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The Effect of Spot Weld Failure on Dynamic Vehicle Performance

2005; Volume: 39; Issue: 4 Linguagem: Inglês

2693-1443

Stijn Donders, Marc Brughmans, L.J.F. Hermans, Nick Tzannetakis,

Advanced Welding Techniques Analysis

Spot welds are the dominant joining method in the automotive assembly process. As the automated assembly process is not perfect, some spot welds may be absent when the vehicle leaves the assembly line. Furthermore, spot welds are highly susceptible to fatigue, so that a substantial number may fail during the vehicle lifetime. The scope of this article is twofold. First, the impact of spot weld quality and design on a vehicle’s functional performance is reviewed, addressing strength and stiffness, NVH and durability. The overview briefly covers both experimental tests and predictive finite element (FE) modeling approaches, explains the complexity of a spot weld design problem and discusses optimization strategies. Second, an industrial robustness study is presented, that assesses the effect of spot weld failure on dynamic vehicle characteristics. Damaged models are generated automatically, by breaking a subset of the vehicle’s spot welds, using a weighted-uniform selection probability. Monte Carlo simulations are then used to assess the scatter on dynamic vehicle characteristics. The role of Computer Aided Engineering (CAE) in the automotive industry is rapidly increasing. Functional performances (NVH, durability, . . .) are fine-tuned on the basis of numerical predictions, so that the expensive physical prototyping phase can be shortened considerably. Traditionally, optimizing a vehicle body starts with improving the fundamental torsion and bending frequencies. These dynamic characteristics should be robust to failure of spot weld connections, thousands of which are present in a typical vehicle body. The first part of this article overviews the use of spot welds in the automotive industry. Section 1 deals with the resistance spot welding procedure and typical characteristics of spot welds. Section 2 describes small-scale experiments and reallife testing of spot weld characteristics in terms of strength and stiffness, NVH and fatigue life, and highlights the complexity of spot weld (layout) design. Section 3 describes a selection of finite element models that are used to predict a spot weld’s functional performance with numerical simulations, and addresses the benefits and difficulties of optimization on the basis of FE models. During the vehicle lifetime, manufacturing inaccuracies, minor accidents and fatigue failures may result in deterioration or even absence of a substantial number of spot weld connections. Also, in a CAD model transferred to a CAE department, some spot welds might be omitted or forgotten. The second part of this article presents an approach to assess the robustness of dynamic vehicle characteristics to this breakage or absence. Automated procedures have been developed, to break a number of spot weld elements with highest strain energy in the nominal (undamaged) model, and also to randomly break a number of welds, with a uniform probability or with a weighted-uniform probability. The latter application allows performing Monte Carlo simulations to assess the effect of random spot weld failure on dynamic vehicle characteristics. Section 4 explains both the input file creation routines and the process flow of required computations. Key results are given in Section 5. Spot Welds in Vehicles Resistance spot welding (RSW) emerged in the 1950s, and is now the predominant assembly technique in the automotive industry. The vehicle components (body in white, cradle, doors, etc.) are made of thin metal sheets that are connected with spot-welded joints (or simply, spot welds); see the example 1 in Figure 1. To create a spot weld, two or more metal sheets are pressed together by electrodes, and an electric current is passed through them. The resistance of the metal generates heat, and the sheets are welded together by means of local metal fusion; a spot weld has been created. No welding material is added in this process. A spot weld consists of three regions, which have different material properties – a weld nugget with a cylindrical shape, a heat-affected zone (HAZ) and the base material sheets. 2 For instance, the yield stress in the nugget is up to three times higher than in the base material, 3 and the plastic properties of the HAZ are non-homogeneous. 4 Due to the applied pressure by the electrodes during the welding, the thickness of the nugget is often less than the thickness of the two metal sheets. This so-called nugget indentation is typically not significant for plates up to 1 mm, but is more pronounced when thick plates are assembled. Stress concentration may occur at the edges where a change of thickness takes place, which may result in crack initiation. 3 The transient heating and cooling results in hardening of the material, and a prestress may remain after cooling. A typical vehicle body-in-white is made of steel sheets and contains about 4000 spot welds. The optimal diameter and distance between two successive spot welds are determined by the sheet thickness. The diameters range from 3 to 7 mm, with a mean of 6 mm. 5 The manufacturing practice of spot welds in the vehicle assembly process poses constraints on the spot weld layout design, as not all positions can (effectively) be reached. Note also that the assembly process is not perfect – sometimes a few spot welds are even missing or broken right from the beginning of the vehicle life.