Effects of spot diameter and sheets thickness on fatigue life of spot welded structure based on FEA approach

American Journal of Applied Sciences, Jan, 2009 by M.M. Rahman, A.B. Rosli, M.M. Noor, M.S.M. Sani, J.M. Julie

INTRODUCTION

Spot welding is a widely employed technique to join sheet metals for body and cap structure in the automotive industry. The strength of the spot welds in the unibody vehicle structure determines the integrity of the structural performance during the vehicle operations. Most spot welds generally carry only shear forces but spot welds can also experience a significant amount of peel force or the force normal to the spot weld in certain loading conditions. The combination of the stress states and geometric shapes of the spot welds lead to stress concentration that can result in fatigue crack initiation around the spot weld. The presence of fatigue cracks can degrade structural performance and increase noise and vibration of the vehicle structure. Therefore, understanding of the fatigue strength for the spot welds is very important in automotive component design.

The static strengths of spot welds have also been investigated. Ewing et al. (1) investigated the strength of spot welds in terms of the specimen geometry, welding parameter, welding schedule, base metal strength, testing speed and testing configuration. Zhang and Taylor (2) reported the thickness effect of spot welded structure on fatigue life. Pan and Sheppard (3) calculated stress intensity factors for crack propagation through the thickness of plate by numerically utilizing finite element analysis. Lee et al. (4) adopted a fracture mechanics approach using the stress intensity factor to model their experimental results on the strength of spot welds in U-tension specimens under combined tension and shear loading conditions. Wung (5) and Wung et al. (6) obtained and analyzed test results from lap-shear, in-plane rotation, coach-peel, normal separation and in-plane shear tests and proposed a failure criterion based on the experimental data of spot welds in various specimens.

Some researchers (7-9) have studied on the effects of base metal properties on the fatigue life of spot welds. They have also studied on the effects of loading conditions with different specimen types such as tensile shear, coach peel and cross tension specimens. These studied showed in general that fatigue life of spot welds depended on the loading conditions and base metal properties.

On the other hand, numerous researchers (1), (10-14) proposed analytical and/or empirical models to predict the fatigue strength of spot welds in the early vehicle design stage. Most of these models were developed based on the relationship between a fatigue damage parameter and number of cycles to failure of spot welds. The objective of this study is to investigate the effect of the sheet thickness and diameter of the spot weld nugget on the fatigue.

STRUCTURAL STRESS PARAMETER

Welded joints experience highly localized heating and cooling from welding processes. As a result, the material properties around the welding joints can be significant variations after welding. The local geometry of the welded joints may have variations due to the amount of heat inputs and welding skills. These variations present significant difficulties for reliable fatigue prediction of welded joints.

Dong (15-16) proposed a structural stress parameter for welded joins based on local stresses at weld toe. A typical through-thickness stress distribution at a fatigue critical location and the corresponding structural stress definition for through-thickness fatigue crack at the edge of a spot weld are shown in Fig. 1 and 2. Stress distribution at the edge of the spot weld nugget is assumed as shown in Fig. 1. In Fig. 1, t represents the thickness of the sheet steel, [SIGMA]x and [TAU] are the normal and transverse shear stress under axial force P respectively. The corresponding structural stress distribution is shown in Fig. 2. The structural stress ([SIGMA]) is expressed in Eq. 1:

[sigma] = [[sigma].sub.m] + [[sigma].sub.b](1)

where, [[SIGMA].sub.m] is the membrane stress component and [[SIGMA].sub.b] is the bending stress component due to the axial force P in the x direction. The transverse shear stress can be calculated based on local structural shear stress distribution, however, the effect of transverse shear stress neglected since the spot weld does not experience significant transverse shear loads in general (15).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The structural stress is defined at a location of interest such as plane A-A in Fig. 3 and the second reference plane can be defined along plane B-B. Both local normal and shear stress along plane B-B can be obtained from the finite element analysis. The distance in local x-direction between plane A-A and B-B is defined as 8. The structural membrane stress and bending stress must satisfy Eq. 2 and 3 for equilibrium conditions between plane A-A and B-B. Equation 2 shows the force balances in x-direction, evaluated along the plane B-B. On the other hand, Eq. 3 shows moment balances with respect to plane A-A at y = 0. When 8 between planes A-A and B-B becomes smaller then transverse stress x in Eq. (3) is negligible. Therefore, Eq. 2 and 3 can be evaluated at Plane A-A in Fig. 3.