Experimentelle und numerische Analyse des Schädigungsverhaltens von Hybridlaserschweißverbindungen

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. 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

Experimentelle und numerische Analyse des Schädigungsverhaltens von Hybridlaserschweißverbindungen

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. 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

Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents

Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 +/- 0.63 N) or the heat treated condition (5.87 +/- 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 +/- 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41% in the as-built and by 59% in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures