@article{WiesentSchultheissLullaetal., author = {Wiesent, Lisa and Schultheiß, Ulrich and Lulla, Philipp and Nonn, Aida and Noster, Ulf}, title = {Mechanical properties of small structures built by selective laser melting 316 L stainless steel - a phenomenological approach to improve component design}, series = {Materials Science \& Engineering Technology}, volume = {51}, journal = {Materials Science \& Engineering Technology}, number = {12}, publisher = {Wiley}, doi = {10.1002/mawe.202000038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-14718}, pages = {1615 -- 1629}, abstract = {Experimental investigations are conducted to quantify the influence of specimen thickness and orientation on the mechanical properties of selective laser melted stainless steel 316 L. The results indicate that the mechanical strength and ductility increase with increasing specimen thickness until a saturation value is reached from a specimen thickness of about 2 mm. Specimen orientation dependency is pronounced for thin specimens (<1.5 mm), whereas only small deviations in strength are observed for thicker specimens with orientations of 30°, 45° and 90° to build direction. The mechanical properties of the specimen orientation of 0° to build direction shows great deviation to the other orientations and the smallest overall strength. A reliable design of selective laser melted components should account for specimen thickness and orientation, e. g. by a correction factor. Furthermore, it is recommended to avoid loads vertical (90°) and parallel (0°) to build direction to guarantee higher ductility and strength.}, language = {en} } @inproceedings{ParedesKeimNonnetal., author = {Paredes, Marcelo and Keim, Vincent and Nonn, Aida and Wierzbicki, Tomasz}, title = {Effect of plasticity parameter on the crack propagation in steel pipelines}, series = {Proceedings of the conference on Technology for future and ageing piplines, Ghent, Belgium}, booktitle = {Proceedings of the conference on Technology for future and ageing piplines, Ghent, Belgium}, language = {en} } @article{WiesentSchultheissSchmidetal., author = {Wiesent, Lisa and Schultheiss, Ulrich and Schmid, Christof and Schratzenstaller, Thomas and Nonn, Aida}, title = {Experimentally validated simulation of coronary stents considering different dogboning ratios and asymmetric stent positioning}, series = {PlOS One}, volume = {14}, journal = {PlOS One}, number = {10}, publisher = {PLOS}, doi = {10.1371/journal.pone.0224026}, pages = {1 -- 25}, abstract = {In-stent restenosis remains a major problem of arteriosclerosis treatment by stenting. Expansion-optimized stents could reduce this problem. With numerical simulations, stent designs/ expansion behaviours can be effectively analyzed. For reasons of efficiency, simplified models of balloon-expandable stents are often used, but their accuracy must be challenged due to insufficient experimental validation. In this work, a realistic stent life-cycle simulation has been performed including balloon folding, stent crimping and free expansion of the balloon-stent-system. The successful simulation and validation of two stent designs with homogenous and heterogeneous stent stiffness and an asymmetrically positioned stent on the balloon catheter confirm the universal applicability of the simulation approach. Dogboning ratio, as well as the final dimensions of the folded balloon, the crimped and expanded stent, correspond well to the experimental dimensions with only slight deviations. In contrast to the detailed stent life-cycle simulation, a displacement-controlled simulation can not predict the transient stent expansion, but is suitable to reproduce the final expanded stent shape and the associated stress states. The detailed stent life-cycle simulation is thus essential for stent expansion analysis/optimization, whereas for reasons of computational efficiency, the displacement-controlled approach can be considered in the context of pure stress analysis.}, subject = {Stent}, language = {en} } @article{WiesentSchultheissLullaetal., author = {Wiesent, Lisa and Schultheiß, Ulrich and Lulla, Philipp and Noster, Ulf and Schratzenstaller, Thomas and Schmid, Christof and Nonn, Aida and Spear, Ashley}, title = {Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents}, series = {PLoS ONE}, volume = {15}, journal = {PLoS ONE}, number = {12}, publisher = {PLOS}, doi = {10.1371/journal.pone.0244463}, pages = {1 -- 30}, abstract = {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.