On-line damage detection and monitoring at thin sheet joints for deriving failure criteria

Determined fatigue strength characteristics of self-piercing riveted, spot-welded and laser beam welded specimens made of steel and aluminum thin sheets as well as their failure behavior are analyzed and compared. Various methods for on-line damage detection and monitoring are applied and evaluated in this context. The analysis of stiffness and frequency are accentuated as methods, which are easy to use and appropriate to detect the initiation of a technical size crack. Correlations between the degree of damage and the change in stiffness or frequency are set up experimentally and verified numerically via finite element analysis (FEM). The failure criteria "initiation of a technical size crack" (a >= 0.5 mm) and "rupture" are recommended for evaluating different joining techniques with regard to their fatigue strength. The presented investigations are based on a multitude of fatigue tests under constant amplitude loading conducted in the context of two joint research projects.