Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

Critical review of automotive steels spot welding: process, structure and properties

The use of metamodeling techniques in the design and analysis of computer experiments has progressed remarkably in the past 25 years, but how far has the field really come? This is the question addressed in this paper, namely, the extent to which the use of metamodeling techniques in multidisciplinary design optimization have evolved in the 25 years since the seminal paper on design and analysis of computer experiments by Sacks et al. (“Design and Analysis of Computer Experiments,” Statistical Science, Vol. 4, No. 4, 1989, pp. 409–435). Rather than a technical review of the entire body of metamodeling literature, the focus is on the evolution and motivation for advancements in metamodeling with some discussion on the research itself; not surprisingly, much of the current research motivation is the same as it was in the past. Based on current research thrusts in the field, multifidelity approximations and ensembles (i.e., sets) of metamodels, as well as the availability of metamodels within commercial soft...

Metamodeling in Multidisciplinary Design Optimization: How Far Have We Really Come?

The use of metamodeling techniques in the design and analysis of computer experiments has progressed remarkably in the past two decades, but how far have we really come? This is the question that we investigate in this paper, namely, the extent to which the use of metamodeling techniques in multidisciplinary design optimization have evolved in the two decades since the seminal paper on Design and Analysis of Computer Experiments by Sacks et al. As part of this review, we examine the motivation for advancements in metamodeling techniques from both a historical perspective and the research itself. Based on current thrusts in the field, we emphasize multi-level/multi-fidelity approximations and ensembles of metamodels, as well as the availability of metamodels within commercial software and for design space exploration and visualization in this review. Our closing remarks offer insight into future research directions – nearly the same ones that have motivated us in the past.

Design and analysis of computer experiments in multidisciplinary design optimization: A review of how far we have come - Or not

Prospects of laser welding technology in the automotive industry: A review

Probabilistic Reliability: an Engineering Approach

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.

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The Effect of Spot Weld Failure on Dynamic Vehicle Performance

Improving the fatigue life of a vehicle knuckle with a reliability-based design optimization approach

Numerical prototyping methods based on numerical simulations and optimization are nowadays a key success factor for cost effectively and timely development, manufacturing of competitive products and processes. The typical numerical prototyping procedures are based on a sequence of numerical simulations/experiments done in an iterative form in order to achieve usually sub-optimal designs. These experiments are usually selected according to the knowledge and experience of an engineer or defined according to experimentally orientated methodologies. The current paper describes part of our preliminary research results of numerical prototyping, specialty focusing on the optimized numerical DOE approach, which leads to substantial reduction of numerical experiments for the specified accuracy of the response surface model.

Advanced numerical prototyping methods in modern engineering applications

Numerical prototyping methods based on numerical simulations are nowadays a key factor to successful cost effective and timely design for optimized products and processes. The typical prototyping procedure would be based on a sequence of numerical simulations/tests done in an iterative form in order to achieve usually sub-optimal designs. These tests are usually selected according to the knowledge and experience of an engineer or defined standard with little or no help from effective methodologies. In contrast, this paper presents the elaborated optimized approach to numerical prototyping based on smart and sequential algorithm. The major advantage of the presented approach is that it can improve the quality of the response model at a fraction of the number of experiments compared to the classical methodologies. Details of the methodology are presented along with a selected example concerning micro-electronic packaging.

Smart and Sequential Approach to Numerical Prototyping in Micro-Electronic Applications

Although the aerospace production process is much better controlled than the process in other industries, it remains true that very small manufacturing tolerances exist in the geometrical parameters (flange thicknesses, hole diameters, …) as well as in material properties. In the current design process, the effect of this manufacturing variability on the structural durability and safety cannot be accurately assessed and is hence compensated for by applying safety factors. This is not an ideal situation, as it may lead to slightly overdesigned structures. A much more promising approach is to include probabilistic models of design variables into the mechanical simulation process. Then, with a new methodology based on reliability analysis, engineers can obtain a better understanding of the actual effect of the manufacturing tolerances and of variability in material properties. Based on the analysis results, the robustness and reliability of the design can be assessed and improved if needed. In this paper, the above-mentioned probabilistic approach is demonstrated on two aerospace applications: a composite wing and a slat track structure. The material properties of the composite wing have been characterized with statistical distributions and their effect has been assessed on the static performance. For the slat track, measurements of different geometrical properties have been collected during the manufacturing process and their variability has been characterized probabilistically with statistical models. Then, a reliability analysis has been carried out using morphing technology and fatigue life predictions with an industrial-sized FE model of the slat track to assess the reliability of the structure in terms of fatigue life. The outcome of the analyses consists of a probabilistic model of the structural performance (e.g. fatigue life for the slat track), given the variability in the geometrical and material parameters.

N. Tzannetakis

Papers

The Effect of Spot Weld Failure on Dynamic Vehicle Performance

Improving the fatigue life of a vehicle knuckle with a reliability-based design optimization approach

Advanced numerical prototyping methods in modern engineering applications

Smart and Sequential Approach to Numerical Prototyping in Micro-Electronic Applications

Reliability-based design optimization of composite and steel aerospace structures