}, subject = {Koronarendoprothese}, language = {en} } @article{BoyceKramerBosiljevacetal., author = {Boyce, Brad L. and Kramer, Sharlotte L. B. and Bosiljevac, T. R. and Corona, Edmundo and Moore, J. A. and Elkhodary, Khalil and Simha, C. Hari Manoj and Williams, Bruce W. and Cerrone, Albert R. and Nonn, Aida and Hochhalter, Jacob D. and Bomarito, Geoffrey F. and Warner, James E. and Carter, Bruce J. and Warner, Derek H. and Ingraffea, Anthony R. and Zhang, T. and Fang, X. and Lua, Jim and Chiaruttini, Vincent and Maziere, Matthieu and Feld-Payet, Sylvia and Yastrebov, Vladislav A. and Besson, Jacques and Chaboche, Jean Louis and Lian, J. and Di, Y. and Wu, Bei and Novokshanov, Denis and Vajragupta, Napat and Kucharczyk, Pawel and Brinnel, Viktoria and Doebereiner, Benedikt and Muenstermann, Sebastian and Neilsen, Michael K. and Dion, Kristin and Karlson, Kyle N. and Foulk, James Wesley and Brown, Arthur A. and Veilleux, Michael G. and Bignell, John L. and Sanborn, Scott E. and Jones, Chris A. and Mattie, Patrick D. and Pack, Keunhwan and Wierzbicki, Tomasz and Chi, Sheng-Wei and Lin, S.-P. and Mahdavi, Ashkan and Predan, Jozef and Zadravec, Janko and Gross, Andrew J. and Ravi-Chandar, KRISHNASWAMY and Xue, Liang}, title = {The second Sandia Fracture Challenge: predictions of ductile failure under quasi-static and moderate-rate dynamic loading}, series = {International journal of fracture}, journal = {International journal of fracture}, number = {198, 1-2}, publisher = {Springer}, doi = {10.1007/s10704-016-0089-7}, pages = {5 -- 100}, abstract = {Ductile failure of structural metals is relevant to a wide range of engineering scenarios. Computational methods are employed to anticipate the critical conditions of failure, yet they sometimes provide inaccurate and misleading predictions. Challenge scenarios, such as the one presented in the current work, provide an opportunity to assess the blind, quantitative predictive ability of simulation methods against a previously unseen failure problem. Rather than evaluate the predictions of a single simulation approach, the Sandia Fracture Challenge relies on numerous volunteer teams with expertise in computational mechanics to apply a broad range of computational methods, numerical algorithms, and constitutive models to the challenge. This exercise is intended to evaluate the state of health of technologies available for failure prediction. In the first Sandia Fracture Challenge, a wide range of issues were raised in ductile failure modeling, including a lack of consistency in failure models, the importance of shear calibration data, and difficulties in quantifying the uncertainty of prediction [see Boyce et al. (Int J Fract 186:5-68, 2014) for details of these observations]. This second Sandia Fracture Challenge investigated the ductile rupture of a Ti-6Al-4V sheet under both quasi-static and modest-rate dynamic loading (failure in 0.1 s). Like the previous challenge, the sheet had an unusual arrangement of notches and holes that added geometric complexity and fostered a competition between tensile- and shear-dominated failure modes. The teams were asked to predict the fracture path and quantitative far-field failure metrics such as the peak force and displacement to cause crack initiation. Fourteen teams contributed blind predictions, and the experimental outcomes were quantified in three independent test labs. Additional shortcomings were revealed in this second challenge such as inconsistency in the application of appropriate boundary conditions, need for a thermomechanical treatment of the heat generation in the dynamic loading condition, and further difficulties in model calibration based on limited real-world engineering data. As with the prior challenge, this work not only documents the 'state-of-the-art' in computational failure prediction of ductile tearing scenarios, but also provides a detailed dataset for non-blind assessment of alternative methods.}, language = {en} } @inproceedings{KeimNonnLenzetal., author = {Keim, Vincent and Nonn, Aida and Lenz, D. and Brinnel, Viktoria and M{\"u}nstermann, Sebastian}, title = {Simulation of the ductile fracture behaviour of high toughness pipeline steels using combined damage models}, series = {Proceedings of the conference on Technology for future and ageing piplines, Ghent, Belgium}, booktitle = {Proceedings of the conference on Technology for future and ageing piplines, Ghent, Belgium}, language = {en} } @inproceedings{NonnKalwa, author = {Nonn, Aida and Kalwa, Christoph}, title = {Simulation of ductile crack propagation in high-strength pipeline steel using damage models}, series = {9th International Pipeline Conference 2012 (IPC 2012), Calgary, Canada}, booktitle = {9th International Pipeline Conference 2012 (IPC 2012), Calgary, Canada}, doi = {10.1115/IPC2012-90653}, pages = {7}, abstract = {The performance of engineering design of high-strength steel pipelines has revealed the necessity to revise current design procedures. Therefore, an improved and detailed comprehension of fracture mechanisms and development of failure prediction tools are required in order to derive new design criteria. In last decades the most successful failure prediction tools for steel structures subjected to various type of loading can be encountered in the field of damage mechanics. This paper aims to describe ductile fracture behavior of high-strength steel pipelines by applying three different damage models, Gurson-Tvergaard-Needelman (GTN), Fracture Locus Curve (FLC) and Cohesive Zone (CZ). These models are evaluated regarding their capability to estimate ductile crack propagation in laboratory specimens and linepipe components without adjusting the calibrated parameters. It can be shown that appropriate parameter sets can be identified to reproduce load-deformation and fracture resistance curves accurately. The strain rate effect on the fracture behavior is examined by dynamic tests on the BDWT specimens. Finally, the shortcomings of the applied models are pointed out with the reference to possible extensions and modifications.}, language = {en} } @article{SadeghpourNonn, author = {Sadeghpour, Ebrahim and Nonn, Aida}, title = {Data-driven models for structure-property prediction in additively manufactured steels}, series = {Computational Materials Science}, volume = {215}, journal = {Computational Materials Science}, publisher = {Elsevier}, doi = {10.1016/j.commatsci.2022.111782}, abstract = {Data-driven models are developed to predict the mechanical properties of polycrystalline materials. The case study is the prediction of the yield strength of a 3D-printed 316L steel from morphological and crystallographic features. Three different artificial intelligence models including feed-forward (FNN), convolution (CNN), and graph (GNN) neural networks are employed to train the data-driven models and are compared in terms of performance and computational requirements. The dataset required for training is generated by performing crystal plasticity finite element simulations. The FNN model has the smallest input size and takes in some statistical parameters describing the material microstructure, but its accuracy is relatively low. The CNN approach inputs voxel-based realizations of the microstructure and is able to give accurate estimations; however, its training process is time-consuming and computationally expensive. In the GNN approach, the polycrystalline material is represented by a graph whose nodes and lines represent the grains and adjacency between grains. It is observed that GNN yields a better performance compared to the other two approaches and has the capability of handling complex tasks.}, language = {en} } @article{KramerJonesMostafaetal., author = {Kramer, Sharlotte L. B. and Jones, Amanda and Mostafa, Ahmed and Ravaji, Babak and Tancogne-Dejean, Thomas and Roth, Christian C. and Nonn, Aida}, title = {The third Sandia fracture challenge: predictions of ductile fracture in additively manufactured metal}, series = {International Journal of Fracture}, volume = {218}, journal = {International Journal of Fracture}, publisher = {Springer Nature}, doi = {10.1007/s10704-019-00361-1}, pages = {5 -- 61}, abstract = {The Sandia Fracture Challenges provide a forum for the mechanics community to assess its ability to predict ductile fracture through a blind, round-robin format where mechanicians are challenged to predict the deformation and failure of an arbitrary geometry given experimental calibration data. The Third Challenge (SFC3) required participants to predict fracture in an additively manufactured (AM) 316L stainless steel bar containing through holes and internal cavities that could not have been conventionally machined. The volunteer participants were provided extensive data including tension and notched tensions tests of 316L specimens built on the same build-plate as the Challenge geometry, micro-CT scans of the Challenge specimens and geometric measurements of the feature based on the scans, electron backscatter diffraction (EBSD) information on grain texture, and post-test fractography of the calibration specimens. Surprisingly, the global behavior of the SFC3 geometry specimens had modest variability despite being made of AM metal, with all of the SFC3 geometry specimens failing under the same failure mode. This is attributed to the large stress concentrations from the holes overwhelming the stochastic local influence of