A structural stress definition and numerical implementation for fatigue analysis of welded joints

Strength tests were performed to reveal the failure mechanisms of spot weld in lap-shear and cross tension test samples. It is shown the while the lap-shear (cross tension) sample is subjected to shear (normal) load at the structural level the failure mechanism at the spot weld is tensile (shear) mode at the materials level. Based on the observed failure mechanism, stress distribution is assumed and related to the far field load for the lapshear and cross tension test samples. It appears that the failure load of the cross tension sample is 74 percent of the lap-shear sample based on the classical von Mises failure theory. The theoretical model is further extended to the mixed normal/shear loading condition. Data from strength tests as well as finite element numerical method are used to validate the model. Finally, the utility of the model in accessing the failure strength of spot welds is discussed. @DOI: 10.1115/1.1555648#

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Ultimate Strength and Failure Mechanism of Resistance Spot Weld Subjected to Tensile, Shear, or Combined Tensile/Shear Loads

A review of T-stress and its effects in fracture mechanics

Aluminum alloys are widely used in the aircraft industries even if such materials present lower ductility with respect to steels, anisotropy phenomena and, more important, they are often “difficult” to be welded or even “non-weldable”. In the last years the friction stir welding process (FSW) was proposed and applied in order to get good mechanical and technological performances of the joints. In this paper, an experimental and numerical investigation on the lap joining of AA2198-T4 aluminum alloy blanks by FSW is presented. In particular the joints strength and metallurgical properties are investigated by varying the joint configuration and the tool geometry and rotational speed. It is found that the use of cylindrical–conical pin tools and the correct choice of the relative sheet positioning increase the welded nugget extension and integrity improving the mechanical performances of the obtained joints.

Friction stir welding of lap joints: Influence of process parameters on the metallurgical and mechanical properties

Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 1: Welds made by a concave tool

The stress intensities (notch stress, stress intensity factors and J-integral) at spot welds under typical loads of tensile-shear, cross-tension and coach-peel are derived as a number of simple formulas on the basis of an analytic solution where the stress intensities at spot welds are generally determined by the stresses around the spot welds and of some analytic solutions to circular rigid inclusions in plates with the inclusions simulating the weld nuggets. The derived formulas show consistently the trends in the stress intensities with the design parameters for spot welds such as nugget diameter and sheet thickness and additionally with spacing of force for cross-tension spot welds and load eccentricity for coach-peel spot welds. The stress intensities at spot welds under general loading conditions are estimated in terms of the forces and moments transferred by the spot welds based on the derivations. The theoretical predictions from the formulas are compared favorably with the finite element results. As an application example, some fatigue test data for spot welds in the form of load range versus life to failure are transferred into the form of stress intensities range versus life to failure with the scatterband of the fatigue test data being substantially reduced.

Stress Intensities at Spot Welds

Notch stress, stress intensity factors and J-integral at a spot weld are generally expressed by structural stresses around the spot weld. The determination of these parameters are then simplified as determining the structural stresses that can be calculated by a spoke pattern in finite element analysis. Approximate stress formulas for structural stress, notch stress and equivalent stress intensity factor are given for common spot-welded specimens. With the aid of the formulas, test data in terms of the original load can be easily transformed into the data in terms of the structural stress, notch stress or equivalent stress intensity factor at the spot weld. The formulas also facilitate the transfer of test data across different specimens. A measuring method is given for lap joints. The strain gauge technique developed for the tensile-shear specimen shows that all the structural stress, notch stress, stress intensity factors and J-integral at the spot weld can be determined by two strain gauges attached only to the outer surface of one sheet. The results presented here should be helpful for the analysis and testing of spot welds and for developing measuring methods for spot welds.

Fracture mechanics solutions to spot welds

The stress intensities (stress intensity factors and J-integral) around a spot weld between sheets of dissimilar materials and different thicknesses are generally expressed by the local stresses around the spot weld. The derivations are established on an interface crack model which represents the radial cross-sections of the spot weld with the local stresses varying from point to point along the periphery of the nugget. The J-integral is determined analytically by applying the equilibrium conditions and the elementary plate theory to the model. The stress intensity factors are then separated by following a load decomposition procedure and an analytic solution to interface cracks. The thermal stress intensities are also obtained for the spot weld. Application examples are given for a tensile-shear specimen.

Stress Intensities Derived from Stresses Around a Spot Weld

Based on a solution to interface cracks obtained earlier by the author, T-stress, energy release rate and stress intensity factors are derived in closed form for some specimen-relevant configurations (four-point bend, one-sided tension and peel-tension) with an interface crack between two elastic layers. The results may be useful for the fracture toughness testing of bimaterial interfaces.

T-stress and stress intensities for the interface cracks in some specimen-relevant geometries

Determination of stress intensity factor (SIF) at spot welds is one of the main problems in fatigue assessment of spot-welded structures using fracture mechanics methods. After a brief overview of the issue, we suggest an improvement in determination of SIFs at spot welds between sheets of unequal thickness, by adding the contributions of transverse shear stresses on the spot weld edge. T-stress at the spot weld is also discussed. A simplified finite element model in conjunction with application of the results obtained is illustrated as well.

Shicheng Zhang

Papers

Stress Intensities at Spot Welds

Fracture mechanics solutions to spot welds

Stress Intensities Derived from Stresses Around a Spot Weld

T-stress and stress intensities for the interface cracks in some specimen-relevant geometries

Stress Intensity Factors for Spot Welds Joining Sheets of Unequal Thickness