Sun Failure Mechanisms of Advanced Welding Processes

1. Auflage 2010
ISBN: 978-1-84569-976-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 336 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

Many new, or relatively new, welding processes such as friction stir welding, resistance spot welding and laser welding are being increasingly adopted to replace or improve on traditional welding techniques. Before advanced welding techniques are employed, their potential failure mechanisms should be well understood and their suitability for welding particular metals and alloys in different situations should be assessed. Failure mechanisms of advanced welding processes provides a critical analysis of advanced welding techniques and their potential failure mechanisms.The book contains chapters on the following topics: Mechanics modelling of spot welds under general loading conditions and applications to fatigue life predictions, Resistance spot weld failure mode and weld performance for aluminium alloys, dual phase steels and TRIP steels, Fatigue behaviour of spot welded joints in steel sheets, Non-destructive evaluation of spot weld quality, Solid state joining - fundamentals of friction stir welding, Failure mechanisms in friction stir welds, Microstructure characteristics and mechanical properties of laser weld bonding of magnesium alloy to aluminium alloy, Fatigue in laser welds, Weld metal ductility and its influence on formability of tailor welded blanks, Joining of lightweight materials using reactive nanofoils, and Fatigue life prediction and improvements for MIG welded advanced high strength steel weldments.With its distinguished editor and international team of contributors, Failure mechanisms of advanced welding processes is a standard reference text for anyone working in welding and the automotive, shipbuilding, oil and gas and other metal fabrication industries who use modern and advanced welding processes. - Provides a critical analysis of advanced welding techniques and their potential failure mechanisms - Experts in the field survey a range of welding processes and examine reactions under various types of loading conditions - Examines the current state of fatigue life prediction of welded materials and structures in the context of spot welded joints and non-destructive evaluation of quality

Autoren/Hrsg.

Sun, X.