the AM voids and surface roughness. The teams were asked to predict a number of quantities of interest in the response based on global and local measures that were compared to experimental data, based partly on Digital Image Correlation (DIC) measurements of surface displacements and strains, including predictions of variability in the resulting fracture response, as the basis for assessment of the predictive capabilities of the modeling and simulation strategies. Twenty-one teams submitted predictions obtained from a variety of methods: the finite element method (FEM) or the mesh-free, peridynamic method; solvers with explicit time integration, implicit time integration, or quasi-statics; fracture methods including element deletion, peridynamics with bond damage, XFEM, damage (stiffness degradation), and adaptive remeshing. These predictions utilized many different material models: plasticity models including J2 plasticity or Hill yield with isotropic hardening, mixed Swift-Voce hardening, kinematic hardening, or custom hardening curves; fracture criteria including GTN model, Hosford-Coulomb, triaxiality-dependent strain, critical fracture energy, damage-based model, critical void volume fraction, and Johnson-Cook model; and damage evolution models including damage accumulation and evolution, crack band model, fracture energy, displacement value threshold, incremental stress triaxiality, Cocks-Ashby void growth, and void nucleation, growth, and coalescence. Teams used various combinations of calibration data from tensile specimens, the notched tensile specimens, and literature data. A detailed comparison of results based of these different methods is presented in this paper to suggest a set of best practices for modeling ductile fracture in situations like the SFC3 AM-material problem. All blind predictions identified the nominal crack path and initiation location correctly. The SFC3 participants generally fared better in their global predictions of deformation and failure than the participants in the previous Challenges, suggesting the relative maturity of the models used and adoption of best practices from previous Challenges. This paper provides detailed analyses of the results, including discussion of the utility of the provided data, challenges of the experimental-numerical comparison, defects in the AM material, and human factors.}, language = {en} } @article{KeimMarxNonnetal., author = {Keim, Vincent and Marx, P. and Nonn, Aida and M{\"u}nstermann, Sebastian}, title = {Fluid-structure-interaction modeling of dynamic fracture propagation in pipelines transporting natural gases and CO2-mixtures}, series = {International Journal of Pressure Vessels and Piping}, volume = {175}, journal = {International Journal of Pressure Vessels and Piping}, number = {August}, publisher = {Elsevier}, doi = {10.1016/j.ijpvp.2019.103934}, abstract = {As part of current design standards, the Battelle Two-Curve Model (BTCM) is still widely used to predict and secure ductile crack arrest in gas transmission pipelines. For modern linepipe steels and rich natural gases or CO2 mixtures, the BTCM might lead to incorrect predictions. On the one hand, it suffers from the insufficient description of the individual physical processes in the pipe material and fluid itself. Furthermore, the model does not account for fluid-structure-interaction (FSI) effects during simultaneous running-ductile fracture (RDF) and mixture decompression. Numerical FSI models allow for a more sophisticated, coupled analysis of the driving forces for the failure of pipelines. This paper deals with the development of an FSI model for the coupled prediction of 3D pressure profiles acting on the inner pipe wall during crack propagation. The coupled Euler-Lagrange (CEL) method is used to link the fluid and structure models. In a Lagrange formulation, the modified Bai-Wierzbicki (MBW) model describes the plastic deformation and ductile fracture as a function of the underlying stress/strain conditions. The fluid behavior is calculated in a 3D model space by Euler equations and the GERG-2008 reference equation of state (EOS). The coupled CEL model is used to predict the RDF in small-diameter pipe sections for different fluid mixtures. The calculated 3D pressure distributions ahead and behind the running crack tip (CT) significantly differ in axial and circumferential directions depending on the mixture composition. The predicted FSI between the pipe wall and fluid decompression in 3D CEL/FSI model provides reliable knowledge about the pressure loading of the pipeline during RDF.}, subject = {Fluid-Struktur-Wechselwirkung}, language = {en} }