Dokument 1.pdf (35.736 KB) - RWTH Aachen University

Kurzfassung / Abstract
Vorteile numerischer Schädigungsanalysen gegenüber der FITNET-Prozedur bei der Auslegung von
hybridlasergeschweißten Bauteilen demonstriert.
Experimental and numerical analysis of damage behaviour of laser-hybrid welds
The recently observed widespread application of laser-hybrid welding process in different fields of
manufacturing industry results from advantages of both laser welding and conventional arc beam
welding processes. With growing application of laser-hybrid welding, the demand for characterisation
of mechanical properties of laser-hybrid weld and of their influence on the structural performance
increases constantly. The accurate fracture prediction is required for the utilisation of possible safety
reserves thus providing economical and safe design of components. In the frame of this work
experimental and numerical analyses are performed with objective to characterise ductile and brittle
fracture behaviour of flawed laser-hybrid welds for three different constructional steels (S355, EH36
und RQT701).
Micromechanically based GTN (Gurson, Tvergaard and Needleman) damage model has been
successfully applied for numerical description of crack initiation and crack propagation. The required
model parameters are determined and verified by means of combined numerical and experimental
investigations (metallographic analyses and tests on the fracture mechanics and Wide Plate specimens).
Subsequently, the phenomena of crack path deviation (CPD) frequently observed in laser and hybrid
laser welds is numerically simulated and analysed by the verified GTN model. It can be confirmed, that
the tendency to CPD depends strongly on the development of stress triaxiality and plastic strains at the
interface between base and weld metal. Regarding component safety, the CPD can be evaluated
positively due to the crack resistance increase resulting from higher energy consumption necessary for
the crack deviation in the base metal.
Besides the ductile fracture behaviour, the influence of the laser-hybrid weld on the toughness
properties in low shelf and transition temperature region has been investigated by means of fracture
mechanics tests and subsequent evaluation with master curve concept. The results show significant
dependence of determined reference temperature T0 on the stress triaxiality varied by different
specimen geometry and initial crack size. The two parameter criterion seems to be suitable to provide a
conservative prediction of brittle behaviour of low constraint fracture mechanics specimens and thus
flawed hybrid welded components. The accuracy of this prediction becomes higher with increasing
failure probability. Furthermore the Beremin-model has been evaluated with respect to describe the
onset of cleavage process of laser-hybrid welds. It can be concluded, that the estimation of the cleavage
failure is only possible by modifying the original model, e.g. by introducing temperature dependence of
the Weibull parameter σu.
The safety assessment of investigated Wide Plate specimens according to FITNET procedure shows,
that the accuracy of failure prediction can be increased by knowledge of measured residual stresses and
toughness values of shallow crack specimens. The advantages of numerical damage analysis for design
of hybrid welded components when compared to FITNET procedure are demonstrated on different
examples for practical application of laser-hybrid welds.
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