Leseproben

1 Mechanics modeling of spot welds under general loading conditions and applications to fatigue life predictions
J. Pan and K. Sripichai,     University of Michigan, USA, P-C. Lin,     National Chung Cheng University, Taiwan D-A. Wang,     National Chung Hsing University, Taiwan S-H. Lin,     Wheel Group, SuperAlloy Industrial Co. Ltd., Taiwan Abstract:
A mechanics model of a spot weld in a finite plate under general loading conditions is presented first. Available closed-form stress, moment and transverse shear force solutions are then discussed for a plate with a rigid inclusion subjected to various types of loading conditions. Based on a strip model, closed-form analytical stress intensity factor solutions for spot welds are presented under various types of loading conditions. A kinked crack growth model is then presented and adopted to predict the fatigue lives of resistance spot welds in lap-shear specimens of dual phase, low carbon and high strength steels. Comparisons of predicted fatigue lives based on the kinked crack growth model with the experimental data indicate that the fatigue life predictions are in agreement with or lower than the experimental data. Key words fatigue fatigue crack propagation J integral kinked crack spot weld stress intensity factor structural stress 1.1 Introduction
Resistance spot welding is widely used to join sheet metals in the automotive industry. These spot welds are subjected to complex multiaxial service loads. In order to be able to predict the fatigue lives of spot welds in vehicles, the fatigue lives of spot welds in various types of specimens have been investigated by many researchers. Many researchers have conducted experiments and correlated their experimental results with empirical relationships. However, these empirical relations are only applicable to the spot welds under particular welding and testing conditions. Since a spot weld provides a natural crack or notch along the nugget circumference, fracture mechanics appears to be a logical choice to characterize or correlate the fatigue data of these welds. Fracture mechanics has been adopted to investigate the fatigue lives of spot welds in various types of specimens or configurations (Pook, 1975, 1979; Radaj, 1989; Radaj and Zhang, 1991a, 1991b, 1992; Sheppard, 1993; Swellam et al., 1994; Zhang, 1997, 1999, 2001; Wang et al., 2005a, 2005b; Lin et al., 2007). Zhang (1999) showed that fracture mechanics solutions can be used to correlate the fatigue data of spot welds in different types of specimens. However, the stress intensity factor and structural stress solutions proposed by Radaj (1989), Radaj and Zhang (1991a, 1991b, 1992) and Zhang (1997, 1999, 2001) have not been adopted widely for characterizing the fatigue behavior of spot welds in specimens and automotive structural components. Pook (1975, 1979) made significant contributions using the energy release rate concept to obtain the stress intensity factor solutions for various configurations of beam and plates with connections. Pook (1979) indicated that, for a class of transversely loaded configurations consisting of two thin plates or beams joined over part of their common plane under symmetric loading conditions, the energy release rate or the stress intensity factor at a crack tip depends on the bending moment acting on the beam or plate in the vicinity of the crack tip. Wang et al. (2005a) conducted a three-dimensional finite element analysis of circular plates with connection under opening loading conditions. Wang et al. (2005b) also conducted a three-dimensional finite element analysis of a nearly square large lap-shear specimen. The results of the three-dimensional finite element analyses of Wang et al. (2005a, 2005b) suggest that using the bending moments and the membrane forces or the corresponding structural stresses to obtain the stress intensity factor solutions for spot welds, assumed to be rigid inclusions in thin plates, can be quite accurate. The closed-form solutions for thin plates with rigid inclusions under shear, central bending, counter bending and opening loading conditions were obtained by Muskhelishvili (1953), Reißner (1929), Goland (1943), Timoshenko and Woinowsky-Krieger (1959) and Lin et al. (2007), respectively. These solutions, except those of Goland (1943) and Lin et al. (2007), were used by Zhang (1997, 1999, 2001) to obtain the structural stresses at several critical locations of spot welds in various types of specimens and automotive structures, where the spot welds were treated as a rigid inclusion in the analytical or numerical solution procedures. Rupp et al. (1990, 1995) used a beam element model, whereas Salvini et al. (1997, 2000) and Vivio et al. (2002) used a spot weld assembly finite element model to represent a spot weld in order to obtain the resultant forces and moments through the spot weld for fatigue life estimations. In this chapter, the theoretical frameworks for spot welds under various types of loading conditions are presented, based on elasticity theories and fracture mechanics. Available closed-form stress, moment and transverse shear force solutions are reviewed for a plate with a rigid inclusion subjected to various types of resultant loads on the inclusion and various types of resultant loads on the plate lateral surface. Based on the J integral for a strip model, closed-form analytical stress intensity factor solutions for spot welds are derived in terms of the structural stresses around a rigid inclusion in a plate under various types of loading conditions. As an example to demonstrate the applicability of the stress intensity factor solutions for spot welds, a fatigue crack growth model is presented that correlates the fatigue lives of resistance spot welds in lap-shear specimens of dual phase, low carbon and high strength steels under cyclic loading conditions based on the experimental observations of spot welds that failed in a kinked crack propagation mode. The fatigue crack growth model is based on the global stress intensity factor solutions for main cracks, the local stress intensity factor solutions for kinked cracks as functions of the kink length, the experimentally determined kink angles and the Paris law for kinked crack propagation. The predicted fatigue lives based on the fatigue crack growth model are then compared with the experimental data. 1.2 Spot weld in a finite plate under general loading conditions
Figure 1.1 shows schematically two metal sheets joined by a spot weld and also shows the surface tractions TU and TL which are applied to the lateral surfaces of the upper and lower sheets, respectively. The weld nugget is idealized as a circular cylinder. Next, we consider the upper half of the weld nugget in the upper sheet as shown in Fig. 1.2. The upper sheet has the thickness t and the nugget has the diameter 2a. Figure 1.2 shows the upper sheet with the upper half nugget with a surface traction TULoad, the resultant loads F and M acting on the lower surface of the upper half nugget, and the self-balanced resultant loads Fs and Ms. Note that the load-balanced part TULoad is statically in equilibrium with the resultant loads F and M. 1.1 Two metal sheets are joined by a spot weld. The metal sheets are under surface traction TU and TL. 1.2 Upper sheet with the upper half nugget is shown with the surface traction TULoad, the resultant loads F and M and the self-balanced resultant loads Fs and Ms. The upper sheet has the thickness t and the nugget has the diameter 2a. Most researchers approximate the general loading condition of spot welds by using the resultant loads applied to the interfacial circular cross-section of the weld nugget, for example, Swellam et al. (1994), Rupp et al. (1995) and Salvini et al. (2000). However, Zhang (1997, 2001), Wang et al. (2005b) and Lin et al. (2007) indicated that the closed-form stress solutions for a rigid inclusion under the self-balanced surface tractions of the plate are important in obtaining the analytic solutions of the stress intensity factors for spot welds. As shown in Fig. 1.2, the resultant force F and the resultant moment M are now decomposed into three resultant forces Fx, Fy and...