axisymmetric and symmetric with respect to the mid-plane, and subjected to thermal load is carried out. A general temperature distribution along the radius expressed by a polynomial relation is introduced. The disk is analyzed as a particular case of the more general disk profile having non-linearly variable thickness given by the power of a linear function of the radius. The differential equation governing the radial displacement field of the Stodolas disk subjected to thermal load is deduced from the equivalent more general equation of this non-linearly variable thickness disk. The governing equation so obtained is analytically integrated and a closed-form solution is presented. Theoretical results are validated by comparing them with those obtained by means of FEA, after performing some significant calculation examples.

Analysis of Static Strength and Failure Mode of FSSW Joint in Aluminum Alloy

In this paper an analysis of Friction Stir Spot Welds (FSSW), applied to lap joint aluminum alloy is presented. The analyses regards the structural behavior of FSSW joints in order to assess the failure mode and its dependence on mechanical properties distribution of the sheets material and, consequently, on process parameters.FSSW joint is analyzed by means of a complex 3D FE model, which allows to evaluate, in a parametric manner, the multifaceted internal geometry of the joint and the actual distribution of material mechanical characteristics after welding. Experimental tests allowed to verify the results and to calibrate the material characteristics in the FE model.Copyright

Theoretical Stress Analysis of Rotating Hyperbolic Disk Without Singularities Subjected to Thermal Load

A theoretical method for the evaluation of elastic stresses and strains in rotating hyperbolic disks without singularity, also subjected to thermal load, is introduced. The differential equation governing the radial displacement field of the hyperbolic disk without singularity has been deduced from the governing equation of non-linearly variable thickness disks given by the power of a linear function of the radius. A general temperature distribution along the radius expressed by a polynomial relation is considered. To overcome convergence problems of the solution, with hypergeometric functions, analytic continuations of these series have been introduced that allow the extension the theoretical thermoelastic analysis to all cases of technical interest. Then, for the first time the authors are aware, closed-form solutions of the governing equation of hyperbolic disks without singularity are presented.

Thermo-Structural Analysis of a New Engine Cylinder Head

An engine head for microcar applications has been analysed and optimized by means of uncoupled CFD and FEM simulations in order to assess the strength of the component. This paper deals with a structural stress analysis of the cylinder head considering the thermal loads computed through an uncoupled CFD simulations of cylinder combustion and in cooling flow passages. The FE model includes the contact interaction between the main parts of the cylinder head assembly and it also considers the effects of bolts tightening and valve springs. Temperature dependent non-linear material properties are considered. The results, in term of temperature field, are validated by comparing with those obtained by means of experimental analyses; the engine has been instrumented with thermocouples on crank case and on cylinder head.

Influence of non-axisymmetric material anisotropy on FSSW static strength

This paper regards the analysis of structural behavior of Friction Stir Spot Welds (FSSW) in order to assess the failure mode and its dependence on mechanical properties distribution of the sheets material and on process parameters. In particular, in this paper is investigated the influence of non-axisymmetric anisotropy of material characteristics on static strengthand failure mode of FSSW. FSSW joint is analyzed by means of a complex 3D Finite Elements model, which allows to evaluate, in a parametric manner, the multifaceted internal geometry of the joint and the actual distribution of material mechanical characteristics after welding. Experimental tests allowed to verify the results and to calibrate the material model in the Finite Elementsanalyses.

One shot failure modes in spot welded structures

Abstract This article focuses on modes of one shot failure of spot welds used to join metal sheets. One shot here means unique static or dynamic load events. After a careful examination of all the failure mechanisms that can occur, the authors outline provisional criteria to establish the likelihood of each of the possible mechanisms in a probabilistic sense. Experimental data from the scientific literature are integrated with other data obtained from special tests on the materials and technologies typical of the automobile industry.

Modelling spot welded joints in elastic-plastic field

In this paper two analytical approaches for the evaluation of spot weld joints behavior in elastic plastic conditions are presented and compared. The procedures are based on two new theoretical models of spot weld region: a circular plate, with a central rigid nugget, having two variable thickness profiles. The complex closed-form solutions allow to describe the displacement of a rigid nugget while plasticity and moderately large deflections are present. These approaches allow to reach a reliable spot weld region model which can be used as the basis to develop a spot weld element in FE analysis even when plasticity and large deflections are in effect. The correct evaluation of the spot stiffness is the basis of a reliable evaluation of fatigue resistance or failure mode of the joint and, consequently, allows to correctly simulate the behavior of an actual multi-spot structure.

Design of Rotating Disks and Stress Concentrations

With the usual assumptions of a plane stress state (σ z = 0) made for rotating disks, the stress field is generally biaxial. Uniaxial stress conditions can only occur locally: at the inner edge of non-rotating disks loaded at the outer edge (Figs. 2.1 and 2.2) and of only rotating disks (Fig. 2.6), and at the outer edge of non-rotating disks loaded at the inner edge (Figs. 2.3 and 2.4), of rotating annular disks (Fig. 2.6) and rotating solid disks (Fig. 2.15). Biaxial tensile, tensile-compressive and tensile or compressive stress occur respectively at the centre of only rotating disks (Fig. 2.15), at the inner edge of non-rotating annular disks loaded at the inner edge (Figs. 2.3 and 2.4) and at the outer edge of non-rotating annular disks loaded at the outer edge (Figs. 2.1 and 2.2). Biaxial tensile, compressive or tensile-compressive stress conditions occur in the intermediate sections of the disk.