13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Characterization of the fatigue behavior of brazed steel joints by digital image correlation (DIC) Michael Koster1,*, Christoph Kenel1, Christian Leinenbach1 1 Empa - Swiss Federal Laboratories for Materials Science and Technology; Laboratory for Joining Technologies and Corrosion; Überlandstrasse 129; 8600 Dübendorf; Switzerland * Corresponding author: Michael.Koster@empa.ch Abstract In the actual work, the fatigue induced damage evolution in brazed steel joints - in particular under the influence of artificial brazing defects - was investigated using digital image correlation (DIC). The brazed joints consist of the soft martensitic stainless steel AISI CA 6-NM as substrate material and Au 18wt.-% Ni as a filler metal. Brazing was performed in a shielding gas furnace with an H2 atmosphere. Besides the lifetime-oriented investigations, the cyclic deformation behavior was investigated using DIC. The measurements show that it is possible to monitor localized accumulating fatigue damage in an early state of the experiment. Besides the determination of the local strains, the deformation zones in the vicinity of strain concentrations could be characterized in detail. A high frequency camera allowed characterizing the damage evolution in the brazing zone during cyclic loading and investigating crack initiation and crack propagation during cyclic loading. The results are in correlation with SEM investigations and allow a more profound understanding of the failure mechanisms of brazed steel joints. Keywords brazing, fatigue, defect, fracture, DIC Nomenclature Roman Symbols a [mm] Defect size A [%] Elongation at fracture BSE [--] Back scattered electrons d [mm] Thickness of the braze layer DIC [--] Digital image correlation E [MPa] Young’s Modulus Nf [--] Number of cycles to fracture Nmax [--] Maximum number of cycles R [--] Load ratio SEM [--] Scanning electron microscopy Greek Symbols σUTS [MPa] Ultimate tensile strength σmax [MPa] Maximum applied load for cyclic testing on the defect-free assumed cross section σy [MPa] Yield strength
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