13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 1. Introduction Brazing has gained increasing importance as a favorable joining technology for many seminal applications in industry in the last years. Generally, brazing plays an essential role because the thermal stress of the joining partners and the processing times can be reduced, compared to e.g. welding. Furthermore, brazing allows joining of dissimilar materials as e.g. metals and ceramics at narrow tolerances. With the use of advanced brazing technologies, as e.g. high temperature (HT) vacuum furnace brazing, brazed joints can also be used for thermally and mechanically heavily loaded components in chemical engineering, power generation and for the production of power electronic components [1-3]. By definition, brazing is performed by heating an assembly over the melting point of the filler metal placed between two substrates without reaching the melting point of the substrate material. The liquid filler metal wets the surfaces of the substrate material and fills the joint gap. Subsequent adhesion and diffusion processes during the cooling of the assembly are essential for the final joint strength. The general differentiation between soldering and brazing is made according to the process temperatures used for the joining process. The above described process at T < 450 °C is referred to as soldering, whereas it is named brazing at T > 450 °C. Generally, brazed joints form heterogeneous systems, consisting of base material, filler metal and diffusion zone. Under mechanical loading, the properties of brazed joints vary significantly from those of the individual joining partners. The complex deformation behavior of the brazed joint is characterized by geometrical and microstructural interactions as e.g. by different elastic-plastic properties of substrate material and thin braze layer. Uniaxial loading and the constrained deformation of the thin filler alloy layer can lead to a triaxial stress state which strongly influences the joint performance [4]. To estimate the influence of defects on bulk materials and on welded structures under quasi-static loadings, defect assessment procedures, such as R6, BS7910 or SINTAP have been developed [5 - 8]. In the scope of previous investigations [9], it has been shown that the R6-procedure can also be used to estimate the influence of defects on brazed joints. In previous studies, a rather unusual fatigue crack growth behavior of brazed steel joints, i.e. extremely steep da/dN-ΔK curves in comparison with the ones of bulk materials, was observed [4]. It could be shown that under defined boundary conditions, the influence of defects on the fatigue lifetime can be estimated based on the stress intensity factor caused by the defect [10], but up to date information on the general influence of brazing defects on the fatigue behavior of brazed joints is lacking. Therefore, a better understanding of fatigue crack initiation and propagation is very important for reliable life-time predictions of brazed joints. Generally, during stress-controlled fatigue tests with metallic materials, fatigue is closely related to the evolution of (local) irreversible strains. In homogenous materials, the strains are relatively equally distributed over a macroscopic length scale and can therefore be averaged using integrating measurement techniques, e.g. with extensometers. However, plastic strains in brazed joints are usually localized in a small volume in or next to the brazing zone. The thickness of braze layers is usually in the range of 50-100 µm, so conventional strain measuring techniques cannot be applied reasonably. To measure local strains during cyclic loading with a sufficiently high resolution, digital image correlations (DIC) were performed. The technique was applied to monitor the fatigue induced damage evolution in brazed steel joints during cyclic loading, in particular under the influence of brazing defects. Furthermore, the fracture surfaces were analyzed by SEM to correlate the experimental results with characteristic deformation features and to obtain information on the underlying failure mechanisms.
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